Kitfox abrupt departure sensitivity to low speed stalls [Archive]

jamesbdunn

12-23-2016, 06:29 PM

:confused: STOL Kit features & Kitfox low and slow sensitivities

I’ve been reading Kitfox accident reports. By far the leading cause of fatalities is low and slow abrupt departure of one wing stall and sometimes a related spin.

A tendency was to cite small wind dying out, or gusting, while flying at slow speeds; especially at high power. Also mentioned were small observed sudden control movements initiating stalling of one wing while in a turning moment. Slow flight turns frequently resulted in uncontrollable stalls of one wing.

The leading cause of aircraft damage is ground looping.

Does anyone have experience with the Sportsman STOL Kit?
It is NOT rated for a Kitfox, or at least not that I know of.

I’ve been reading about their performance on Cessna 170 aircraft; phenomenal. Even in slow and tight turns the wings will not stall. The approach angle of attack was more flat, providing greater visibility. Also, there were reports of the planes having a feeling of being more stable at touchdown.

Any PIREP’s to share here related to:
1) Sportsman STOL Kit actual performance
2) Retrofitting Sportsman STOL Kit features onto a Kitfox wing
3) Stuff I’m not considering in-general

The Sportsman STOL Kit is reported not to affect cruise speed.

jonstark

12-23-2016, 07:35 PM

I operate three Cessna 180/182. The 64G has the standard wing, the 56 182 has MASA and WingX, and the 56 180 has Sportsman. The Sportsman equipped is my personal favorite. Slow flight is simply amazing! Stall horn blaring at 45mph indicated and still very controllable. If anything it increased my cruise speed. A Sportsman wing WILL stall though. Any wing will. Don't kid yourself!

Sportsman isn't available for any Kitfox aircraft. It is a certified modification. I'm sure Will Stene there can answer any questions you may have though.

You might contact Joa Harrison with landshorter.com about his Vortex Generators. I intend to install a set soon after flying my IV to compare before and after characteristics. They have worked wonders on another aircraft I fly.

Jon

Av8r3400

12-23-2016, 08:02 PM

I think you'll find that the "stall spin" accident scenario is common across the board on all aircraft. There is nothing inherently wrong with the Kitfox wing or it's configuration that proper pilot training wouldn't solve.

These are STOL designed aircraft meant for these activities. The most important part is proper training and experience at altitude prior to doing these maneuvers near the ground.

A sportsman cuff is an attempt at fixing a non-problem with this plane.

Esser

12-23-2016, 08:15 PM

Don't mean to be rude James but get some spin training. I can't recommend it enough. It still blows my mind that this isn't a basic requirement to get a pilot license in the US of A. I don't even consider myself checked out in an airplane unless I spin it and get a feel for how it feels behind the power curve. Of course some planes you can't safely do this. For sure this one thing won't save you completely but it's a good start. For example, after I spun an SZD-55 glider, I had a tonne of respect for getting it low and slow cause it will drop 800' in a blink of an eye. And after that. I never ever put it close to a wing drop situation because I knew the outcome.

I also think that most plans max accidents are low and slow. Kitfox has zero documented inflight structural failures. The rest comes down to pilot error or power issues.

If everyone wanted a plane that flew like a 170, we would all own 170s.

av8rps

12-23-2016, 08:28 PM

I'll take a stab at this...

A bunch of years ago I had read a report that said the Piper Super Cub had the highest Stall-Spin fatality rate of any single engine aicraft. The reason for that was not that the Super Cub was a bad airplane, as everyone knows a Super Cub is one of the nicest flying, most docile airplanes you can own. So why so many bad accidents?

The answer as explained in the report was amazingly simple.... because of the exceptional capabilites of the Super Cub, it is being operated at its limits in challenginge operating environments. So in effect, if the design wasn't so much better than most others it probably wouldn't be subjected to those kind of situations, and then it wouldn't be having all those accidents.

The more I thought about that after reading it, the more that made sense to me. And now because STOL flying is becoming more popular than ever, that accident rate is likely to go up unless the pilot operators get better training to develop their skills, AND they improve their ability to use better judgement so as to avoid operating the aircraft too far at the outer limits of the envelope. It isn't the airplane...

I've been flying these planes since 1986 (Kitfoxes, Avids, Highlanders). And not that all the models operate exactly the same, but in my experienced opinion, the Kitfox is a docile, easy to fly airplane.

BUT like any airplane, if you fly it too slow and push it too hard you can get in trouble with it. So you need to be properly trained, being taught what you can do with it, and what you can't do with it. If you were to read all of my past years of posting you will stumble on my comments at least a few times that say I tend to carry a little extra speed in takeoff and landing modes than most, as I've tested my Kitfox enough to know at what point it will drop a wing in the stall. But again, every airplane I've ever flown has a limit that needs to be respected. And just for the record, my current Kitfox will drop a wing sooner than most, as it was built with less washout in the wingtips than what was recommended in the building manual. But even with less washout than suggested it is a really nice flying, predictable airplane.

I'm sure you could put some mods on the wing to make it even more docile than it already is. But I think to bring accident rates down it would be better to have new Kitfox owners get some Kitfox type specific traing from people like Paul L.at Stick and Rudder Aviation.

In a nutshell, I think it's likely the Kitfox is flying better than we are ;).

PapuaPilot

12-23-2016, 10:28 PM

I think your questions have been answered.

I have flown both the Sportsman and Robertson equipped C-206. Both of them are much better in doing STOL then the with standard Cessna wing. The two fly different from each other. I don't recall any noticeable reduction in cruise speed.

The Sportsman is a drooped leading edge that is riveted on the existing Cessna wing, which makes a different airfoil that stalls at a lower airspeed. It is docile, but still stalls. Most often when a plane drops a wing in a stall it is because the plane is not coordinated. That is simply pilot error, whatever the reason.

Without a doubt modifications can be done to the standard KF to make it land slower. You could do things like adding vortex generators, gap seals, a different wing (Kitfox has the STi wing) or slotted leading edges (get the Just Aircraft Super STOL). It all depends on what performance you are looking for. If you want to land slower than don't expect a fast cruise speed.

I really like the balance that the Kitfox gives; good STOL ability and good cruise speed too. I found it funny that I passed a 180HP Super Cub in cruise (this past summer). Granted the Super Cub could land a little short, but not by much.

I enjoy doing STOL approaches at 1.1 Vso, but it has to be very calm to do so. Like others have said you have to get to know your aircraft. Every aircraft has it's capabilities, but it takes a pilot that has been trained, practices and has the skill to fly the plane. Not all of us are going to be a Bob Hoover. It is important to know our limitations and to stay within them.

airlina

12-24-2016, 06:31 AM

I remember an old saying I once heard-" The Piper Cub is the safest airplane in the world, it can just barely kill you! "

av8rps

12-24-2016, 07:40 AM

For what it's worth...

Steve Henry (the deadstick takeoff Highlander guy) has recently put a leading edge cuff on his Highlander and says it is the best mod he's done to it (and he's done a bunch). As I recall it dropped his already ridiculously low stall speed so much that he can land his modified Highlander within 10% the distance of his Super STOL. And he said it did nothing to his cruise speed. So he's pretty happy with his highly modified (now 145 hp Yamah Apex powered too) Highlander. However, the Highlander has the same airfoil the Model 1-3 Kitfox used, not the newer Riblett airfoil that is used on the newer Kitfoxes. So I don't know that it would do much on the newer airfoil, which ironically already uses a plastic leading edge extension, but for speed, not STOL reasons. I would've thought Mr. Riblett would have done that when he redesigned the wing as he certainly was a smart aerodynamicist. But who knows, maybe just drooping the leading edge on the Riblett wing would help to lower the stall without affecting the top speed?

With all that said, I have to say this: I really like the way my Kitfox flies, and performs. So I'm gonna leave it just the way it is. But who knows, maybe a sportsman type cuff could make it an even better airplane?

I told my buddy once who was insistent on streamlining his airplane but was overwhemed with all the details required to make fairings. "All it takes is some temporary plastic sheeting or cardboard and a bunch of duct tape to know if it works. Oh, and a test pilot" :)

jamesbdunn

12-25-2016, 05:06 PM

Thank you ALL for your individual insights.

I'm taking them all in.

Bud Davidson

12-26-2016, 06:09 PM

I am fascinated by the references to Harry Riblett's airfoils. Harry was, and may still be a member of EAA Chapter 240 in Wilmington, Del. When his book on airfoils came out I read it with intent of applying his work to the wings I was doing at the time. I happened to be President of the Chapter at that time.
What Harry indicated he had done was finish some work on airfoils done by his associates at what I think was NACA at the time. This was a lot of years ago so my memory may be faulty here. But, what I understood was he interpolated the original data thereby creating new light aircraft airfoils which work was abandoned when the industry suffered a downturn.
The point is, that I do not believe Harry ever conducted experiments with airfoils. The Avid/kit fox application is unique. Harry led the construction of a WAG Aero CUBY for our Chapter but did not incorporate a revised airfoil. He recommended I not redo my wing ribs suggesting the benefit would not outweigh the effort. So, that experimental application also never occurred. I have never heard of any other applications. But I left that Chapter 15 yrs ago.
Any of you who have read his book will observe his science and engineering without benefit of an editor. That does not detract from his remarkable work.
Bud

av8rps

12-26-2016, 08:46 PM

Interesting info about Mr. Riblett. Pretty cool that someone here on the forum knows him.

Somewhere in my archives I have his letter where he explains to Avid what he did with their airfoil and why, but they never took him up on his suggestion to change their wing. But Kitfox did, which ultimately proved he was right about what he said in the letter. And I guess we can say the rest is history...

Rooster

12-27-2016, 06:41 AM

AV8RPS has mentioned Steve Henry a post or two down.
What People must take note of is that he flies upwards of 5 hours per week, I'm told....

DesertFox4

12-27-2016, 12:15 PM

I was told by Murle Williams some time ago that Harry Riblett passed away. :(

I found his obit. in a Delaware paper. See below. He and Murle communicated frequently. I've seen original drawings of new Riblett wing designs in Murles file. Very interesting work. I beleive Harry was instrumental in Murle adding the extrusion on Murle's earliest Kitfox build' wing, a model 3 that was awarded Grand Champion back in the late 80's or very early 90's. This leading edge extrusion was simply a pvc pipe of about 3/4 inch cut in two length-wise and bonded in the proper location to the front spar. My impression is it increased the wing's speed. I hope my recollections are accurate but it has been 10 years or more since Murle discussed this topic with me. I may inquire next time I stop at Murle's to varify specifics. Suffice to say that Harry was very skilled at wing designing. Would have enjoyed meeting him.

" Harry Calvin Riblett, Jr. was born in California on November 14, 121061929. He passed away in Wilmington on December 23, 2012.

Harry was a pilot and built two airplanes at home, a Starduster II 150 HP, 2-seat biplane and an 85 HP Piper Cub replica (as a group project with other pilots). He also researched airfoil designs, convinced that safer airfoils would save lives. His first book, "GA Airfoils" shows airfoils superior to the currently available NACA (government) airfoils. Both his first and second book, "Spin Resistant Airfoils", are currently being used by aircraft designers in the Experimental Aviation Association (EAA). Harry's expertise was renowned worldwide and he served as an expert witness on several legal cases involving airplane crashes."

efwd

12-27-2016, 06:25 PM

Steve! Do you get "time" from some source in the universe that the rest of us aren't aware of? I mean, the work going on in skunkworks with the carbon fiber and all and you manage this forum with all the research of various stuff. Love it. :)

jamesbdunn

01-01-2017, 10:32 AM

Don't mean to be rude James but get some spin training. I can't recommend it enough.

Based on everyone's input, I'm definitely going to investigate the details of installing the equivalent of a Sportsman Stall kit. Which I guess begins with contacting my local FAA Inspector.

For those who advised against modification, your input was equally as valuable. Providing me with perspectives I might not otherwise have considered. My choice is biased by my personality, to take on new projects.

I understand to avoid a low altitude spin, that about 20 mph of additional speed is desired above Vso before climbing out of ground effect, to offset the likelihood of a tail wind gust; in the case of a headwind, wind dying out; and in case of turbulent crosswinds and different wind directions above the treeline.

On takeoff the likelihood of recovering from a spin is unlikely.

My interest is in being better "equipped" to deal with rogue conditions. And be able to fly closer to the edges of expected performance, without abrupt consequences. I want to feel an expected approach to a flight limit, and have the time and performance resources available to respond.

I have received spin training, and I agree it should be required as part of certification. However, I can see how it might cause the cost of liability insurance for flight instructors to increase. Once a student learns spin recovery, bad judgement might cause more accidents during training. Where the student because of the fun factor, attempts spins during training on their own. Or recently certified pilots taking joy rides with passengers and doing spins without sufficient practiced experience. Hard call, because of legal system, insurance, and human factors.

I remember my first spin. The instructor asked if I studied the procedure; I had. He talked me through entering the spin, but did not prompt me on what to do. I knew what I was "supposed" to do. But my senses were overwhelmed. After struggling a couple of rotations I asked the instructor to recover; which he did in about 8 seconds.

He then did a spin and talked me through what he was doing, and why. He recovered.

Then he had me set up and enter a spin. This time my senses knew what to expect and it was cake. .... More like cake with frosting topped with Skittles !!

I really liked it and asked to do another spin. I couldn't help it ... I yelped Yeeee Haaaa

I did a couple more and our instruction was finished.

I felt so empowered to learn something in so short a time that previously put me on edge, to give me a total feeling of control.

But I'm not arrogant (loathing of others), I know my feeling of control can come back and bite me unless I stay diligent, and learn more from others.

===========

I'm a newbie. So if more experience chimes in here, I certainly take no offense.

When I sold my Varieze the pilot had many more hours of experience than I had. I warned him that to set down on the numbers he had to set up to land long before the numbers and bleed off ground speed because the Varieze floats in ground effect forever if above 70 knots. I warned him of the need to pump the brakes to minimize gear resonance during hard braking, and advised him to do his first landing on the widest and longest runway he can find. But he took off the following day with ice and snow on the short 3000' runway, clipped the top of a tree (witnessed by local A&P) most likely because of excessive braking to steer, flew to near fuel exhaustion to around St Louis somewhere, came in on a short and narrow runway too hot, floated to past the middle of the runway, tried to force it to stop instead of going around, went off the end of the runway into a ditch where a log severed the landing gear and the propeller chopped into the log. I know this because the pilot called me at work and asked how to take off the wings.

I have personal insight as to the importance of listening to experience.

I not only take no offense to your suggestion, I appreciate your concern.

James

Av8r3400

01-01-2017, 12:34 PM

James,

First off, do you have a Kitfox now or how much time do you have in one? Or are you in the investigation to buy phase of things?

Second, before delving headlong into fixing a non-problem with an untried, non-solution, fly and get to know the airplane as it was designed. Kitfoxes work and perform very, very well as designed. This current airfoil has been in use on the Kitfoxes for over 25 years and has earned a very good reputation.

Adding 20 mph to your airspeed before climbing out of ground effect is ludicrous. This is not a VariEz. It is a very light and very forgiving airplane that will do just what you tell it to do. If you are flying in conditions of 20+ mph gusts, you need to pick another day to go flying.

Basing your course of action on accident records of questionable people doing questionable things with an aircraft and the trying to design out their mistakes, imo, will only lead to frustration if not outright trouble.

Rooster

01-02-2017, 12:24 AM

jtpitkin06

01-02-2017, 05:32 PM

The FAA has long had a list of hot items related to accidents. At the top of the list for several years is loss of control. Instructors, check airmen and designated examiners are encouraged to address the problem during instruction and check rides.

The good news is that we are getting better in the experimental community. Loss of control accidents have decreased significantly.

Although slow speed low altitude loss of control is the leading cause of accidents in the Kitfox, it does not mean the aircraft is prone to stall and spin more than other aircraft. The number of accidents per hour flown is actually lower than many other aircraft to include both experimental and certified aircraft.

For those not familiar with the flight characteristics of the Kitfox, do not be alarmed.
In simple terms, the Kitfox is one of the safest aircraft you can fly.

John P
Greenville Tx

jamesmil

01-04-2017, 06:07 PM

If I may I would like to add to this discussion on the Kitfox stall spin behavior,
My fox is a model 7 with the "laker fiberglass leading edge" it has the 912uls and 850x6 tires and the speed kit option for the tail.
I have found that with the stick full back and in full stall it is very easy to control a wing falling off with small rudder inputs. Also the ailerons / flaperons remain effective and will pick up the low wing without the rudder, not the right way to pick up a stalled wing on an airplane with just ailerons. In a full power on stalled configuration with half flaps it is still very controllable with rudder in to pick up the wing in fact it shows about 30/32 mph ias. I have tried to spin it but find it hard to get more then a spiral,not a fully developed spin.
I feel that the laker leading edge is the reason my airplane is so docile at slow speeds. This is the only Kitfox I have any time in, it just turned 350 hr, after first flight in April 2012 but i do have about 600 hr, acro time so am comfortable with spins.
A word of advice, " know your airplane" and always have an out.

av8rps

01-04-2017, 06:47 PM

Can you please describe for all of us what the Laker leading edge is, and what it does?

Av8r3400

01-04-2017, 07:43 PM

A thin fiberglass leading edge cover, under the fabric.

It does not change the airfoil shape, just make it smooth over the leading edge across the ribs. Similar to the old Avid aluminum leading edge without the oil-canning.

DesertFox4

01-04-2017, 08:18 PM

Paul, a couple photos from my build thread that shows highlights of a Laker Leading Edge install. It does slightly change the leading edge by bridging and maintaining the profile between ribs as originally designed by Harry Riblett. As we all know the wing as built without the L.L.E. works exceptionally well in all speeds. The L.L.E. wing does feel different than the stock wing. Certainly not needed for a fantastic performing Kitfox, just interestingly different.
12158

12159

av8rps

01-07-2017, 08:41 AM

Thanks for clearing that up for me. I have seen those before, just didn't know what they were called.

I remember when Avid started doing that with their leading edges. They really make the wing look nice, we just never could determine if they did anything but add weight.

I really like the look of them on the Kitfox wing and would think they may improve top speed as it looks like it would reduce drag on the leading edges. (The Kitfox wing is much cleaner than the old Avid wing, that's why I say that).

tx_swordguy

01-07-2017, 03:52 PM

James, while I am a low time pilot and I have an avid not a kitfox I think the differences in the flight envelope between the two are not very far off. While I am all for experimenting, do so after getting a few hours in the plane. I was told by the PO that after 10 hrs I would have it down. While I still feel I am learning every time I climb in, there was indeed a change at the8-10 hr mark where things really started clicking. I still put VGs on the wing and tail at about the 20 hr mark not so much to slow my stall but to keep a more stable feel on the wings right up to landing. It worked for me and the stall breaks very clean (might have dropped stall a hair).As for a climbing spin-good luck. I have the 582 and at 40 I can pop the flapperons and pull up off the deck in under 500' and be 40' in the air almost immediately(again a low time non-STOL trained pilot). I would be more concerned with a spin on landing than taking off. I land and take off on a 1000' grass strip with a power pole on the north end. I practiced for a month at the local airport before ever trying my stip. When I did I made numerous approaches at different speeds and slips before I ever tried putting the wheels on the ground. First landing I dropped over the power lines and slipped to a landing with 300' to spare. I was duly impressed with myself and the plane. Just get in it and fly, from there figure out if and what you want to do to it. Take it for what it is worth, many, many more here with loads more time and experience than I have. listen to them
Mark

Av8r_Sed

01-07-2017, 06:19 PM

I have the 582 and at 40 I can pop the flapperons and pull up off the deck in under 500' and be 40' in the air almost immediately
Mark

^^^ This is OK as long as you don't try to continue the climb at that low of an airspeed. People can get in trouble with this design on departure with slow climb speeds. There isn't a lot of momentum in these light birds and if you do stall due to wind shear or whatever, recovery will be very difficult if not impossible. The climb rate at 50 or 60 is still pretty impressive and there isn't much reason for any slower climb.

tx_swordguy

01-07-2017, 06:41 PM

I agree 100% I will usually get up about 40' and then let the nose down/ bring in flaperons and pick up speed to get over the power lines. by the time I am over the power lines I am pushing 50+ and climbing much slower but gaining speed. Once I hit 60 I increase my climb angle and hold 60. I only take off to the north over the power lines with a north wind usually only between Nov- april with the temps usually not above 70degrees. All other times I take off southbound with only a barb wire fence at the end. So far it is working out fine. Not saying something cant happen but I feel pretty good about the way things are working out so far. I am pretty particular about my wind patterns taking off to the north though.
Mark

jamesbdunn

01-08-2017, 07:38 AM

...while I am a low time pilot

I practiced for a month at the local airport before ever trying my strip. When I did I made numerous approaches at different speeds and slips before I ever tried putting the wheels on the ground. First landing I dropped over the power lines and slipped to a landing with 300' to spare.

Just get in it and fly, from there figure out if and what you want to do to it.

Take it for what it is worth, many, many more here with loads more time and experience than I have. listen to them

Mark

Mark, your experience resonates with my experience with the Varieze; carefully pushing things around near limits to get the feel of impending departure from expected performance; in forgiving situations.

I respect everyone's offered experience. But if I replied individually to everyone, it would be messy; and a bit weird :rolleyes:

But as the new guy, I want you to know your contributions are equally appreciated. It's up to me to use common sense to combine everything into a useful course of action; or in-action.

Thanks

jamesbdunn

01-08-2017, 07:51 AM

...I will usually get up about 40' and then let the nose down/ bring in flaperons and pick up speed to get over the power lines.

Mark

That's interesting. I was reading an article about a fighter pilot in jet aircraft that studied aerodynamics and found a way to out maneuver his friendly opponents. This technique was later adopted and taught in fighter pilot training. I don't know if it would work in propeller driven aircraft, as these jets are moving at speeds unrelated to an unrecoverable stall because of the massive engine thrust.

But what he discovered was in a clean configuration he could gain altitude much faster by porpoising the climb, instead of a constant climb.

The engine could not sustain the most efficient climb speed, so by porpoising, the aircraft speed transitioned through regions of greater lift while being more aerodynamic in a clean configuration.

For SEL aircraft near stall speeds this is a BAAAADDDD idea.

But once well above stall speeds and with no flaps, I wonder if porpoising at cruise power is as effective as sustained climb at full power?

I remember there was something about the climb profile, but don't remember clearly.

av8rps

01-08-2017, 09:05 AM

Another point on this interesting discussion;

We are not talking about identical airplanes here, so we really need to take that into account.. My early Avid Flyer weighed 400 lbs empty and would safely climb out at 45 mph at a 47 degree angle with no issues at all since the stall speed was 22 mph. But it (like Kitfox 1 -3 Models) had a completely different airfoil than later Kitfoxes. That airfoil will fly (and stall) much slower, and land much shorter than the newer airfoil design. So in this discussion it is almost like talking about two completely different aircraft.

By comparison, I climb out in my Model 4 Kitfox amphib at 65 to 70 mph. I can climb at 45 mph, but it doesn't feel right and makes me a bit uneasy since my stall in that configuration is approaching the 40 mph range. So I avoid using techniques like that.

And even though we need to consider the huge variety in our airplanes, this has been a very good discussion that could prove beneficial to all of us.

PapuaPilot

01-08-2017, 11:02 AM

[QUOTE=tx_swordguy;61792] I have the 582 and at 40 I can pop the flapperons and pull up off the deck in under 500' and be 40' in the air almost immediately.

I would encourage you do think about what would happen if your engine has a hiccup or dies during this maneuver. I don't think things would end well. Where I work we do not allow simulated Vx power losses during takeoff because we have had a couple of serious incidents doing them. I'll never forget doing simulated power loss at Vy on the amphibious Caravan at 100'. It was almost unrecoverable and the pushover at the top was drastic (to keep it from stalling). It was like going over the top of a roller coaster.

One way to check this would be to simulate it at a safe altitude; note your altitude, slow down to just above stall, add full power, pull on the flaps, pitch up, then cut your throttle and see if it is possible to recover without stalling or going below the initial altitude.

av8rps

01-09-2017, 12:29 PM

I'm an advocate of everyone knowing what their airplane will do in a simulated power failure on takeoff.

I typically will go to 3,000 agl and use that as my imaginary runway along with the runway compass heading. I apply full power and start climbing at whatever airspeed I want to test, so I know what speeds and weights it will stall at with full power. Once I establish that I do simulated power failures so I know what to expect.

Once I know what to expect from an engine failure, at the same altitude I practice what altitude is needed to get back to the runway safely (aka "The impossible turn" made possible).

And if any of you think this is dangerous, think about the consequences when you have a real power failure on takeoff and you don't know what to expect. I've had 5 full engine failures in 4 decades of flying, 3 of them on takeoff. I'm here to tell you that you can't know your airplane well enough...

(and for anyone wondering, I landed successfully every time with no damage to me or the airplane. I always say "Even the space shuttle that weighs tons upon tons with short stubby wings glides. But you better know what speed it glides, and even more importantly what speed you need to flare and touchdown. ;) )

Geez, I hope I'm not jinxing myself now???

airlina

01-09-2017, 05:53 PM

Sound advice there Paul-as I was reading your post , I was thinking that guy does the exact same thing I do to practice. I have found that even with a"holy cow" what was that delay factor thrown in- 400'AGL is my number to make the 180 back. the actual manuever takes about 200 ' but I double it for mom and the kids. I have gone to a quiet grass strip and fooled around with the turn back for real with a pull to idle at 400' after takeoff and found that turn around wasn't the problem but having enough runway to land on was. I wouldn't try it from a short strip unless it was my only choice. The key is to have all this figured out BEFORE you add full power to takeoff-know what you are going to do before leaving the ground on every takeoff. Bruce N199CL

av8rps

01-10-2017, 11:53 AM

I'm glad to know others practice for power outs and turns back to runway. It truly is one of the most important things to know when operating any aircraft imho.

If anyone thinks this is difficult or dangerous, the Kitfox, even on 230 lb amphib floats is a ***** cat compared to the same maneuver in my Lake Amphibian. But I practice it in that just the same. By comparison the Lake sinks like a stone with the gear down, flaps down and the prop at fine pitch. And because the engine is mounted high above the fuselage on a pylon, and because the super slow hydraulic trim tabs are 1/3 of the total elevator area and pitched way up for takeoff, when you lose power the drag of the prop along with those huge trim tabs forcing the nose up, will pull the nose pretty much straight up. So you instantly need to push full down on the elevator and get those trim tabs down as quickly as you can. But amazingly, with lots and lots of practice the Lake can actually do a turn back to the runway safely from only 500 agl. One more reason it pays to practice. However I'd double that number to 1,000+ feet agl if you are not well practiced.

The Kitfox and Avids have an amazing capability to do a safe 180+ degree turn in a very short time, which provides an amazing advatage to get back to a runway in the case of a power failure. I don't know of an airplane that could do that as well, and I've flown lots of aircraft. But unless you are well practiced, if you have an engine failure on takeoff and you aren't sure if you can make the turn back with a large margin of safety, plan to go straight ahead as even these super docile airplanes have their limits.

HighWing

01-10-2017, 02:13 PM

Can you please describe for all of us what the Laker leading edge is, and what it does?

During the initial discussion of the vortex generators that some began putting on their wings, I seem to recall that most found a much more docile control feel nearing stall, but not much lowering of stall speed. Discussion then suggested the possibility that the shrink of the fabric between ribs and and false ribs gave some repeated variation of the original Riblett airfoil design across the span. Some buddies and I talked about it and since I was in the middle of a build, I became the Guiney Pig. I used aluminum and with some tweaks in the the installation hoped to eliminate the oil canning. One of the guys in our group flights had an early Rans S-7 and he got a lot of ribbing from the occasional appearance of his wings while on the ground and I knew what to try to avoid. He never had any issues in flight however. Being aluminum, what I did was heavier than the LLE, but it was not available yet.

jiott

01-10-2017, 04:48 PM

I had heard a while back, maybe it was from John McB at the factory, that there was some thought that the standard wing (without LLE) with its leading edge "shrinkage between ribs & false ribs" gave a similar effect of vortex generators. It would sure be interesting to get some side-by-side testing to answer some of these questions.

jamesbdunn

01-11-2017, 12:27 PM

Practicing power out return to runway.

What does experience show?

I'm not expressing an opinion here. I'm just trying to work the numbers a little.

When taking off on an asphalt strip, 2000 feet down the runway, the altitude is going to be between, what?

about 40 seconds down the runway

A medium pilot, full fuel: 700'/min or around 500' above the runway
At gross: 500'/min or 330' above the runway

Dumping the nose forward an air speed of what, 50 mph is needed to make a safe turn? Especially since you're going to be looking outside the cockpit more than inside.

If power out is before 2,000 feet it seems like a turn is not advised
just do your best to set it down before running off the end of the runway.

if power out is after 2,000 feet it seems like choices change a lot as altitude is increased before power out.

Is the runway long or short. Density altitude. Winds & Gusts. Grass strip next to runway as an alternate landing not requiring as much of a turn.
...

Then there is the bush.

Potentially no alternate landing zone other than runway without damage; but, perhaps safer than trying to get back to the rough strip.

Bush power out return to unimproved runway.

Are there seemingly acrobatic techniques when the choice is possible death regardless of which you choose? Choosing between extreme choices.

I'm guessing a full nose over well above stall, followed by full left rudder, full left aileron, while still nose over. then neutralize hard, wait a second, then enter stable glide. Just trying to get closer to being in the clear.

Obviously practiced at significant altitude to see if it would work before actually trying to use this, or something similar, in an actual situation.

This was just to frame the question.

What are bush strategies for power out return to runway?

av8rps

01-11-2017, 07:54 PM

Wow, lots to talk about.

Regarding fabric shrink between ribs vs VG's, I think proving that will be very, very difficult as no two Kitfoxes are really alike. So I'm not sure exactly how we could prove that one way or another? Maybe if no one could catch a smooth leading edge Kitfox unless they put smooth leading edges on theirs, or no one could fly next to a VG equipped Kitfox unless they added VG's to theirs, how else would we ever know? Unfortunately (or, fortunately) for us, I think our Kitfoxes were so well designed that it is hard to see huge improvements with mods that may affect other aircraft designs very significantly. Or at least that's my take on it.

But I do want to comment on VG's: For anyone installing them you need to remember that VG's make the aircraft more controllable up to the point it stalls, but when it does stall it typically happens all at once as the wing is totally done flying then. Unfortunately, that means you don't have any real advance warning before finding yourself in a fully developed, deep stall. So for me, I'm admittedly not a real huge fan of VG's unless I can get a much lower stall speed benefit from the installation (and I've never experienced much VG benefit on a Kitfox).

Regarding the numbers for making a turn back to the runway; I don't know if anyone here can give you the numbers asked for as there are just too many variables in our Kitfoxes. The numbers can only accurately be determined in your own Kitfox by practicing with it at altitude as I suggested earlier in this thread, so you are prepared if you ever do have an engine failure. I suppose that might sound a little like I don't want to answer the questions directly, but I'm doing that intentionally because I wouldn't want anyone to hurt themselves in an engine failure situation they never practiced and only remember the discussion here where I was making reference to a very different Kitfox than what they own. I will however throw out one number I have used a few times that seems rather universal in the Kitfox and Avid / Highlander design - 60+ mph is a good general glide number, but be prepared to increase that as you get close to tbe ground so you have enough speed to flare. These are docile aircraft that glide exceptionally well, but if allowed to get too slow the high lift airfoil can develop a really high sink rate. So carry a few extra mph for the wife and kids :)

Engine failures in the bush flying environment? Well, that is a really huge, difficult subject. There is no good answer in my opinion as to what one should do unless we knew all the details of the situation. Best answer, train with Paul L. at Stick & Rudder. He'll stand the best chance of preparing you for a tough situation in the bush.

But I do have to say this, if I had to have an engine failure on takeoff in the bush, I'd prefer to be in an airplane like a Kitfox. Not only will it fly and land slower than many other aircraft, it also will out turn just about anything flying. A Kitfox is very nimble & agile, which is a huge advantage in a bad situation. And it has very little inertia compared to many other bush planes that typically weigh 2 or 3 times more. Plus the pilot and passenger are surrounded by cross triangulated chromoly steel, which is highly regarded for crash survivability not only for aircraft, but also race cars.

That's a lot of typing to be so vague, but I hope it helps to answer some of the questions asked.

Ramos

01-12-2017, 10:11 AM

I know less than just about anyone here. I am just smart enough to know that the last post (#37) is not at all evasive. What it is, is chock full of wisdom that should be taken to heart!

jamesbdunn

01-12-2017, 12:04 PM

...train with Paul L. at Stick & Rudder. He'll stand the best chance of preparing you for a tough situation in the bush.

Thanks, thought I'd drop in what I found about Paul L.

http://stick-rudder.com/index.php/ct-menu-item-11/ct-menu-item-13

So VG's remove the perceptible feel of impending stall? I had no idea they did that. So a stall indicator device becomes necessary if planning to operate near stall conditions, if using VG's?

Or, is there still sufficient indication of impending stall?

av8rps

01-12-2017, 06:40 PM

James,

I like what the video below shows about the benefits of installing VG's on a Wheeler Express aircraft, as the Express benefits significantly from VG's. So in that case I am all for installing them. You will be able to land much slower, much safer than without VG's. A 200 knot aircraft that stalls at 40 knots is certainly impressive;

http://www.bing.com/videos/search?q=how+vortex+generators+work&view=detail&mid=41FCE8371ED381DD818141FCE8371ED381DD8181&FORM=VIRE

And while I have not found a great benefit from VG's on any Kitfox I've ever flown, here's a really great video of an early Kitfox with VG's where the poster claims he lowered the stall by 5 mph with the addition of VG's. I've never seen that, but maybe the planes I flew didn't have the VG's in the most advantageous position? Take a look and decide for yourself. And make sure to read his comments;

https://www.youtube.com/watch?v=bIsWseMbDQU Wing without VG

https://www.youtube.com/watch?v=gor7LhsAILs Same wing with VG

Now I personally didn't see a lot of difference between with and without, as the pilot never let the stall fully develop in either video. And frankly, I don't believe we will ever see much improvements with our Kitfox wing as it uses flaperons and that makes a huge difference when talking about controllability and stalls. A good example is that the Highlander wing (same as early Kitfox airfoil basically) benefits greatly from VG's. Why is that?

Simple, because the flaps and ailerons are attached to the trailing edge of the Highlander wing, so they are affected immediately when the wing begins to stall.

So now compare it to the Kitfox wing with a flaperon that is flying in its own clean air under the trailing edge of the wing. Even with the wing stalled the flaperon isn't. And in fact, the flaperon is not only adding control authority when a normal aileron would be ineffective, but it also is providing some additional lift on the trailing edge of the wing where it tends to automatically reduce the angle of attack of the wing while also lowering the nose of the aircraft, which all helps to get the aircraft flying sooner. So in a nutshell, when the wing does finally stall on the Kitfox the recovery is going to be quick. And that's always been my experience in these airplanes. They typically will be flying again before the nose gets below the horizon. At least that's my take on it after flying these type of planes for decades, and owning both a Highlander and a Kitfox.

That tuft testing video of the Kitfox does a really good job to prove that the Kitfox has an extremely gentle and easy to recover from stall, whether VG'd or not. So again I'm not so sure we really need any VG's on our planes. I tend to think putting VG's on the bottom of the tail is the most beneficial thing to do with a Kitfox, as losing elevator authority is probably one of the more obvious areas we could help improve if we want to land slower. But maybe I'm wrong? Maybe others have learned how to better place the VG's on the wings and are seeing more benefit from them? If he really lowered stall by 5 mph as he claims, even though nowhere near as significant as the Express, 5 mph is still pretty good.

I'm admittedly pretty old fashioned and believe in flying by attitude and by the seat of my pants. I prefer flying a Kitfox that way as I think it is more fun. So I tend to like my Kitfox just the way it is. But for those that want to max out their Kitfoxes short field capabilities, maybe VG's are still worth it? But if I were to add VG's and planned to max out my Kitfoxes short field abilities (read as flying super slow close to the ground) I would for sure install a very accurately calibrated (to my specific Kitfox) angle of attack indicator so when I wanted to fly close to the stall I would know exactly when to expect it to happen. You do not want to learn unexpectedly that you just stalled when you are at tree top height above the runway.

With all that said, I'm never so proud to admit I might be wrong about any of this. So if others are seeing more benefits from the installation of VG's on their Kitfox than I know of, please chime in on this subject. There have been a lot of advances in aircraft design over the years from those willing to experiment. While I typically tend to fly my airplanes in a stock condition and work to better improve my abilities to operate it better, that doesn't mean we can't work better to improve the pilot and the plane.

Ramos,

Thank you for the kind words. Sometimes when trying to explain some of these things I feel like I might need to put on my flame-proof suit. It's nice the group here can have productive discussions without anyone feeling a need to defend themselves. That is not always the case when on many other forums...

We have a great group here :)

(note: Vg's on Kitfoxes have been discussed before here. Check it out at http://www.teamkitfox.com/Forums/showthread.php?p=49166 There is some really good additional info with testimonials and results there)

Ramos

01-13-2017, 10:47 AM

No doubt, this site is awesome. I am following this thread and have followed many others here with great interest. The honest, friendly sharing of knowledge gained through experience that takes place here is invaluable to someone such as myself. I have pursued Kitfox information since 1985. First in magazines and now the internet. Actually got to see a few in person for the first time this last Summer at the WAAAM fly-in. Have yet to sit or fly in one. None the less, I DO possess the necessary passion and am close to having the means to finally turn the dream into reality. What you folks provide to me is both knowledge and inspiration. Thank you all for being so welcoming and patient.

I apologize for the extreme 'thread drift', here. We now return to the regularly scheduled program...

DesertFox4

01-13-2017, 03:54 PM

Kind words Ramos for the kind and generous folks on this website. Glad they keep you inspired to reach that goal of owning your own Kitfox soon. My first Kitfox took 15 years to obtain. Best decision aviation wise I made in 34 years of flying. Changed my life in so many positive ways.

jamesbdunn

05-24-2017, 07:32 AM

... I used aluminum and with some tweaks in the the installation hoped to eliminate the oil canning. ... Being aluminum, what I did was heavier than the LLE, but it was not available yet.

I visited Lowell and his Kitfox; a beautiful plane. Obviously a passion for the details.

jamesbdunn

05-24-2017, 07:55 AM

I understand that Vx depends upon the Kitfox model and modifications.

However, I see videos and stories of Kitfox planes and their pilot "hanging from the propeller" [exaggeration]. But it brings into question [for me] what happens when an engine fails [gas starvation scenarios, mechanical, propeller...]?

Can an airplane hanging from the propeller have enough airspeed to throw the stick forward in-time to enter a glide? Or, does the airplane immediately enter a severe stall and not have sufficient altitude to enter a glide before hitting the ground?

What goes on in my mind, at times is only a sampling of a broader understanding of related physics and experience. So please do not read this as questioning pilot experience. I pose this as a question to learn about potentially other influences in play.

Seems like hanging on the propeller and having an engine failure potentially can instantly transition from high angle of attack to severely stalled.

Is this the purpose of Vx?

To insure sufficient inertia into relative wind,
gravity not being a super major factor in reversing the direction of acceleration in relation to relative wind,
sufficient margin to a stalled condition in terms of providing the pilot with sufficient time to respond,
not having to make large control movements and causing stall/spin

Is the purpose of Vx to have sufficient margin for engine out to ensure entering into a stable glide?

jtpitkin06

05-24-2017, 09:45 AM

Wow.., a lot to answer.

No kitfox has ever been powered to be able to "hang on the pro" as some of the aerobatic overpowered wonders like the Extra. Helicopters hang on the prop. Kitfoxes do not.
--------------

Q.
Can an airplane hanging from the propeller have enough airspeed to throw the stick forward in-time to enter a glide? Or, does the airplane immediately enter a severe stall and not have sufficient altitude to enter a glide before hitting the ground?

-------------

Ans.

At Vx there is sufficient energy in a Kitfox to lower the nose and enter a glide. However, there may not be sufficient altitude. There is also a delay in the pilot reaction when an engine quits of about three seconds. The danger comes when the nose pitches down and the pilot over-reacts by pulling the nose up, creating an accelerated or secondary stall.

There is never any reason outside of aerobatics to climb at a speed less than Vx.

For normal and short field take offs the obstacle is assumed to be 50 feet. So unless you have giant sequoias off the end of your runway you clear the obstacle and then accelerate to Vy without losing altitude. The Vx climb is very short in time. If your aircraft climbs at only 600 rpm or 10 feet per second, then the obstacle climb is only 5 seconds. Probably half that in most Kitfoxes.

----------

Q. Seems like hanging on the propeller and having an engine failure potentially can instantly transition from high angle of attack to severely stalled.

-----------
Ans. At speeds below Vx, possibly. But this is assuming the engine fails and the pilot tries to hold the nose up during the reaction period before establishing a glide. All the more reason to transition to normal climb as soon as the obstacle is cleared.

----------

Q. Is this the purpose of Vx?

To insure sufficient inertia into relative wind,
gravity not being a super major factor in reversing the direction of acceleration in relation to relative wind,
sufficient margin to a stalled condition in terms of providing the pilot with sufficient time to respond,
not having to make large control movements and causing stall/spin

Is the purpose of Vx to have sufficient margin for engine out to ensure entering into a stable glide?
-----------
Ans. None of the above. Vx is best angle of climb for the purpose of clearing an obstacle. Nothing more. It is a calculated number achieved by plotting speed (x axis ) vs rate (y axis) and drawing a line from 0/0 to tangent on the curve. Where the tangent contacts the curve is Vx. Best rate is the highest point (y value) on the speed/rate curve.

----------

Summary. If your runway has an obstacle, lift off so as to rotate into Vx without a speed overshoot. Clear the obstacle then you accelerate to best rate or cruise climb.
If the engine quits, establish a glide pitch and aim for your emergency landing point.

John Pitkin
CFII 47 years.

jiott

05-24-2017, 10:06 AM

Just my understanding here:
The purpose of Vx is maximum sustainable climb angle at full power; it has nothing to do with engine out or stall recovery.

In my Kitfox flight training we did many power-on stalls where it felt like you were hanging on the prop. As long as you kept the ball reasonably in the center, the stall was a very straight ahead nose drop that was easily recovered by enough forward stick to regain flying speed. If done properly there was nearly no loss of altitude. Response time had to be immediate but not a panic situation. A stall in a Kitfox (I'm speaking of an SS7) is nothing to be feared, ball centered, unless you are very close to the ground. I know of a friend who actually purposely stalls occasionally on a short approach over trees to quickly lose altitude, then a quick forward stick to break the stall and land. I am not recommending this and have not done it myself, but apparently the Kitfox stall is so benign and controllable it can be done.

So an engine out at Vx may or may not cause a stall, depending on how quick you are on reacting to it, but in either case is a very controllable event.

Esser

05-24-2017, 10:30 AM

Well I'm sure this will start an argument but I never ever use Vx in any plane I fly. There is really no need. If you are flying on such a short runway that you have to follow the POH to the letter to clear an obstacle or you won't make it, that runway is too short. There is no way you can guarantee your engine is making the same power as the day the poh was written. A fresh engine and a 2000h engine aren't the same.

Originally being a glider pilot, my flying style is all about energy management. My shortfield technique is get it off the ground as soon as possible, stay in ground effect and build up excess speed until I feel I have to pull up to clear the obstical and then bleed off the excess speed in the steep climb until I hit Vy then climb out as normal. You gain more more energy in less time thus way. Practice it yourself at a regular airport and you will be amazed how high you can get in the same distance as if you just pulled it off at Vx and climbed out.

Mileage may vary.

I also recommend reading Contact Flying by Jim Dulin. He talks about techniques like this and energy management at length. Best $10 you'll spend on flying ever.

jiott

05-24-2017, 12:41 PM

I agree with you Josh. Nearly all Vx climbs are unnecessary and are for show off at fly-ins or when you are flying with your Super Cub buddy. I'll admit I do them often just to keep in practice and make sure I can hold a nice steady Vx airspeed in the rare case I would ever really need to do it. I fly a lot in the mountains, but would never deliberately use an airstrip where Vx is required. However, that said, I can see where a pilot thru his own stupidity or inattention could get trapped in a blind canyon with no turnaround room and maybe need to do a Vx climb to clear the ridge. Stuff happens, so we do need to have the skills to fly at the edge of the envelope. Just my 2 c.

flflyer

05-24-2017, 06:00 PM

I just read this whole thread. Lots of points being brought up. I also think my model 6 wing works fantastic at slow speed as is. I also agree that some gap seal or VG's on the tail could help it flare better with an engine out.

I've had the displeasure of an engine failure on takeoff also, many years ago, and a partial power loss on takeoff just a few years ago. If I'm taking off, I'm planning which way I'm going to turn at which altitude. I even will takeoff slightly downwind to get a better scenario if I lose power at low altitude. The Kitfox handles normal airports so easily that you can easily find a place to put down, but on small strips, it really pays to think about what's in front, to the side, and to have a turnback strategy.

One thing that will help is to drift crosswind, or even turn 10 degrees crosswind on climbout. The turn should be away from the crosswind if there is an actual crosswind component (there usually is). Obviously don't do it over houses or hangars, etc, but in general, if you drift crosswind on climb, you will need less room to turn back. This is because you only need a 180 turn, instead of a 235, then a 45 to get lined back up. If you lose power, and you've hit your altitude that you've worked out as a reasonable one, then you just turn into the crosswind (if there is one of course) and by the time you are on the runway heading you will be generally lined up with the runway.

I even make one more lap around the airport while gaining altitude if the conditions aren't good for landing nearby. That way, if there is some overheating, low oil pressure, or something giving clues, you are ready to do an engine out pattern. And you have time to double check the engine temps are stable.

I love flying sailplanes, and every landing is engine out. Everybody practices a rope break on takeoff, which is just like an engine out; the instructor intentionally releases the tow rope at 200' AGL, and you turn around and land. Easy in a sailplane, although they take a long time to turn compared to a Kitfox. In that exercise, you have to get the nose down right away, which is very much like a powerplane losing an engine on climbout.

BTW, I had the Sportsman cuff on my C180, it was fine. No matter how slow your plane will fly, you can always exceed the angle of attack that keeps the wing flying. I would get an angle of attack gauge before modifying the Kitfox. They are a lot of fun in slow speed scenarios, and will always tell you how much margin you have before stall.

Last point, I do believe that STOL planes that are 'hung on the prop' are in a risky situation, even though I've seen plenty of guys flying that way for super short landing demos. And yes, if you lost the engine, the critical angle of attack does change somewhat, and you need speed that you don't have. No reason to fly a Kitfox that way, on the backside of the power curve, where the wing is producing lots of drag. The reason the demo pilots do it is because the instant you chop the power, the plane will go down, which hopefully, is on the X during the landing contest, 1 foot below. And, it's about as slow as you can get a plane, at the cost of exposing it to big problems with an engine out.

jamesbdunn

05-25-2017, 09:29 AM

Is the purpose of Vx to have sufficient margin for engine out to ensure entering into a stable glide?
-----------
Ans. None of the above. Vx is best angle of climb for the purpose of clearing an obstacle. Nothing more. It is a calculated number achieved by plotting speed (x axis ) vs rate (y axis) and drawing a line from 0/0 to tangent on the curve. Where the tangent contacts the curve is Vx. Best rate is the highest point (y value) on the speed/rate curve.

John, and others

Thank you for clearing up my over-thought misconception of Vx. A helicopter as someone alluded to is flying Vx straight up (hanging from the propeller), and when power dies the aircraft drops (unless auto rotating). Vx has nothing to do with margin to stall during engine out.

I appreciate everyone's voiced experience.

My take-away: is that when taking off, and there are no realistic obstacles on long runways with reasonable density altitude, to lift off well above Vx and climb out at Vy to transition to a safer gliding altitude as soon as practical.

To get to an altitude where a non-urgent power off landing can be initiated.

To have an engine out plan of action for each phase from lift off to cruising climb.

I'm not well experienced. So what is obvious for the experienced pilot, I have yet to experience and pay attention to the details.

From the comments provided, there seems to be regions of the transition from wheels-off to cruising climb.

Wheels-off,
transitioning towards Vx while in ground effect,
since at full throttle the pulling back to change AoA,
sustained climb at Vx,
relaxing the elevator to transition to Vy (or lift off near Vy and avoid Vx)
climbing Vy,
Vy extended to clear mountains or other tall obstacles (circling)
attaining a safety margin altitude AGL,
transitioning from Vy to climbing cruise.

So it seems that depending upon in which phase the engine out occurs, how much runway is left, and the airport configuration, the pre-planning is significantly different. Your options for engine out landing sites and what the plane needs to accomplish are different.

On a long strip in a 100 hp Kitfox, the altitude for climbing cruise might occur before leaving the end of the runway.

I'll have to think about the gliding altitude lost in a banked turn, and what height AGL is needed for each phase toward climbing cruise.

I have seen posted 1000' lost to glide 1.5 miles for a Kitfox 4, straight and coordinated.

How much altitude is lost in a gliding turn (plane effectively heavier)?

Effect of density altitude on gliding turn?

Other engine out influences to consider for flight planning?

jamesbdunn

06-05-2017, 09:02 AM

I'm just playing with semantics now. I have no intention of flying below Vy except to clear obstacles.

I now understand that operating below Vy adds risk to flight operations. I would NOT have had this revelation without everyone contributing your experience. Thank you.

I can see myself landing and taking off in a farm field with obstacles.

So do you clear obstacles faster by getting to Vx and climb at Vx. Or, entering ground effect, accelerating, overshooting Vx, then climb with a higher angle of attack with an airspeed dropping towards Vx with the stored energy built during ground effect. (the later was taught to me at one time by a CFI, though managing when to make transitions for me is vague).

Does available HP at gross weight change Vx ? Especially from one Kitfox to another.

I happened to think that a 180 hp Kitfox (and the weight of its engine) is able to sustain a climb at lower airspeed than a runout engine making a 40 hp Kitfox. Lots of different engines are used, from automobiles, motorcycles, snowmobiles, aircraft engines; with influences from CG, humidity, and % of TBO (loss of HP).

Does higher HP just allow for accelerating faster to Vx (20 feet), and does the propeller wash lower Vx? Is Vx only affected by a couple of knots by HP, or is engine HP a significant influence in Vx?

https://generalaviationnews.com/2016/02/01/titan-powered-kitfox-makes-first-flight/

Vx was never a real consideration with my Varieze, short fields and grass strips were never considered. With the blade in the rear, much of an angle of attack meant trimming the propeller on asphalt, or digging ditches.

I do NOT have a Kitfox, and am training in a Taylorcraft getting ready for taildragger operations. Depending upon the configuration of the Kitfox I buy, I am anticipating the V speeds are going to be different.

I understand that because the Kitfox "Wants to Fly" well below Vy on takeoff, get up into ground effect and let speed build.

From the Youtube videos of Kitfox, these are often transitions measured in seconds, and even fractions of a second (https://www.youtube.com/watch?v=-8Qf_axzjHw).

Kitfox 582 and 912 competition
https://www.youtube.com/watch?v=vpxDqe5gbcs

These were both racing and flying likely at or above Vy. The extra HP provided certainly different performance. Potentially indicating that engine HP may influence angle of climb and not just airplane structure.

How would a pilot determine Vx for their particular aircraft WITHOUT experimenting near the limits of aircraft performance?

========== Proposed Target to blast holes in ==========

I haven't found Vy or Vx published for Kitfox aircraft.

To determine Vx for a particular Kitfox:

Load near center of CG at MTOW

At greater than 1000' AGL, watch engine temperature closely, and anticipate the potential to enter a stall.

Enter a sustained climb at what is believed to be Vx at full power. Using a GPS, record the temperature, ground speed, airspeed, and vertical change in altitude (indicator, or change in pressure altitude and start & finish times).

Do the ground speed vs change in altitude calculation.

Get set up like before and repeat at a slightly higher airspeed.

Get set up like before and repeat at a slightly lower airspeed.

By using this method, search for Vx at that Density Altitude

Repeat process again by increasing altitude by 3000 feet and repeat.

Find Vx at that increased altitude.

Use the two points to derive if Vx changes with density altitude, and what airspeeds are actually associated with Vx for that aircraft.

Or, possibly there is an Android App that does glide slope automatically (angle with reference to ground), and just reference airspeed.

In this way, a pilot is not deceived by assumption.

Correction: I am not deceived by assumption.

Vx is best angle of climb for the purpose of clearing an obstacle. Nothing more. It is a calculated number achieved by plotting speed (x axis ) vs rate (y axis) and drawing a line from 0/0 to tangent on the curve. Where the tangent contacts the curve is Vx. Best rate is the highest point (y value) on the speed/rate curve.

----------

Summary.

If your runway has an obstacle, lift off so as to rotate into Vx without a speed overshoot. Clear the obstacle then you accelerate to best rate or cruise climb.

If the engine quits, establish a glide pitch and aim for your emergency landing point.

John Pitkin
CFII 47 years.

Dave S

06-05-2017, 02:04 PM

James,

Regarding your question:

How would a pilot determine Vx for their particular aircraft WITHOUT experimenting near the limits of aircraft performance?

I don't think that is possible - fact is - proper testing does mean testing to the limits of performance. Vx and Vy are determined by testing; and, that testing does not have to be in any way unnecessarily risky.

In testing an aircraft for these parameters, best practices call for taking it up high enough so a person can un-screw any maneuver that could get screwed up. The procedure for Vx and Vy determination is pretty simple and is contained on pages 64 and 65 of Advisory Circular 80-89B.

One of the truly terrific revelations for me when I went from standard certified aircraft to building, testing and flying our kitfox is how the experience genuinely enhances a persons understanding of how it all works. On a standard aircraft, we just do what the book says, not necessarily with much understanding about how and why....with an experimental, we write the book. While I always did my weight and balance work with rental airplanes, I have to admit that I had never stalled one of them at max weight and aft CG - a person knew the limits, but never exactly knew how it worked; however,I know exactly how the kitfox behaves in that configuration because it was tested. Much to be said for our experimentals.:)

jiott

06-05-2017, 04:44 PM

James, we are all happy to answer questions, but you have so many that I believe you would be better served to sit down for an hour with a competent CFI (preferably one who instructs Sport Pilots in LSA's) and hit him with all these questions. He should be able to answer nearly all of them, except possibly some very specific to the Kitfox. Many of your questions are covered in basic pilot training.

I also think you are way over-estimating the dangers of Vx. Most of us do it all the time, even for fun. The approximately 60 mph Vx airspeed of the Kitfox SS7 is a long way from the 42-43 mph stall speed with 1/2 flaps.

efwd

06-05-2017, 06:50 PM

Dave, I think ahead to the day I have to load my ship with full weight to do the tests discussed here. That must be a little unnerving to do. I look forward to gleaning the information and knowledge you speak of but man, Im not sure about the max weight testing.:o
Eddie

Dave S

06-05-2017, 07:48 PM

Eddie,

I think the approach of doing the testing in increments keeps a person safe and sane.

The load testing takes some time, I found, starting with a moderate load at mid CG...add some more weight....go out and fly the stalls again - for power stalls I started at a lower power setting then inched up.....then moved the weight back, then flew it again, then more weight....then more rear CG.....flew it again. again and again etc.... by incrementally increasing the load and power - a person can understand the handling characteristics as the testing progresses and know when to stop.

Bud Davidson

06-05-2017, 08:09 PM

The approach Dave S describes is familiar. My earlier homebuilt was one-of - a-kind with no available data. It was 1450 lbs empty and should have had a 2300 gross. I used 50 lb bags of horse feed... clean weight source... and did just as Dave described. When it reached 2250 with full loading of the baggage area it did not recover normally from a stall as up to that point it had. That's where I quit and set the gross at 2200. Takes time but flying is fun after all.

Bud
IV 1200 soon to be painted
912UL

jiott

06-05-2017, 08:10 PM

Eddie, will you be testing to the light sport weight of 1320 lbs, or the full engineered weight of 1550 lbs? The 1320 was kind of a non-event for me, but I didn't need to go to 1550 so can't speak to that. I like Dave's slow and easy approach.

efwd

06-05-2017, 08:29 PM

I'll be doing 1320#.
Eddie

rv9ralph

06-05-2017, 09:37 PM

jamesbdunn

06-11-2017, 06:29 PM

As Dave and John stated. To determine Vx and Vy, you need to do the test for that specific Kitfox, not by model by specific airframe. The process for determining the Best Rate of Climb and Best Angle of Climb are in AC 90-89B, which is at this URL: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-89B.pdf

Reading the test procedure, it is only a few paragraphs, should clarify in your mind how it affects flight. Also, reading the rest of the AC, which is recommended before first flight of a new E/AB or Light Sport aircraft. The whole test period is explained for each hour of Phase 1.

Ralph

Page 64 & 65

I find it interesting that the recommended procedure cites at full power 10 mph/knots above stall for the particular aircraft being tested.

published 37 mph for a VW with vortex generators

http://www.teamkitfox.com/Forums/showpost.php?p=18343&postcount=36

published 30 mph stall speed for Kitfox 3

http://www.landshorter.com/page3.html

To be able to detect the tangent requires a sufficient number of data points to provide detectable resolution. One data point below the peak is not enough to determine best angle of climb.

I believe it was a helio courier with turbine engine that during climb out at positive angle of attack, passes over its point of takeoff; flying backwards in zero wind. Wing design, not hanging on the propeller.

Running a line from 0 through the tangent of the curve does not account for transverse forces on the wings, only the change in altitude. Without a ground reference, Vx real cannot always be correct in relation to the point of takeoff. AC determined Vx only relates to free flight not referenced to the point of takeoff.

Also, according to the AC then, Vx is NOT purely the maximum angle for climb, as angle of climb close to power-on stall speed are not considered. AC determined Vx does have safety margin built into it. I can only guess this was done to provide a safety margin in the event of a loss of power during Vx, safety related to gusting winds, and/or other considerations.

Please don't take me wrong. I have an increasing level of respect for the Kitfox, especially near stall speeds. The more I question, the more I find operations near published stall speeds are on the safe side of design.

rv9ralph

06-11-2017, 08:59 PM

James,
I am not an aerodynamicist nor am I a test pilot. However, I think you are getting in the weeds trying to decipher this.

The testing of an aircraft starts using a stabilized and constant airspeed at least 15 knots/mph above the tested and demonstrated stall for that aircraft. Then the timed climb is conducted starting at a predetermined altitude maintaining a constant airspeed for one minute noting the altitude at the end of one minute. Repeat starting at the same altitude, with a decrease in airspeed by 5 knots and climb one minute then note the altitude. Repeat again, and again When this data is plotted, there will be an increase in the rate or angle of climb up to a point and then it will drop off. This is the data point that will give the appropriate Vx and Vy for that tested aircraft. With certified aircraft, it is considered to apply to the whole fleet of the same type of aircraft (hence the purpose of "type certified") with experimental aircraft, each one is considered unique and needs to be tested to demonstrate their performance.

This climb is not hanging on the prop, just above stall. The test is conducted using airspeeds (remember, you start 10 knots above stall), in increasing increments until the performance drops off. The position over ground is not considered in making these tests.

That is it for now.
Ralph

PapuaPilot

06-11-2017, 10:13 PM

James,
I have to agree with what Ralph is saying. An aircraft's stall speed has nothing to do with the Vx flight testing. The reason the 10 mph is mentioned in the AC is to keep you from stalling during the testing.

The AC tells you to start at the testing at 15 mph over the estimated best rate of climb. You do a bunch of successive climbs while reducing the speed in 5 mph increments until you get to 10 mph above stall speed. You write down all of the times to climb and plot them on a graph. The highest point on the graph is Vy and the tangental point from 0 is Vy.

Ideally you want to get more than one point slower than Vy to make a more accurate curve. Even if you only get one data point you can always plot 0 rate of climb at your aircraft's stall speed and estimate the curve it would make. With my Model 5 the estimated Vy was 65 mph and my Vs was 46. I flew it at 5 mph increments from 80 down to 55 (following the guidelines in the AC) and I got a pretty accurate graph.

You are correct that you do not reference Vx from the takeoff point.

Vx is used for obstacle takeoffs or out climbing obstacles/terrain in flight. If you have an obstacle on take off you accelerate, liftoff, continue to accelerate to Vx, pitch to hold Vx until you are over the obstacle. Then you accelerate to Vy or a cruise climb.

Vx doesn't have anything to do with stalling or stall margin, it is merely for climbing over something.

Using Vx puts us in a vulnerable position at low altitudes if the engine were to quit or if you encounter a gust. There may not be time to push the nose over and regain your speed before hitting the ground. You also need to consider reaction time, the possibility of doing nothing or the wrong thing.

When you do an obstacle takeoff in a certified aircraft you should use the POH's takeoff performance charts and calculate the distance it will take to clear the obstacle using the actual conditions. The stated distance assumes you will be doing a maximum performance Vx takeoff.

kitfoxJeff

06-25-2017, 07:23 PM