A much faster Kitfox?

[av8rps]

I often wondered if we were to really clean up (aerodynamically to reduce the drag of) our Kitfoxes, how fast and efficient might they really be? I'm convinced they have a lot more potential for speed than most think they do.

So I think it might be worthwhile for our group here to discuss ideas on this subject.

Now, I know that there are those out there that have already done an amazing job cleaning up their Kitfoxes. But I'm advocating going beyond the norm by adding aerodynamic fairings like these; (as an example)

- The outer wing strut connection to the wing, covering up the rod ends. I've always thought something like they used on Monocoupes and air racers could be used, and yet still be easily removed for inspections?

- Really nice deep jury strut fairings (that meet the 4.25:1 ratio). Kitfox built them for the M4 Speedster, but they weren't very deep, and we haven't seen them since (as far as I know).

- A nice fairing for the horizontal stabilizer strut rod ends and where the strut intersects the fuselage. Kitfox built steamlined struts for the Horizontal, but never for the attach areas.

- A fairing for the lower wing strut to fuselage attachment. But this one would have to be easily removed for wing folding.

- A fairing for the landing gear attach area on the fuselage. This one would likely have to be flexible so it can allow gear movement.

- A custom fairing that would clean up the area under the butt rib / top of door area, and would extend to the bottom side of the turtledeck, cleaning up the area where the flaperon tube comes out as well as the bottom of the wing root. By filling that area in we would also be able to keep our cabins warmer in the winter.

I know that many will think this is overkill for a Kitfox since it is generally viewed more as a fun backyard STOL aircraft than as a cruiser. But let's face it, ever since the new Riblett wing was put on the Kitfox, with the right power plant and prop our little backyard STOL planes make pretty reasonable cruisers. But I'm convinced they could be a lot faster with the right aerodynamic improvements...

Plus, in addition to the improved speed there are other benefits we will get from more aerodynamic "clean up":

* Less drag also provides a better climb rate on the same thrust
* Less drag requires less fuel, less horsepower / thrust
* Less drag, less fuel, also makes for better range (you can pull power back and yet go same speed as before)
* Less drag means you can get places more effectively, flying faster on the available horsepower (e.g.; Dealing with headwinds)
* Less drag can also equate into more safety. Being able to go around the bad weather when needed (I'm not advocating flying in bad weather, but if you find yourself in that predicament you have more options...)

Now don't get me wrong, as I know our Kitfoxes are already a great combination of back country abilities while also being able to cruise. There really aren't many aircraft that can do both so well on such little power and fuel. But because I happen to own a really fast little 80 hp Model 4 (WOT= 125 mph on amphib floats no less!), it makes me realize that with a few more aerodynamic improvements, it has a lot of potential to be an even more AWESOME aircraft.

I know the current trend is towards more back country abilities. But for the average guy, our Kitfoxes are already pretty good in that realm. Getting places faster I believe is more important, and more practical to more people. And heck, if we could just prove these clean up ideas to be effective, and the parts could be made cost effectively, Kitfox probably couldn't make airplanes fast enough. (although that might already be the case?)

Why am I not doing it? Simple, too much going on. So I thought I'd share this with the group to see if someone wants to try doing some of these ideas to prove them out.

Now, if I were doing it, I wouldn't make it a huge ordeal by making complex molds and forms, etc. Rather I would just obtain some of the material they use to make casts in the medical world (plastic that can be formed with heat?) and then tape them onto the plane, one at a time until I got all of them on it. And as I added them, I'd try to verify the results of each fairing until I came up with a final total MPH increase number. I doubt any one area will make for a huge change, rather it is likely that the combination of all the areas will give us the increase we desire. In other words, there's no magic bullet for gaining speed. Rather, it is a combination of little things that are likely to improve speed.

And sure, this could be just a brain fart I am having that proves to be a big waste of time? But I really don't think it would be, as everytime I put the nose down on my Kitfox with the new wing, I'm reminded that unlike the Kitfoxes of past, it isn't the wing that is holding us back from going faster. So if we could now just make the rest of the airframe as efficient as the wing is, we are bound to go faster. :)

[Peteohms]

Wouldn't that cause you to easily exceed VNE? Maybe over stress the airplane? Just a thought.

I was discussing making some streamline changes to a plane with the designer. He said the plane was draggy to prevent over speed. Wasn't a Kitfox, but same argument might apply.

[DesertFox4]

Paul, a couple of the items on your Kitfox speed mods. wish list have been done.

The strut to wing attach points for instance. Murle Williams had them on his Model 5 years ago. I have had a set of the same plastic fairings sitting in my hangar since before I first flew my Model 4 back in 2003. I just never painted them and installed them. They do look nice though.

Also the landing gear (Grove) have been faired into the fuselage on a couple Arizona Kitfoxs going back to Murle Williams 1997 model 5 Grand Champion. Phil Laker I think did that mod to his Vixen also.

All those mods should be effective for increasing speed. We just don't know exactly how effective. It would add build time to the aircraft but for some that isn't a concern. Weight has to be factored in also but most of the mods you mentioned can be, and some have been done, with careful attention to minimizing weight gain.

It is always nice to pick up a few mph to your cruise speed and or reduce your power setting and leave more fuel in your tanks for the same cruise speeds.

[av8rps]

Hi Pete,

I agree that we need to always be aware of not exceeding VNE. And I also agree that we need to not go nuts on this speed stuff. And that we need to respect design limits. But I just recently was flying with a friend in his multi million dollar Pilatus PC-12, and the whole time we flew he was working to stay under the VNE, as he could exceed it easily if he wasn't careful.

For me, I only wish I could fly my Kitfox having to worry about trying to stay under the the VNE constantly. Just imagine how efficient that would make our planes? If the Super Sport as an example could do that, Kitfox would be the talk of the town. I'm not sure, but I think that's the reason people buy the planes like the RV-12. I've heard it can cruise easily at the max LSA of 120 knots. Same goes for the others like the CT. So why can't the Kitfox join those ranks? It's already close. And actually, I'm thinking the Kitfox is probably stronger than those other airframes.

I think a more critical concern would be keeping under our maneuvering speed limit, as even with floats on my plane I do have to watch that number. But hey, I don't mind that at all, as that number on my plane is faster than most of the competition can cruise at (on floats).

I suppose its possible that the airframe design could be intentionally draggy to force staying within VNE? But if so then I'd be confused as to why they went to the Riblett airfoil, as that airfoil changed everything about the airplane. We went from that slow little backyard flyer to a 130+ mph Speester, that did a regular aerobatic demonstration at air shows all across America. And if any one thing sold the plane back then, it was the simple fact that not only was the airplane versatile enough to do just about any role an owner could want, but it also had a reasonably fast cruise speed to get you from place to place.

And as I recall, Kitfox sold a whole bunch of planes in those years.

But again, I agree with being extra careful with too much speed. So your point is well taken.





Wouldn't that cause you to easily exceed VNE? Maybe over stress the airplane? Just a thought.

I was discussing making some streamline changes to a plane with the designer. He said the plane was draggy to prevent over speed. Wasn't a Kitfox, but same argument might apply.

[av8rps]

Yes, I remember Murle doing some of those mods. And I can't say I know this factually, but I've always heard that not only were Murle's Kitfoxes beautiful, but they were also light weight, and fast. So when I was thinking about what I initially wrote, I was thinking alot about what Murle has done over the years.

However, using the wing strut fairings he made as an example, while lightweight and very simple, I believe they could be made much more effective if we just studied some older airplanes.

My mentor at EAA's Oshkosh Seaplane Base is Bill Brennand, who I've come to know very well now for over a decade. Bill not only founded that Seaplane Base, but also was a world class winning air racer, and Steve Wittmans right hand guy (Bill and Steve's original air racer "Buster" hangs in the Smithsonian today). So he knows a thing or two about making an airplane go fast. One day with my Kitfox in front of us, I asked Bill about a bunch of my ideas about how to make a Kitfox fly faster.

In Bill's normal fashion, he was quiet for a long time, and then responded by saying "Just look at what we did back then to our racers. We didn't always have the option for more power, so we typically used small engines with low drag airframes. So if you look at what guys like Wittman did, you'll find all your answers." Then he went on to say he was surprised that as popular as the Kitfox was that anyone hasn't already done that. He laughed a bit while half joking "If Wittman would've owned one, it would've probably been 40 or 50 miles an hour faster with the same engine".

So I've been studying all that ever since he told me all that. And I've concluded that while any fairing is probably better than no fairing, the superior fairing ideas are likely to surprise people with their amazing results. But again, there is no one magic bullet for speed. It is a combination of effective airframe cleanups as well as lightening techniques. Heck, just look at what wing strut fairings do for our faster models (10-12 mph is pretty common).

On an entertaining note about Bill, one day he told me to look at one of his old air race photos that was taken right after a race he'd won. Prior to the race Wittman had decided that to save weight they were going to make the side glass out of aluminum rather than a heavier transluscent material. Bill admitted he could see almost nothing to his side, describing it like a race horse with blinders on. But ultimately, he won the race ;). He said the only time that caused him real concern about not having side visibility was when he was in that little 15 ft winged racer during the qualifying laps and the prop disintegrated in flight, forcing him to land between a bunch of automobiles and people in a busy parking lot :eek:. He said it was uneventful, and they still qualified for the race the same day, and won. Wittman of course proved his point that every clean up one can do, and every ounce of weight savings, can be the difference between winning and losing.

But Bill chuckled with his final statement, shrugging his shoulders and saying "But he didn't have to fly the airplane... I did".

Based on all the things Bill has taught me over the years, I feel like I still have a lot I need to know :)


Paul, a couple of the items on your Kitfox speed mods. wish list have been done.

The strut to wing attach points for instance. Murle Williams had them on his Model 5 years ago. I have had a set of the same plastic fairings sitting in my hangar since before I first flew my Model 4 back in 2003. I just never painted them and installed them. They do look nice though.

Also the landing gear (Grove) have been faired into the fuselage on a couple Arizona Kitfoxs going back to Murle Williams 1997 model 5 Grand Champion. Phil Laker I think did that mod to his Vixen also.

All those mods should be effective for increasing speed. We just don't know exactly how effective. It would add build time to the aircraft but for some that isn't a concern. Weight has to be factored in also but most of the mods you mentioned can be, and some have been done, with careful attention to minimizing weight gain.

It is always nice to pick up a few mph to your cruise speed and or reduce your power setting and leave more fuel in your tanks for the same cruise speeds.

[HighWing]

I find this thread very interesting. Paul's thoughts seem to pretty much parallel mine since the beginning. My current Model IV has the 912UL just like my first one. I put a lot of fairings on that one and the biggest thing I noticed was the fuel burn when on our occasional flights of six to eight. We always flew as a group so the cleaner airplanes would fly at lower engine speeds resulting in some interesting observations at fueling times. Once on a three hour flight on our way to Idaho the range at Winnemucca for fuel in our flight of six was a tad over 7 gallons to a tad over 12 gallons. I have put the same fairings on the new one largely because I like the simplicity of the UL and hoped to keep up with the ULS guys.

I would also agree that climb at clean is better than climb at dirty.

In the other thread about 7SS's empty weights I couldn't keep from thinking about what we like on our airplanes. Mine is heavier than a typical IV, but I tried to make the added weight a functional addition rather than just a look good addition, though I think they look good.

I have done several of the mods Paul suggests.. I added a little over six pounds to the empennage with full airfoiled horizontal and vertical surfaces with gap seals and the strut fairings. Also the over the horizontal fuselage gap closure. I am embarrassed to say, however, that I haven't done enough flying to really report on the benefits.

Just today, I was looking at the 21" tires and thinking what I would need to mount the aft wheel fairings like those used on the Ercoupe. I've thought about that one for years

Helping my buddy on his Lancair IV taught me some interesting tricks working with fiberglass. Most of these fairing mods can be done over a finished airplane. More on the tricks later. A couple of pictures for now.

[kitfox5v]

Lowell,
You need to add this to your list of toys on HW LLC. I would like to put those on my 4. I've got the ones kitfox sells but the longer teardrop will create less drag.
How did you make the seals for the farings? What are they made of? Thanks
Hope you had a great birthday. Eddie

[av8rps]

Lowell,

That is exactly what I was talking about! I had no idea anyone was doing it to the extent you are. I also think your results from trips with similar aircraft, faired and unfaired, with varying fuel burns proves this is worth doing to our airplanes.

Paul

[HighWing]

Eddie,
I've thought about kitting the parts, but most have to be custom fit. Example - the ribs I used in the horizontal tail are patterned somewhat after the ribs in the speedster tail, but not exactly. The fairings on the struts fit the profile of my horizontal stabilizer and won't fit anyone elses

Below are some photos of the Horizontal stabilizer strut and jury strut fairing tools and procedures.

1 The fairing tooling and a couple of glass fairing pieces. The tool was made by gluing a couple of pieces of balsa of the appropriate width to a strip of thin aluminum. It is then sanded to shape and covered with a single layer of light weight glass - see below for the method. It is then sanded and primed and sanded smooth. One of the fairing pieces is glued at the trailing edge and one is still open. The airfoiled foam piece for fitting to the tubing can simply be press cut using the end of the fairing piece. The fairing section on the right is 20-1/2" long and weighs 1.005 oz.
7642


2. One of the Lancair techniques. A single layer of glass cloth is laid down on a piece of vinyl sheeting and the resin is added to the cloth. After it is saturated a sheet of thin Polyfiber or other Dacron covering fabric is laid on top of the glass.
7637



3. another sheet of vinyl is placed on the top of the glass strip and excess resin is squeegeed out.
7638


4. The bottom layer of vinyl is removed and the glass is laid out on the form - previously treated with a mold release - and all air bubbles are removed by finger pressure. The top layer of vinyl is then removed and it is rechecked for bubbles.
7639


5. The clamp - couldn't find it for a separate photo - is placed to hold the glass - Dacron sandwich so it wont lift up at what will be the trailing edge and secured with a couple of spring clamps. The clamp is made from a length of piano hinge with aluminum angles riveted to the free edges forming a sort of a long clamshell clamp.
7640

6. The finished fairing is trimmed slowly with a table saw
7641

[SWeidemann]

Kitfox Folks,

My Vixen may be one of those Arizona airplanes you are referring to. It has a built up bunch of wood framework & stringers starting just aft the firewall, running to a channel for flush mounting the spring gear, and continuing aft & tapering back into the bottom longerons a ways back. The smooth fabric on the bottom I am certain adds some speed. I get about 120 mph with a 912 ULS. I've also got my mind on some streamlined horiz strut fairings, wing strut/ fuselage fairings & wing strut-to-wing fairings. I already have a set of gas cap fairings to install.

"Also the landing gear (Grove) have been faired into the fuselage on a couple Arizona Kitfoxs going back to Murle Williams 1997 model 5 Grand Champion. Phil Laker I think did that mod to his Vixen also."

Skot

[av8rps]

Skot,

I'm curious, have you ever tested your Vixen to see just how fast it will go wide open? So many times when we are talking about speeds one guy means at x rpm, while another is using y rpm. So I've learned to ask people "What will it indicate as well as show for true air speed when wide open throttle, straight and level, at a reasonable altitude of say 7,500 ft? And what is the engines rpm at your max speed".

Doing it this way is a really good way for us to compare our fleet so we can figure out what mods really makes them go faster.

Now I don't want anyone tearing the wings off their airplanes in an effort to find out that number, but it would be interesting to know if most of our fleet could even come close to their VNE.

[DesertFox4]

Skot- I couldn't remember if your Vixen's gear had that fairing mod or not but sure sounds like it has.


Paul- I've had my model 4 with a 912uls to 132 mph wide open throttle many times. Usually at 3,500 feet or so altitude with the Ivo Lite prop. with rpms
in the 5,600 to 5,700 range for just a minute, two at the most. I have no wheel pants, spinner, jury strut fairings or horizontal stabilizer fairings. I do have gas cap fairings and strut fairings and the wide body modification along with the razorback mod. Empty weight 659 lbs.
My airspeed indicator and gps are within 1 mph on a zero wind day. We actually have quite a few zero wind days here in Phoenix.;)

Some years back Michael Gibbs had the exact same model 4 Kitfox with the same engine but with wheel pants, large spinner and streamlined jury struts and an Ivo Medium prop and he could hit VNE (140 mph) in level flight under the same conditions. I can't recall what his empty weight was but I think slightly heavier than mine. He also climbed that Kitfox up to 17,999 feet in altitude and still had a little more ceiling left.

[Esser]

I don't think anyone will rip their wings off if it's within reason. A really good advantage to the Kitfox is that is has push rods which helps with flutter. The bad thing is it has a flexible wing which is not good. Also, I don't know how well the flapperons are balanced as I havent got that far yet. On that note, the flapperons don;t seem that stiff. You could inject a dense foam in them but you will certainly get corrosion issues if you do that.

All that being said, I do think the Kitfox has some efficiencies to be gained. If you want to go faster, that is a different story.

[desertfox1]

Murle and I built our planes side by side in his shop(94-96), good times!
We enclosed the landing gears and did a bunch of other mods. I'm in the
plastic business so was able to form the parts as we came up with ideas.
I have done the gear enclosure mod on several planes since then, including yours Skot. I think the exposed gear is the biggest source of drag after the
round tube struts.

I'll try to post a pic of the latest fairing Bill and I are working on. Just what
you were talking about Paul, 4.5 inches and under a pound per strut.

Phil

[SWeidemann]

Phil,

Do you, or will you make these parts for other Kitfox fans?

Skot

[n85ae]

I can easily exceed VNE in level flight with N85AE if I firewall the throttle
with my IO-240B, I have a 74" Sensenich prop on it. The engine is very
smooth thanks to recent dynamic balance of the prop, but ...

It feels like a runaway train at 140, and it's NOT a comfortable speed at
all. The plane is all wrong for going fast ...

My Dad (the one with the PhD in Aero Engineering) told me once - Don't
waste your time trying to make it go fast the shape of the plane is all
wrong. If you want to go fast build a different plane.

Jeff






Now I don't want anyone tearing the wings off their airplanes in an effort to find out that number, but it would be interesting to know if most of our fleet could even come close to their VNE.

[av8rps]

Jeff,

I'm not surprised that a IO-240 powered Kitfox will easily exceed VNE. The non rotax aircraft engines still seem to be all around the best for flat out speed. Granted, I do remember an article years ago where they did a fly off between a Kitfox 5 IO-240 vs a Kitfox (4?) with a Rotax 914, and they were both really fast airplanes, with the 914 being just a bit faster (150+ mph as I recall), but it was using higher altitudes to get it.

My model IV seems to not hit the "wall" that previous models I've flown do. I attribute that 90% plus to the Riblett airfoil. One day I put my Kitfox amphib in a long shallow dive while trying to catch up with some buddies that were flying about 500 ft below me, and because I was a bit fixated on where they were, I wasn't paying attention to my airspeed indicator. When I finally leveled out and passed them like they were standing still I looked down at my ASI to see I was going 155 mph! A really DUMB mistake on my part (and I won't ever do that again), but I have to say I never noticed that the airplane was struggling to go that fast. In fact, I'm pretty confident it would have gone faster had I not leveled off.

One of my favorite antique planes is a Monocoupe. The Kitfox so much reminds me of a Monocoupe. And when you study them, they went like hell on not a lot of power. Yet, they didn't use any kind of a special airfoil, and the general concept of the airplane was pretty much like our modern day Kitfoxes. So I really do believe our Kitfoxes have much more potential for speed.

Again, I'm not saying we need a 200 mph Kitfox, but it sure would be nice if we could cruise closer to VNE without having to run wide open. Your Kitfox is what I believe to be the exception to the fleet, and probably a good example of what we can accomplish if we clean it up more, put more power on it, or both.

[av8rps]

Those look awesome Phil! Yup, just like I was talking about.


Murle and I built our planes side by side in his shop(94-96), good times!
We enclosed the landing gears and did a bunch of other mods.....

Phil

[av8rps]

Steve,

Well, you and Michaels Model 4's are just more proof of the potential our planes have for higher speeds. So just imagine if we could do more clean up on them and go even faster...:D

I think there is something special that happened when the Murle Williams wide cabin mod was done on some of those Model 4's. I'm thinking it must give the short fuselage better aerodynamics / taper, much like the new Kitfox has being wider, and not requiring doors that are bubbled out.

So how about short or long wings? I'm pretty convinced the 4's with the Speedster (short) wing are faster than the long wing. But with the newer and heavier Kitfoxes, the short wing wouldn't work as well as the long wing for speed as it has to fly at too high of an angle of attack even when at cruise. Anybody have any thoughts about that?





Skot- I couldn't remember if your Vixen's gear had that fairing mod or not but sure sounds like it has.


Paul- I've had my model 4 with a 912uls to 132 mph wide open throttle many times. Usually at 3,500 feet or so altitude with the Ivo Lite prop. with rpms
in the 5,600 to 5,700 range for just a minute, two at the most. I have no wheel pants, spinner, jury strut fairings or horizontal stabilizer fairings. I do have gas cap fairings and strut fairings and the wide body modification along with the razorback mod. Empty weight 659 lbs.
My airspeed indicator and gps are within 1 mph on a zero wind day. We actually have quite a few zero wind days here in Phoenix.;)

Some years back Michael Gibbs had the exact same model 4 Kitfox with the same engine but with wheel pants, large spinner and streamlined jury struts and an Ivo Medium prop and he could hit VNE (140 mph) in level flight under the same conditions. I can't recall what his empty weight was but I think slightly heavier than mine. He also climbed that Kitfox up to 17,999 feet in altitude and still had a little more ceiling left.

[rocketman2tm]

Seeing as a 200 mph Kitfox was mentioned, I feel that I should share an article about a turboprop Kitfox I found.

http://www.aero-news.net/index.cfm?do=main.textpost&id=7d1cc9e3-ef1b-4efc-b0b1-902f64bdadf0

This was from the Skystar days. I doubt that they ever sold any. Did anyone ever see this beast or see it fly?

[GuppyWN]

Seeing as a 200 mph Kitfox was mentioned, I feel that I should share an article about a turboprop Kitfox I found.

This was from the Skystar days. I doubt that they ever sold any. Did anyone ever see this beast or see it fly?

"We've derated the puppy to only 300 horsepower or so, added some rudder, and are requiring a factory checkout... but for those who are willing to part with $150K, you'll get an airplane that takes off, at full gross, in under 20 feet, climbs 5000 fpm, and will cruise at 200 kts at 18K."

Holy cow!!

[Av8r3400]

Anyone have a photo of that beasty thing?

[t j]

Here's my speed story. Kitfox classic 4 with 52 HP 503 Rotax with 3:1 gear box. First built, 68" 3 blade GSC taper tip prop. No fairing on struts or the stock tube landing gear. 6000 RPM was 65 mph.

Put the wood fairing on the landing gear and changed to a 72" 2 blade Square tip Warp prop. 6000 RPM was 75 mph.

Added the PVC fairing to the wing struts. 6000 RPM went up to 85 mph. Full throttle level flight, 6600 RPM and gets 87 mph.

I have the prop set for climb not cruise. I think I have hit the max speed wall for this prop/pitch and the little 503 engine. I have the prop set for climb not cruise. I can get it up to 100 mph in a dive but I have to reduce the throttle to less than 4000 RPM or the EGTs get too high.

[t j]

Anyone have a photo of that beasty thing?

I remember reading that article and got pretty excited. Then I remembered what day it was. Check the date.

[DesertFox4]

I remember reading that article and got pretty excited. Then I remembered what day it was. Check the date.

Tom you nailed it. ;)
The closest they ever got to something like that was the turbo charged 7 with a Hoffman C.S. prop.

[kmach]

Does the 2004 date have significance ?

Other than being 10 years ago, and Skystar in the article.

[DesertFox4]

Does the 2004 date have significance ?

April 1st does. It's an April Fool's Day joke from Ed Downs.

[kmach]

I missed the April 1st part ! Doh!:)

[Slyfox]

this is my story. I have the kitfox 4 1200. speedster wings. empty weight 630lbs, my horizontal has the wood inserts, so it's not a flat horizontal. vertical is normal, flat. I have all gaps sealed including the rudder, I have the plastic fairings on the struts. I have the 912uls with the medium IVO inflight. I can take off within 100 ft. I can climb out @ 60mph with a vsi of 1800ft min. I cruise all day long at 120mph. I have thought of cleaning her up a little more. I think 120 is just fine and the take off is great. Just ask Scott. My thought is if you clean her up too much than slowing down coming into an airfield will change, meaning it won't. at least like I like. I love the fact that I can pull the throttle and flatten the prop and do a real nice short approach. unless you have witnessed my short approaches you don't know what a true kitfox can do. I personally don't want to screw with this one. so my dirty little plane, both visual and actual flying is just fine with me. I love the way she is, I'm not changing her. so you can have your fast kitfox if that's what you want. for me if I want fast, which I do, I go fly my rv. she does very much right there at the 200. now that one, if I can make her go faster you bet, that is what she is designed for.

as far as going up high. I believe the kitfox is a low flying machine, you know, flying through the trees and along the river bank. I've personally never flown to 17000 ft even in the rv. do you have oxygen. I've got it in the rv. just asking. the only time I've taken the rv up high is to go over weather.

have a nice day:D

[av8rps]

I agree we don't want to lose our short field ability while trying to gain cruise. My 4-1200 even on floats doesnt like to slow down (especially if I compare it to my Highlander) but I've learned how to land it short by slipping it really hard (in initial testing hard enough that I blew out my side windows :D), which offsets an airfoil that is too clean to slow down easily, an idle that is a bit too high, and a prop that's not long enough. I'm guessing you are running a 72 inch prop? A med IVO in a longer length I'm sure would help slow my plane down, but with an 80 hp 912 that really isn't an option.

Yeah, our planes will never go as fast as the bigger RV's, but I'm advocating that they should be able to go as fast as the small RV 12, while still being more versatile (skis, floats, big tires, etc). And heck, some of our Foxes are already close to the speeds of that 12, so I think it can be done.

Fast? I once flew a turbo 330 hp Glasair 3 that cruised at 75 percent over 300 mph. And one like it clocked over 400 mph at Reno this year. So if I ever REALLY want fast, I'm probably going that route.

What I'm getting at with a faster Kitfox is having the ability to go 10 or 20 mph faster when trying to get somewhere when dealing with that all too common headwind. And I'm convinced that our little Kitfoxes have that potential if we can just share some ideas.

[Slyfox]

not sure what my prop is 70 or 72. I called and ask for and got one. have to go measure. it's perfect in my opinion. I like to slip hard as well, that's how come I can do the 10ft off the numbers for a base turn:eek: and to slow down more on the home stretch I flatten the wings and keep in the hard full rudder :eek: until I have the right speed to touch down, than I let go of the rudder and full stick back :D ya baby. gotta love that fox

[SWeidemann]

Getting back to the faired in fuselage bottom to enclose the landing gear, does anyone know of drawings, plans or a parts modification available? (For a retro-fit or new build).

Thanks much,

Skot

[Slyfox]

I personally have thought of doing this. would like to hear more. also the fairing on the rear horizontal brazes sound good as well. now I have the nice cowl for the front of the radiator, haven't put it on yet either. too much time flying not enough time to do things I guess.

[Kurt.A]

I think this is a great idea. Anything that reduces drag certainly has to help fuel economy which means more flying time, more accessories etc. With my limited knowledge of aerodynamics I do have a question. What effect does reduced drag have on stall speed? Does it remain the same speed? Will the warning indicators prior to stall change?

[PapuaPilot]

Kurt,

Reducing drag will not affect the stall speed. Without going deep in the aerodynamics the only real factor on a Kitfox would be the weight.

A plane with less drag will be harder to slow down for landing.

[HighWing]

Kurt,

Reducing drag will not affect the stall speed. Without going deep in the aerodynamics the only real factor on a Kitfox would be the weight.



I thought on this thought. The guy I helped modify his Avid reported on the effects of the mod. He had presented his elevator with a set of full size drawings that I followed while doing the mod. He reports that it increased elevator area by 30% and reduced stall apeed by 8 mph. Lots of talk in the past on the Kitfox sites on increasing elevator authority so as to be able to do a true three point landing. Increasing elevator area might be the best solution.

[PapuaPilot]

High Wing,

The increased area of the elevator didn't change the stall speed. It only helped to get more pitch authority and a greater angle of attack (i.e. to slow down).

Stall speed is a function of weight, wing area, air density and coefficient of lift. Here is the stall speed formula: V = sq root (W/(1/2p x S x Cl_max))

Where:
V = Stall speed
p (rho) = air density
S = wing area (Ft2)
Cl max = Coefficient of lift at stall
W = weight

In the case of your friend's Avid nothing in the stall formula changed. The wing area and Cl max (the lift of the airfoil at the max angle of attack - determined in wind tunnel testing) didn't change. Stall speed assumes that the wing is an its maximum angle of attack. The Avid simply could not achieve the needed angle of attack to stall the wing due to an undersized elevator.

Like I said in the first reply, the only thing on a Kitfox that can change the stall speed is the weight of the plane. A heavier plane will have a higher stall speed. The other factors are part of the aircraft's design (wing area and type of airfoil). We can't do anything about the air density, it is the mass of the air molecules. Don't get this confused with density altitude. They are not the same and is a whole other topic.

I purchased the larger elevator that is being used in the current production Kitfox for my Model 5 for the very reason you stated. I have a heavier engine and was told by John McBean that I would run out of elevator in the flare. The only other way to keep the elevator effective is to keep a little power in so there is prop wash over the elevator, which will only make you come in faster. I want elevator authority so as to be able to fly STOL landings and bought the larger elevator.

[HighWing]

I think I understand your point, but still have a question or two. Are you suggesting that when we complete the POH we do some math and enter a number on the performance page under the stall speed heading? When I did my Phase I, I took the airplanr to altitude and did several stalls at different throttle settings. Those are the numbers I put in the POH. My friend did the same and now with the larger elevator he finds the numbers 6 mph lower. What do you suggest he put in his POH?

[PapuaPilot]

I haven't done any Phase 1 testing so it's hard to know what to say. I think you would have to put the numbers that you found during the flight testing.

[PropellerHead]

Could it be that, with the smaller elevator, your friend was just running out of elevator authority and not really stalling?

What about some rigid fairing attached directly to the landing gear legs? Maybe leave a little gap to account for gear movement?

[PapuaPilot]

[QUOTE=PropellerHead;43847]Could it be that, with the smaller elevator, your friend was just running out of elevator authority and not really stalling?

That's what I think was happening.

[av8rps]

Hi Tom,

Your information here is a really good example of what one can do to improve efficiencies of our planes. By cleaning up your airframe and replacing a prop you in effect have improved the cruise speed of your 503 Model 4 Kitfox by 34%.

That is a pretty amazing number as there are some unfaired Kitfoxes flying around with almost double the horsepower you have and not going as fast. So your modifications prove exactly what I was getting at by saying I think we have some additional opportunities to improve our speeds. Granted, once a person installs the most obvious fairings, the changes and improvements tend not to be as noticeable. But they do all add up.

I used to fly an Avid Flyer A-model that was super light with a Rotax 532 that produced 74 hp (I know that because we dyno'd the engine on three different dyno's). On a set of straight fibreglass floats and a super efficient propeller it would go 115 mph wide open straight and level. My buddy with his 150 hp Super Cub on straight floats couldn't even think about keeping up with me if I was on the throttle. But that little Avid was pretty well streamlined, and it was super light (empty weight on wheels was only weighed 396 lbs, and approx 500 lbs on floats). And the prop was a hand carved scimitar shaped, multi-lamination thing of beauty.

Now most would never believe you could get an old draggy Avid Flyer to go that fast, but they're wrong. It is possible with everything being right. That airplane and yours proves that.

Paul


Here's my speed story. Kitfox classic 4 with 52 HP 503 Rotax with 3:1 gear box. First built, 68" 3 blade GSC taper tip prop. No fairing on struts or the stock tube landing gear. 6000 RPM was 65 mph.

Put the wood fairing on the landing gear and changed to a 72" 2 blade Square tip Warp prop. 6000 RPM was 75 mph.

Added the PVC fairing to the wing struts. 6000 RPM went up to 85 mph. Full throttle level flight, 6600 RPM and gets 87 mph.

I have the prop set for climb not cruise. I think I have hit the max speed wall for this prop/pitch and the little 503 engine. I have the prop set for climb not cruise. I can get it up to 100 mph in a dive but I have to reduce the throttle to less than 4000 RPM or the EGTs get too high.

[HighWing]

Could it be that, with the smaller elevator, your friend was just running out of elevator authority and not really stalling?


I think you nailed it. The math likely defines the minimum theoretical stall speed. What we have in a practical situation are many deviations from the ideal. Another example would be the airfoil where the only areas exactly on the Riblett design are over the rib capstrips and the eighth inch wide upper false ribs. The rest of the wing is a compromise determined by the geometry of the shrunk fabric surface.

A good friend and Model IV builder tells of the time he heard Harry Riblett discuss the Kitfox use of his design. He mentioned that the most critical part of the design was the first five inches. The LLE from Kitfox adresses that issue. I put an aluminum leading edge on my wings and compared with my first Model IV, the new one stalls three mph lower - 40 vs. The previous 43.

Then as to the elevator, for years fliers have been doing all sorts of things to increase elevator authority in the early Kitfoxes. The most common are the tape gap seals. The more recent availability of the larger elevators has attacked the deficiency at its source. That is essentially what we did for my friend's Avid.

I guess it would be safe to say that, although a great airplane, the Kitfox is not perfect out of the box.

[av8rps]

I think weight is the most critical component for stall speed, assuming the airframe is the same.

My early Avid Flyer would stall around 20 mph solo. Yup, I did say 20 mph (it was even advertised at the time at 22 mph. My old flight instructor didn't believe it until I demonstrated it to him).

By comparison I did all the initial test flying on a friends Avid Flyer Mk 4 and it stalled solo at 36 mph. But the Mark 4 weighed 55% more empty than my A-model did (400 lbs vs 625 lbs). So the empty weight is 55% higher, while the stall speed is up 75%. Oh, and my old Avid has a small elevator, while the Mark IV has a much larger elevator. AND, the center of gravity on the early airplane has a forward CG, while the Mark IV has a rearward CG. Typically nose heavy planes stall sooner than ones with a rearward CG. So if one could move the cg back on my old A-model, it might actually stall even slower? :eek: (hard to believe, but maybe?)

Now this isn't very scientific, but rather is based on some real life comparisons to almost identical airframes that have very different empty weights.


I think you nailed it. The math likely defines the minimum theoretical stall speed. What we have in a practical situation are many deviations from the ideal. Another example would be the airfoil where the only areas exactly on the Riblett design are over the rib capstrips and the eighth inch wide upper false ribs. The rest of the wing is a compromise determined by the geometry of the shrunk fabric surface.

A good friend and Model IV builder tells of the time he heard Harry Riblett discuss the Kitfox use of his design. He mentioned that the most critical part of the design was the first five inches. The LLE from Kitfox adresses that issue. I put an aluminum leading edge on my wings and compared with my first Model IV, the new one stalls three mph lower - 140 vs. The previous 143.

Then as to the elevator, for years fliers have been doing all sorts of things to increase elevator authority in the early Kitfoxes. The most common are the tape gap seals. The more recent availability of the larger elevators has attacked the deficiency at its source. That is essentially what we did for my friend's Avid.

I guess it would be safe to say that, although a great airplane, the Kitfox is not perfect out of the box.

[AirFox]

I'm pretty happy with the Super sport speed with 26" Airstreaks.
7674
This pic was today with a nice tail wind. I average 26 mph cruise at 5200 rpm. At 5400 I'm in the 130's.

[PapuaPilot]

Typically nose heavy planes stall sooner than ones with a rearward CG. So if one could move the cg back on my old A-model, it might actually stall even slower? :eek: (hard to believe, but maybe?) Now this isn't very scientific, but rather is based on some real life comparisons to almost identical airframes that have very different empty weights.

That is true, a plane at aft CG will have a lower stall speed then the same one with a more forward CG. The reason is because the elevator & horizontal stabilizer provide down lift. With a forward CG the tail has to provide more down lift to balance these forces. The total lift of the wing produces is equal to the weight of the plane plus the downward force of the tail. When you go to an aft CG the total lift the wing is reduced lift due to the lower down force of the tail. When the total lift of the wing is reduce so is the stall speed, it's like flying at a lower weight. A lighter plane stalls at a slower speed.

There are many advantages of an aft CG. The nose will pitch easier so you can get a deeper flair. The elevator is more responsive, but the plane is slightly less stable in pitch. With a slower stall speed you get a reduced ground roll. This can help too when you are doing a soft field takeoff (easier to get the nose up and a lower liftoff speed). You also get a higher cruise speed because the wing has to make less lift (which means less drag too). All things considered an aft CG is better choice.

I am not promoting flying a plane outside of its CG limits. That can be deadly.

[Kurt.A]

Excellent information guy's! All the more reason to build as light as possible when you can and pay close attention to weight placement. Which covering/painting system provides the lightest end result?

[av8rps]

Kurt,

The lightest covering system I am aware of is dope and fabric. But you can't get into wanting a deep shiny finish. Just put the paint (butyrate or nitrate dope) on heavy enough so it covers just barely, and stop. It's going to be flat finish and kinda boring looking, but its going to be lightweight.

The only other very light process is this new stuff from germany that you iron on like we used to monokote model airplanes. But expensive and limited life.

Paul

[Danzer1]

The only other very light process is this new stuff from germany that you iron on like we used to monokote model airplanes. But expensive and limited life.

What process are you referring to and what information can you share on it's "limited life"? Or is this an opinion?

[av8rps]

Oratex covering...see www.betteraircraftfabric.com

I read a recent article about this fabric process as it looked interesting. But I somewhat lost interest when in the article the writer said they expected a 6 year life, and it would cost 5 or 6 grand to cover an airplane.

But I do see on the Oratex website they now say 10 yrs or more.


What process are you referring to and what information can you share on it's "limited life"? Or is this an opinion?

[Danzer1]

I'm familiar with Oratex and have read everything I can find on it, but have never read anyone referring to such a short lifespan. Do you have the article reference? There are many, many aircraft using this system, I doubt many would if they expected 6 years. Really would like to hear more.

Greg

[Kurt.A]

Kurt,

The lightest covering system I am aware of is dope and fabric. But you can't get into wanting a deep shiny finish. Just put the paint (butyrate or nitrate dope) on heavy enough so it covers just barely, and stop. It's going to be flat finish and kinda boring looking, but its going to be lightweight.

The only other very light process is this new stuff from germany that you iron on like we used to monokote model airplanes. But expensive and limited life.

Paul

So a quick look through Aircraft Spruce online catalog came up with several options for covering and painting using fabric and dope. Can you steer me in the direction of a book or someway of doing a comparison between brands and techniques? What would be the weight savings performed the way you have described vs the shiny method? When you say fabric are you referring to Dacron, Ceconite, etc????

[SWeidemann]

How about good old Poly Fiber System? Mine has at least 11 years on it, and looks good. Of course, with any fabric plane it should live in a hangar when it's not out flying.

Skot

[av8rps]

I knew someone would be asking me that, but I honestly cannot remember where I read it . It might have been on the web? Or since I get almost every aviation magazine known to man, if it was there it was in the last 6 months somewhere? I seem to recall the info was associated with Frank Knapp's Lil Cub in the Valdez STOL/Alaska stuff.

I still find the Oratex product interesting, and believe it will become more popular in time. And for anyone considering the product I would recommend using the information from their FAQ area of their website, which does talk about lifespan. I'm sure their answer to that is more reliable than what I read.

Paul



I'm familiar with Oratex and have read everything I can find on it, but have never read anyone referring to such a short lifespan. Do you have the article reference? There are many, many aircraft using this system, I doubt many would if they expected 6 years. Really would like to hear more.

Greg

[Danzer1]

Thanks Paul,

I'll keep digging. Just to clarify though so as not to slight Oratex, they don't just say 10 years or more. They guarantee it for 10 years. I'm still weighing the cost/weight/process/paint with all the others.

So far I like the Oratex tape process better and the ability to use non pinked (straight edged) tape for a cleaner look. We'll see more longevity data as time goes on, but to me it looks good. I'd like to see the price more competitive though and that is changing with the dollar/euro exchange rate. So we'll see.

Greg

[Kurt.A]

Getting back to the original intent of this thread has anyone information regarding surface finish and how it affects drag? Does the boundary layer come into effect and negate a rough surface causing more drag? Does the boundary layer become thinner with a smooth finish and in essence provide a smaller frontal area?

[HighWing]

I think weight is the most critical component for stall speed, assuming the airframe is the same.

This makes a lot of sense. Then I have to wonder. Kitfox reports their 750 lb. Super Sport stalls at 41 mph. That is one mph. higher than my 650 lb. Model IV. The math - 15% higher empty weight 2.5% higher stall speed.

I realize the only thing that is identical on the two airframes is the Riblett wing, but there are significant differences elsewhere. Any ideas what they have done with the design that results in the extra efficiency in stall speed?

[av8rps]

Lowell,

Is the stall on your model 4 at 41 mph clean, or with flaps? Do you have the full length wing? If you do have the 32 ft wing and your are using flaps, then I'd suspect something else being wrong with your plane like an inaccurate ASI at low numbers, a forward CG etc. The heavier Super Sport should most definitely stall sooner than your Model IV. I just don't believe there is enough airframe difference between the two to make affect the stall that much.

At least that's my take on it. Maybe others have some ideas?

Paul


This makes a lot of sense. Then I have to wonder. Kitfox reports their 750 lb. Super Sport stalls at 41 mph. That is one mph. higher than my 650 lb. Model IV. The math - 15% higher empty weight 2.5% higher stall speed.

I realize the only thing that is identical on the two airframes is the Riblett wing, but there are significant differences elsewhere. Any ideas what they have done with the design that results in the extra efficiency in stall speed?

[av8rps]

If you are interested I can connect you with a friend of mine that has built a whole gaggle of airplanes, including a Kitfox and an Avid. And every one of them were super light, and all were done in dope and fabric. In fact, here is the Kitfox he built a while ago in his Barnstormers Ad (he's almost done with another plane so he's selling that one). Note the empty weight

" Kit Fox new build 11 light sport stol, n numbered, in phase 1 582, only 429lbs,521 useful,42 1/2 at shoulders,20"trunda tires, line bored Grove gear, s/s taper warpdrive prop, bagage compartment.streamline struts full electric, starter sealed battery, Maul locking tailwheel, HD I beam struts, us coastis vinly easey removeable,9core rqdiator, trailer included, turtleback cover for storage or towing with wing folded, built by a little old seaplane pilot • Contact John Knapp (http://www.barnstormers.com/contact_seller.php?to=2548&id=949731&title=kit+Kox+11+new+build&return=%2FExperimental%2C%2520Kitfox%2520Classifie ds.html), Owner - located Bath, NC USA • Telephone: 252-923-5145 • Posted October 14, 2014

Now, I'm sure someone is going to say that maybe his scales are off? But if they are, then it sure is interesting to see how his airplanes perform so much better than most others. If you have a chance, look at this video of his Avid Flyer he flew on floats for years;

http://www.youtube.com/watch?v=qjDSatUSoCY

If I remember correctly, that Avid on floats in the video with a 2 stroke Rotax 532 weighed only 387 lbs ON FLOATS. So he knows how to build light. But I would attribute most of that to his dope and fabric finish. Oh, and it seems to have held up well. He built that Avid in 1984.

Paul





So a quick look through Aircraft Spruce online catalog came up with several options for covering and painting using fabric and dope. Can you steer me in the direction of a book or someway of doing a comparison between brands and techniques? What would be the weight savings performed the way you have described vs the shiny method? When you say fabric are you referring to Dacron, Ceconite, etc????

[t j]

Any ideas what they have done with the design that results in the extra efficiency in stall speed? Just thinking out loud. Vso is indicated airspeed at maximum gross weight and max aft CG. I suspect it was loaded for an aft CG which may account for the efficient stall speed.

Just for drill, here's the stall speeds for different weights and CG I recorded for my classic 4 during phase one.

WT----CG----Stall Clean ----Stall Full Flaps Vso
765-- 12.4-----38 mph
898-- 13.68----43--------------------39
954---14.42----45--------------------40
1049-14.68-----46--------------------42

[HighWing]

Paul, actually my stall was the 40 mph one. I hope my tendency to mix numbers didn't occur here.

My first Model IV stalled at 43 and I flew it for nine years so that number was pretty much verified as I piled on the hours - 900.

The ASI might not be accurate at low speeds, but I did check it with a water manometer when building and it followed the theoretical curve quite closely - image of the spreadsheet attached. If there is an error, I suspect it would be common across the fleet using the factory supplied pitot tube.

I find this thread very interesting and it brings to mind some more things I might want to try on my current airplane. Thanks to all who have contributed and hope for more ideas in the future.

[av8rps]

Lowell,

I guess I should have known you did a scientific test of your ASI. I'd be disappointed at anything less from you ;).

So, maybe, just maybe... the advertised stall speed of the newer planes are a bit optimistic? Or just maybe, they are being tested with an aft cg and a very light load?

Either way, our Kitfoxes generally speaking still have a really nice speed range from slow to fast. And since I am overall ok with the low speed end of my Kitfox (how slow does one really need to land anyhow?), I'm still curious how much a little more aerodynamic clean up might help our airplanes.

Since our Kitfoxes generally resemble the old Monocoupes of the past, why can't they go as fast? I think they can...

Paul

[mscotter]

I find this topic fascinating. It would be interesting to hear some more specifics on some of the mods. For example, the wide-body mod on the model IV's. I read on another post somewhere that some believe this mod actually improves cruise speed? Any data to back this up?

[av8rps]

This is still a great thread as there was a lot of great info shared here.

There is info earlier in this thread about the Kitfox 4 tri gear that had the wide body mod that would go 140 mph. So yes, there is something about what that does to the drag on the stubby little Kitfox 4 fuselage. Unfortunately it will take someone with a better knowledge of aerodynamics than me to explain why that would make sense.

But knowing that I am beginning to wonder if the bubble glass doors on my Model 4 amphib might be attributing to the extra speed I seem to enjoy. Maybe those doors are having a similar effect as the wide body mod? I've never flown it with conventional doors, so I can't compare.

All good food for thought...

[Av8r3400]

Short, Stubby and really, really FAST...

12860

[Micro Mong Bldr]

Isn't the best fineness ratio for subsonic about 6:1?

[Guy Buchanan]

I couldn't find that anyone's mentioned a great book on drag reduction: Speed With Economy, by Kent Paser. (Probably available at EAA.) He does major mods to a Midget Mustang to gain dramatic speed improvements. (64mph top speed improvement.)

[jrevens]

I couldn't find that anyone's mentioned a great book on drag reduction: Speed With Economy, by Kent Paser. (Probably available at EAA.) He does major mods to a Midget Mustang to gain dramatic speed improvements. (64mph top speed improvement.)

I've known Kent for many years, and his book is filled with good hints and ideas. His airplane was pretty drastically modified - it's a Mustang II by the way - and many of the things he did probably wouldn't be copied by most builders (like the tiny little canopy and other mods in that area). It sure was/is? a fast airplane, but I don't think he has flown now for quite awhile... I hope he's still around. I haven't seen him for a few years. He admitted at one time that some of the things in that book didn't prove to be beneficial like he thought... for instance the inversion cones in the exhaust system. It is an interesting book, worth having.

[jamesmil]

Hi guys, it's been a while sence I have posted on the forum but as I saw this discussion on cleaning up our kitfoxes for more speed I wanted to post what I did to get more performance from my model 7SS with 912uls.
I have to admit I took the easy way out but much more expensive way as I changed props from a ground adjustable to a 3blade electric constant speed by
Full Torque propulsion. Now I am getting full rated power on take off "5600 rpm" With much higher climb angle at 55kt and about 50' shorter take off. As far as cruise speed I am seeing 10kt,s better at 5200 rpm and 28"mp, at 6000' and 5150 rpm and 24.2 mp,"wot" I am indicating 103kt with a true air speed of114kts as computed by the garmin g3x. Also I am running 850x6 tires so there is a drag penalty there. I got about 20 hrs on it and am really happy with it, I will try to post a video of the g3x in testing at 6000'

[av8rps]

http://www.sportclass.com/jeff-lavelle-repeats-at-reno-2013/

Stubby and fast is nice, but I'd prefer the 400+mph Glasair 3 that races at Reno regularly. Tom Hamilton (Glasair 3 designer) told me that guy is making his Glasair 3 faster every year, so he reminded him that the G3 has only been tested to 523 mph for flutter, so be careful in shallow dives :eek:



Short, Stubby and really, really FAST...

12860

[kmach]

Hi guys, it's been a while sence I have posted on the forum but as I saw this discussion on cleaning up our kitfoxes for more speed I wanted to post what I did to get more performance from my model 7SS with 912uls.
I have to admit I took the easy way out but much more expensive way as I changed props from a ground adjustable to a 3blade electric constant speed by
Full Torque propulsion. Now I am getting full rated power on take off "5600 rpm" With much higher climb angle at 55kt and about 50' shorter take off. As far as cruise speed I am seeing 10kt,s better at 5200 rpm and 28"mp, at 6000' and 5150 rpm and 24.2 mp,"wot" I am indicating 103kt with a true air speed of114kts as computed by the garmin g3x. Also I am running 850x6 tires so there is a drag penalty there. I got about 20 hrs on it and am really happy with it, I will try to post a video of the g3x in testing at 6000'

I tried to google search this propeller , no luck , do you have a website or more info.