Kitfox abrupt departure sensitivity to low speed stalls

[jamesbdunn]

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...-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.


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]

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]

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]

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.
Eddie

[Dave S]

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]

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]

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]

I'll be doing 1320#.
Eddie

[rv9ralph]

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/.../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

[jamesbdunn]

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/.../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/sho...3&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.