Kitfox abrupt departure sensitivity to low speed stalls

[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.