Interesting comparison of descent rates in Avid verses Kitfox

[1avidflyer 1983]

Hi all,  I was looking at some of my videos of flying the Avid MK IV and my Kitfox 4, and I knew that I showed a much bigger descent rate in the Kitfox 4 (long wing) than in the Avid MK IV.  Kitfox weight is about 650, Avid is about 610 empty.  Stall speeds are about the same in both planes.   Kind of figured maybe the verticle speed indicators were not very accurate as I was seeing nearly double amount of descent in the Kitfox.  That was on base leg, at about 60 mph in each plane.  Well when I looked at videos, and timed the descents, I showed 200' descent in 30 seconds in the Avid  (400' per minute) , and 400' descent in the Kitfox in 37 seconds  (almost 650' per minute.  I would have expected the Kitfox would have had a better glide than the Avid, but it seems not.  JImChuk

[TJay 1041]

I noticed that with My new wing too. Sink rate is faster but floats more in ground effect. That could be because I come in faster with the new wing because it sinks faster. Another thing I noticed is the CG range on the new wing is way bigger. With my old wing 25lbs max in my rear baggage compartment would make it fly nose high and be all squirrely in the sky the new wing you hardly notice the extra weight. These small airplanes will talk to you .

[dholly 718]

My experience as well re: sink rate. And the difference between my KF3 vs. KF4-1200 was, not surprisingly, just as pronounced due to the Avid and early KF airfoil similarity. The 1st flight log entry for my KF4 actually says “sink rate at less than 60mph is dangerous”. But it sure will float a looong way, mostly when you don’t want it to lol. 

[1avidflyer 1983]

Hi Doug, good to see you on the forum!  JImChuk

[nlappos 413]

I suspect that engine idle and prop pitch angle are big drivers here. If you have the nerve, can I suggest you kill the engine and stop the prop and do the descents again?

[1avidflyer 1983]

I have a 69 1/2" two blade NR on the Kitfox, and a 64" two blade warp drive on the Avid.  Engine idle rpm is close to the same in both at that point, but of course the Kitfox had the  gearbox, and the Jabiru in the Avid is direct drive.  Maybe if it was winter and I had a large lake under me and I was on skis, I would shut off the engine and see what the difference was.  With trees everywhere around me I'm not sure I'm up to shutting down on purpose.  The one engine out landing I had in the Avid, it seemed to glide just fine at 60, but I can't say what the rate of descent was.  JImChuk

[nlappos 413]

Jim, it is careful thinking like yours that lets you earn your gray hairs!