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av8rps
Joined: 05 Jan 2008
Posts: 33
Location: Central Wisconsin
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 Posted: Mon Jan 26, 2009 8:47 pm Post subject: Re: 26 foot Clipped Wing Kitfox |
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Ok gang, if you haven't already seen what Michel has done using X-plane software to simulate clipping the wing down to 26 ft, here is a copy of his results of clipping the wing on his model 3 (this was posted elsewhere on the forum as well);
[Quote:
From: av8rps [paul676(at)tds.net]
I know nothing about X-plane, but what we are trying to find out here is how a Kitfox
IV will perform if the standard Ribblet 32 ft wing is cut down to a 26 ft span.]
Michel wrote:
Hello Paul,
First, a word of warning: although this is an excellent simulator as it divides everything to a blade element with own coeficient of lift, drag and moment ... it is only a simulator with its limitations.
For example, to simulate the flaperons, I had to cheat. Why? Well, a flaperon stall much later than the wing because the air passing under the wing is pressed on the top of the flaperon, keeping it laminar.
If a simulator was to do exactly the same thing, the interaction between different surfaces should be calculated using Computerized Fluid Dynamic (CFD) that divides the entire volume of space into independant parcels. That can't be done in real time.
X-Plane takes into account simple things like the prop wash effect on the control surfaces. But not much more. So, to make my flaperons, I had to manually change the lift coefficient and stall angle that exceeds then 20 degrees AOA.
Anyway, I made a copy of my Kitfox model 3 with Jabiru 2200 and shortened the wings to 26 feet span. The result was interesting!
I can't really measure the take-off distance. To do that, I should mark the ground with lines and use replay to see when it lifted. What I did is:
Stall speed at idle: normal: 34 mph. Short wing: 39 mph
Max speed at WOT: normal: 110 mph. Short wing: 115 mph.
Climb rate at 60 mph: normal: 1,000 ft/m. Short wing: ... also 1,000 ft/m!
Please, take those figures not as absolute but relative values.
The general feeling is this: performances weren't very much affected but the feeling of flying was quite different! First, I was surprised to see how much roll power I had! More than with the long wings. (Yes, the flaperons were also shortened). On second thoughts, it may make sense: shorter flaperons but also shorter wings that present drag to roll motion.
The general feeling of flying was more flimsy with the short wings. For example, with the normal wings, a stall induces a soft mush-down; something I also experience in real life with my plane. But the short wing model stall harder. Uncoordinate stall went much faster in a wing drop and possible spin with the short wings. It felt better to fly the unmodified plane but perhaps that's because I am used to it.
I hope it helps.
Cheers,
Michel Verheughe
Norway
Kitfox 3 - Jabiru 2200 ... flying as PAX
Michel,
****My reply****
Thanks so much for doing this test on your X-plane software. The results were pretty much what I had expected based on what I had read, with the exception of the flimsy handling characteristics. I would have thought the higher wing loading would actually make it feel stabler. But as you said, it's not necessarily an exact science, so once again the real test is just that...
I do wonder how the newer airfoil used on the Model IV along with the wider symetrical-airfoiled flaperons might change the numbers, as there is a significant difference between a real Model 3 and a Model 4 for top speed. I learned that the first time I put the nose down on my Model IV with the long wing...it builds up speed fast. Much faster than does the older higher lift airfoil used on the Avids and the Kitfox 1-3 models, which feel like they hit a wall once you exceed 100 mph. At least that's been my experience.
I'm curious, does X-plane accurately simulate your jabiru powered Model 3 for performance specs? It sure sounds like it does a good job with handling and control feel. I'm pretty impressed that you could actually sense and feel the increased stall characteristics with the shorter wing. That's pretty cool!
Thanks again Michel.
Paul
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_________________
Model IV-1200 912ul Amphib
Avid Flyer Sn#1 and Sn#26
Highlander 912s taildragger
Lake Amphibian
Central Wisconsin
paul676@tds.net |
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