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        Discussion Cheranovsky highly tapered planks

#1 g_munu Aug 31, 2012 12:02 PM

Cheranovsky highly tapered planks
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The BICh-21 racer and its predecessor the BICh-20 "Pioneer" were tailless aircraft built in the USSR just prior to the second world war. They're very stylish little planes (especially for Soviet ones) so naturally I thought about building a flying model.

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But look at the wing taper. Tip stalls could be a real problem. So I sketched a 1 meter wingspan BICh-21 in Ranis to see what could be done.

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Took about 12 degrees of twist at the wingtips to get a sin cubed gamma distribution. Stability margin is decent at 10%. The only way to ensure proverse yaw is by using only the most outboard part of the trailing edge as roll input. You're going to have adverse yaw if you use full-span elevons for roll.

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So what happens as it approaches stall? Looks like it should stall in the center of the wing. Even if you try to stall the tips first by holding the outer "aileron" surfaces neutral, maximum cl occurs inboard. So that's good, at least. This does not appear to be a candidate for crazy 3D-style control surface throws.

So what do y'all think? Don't worry, I'm still noodling around with an electric Manatee too. I build way more airplanes on paper than I fly. Hey, it's still a hobby, right?

#2 g_munu Aug 31, 2012 04:16 PM

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Chuck glider is a success!

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Directional stability is fine. At this scale, it doesn't want to tip stall that badly, although it exhibits "pecking" if trimmed to fly too slowly.

#3 Quick61 Aug 31, 2012 09:58 PM

So the next steep is going to be built up, bagged or molded? :) That was a quick turnaround from software to tossing across the yard. Should be a great looking ship.

:popcorn: Mark

#4 g_munu Sep 01, 2012 11:30 PM

Thanks to the vertical tail, I dont think adverse yaw as as much of an issue. I flung the heck out of that cardboard glider and it behaved alright with less twist. Hopefully the pecking will go away with a real airfoil (and maybe an upper surface trip).

The original had composite wooden wings sheeted with plywood/phenolic "shpon". May as well take the same approach for the model.

#5 g_munu Sep 04, 2012 10:01 PM

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Forgot this was meant to be a 1 meter wingspan. Plywood... no thanks. Besides, compound curves are so much easier with stick and tissue. I think it'd look swell with aluminized dope over silk. More comments are in the attached pictures.

#6 g_munu Sep 05, 2012 04:21 PM

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Today's efforts are attached. Elevons will be hinged near the bottom surface.

#7 Jon Snow Sep 05, 2012 04:36 PM

Are you looking to fit retracts?I hope you build this one,interesting project.

#8 g_munu Sep 06, 2012 11:35 AM

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No plans for retracts, Stu. That's a lot of weight for a small plane that's already hauling around a pretty big fuselage. Besides, with wheel pants this plane ought to hold a knife edge pretty well. Bet your Horti can't do that ;)

#9 g_munu Sep 14, 2012 03:28 PM

project is still alive
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Just wanted to let you fine tailless flyers know that the Cheranovsky Самолет Гоночный 1 is still coming along.

I threw out the original wing. It was too thin for the speeds I care to fly at and needed to be re done. So I started over and arrived at something worth posting pictures of once again. Fewer ribs this time around, too. The airfoil's a bit thicker and has slightly more positive Cm.

#10 g_munu Sep 14, 2012 03:39 PM

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The revised wing spar as viewed head-on. No more kinky stress risers. Spar caps can now be steam bent :D

#11 g_munu Sep 15, 2012 11:23 PM

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At least nine million years ago I once built a rubber powered Guillows Spitfire. The memory vaults are rusty but some of the concepts are coming back to me. Lots of 1/16 inch square balsa stringers running between formers.

Will likely end up laminating and blocking wood for a firewall and motor mount. As a prototype, I'd like plenty of room to shim the motor mount to adjust up-thrust or down-thrust.

#12 Jon Snow Sep 16, 2012 03:11 AM

:popcorn::popcorn:Jens will love this:rolleyes:

#13 kleinaberfein Sep 16, 2012 03:19 AM

It is an unusual bird to be sure but the construction technique warms my heart to no end as an old free flighter.


#14 g_munu Sep 19, 2012 11:32 PM

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Attachment 5169694

Well, it's coming along. Re-did the fuselage formers with a hopefully more scale-like hexagonal cross section, which also will permit sheeting if desired along part of the fuselage.

#15 Jon Snow Sep 20, 2012 03:13 AM

Keeps the pics coming

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