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Author Topic: Biplane design  (Read 1973 times)

Jim Roselle

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Biplane design
« on: July 21, 2014, 10:15:55 AM »
Hi all,
 
I recently purchased two sig akrobats and I'm kicking around the idea of combining them into a biplane. I would have two 36"ish span wings with a combined area around 500sq".

My first question is in regards to engine choice. My 500sq" twister flys great with an LA .46, would that be ok on a biplane with the same wing area?

Secondly, horizontal stab/elevator. Does the 20-25% of wing area rule apply to biplanes?

Lastly, if I do flaps do I do both wings? Just top? Just bottom? Full span?

Thanks for your time and advice guys,

Jim
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Offline Tim Wescott

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Re: Biplane design
« Reply #1 on: July 21, 2014, 11:28:36 AM »
Some of this is badly filtered through aerodynamics textbooks.  Some of this is outright guessing.  Take it with your own personal grain of salt:

If the textbooks are right, it's going to have the induced drag of a plane with a 45" wing span or so, and probably less lift overall than a 500 square inch single wing.

The LA 46 is so renowned for versatility that yes, I think it'll work well.

A 25% tail set back the "right" amount for a 36" wing will be deficient in tail volume, I think.  A 25% tail set back as if the wing were 50" span would probably be better, if you can make it look good.

I'd do full span flaps on both wings.  I'd also extend the wingspan to 40" or so with spliced-on spar tips and a few extra ribs.

Lastly, figure that it's either an experiment or a fun plane, but not something that you should beat your brains out making super competitive.  If you do luck out and get something super, let us know...
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Offline Dennis Toth

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Re: Biplane design
« Reply #2 on: July 22, 2014, 10:01:39 AM »
Jim,
I did this with two Yak's (about 590 sq in), I didn't do flaps but did increase the tail to about 20% of the total wing area. I also increased the tail moment a few inches. I am using a K&B 40 turning an APC 11x5 at around 9500 rpm launch, gives 5.0 lap on 60' C to C. For the CG a simple way to get a starting point is to do the calc as if you are looking down so you see one wing outline that includes the offset (if you use this) then start at 15 -18% back at the average cord. It is lots of fun to fly once trimmed, be patient and work through the little issues that Bipes have and you will get a fun ship. Search bipes on this forum for other tips.

Best,       DennisT
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Offline Avaiojet

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Re: Biplane design
« Reply #3 on: July 23, 2014, 12:18:28 AM »
Dennis Toth's Yak Yak!
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Amazing how ignorance can get in the way of the learning process.
If you're Trolled, you know you're doing something right.  Alpha Mike Foxtrot. "No one has ever made a difference by being like everyone else."  Marcus Cordeiro, The "Mark of Excellence," you will not be forgotten. "No amount of evidence will ever persuade an idiot."- Mark Twain. I look at the Forum as a place to contribute and make friends, some view it as a Realm where they could be King.   Proverb 11.9  "With his mouth the Godless destroys his neighbor..."  "Perhaps the greatest challenge in modeling is to build a competitive control line stunter that looks like a real airplane." David McCellan, 1980.

Jim Roselle

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Re: Biplane design
« Reply #4 on: July 23, 2014, 05:30:12 AM »
Thanks for the input guys!

Jim
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