What is the formula for calculating the size of a servo needed to controll the elevons on a flying wing? I am building a 96 inch draco out of all balsa. I have a 96 inch draco built with coroplast and i used two servos on each elevon because the coroplast likes to bend at such long lengths, the elevons run the full length of the wing.I would like to only use one servo on each half on the wood verson. Any help would be great.I can post pics of the coro version if anyone is interested. It flys great.

I can't answer your question about force, but I would have 2 control points per flap. Break the span down into thirds and station your control horns like so. I think this would be the lightest and most trustworthy option, instead of building a massively rigid flap controlled at a single point. If this plane is to fly with any speed at all, you want to inhibit flutter.

There is an excel spreadsheet that has formulas for calculating servo forces available to download for free.  I can't find a link to it right now so you might do some searching on google but here is a website that does pretty much the same thing.

http://www.csd.net/~cgadd/eflight/calcs_servo.htm

Take a guess and watch roll rate at speeds for blow back.

Thanks. I did use two servos per elevon on my coroplast version of this plane and I think that I should do the same on this one.

Check for the on line  calculators in the "<span class="oldpost"><font face="Verdana">Aerodynamic tools, calculators and test links </font></span>  " sticky thread.  There's a couple of calculators there for you.

These will provide the load on the servo.  But you need to figure on getting a servo that will provide 50 to 100% more torque than what the calculator says.   You do not want to be operating too close to the stall torque on any servo and the max torque given in the specs is always the stall torque.  Sizing the servo to keep the max surface load to 50 to 60% of the stall torque will ensure you keep the control surface response speeds up.  If you're using two servos per aileron to help avoid flutter issues then the total torque should add to the same spec.

I found the Excel file I referred to above that has the servo calc functions.  Try this link <font color="#00265e">http://www.geistware.com/rcmodeling/...calculator.xls

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This site may be useful:

http://www.fayettevillercclub.com/id26.html

I like to use about 70 percent of top speed when calculation required servo torque.  High aileron and elevator travel is very useful when fighting wind gust during landing approaches, but the same travel may produce excessive roll and pitch rates at top speed.  Undersizing a servo results in momentary servo stall at less than full control surface deflection, which may prevent wing breakage upon panic application of full elevator in attempted recovery from a steep dive.

Over in the 3D section there is a thread on the Aeroworks Extra 260 .60 size and some are saying theirs drops a wing when they snap it into a wall or in some other way snap full up into it.  It makes me wonder if they aren't experiencing some form of aileron servo blow back as my Extra does not do it at all.  As Mr. Matthews mentions above it is wise to go with more servo than recommended and mine are sized accordingly.  The manual recommends 80oz/in and mine are 144.  Just a guess but that may be their answer.

Just an idea, but I would use a single high-torque servo, per wing, with a control "wire" that would span to the 1/3rd points of the flaperons... giving you dual-position control with only a single position force. You are familiar with the means most planes use to control the elevator?... a "wire", bent into the control surface on each end, with a "control horn loop" at the center. (or sightly offset to the side of the fuse.) This same type of design worked for me, using a single servo to move 30" flaperons. I can provide a detail illustration if requested.