Design => Stunt design => Topic started by: Matt Spencer on August 11, 2017, 09:43:55 AM

Title: Design Information . Fancher .
Post by: Matt Spencer on August 11, 2017, 09:43:55 AM




Shamelessly purloined from the A.M.A. .


Title: Re: Design Information . Fancher .
Post by: Matt Spencer on August 11, 2017, 10:51:19 AM
Just transfering the info here . Hopefully Rootbeard will stuff it up the top , for foture referance .


Title: Re: Design Information . Fancher .
Post by: Peter Nevai on August 11, 2017, 04:38:09 PM
Great Stuff
Title: Re: Design Information . Fancher .
Post by: Matt Spencer on September 02, 2017, 05:45:29 AM
YOU want more GRUEL %^@

let me give you a quick way to get the CG almost spot on.

First figure out the wing area. I'm guessing it's about 700 square inches or so ... but measure it and find out. Simple measure the span tip to tip, find a spot halfway out one wing from the fuse centerline and measure the chord at that point (yes, including the flap). It'll probably be around 11 or 11.5 inches or so. Multiply the span by that "average chord" and you'll get the wing area close enough for Gov'm'nt work or fixing the proper location of the CG.

Do the same thing with the stab/elevator. I'm guessing the area will be in the vicinity of 175 square inches or so.

Divide the wing area into the tail area and you'll come up with a number right around 25% or so ... smaller if the tail is smaller and larger if it's larger. It will almost certainly be somewhere between 20 and 26 or 7 percent.

Whatever that number is, go back to that average chord you located and multiply the chord length by the tail percentage of the wing. Let's continue with our "supposed numbers" and divide the 175 square inch tail by the 700 square inch wing. Son of a gun, that's exactly 25%.

Now multiply the chord length (we're using that 11 inch number I made up) by .25. 25% of 11" = 2.75".

At that average chord location measure back 2.75" on each wing and make a removable mark on the bottom of each wing. Adjust the nose and tail weight until the ship balances at that 25% of the average chord. I guarantee the CG will be perfectly safe for first flights and will most likely end up within a fraction of an inch or so after you've completely flight trimmed the airplane.

The last important part of that "bench trimming process" is to properly locate the leadouts in relation to that computed CG.

Pick the airplane up by the wingtips with your index fingers as close as you can to the same fore and aft location of each wingtip. The ship will, of course, balance in the same fore and aft "plane" as it did when you set the CG at the Average chord location. Make another mark on the inboard wingtip exactly where that point is on the tip. Measure 1.25" aft of that point and make that point the middle point between the up and down lines where they exit the tip. Keep the lines close together for best flight trim.

Make sure you've got an appropriate amount of wingtip weight based on how much longer the inboard wing is than the outboard. Make sure the wings and tail and rudder are straight and the hingelines sealed and go fly.

Don't make CG changes initially based on how quickly the ship turns. Your best clue to the proper CG is how the ship glides after the engine quits. If it's correct approaches and landings should be reasonably controllable and gliding a lap or so should only require a modest amount of whipping.

If the airplane slows quickly and/or balloons up into a headwind, you need to move the CG forward. If you can easily whip the airplane more or less as far as you want, the CG should probably be moved back a bit to optimize maneuvering performance. Most likely, the glide will be just about as described in the first instance, controllable with no tendency to balloon into a headwind and 'whippable" with some effort on the part of the pilot.

If the plane is too responsive or not responsive enough with the CG set to provide the proper glide, use your adjustable handle spacing to make you happy with the rate of response.

You will almost certainly NOT have to add or subtract large amounts of weight based on your bench set CG. It will be "that" close. There are reasons for making very small changes but they would be determined only after significant numbers of patterns have been flown and would be the result of observed tendencies to do things less than perfectly on a repeating basis.

A subject for another time.

L .