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Old Sep 20, 2014, 01:09 AM
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[QUOTE=BWX_;29477855]Y

Do you put the nail in the right wingtip or both? or?.. Never heard of that one. I notice that if I try to hang the Radian on it's prop, the left wing always tried to fall first. It might be a trim issue though, or even a balance issue, I haven't investigated it yet.

/QUOTE]

What I do is hang the Radian from a heavy thread that is hooked to the basement ceiling with the free end tied into a noose. I wrap the noose around the motor shaft and then balance the tail of the Radian on a hand held pencil eraser under the blade of the skid that is propping up the tail. I then tape (small piece of blue painters tape) a 6 penny nail to the top of the wing along the CG line as far out as possible and retest the balance. If necessary, I adjust the location of the nail, outward and back along the wing, until I find the optimum location.

My most recent build required a 6p and two 4p finishing nails to get it right. After determining that the 6p nail alone would not do the job, I "injected" it into the bottom fleshy part of the wing just as a doctor would when giving a patient a shot as far out as I reasonably could. I then retested with a 4p nail which I ended up inserting just forward of the 6p nail, also along the CG line. After that I found that I needed another 4 p nail inboard of the others to get it right.

Getting a perfect balance around the longitudinal axis is just as important as getting the right CG. While you could correct for imbalance using the rudder, you can only do that for ONE airspeed.

Which brings me to another point that is often misunderstood (This may get a little blurry. It is late at night and I've had a couple of glasses of wine): Mixes that are in the same domain, ie. compensating for airfoil differences as elevator and flap mixes do, vs mixes that cross domains as power and airfoil or weight and airfoil mixes do. Huh?

If you create a flap/elevator mix, it will work at any airspeed because the two airfoils are affected by airspeed in a directly proportional way (well very closely anyway) and balance each other regardless of airspeed.

A throttle/elevator or throttle/rudder mix, on the other hand, can only work at one airspeed because you are mixing apples and oranges. The flight attitude (airspeed) changes the forces exerted by the control surfaces but does not have any affect on the power component. So if you setup your mix to work for climb attitude, it will not work well in a level flight, higher airspeed, attitude. I've tried power mixes with varying success and finally abandoned them in favor of dual/triple rates and expo. I have one rate for power and another for thermalling. The pilot does the rest. It is much easier and you are never fooled.

Now to my point, this idea also applies when trying to compensate for weight imbalances with trim. You will never be able to get an airplane to fly straight at all air- speeds with one trim setting UNLESS you first balance around the longitudinal (fuselage) axis. Because of the long moment arm (wingspan) you don't need much weight, a couple of grams will do nicely, hence the finishing nails. It is real easy, guys and makes a world of difference. It only takes about 5 minutes, and unless you know where to look, you won't even see the nails.

Good night
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Last edited by cliffkot; Sep 20, 2014 at 02:23 PM.
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Old Sep 20, 2014, 03:52 AM
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United States, NY, Big Flats
Joined Nov 2013
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Yeah I felt that in a big way when on a whim I stuck the Mobius on my wing sideways looking at fuse (pretty far out too) without a counterweight for one flight.
-on that spot-



I knew I would need to trim rudder a huge amount, which I did, but that didn't fix the problem completely by any means. When climbing the plane acted different than going straight and in turn different when turning left than right and also different when diving.. and also different at different speeds. Of course the Mobius crated drag on one side too.. It was really bad.



I'll have to edit and upload that vid.. it was a nasty gusty cloudy day too.

So yeah on a much smaller scale I can see where an imbalance from side to side could have bad affects. My Radian flies great, I can't complain, but I will balance it from side to side before my next flight and I am sure that will only make it fly better.
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Old Sep 20, 2014, 01:02 PM
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USA, CA, Pismo Beach
Joined Oct 2007
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Very good points and recommendations cliffkot. I would like to suggest one correction in your description though; you are doing a lateral balance along the longitudinal axis, not a longitudinal balance. The longitudinal balance is what we do when setting the CG along a lateral axis.
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Old Sep 20, 2014, 02:20 PM
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Quote:
Originally Posted by Greywing View Post
Very good points and recommendations cliffkot. I would like to suggest one correction in your description though; you are doing a lateral balance along the longitudinal axis, not a longitudinal balance. The longitudinal balance is what we do when setting the CG along a lateral axis.
Yeah well you know what a little wine will do....

(fixed)
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Old Sep 20, 2014, 02:31 PM
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I know, I know. My favorite anesthetic.
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