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Old Nov 12, 2012, 09:14 AM
JRuggiero is offline
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pardus,

Am I seeing some tuck-down of the leading edge of the left wing during the dive tests? Or am I being confused by aileron deflection? If the leading edge near the tip is, indeed, twisting down, that would account for some (all?) of the acceleration and increasing dive angle.

Jim R.
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Old Nov 12, 2012, 09:22 AM
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Graham Dyer
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Originally Posted by JRuggiero View Post
pardus,

Am I seeing some tuck-down of the leading edge of the left wing during the dive tests? Or am I being confused by aileron deflection? If the leading edge near the tip is, indeed, twisting down, that would account for some (all?) of the acceleration and increasing dive angle.

Jim R.
Sharp eye Jim, it is a bit of the iron on covering on the leading edge that's come loose, next flight (without the cam) it was taped up. In this videod flight and the next the tuck-under problem seems to be solved.
Old Nov 12, 2012, 08:21 PM
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I fly several twin boom pushers, swapping the wing-boom-tail among different fuselages.
For this one, I had to add 3/8" shims to the trailing edge attachment of the booms to prevent a violent pitch-up on launch. In essence moving the nose of the horizontal -up-.
Don't recall if I dive tested it, I've had a few problems with doing that, when the download on the horizontal bends the booms to add down to the horizontal, which overspeeds the plane into flutter.
Old Nov 13, 2012, 09:33 AM
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Sparky Paul,

The booms on your aircraft are very small diameter. Have you built or rebuilt any airplanes with larger diameter booms that are less likely to deflect at high speeds? Naive question...!

Jim R.
Old Nov 13, 2012, 10:20 AM
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I've used aluminum arrow shafts, and 3/8" CF tubes, going for heavier walls on the newer planes.
The arrows work well.. Can't put too much weight aft with a pusher configuration. That requires a longer nose to balance the motor and the tail end.
Old Nov 13, 2012, 11:17 AM
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I've used aluminum arrow shafts, and 3/8" CF tubes, going for heavier walls on the newer planes.
The arrows work well.. Can't put too much weight aft with a pusher configuration. That requires a longer nose to balance the motor and the tail end.
They can be a problem-- some fishing poles have tapered sections -
I used a 1/2" carbon fiber boom- very rigid -for my camera plane -
My friend Ed Skorpea made up a tapered mandrel recently, to do some nice booms .
His work is absolutely the best you will see. booms carbon fiber pipes etc..
Old Nov 13, 2012, 01:58 PM
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Arrow shafts are probably stressed to sustain compression rather than bending loads. Fishing poles will be engineered to resist bending loads, but are naturally built to be flexible. Probably nothing beats a purpose wound boom.
Old Nov 13, 2012, 03:15 PM
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Originally Posted by pardus View Post
Thank you for the suggestions.

Put a camera on the horizontal stab over the weekend, no noticable deflection.

Upthrust on the motor has seemed to cure most of the ills, as now need much less down trim to maintain gentle climb with power which has eliminated the increasing dive in the dive test.
If changing the motor thrust line affected the power-off dive test results, it must have been out of trim in glide before the dive test. The dive test is only meaningful at all if the airplane is in trim near best L/D speed for a hands-off glide.

A better test for CG position is inverted flight. The amount of down elevator required inverted for level flight at the same speed as upright is a good indication of the stability margin - less down elevator, lower stability margin.

Thrust issues are easy to confuse with pitch trim and stability. The pattern guys have systematic trimming routines in a particular order to sort out which is which.

http://www.australianpatternassociat...tholdsmith.pdf
Last edited by kcaldwel; Nov 13, 2012 at 03:28 PM.
Old Nov 13, 2012, 05:05 PM
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Thinking about a twin boom pusher. Would like to scratch build from EPP. Thinking aileron and elevator control, with a 2S lipo, Blue Wonder-type motor, 8 x 6 prop.

Never build a twin boom style plane before.

Suggested plans?
Old Nov 13, 2012, 05:26 PM
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I use wings from normal airplanes, with the tail area and position the same, just modified for the vee and keep the c.g. the same.
.
http://www.charlesriverrc.org/articl...tailsizing.htm
.
Old Nov 14, 2012, 02:51 PM
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Having almost lost a new pod n' boom style model to flex in the tail boom I'm not as much a fan of small section single or twin booms anymore. Or if used I like the idea of doubling up on the tubes used to produce a vertically oriented figure "8" form with a little bit of filler in the side grooves.

Or obviously if a larger diameter tapered format boom is used that works nicely as well. But I ain't risking any more airtime on small diameter booms that can flex. One near crash was enough.

In my case it was a T tail electric that used a carbon tube boom. I was lucky that I'd gotten the model well up in a thermal. At that point I decided to speed up the descent and play around. Once the speed built past a point the drag on the high mounted stab bent the boom and I had NO up response to allow a recovery. Since "up" didn't work I pushed full down which did work. I then rudder rolled (polyhedral wing) back to upright and gingerly landed.

The next outing for that model was the following week at a big electric flyin. As a bandaid solution I tied a length of control like steel cable from the wing trailing edge to the top leading edge of the fin. Yes, it was a suspension bridge solution That got me through the meet. Shortly after that I bulked up the boom with some epoxy and carbon tow molded around the original boom to stiffen things up. It was then about as stiff as a carbon golf club shaft and gave no further issues.
Old Nov 14, 2012, 04:23 PM
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Interesting fix, Matthews.

Anyone have any suggestions / plans for good, rugged, simple twin boom planes for me to try?
Old Nov 15, 2012, 11:15 AM
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Steve, any are fine. Just don't opt for too skinny a tube for the booms. Or go with modifying it with that figure 8 stacked double tubes to get the vertical stiffness you want and need to ensure alow flexing and maintaining elevator consistency over a wide speed range.

If I were making a twin boom and being a trifle nervous over that last fiasco I'd suggest two pieces of .375 OD pultruded tube for models up to around 40 to 45 inch span. For larger ones up to 6 foot span I'd go with .5 or bigger. For anything over 6 foot I'd go with two stacked tubes of .5 or larger or just give up and make the booms up.

For someone that insists on using arrow shafts or other pushrod size options I'd say go with a glued together stacked figure 8 setup for the booms.

And of course all this doesn't help much if the wing center section out to the booms is also too flexy. Designs which try to get away with using flexible foam such as Depron or fan fold floor underlay need to cover the center area of the wing with something that stiffens up the foam at least out to the booms. There's lots of options but an easy and cheap one is to use one of the water based varnishes and "dope" on a couple of layers of art store tissue paper or newsprint. Tissue for the smaller models and newsprint for the bigger ones. Or in the case of built up balsa wings either additional load bearing spars in the wing or sheeting the center area out to the booms again.

All this may sound like overkill. But I wonder how many folks with flexy tail booms have had flexure based elevator response issues and simply don't realize that the model is reacting oddly and inconsistently due to the flex of the booms as speed changes bend the booms. We go to a lot of trouble to build our models accurately and place a good degree of importance on setting the wing to stabilizer angles to within a half degree to get the models flying correctly. What good is all that if we build the model with booms that allow the tail to flex relative to the wing over a two or three degree arc from only a few ounces of force?


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