I built a DLG style glider with a v-tail. This is not a normal DLG but an S8 rocket glider for FAI SM competition. The issue it has is that the tail flutters, not up and down or side to side but clockwise/anti-clockwise. Hence the questions; what causes this, what should be done to avoid or minimize it?


Richard

add a sub-fin to create a Y configuration to balance and damp all vertical, lateral and twisting forces

is it just harmonic thing where boom flex is causing this flutter ? what is the swist resistance of your tail boom ?
it has me curious.

My guess is that any slight imperfection in any of the flying surfaces, or tailboom (if not spirally wound, for ex.) and/or thrustline can start a V which is still like a (_) to start rocking in an arc as part of the circle O into which the V can be projected

I would love to see a video or a more detailed description.

Maybe the shaft you used simply needs to be larger so it's stiffer. And in this case I'd guess that if it WERE spirally wound as well as having some good longitudinal fibers that it might be stiffer in torsion and not get into that flutter issue.

Is there any play in the control linkages? If the movable surfaces can shift a little that would be enough that if they start oscillating within the "softness" of the linkage that they would twist the tail and thus the boom in sympathy.

And finally are the fixed surfaces stiff enough by themselves that they resist twisting and flutter and end up twisting the tail boom like you're finding.

Stuff like this is seldom any one factor.

The boom is a pultruded arrow shaft which has been adequate with earlier iterations save that they were all cross tails. It did not seem to be weak when I selected it but was easy to twist using the v tail as a lever. I wanted to make the tail as light as possible and thought the v arrangement would be better in that regard.

The maiden flight had an over powered motor in it which produced over 5 pounds of thrust for 1 second. Enough to do about 300' and the flutter began at about 60'. One fin [epoxy& glass laminated] broke at the root, the other remained attached.

For the second flight the balsa fin was reattached with a 4" brace between the two to make a small triangle and the control surfaces themselves were reduced in cord 20%. The motor this time was a proper one as used in competition and made about 1.3# of thrust for 7 seconds adequate for a 10 0z plane. It did about 800' and all looked good till the very top where I could see the tail fluttering. Its probably doing 150 MPH or better during the accent. Fortunately nothing broke but I would like to do what ever can be done to remedy this problem.

I'm building a second one with a much stiffer boom and would like to use the v-tail configuration again. You all have given good points to consider and any further help or thoughts on this is appreciated.


Richard

P.s. Its pull-spring like many DLGs so no play and the surfaces were stiff enough not to be easily bent by hand.

Flutter is as much about structural stiffness and unbalanced masses as it is about aerodynamics. The Y-tail suggested above will not only result in more balanced aerodynamic moments, but will bring the center of mass of the tail closer to the axis of the tail boom.

Minimizing the mass at the tips of your tail surfaces will also reduce the tendency to flutter.

You can increase the torsional stiffness of the tail boom by wrapping fibers around it at +-45 deg or by increasing the diameter of the boom. Given the small diameter of the arrow shaft, a modest increase in diameter can result in a huge increase in torsional stiffness. This article is about rolling tubes for wing joiners, but it would also apply to rolling a thin walled tail boom: http://www.charlesriverrc.org/articl...s/kev_tube.pdf.

If you roll your own, you could make the boom tapered, which would minimize the weight at the tail. The arrow shaft is way heavier than it needs to be at the very location where weight is most critical.

even the cheap 0-90 rolled fabric tubes from ebay are far stiffer in torsion than pultruded tubes. usually lighter as well as they can get away with having thinner walls.

re v-tail config; anyone who has ever shot an arrow with one of its 3 feathers gone will notice how the back end wobbles and rotates around in a circle

Arrows wobble and rotate anyway, but yes an arrow with a flight removed will tend to be unbalanced aerodynamically and rotationally, but this is due to the way the flights are setup. the aircraft with a v-tail will not have a desired rotational lift(Force) pattern around the shaft. both tail feathers will be setup so any aerodynamic force under normal pitch change will be equal and in the same pitch direction (Not including tail Tilt) not in the same direction around a shaft like an arrow.

WORLD’S MOST AMAZING ARCHER in Slow Motion - Smarter Every Day 130 (6 min 44 sec)

watch around the 2:30 mark onward.

The Archer's Paradox in SLOW MOTION - Smarter Every Day 136 (8 min 51 sec)

go to the 4:50 mark in that one to see how the spine determines the wobble factor of an arrow on a traditional bow and to some lesser degree a compound bow of modern setup.

As you said, it was enough to get by on earlier versions. But pretty obviously the V tail setup has different forces in action. And now the tube that worked before isn't good enough.

Consider too that arrow shafts come in a variety of diameters and stiffness ratings. I had not realized this before until I got into some archery myself about a year and a half ago. The difference from the softer to stiffer shafts is a big one. If you didn't know about this and bought the wrong grade and size it may well be the source of this problem.

Originally Posted by tspeer

Flutter is as much about structural stiffness and unbalanced masses as it is about aerodynamics. The Y-tail suggested above will not only result in more balanced aerodynamic moments, but will bring the center of mass of the tail closer to the axis of the tail boom.

Minimizing the mass at the tips of your tail surfaces will also reduce the tendency to flutter.

You can increase the torsional stiffness of the tail boom by wrapping fibers around it at +-45 deg or by increasing the diameter of the boom. Given the small diameter of the arrow shaft, a modest increase in diameter can result in a huge increase in torsional stiffness. This article is about rolling tubes for wing joiners, but it would also apply to rolling a thin walled tail boom: http://www.charlesriverrc.org/articl...s/kev_tube.pdf.
.

This is the info I needed to look at. BTW the Y configuration is not workable because the rocket motor will roast anything in its exhaust path. I see now there was too much mass and too much area at the tips. Previous V tails built did not have to deal with this kind of speed so I have learned something.

Thanks for the arrow info too, I've always thought the wobble was because it was a push as in pushing a rope where this glider is in tractor configuration.I shall put on a wrap of Carboweave to help it that way. Generally the least expensive tubes were what was sought but with this info on the grades of shafts that will change. A SkyShark Nitro Pro UL will be fitted to the next iteration.

Thank you gentlemen all.


Richard

Speaking of arrows and stiffness I just recalled seeing some "Fat" or "Phat" model shafts in the sporting store a while back. Thinner wall and larger than typical diameter but still very stiff as any arrow needs to be. The larger OD would inherently be stiffer in torsion as well so it might aid you on your boost glider issues.

PS; From talking to the archery guy you would be far from the first one asking about arrow shafts for things other than archery. So by all means tell the archery counter guy that you're after qualities that are not directly archery related and they might well have some further suggestions for you to ponder. The ones that know their stuff are often interested in such other uses.

Thanks for the info. I have been bequeathed the Nitro Pro which may be the solution given its a tapered shaft with the big end 9+ mm certainly bigger than any of the arrow shafts I have now. It will have a Carboweave wrap added at the front as it appears to be unicarbon. Although these planes are usually less than DLG size the required dork landings have often shattered the shaft.

Ok maybe its my flying.


Richard