Torsional Stiffness of Wing

[Bill Harris]

One problem I have had with the GWS SlowStick wing when using it at higher speed with a stronger motor is that it tends to flutter chordwise. The wing will flap and twist about the span axis. I need to increase the torsional stiffness of the wing without adding weight.

As shown on the attached picture, I think I can do this by adding diagonal carbon fiber rods attached to the existing cross rods and the TE and LE CF rods. Seems to me that this would stiffen it without adding weight, although drag would increase.

What do we think? Any other ideas?

--Bill

[BMatthews]

....What do we think? Any other ideas?

Yes, move on to better models for higher power flying.... :D

Mod the SS if you really must but I can't help thinking that you're trying to turn a sow's ear into a silk purse. That wing is a drag bucket at the best of times thanks to the airfoil shape. The use of single surfaces arced soft foam also means that the darn thing is super flexy.

The carbon rods obviously add SOME weight but I would agree that it wouldn't be much. The outer portion is still subject to fluttering though but your mod may stiffen it enough to avoid it at the speeds available to this motor and prop. But try to push it faster and the flutter will probably come back. Or it may still be there even with the rod mod. It all depends on how fast you're trying to push it.

If you really must push the limits on this thing I would suggest a full bracing pattern of low strech 2 lb test Spectra fishing line and bracing towers in a pattern similar to the original Eindecker. The double pattern of lines will support the foam at the mid span point and out near the tips and should pretty much eliminate any flutter for a low weight cost. The lines will produce some drag but if drag is a problem then you shouldn't be trying to make an SS fly fast. The airfoil is going to prevent it in any case.

[Bill Harris]

Actually, I _am_ trying to make a purse out of a sow's ear. Although the SS "vacuformed" wing is pretty quirky, it does have good lift and low speed manners.

I should have explained more: by "stronger motor" I'm thinking something along the lines of a 400 brushed on 3s LiPo's and increasing the pitch speed to increase the top speed from 23-ish to 28-ish MPH. This will increase the acceleration and wind penetration, so I'm not looking for "high speed" as such but to widen the performance envelope somewhat.

I think we are onthe right track with bracing- either the Eindecker or the CF rod.

--Bill

[vintage1]

silly thought.

Iron some solite on the underneath?

[BMatthews]

silly thought.

Iron some solite on the underneath?

Where's that "I'm with Stupid^" smilie.... :D

A fellow up this way has a Slow Stick style and size of model but with a built up tissue or Solite covered wing using a basic Clark Y or close facsimile airfoil. But the point is that his flat bottomed proper airfoil version flies like a champ. It's faster, climbs quicker and basically out runs and outflies the Lite Stike with the same GWS IPS motor and yet slows down for landing to the same speed as the Lite Stik. A PROPER airfoil is full of advantages.

....oh, and the full covering provides all the torsional stifness needed so that no external bracing is required.

Play as you will and I'll even help and encourage but I feel it's my "duty" as your conscience and nag to point out the above... :D

The small carbon rods for the A frames above and below with either the Spectra line or medical suture thread should provide the low stretch bracing you need. I know that tieing knots in teh SPectra is darn near impossible to do and not have them slip but give it a try. The other option would be kevlar thread perhaps.

Actually even some doped or varnished button thread may work. It's lightly loaded enough and SOME stretch shouldn't hurt as long as it's all symetrical.

[roidspop]

Interesting thread.

I've been flying my Slow Stik with a Park 400 brushless, 4.4:1, 7x6 and a 3s lipo. No flutter, although I do have to be careful of overspeed in dives and abrupt pullouts. So far, the wings continue to fly in formation with the fuselage, but they do try to applaud occasionally.

The wing was fluttery and bendy on its maiden flight. I hot-glued the CF spars and wing-joint and then covered them with packing tape. I haven't observed any flutter since then.

I have attempted to clone a Mountain Models Duskstik using a single surface foam wing with a single 1/8 " CF spar. First version didn't fly well...not enough dihedral. Second version had 8" cranked tips and flew very nicely, but in turns, the tips would flutter. I stiffened the wing with filament tape along the LE and TE. With no more change than this, the plane became a real dog...very difficult in all axes. It seems that the flexy wing had been automatically adjusting to changes in loading and somehow producing good flight characteristics. Now I think I may go over to a two-surface flat bottomed wing of the same area and see how that does. 10 degrees dihedral and no cranked tips?