Control Surface Loads

[Reed]

So your plane is ripping around the circle at previously unknown speeds and it starts to get mushy, especially on the bottom turn. Hmmmm....

Obviously there could be several causes for this, but I'm interested in the idea of dropping a hair's worth of camber into the TE surface(s) in order to bring them to actual airfoil shape under load. Does this make sense? I think Craig told me he does this sometimes - anyone else?

[bjaffee]

Would the flaps and elevators tend to pull down under load already, or get sucked up?

[Reed]

Brett,
I've been reading an excellent book called "Understanding Gliding" that has helped me, well, understand gliding. It's clear that air is pushed down off the TE of a wing as it's creating lift. And we've seen that all the wingload failures in DS come at the bottom turn, usually just as the plane begins to head back home. So I assume there are a lot of lifting forces at that point. My very limited DS experience tells me that sometimes the plane does not give great elevator response down in the dead air and it takes some getting used to. There are certainly other factors at work and it may be a BS question, but if anyone knows whassup or has any experience trying this...

[Craig Toutolmin]

The wing is bending up from the aero load. The surfaces want to go up, reflex or negative camber. The flaps are harder to control because of the geometric arrangement or servo programming to get max throw for landing. As the loads increase the flaps deflect sooner and as such should have a bit more snap-flap throw. Joe showed me a simple way to check for moderate DSing. Put a finger under the flap and aileron and lift up to see if they are even under load. Digital servos have an advantage here because they give all of their torque after the first few degrees where analog deliver it at around 30 degrees.

If set correctly for the airfoil you should be able to get more lift without additional drag at the upper end of the airfoil's drag bucket.

Most softness comes from the elevator. With V-tails there is a tremendous force to flatten the V. This give in the V takes away from the elevator effectiveness.

The ruddervators also twist towards the tips under load if they are not strong enough in torsion.

Elevator push rods usually (free floating types) will buckle and the housings of sleeved rods can be torn from the side of the fuse if not properly attached.

Another cause of would be boom flex. Ollie rans some numbers for the downward force on the elevator and it is pretty impressive. You quickly see why V-tails flatten. The boom flex reduces the wing/horiz stab decalage.

Another cause of softness can be CG. Depending on the plane, if you have a CG set for F3F and start cranking circles the plane can begin to tuck and results in a somewhat similar feel on the stick.

[Yochim]

On a similar note I am wondering about layups for DS planes and how to use carbon uni most effectively?

I am planning a 2-metre DS plane and my thinking was to use a carbon capped spar and then 4.7oz. uni-carbon at 45degrees on the wing with a layer of 2.2oz. glass over it.

Ailerons only, no flaps and close out the ailerons with aluminum tubing.

Any thoughts?


Y.

[Reed]

I think I have a combination of all of the above since it is a lightly bagged vtail and the boom has been cracked once with only a mediocre repair. It softens to where I've run out of throw a couple of times on the bottom turn - scary and inefficient. On the front side I can crank turns as hard as I want and take very quick loops at the bottom of a speed run with none of this softening in evidence. DS is different!

Yochim,
If you look at Craig's thread re: fav DS planes you'll see that I have a plan very similar to yours with the addition of slightly heavier woven cf on bias and 1/64" ply over all that. IMHO, you're on the right track. Airfoil? Planform? New thread?

[Yochim]

Hi Reed, sorry about the Thread Hijack.

My wing uses the RG-14 and I got the cores from my cousin Tony D., 88" span with 560 square inches of area, 8" root chord.

The wing he built uses 4.7oz. uni at +/- 45degrees top and bottom of the wing but this is a bit expensive for me although it is brutally stiff.

I thought of using some 5oz. woven carbon at +/- 45degrees with a layer of 2.2oz. glass over it but I was not sure if it would be stronger/as strong than the carbon uni at 45degrees. Any thoughts on that?

I had a look at some photos of Paul Natans' Speed Runner and that is the way I am going with the control surfaces, short ailerons and a standard tail using a Hitec HS615 on the elevator.


Y.

[Craig Toutolmin]

"I thought of using some 5oz. woven carbon at +/- 45degrees with a layer of 2.2oz. glass over it but I was not sure if it would be stronger/as strong than the carbon uni at 45degrees. Any thoughts on that?"

The carbon uni should be much stronger. If the weights and tow size were similar (2.5 oz uni and 5.0 oz plain weave) on a +-45 the uni should still be stronger and lighter. The fibers in the uni will be nice and straight compared to the woven cloth. This should increase the strength and minimize spaces for epoxy to collect - reducing weight. After I run out of my stash of plain weave carbon I'll start looking for a lighter weight (2-3oz) uni.

[Ollie]

Increasing camber by drooping the trailing edge increses the nose down pitching moment coefficient of the wing. If the elevator compensation is mixed in to keep the plane at the same pitch attitude, the down load on the tail is increased.

I strongly suspect that tail boom downward flex is the culpret that results in less nose-up pitch authority. At 175 MPH the down load on a horizontal tail of 100 square inches could be as high as 40 pounds or so. Try suporting the fuselage so that the tail boom overhangs the support and start hanging weights from the tail. I think you will see that most tail booms flex excessively under load. Also, notice the effect on elevator neutral because of boom flex effect on the control linkage. If the tail boom survives a similar tail loading without excessive bending or control deflection then tail boom flex is probably not the cause of the lack of up elevator authority at speed.

[Reed]

Fantastic information here, Gents!

I'd never have suspected that the uni would be stronger for torsion but it makes sense once explained. My main reason for using the woven was alluded to by Craig - it's what I have in the shop already.

Good advice on the tailboom, Ollie, I'll try it once I put it back together after my latest encounter with the planet's surface. This time the fix will be stronger, but my poor Vindy is having the Hobbesian existence Ken Russell warned me about. After 3 years of babying it I started DSing and it's life has become nasty, brutish, and short.

[shoe]

uni carbon is stronger in whatever orientation it's layed right? so which is better for DSing? +45deg or -45deg? ie if the TE /fuse intersection is considered the origin should the fibers run from TE to LE or vice versa?

[Ollie]

If the purpose of the unicarbon is to give torsional stiffness to the control surface or the wing, then each skin should have two layers, one at right angles to the other. In other words, one layer at plus 45 degrees and the other layer at minus 45 degrees to the long dimension.

[Yochim]

The designer of the DS wing I am building uses this system of +/-45 degree carbon top and bottom over a Mark Drela type spar and I was gob-smacked when I tried to twist the wing panel, it is STIFF but actually a lot lighter than I thought it would be.

The only thing keeping me from using this technique is the cost, it takes a fair amount of carbon to cover an 88" wing with carbon at +/-45 degrees.

Then again I saw the plane fly (he calls it the Rimmer) and it looks like the extra effort and money is worth it.


Y.