Pre-Preg

Here's the key thing: Glider structures are bound more by stiffness than by strength. True, pre-pregs can be much stronger than more conventional laminates. However, they allow only a rather modest premium in stiffness.

Think of it like this: Suppose you have a magical material to make wing spars out of that just as stiff as what we use now, but is is twice as strong. Then you use half as much. The result is that you'd have twice the wing deflection per unit g. Your new wing at 2.5g looks like your old one at 5g. And 4g looks like 8g.

That might work out OK, and it would have a pretty soft ride, but the aero effects can be unpredictable, and it gets pretty hard to make control surfaces that work smoothly while following the curvature of a g-bent wing.

Furthermore (and probably much more importantly), stiffness is much more important than strength when mitigating elasticity. So you'd end up using way more of your magic material than dictated by strength just to get the stiffness up where you need it to have Vne with a usable margin against flutter..

And, as others have already touched on, extra mass often results in a structure that is more resistant to handling, assembly, and operational damage. And gliders aren't much fun unless they are operational.

The full sermon on these topics runs around an hour. If you want the whole thing, come to our 21-27 January 2017 Akaflieg:

https://www.facebook.com/events/335703193452266/

Thanks, Bob K.
https://www.facebook.com/pages/HP-24...t/200931354951

Op zondag 4 december 2016 18:14:46 UTC+1 schreef Bob Kuykendall:
Here's the key thing: Glider structures are bound more by stiffness than by strength. True, pre-pregs can be much stronger than more conventional laminates. However, they allow only a rather modest premium in stiffness.


Bob,

I don't follow. Assuming one type of fibers, both stiffness and strength are only driven by fiber straightness and fiber volume fraction, so you'd expect both to rise comparably. Obviously, pultrusions and unwoven (UD, BIAX, TRIAX) are better at both.

So aside from moving to unwoven fibers, fiber fraction is the big elephant in the room.

If we compare our typical 200 g/m2 carbon cloth, these are typical weights for the full laminate:
Hand-laminated, 32% fiber volume fraction, 520 g/m2 total areal weight.
Vacuum-bagged; 40% fiber fraction, 415 g/m2 total areal weight.
Infusion/prepregs, 55-60% fiber fraction, 325-300 g/m2 total areal weight.

Getting very light weights with prepregs can be tricky, once you formulate resin fraction approaching or below 40%, the laminate gets very dry. Even with an autoclave and de-bulking in the process, getting voids is a risk. With infusion, as long as you have wet-out you're fine. In practise both are in the 55-60% Vf range.

Prepregs are a no-brainer for small parts. For big parts, it quickly gets prohibitively expensive. Even for modest amounts of fabric (a few hundred square meters/yards), expect to pay hundreds of US$/Euro's per square meter for a typical aerospace qualified combination, like T700 or T1000 plus a qualified resin.

Both also yield extra weight reductions since the adhesive joint to the core is way lighter.

Bottomline, using either technique can spectacularly reduce empty weight and weight reductions up to 1/3rd of the structural weight are possible.