Pre-Preg

gliders are cured in an oven at SH anyway. i think it's most probably a cost thing. even though these new gliders are ridiculously expensive at face value, the profit margin the manufacturers are getting is very small, so they keep costs down wherever possible. that's part of the reason schempp-hirth (as well as many others) re-use fuselage designs. take the ventus 2 versus the V2X. you got a performance increase for only a new fin design, new horizontal tailplane, and new winglets.

On Thursday, December 1, 2016 at 5:55:38 PM UTC-5, Tony wrote:
Yes the Windward gliders are carbon prepreg.

I guess the other manufacturers aren't that concerned about empty weight and don't want to have to deal with ovens for curing?

Most gliders work out at about 6 (30 kg/m^2) dry with a 240 lb pilot, don't they?


An ASW27 would be about 40kg/m^2 with a 240lb pilot weight

L

On Friday, December 2, 2016 at 6:28:43 AM UTC-8, Luke Szczepaniak wrote:

Most gliders work out at about 6 (30 kg/m^2) dry with a 240 lb pilot, don't they?


An ASW27 would be about 40kg/m^2 with a 240lb pilot weight

L

Cost and repairability are factors with pre-preg although power aircraft like the Lancair use prepreg extensively. Newer spread-tow pre-pregs are amazingly light and stiff.

From a performance standpoint, span loading is a significant factor. For an equal wing loading and span, the lighter sailplane with the lower span load has lower induced drag. Reynolds numbers also play a factor when the chords get so small, but the aero community seems to be getting better at developing profiles that aren't hurt too much by this.

Cheers,
Craig

On Thursday, 1 December 2016 22:03:22 UTC+1, Jonathan St. Cloud wrote:
Just wondering why glider are not made with pre-preg. Seems like it would save weight.

Having done 10 years working in Formula 1 making the cars solely from Prepreg and then another 6 years in aerospace making a payload fairing for a well know rocket launcher, I can say that the preconceived idea that prepreg is expensive is wrong.
You also don't need an autoclave to cure it

If you were willing to invest just a little money your raw material usage, and you reproduce-ability would increase no end.
Also, you could happily do a post cure on the parts and paint them any colour you like, just like in Formula 1.

On Friday, December 2, 2016 at 1:25:16 PM UTC-5, Craig Funston wrote:
On Friday, December 2, 2016 at 6:28:43 AM UTC-8, Luke Szczepaniak wrote:

Most gliders work out at about 6 (30 kg/m^2) dry with a 240 lb pilot, don't they?


An ASW27 would be about 40kg/m^2 with a 240lb pilot weight

L

Cost and repairability are factors with pre-preg although power aircraft like the Lancair use prepreg extensively. Newer spread-tow pre-pregs are amazingly light and stiff.

From a performance standpoint, span loading is a significant factor. For an equal wing loading and span, the lighter sailplane with the lower span load has lower induced drag. Reynolds numbers also play a factor when the chords get so small, but the aero community seems to be getting better at developing profiles that aren't hurt too much by this.

Cheers,
Craig

I've heard conflicting sides about pre-preg repairability. Greg said its a misconception and can be repaired fairly easily and I have seen a Sparrowhawk that has been repaired by him.

https://www.youtube.com/watch?v=JYcHDPzx1ao

Easy enough to fix if you know what you are doing.
Not quite as easy as wet laminate that the gliders are made of today

run of the mill pre-pregs are heavier then a careful wet layup but the pre-pregs allow for a less skilled labor force. When reducing the amount of material, the orientation and wetout become super important. With prepregs the resin/matrix ratios are more predictable and variations in skill are all but eliminated, The shape of the pieces is also more fixes as the resin holds the fibers in place and there is much less distortion. Pre preg is also much faster to construct. If sailplanes were to be wet layup the costs would be in the range of 15-20% more.

So what you are saying is that if we all move to pre preg sailplanes we save 15-20%? Why is no one in Europe doing it?
I personally see no reason why a wet laminate would be better or lighter
Based on the square meterage of the material vs the time saved you would probably save a little on prepreg, and open a whole new window of opportunity in other areas.

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.