Pre-Preg

In Europe there is probability that it has much do with the cost of
certification of a new manufacturing process. Here where the weather is
often cr** people do go for light weight empty.

On Friday, December 2, 2016 at 4:15:15 AM UTC-5, Jim White wrote:
In Europe there is probability that it has much do with the cost of
certification of a new manufacturing process. Here where the weather is
often cr** people do go for light weight empty.

If it's good enough to fly, it's good enough to fly at 7 lbs / sq ft. Otherwise it's a better day for a bicycle, or hiking boots or maybe sitting indoors in the rain annoying the folks on r.a.s.

From the XC performance standpoint, there's little advantage to empty weight very much below 500# in a 15m sailplane. For self launching, well, weight is really important...

-Evan Ludeman / T8

On Friday, 2 December 2016 14:33:01 UTC+2, Tango Eight wrote:

From the XC performance standpoint, there's little advantage to empty weight very much below 500# in a 15m sailplane. For self launching, well, weight is really important...

-Evan Ludeman / T8

With lighter structure and materials you can build smaller wing and still have acceptable min. wing loading. Smaller wing - higher aspect ratio - higher performance (Diana-2 for example)?

On Friday, December 2, 2016 at 3:33:01 PM UTC+3, Tango Eight wrote:
On Friday, December 2, 2016 at 4:15:15 AM UTC-5, Jim White wrote:
In Europe there is probability that it has much do with the cost of
certification of a new manufacturing process. Here where the weather is
often cr** people do go for light weight empty.

If it's good enough to fly, it's good enough to fly at 7 lbs / sq ft. Otherwise it's a better day for a bicycle, or hiking boots or maybe sitting indoors in the rain annoying the folks on r.a.s.

From the XC performance standpoint, there's little advantage to empty weight very much below 500# in a 15m sailplane. For self launching, well, weight is really important...

-Evan Ludeman / T8

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

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 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

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

On Friday, December 2, 2016 at 6:48:16 AM UTC-6, Bruce Hoult wrote:
Most gliders work out at about 6 (30 kg/m^2) dry with a 240 lb pilot, don't they?

My LS6 runs around 8psf (39 kg/m2) dry with me in it (240 on a good day). I would think you would have to get into open class or older std gliders to get as low as 6 psf...

Works great for no-ballast (Beer ballast?) contests...

Kirk
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