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"Bob Kuykendall" wrote in message
om...
Eric Greenwell wrote:

A few have suggested 40:1 is not possible at less than 15 meter span,
but when 15 meter spans can now do 48:1 or better, this is not sensible.

Now we're getting to the crux of the matter. In order to continue this
discussion, we need to avoid comparing apples to oranges.

The 40:1 best L/D performance that you're seeking to match for an LS4
equivalent is the actual Johnson-tested and verified best glide
performance of the LS4. It really does go 40:1 at best glide. I can't
remember what the factory claimed for it, but it is probably on the
close order of 43:1, right in the ballpark of what I'm claiming for
the HP-24.

The 48:1 best L/D performance that you say that 15m ships can do or
better is just a claim. It is salesspeak. It is not verified by any
impartial body. It is, in my mind at least, false until demonstrated
true. When a well-designed 18 meter ship like your ASH-26E can barely
hit 50:1, even when you feather in the squeakiest data points, you
just have to wonder how good you really can do with three whole meters
less.

Now, I'm not going to say that I think that 48:1 or 50:1 is outside
the realm of possibility for a 15m ship. But I will say that I don't
see it in the impartial test reports that I have read.

And I will note that this is a pretty sore topic with me. I'm
developing a 15/18m sailplane for which I have released what I think
is a reasonable best L/D claim. It has a well-designed wing, courtesy
of my Stanford phd friend who does low-speed aero engineering for NASA
Ames and campaigns in 15m national competition. So I think I have good
reason to believe that my ship will meet my claims. And I also have a
pretty good idea that there's no magic out there that is going to let
you pull 50:1 out of 15 meters like you'd extract a rabbit from the
eye of a needle.

As for real, tested best glide performance available from a 15m ship,
a good example is the Ventus. When Johnson tested the Ventus A in
1981, he got 45:1 out of it. Just incredible. Not many people were
particularly enamored of its handling characteristics, but it went
like stink.

And after 15 years of evolution that produced a new fuselage and a new
wing, Johnson tested the Ventus again in 1996 - this time the 2B
model. His results show that the best L/D increased an entire negative
1 to 44:1. It was definitely a nicer glider, with the auto-connects
and nicer cockpit and other improvements, but somehow the best L/D
didn't go up.

Going forward, I propose that if we're going to be tossing a lot of
best L/D claims around, that we restrict ourselves to tested, verified
best L/D performance values, for example the idaflieg or Johnson
results. Otherwise I'm just going to have to join the Liar's Dice game
and claim a patently unobtainable 50:1 for the HP-24. And I'd like to
think of myself as a more honest person than that.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

Well said, Bob.

All else equal, span determines performance. Clearly performance is a
highly desired quality for the buyers of new sailplanes. I think is true
that any cost savings due to a shorter span will not offset the lesser
performance with the majority of buyers.

If you really want a short span glider, wait until the first generation of
owners dump them on the used market when they move up to larger spans.
Then, short span gliders will be really cheap.

15 meter gliders are popular for a reason. They are still small enough to
be easy to assemble while delivering excellent performance. However, I
suspect that if costs were equal, 18 meters would be still more popular.

Bill Daniels
(20 meter driver)