On Wed, 10 Nov 2004 22:57:27 -0800, Eric Greenwell
wrote:


But here's question: we know a modern glider can be smaller than the 24
year old LS4 design and have the same performance.

Do we really know that?

I believe it when I see one. Frankly spoken, I doubt that this is
possible without major compromises concerning cockpit size and crash
protection.

I need a certain cockpit cross-section to be able to sit comfortably,
so the cross-section of the fuselage (which defines most of its drag)
is fixed, independent of the wing span. Fuselage surface area is also
fixed (apart from the fact that the fuselage will be a little
shorter), so there is very little potential to reduce the fuselage
weight compared to a current glider (say, ASW-28). Proof is the PW-5
which is only slightly lighter than an ASW-28 despite the fact that it
has much lower Vne and maximum weight.


Fixed fuselage cross-section with a smaller wing means that the
fraction of fuselage drag on total drag is going to be greater. As a
consequence the wing needs to save drag - and the only chance to do
this is increased aspect ratio... which will lead to wing loading
problems.

One solution could be to build the whole glider extremely light (like
the Apis or Sparrowhawk) to get normal wing loadings of about 33
kg/m^2 at a high aspect ratio, but this is going to result in the
inability to carry water, low Vne (hence the comparably bad
penetration of the Apis compared to club class gliders with similar
L/D and wing loading) and questionable crash protection.

The Sparrowhawk and Apis look really good and are definitely state of
the art - but to be honest, I would not like to rely on their cockpit
shell strength when I impact at 50 kts or above.



I suggest 13 meters
would do it without heroic efforts by the designer, but what do the
citizens of RAS think is the minimum?

It's not the wing span or weight, its acceptance. And I think history
has shown what kind of glider will be accepted (and bought) and which
not.
How many LS-4 have been sold? 1.400?


Let's face it:
At the moment the Sparrowhaw is sold for $33,950, the LS-4 for 39.500
EUR (VAT not included).
The Sparrowhawk is not that much cheaper, especially if we consider
the fact that it is much smaller and much simpler (no retractable
gear). And, of course, it's not certified (the certification alone is
the major part of the development costs - this is what makes an
aircraft so expensive). Shall we bet that if it was certified the
Sparrowhawk would be at least as expensive as an LS-4?


If we had an exchange rate of 1:1 as we had two years ago, you'd get a
lot more bang per buck with an LS-4, wouldn't you?







Bye
Andreas