LS6 vs LS3 vs Discus question

Looking at sailplanes. Is a 15m LS6 really any better
than a 15m LS3? Both are flapped and have 40:1 glide
slopes. In fact why does the LS6 have a better handicap
than say a Discus, when the Discus is 2 points better
in the glide? Is the 15m LS6 any better than an LS3
or Discus??
Thanks

Kay

If you never plan on flying faster than 80 knots or so, then there is
indeed no advantage to the LS6.

Earlier, Kay Scott wrote:

Looking at sailplanes. Is a 15m LS6 really any better
than a 15m LS3?

Yes, no doubt about it in my mind.

The biggest difference is that the LS6 actually achieves 40:1, while
the LS3 just claims it. Of note, the LS3 uses the old FX67 profiles,
which are quite sensitive to bugs, rain, dust, and other real-world
elements. The LS6, on the other hand, uses FX81-series profiles, which
are more conservative but also more robust. The FX81s tend to maintain
more laminar flow under real-world conditions, and they also have lower
pitching moment and consequently less trim drag.

Earlier models LS6 have manual-connect controls and a somewhat hokey
aileron damper system retrofitted to combat flutter. Later models have
a neat mechanical mass-balance system and auto-connect controls.
However, at least one operator complains that later LS6s have more
aileron friction than earlier ones. This could be due to the switch
from stainless steel to mylar for the lower flaperon seal.

Both are flapped and have 40:1 glide
slopes. In fact why does the LS6
have a better handicap than say a
Discus, when the Discus is 2 points
better in the glide?

My take on that is that it's because the LS6 flaps allow better
profile-to-speed optimization, and that the LS6 has a somewhat thinner
wing in T/C.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

Bob Kuykendall wrote:
Earlier, Kay Scott wrote:


Looking at sailplanes. Is a 15m LS6 really any better
than a 15m LS3?


Yes, no doubt about it in my mind.

The biggest difference is that the LS6 actually achieves 40:1, while
the LS3 just claims it.

I have to disagree with this statement. The LS3 certainly achieves 40:1,
my LS3 does and so did the one that Dick Johnson tested:

http://ssa.org/Johnson/25-1978-11.pdf

The interesting thing is that the LS3a and the LS3/17 didn't
seem to be able to achieve the same results.

Obviously the LS6 is better than the LS3 up in the very high speed
range and the LS6 can be flown over a wider range of wing loadings.

Of note, the LS3 uses the old FX67 profiles,
which are quite sensitive to bugs, rain, dust, and other real-world
elements. The LS6, on the other hand, uses FX81-series profiles, which
are more conservative but also more robust. The FX81s tend to maintain
more laminar flow under real-world conditions, and they also have lower
pitching moment and consequently less trim drag.

In practice the LS3 doesn't seem to be as sensitive as other gliders of
this era.
Is this trim drag as much of an issue for flapped gliders?


Earlier models LS6 have manual-connect controls and a somewhat hokey
aileron damper system retrofitted to combat flutter. Later models have
a neat mechanical mass-balance system and auto-connect controls.
However, at least one operator complains that later LS6s have more
aileron friction than earlier ones. This could be due to the switch
from stainless steel to mylar for the lower flaperon seal.

The LS3 autoconnects unlike the later LS3a and LS3/17, which have
separate flaps and ailerons. This feature comes at the expense of a ton
of mass balance lead on the flaperons.



Both are flapped and have 40:1 glide
slopes. In fact why does the LS6
have a better handicap than say a
Discus, when the Discus is 2 points
better in the glide?


My take on that is that it's because the LS6 flaps allow better
profile-to-speed optimization, and that the LS6 has a somewhat thinner
wing in T/C.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24


Greg O'Sullivan

At 02:06 02 September 2005, Greg O'Sullivan wrote:
Bob Kuykendall wrote:
Earlier, Kay Scott wrote:


Looking at sailplanes. Is a 15m LS6 really any better
than a 15m LS3?


Yes, no doubt about it in my mind.

The biggest difference is that the LS6 actually achieves
40:1, while
the LS3 just claims it.

well if the LS6 did only 40:1 I'd be worried!!! More
like 44:1

As a former LS3 owner, my experience was that a well-prepared '3 was
equivalent to an ASW 20, Ventus 1, or LS6 in both climb and cruise. The
operative words are "well prepared." The LS3 uses an older airfoil that
is, indeed, more sensitive to bugs, rain, and waviness. Worse, the LS3
wing has a tendency to get flat spots over the spar caps cause by
shrinkage. Initially I was able to smooth these out by sanding. Later
it was necessary to build up the profile over the spar caps. This
eventually stabilizes but I suspect few LS3s have had this shrinkage
corrected even if they have been refinished.

A few other U.S. owners did the same thing and one or two also profiled
the leading edge (the outer wing, especially, was apparently too blunt
from the factory), with spectacular results.

This is only a problem at high speed, by the way. Climb is unaffected
and is excellent.

The early LS3s were highly respected for their performance when new. As
one poster observed, however, many of the later LS3a types didn't seem
to go as well. There was a debate in SOARING magazine, sparked by Dick
Johnson's testing, about whether the wing molds had warped as time went
on, but I don't know the true story.

In any case, the LS3 handles like a dream (as do the LS6 and Discus, by
consensus) though it is more pitch sensitive than some if the CG is at
the aft limit, which it should be for maximum performance. And it's
very strong. Alas, the wings are heavy: about 175 lbs. each for mine
(125 at the root, 50 at the tip). So a strong crewperson or a
one-person rigging system is helpful, in particular because the wing is
trailing edge heavy due to the lead mass balance for the one-piece
"flaperon".

Chip Bearden