As a diversion from the LSF angst, I'd like to briefly resurrect an old
thread. Shortly after NATS there was a flurry of dialog regarding 2M
design. My comments, in general, suggested there was still some room to
upgrade 2M designs, look more closely at the Drela airfoils, optimize
planforms at the wing loadings we could achieve, etc. etc. So for the
last 6-8 weeks that's been a fun project. The result is a poly, aptly
named the Kahuna 1.8 (2M doesn't quite fit in the bed of my truck so we
opted for 72" span). Although a poly, it's intended to be a validation
platform for trying a few things out. Basically a V-tail, pod and boom,
Drela section, optimized planform for the wing loading and still
winchable at an AUW of 19.5oz. The structures are basically similar to
DLG but without the need for a sub-rudder. Other than that (and swapping
out the airfoils), it's more or less a discus on steroids
Unfortunately, as Ben Wilson has noted, the dwindling number of
sailplane guys makes the following unverifiable as there's only one
active soaring pilot in Oklahoma that I'm aware of (me). So no
witnesses. But there's no reason to make any of it up.
The last couple of evenings were spent doing hi-start trim flights,
checking incidence, CG, control settings, etc. etc. Today was the first
time to really winch it. The soccer field is somewhat small (three
fields wide by one field long) and surrounded by trees on three sides
and a major road on the 4th. So flights are pretty much restricted to
flying over the open field area. First hard winch launch was a bit
downwind. No wing flex, nice zoom, good stability, etc.
Second winch launch was similar - maybe 300ft total altitude at
pushover. Launched into a nice little thermal and the adventure began.
It became very obvious that the Drela sections were performing well.
Just enough airfoil transition and washout to minimize tip stalls. The
wing could pull a ridiculous AOA and just mush straight ahead. Basically
very good pitch and yaw response with the V design chosen. After
checking it all out and finding about three more thermals the ship is
pretty well settled in and we've got 10 minutes on the flight and good
altitude.
Wonder how long this can go on, sez I. Five thermals later and we're at
25 minutes. Gee, sure would be fun to do that 1 hr Level IV flight. Just
about then the lift cycle quits and at 29 minutes, and again at 34
minutes, sub-tree top saves are needed to keep the flight going. The one
at 34 hit a bubble on landing approach and off she went. Discus
technology and methods show promise in this size class.
Somewhere around 45 minutes into the flight, it's looking really grim
with a large sink cycle everywhere. I have no idea why those two
adolescent hawks decided to have a bird fight over the field just then.
But the two of them came tumbling out of the sky ass over tail feathers
and pulled out at about 50ft right at the edge of the west tree line.
They both started circling down wind - and were going up. Following the
fighting falcons paid off with one of the better small thermals of the
day and we're off to the races again.
Finally at 55 minutes into the flight it's time to relax until, at 58
minutes, another sink cycle comes roaring through and it's looking grim.
Right then the little flag on the Tx antenna starts pointing north.
'Follow the flag' is the lesson given by Doug Harnish at the NATS so in
desperation we're using what little altitude is left to head for the
north tree line. And right there on a bead with the little flag is a
tight little twister kicking off from the only tree on the line that's
wiggling it's branches. The hour flight is made.
That little thermal worked so well that we were well and truly skied out
(for a 2M) when it quit. Hmmm, wonder if a 2hr is in the cards? Nope
(common sense kicks in) - it's a 400maH pack and the best we've got from
it in the other plane is about 100 minutes and we've already used 15 on
top of this so....
Brought it home at the end of the next lift cycle for a 77:06 flight.
Not bad for a trim flight on a 1.8m home design (and build). Sure
would've been nice to have a witness or two for that 2nd L-IV flight but
to my knowledge the nearest witnesses are 250+ miles away.
Bottom line - I still submit we are not optimized in 2M. Yes there are
some good ships out there but I do not see any reason a 2M needs to come
in at 46 oz or more. 30ish max if you're really heavy on the structures.
Although this one is a poly, the same structures will be used for a
flaperon version which shouldn't weigh more than 24oz. Yeah, it probably
won't take a pedal-to-the-metal-full-bore-stomp-on-it winch launch. But
the way this one is flying makes that a reasonable compromise.
This isn't hard to do, guys. I still believe the reason 2M performance
is generally inferior to open class is that most 2M ships are either
converted slopers or small open class ships, neither of which are
designed to the 2M class itself. Look to marrying some DLG technology
with more appropriate (Drela) airfoils to deal with the wing loading.
Dang, I sure could have used a couple of witnesses for the L-IV. Maybe
next year........
Come to think of it, my first (witnessed) 1 hour flight was also done
with a 1.8m ship about 2 years ago. But it was a lot harder flying that
over-weight glass job compared to this one.
Back to the drawing boards.
- Dave R
PS - Thanks Dr. D. These sections are REALLY good.
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