You may recall I finally got tired of waiting for some kit builder
to market an honest to Gordy 2 meter floamy thermal flying wing. Terry
Tiesan, Mark Mech and MAD Aircraft all talked about doing it. Nobody did.
So I bought a Mongo Jr. and built it to fly thermal.
Nobody at BASH Enterprises could tell me much about flying a Jr. off
a high start or winch. Those folks are all hard-wired slopers. B2 thought
it might work.
So far, results are mixed. It will thermal, and I have thermaled it
high enough to lose orientation and wound up diving back to the field minus
one winglet. But if you slow down too much in a thermal turn, the
Ther-mongo will slip into a death spiral. Luckily, it bounces, unlike my
late 2 meter Klingberg Wing. And with a 30 feet of altitude or so, you can
pull out of the death spiral.
High start launches are no problem at all. The Mongo Jr. tracks
well and launches fairly high. Winch launches are tougher for some reason,
and high speed launches have resulted in some elevon flutter, from the
servo linkage I think.
Here's what I did initially: I inlaid a 1/4" carbon fiber tube
(picked up at Toledo a few years ago as a freebie) in a groove in the bottom
of the wing, just an inch or two aft of the recommended CG. I inlaid it
into the bottom of the wing because it is flat at that point, and easier to
inlay than in the top.
I pitched the Coroplast winglets and used thin styrofoam sheet from
the restaurant carryout boxes to make up winglets.
I installed the full sized JR servos as far forward and as far
inboard as I could, laying them down flat, and running thin Sullivan cable
pushrods in an arc from the servo wheel a spot about 1/3 of the way out on
the elevon span. And I used Kibrough servo savers on the servos, just in
case. The buried Sullivan cables were to reduce drag across the wing top.
(But I screwed up by angling the emerging cables and servo horns at right
angles to the elevon hinge line, instead of parallel to the center line.
Extra drag. Darn.)
The thin styrofoam winglets didn't work out. The stuff was OK on my
Zagis, but Mongo Jr. is bigger, the winglets are bigger, and at speed they
would flutter. I went to 1/4" styrofoam sheet from the bottom of chicken
trays. The guys at the club thought I had renamed the plane "Tyson."
The 1/4" styro winglets worked fine but felt draggy. I did the math: 1/4"
x 2 x 12" tall = 6 square inches of frontal area.
I am now using winglets made of 1/16th balsa sheet, covered with
monokote, in a "boot-shaped" profile rather than the truncated right
triangle that the coroplast winglets featured. I like this setup.
The curved Sullivan cables did not work out either. They did reduce
drag I suppose, but there was a lot of slop in the linkage. It was like
flying a plane with the servo connections made out of old garter belts. You
had to fly with a lot of advance planning. And I got some elevon flutter
in high wind launches and heavy winch launches.
I moved the towhook back about 3/8" from my initial guess. (If you
build one of these, make sure to provide for a moveable tow hook.)
I broke down and added a couple of ounces of nose weight, to get the
CG about 11 to 11 1/4 back from the nose.
I added trip strips to the top of the airfoil. This is an on-going
experiment.
I got my hands on some Harley Michaelis' RDS couplers and two weeks
ago we had he first rainy weekend for a long time; I used the opportunity
to retrofit the Ther-Mongo with RDS servo linkages for even less drag, and
a much more positive control linkage.
When I tore into the Mongo to do the RDS installation I found out
that a big part of the elevon slop was due to our old friend the inverse
cube law (or as Joe Wurts has called it, the "scale effect."). Big planes
look like they are flying slower, but really they are flying faster. And if
you increase the dimensions x 2, you triple the overall weight. This is why
ants can lift several times their own weight and I have trouble getting the
winch battery out of the car.
In comparison to the 48" Zagis and Boomerangs,, the bigger, heavier
Mongo Jr was doing serious internal damage to itself on rough landings. The
servos had smashed themselves a whole bunch of additional room inside their
styrofoam pockets. The servos were flopping around in there looser than an
idle thought in Dan Quayle's brain.
Tightening up the servo pockets and retrofitting the Ther-Mongo with
RDS linkages got rid of a whole bunch of the elevon slop. Any slop left is
due to my building skills which are second to Nunn.
That's Sam Nunn, the Senator from Tennessee, who beat hands down at
a paper airplane contest some years ago. But I digress.
I was able to retro-fit the RDS linkages without un-hinging the
elevons! The original elevon hinge was 3M brand Blenderm medical tape. I
simply cut off about 3" of hinge tape on each elevon where the RDS pocket
was to be inset. Then I peeled back 3" of monokote on the elevon and used a
grinding stone on a Dremel tool to grind out a shallow hole to inset the RDS
pocket.
I drew a line on the top of the wing from the servo pocket to the
place where I wanted the RDS rod to exit the foam TE. That done, I let the
elevon flop full down, and started a tunnel for the RDS control rod from the
foam TE, using a very long 1/16th drill bit that I had picked up at a hobby
show years ago. A long piece of pushrod with a sharp chisel shape on the
end would work as well. I sighted along the drill rod and the line on the
upper surface of the wing, and tunneled slowly. One one tunnel I was a
little off, so I compensated for that when I re-set the servo. No big deal.
I slid the RDS control rods in from the foam TE, leaving the 45 deg.
bend hanging out over the elevon. Getting the control rod length right took
a few tries, gradually removing length from the couple end. I slid the
pre-fabbed pocket over the 45 deg. bend, and then used masking tape to hold
the pocket in place in the elevon recesses. (I know--I should have used
duct tape. Nobody is perfect!) Then I adjusted the elevons to what I
thouht was the correct angle and tightened the RDS coupler setscrews. After
testing for a while with the TX and Rcvr turned on, I finally shut down and
removed the masking tape and glued the pockets in place in their elevon
recesses. I used spring clamps to hold the pockets tight in place overnight.
Now the elevon movement seemed smooth and positive.
I put a 3" patch of Blenderm hinge tape over each pocket, but did
not otherwise cover them up. I want to be able to explain to folks how this
RDS system works. I could hardly wait to get the Ther-Mongo back in the air.
I still haven't had a chance to fly the Thermongo thermally, but
yesterday we had a great slope day at Newark, OH. The RDS couplers made a
wonderful positive change in the way the Thermongo flies. It was very
smooth, very fast, very controllable. Rolls were easy, loops trivial. The
RDS coupler seems to also change the "linearity" of the control movements.
Flying felt much more instinctive and "right." I even let an 11 year old
friend of my son's try his hand at the Mongo--with no prior RC experience.
He was doing OK until the wind died down on us.
On the slope, the Mongo Jr. doesn't even notice MOGs. If flys right
through them. On the flat field, it has been a fun flyer for medium to
strong lift, if you don't mind a sort of wierd thermal turn. I expect it
will be even better with the RDS couplers.
Finally, a couple of RDS coupler hints:
read the updated web site first
build a mock up first
you really do need both set screws
Get a prayer rug: if you try the couplers, you will find
yourself bowing in the general direction of Harley Michaelis five times a day.
And some day, somebody is going to kit a real thermal foamy 2 meter
flying wing. When they do, I'd recommend RDS couplers on the servos.
_________________
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Tom Nagel new email addr /O\ [EMAIL PROTECTED]
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