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Wingspan:
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39.30" (1 meter)
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Wing area:
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155 sq in.
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Weight:
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4.20 oz.
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Wing Loading:
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3.90 oz./sq. ft.
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Controls:
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Rudder, elevator, tow release
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Receiver:
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Great Planes Electrifly
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Servos:
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2 Hitec HS-55
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Battery:
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110mAh NiCad, 150mAh NiMh
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Manufacturer:
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Scorpio
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Available From:
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Hobby Lobby
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Introduction
What, another bent wing sailplane, you say? Those of you who read my story about the Super Soarer
might now think I have a thing for gull winged airplanes. Actually, what really intrigued me
about this little scale sailplane was the idea of aerotowing it with a small park flyer tug,
which I got from the Hobby Lobby web pages.
The Hobby Lobby Habicht is scale model of the DFS Habicht, a German single seat, open-cockpit
sailplane of the 1930's. This diminutive (dare I say "cute?") foam ARF spans only 1 meter, and
tips the scales at around four ounces, ready to fly.
Kit Contents
The ARF consists of just a few parts. The major components are all made of extruded polystyrene
foam (EPSF), and include the fuselage, wing, and the horizontal and vertical stabilizers.
The fuselage is hollow: it's actually made from two halves that have been joined together, so
it's built almost like a composite molded plane. It has lots of compound curves, and looks quite
scale. One thing immediately noticeable is that there isn't a whole lot or room to place the
radio equipment!
The gull wing comes as a single piece. The airfoil is a constant thickness slab, but it
is curved to form an under-cambered shape.
The tail feathers are both made of thin foam, and are precut. The elevator and rudder hinge
lines are pre-scored, making it easy to separate them later.
All the foam parts have a very nice, smooth finish. In fact the finish is so smooth that, from
a distance, one gets the impression that the parts have been painted pearl white. The wing and
the fuselage also come pre painted with blue trim exactly as you see in the photos. Note: be
careful with using tape around the blue areas, as it may lift the paint.
In addition to the foam pieces, there are a few plywood parts, such as the main former, servo
tray, belly skid and tail skid. All the hardware needed to complete the kit is also in
present. Finally, the ARF includes some finishing pieces: a clear windshield and decals.
Assembly
Assembly is fairly simple, though as you might expect, it's a bit complicated by the fact that
everything is so small. Care must be taken with the foam parts, as they can dent easily. The
instructions recommend working on a foam-covered surface to prevent damage, which is a good
idea. I used 5-minute epoxy to assemble most of the parts. You can use odorless (foam-safe) CA
or polyurethane glues too.
Fuselage
The first step is to cut out the "wing fairing" (the upper part of the mid fuselage), so that
the shoulder-mounted wing can be attached in a later step. This is fairly simple, because three
of the cuts have already been made. All that's needed to free the fairing is to cut out the
rear portion. Make sure to always use a good sharp blade when cutting the foam, otherwise it
may tear.
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The wing saddle is precut, but the upper portion of the fuselage must be removed.
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The wing fairing removed.
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With the fairing removed, the main former can be installed just behind the cockpit. I found that
the ply former needed quite a bit of sanding before it would fit into the fuselage (as was the
case with almost all the wood parts in the kit).
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Fuselage former.
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The former glued in place.
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The next step is to install the nose-mounted tow release hardware. A wood reinforcement block
is first glued to the inside of the nose. Next, a small wire hoop is mounted through
the outside of the nose and into the block. A small tube gets installed on top of the block, so
that it protrudes horizontally out the nose. The tube acts as a guide for a pushrod that
engages the hoop to capture the towline. When the pushrod is pulled back, it disengages from
the hoop, freeing the line (see the picture for a better idea of how this works).
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The tow release in the closed position. When the horizontlal piece of wire pulls back,
the towline is released.
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The servo tray gets added next. I was a bit worried about being able to fit the tray through
the hole in the cockpit, but as it turned out, it fit fine. As with the former, the servo tray
required some sanding to fit correctly in the fuselage.
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The servo tray.Yes, it actually fits through the cockpit opening.
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A thin ply keel/skid needs to be installed along the bottom of the nose. There are no slots for
this in the fuselage, so cutting them is a job for the builder. There is also the option to
install a small high start tow hook (which I did). A small ply plate installed on the floor of
the fuselage provides reinforcement for the hook. Finally, at the rear of the fuselage, a small
ply skid is installed to protect the tail and the rudder.
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Belly skid.
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Next we move to the tail feathers. The control surfaces are joined to their respective
stabilizers, but the hinge lines come scored to the point where they are just about cut
free. A bevel gets sanded into both the control surface and the stabilizer, which frees up the
surface enough to move freely in both directions. After beveling, I hinged the surfaces with
small pieces of transparent tape. Finally, the small ply control horns are glued in place in
their precut slots.
Once the tail feathers are all assembled, they can be mounted to the fuselage. The mount for the
horizontal stabilizer was a bit crooked, but a bit of sanding sorted that out. I glued the
horizontal in place with 5-minute epoxy, using a couple of pins in front and back to hold the
piece in alignment.
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Attaching the horizontal stab.
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The vertical stabilizer gets mounted into a tab on top of the horizontal, making alignment
easy. Before gluing, I made a small triangle out of some scrap ply, so that I could make sure
the part was positioned at ninety degrees to the horizontal stab. Again, I used 5-minute epoxy
to glue it into place.
The pushrods included with the Habicht come with music wire already installed in the housings.
The ends of the wires are pre-bent, so there's no way to adjust the length of the pushrods, or
the linkage. Adjustment is actually accomplished by the positioning of the servos, which I'll
get to later. Small dimples in the rear fuselage mark the pushrod exits, so holes can be made in
the proper places.
The pushrod housings get glued in at the tail, and at the former in the forward fuselage. Care
must be taken to make sure the correct pushrod is going to the correct surface. The pushrods
are the same length (though the instructions incorrectly state that the rudder pushrod is
longer), but they can be told apart because the wire bends on the elevator pushrod are at ninety
degrees to each other. On the rudder pushrod, the bends are parallel.
Once the pushrods are installed, the bent ends can be placed into their respective control
surface horns (you can also do this before gluing them in place to check the alignment). They
are secured in place on the surface horns by gluing a small piece of extra pushrod housing
material to the ends of the wires.
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Rudder horn and pushrod.
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Elevator horn and pushrod.
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Time to mount the servos. I used Hitec HS-55's, which are the servos recommended in the Hobby
Lobby catalog. They fit perfectly, though you could probably also use HS-50's to save a bit of
weight (about 0.16 oz total savings). However, when the wing is installed, it actually pushes
down on the pushrods, causing some extra friction. I couldn't get rid of the friction; so I
figured the higher torque of the HS-55's was the way to go (and it turns out they work well).
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The pushrods are visible at the rear of the wing saddle. The wing will push down on them
when it's mounted.
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As I said earlier, there's no way to adjust the linkages on the pushrods (because there are no
linkages). That means that everything must be adjusted by the positioning of the servos. To
accomplish this, the cutouts in the servo tray are made extra long. The servos themselves are
first mounted (using a bit of CA) into their own little ply sub-frames. The servo/frame is then
placed into the overly large slot in the servo tray. With the pushrod hooked up to the servo
arm, the servo/frame can be slid fore or aft in the tray until the control surface is in
neutral. Once everything is lined up, the frame gets glued to the tray with a CA.
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A servo in its sub-tray.
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Servos and pushrods in position.
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Once I was satisfied with the movement of the rudder and elevator, I set about hooking up the
tow release. There's obviously no room for a dedicated tow release servo, so the Habicht is
designed to have the release coupled to the elevator servo. A small pre-bent pushrod gets
attached to the elevator servo arm, on the same side as the elevator pushrod. The rod goes
forward, into the small tube in the nose, where it exits the fuselage and engages the
wire hoop. So, whenever the pilot inputs up elevator, the release pushrod is pulled back away
from the hoop. The idea here is to set it up so that full up elevator will pull the pushrod
back far enough to completely disengage from the hoop, thus releasing the tow line.
The release pushrod that came with the kit was actually a bit too short to reach the servo arm
and still engage the hoop. This wasn't too much of a problem, as I was able to take some
similar sized metal wire I had on-hand around and bend it to the proper shape, then cut it to a
good length.
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The elevator servo, with the elevator pushrod (left) and the tow release pushrod (right)
attached.
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The instructions suggest setting up the release so that it disengages at full up elevator. I
decided to get a little tricky with my computer radio, though. I set up the elevator throw so
that pulling the stick back all the way would give enough up throw for flying, but still not
allow the tow release to open up. I then programmed a mix in my radio that would move the
elevator servo full travel only when I pulled back on the spring loaded trainer
switch…more like a true release switch. Of course, hitting the switch would still cause
full up elevator, but at least there would be no chance of accidentally causing a release when
too much up elevator was applied.
Wing
There's not much construction involved with the wing. There are a couple of thin ply dihedral
braces that need to be glued into a slot on the bottom. I had to sand these to
fit first, and then I used wood glue to glue them in place. I've found that wood glue works
well to bond wood to foam. Any excess glue that gets squeezed out of the slot can easily be
cleaned up using a damp rag. After the braces are glued in place, a flat piece of precut ply
(sort of a spar cap) is glued over the slot. I used epoxy for this step.
The Habicht's wing has a squared off leading and trailing edge, but I decided to round off
former. I doubt this has much aerodynamic effect, but it looks nicer.
Next, the instructions show how the wing fairing, which was cut out in the very first step, is
to be glued to the top of the wing. The wing is then supposed to be glued to the fuselage.
When I trial fit the wing to the fuselage, though, I found a problem…the fairing did not
sit flush with the top of the rest of the fuselage. The problem is that the wing saddle is not
cut with the dihedral angle of the wing accounted for. I had to sand the edge of the saddle to
an angle that matched the dihedral, so the wing would sit flat, and thus a bit lower. I also
had to sand out a notch for clearance of the ply spar cap on the bottom of the wing This
allowed the upper fairing to fit flush.
I was about ready to glue the wing to the fuselage, but I was somewhat concerned about getting
the center of gravity correct. I began to suspect that I might have to place either the
receiver or the battery under the wing to get the plane to balance properly. Just in case that
was necessary (and it was), I placed a small balsa bulkhead behind the main former. That way, I
could place the battery or the receiver in the space under the wing without worrying about it
sliding backwards into the tail.
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I added this small balsa bulkhead below the wing saddle.
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I glued the wing to the fuselage with five-minute epoxy. Once that was done, I checked the fit
of the wing fairing again. I did a bit of sanding along the bottom of the fairing to get it to
fit flush, and then glued it in place with epoxy.
Finishing
The Habicht comes with some self-adhesive decals, which are easily applied, and add a nice
finishing touch. It also comes with a small clear plastic windshield. Unfortunately, I did a
bit of a bad job installing this. My suggestion for installing it is to first remove the small
"spikes" that are included at the bottom of the windshield (I attempted to use the spikes to
anchor the windshield into the foam before gluing it, which did not work out too well). Just
tape the windshield in place at the corners to hold it down temporarily, and then apply a thin
bead of medium foam-safe CA around the base. Finally, remove the tape, and apply a bit of glue
underneath the corners.
Final setup
I set the elevator for the recommended throws, and set the rudder for as much throw as I could
get, which is my standard practice for rudder-elevator controlled planes.
My intention was to use a very small 150MaH NiMh battery pack for the Habicht. This would be
compact enough to fit easily in the nose, and hopefully not cause the plane to be too nose
heavy. Unfortunately, the pack took a bit longer then expected to arrive, so I decided on a
temporary compromise, and used a physically larger 110MaH NiCad pack for the initial flights.
This pack was large and heavy enough that I had to place it under the wing in order for the
plane to CG correctly. I placed the Great Planes ElectriFly park flyer receiver in the nose.
The plane was a touch tail heavy, but placing a couple pennies up in the nose fixed that.
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110mAh NiCad in place under the wing. Balsa stick glued to the former is a temporary
measure to keep the battery in place.
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Looking forward into the nose from behind the cockpit. The tow release pushrod can be
seen to the left, and the receiver to the right. There is not a lot of room, but it
does fit.
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Final flying weight with the large battery was still less then 4.2 ounces, a touch under the 120
gram specified weight. Wing loading at this weight is 3.90 oz/sq foot.
A while after the first couple flights, the little NiMh square battery pack arrived. It turns
out that I still had to place the smaller pack underneath the wing to get the plane to balance
out. I also had to remove a small portion of the leading edge where it passed through the
fuselage to get the battery to fit through the opening in the former (in retrospect, a flat pack
would have worked better). Overall, it shaved off a few grams of total weight.
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The 110mAh NiCad (below), and 150mAh NiMh pack(above).
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The NiMh pack, under the wing. Tissue paper holds it in place.
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Flying
I did not want to aerotow the Habicht on its maiden flight for obvious reason. Instead, I
figured I'd either look for a really small slope to test it out on, or else fly it at one of my
normal slope sites when the wind was really light.
An overcast afternoon without much wind arrived, so I figured it was a good time to test the
Habicht. Turns out that the wind at the top of the slope was a bit more then I would have liked
for this (5-10mph, I'd guess), but I still figured it wouldn't be too bad.
A gentle toss and off she went. The CG appeared to be just right, and it required just a tad of
down elevator trim. I must say there were no surprises. The Habicht behaved exactly as I would
expect a lightweight rudder-elevator sailplane to fly. It flew quite smoothly at first,
cruising back and forth across the slope, and seemingly right at home in the light lift (it
looked quite nice too). Rudder authority seemed good, though I did find myself using a lot of
it, especially when I was flying fairly close to the slope.
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The wind started to pick up and get a bit gustier. There was no problem controlling the little
glider, but I started noticing the wings flexing up and down quite a bit more then they had
been, so I decided to bring it in. I ended up landing in a patch of wildflowers. I'd
definitely advise landing the Habicht on something soft. Although the plane's light weight
probably makes it unlikely that it will break outright too easily, it would still be easy to
puncture the thin fuselage should it land on a sharp stick.
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On another light-wind day, I flew the Habicht at a different slope to wring it out a bit more.
This time, I removed the pennies from the nose to move the CG back a tiny bit. Starting off in
3-4mph wind, the Habicht again had no problems. The wind increased a bit, so I decided to try a
few aerobatics. Rudder rolls are pretty much impossible...the plane is so light that it just
can't pick up enough speed in a dive to complete a roll. Loops are possible, though they tend
to be very tight and not too scale looking, again because of the almost non-existent weight. I
had the most fun just cruising it low, skimming the flowers and brush on the face of the
slope. I had to laugh when the occasional bee or insect would chase the plane as it flew by.
One thing that surprised me was the penetration this little plane has on the slope. Now,
obviously this thing is no speed demon, and the wind was still relatively light. Still, I had
halfway expected to be flying backwards most of the time. This was not the case. I don't think
that flying on large slopes is what the Habicht is really intended for, though. It would
probably be much more at home above a small grassy hill or sand dune in a gentle breeze.
Aerotowing
Now I felt I was ready to do what I had originally had in mind for the Habicht: aerotow! Before
I begin, I should say that this was definitely a learning experience. Neither my friend Ken
(the tow plane pilot), nor myself had any experience aerotowing. Consequently, we did lots of
stuff wrong, despite my efforts to read up on the subject beforehand.
The tow plane we used was Ken's Hobby Lobby Speed 400 Super Cub. I was a bit worried that the
tug might not be able to fly at a slow enough speed that would be safe for the Habicht. The
Habicht has no spar beyond the inner section of the wing, so flutter and/or structural failure
was a concern if the tug flew too fast. As a bit of additional insurance, I put a length of
fiber tape on of the bottom of each wing, from root to tip, hoping this would give a bit more
strength.
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Belly view. Fiber tape reinforcement can be seen on the bottom of the wing.
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The Cub was converted into a tug by adding a tall towing mast on top of the wing, right at the
center of gravity (which is, as it turns out is WRONG). The hollow carbon fiber mast had a
notch on the top to accept the towline. A servo-actuated music wire rod running through the
tube engaged the towline. If an emergency release was required, the rod moved downward,
disengaging the line. The mast itself was so tall because my friend was worried that the
towline might foul the balanced rudder of the Cub. That extra height and the mast's position
would prove to be an issue later.
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Hobby Lobby Super Cub tow plane. Note large towing mast above the wing: It's way too tall
and in the wrong position.
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The final preparation was to build a dolly to use for the initial ground roll of the glider.
Taking a clue from the Hobby Lobby website, I made a small two wheel dolly, using scrap blue
foam, a carbon fiber rod for an axle, popsicle sticks, and some light foam wheels.
With everything ready, we had only to wait for the wind to die down before our first aerotow
attempt. When conditions were good, we set up on a small, rough, asphalt service road in the
middle of the field. Following the Habicht's instructions, we used about 15 feet of
monofilament line as our tow cable.
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The Habicht rests on its dolly, waiting for the next tow..
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The first aerotow was a semi-success. We set the Habicht and Cub up on the runway. Ken eased
the throttle forward on the Cub, and the Habicht/dolly began rolling forward. Ken added a tiny
bit more power. The Habicht tilted forward as if it were a wheelbarrow, then promptly pulled
itself off the dolly and skidded onto the ground. Good thing it had that plywood skid! The
little glider was now being dragged across the asphalt, but doing so in a pretty stable fashion,
so I told Ken to just keep going. A few seconds later the glider lifted gracefully off the
ground and assumed the classic high tow position behind the tug.
The tug lifted off smoothly and accelerated, while the Habicht remained stable, without any
substantial wing flex showing. Tug and glider continued to climb out, and Ken began a slow turn
to the left. At this point, there was a break down in tug/glider pilot communication. I
expected Ken to complete a 180-degree turn, and then fly parallel to the runway for a bit before
turning back the other way. Instead, he continued his turn, intending to circle continuously.
At some point, the tug turned a bit too steeply, and the towline must have gone slack. When
the slack was taken up again, the Habicht was yanked violently nose down, and then rolled
inverted. In the meantime, the tug, being attached to the towline by a huge lever at the
c.g. (that tall mast), got yanked nose up. I hit the tow release...it worked! The Habicht
almost immediately flipped upright and assumed a slow glide, as if nothing at all dramatic had
just happened. The tug immediately recovered as well, and both airplanes landed without harm.
Okay, chalk that one up to experience.
Did I say both planes landed without harm? Well, not quite. The glider yanked the tug hard
enough that the tow mast was nearly ripped from it's mooring on top of the wing. So, w e make a
few changes for the second day of aerotowing. The tug's tow mast was cut down to a little less
then and inch high. On the dolly, I added some "stoppers" toward the front of the cradle to
block the leading edge of the glider's wing and prevent it from pulling itself off too early.
Finally, we extended the towline from 15 feet to about 25 feet.
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Sailplane and tug on the runway.
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Weather conditions on the second day were not quite ideal. It was overcast and there was a
slight and variable breeze. We tested the air out first with our electric gliders, and it
seemed smooth, with the occasional patch of very weak lift, and little or no sink. The problem
was that the breeze was blowing across our little runway.
As light as the crosswind was, it proved to be a problem for our tug. The Cub, while a good
flier in its own right, has no ailerons, and it is not an ideal glider tug...but it was all we
had. We set up the planes on the runway, and waited for a lull in the wind. When it arrived,
my friend powered up the tug and we were off. The glider rolled forward on the dolly about 5
feet, at which point the glider and dolly "wheelbarrowed" again. This time, instead of the
falling off the dolly and getting dragged across the pavement, the glider jumped into the air.
Not exactly a smooth liftoff, but it worked. Unfortunately, a gust of wind arrived just as the
tug was leaving the ground, picking up one of its wing tips and sending it veering off the
runway. It appeared the Cub wasn't going to make it, so I hit the release switch, and the
Habicht immediately let go of the towline. Of course, hitting the release also gives full up
elevator, and the Habicht ended up pointing it's nose straight up into the air, stalled, and
went straight down into the dirt. In the meantime, the tow plane continued into the air
successfully. The nose of the Habicht was a bit creased, but there was no other damage (later
on, I followed the instructions, and removed the creases with careful application of
heat from a sealing iron and a bit of light sanding).
Once the Cub had returned, we hooked it up again and waited for another lull. This time, we
decided that the Cub should use more power on take off. We had been worried about overstressing
the glider's wing, but it appeared pretty solid in the previous days' aerotow. We figured it
would be safer to take off with a bit more airspeed, given the prevailing conditions. Another
lull arrived, and the tug powered up quickly. The Habicht leapt from its dolly again, just as
the tug once again veered off to the left. This time, though, the tug got off the ground
cleanly and I remained on tow. Ken established a stable climb and we were off.
As Ken began the first turn to the left, I applied a bit of right rudder to keep the glider on
the outside of the turn and keep the towline taut. This, in combination with the increased line
length, smoothed things out quite a bit compared to our first aerotow flight. We continued
parallel to the runway for a while, and then Ken started another left turn, which again went
smoothly. This was fun! The tug and glider continued upwards in one more racetrack pattern
before I toggled the release.
The Habicht released cleanly. As you might expect, the sink rate of the Habicht is pretty low,
and it smoothly floated around up there for good amount of time, especially considering there
was little if any lift to be found. As I got low enough to start think about a landing, I
encountered some very small patches of lift. I was already a bit too far downwind to really
work the lift, and I was worried I might not make it back (the wind was picking up at this
point). However, based on the amount of vertical movement I was seeing as I flew through it,
I'd say it wouldn't take very much of a thermal to quickly get the Habicht to speck-out
height. The main limitations would be the plane's small size and the lack of visibility this
imposes, not too mention the light weight and not exactly slick wing profile, which would
prevent it from penetrating upwind very far. Despite the headwind, though, I made progress,
skimming across the access road and landing almost right in the spot where I had started.
Finally, a fully successful aerotow!
The next flight was pretty similar to the first, including the rather sketchy takeoff. When we
got to a good height, I waited to release until the planes were almost overhead, going upwind.
It looked like they were going fairly slowly, but I didn't really take the increased wind speed
into the equation. Consequently, when I released, the glider had quite a bit of airspeed.
Since hitting the release also results in full up elevator, the glider immediately did a
complete loop. Oops! Must remember to slow down the tug a bit before release next time. The
actual gliding flight went very smoothly again, and ended with a landing on the runway.
When we were done flying, Ken commented on how hard it was to control the tug while towing the
glider. The Cub had a tendency to go nose up when the power was increased. Also the rudder was
over controlling the Cub, to the point where it would turn much to steeply when even a small
amount was applied. I took a look at some aerotowing information on the Internet and in some
magazines, and realized that the tug had the tow release mounted in the wrong place. It should
have been at or near the trailing edge of the wing, not on the center of gravity. This was
probably the reason for the Cub's weird behavior. We decided it would be a good idea to move
the towline mounting point back, but unfortunately, we did not have time to try it out before
this article was finished.
In the mean time, here are a few things we learned about aerotowing. 1). It's lots of
fun. Actually, the towing part can be more fun then the gliding part. 2). Mount the towline
release on the tug near or behind the trailing edge. 3). A longer towline is better then a
shorter one. 4). Finally, if you use a dolly, don't cut corners: use four wheels, not two.
Although we were unable to try it, I'm fairly confident that if you are taking off from short
grass, you may not even need a dolly.
Conclusion
I definitely got a lot of enjoyment out of the little Habicht. It's a neat-looking glider
that flies quite well, especially for such a tiny plane, and a scale one at that. If you want a
scale airplane for a very small slope, you'll want to take a look at the Habicht for sure.
The whole electric aerotowing thing was quite a lot of fun. It was a bit challenging at first,
but the Habicht performed wonderfully. I think if you are into both gliders and park fliers,
and you have a suitable field, you owe it to yourself to give park-aerotowing a try!
For more information on aerotowing, check these sites:
Aerotowing primer: www.scalesoaring.net/tow.html
Aerotow releases: www.sailplanes.com/asher.htm
Aerotow links: www.scalesoaring.net/aerotowsheet.htm
Jun 27, 2003, 11:56 PM
Mike Heer
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