Recommendations From an Experienced Elektro-UHU Pilot
by Steve Kranish.
Recommendations From an Experienced Elektro-UHU Pilot
May 01, 1996, 01:00 AM
The Graupner Elektro-UHU is an intermediate level electric powered sailplane
with a conventional built-up wing and a moulded plastic fuselage.
The Elektro-UHU is very similar to the Elektro-Junior (V dihedral) and
Elektro-Junior Sport (polyhedral), sharing an identical fuselage and similar
wings. The differences are the dihedral setup, wing area (they are about 5%
larger), and level of prefabrication (they come with prebuilt and covered
The most recent Hobby Lobby catalog shows a new version called the
'Bussard', that is a bit larger still (584 sqin).
There was also a version called the 'Chip' which had a smaller wing and
ailerons. Hobby Lobby no longer sells it, but you may find one in a hobby
shop. There may be other versions sold in Europe - Graupner gets a lot of
mileage out of its fuselage molds.
The comments below should apply to all versions of the Elektro-UHU.
Some may criticize the E-UHU for being a bit heavy - 50 oz for 500 sqin -
but the additional weight is in a sturdy fuselage, which is the most
vulnerable part of an electric plane. Mine has survived a few very abusive
landings, and if you follow the suggestions below, it is very repairable.
The E-UHU goes together very quickly. I built mine in about a month of
evenings, and successfully flew it the first time I took it out. That was in
November 94, and it is still in use, generally getting 4 to 8 flights a week
when the weather is cooperative. It is a VERY easy to deal with airplane.
The wing goes on with two screws, and the motor battery goes in through the
'canopy' hatch. It takes only a few seconds to swap the battery, so I go to
the field with four charged batteries, and put in 4 trouble free flights in
45 minutes to an hour. Flight times on a 1000mAh pack range from 7 minutes
in totally mediocre air to over 20 minutes in thermals.
While no plane is truly perfect, I have to say that I _LIKE_ this plane, and
it has the mileage on it to prove it. If anything happens to it, I will
probably buy another to replace it, simply because it is a low hassle plane
to work with.
Recommendations from an experienced Elektro-UHU pilot:
WHAT YOU NEED IN ADDITION TO THE KIT:
Graupner Speed 600 BB 8.4V motor
Graupner noise suppressor recommended
Speed control with brake, Astro Flight 211 or similar
SPST safety interlock switch
Astro Flight 13 gauge superflex wire
2 pairs Astro Flight zero loss connectors
Graupner 8x4.5 Folding Prop OR all the parts for Aeronaut 8x4 Folding Prop
Sanyo 7 Cell 1000mAH SCR battery pack(s)
a good _PEAK_ charger
150mAH Sanyo Flight pack
2 Futaba S133 or HiTec HS-80 or similar micro servos
Hitec/RCD 5 Channel Micro RX
Aluminum or brass tubing to line the wing holddown bolt holes
2 Oz Fiberglass cloth
Poly ZAP CA glue
additional 1/16" plywood
spare wing bolts
spare wing pin
If you have bought the kit version, that requires building the wings:
Approximately 1 roll _STIFF_ covering material, such as Oracover or Monokote
Smaller amounts of covering material in contrasting colors
your favorite balsa glue; I still like Ambroid
The suggested power systems listed on the box are usable only in still air,
and if you don't really want to gain any altitude. The plane does not need
to be overpowered, but there is _NO_ reason to underpower it. You do need
power for a reasonable climb rate. If the climb rate is slow, your flights
will end up being short.
I have gone through many of the suggested systems, and can recommend:
7 Cell 1000mAh SCR Sanyo battery pack. (Don't even think about 6 cells.)
Don't waste your time or money on the so-called 'Sport' packs. Use SCRs
until someone comes up with a better cell.
Graupner Speed 600 8.4V BB motor. (The 7.2V motor is a wimp.)
Use the Grauper 8-4.5 prop. Balance it carefully - it will need it - and buy
some extra spinners. You will break a lot of spinners when the prop blades
catch on the grass and bend forward.
Much better yet, forget about the Graupner prop and use the Aeronaut
equivalent. It is a pain to purchase, because you need a whole lot of
separate parts, but the resulting prop is quieter, may produce more power,
and will not break the spinner. The prop nut will ONLY fit in the spinner if
the sides are parallel to the screw holes. The best part is that mine
balanced perfectly straight out of the bag.
Use Astro flight Connectors. The Tamiya (Molex) connectors should be cut off
and thrown away without even trying them.
Use VERY HEAVY gauge wire, such as the Astro Flight 13 gauge super flex.
Although this is not a very high performance system, you CANNOT afford to
waste energy on inadequate wiring or connectors.
I have purchased, but not tried, the new Graupner gear motor and prop. This
should improve the climb and motor run time. It was not available at the
time the kits were introduced, so it is not mentioned in the suggested motor
setups. It will add about 1oz to the nose, so I have been waiting until I
put together a motor control with BEC.
Before building the wing, stack the ribs, pin them together, and sand them
to a uniform shape. The die cutting is so-so at best. Note that the middle
rib is oddly sized because it is suppposed to use a cap strip under the
Don't even thing about leaving out the shear webs. A wing without shear
webbing will be reduced to scrap very quickly. I installed the shear webs on
to the lower spar, and then made up a tiny sanding block out of piece of
scrap spar with sandpaper glued to it. Use this to make the top edges of the
shear webs a tight fit to the underside of the top spar. WORTH THE TIME AND
Put aluminum or brass tubing in the wing hold-down screw holes as liners for
the holddown screws. If you don't, the screws will not shear - they will
just rip the wing apart. Good idea for all planes.
If you use the nylon screws in the kit, order some spares from Hobby Lobby.
You will need them, and being metric, they are hard to find elsewhere. If
you don't have spares, the day will come when you wish you did.
Order a spare wing pin, too. You will probably lose on in dry grass, and
never find it.
Keep it light. It is heavy enough WITHOUT your help. I used Oracover on the
wings - a bit heavy, but the thin Graupner wings need the added stiffness. I
would not recommend the overly flexible low temp films. The wing on the
'Bussard' should be stiffer.
I made a very nice wing bag out of felt cloth. I traced the wing onto a
piece of paper, added about 3/8" to 1/2" as a seam allowance, and cut out
three pieces of felt (the piece in the middle is a separator). Stitch
together around all but the root end, and add a flap to cover the root.
Makes it MUCH easier to handle the wing.
Don't think about replacing the tail surfaces with built up structure. The
plane will be nose heavy; you will need the tail weight.
Use covering material hinges so that there are no hinge gaps. I actually
hinge with the covering on one side; on the tapered side I use an extra
strip of covering material about 3/4" wide as the hinge. Yes, the hing is
slightly off center, but it does not matter. I have never had one of these
FUSELAGE AND PUSHRODS
If your inner pushrods do not have a thin wire for their entire length,
throw them away and purchase a replacement set of pushrods from Hobby Lobby.
The pushrod material is surprisingly brittle; the wire keeps it from
breaking. I have had one of the original pushrods fail in flight (at about
10 feet, I think).
Use a 2mm nut (from the servo end of the pushrods), or if at all possible,
use a 2mm die, to cut the threads on the rear of the inner pushrods. Don't
try this after they are installed in the plane, and treat the pushrods
gently - they are actually brittle, and you don't want to weaken one enough
for it to break in flight.
Make sure that the flexible pushrod housings are secured at BOTH ends. The
plans do not indicate how to do this - you need to add a piece of wood
across the inside of the fuselage in back of the servos, and run the
pushrods though it. Glue securely. If you don't, you won't really have any
control over the plane. Don't expect a friction fit to be adequate unless
you like REAL excitement.
Make sure that the wing hold down plate cannot cause the fuselage to flex. I
would HIGHLY recommend reinforcing the wing saddle area with two layers of
fiberglass (2oz) applied with Poly-ZAP. Sand first with 220 sandpaper to
roughen up the wing saddle surface. If the fuselage can flex at the wing
holddown, it will eventually break. I speak from experience! I now have a
strip of FG across the top of the wing holddown plate, pieces on the inside
of the fuselage to hold the holddown plate to the fuselage sides, and a
piece covering the entire wing saddle. Makes a HUGE difference in stiffness.
The same is true of the motor mount on the nose. My UHU now has 2 layers of
2oz cloth plus a layer of 1/16" plywood on the OUTSIDE of the nose - makes
the motor mount VERY stiff. Cut two pieces of large dowel (1 1/8" - like
trashy closet rod material) to use as spacers so that a C-Clamp can be used
to clamp the entire nose assembly together. If you put on a heavy layer of
Poly Zap it will not cure until under pressure.
Poly-ZAP is probably the best glue to use for repairing fuselage cracks.
Using anything else first only makes it impossible to get a good bond with
Poly Zap after the first repair fails.
If you fly on wet grass or snow, add a skeg. Most of the damage done to my
UHU has occured AFTER a sucessful landing, sliding across the snow or grass
with no control. A skeg can be made from a strip of spruce on the existing
plastic fuselage skid, and two wing tip skids glued into the wood. DO NOT
drill holes in the fuselage here; holes will instantly become a source of
Remember that every opening or hole in the fuselage is going to be a source
of cracks. Use ROUND holes if you must. I use a safety interlock switch
mounted through a 1/4" hole in the fuselage, but my radio on/off switch is
mounted inside. The square hole it would require is a recipe for disaster.
Think about reinforcing the openings for the hatch and wing. A lot.
EQUIPMENT INSTALLATION AND BALANCING
Watch the CG. You will probably have to put both the radio battery and rx
under the mezzanine (servo mounting )plate. I have nothing but the motor in
the nose compartment. The controller is below the wing LE, and the rx
battery (150mAH) is behind the servos. If it is nose heavy, the climb is
I used two Futaba S133 servos, mounted in the rear of the servo opening in
the mezzanine plate. Don't even think about larger servos. I removed the
piece of plywood from the servo opening, trimmed it to fit my servos, and
glued it back in.
With small servos, the servo leads are too short to reach the receiver after
they have been installed. As a result, radio installation is rather tricky.
Connect the ENTIRE radio system together on the bench. Lay it out on top of
the plane to make sure the wires can reach - you may need an extension on
the battery or switch. Don't forget to connect the motor controller. Push
the radio battery under the mezzanine plate - it will go all the way to the
rear, behind the servos. Next push the servos in, then the receiver. Gently
push everything through until you can fish the servos up through the servo
opening, one at a time. _YES_ it is a pain, but it works. I have done it
many times. I have the switch mounted on a cross brace in the rear of the
I put a small piece of balsa triangle stock in front of the servos as a stop
for the motor battery. You need a fixed stop to make sure the CG comes out
the same each flight. I put some styrofoam strips, with holes cut in it, on
either side of the battery. Put a few rubber bands on the battery to hold it
tight in between the styrofoam strips. The battery should _NOT_ move in a
normal landing. If it comes out, it will probably wreck the canopy, which
Hobby Lobby charges $25+ to replace. Ouch!
You MAY need additional downthrust, especially if the CG is forward.
Check the wind direction. If there is any breeze at all, make sure you are
launching into the wind. Give the plane a smooth, level heave. It may sink a
bit before picking up flight speed.
Climb angle is reasonable, but not steep. Don't let her hang on the prop.
If the nose gets too high during the climb, the UHU tends to wobble off on a
wingtip. Not really stall, just kind of wobbles off.
I have never been entirely happy with my trimming of the UHU, and I really
don't know why. Windy weather is more of a problem - in calm weather she is
very gentle and easy to fly. I just have not figured out the appropriate
trim changes for windy weather.
Power off transition will require some uptrim. If it requires a LOT of up
trim, like more than you have available, something else (like the CG or
downthrust) is wrong. Remember to take out the uptrim before restarting the
I have done many, many 12 minute+ flights with 2 to 3 climbs and minimal
thermal activity. I have done a number of 20 minute+ flights in thermals.
Even in dismal air I can get 7 to 8 minutes on a battery pack.
The UHU will pass right through a good thermal without giving you so much as
a hint. It will not do the typical raised wingtip or wobble that you may
experience with a 'floater' type glider. You need to know where to expect
rising air. She will thermal, but you have to find them.
In general, the E-UHU is a nice plane to fly and work with. Lands faster
than a floater glider, but not at all difficult. Be aware that she will
REALLY accellerate in a dive, even a shallow one. I find it best to take OUT
the normal gliding uptrim when the plane is near the ground, either in
preparation for a restart or for landing. During landing you _REALLY_ need
to be careful to keep the airspeed _UP_ to avoid stalling.
If you have to do tight circles to burn off altitude when setting up a
landing, keep the circles _FLAT_. The E-UHU will stand up on a wingtip
easily - but when the breeze is directly at the TOP surface of the wing, it
will effectively stall out and drop like a rock. This is why you need to
keep the airspeed _UP_, and it is probably the worst habit of an otherwise
very nice airplane. If you zoom the plane into a shallow dive, you should
have enough airspeed to do tighter circles.
The prop can be used as an airbrake during the landing approach by just
bring it up to idle. It will produce more drag than thrust, and help to slow
the plane down. Just make sure you cut the motor before it touches down.
Stay charged, and Good flying!
Steve Kranish, Beverly MA USA