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Recommendations From an Experienced Elektro-UHU Pilot

by Steve Kranish.

Recommendations From an Experienced Elektro-UHU Pilot
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 
wings).

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
prop balancer
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
Switch Harness

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

POWER SYSTEM

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.

WING

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 
sheeting joint.

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 
TROUBLE!

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.

EMPENNAGE (TAIL)

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 
fail.

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 
cracks.

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 
poor.

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 
hatch area.

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.

FLYING

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 
motor.

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
skranish(at)world.std.com
 

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