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:
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
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 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.
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.
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.
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.
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
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