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Old Dec 18, 2010, 04:27 PM
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USA, NC, Matthews
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HobbyCity/Prodesign 70mm L-39

I saw the HobbyCity L-39 was available in the HobbyCity USA warehouse for around $195 delivered. Since I already had a power system and radio components that would work, I decided to give it a try. I found a thread on it but it didn't give much info. I also found out that this was a ProDesign kit and found the instruction manual for it.

Component used;

Sapac 70mm fan
Sapac 3200kv motor
HobbyKing 90/100 ESC
Futaba FASST radio
Hyperion Digital Servos (DS13TMB for elevator and DS09SCD for ailerons)
HobbyCity Micro Retracts
Castle Creations 10A BEC
Turnegy 4s 3000 20-30c & Turnegy 4s 4000 40-50c lipos

Initial impressions of the plane are very good. The fuselage all fiberglass and is very nice and has only slight imperfections. The horizontal stab and wings are covered balsa (the wings balsa sheeted foam). The paint job is very nice as well. I was especially surprised at the quality of the covering on the flying surfaces. There wasn't a singe wrinkle anywhere. The attention to detail was not expected. For example, the balsa servo covers for the wings are already covered and fit into the countersunk opening perfectly.

The overall design is very nice as well but there are a couple issues. The horizontal stab sits about 1/4 inch too low and is in the flow of air from the fan. I modified the stab so it isn't as bad now. Hopefully it will not be an issue when I make my mylar thrust tube. Also, the Sapac fan shroud would not fit. The one I have doesn't have the removable lip but I think the one with the removable lip would work. Fortunately the shroud that comes with the kit fits the Sapac motor and rotor very nicely. I did have to dremel out a hole for the motor wires to come though but it worked out perfectly.

Another thing I wasn't expecting is the size. A friend of mine is building a Sapac L-39 with the same power components and I was expecting this plane to be similar in size. It is about 7 inches shorter than the Sapac. Whats interesting is that the intakes are about the same size though. This think should perform very well.

Anyway, here are some pictures. I am waiting on the retracts and a couple other pieces so I am pretty much done with what I can do for now.
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Old Dec 19, 2010, 07:09 PM
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WoW!!!!! she went together fast!!!! Looking foward to the flight report!!!!
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Old Dec 19, 2010, 08:45 PM
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Crappy weather here so not sure when the maiden will be.

Anyway, a little more progress while still waiting on the gear and motor wires to arrive.

I have the fan mounted and the thrust tube installed. The thrust tube presses against the elevator control. I haven't tested with power on yet but it doesn't seem to effect the servo with power off. I have used an oversized metal gear ball bearing servo so it make work ok as is. I will test with wot and see how much stress that puts on the servo. If it is a problem, I will have the push rod exit the side and use a normal control horn on the elevator.

I have the struts and wheels selected and put together. I am using way oversized main gear and using dual wheels for the nose as I will be flying off of grass. These may or may not work on the retracts so I might end up using plain old wire struts.
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Old Dec 20, 2010, 01:54 AM
deltas are cool
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Tehachapi ,CA.
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the pic of the "stock fan" below the red wing tanks ..that is a WM 400 fan ..or a copy of one .is that the fan that comes with it ?
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Old Dec 20, 2010, 05:17 AM
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copenhagen Denmark
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It is a copy " Pure Rip Off" done by prodesign/proedf of the first wm 400 mk 1.
Claus
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Old Dec 20, 2010, 05:58 AM
Aka: Tom Jenkins
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Palm Beach County, Fl.
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The Jet looks like a SAPAC reject, take that back, it doesn't even resemble a SAPAC
Where did the diverter plates go?
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Old Dec 20, 2010, 07:52 AM
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Definitely not a Sapac copy/reject etc. I compared it side by side with a Sapac and there is nothing the same, not even close in size either.

Yes that fan came with it but I would never use the included stock rotor. It didn't even come with a nose spinner and has a nic in one of the blades. It may be a clone of a proven design but it isn't anywhere near the same quality.
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Old Dec 20, 2010, 10:23 AM
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What is its all-up-weight with battery? Mine is 3lbs 15 Oz. with air retracts and 4S 4200mAh battery and 3 ounces of lead in the nose. I put a real Wm400 Mk2 in it. Has not flown yet.
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Old Dec 20, 2010, 10:37 AM
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Don't know the weight yet as the batteries and gear haven't arrived yet. What retracts are you using? You are using a 4200mah 4s and it is still tail heavy? I was thinking the 4000mah may be a bit too heavy and was considering using some 3000mah 4s I have.

How do you have your elevator linkage setup? Does it rub the thrust tube?
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Old Dec 20, 2010, 05:05 PM
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I have a WM400 Mk2 with a Neu 1112/1y. The elevator servo (HS65MG) does not rub (barely). I'm using the thrust tube for WM400 and it is just right. I had to sink the elevator servo pretty deep into the rudder and grind away some of the trailing edge of the fan shroud but the elevator is free and smooth.

My concern is the weight of this tiny jet. I haven't flown it yet due to the weather (or maybe that is a convenient excuse). I have run it up full throttle and it makes good thrust. My retracts came from China from from PTTSE (he sells here on RC Groups). They fit without any issues and look pretty good. We'll see. I had to reinforce the retract mounts with better wood and epoxy. The way it was delivered and my landings would have ripped the retracts out on the first try. Take a good look at them.

Ron
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Old Dec 26, 2010, 07:32 PM
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Not much new to report, still waiting on retracts and lipos from HobbyCity. FYI, they send any lipo orders that total 3200mah or more, via Singapore and it takes quite a bit longer. Probably won't order lipos from them anymore unless they are in stock in the USA warehouse.

Anyway, I wired up the motor/esc and ran it up in the plane. I noticed what sounded like slipping at wot. It may have been it losing timing but I am not sure. I also saw it shooting some sparks from the motor at wot. Maybe a bad bearing? I pulled it apart and all magnets seemed ok. Since I had another motor I swapped it out and it seems to work fine. Feels like it has almost 1 to 1 thrust ratio but that is with a 3000mah lipo instead of a 4000mah. Plus the retracts aren't in either. Still, it should go pretty good.

Also, the thrust tube doesn't effect the elevator controls at wot so it will stay as it is.
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Old Dec 30, 2010, 08:37 AM
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What did you use for the thrust tube?
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Old Dec 30, 2010, 08:45 AM
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Some mylar I had left over from another build.
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Old Jan 03, 2011, 07:52 PM
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Well, the retracts were a no-go. I got them installed ok but by the time I made clearance for the oversize wheels (to be able to fly from grass), they interfered with the battery and left no place for a steering servo. So, I pulled them out and taped over the cut-outs (I will fix the cut-outs correctly if it turns out to be a good flier). I was contemplating a hand launch and ran it up a couple times while holding it and my TX. It seems to have plenty enough power for a hand launch but I think because of the wing area that it will sink too much before getting on step. Oh, by the way, it weighs 1348 grams (2.97lbs) ready to fly with a 3000mah 4s. I was able to get the battery mounted where it needs no dead weight added for the correct CG. I could make the 4000mah fit, but I think that would be pushing it.

So, I made a dolly for rolling take-offs and belly landings. I have an extra RX on the dolly and use the rudder channel for steering. It is set so it should rotate quite easily. I can adjust the dolly CG at the field as well. Anyway, hopefully this will work ok cause I really don't want to use a bungee. Will try to maiden this weekend, weather permitting.

After watching some of the Sapac L-39 videos with the same power system and being a pound heavier, this thing should perform very well...assuming it flies that is.
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Old Jan 04, 2011, 03:15 PM
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It flies. I've flown mine. But a short 20 seconds flight, it was too sensitive on aileron (+- 12 mm throws-copied from SAPAC, my kit had no manual), I was concentrating on keeping aileron under control, and had a nasty tip stall, which ended the flight prematurely. I had haoye mini retracts on mine, yours will fly better without the weight of retracts, I think.
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