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Apr 03, 2016, 10:01 AM
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Build Log

Scale Build-Off 5 - Jemco 51" P-39 Airacobra


This kit came up for sale a couple of months ago. There is not much on the P-39, but threads on the P-51 from the same line seemed to indicate it would be a good intermediate build in a larger size - exactly what I needed to feel confident in tackling some other kits in the wings. So, please do not be shy with your tips and advice.

The fuselage is an early self-rigged affair with assembly instructions but no plans. There is a plan for the wing. I am making templates for all parts and intend to add them to the OuterZone. (The Jemco P-51 is there already).


Full-specs from the box.

Other Links:
Build-off 3 thread for different P-39 that has lots of reference material
Last edited by kdahlhaus; Apr 03, 2016 at 10:21 AM.
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Apr 03, 2016, 10:07 AM
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Beginning the Wing


I've been cutting out, cleaning up, and tracing most of the parts over the last few weeks. Last night I was able to get some time in. Took about two hours to go from the parts to the partially finished wing. Following advice from a Harry Higley book, I ensured that the ribs slid into place with no binding or using the magnets to coerce them into place. Only the main spar and trailing edge are glued at this stage. The other spars are there to ensure alignment.

Only broke two ribs in the process!
Apr 03, 2016, 10:16 AM
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TBolt's Avatar
Great, I have one in my " one day I will build it" collection.
Apr 04, 2016, 11:25 AM
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I got hooked on Jemco/Mark's Models/Dynaflite "Fun Scale" P-51's with .40 glow power. They were aerobatic but flew like trainers. I still have a fun scale P51 and P40 in my stash and will convert them to electric power. The wing designs were not intended for retracts and would require modification of structure for them. I used those dummy landing gear covers that made them look scale. I also added balsa block to nose and rear fuselage for more scale-like look. Take care to avoid banana-shaped rear fuselage. You can enhance the scale appearance with some ingenuity.
Apr 04, 2016, 02:42 PM
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Thanks E-Chal. I'll pay attention when starting the fuselage. The outerzone plans page has a photo of the P-51. It looks good.

Mine will be electric. I haven't run the numbers yet, but have some ideas.
Apr 08, 2016, 08:40 PM
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Started on the left wing tonight.
Apr 08, 2016, 11:23 PM
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No Retracts?
Apr 09, 2016, 07:39 AM
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Not this time. Time, cost, the plane being stand-off scale, and it not designed for them are the reasons.
Apr 09, 2016, 11:30 AM
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Fun scale model series are light weight with generous wing area which makes them easy to fly, aerobatic and easy to land. You will need to build a battery compartment with access in the top of fuselage. Battery pack will need be just behind the firewall and you may need to add some lead to the nose to get CG right. Expect that nose wheel mount may interfere with battery compartment space, battery compartments and hatches are always somewhat of a problem in converting gas models to electric power.

Pretend that your model is always in a traffic pattern and has gear down and locked. Dubro (?) plastic scale landing gear parts dress up naked LG wire considerably. Add LG covers of 1/16 ply to main gear for looks.

Try to locate servos, etc. forward and keep tail as light as practical. ( Weight of nose wheel/mount should help with CG ) . Will watch this build with much interest.
Last edited by E-Challenged; Apr 09, 2016 at 11:56 AM.
Apr 09, 2016, 01:18 PM
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Thanks for the tips - I appreciate them very much.

I've never added a hatch to an existing plane before - do you have any more suggestions, details, or pics?

I've been considering replacing the solid tail surfaces with built-up ones. Siriuis80 cut over 50% of their weight by doing so in his Bristol M1.
Apr 09, 2016, 02:56 PM
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In my conversions I usually place the battery where the tank would be located, and make a hatch.
Apr 10, 2016, 09:16 AM
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I am cutting out and tracing the fuselage parts now so I'll keep your thoughts on the hatch / battery location as I do that.


Starting to work on the drive system. The box says 3 3/4 - 4 lbs weight. I am assuming that is AUW with about 8oz fuel. .19 to .40 engines were recommended. So the estimate I'm using for weight of the frame and electronics is about 2.9 lbs:

upper end:
4.0 lbs - 0.805 lbs (enya .40 engine) - 0.44 lbs (8 oz fuel) = 2.76 lbs frame and electronics

lower end:
3.75 lbs - 0.375 lbs (enya .19 engine) - 0.44 lbs (8 oz fuel) = 2.93 lbs frame and electronics
Apr 10, 2016, 09:26 AM
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The first candidate setup so far is a Turnigy G25 Brushless Outrunner 870kv turning a Master Air Screw 10x7 3 bladed prop on 3-cells.

MotoCalc thinks it will work well:

Motor: Turnigy G25 Brushless Outrunner 870kv; 870rpm/V; 2.4A no-load; 0.03 Ohms.
Battery: Zippy-H 2200 2C (20C); 3 cells; 2200mAh @ 3.7V; 0.0125 Ohms/cell.
Speed Control:Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Master Airscrew 3 Blade 10x7; 10x7 3-bladed (Pconst=1.31; Tconst=0.936) direct drive.
Airframe: Jemco P-39; 490sq.in; 60.2oz RTF; 17.7oz/sq.ft; Cd=0.056; Cl=0.47; Clopt=0.67; Clmax=1.24.
Stats: 75 W/lb in; 62 W/lb out; 19mph stall; 27mph opt @ 59% (23:53, 88F); 32mph level @ 67% (19:30, 92F); 963ft/min @ 24.2; -247ft/min @ -6.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (28.2A) falls approximately between the motor's maximum efficiency current (28.1A) and its current at theoretical maximum output (165.8A), thus making effective use of the motor.

Aerodynamic Notes:

The static pitch speed (51mph) is within the range of approximately 2.5 to 3 times the model's stall speed (19mph), which is considered ideal for good performance.
With a wing loading of 17.7oz/sq.ft, a model of this size will have trainer-like flying characteristics. It would make an ideal trainer, for use in calm to light wind conditions.
The static thrust (42.3oz) to weight (60.2oz) ratio is 0.7:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (25.2oz) to weight (60.2oz) ratio is 0.42:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.

Although the estimated top speed is only 46 mph. That might be a little slow but this will be done as a fighter and not a pylon racer.
Apr 17, 2016, 11:47 AM
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The aileron hinges sit in a narrow strip of wood. That did not seem good enough so I added small blocks so they have more wood to anchor to.

Working on the leading edge. No pics of that though.

The weather broke this weekend so I may take a build session to put together a parkzone t-28 I picked up last Black Friday.
Apr 17, 2016, 11:56 AM
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I don't think that built-up tail feathers will be needed for CG reasons with this design nor would they be much lighter.


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