Suitable motor for a topflite P39 Airacobra - RC Groups
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Jul 02, 2004, 09:49 AM
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Suitable motor for a topflite P39 Airacobra

Hi All
I'm building a Topflite P39 airacobra. Topflite suggest that the AUW is between 8-10 pounds. Wingspan is 63 inches.
The recommended engines are 61-75 2 stroke or 70-91 4 stroke...however, a glow motor will spoil the clean lines of the plane so I'm looking for suggestions on suitable motors, props cell count etc.....all suggestions greatly appreciated.............

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Jul 02, 2004, 04:07 PM
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ED209's Avatar
I'd suggest calling Kirk Massey at New Creations R/C ( ). Just tell him the specs for your plane, and he'll give you a recommendation based on the kind of flying you plan to do. I'm not sure what software he uses, but he can give to an estimate of flight duration, climb-out, etc. depending on the motor/esc/prop/battery combo you choose. He's helped me out with several of my planes, and I'm very happy with the results. He also is a great source for info about most electric flying products. Just be careful, because if you let him, he'll keep you on the phone for hours and run your long distance bill through the roof.

BTW - I'm not affiliated with Kirk in any way. I'm just very happy to recommend someone who is as excited about the hobby as I am.
Jul 05, 2004, 05:51 PM
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Sabrejock's Avatar
Mine flew at 8 lbs RTF. Astro 40G with 22xCP2400 and 12x10 APCe. There was extensive lightening during construction with swiss cheesing everywhere in the ply. Balsa spars. Nothing in the wings for landing gear. (Dolly or bungee launch). It was a real pleaser in flight with great performance and easy 5 mins of "worm burners".

I think that with today's system of a BL capable of ~800-1000W and LiPo's a weight of 10lbs could be targetted including the retracts and flaps. Unfortunately the retracts for this model are quite substantial, and the wheels are large. I needed my battery well forward so there would have been no space for the retract nose gear anyway.

With a lighter motor and LiPo's the problem might be more severe. Tex.
Last edited by Sabrejock; Jul 05, 2004 at 08:37 PM.
Jul 05, 2004, 07:58 PM
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vintage1's Avatar
Cheap grunt might be astro 60, and then spend the money you saved on brushless on a decent LIPO pack. If it needs noseweight anyway, whiy not slap in a heavy tough cobalt lump?

Pretty sure the 60 will do a kilowatt, and direct drive too, and its a slendrered motor than a big outrunner or geared Hacker etc. And can be mounted well forward.
Jul 05, 2004, 08:56 PM
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Sabrejock's Avatar
Regarding my earlier comments: I certainly don't advocate using LiPo's in any application where their attributes won't be fully utilized. Less weight won't be important in this case nor would increased duration. I gave the example solely as an option. This bird will fly gangbusters at twelve pounds for a good five to six minutes safely with GP3300's. Why do more? (You'd run out of worms to scare).

The DD AF 60 is a good combo, I'm using it myself in a scale bird. The thing with them is heat dissipation. They tend to get a bit hot and the Airacobra is a tough frame to ventilate from the front. I managed to do a fair job by holing the spinner backplate and slotting the actual spinner to allow air to flow through. Not noticeable when the motor is running!

Keep thinking, though, it really is a great bird in the air. Tex.
Jul 06, 2004, 04:18 AM
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vintage1's Avatar
I don't think I can agree that 'less weight won't be imporatnt' in ANY aircraft frankly.

Lower stall speeeds and freeedom from tp stalling at low speeds, or freedom from snapping at high are just two things on a warbird, plus better climb rate and teh ability to pull larger loops.

Geared Hacker B50L will pull similar powers tho, and so witll the MaxCim as far as I know. I don't fly stuff that big - can't land it round here - but those are the successful power trains I have seen people reporting on.

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