Alternate power system for P38 ( is it possible?) - RC Groups
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Nov 19, 2001, 12:34 AM
Registered User

Alternate power system for P38 ( is it possible?)

I have a P38 and althought the preformance is good the flight time are short (approx. 2 min.). It weighs 2.5 lbs ready to fly, wing span is 50", wing area is 378 sq. in., motors are 7.2v 400, and battery pack is 800milliamp 10 cell pack. Can anybody come up with an alternate setup that wil give me longer flight times?
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Nov 19, 2001, 01:12 AM
high-speed freak
opualuan's Avatar
15oz/ft^2, huh? even with an 800mah pack? was this a gas arf?

well, the easiest way is to swap out that battery pack.. zagis (not the x) on speed 400's with a pack near that size only fly for ~5 minutes. maybe a 1700? at 2.5lbs AUW, twice the battery won't effect it too much...

any components you could swap for lighter ones?

are you running the 400's direct? I have heard that few people run 400's with a gearbox, but my be worth investigating... also may want to consider running a hot geared 300 setup, I don't have numbers but it may be within the possibility...

how much are you willing to spend? brushless motors will be more efficient, but for a dual-motor larger plane... get out your wallet!
Nov 19, 2001, 01:50 AM
Registered User
Brad Trent's Avatar
I fly a similar size twin, 48" span, 32 oz., using 2 x 7.2V 400's, geared 3:1, with Grish 9x7 props, using 10 x Sanyo 1400 AE's, and routinely get 7 to 10 min. flights, with lots of aerobatics. Are your motors direct or geared? Do you r.o.g., or handlaunch? Chucking it will save a lot of power for flight that is otherwise used in takeoff; not as cool looking, but lots more juice left for loops and rolls. With the gearing, mine is not super fast, only 45 - 50 mph per Motocalc, but thats fast enough to do anything I can handle.

Stating your prop size, gearing, and amp draw would help us zero in on your problem, and provide more specific answers. opualuan's suggestion on the 1700 AE's or the Radio Shack 1600 Nimh is a good starting point.Help us help you.

Nov 19, 2001, 11:51 AM
Official Boat Bum
Eddie P's Avatar
Blue, check out the new Panasonic 2000 NiMh cells. Apparently they weigh in close to what the old 1100's weigh. Also, the CP1300's weigh as much as your 800AR's and would give you longer flight times.

Generally look at the new cells that are out there. The heavy end would be the CP1700 (don't do the 1700AE) for this type of DD s400 setup. If you want to keep flying fast and still get duration stay w/ DD and go with one of these new cell types. But if you just want more duration a geared setup is going to give you that.

Nov 19, 2001, 01:57 PM
Gravity sucks.
mrittinger's Avatar
My twins run 2x6v400,Direct drive,with 5/5 APC props, and 8 1250 nicads or 8 2000 nimh cells.
Great performance, good run time....

Check out the P-calc program , run some numbers..
Nov 19, 2001, 02:13 PM
Registered User
Jim Ryan's Avatar
I fly my trusty old P-38 (similar size to yours) with 8 X CP-1300 cells. These weigh almost exactly the same as 800AR cells. I'm using 7.2V Speed 400 motors wired in parallel and 6 X 4 props. Flight duration is about 8 minutes with this setup.

As Mark commented, they've broiught out some great cells since this model first flew 5 1/2 years ago.

Nov 19, 2001, 04:00 PM
Registered User

P38 setup

Oops, forgot about the gearbox and prop. I am using 3:1 gear boxes with APC 9x6 props. Current draw is approx. 18 amps.
Last edited by BlueAce; Nov 19, 2001 at 05:13 PM.
Nov 19, 2001, 08:53 PM
Registered User
Brad Trent's Avatar
BlueAce, thanks for filling in the details. Your 18A draw sounds a little high, as my twin 7.2 400s only draw 14A while using a 9x7 prop and the same gear ratio and cell count as you, so a check of the gearboxes is in order, to make sure that they really are 3:1, and to make sure that the gear lash is set correctly, not binding or dragging. Lube the bushings and/or bearings of both the gearbox shaft and the motor with light machine oil, while you are checking them out. The real problem, though, seems to be your packs. They may not be performing up to spec., try a slow discharge/charge cycle at C/10, then check for changed times. Going to any of the newer high capacity, light weight cells will be the easiest fix, though not the cheapest. Several good suggestions on new cells are in the previous posts above.

Good luck , and let us know what you do and how it works.
Nov 20, 2001, 04:48 PM
Registered User

Thanks for the help

Thanks to all for the speedy response to my question. I won't be able to try any of the suggestions any time soon, but I will post the results as soon as I can.