I've recently installed two Graupner Speed 480 ball raced motors into my Twinjet. It's always run fine on a Jeti JES 350 speed controller with Speed 400 motors, but is now cutting out after a few minutes flying. I'm using a 2300maH 8cell nicad pack. Could it be an overheating issue? I'm using Cam Speed 5.5 x 5.5 props if that's an issue. Help! the weekends going to be sunny and we don't get many of them over here.

Chocky.

Are the motors in series or parallel?

Motors are in parallel. having spoken to some electrically minded guys at work, they think that the Jeti 350 Speed controller is not able to cope with the Current demand from the two motors and therefore cuts out. I'd still like some confirmation before I replace the controller with a beefier one.

Define "a few minutes."

Also, do you still maintain control of the airplane, or does it completely shut down?

The new, hotter motors will draw more current from the system and drain the batteries down faster. Full throttle flight times will probably be cut roughly in half. Maybe you're seeing the normal low-voltage cutoff?

What kind of batteries are these? 2300mAh is an odd size for a NiCd, so I assume they're not brand-name cells like Sanyos, which are typically 2400mAh. It may be that these are cells are of dubious quality, and cannot handle the current you're asking from them. As the current draw on a battery goes up, the voltage drops. A good battery will hold its voltage, but cheap off-brand cells will often curl up and die under any serious stress.

That Jeti speed control is rated at 35 Amps continuous, right? I can only find reference to a Jeti 35, which is also rated at 35 Amps. Hobby Lobby's site claims 14 Amps per motor with 4.7x4.7 props on 7.2 Volts. It's entirely plausible that with more cells and/or larger props, these motors are in the 18+ Amps per motor range, borderline for that speed control.

Try a Great Planes 50A speed control or something similar.

With a 2000mah pack the plane flew for about 3 minutes, slightly longer on the larger pack (which incidentally is a 2400mah - my mistake) both pack are Sanyo. Upon shut down, there was no noticable drop in power, which is what I would normally have experienced with 2 Speed 400 motors. The power just cut out in an instant, like someone had flicked a switch. Control surface power was maintained.

I've now had permission from upper management (the Wife) to purchase a 50 Amp speeed controller. Off to the model shop tomorrow evening and to my flying sight on Chobham Common first thing Saturday.

Thanks one and all for your comments thus far. I'll be sure to report back.

Chocky.