I have one of your P-38 kits that is still in the box. It seems to me that reversing one of the engines would be a simple job of wiring terminal connection.

From : Don Stackhouse

That's correct, but read on...

If so, do you sell or have knowledge of a similar-sized left-handed propellor I could use?

(The whole idea would be to cancel the P-Factor affecting all twins with props which turn the same direction...).

The reason counter-rotating props are such an issue on full scale twins is because of the combined effects of P-factor plus asymmetric thrust in the case of an engine failure. Assuming right-handed props, if you have a failure of the left engine, the P-factor of the right engine is trying to yaw the plane to the left, and the asymmetric thrust from the right engine is also trying to yaw the plane to the left. The asymmetric drag of the windmilling left engine and prop adds to this yawing effect. Any yaw-to-roll coupling due to wing dihedral and/or sweep is trying to roll the airplane to the left, and the torque of the engine is also trying to roll the plane to the left. The net result is that the airplane wants to roll over and play dead. If it happens on takeoff, right after liftoff, at low airspeed and low altitude, the results can be very nasty. The pilot needs to be ready to apply a bootful of rudder and immediately feather the prop on the dead engine, while simultaneously getting the gear and flaps at the correct setting and holding the airspeed precisely at "Vyse", the best single-engine rate of climb airspeed. Any delay or mistakes on any of these can result in an unplanned and possibly un-controlled termination of the flight.

If you calculate the volume coefficients ( a measure of tail size and authority - check the "Ask Joe and Don" section of our website for instructions on how to calculate it) for the vertical tail on typical twins vs. typical single-engined aircraft, you will find that twins tend to have two or three times more vertical tail that do singles. The need for enough rudder authority to counteract the yawing problems with the critical engine dead is what usually determines the vertical tail size for a multi-engined aircraft.

If the right engine fails, the asymmetric thrust of the left engine counteracts the P-factor and torque, making the control problems much less severe. For this reason, for two right-handed tractor props, a failure of the left engine is a much more critical problem than a failure of the right engine, so the left engine is termed the "critical" engine.

If the airplane has counter-rotating props, with a left-handed prop on the right engine, there is no "critical" engine. A failure of either engine results in the asymmetric thrust helping to cancel the effects of the P-factor and torque, instead of adding to them. It's still a demanding situation because of the power loss, but the control problems are much less of an issue.

These are all very important issues for a full-scale twin with wing-mounted engines, or a gas-powered twin model. However, engine failures are so rare for an electric model that for these it's rarely a significant issue. Most of the failure modes on electric models will result in the failure of both motors simultaneously. In addition, the power loadings on electrics tend to be lower, so the effects of a single motor failure tend to be less severe.

Single-motor failures can occur on electric models. I had a motor go bad on my RK series DC-3 prototype a while back (I tried to solder the wires to the right engine before the iron got fully up to temp, which took longer, overheated the plastic end-bell, and caused the brushes to shift enough to lose contact pressure). The right engine would only put out about 20% power. The plane had a bad swing to the right when the throttle was first opened for takeoff, but once it was in the air the discrepancy in power was barely noticeable. Of course the RK DC-3 is extremely efficient, takes very little power in flight, has the engines mounted relatively close together, and has a long tail moment and plenty of wingspan. All of these things help when one engine decides to call in sick. The RK P-38 would be a little more of a handful, but still should be reasonably well behaved.

The bottom line is that for a typical electric model, especially an indoor/backyard type, counter-rotating props are usually not worth the trouble. If your single-motor planes don't need contra-rotating props (i.e.: two props on the same axis, spinning in opposite directions) to counteract P-factor and torque, a twin model of the same size and power will need counter-rotating props even less!

I also need to know the maximum current draw of the motor/prop combination at 7.2 and 8.4 volts so I won't exceed the Pixie P-7 rating...

Smaller packs (still seven cells) are nice and light, but flights tend to be short, and many of the brands of cells in the smaller sizes have trouble putting out enough current. There is only one brand of 110 mah NiMH cells we've found that truly perform well in that regard, and we sell those too, especially for our single motor models. For our new WW I series of Roadkill models (yes, I'll be officially announcing those, as well as the DC-3 and some others, as soon as our webmaster has time to get the photos and specs posted on our website) those little 110 mah packs are perfect. However, for the P-38 we recommend the seven cell 170 mah NiMH packs.

Don Stackhouse
DJ Aerotech