I am planning on making a small model powered by four pager motors, either with or without gear reduction.

I'd like to apply any thinking regarding a 4 motored plane, based on your experience with your B-17.

Basic question: with all 4 props going in the same direction, is there much of a torque problem? Assuming there might be, do you have a recommendation for coping with it?

Because of the problem of finding matched right and left hand props, I'd prefer to go with right hand only.

From : Don Stackhouse

In a word: "No". There is not a torque problem. In fact, torque is generally less of a problem on a 4-engine model than it would be on a single-engine model with the same total installed power.

"Torque" in the sense we refer to it here is actually what in engineering term would be more properly called a "couple". In engineering, a "torque" is a force on the end of a moment arm, such as the thrust of a helicopter's tail rotor acting on the moment arm of the tail boom. A couple is purely a twisting force, such as the twisting effect in the helicopter's main rotor shaft, or the twisting in a prop shaft.

The key here is that a couple applies the same pure twisting effect to the structure regardless of where on the structure it is located. You could mount a prop + motor on the tail, on the nose, all the way out at one wingtip, or down on the landing gear, and the plane would still experience the same twisting force from it. In the case of a helicopter's rotors, the pure twist ("couple") in the main rotor shaft is balanced by the sideways force of the tail rotor acting on its moment arm ("torque"), so the twisting forces cancel out, but we still have to account for the sideways force of the tail rotor that is trying to make the helicopter drift sideways across the ground. Typically we bank a hovering helicopter the other way a degree or two, so that the resulting sideways component of the main rotor's thrust can counteract the equal and opposite sideways thrust of the tail rotor.

The torque ("couple"?) made by a propeller as it absorbs power from the motor depends in part on its diameter. Each of the individual segments of the blades makes lift and drag. The individual lift forces add up to make the total thrust of the prop. The individual drags times their distance from the propshaft create individual torques, and these torques add together to make the total couple felt by the propshaft. The tangential forces from the individual drags of the different blades (assuming it's not a one-bladed prop!) cancel each other out so that what the propshaft feels is a couple, not a torque.

For a given amount of power and a given required efficiency, you need about the same total amount of propeller disk area. You can have all that area in one large disk, or in two smaller disks, or in four even smaller prop disks. As the disk diameters get smaller, the moment arms that the individual blade segments have to generate torque with get smaller. For this reason, four small props absorbing the same total power with the same efficiency as a single large prop will have less total torque. They will also have a greater RPM, but we won't go into that right now.

The problem with multi-engined airplanes is asymmetric thrust in the case of an engine failure (and yes, that can and does happen with electrics, just not as frequently as it tends to happen on gas-powered models). If you lose an outboard engine, its windmilling drag acting on the moment arm of the distance from it to the C/G creates a yawing moment, which in engineering terms described above fits in the category of a "torque". Meanwhile, the thrust of the still-running corresponding powerplant on the other side creates a yawing moment in that same direction, and together they can in some cases create a bigger yawing effect than what the rudder's sideways lift acting on its tail moment arm can overcome. The plane can lose control in yaw, the yaw couples with any dihedral or sweep to create a rolling effect that can overpower the ailerons (the loss of lift in the section of wing behind the windmilling prop can add considerably to this effect), following which everyone associated with that particular aircraft tends to have a very bad day.

Counter-rotating props can help a little, since the directions of rotation of the individual props can be chosen so that the rolling effect of each prop tends to cancel its asymmetric thrust in an engine-out situation, but for models this tends to be a very minor effect. It certainly isn't significant enough to warrant hunting all over the place for matching right and left-handed props.

We have seen engine failures on both the B-17 and the DC-3. I vaguely recall some engine-out troubles on the Northrop XB-35 prototype, and even with its larger size, wider engine spacing and far less yaw control authority, it still wasn't a big issue. In the case of the Boeing it was due to mounting screws vibrating loose, resulting in one motor flopping around until it fatigued the wires to the motor and broke the circuit. When you have four motors all running together, the resulting rather complex vibrations can cause some really strange things to happen. On the big Boeing (and all of the other RK Series models for that matter) we now recommend safetying the motor mounting screws with a drop of white glue on the exposed threaded tip of the screw where it sticks out through the gearbox assembly's plywood plate. When the motor came loose, we noticed no significant effects in the aircraft's in-flight handling, other than a small loss of total power. The loss of a single motor is only 1/4 of the total, and isn't that big a deal for that airplane.

On the DC-3, it was a case of trying to solder the wires to the motor with an iron that wasn't fully up to temperature at the time, so it took longer to make the connection. This overheated the motor terminal, causing the motor's plastic end bell to soften, which let the brush shift in the plastic and lose most of its contact force with the commutator. The motor would run, but at more than about 1/3 throttle the brush would start to bounce on the commutator, which limited the motor's max power output to about 20%. The problem was not obvious in a static check unless you got tachometer readings form both motors at full power. Meanwhile the other motor would try to make extra power, so the resulting asymmetric thrust was nearly as bad as that of a total engine failure on one side. In flight it had little measurable effect on handling, mainly just a reduction in cruise speed and especially in climb rate due to the loss of power. On the ground was another matter. The plane would try to swerve hard to the right, requiring nearly full left rudder (and therefore tailwheel steering) to keep it straight in the early part of the takeoff run. Once the airspeed built up and the rudder became effective it wasn't an issue.

The bottom line is that engine-out situations in multi-engine models can be an issue regardless of whether you have counter-rotating props or not, the longer moment arms of the outboard engines on four-engined models can make that worse, but dividing the power up among four smaller motors instead of one or two big ones can offset that problem to some extent. In any case, the relatively small installed power and low wing loading of typical backyard/indoor electrics tends to make this much less of an issue for them than for larger, heavier and more powerful models. Make sure your rudder is very effective, but don't bother worrying about counter-rotating props.

Don Stackhouse
DJ Aerotech