# How do propeller-driven, fixed-wing UAVs counter negative torque from engine rotation?

What cancels the rolling moment produced from the rotation of the engine?

• I don't understand why UAVs would be any different in design from ordinary propeller aircraft here. Aerodynamically, you have all the same considerations. – Nathan Tuggy Sep 13 '15 at 21:37

## In exactly the same way as any conventional, single-prop aircraft does

Which is to say, by using stabilizer and aileron/rudder trim, and the natural resistance that wings/stabilizers give.

The wings/stabilizers will naturally resist rolling forces to some extent, particularly at flight speed. At lower speeds (eg when taking off or landing) the control surfaces can counter the force quite easily - a little opposite rudder and aileron is all that's required.

• People underestimate how MUCH wings counter torque just by being there (no asymmetric modifications, no aileron adjustments etc). Try this. Get yourself a rubber powered toy plane. Does not have to be anything fancy. Wind it up without the wings on and let go. See how fast the body spins? Now put the wings on (no need to assemble the tail etc, this is a simple experiment) and let go. Notice that the plane would not make even one full roll before hitting the ground. In reality the pilot subconsciously adjust banking angle. The adjustment is so small that you won't really notice it. – slebetman Oct 19 '16 at 16:34

The design considerations for UAVs are the same as that of other aircraft.

The torque applied by the motor (or engine) to the propeller causes an equal and opposite torque applied by the propeller to the UAV body. This torque, $T$ is opposite to the propeller rotation and is proportional to the square of propeller angular velocity ${\Omega}$, given by,

$T = -K_{T_{P}} * {\Omega}^{2}$,

with $K_{T_{P}}$ being an experimental constant. The effects of this torque are relatively small for small UAVs. In case of large UAVs, these effects are nullified by the same control inputs(aileron) as in other aircraft.

In case of multi-propeller UAVs, it may be desirable to use counter-rotating propellers so that the net torque generated by the propellers as they rotate is canceled to zero. Using of contra rotating propellers is also an option.

Source: www.unmannedtech.co.uk

A unique case for UAVs is the quadcopter, where two propellers (rotors) spin clockwise and two spin counter-clockwise, canceling the net torque on the aircraft.

• Quadcopters, hexacopters, octacopters... but yes. – Nathan Tuggy Sep 14 '15 at 18:35
• How about quintacoptors and septacoptors? :) – FreeMan Jul 29 '16 at 14:01

As noted above, whether piloted or UAV a/c, it makes no difference. An aircraft design counters engine torque in one of several ways.

ASYMMETRICAL WING TWIST - In many light GA aircraft, the angle of incidence of the wing opposite the descending side of the propeller arc is greater than the angle of incidence for the wing on the descending side of the propeller arc. This creates an imbalance in lift between the two wings and helps to counteract the propeller torque. However the amount of counter torque is fixed and may have to be augmented by control inputs at higher speeds.

AILERON TRIM - Some aircraft, in particular large twins which do not have counter-rotating propellers have trim tabs on the ailerons which allows the pilots to counteract roll torque about the longitudinal axis of the aircraft. This is more advantageous than the asymmetrical wing twist described above in that the roll trim can be adjusted for takeoff/climb/cruise and landing airspeeds and engine power settings.

COUNTER-ROTATING ENGINES/PROPELLERS - Many twins have engines which are geared to rotate their propellers in opposite directions such that each engine's torque cancels out the torque of the other. This has an additional benefit of reducing thrust asymmetry during an engine failure by eliminating a critical engine from the design.

REACTIVE COUNTER TORQUE: In some turboprops, mainly the PT-6 and other reverse flow, free turbine designs, the exhaust stubs are designed to divert exhaust gases from the power turbine in opposing directions offset from the centerline of the powers haft, creating a reactive torque which counters the torque of the propeller. This design concept is popular with turbine conversions of existing reciprocating engine powered aircraft eg. the PA-46 JetProp conversion, etc., where the original airframe was design around a lower power engine which produced less torque.

Another design, virtually unheard of in aviation today, is to turn the propeller by means of a stream of high pressure gas emanating tangentially from the propeller tips. This eliminates any kind of torque effect about the airframe from the turning of the propeller but is a superfluous design in an age of jet aircraft.

CONTRA-ROTATING PROPELLERS - This option was explored during WWII on military fighters and is currently used today on several Unlimited-Class Air Racers eg Precious Metal, the old Red Baron RB-51 prior to its destruction in a crash. It consists of two sets of propellers mounted to a gearbox which rotate in opposite directions. The design has several advantages in that it not only eliminates engine torque, but also the effects of P-Factor, gyroscopic precession, and propwash on the fuselage and empennage of the aircraft. The downside of the Contra Rotating Propellor is the expense and the excess weight of the gearbox and extra propellor.