9
$\begingroup$

How would you design an airplane to counteract a single engine’s left turning tendencies caused by torque, p-factor, spiraling slipstream and gyroscopic precession without pilot input or use of rudder?

$\endgroup$
2
  • 1
    $\begingroup$ Are there detectable forces on the aircraft, such as engine mount strain, which would be proportional to the forces involved? $\endgroup$ Dec 10, 2021 at 6:44
  • $\begingroup$ Make it jet powered and centerline thrust? $\endgroup$ Dec 11, 2021 at 13:45

8 Answers 8

13
$\begingroup$

Look at the designs for literally hundreds of powered free flight model airplanes. Every one is designed to fly in a stable turn while both climbing under power and gliding after the engine run stops. And all do this with very subtle-seeming trim adjustments in roll, yaw, and thrust line. Very common is to adjust the glide by warping or adding small tabs to the flying surfaces (usually giving a large circular glide path), then adjust powered flight by adding down and right thrust (to counteract the left turning and climbing tendencies from torque, P-factor, and increased flying speed).

Very little adjustment is likely to be needed to a light plane anyway -- pilots habitually trim aileron, rudder, and elevator for "hands-off" stability, with the result that there are a large number of accidents that have occurred where a running airplane (due to engine being warmed or hand started by a single pilot) has gotten unchocked and taken off by itself, sometimes flying without crew for half an hour or more (I think the record was one in Australia that circled near the airport for four hours, until running out of fuel). The biggest consideration is that it may be necessary to set up at partial throttle, or the airplane will tend to climb to its service ceiling.

$\endgroup$
0
11
$\begingroup$

This can easily be done by mounting the engine at a very slight yaw angle to the fuselage, of order ~ 1 or 2 degrees, in the direction opposite to the turn. This tips the thrust line off-axis and helps counter the turning tendency.

$\endgroup$
1
  • 1
    $\begingroup$ Single engines on model aircraft are typically mounted yawed and pitched downwards a bit. $\endgroup$
    – rcgldr
    Dec 9, 2021 at 23:21
10
$\begingroup$

Single Engine Twin Prop or Contra-rotating Propellers.

It isn't the engine that causes the turning, it's the propeller. In the twin prop case I'd specifically make the two props rotate in opposite directions.

Of course such a design has other major disadvantages, but it would achieve your desired objective.

$\endgroup$
2
6
$\begingroup$

Well, if rudder deflection is required to counter yaw when full power is applied, it stands to reason you could simply angle the vertical stabilizer by a corresponding amount.

However, the logical follow-on question would then be: "how would you design an airplane to counteract this new airplane's right turning tendencies at less than full power?"

There are tradeoffs to be made in all designs, and a moveable control surface is simply the easiest and best solution to this momentary "problem". (Let pilots be pilots!)

$\endgroup$
2
  • $\begingroup$ But power is not at constant all the time, is it? Is the v.stab angle that's correct for WOT also correct for idle? $\endgroup$ Dec 10, 2021 at 2:35
  • 1
    $\begingroup$ @Harper-ReinstateMonica, that’s precisely my point! Meaning that the pilot needs a moveable control surface to counter the effect. $\endgroup$ Dec 10, 2021 at 5:45
4
$\begingroup$

Disclaimer: This answer borrows liberally from this one, but since it could not be found when the question was asked, I add parts of it here.

The direction of rotation of the propeller introduces an asymmetry which designers try to mitigate by adding more asymmetries. Specifically, a right-turning tractor propeller (clockwise from the pilot's viewpoint) will swirl the propeller wake in the same direction which adds a negative sideslip angle on the vertical tail (which sticks up into the upper half of the wake only). By pointing the axis of rotation of the propeller slightly to the right (right sidethrust), this sideslip angle can be reduced.

When MBB tried to rebuild an old Me-109, they initially replaced the missing vertical with a symmetrical one, including a symmetrical airfoil. Years later, original drawings surfaced (in Spain, IIRC) which showed that the original vertical had camber. When the new vertical was replaced with the original design, flight characteristics improved markedly.

The propeller axis can also point slightly down in order to move the thrust line closer to the center of gravity, reducing thrust-related trim changes, and to reduce the P-factor effect (sideways shift of the thrust vector) at high angle of attack. This is called downthrust. One example would be the RR Griffon-engined version of the Supermarine Spitfire: When the new engine was installed, its lower thrust line required 2° more downthrust.

This is one of the few topics where the English version of Wikipedia stays schtum; only the German version has an article on it (Motorzug and Motorsturz).

$\endgroup$
2
$\begingroup$

How would you design an airplane to counteract a single engine’s left turning tendencies caused by torque, p-factor, spiraling slipstream and gyroscopic precession without pilot input or use of rudder?

Make the airplane asymmetric. I'd make one wing longer than the other, put more tilt on one wing, put the rudder off center, sweep one back more than the other, or all the above and possibly more. We expect aircraft to be symmetric but they don't have to be, and there are historical examples of aircraft that are asymmetric. https://en.wikipedia.org/wiki/Asymmetrical_aircraft

Why make aircraft symmetric? Because it's easier to design it that way, I guess. Another is because if engine power is lost a symmetric glider would be easier to control than an asymmetric one.

My guess is that there's a little truth to all the reasons I thought up on keeping the airplane symmetric. A big reason to keep the airplane symmetric, at least by my estimation, is that the off center thrust of a single propeller is going to be so small in most cases that it's not a real concern. If an airplane is built to be asymmetric then the direction of rotation on the propeller is fixed, putting in a different engine will throw off the design. Just how much turning torque will have to be designed into the asymmetry may be difficult to know while designing the airplane, then if you get it backwards (because the engine turns the opposite direction from what was expected) there's going to be big problems for someone.

The most common method to counteract the single engine torque is to use counter rotating propellers. This can be with coaxial propellers on the center line or symmetric pairs off center. A single engine does not mean it must have a single propeller. The Wright brothers used a single engine to drive two propellers by belts, one belt had a twist in the path to the propeller to reverse the rotation. This solution is as old as the airplane.

I will say that the answer offered by @Zeus, some kind of autopilot to manage the turning moment of the propeller, is perhaps better than the two I offered above. Designing asymmetry into the aircraft is going to be difficult to get right. Counter rotating propellers is complicated. An autopilot is going to be complicated too, but perhaps far less complicated than any other option.

$\endgroup$
1
$\begingroup$

You are putting forth an impossible task. For example, as an aircraft begins to accelerate, then rotates to take off, different factors are in play in different amounts at different times, as the "angle-of-attack" of the fuselage (relative to the airflow) changes, and the motor rpm changes, etc.

In at least one case, engine-related turning tendencies were countered by making one wing (or one set of wings if it was a biplane) longer than the other.

$\endgroup$
0
$\begingroup$

Of all tricks, this hasn't been mentioned yet (and is not excluded by the question):

Autopilot

Or more specifically, any sort of artificial control/stability augmentation system.

On many airplanes, there is a dedicated part of it specifically for the stated problem. Usually it is called Yaw damper, although this term is somewhat misleading: damper is only meant to dampen yaw oscillations. Yet in practice, the same subsystem (with the same switch) often also provides the autorudder functionality, negating sideslip.

Of course, it is useful not only for single engine airplanes: it works for any asymmetry, including a turn.

$\endgroup$
1
  • $\begingroup$ The question says "without pilot input or use of rudder", so I guess this is excluded. $\endgroup$
    – Bianfable
    Dec 13, 2021 at 13:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.