# Can prop wash from a live engine contribute a sideways force on the aircraft?

A recent response to this question, it was posited in the accepted answer that: the rudder "also creates a side force" (as well as a torque) that is countered by banking the aircraft, allowing it to fly straight.

While the answer did indeed explain the control inputs and positioning of the aircraft for lowest drag with an engine out, it did leave me wondering if other forces were in play.

Specificly, any force acting on the center of gravity will tend to move the mass in that direction (unless opposed by another force). Forces away from the center of gravity tend to produce rotation from their torque.

Realizing there may be points in between where both "push" and torque occur, is the possible that another force, from the propwash of the live engine, is also responsible for creating additional sideways force on the aircraft during an engine out, or is it only from the rudder input used to control the thrust asymmetry?

• @quietflyer the yaw string, if the plane rolls (without turning) but has an AoA, won't be straight back. Just thinking about what you were saying: there is some side force on the fuselage that bank must compensate. In order to staighten a yaw string on a plane that is rolled and pitched, a little more rudder is needed, which actually puts the plane into a slight skid. Jan 11, 2022 at 15:51
• I think that the spiralling propwash definitely can contribute a yaw torque by striking the vertical fin and by definition this must involve some sideforce, just wondering how significant it would be on twin/multi engine aircraft, wondering if the question might be improved by asking that more specifically, just a thought. It looks your comment above is getting at something else maybe, the fact that any rolling motion around the aircraft's longitudinal axis automatically converts angle-of-attack into sideslip. My hunch is that unless we are talking about rather rapid roll rate, this Jan 11, 2022 at 17:50
• My hunch is that unless we are talking about rather rapid roll rate, this doesn't usually play a very significant roll in aircraft's slip/skid dynamics, but it is something to consider-- Jan 11, 2022 at 17:51
• This is a good question that stimulates a rather interesting analysis-- Jan 11, 2022 at 20:40

On a twin with a single fin and rudder, the only significant propeller wash factor is propeller P Factor, as it offsets the thrust line to the right of the propeller axis (for a clockwise-from-behind propeller).

So the right engine's net thrust line is farther away from the airplane's longitudinal axis than the left's, and more rudder is required to counter the thrust offset with the longer arm, all else being equal, which is why losing the left engine (the critical one) is a bigger deal than losing the right (along with the fact that a lot of twins had a single hydraulic pump powered off the left engine, so if you lost the left side, you had to manually retract the gear with a hand pump AND deal with the control issues, all while doing the engine-out drill - a pretty intense exercise). Of course if you have counter-rotating propellers, conditions are the same with either engine out, especially if you had dual hydraulic pumps or an electric backup pump instead of a hand one.

The spiral wake of the prop is of little significance because the wakes generally miss the fin/rudder, being off to the side.

In a plane like a Beech 18 with two fins/rudders that are directly in the propeller slipstream, you have improved rudder effectiveness because the surface is in the prop blast, and the spiral slipstream may apply its own lateral component to the fin/rudder depending on where the aerodynamic center of the surface is within the spiral flow field, that determines what the overall angle of attack of the surface is. In any case, it may help or hinder the rudder, but mostly the effect will be lost in the noise of all the other effects.

All that is going on is, the thrust line is offset, which wants to take the plane around in a flat turn, rudder is applied to stop the turn, and you end up with a resultant thrust vector, the sum of the engine's thrust and the rudder's lateral lift, slewed to one side, and the bank applied reorients the thrust vector back to being straight ahead.

The rule of thumb is 5 degrees of bank because most twins don't have yaw strings and the ball doesn't help you much, and 5 degrees is a good ballpark number that works well enough when at blueline speed. If you had a yaw string, you would just apply whatever bank it takes to align the yaw string, 4 degrees of bank, 6 degrees of bank, whatever.

Imagine taking off in a glider with the tow line connected to the wing leading edge a few feet from the fuselage. You would have tons of rudder in just trying to keep the nose pointed at the tug, but you would still slew way out to the side opposite to the tow line connection. You would have to lower the wing with the tow line connected, and at some angle, you would find the glider lined up behind the tug, with the yaw string straight, with the wings banked 5 or 7 or 10 degrees into the side that has the two rope connected.

• "The spiral wake of the prop is of little significance because the wakes generally miss the fin/rudder, being off to the side." -- no doubt true-- but the question didn't really address significance -- Jan 11, 2022 at 22:49
• Significance is implied in the context. Otherwise, why bother asking?! ;) Jan 12, 2022 at 4:54

Forces away from the center of gravity tend to produce rotation from their torque.

While technically true, I feel that this sentence can be a little misleading. If you push on the side of a body (such as an airplane), assuming there are no other forces acting on it, the body will both rotate around its center of mass, and move sideways. In other words, there is always going to be a "push" as you called it. Now, the ratio of "push" to "rotation" will slide more toward rotation the further out you go from the center of gravity, but "push" will never reach zero.

To directly answer your question, no, there wouldn't be any noticeable sideways aerodynamic force on the fuselage in the scenario described. The prop wash from the good engine might contribute some sideways force, but it could be to either side depending on which way the engine is spinning. There might also be some small momentary forces due to the pilot being human and therefore not able to fly perfectly coordinated, but they generally won't favor one side or the other. The bank needed is almost entirely due to the sideways force generated by the rudder.

I think it might be helpful to look at a helicopter in normal flight. The main rotor generates a torque, which needs to be countered by the tail rotor out at the end of its boom. But the tail rotor also generates a sideways force, which needs to be countered by the main rotor.

• It was the comparison to helicopters in the referenced question that got me interested. I wonder if a clever 'copter designer would make the rotor a little high to generate a bit of roll torque. Having to tilt the rotor a bit (the other way) may lessen the chances of forming a vortex ring above the rotor. +1 for your "push and torque" real world analysis. Jan 11, 2022 at 21:11

Can prop wash from a live engine contribute a sideways force on the aircraft?

Absolutely.

For a twin-engine aircraft with wing-mounted engines with propellers:

If the failed engine is the "critical engine", then any sideforce and yaw torque from the spiralling slipstream will increase the yaw torque and sideforce that the rudder must generate to counteract the yaw torque from the thrust asymmetry, but will also contribute a sideforce that very slightly decreases1 the amount of deflection of the slip-skid ball associated with zero-sideslip2 flight.

If the failed engine is not the "critical engine", then any sideforce and yaw torque from the spiralling slipstream will decrease the yaw torque and sideforce that the rudder must generate to counteract the yaw torque from the thrust asymmetry, but will also contribute a sideforce that very slightly increases1 the amount of deflection of the slip-skid ball associated with zero-sideslip2 flight.

These effects may not be easily detectable, since the fuselage and vertical fin are not in the core of the propwash in a twin-engine aircraft of this configuration. This answer does not intend to imply that these subtle effects are the reason that one engine is designated the "critical engine", and the other engine is not.

If both props rotate in the same direction, then if the props rotate inward at the top of their arcs, consider both engines "critical engines", and if the props rotate outward at the top of their arcs, consider neither engines "critical engines".

Footnotes:

1. The fundamental reason why the spiralling slipstream produces a change in the net sideforce generated, in the zero-sideslip condition, by the fuselage, fin, and deflected rudder combined, rather than simply being cancelled out by an equivalent sideforce in the opposite direction contributed by the rudder itself, is that the rudder acts at a longer moment-arm from the CG than does combined effect of the rear portion of the fuselage and the vertical fin, and thus the rudder requires less sideforce to create an equivalent yaw torque.

2. For the purpose of this answer, consider the definition of "zero sideslip" to be that the fuselage is aligned with the free-stream (i.e. unaffected by the propwash) "relative wind", rather than yawed to point to the left or right of the direction of the incoming free-stream "relative wind".

This answer is based on the ideas expressed in this related ASE answer.

• @RobertDiGiovanni -- see footnote 1. That's really it in a nutshell, not sure any more need be said. Jan 11, 2022 at 21:55
• I think the paragraph about counter-rotating propellers and critical engine is reversed -- outward rotating is both critical (e.g. P-38), inward rotating is no critical (e.g., Beech Duchess). Jan 11, 2022 at 22:32
• @FredLarson -- the definition of "critical engine" is "the good engine-- the one you'd least prefer to lose". Thinking of it that way, do you still think the answer contains an error? PS my understand is that in the production P-38, both engines rotated inwards at the top of the prop arc, not the other way. Jan 11, 2022 at 22:37
• Yes, I do. For example, if the engines rotate outwards, the left engine will have right-turning tendencies. If the right engine fails, these right-turning tendencies will exacerbate the off-center thrust yaw toward the dead right engine. Same thing in reverse for the left engine failing, so both engines are critical. Correct? Jan 11, 2022 at 22:40
• @FredLarson -- in the situation described in your last comment, you describe the turning tendencies correctly-- but that means that each engine is turning in the least unfavorable direction for single-engine operation. The definition of "critical engine" is the engine that is turning in the most favorable direction for single-engine operation-- that's why you'd prefer to lose the other (non-critical) engine. Jan 11, 2022 at 22:45