Is the torque effect so pronounced in slow flyers that it affects the steering even after takeoff?

I took my Gnat 280 slow flyer on it's maiden flight yesterday, and i could not get it trimmed right. Before takeoff, I made sure all control surfaces were dead-on straight and the CG was just right. Taking off from the ground, the plane veered to the left, which I expected because of the torque effect of the propeller spinning in the opposite direction. However, once in the air it continued to veer left, even after I set the rudder trim all the way to the right (which helped only a little).

Since the prop is so large relative to the size and weight of the plane, it would stand to reason that the torque effect would be worse in this plane than in others. also, I believe speed helps negate the torque effect, so the low airspeed of this plane could also be exacerbating the problem. Can anyone confirm this line of reasoning, or is my problem being caused by something else? Is there anything I can do about it besides dialing in some counter-steer?

From : Don Stackhouse

It looks from this question and from some of the other replies to this thread that we need a discussion of prop and torque effects here.

There are actually a whole shopping bag full of effects from the prop on aircraft flight trim.

Let's start with torque. Power is proportional to torque times RPM. If you have power, then you have torque. For a given amount of power, a lower RPM (such as using a gearbox to slow the prop down, allowing a larger prop) means that torque must go up. All of this is true regardless of whether the model is on the ground during takeoff run, or in flight.

The torque applied to the prop by the motor inspires an equal and opposite torque applied by the prop onto the airplane, in accordance with Newton's third law, the one about action and reaction. To counteract that torque, one wing must make a little more lift than the other. For a right-handed prop (i.e.: the prop turns clockwise when viewed from behind), the left wing has to make a little more lift in level flight. In most cases, the difference is very small, but at low speeds or with very high installed power, it can become significant. It's part of the reason for the typical left-wing drop in a power-on stall.

Full scale pilots transitioning to the P-51 Mustang, once they became moderately proficient, were told to go to a good altitude (like about 15,000 feet AGL), slow down to about 125 knots airspeed with the gear and flaps down, then apply full takeoff power. Even with all the controls shoved as far as they would go to the right side of the cockpit, the airplane would inexorably roll over to the left.

The next effect is one that has been mentioned in some of the other posts to this thread. The torque from the motor, applied to the air by the prop, causes the air to rotate behind the prop, sort of like a horizontal tornado. The amount of swirl depends on the disc loading (i.e.: how much power you're trying to pump into what size of propeller disc). If the prop is relatively small compared to the power, there will be a lot of swirl. In extreme cases the helix angle of the swirl might be as much as 15 degrees or so, but usually (particularly for models, with their relatively low installed power) it's much less than that.

Making the air swirl like that takes energy, and that means efficiency loss. On some aircraft with extremely high installed power, a well designed contra-rotating propeller system (i.e.: two props rotating in opposite directions on the same axis; the term "counter-rotating" refers to two props rotating in opposite directions on two DIFFERENT axes, such as on the P-38) can take the swirl out with the second prop and recover most of that energy. It also cancels out the torque and gyroscopic effects, which can be very important on small airplanes with big motors, such as the later model Spitfires with RR Griffon engines.

However, the weight, vibration and complexity issues for such a system are a HUGE problem, and there must be an awful lot of torque effects and/or efficiency losses in order for this approach to be worth the trouble. One of the single biggest factors that kept the Northrop XB-35 flying wing from being ready before the end of WW II was the development problems of the gearboxes for its contra-rotating props.

For a normal single-rotation prop, that swirl can interact with other parts of the airplane to cause trim changes. Essentially any surface that sticks out to one side of the thrust line and not to the other can cause this. For example, the fin/rudder sticks out above the thrust line, but generally not very much below. For a right-handed single-engine prop, the slipstream curls around the fuselage and strikes the left face of the fin/rudder, pushing the tail to the right and the nose to the left.

On some of the power free-flight designs, the wing is mounted on a pylon above the fuselage, just behind the engine. These models also often have huge lifting stabilizers, so the C/G tends to be well aft, with much of the wing pylon ahead of the C/G. The force of the swirling slipstream against the left face of the pylon tends to roll the airplane to the right (counteracting torque), and also tends to yaw the airplane to the right. These models sometimes have an under-slung fin as well, which also makes the slipstream effects tend to make the model yaw to the right.

Twins with wing-mounted engines don't generally see this effect, because the slipstreams are roughly symmetrical, and located on both sides of the fin/rudder. The amount of this effect that appears on any particular aircraft will depend on the amount of power and on the geometry of the aircraft.

Now let's talk about "P-factor". This is the result of the airflow into the prop not being parallel to the propshaft, such as when the angle of attack of the aircraft is high during a climb. In this case, both the airspeed and angle of attack of the blades on one side of the prop disk (the right side in a typical right-handed prop in a steep climb) is greater then the airspeed and angle of attack of the blades on the other half of the disk. The blades with the greater speed and angle of attack will make more thrust than the others. In the case of our climbing right-handed prop, the right side of the prop disk makes more thrust than the left, and therefore the prop tries to yaw the airplane to the left.

Note that this effect requires both airspeed and skewed airflow into the prop disk. It will not be significant during the beginning of takeoff run, because the aircraft's airspeed is too low. It will also not be significant on nosewheel airplanes prior to rotation at the end of the takeoff run, because the inflow to the prop will not be angled significantly before then. It will be most pronounced AFTER takeoff, during climb. Airplanes with very large diameter props in proportion to the rest of the airplane will also see more of this effect.

All of this explains why it is that, although this effect most definitely exists to some degree on ANY single rotation prop (including the props on wing-mounted twins) any time that the airflow into that prop is not exactly parallel to the propshaft, you might not notice it very much, particularly at the beginning of takeoff run. If you were in a twin that had little or no slipstream effects from swirl against the fin, and a small ratio of prop diameter to wingspan and length, you could be forgiven for not noticing it. However, even on those sorts of aircraft, what direction do you have to hold rudder during a power-on stall entry, and which wing tends to drop at the stall break?

There's one other related effect I'd like to mention here. Remember how P-factor is the result of the downward-moving blades on the right side of the disk making more thrust because they're seeing a higher airspeed and a higher angle of attack than the upward-moving blades on the left side? Well, if something is moving faster and also making more lift (or thrust, in the case of a propeller blade), then it is also making more drag. In the case of those downward-moving blades, that extra drag is acting nearly upwards, parallel to the plane of the prop disk. The airplane s ees this force as LIFT, even though it's coming from the prop. For a nose-mounted tractor-propped airplane flying with its nose high, this "lift" is pulling the nose even higher, de-stabilizing the airplane. The same is true for yaw stability, except that the lift in that case would be sideways, and be therefore de-stabilizing in yaw. For aft-mounted props (any prop mounted significantly aft of the C/G, regardless of whether it's a tractor or pusher), the in-plane forces from the prop increase pitch and yaw stability.

It's even true for contra-rotating props. The little fins on the Northrop YB-49 were there to replace the lost stabilizing effects when they removed the XB-35's props and replaced them with jets. Another example of this effect is on the V-22 Osprey. The lift from the rotors supports the aircraft when hovering, the lift from the wings supports the aircraft when in full forward flight, but it's this "translational lift" from the in-plane forces in the rotors that supports much of the aircraft's weight when halfway through the transition from helicopter to aircraft mode.

Thrust lines and aircraft rigging enter into the whole prop effects issue as well. Assuming a right-handed prop, P-factor tends to swing the nose to the left, and depending on the airplane's layout, the swirl of the slipstream may or may not do the same. To counteract this, some airplanes have the fin, rudder, and/or thrust lines canted to offset these effects. The right thrust that is common in models is one example. The full-scale Aeronca 7AC Champion that I used for some of my flight training had the leading edge of the fin offset by about 1/2" to the left, acting like built-in right rudder.

Typically these approaches can only compensate within a very small range of airspeeds and power settings (as closely as possible) for the flight condition where the prop effects are most annoying, typically that flight condition where the plane spends most of its time. For example, on the "Champ", that 1/2" of fin offset was just about right for cruise power and airspeed. At climb power and speed, the pilot (me) had to hold some right rudder, while in a glide (no power, no torque, no swirl, and no P-factor, but the fin is still welded on with that 1/2" offset) it was necessary to hold a little LEFT rudder to keep the fuselage in line with the airflow.

In some regards a thrust line adjustment is a better fix, since the effects of the adjustment naturally vary themselves in proportion to the prop effects they're supposed to counteract. However, changes in aircraft angle of attack will cause changes in P-factor that thrust line changes still can't exactly compensate for outside of a very narrow range. The bottom line is that if the airplane has a propeller on it, there's almost certainly going to be some place in the aircraft's operating envelope where the pilot will have to use the rudder to keep everything perfectly straight.

Gyroscopic effects only occur while the aircraft is changing its pitch or yaw attitude. For example, if you pitch sharply nose-down with a right-handed prop, it will tend to yaw the plane to the left due to the gyroscopic precession effects. If you turn left with a right-handed prop, the prop will try to pull the nose up.

The Sopwith Camel is one example of this. Some versions of the Camel had a Clerget rotary engine. Rotaries are radial engines where the prop is bolted to the crankcase, and the crankshaft is bolted to the firewall. The whole engine spins around with the prop. In the days when spring manufacture was still more in the category of blacksmithing than science, and few, if any, good high-temperature alloys had been developed, valve cooling was a big problem, and the rotary was a very good solution given what they had to work with. However, this also meant that this tiny, lightweight stick-and-rag airframe now had a 400 pound gyroscope spinning around on the front of it. To make matters worse, the carburation on the Clerget rotary required a small adjustment to the fuel mixture at about 200 feet of altitude after takeoff, or the engine would start to sputter.

Now imagine you're a new pilot out for your first flight in a Camel, with all of your pathetically few flying hours prior to this logged in a low-powered two-seat trainer with an in-line (NON-gyroscopic) engine. You make what feels like a frighteningly fast but otherwise normal takeoff, you're trying to keep this unstable beast on a more-or-less smooth (but alarmingly steep) climb (at least compared to what you're used to), at an airspeed that feels about right to your trainer-accustomed flying skills, but probably a little too slow for the Camel's higher wing loading, when, just as you start your first turn in the pattern, the engine starts to cough and die.

Your inexperienced head is instinctively drawn down to the sparsely populated instrument panel to attend to the engine's problems, and you don't notice that, as your plane continues to turn, the nose inexorably begins to rise. By the time your attention is drawn back outside of the cockpit by the shudder from the now stalled airflow separating from the top of the wings, it's already too late as the nose suddenly and violently pitches down. The hard yaw to the left from the gyroscopic effects of the pitch-down gets the rotation firmly started and finishes the job. With only 200 feet to fall, you don't even have time to figure out what killed you.

However, eight decades later, the astute reader of a book that contains a photo of your wrecked Camel will know. One wing is wrapped around over the top of the fuselage and the other underneath, the telltale indication that the plane was rotating in a spin when it hit.

In the "War to end all wars", the British lost about 1000 Camel pilots in combat and about 1700 in training accidents, many to stall-spin accidents that were set up for their hapless pilots by the gyroscopic precession effects of that heavy engine and its enormous prop.

So what's the bottom line on prop effects? It depends on how all the factors I discussed above, plus some others I didn't go into, all add up to determine the plane's behavior. Each different airframe/engine/prop combination at each different flight condition will have its own unique set of characteristics.

However, if the basic design of the airplane is even close to reasonable (with the exception of some extremely high-powered aircraft operating at the extreme low speed and high power corner of their operating envelope), you should have enough control authority to overcome any prop effects and still have plenty of control travel left to spare.

v In Ben's case, it sounds like there's more going on there than prop effects alone should explain. I'd verify by seeing if these effects go away with the throttle at idle, and double check for warps, particularly in the wings.

Don Stackhouse
DJ Aerotech