Torque
April 14th, 2007 by lex
Since we’re on a bit of a flying jag (and heads “thunk” on computer desks across the land), SJS has a series of “getting back to basics” (i.e. prop-powered aircraft) vids up which he alerted us to over at The Other Site. To which I only replied, only partly in jest, “Rudder pedals. That you actually use when you’re not taxiing. Ugh!”
For what you mayn’t have considered, constant reader, is that - rotate though they do? - get engines generate no noticeable torque, neither on the run up nor in level flight.
Prop planes? Oh, so very.
When I was starting Primary flight training in Pensacola, Florida back in my long-lost, oft-lamented, egregiously misspent youth, we had two choices of machines to fly. The first was the venerable (that’s a code word in aviation meaning “old”) T-28 Trojan, a real beast of a machine with performance comparable to a World War II fighter - the Navy version had over 1,400 horses under the cowl and red-lined at over 300 knots. Not to mention the mechanical complexity of a radial engine trainer - say “cowl flaps and blower” to a fresh-faced flight student and watch him turn quizical - that’s a lot of aircraft to learn how to fly in.
Our other option was the T-34C Turbo Mentor - an aicraft known universally to the Trojan set as the “Turbo Weenie” because of the fact that, while it had a miniature jet engine turning it’s propeller - hence the “Turbo” in the aircraft name - it had barely a third the static horsepower of the T-28.
T-28 students had what passed among primary flight students for a devil-may-care panache: We used to joke that if there wasn’t any oil leaking from a T-28 the aircraft had to be down - the only reason could be that there wasn’t any left in the casing to leak - parts were hard to come by, and there was that great big, monster of an engine up front. Another common piece of wit was that you could tell the Trojan student by the bulging of his right thigh from battling all that torque.
There are three elements in play that make a prop-driven aircraft want to yaw the left during a takeoff roll: Torque, prop wash and p-factor. Torque is Newton’s old saw about “equal and opposite reactions” in play. It’s also what spins the prop, so it’s not like it’s a bad thing, but anyway.
Seated in the cockpit, the propeller is seen to rotate in a clockwise direction. If it was suddenly liberated from the engine mount, a high speed prop would want to walk briskly to your right until it either ran out of energy, or ran into something unyielding. Since it’s a fundamental part of the maintenanceman’s job description to ensure that it can’t get free, the opposite reaction generated is for the airframe attached to the prop to want to walk in the opposite direction, i.e., to the left. This is sensed as a yawing moment around the airframe’s horizontal axis. On the ground this necessitates a healthy dose of right rudder to keep the plane tracking down runway heading for the take-off roll. Conversely, a passive pilot could allow the plane to drift off the prepared surface and into the turf, but that attitude is robustly discouraged.
The same thing happens in flight, but the phenomenon gets a different focus - since the wheels are liberated from contact with the earth, the tendency is for the aircraft to want to roll in the opposite direction of the prop’s spin rather than “walk” (i.e. yaw). Roll is counteracted not with rudders, but with ailerons - in this case, right aileron to counter a left roll. A right-roll stick command puts the left aileron down (increasing lift on the left wing) and puts the right aileron up, decreasing lift on that wing. As we increase lift we also increase induced drag, which is that component of total drag generated as a function of lift (the other main component of total drag is parasitic drag, which is the resistance of the airmass to the airframe). Against the increased drag on the upgoing left wing, right rudder is applied to keep the nose and wings in “coordinated flight.”
The instrument below is found in prop aircraft to keep the pilot honest (and an analogue is found in jet aircraft to remind him of his roots, like):
In an uncoordinated (or “skidding”) turn to the right, the aircraft wingform symbol would reflect a right angle of bank but the ball - the black disk in the center of the liquid gauge - would be out to the right. “Right ball, right rudder” is used to bring it back to center. IP’s who considered themselves wags would sometimes pound on one side or another of a grossly out of balance aircraft with their fists, often causing their wild-eyed students to ask in alarm on the intercomm, “What is that banging sound?”
“Oh, that’d be the turn needle ball, trying to get back into the airplane,” he’d reply. Whether or not you found that sort of thing funny depended greatly on your perspective, I suppose.
Prop-wash is an effect related to the helical flow of air coming back off the propeller. It moves in the same direction as the prop, so the air on the right side of the airframe is moving “away” from the vertical tail while that on the left is moving “towards” it. Impinging on the left side of the tail it generates a left yaw moment around the horizontal axis which must be countered by, you guessed it, right rudder.
Finally, there’s the dreaded “P-factor.” The downgoing prop blades on the right side of the disk (seen from the cockpit) have a higher angle of attack - and therefore generate more thrust - than the upgoing blades do on the left. That provides a relatively weak (compared to the other factors) push to the left that once again must be countered by right rudder.
All of these effects reach their maximums at slower speeds when aerodynamic controls are less effective. Like on a carrier landing, or, heaven forbid, a bolter or waveoff wherein the guy who’s cobbing the throttle(s) up from an underpowered condition doesn’t have enough right rudder authority left to get the damned thing safely airborne again without going over the side.
Which, pace our too-long neglected E-2 brethren, is why the Navy was so very keen to get out of the whole propeller driven flight operations scheme just as soon as we could. Not even going there on the whole “critical engine” thing. And even if it weren’t easier to impress the chicks at the O’Club as a jet guy.
Which, you know: Priorities and all.
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