Engine torque and rolling.

[Cuirassier]

It is often remarked that each aircraft has it preferred direction of roll, which is in the opposite sense to the propeller's rotation.

Why should that matter ? While the aeroplane is in its trimmed condition there should be zero moment along the roll axis regardless of the engine torque.
To depart from this orientation force must be applied to the stick for rolling in either direction. Then why should it matter in which direction the plane is being rolled ? It does it from its state of equilibrium in both the cases.

I hope I got my question across.

 

[Shortround6]

at high speed it doesn't matter much.

at low speed, take off and landing, it matters a lot. And it is not quite as simple as you state. While the plane can be trimmed to a steady state of equilibrium once you start to roll you are either fighting the torque effect or it is aiding you. in other words you need more aileron deflection to roll one way than the other.

The most famous case of engine torque was the Sopwith Camel although that might be more properly gyroscopic effect. it would turn 270 degrees one way as fast as it would turn 90 degrees the other way. having several hundred pounds of engine spinning at 1250rpm does strange things to the handling of a 1500lb airplane.

 

[mikewint]

Equilibrium is not a "steady-state" condition unless the conditions themselves are fixed. Any change in the conditions voids the "steady-state". An aircraft which is "trimmed" for one set will be un-trimmed when something changes. Since torque is not a fixed force it can't be trimmed out under all conditions.

 

[mcoffee]

Newton's Third Law of Motion.
An aircraft with a clockwise turning propeller (viewed from the cockpit) is having to resist a tendency to roll counter-clockwise. To roll clockwise, it has to resist the tendency even more; to roll counter-clockwise, resist the tendency less. It will have a faster roll rate to the left.

 

[BiffF15]

at high speed it doesn't matter much.

at low speed, take off and landing, it matters a lot. And it is not quite as simple as you state. While the plane can be trimmed to a steady state of equilibrium once you start to roll you are either fighting the torque effect or it is aiding you. in other words you need more aileron deflection to roll one way than the other.

The most famous case of engine torque was the Sopwith Camel although that might be more properly gyroscopic effect. it would turn 270 degrees one way as fast as it would turn 90 degrees the other way. having several hundred pounds of engine spinning at 1250rpm does strange things to the handling of a 1500lb airplane.

I will never forget my Dad explaining how those planes had the crankshaft bolted to the firewall and the prop and engine together spun. I thought he was totally BS'ing me!

Cheers,
Biff

 

[Cuirassier]

Newton's Third Law of Motion.
An aircraft with a clockwise turning propeller (viewed from the cockpit) is having to resist a tendency to roll counter-clockwise. To roll clockwise, it has to resist the tendency even more; to roll counter-clockwise, resist the tendency less. It will have a faster roll rate to the left.

That is all true, but that tendency to roll in the opposite sense exists in completely level flight too. Only it is balanced by aerodynamic forces.
Once the plane departs from this position, the extra moment being applied is on top of the already existing moment used to counter engine torque.
At trim (Mengine + Maerodynamic )1 = 0 then it becomes (Mengine + Maerodynamic)2 + Mroll = Inertia*angular acceleration
(Mengine + Maerodynamic )1 = (Mengine + Maerodynamic )2 should be true, but it is not. Why ?

Since torque is not a fixed force it can't be trimmed out under all conditions.

True, but none of the talk about an aircraft in roll mentions accelerating the engine. Unless speeding up the engine is mentioned, it should be fair to assumed that the engine torque does not change drastically.
Why should rolling change the engine torque anyway ?

you are either fighting the torque effect or it is aiding you.

Thus my question. How would you physically define this torque effect ? Why does it aid rolling in one direction but not the other ?

 

[BiffF15]

Thus my question. How would you physically define this torque effect ? Why does it aid rolling in one direction but not the other ?

As mentioned previously, the engine wants to spin the opposite direction of the prop. When you are in level flight (stabilized) the plane is trimmed against this and therefore not rolling. When you roll the same direction as the prop you have to overcome aerodynamic forces as well as engine torque. Roll opposite from the prop and the torque helps.

Most SE piston fighters have a prop that spins CW from the pilots perspective. The stick is also held in the right hand and is easier to push left than pull to the right. Pilots end up sitting angled to the left just a touch, which combined with the effects of torque results in pilots preferring a hard turn to the left over one to the right. It's easier, faster, and more "natural" feeling. I preferred left hand patterns, and left turning fights in the Eagle as it was more comfortable or easier to look over the left shoulder while fighting than the right.

Cheers,
Biff

 

[mcoffee]

That is all true, but that tendency to roll in the opposite sense exists in completely level flight too. Only it is balanced by aerodynamic forces.
Once the plane departs from this position, the extra moment being applied is on top of the already existing moment used to counter engine torque.
At trim (Mengine + Maerodynamic )1 = 0 then it becomes (Mengine + Maerodynamic)2 + Mroll = Inertia*angular acceleration
(Mengine + Maerodynamic )1 = (Mengine + Maerodynamic )2 should be true, but it is not. Why ?

True, but none of the talk about an aircraft in roll mentions accelerating the engine. Unless speeding up the engine is mentioned, it should be fair to assumed that the engine torque does not change drastically.
Why should rolling change the engine torque anyway ?

Thus my question. How would you physically define this torque effect ? Why does it aid rolling in one direction but not the other ?

If the aircraft is using (pick a number) 5% of its available roll control to resist the prop torque, then only 95% is left to actually roll right. The aircraft has effectively 105% roll control to the left.
If you are standing on the beach in a 30mph wind, you are at equilibrium by leaning into the wind. Can you then run into the wind as fast as you can run downwind?

 

[Timppa]

at high speed it doesn't matter much.
The most famous case of engine torque was the Sopwith Camel although that might be more properly gyroscopic effect. it would turn 270 degrees one way as fast as it would turn 90 degrees the other way. having several hundred pounds of engine spinning at 1250rpm does strange things to the handling of a 1500lb airplane.

One of the persistent aviation myths.

One of the legends clinging to the Sopwith Camel is that it was so reluctant to turn 90 degrees to the right that pilots preferred making a 270 to the left. Now, this is
being said about the airplane that is widely regarded as the premier dogfighter of World War I. You have to wonder whether such roundabout tactics were practical when you
had a Fokker on your tail.

The rotary's most notorious vice was its gyroscopic couple. Like any spinning mass, the engine and its attached propeller resisted efforts to change their orientation; when
forced, they pulled at a right angle to the pressure applied. In a steeply banked turn to the right, for instance, a Camel wanted to drop its nose toward the ground; you
had to use left rudder and back stick to hold it up. In a left turn, the nose wanted to slice upward; you corrected with left rudder and forward stick.

I have the good luck to be friends with Javier Arango, who owns a couple of Camels — one a reproduction, one original. They form part of The Aeroplane Collection, based in
Paso Robles, California. Their stablemates include Fokkers, Nieuports, Sopwiths, a SPAD, an S.E.5a and the oldest flyable airplane in the world, a restored Bleriot
originally built in the United States in 1911. Almost all are airworthy and use original engines. Javier, who has a Harvard degree in history of science, says the mission
of his collection is to gain a deeper understanding of the very rapid evolution of aeronautical technology during the war. The builders of the airplanes, unfortunately for
us, were too busy to document it.

A few years ago, Javier and I set out to collect some flight data in order to compare contemporary accounts with objective measurements. One goal was to assess the
magnitude of the rotary's famous gyroscopic couple. Javier himself had not found the Camel's flight behavior disturbingly asymmetrical, but he knew he was probably
unconsciously correcting for it.

We have progressed rather slowly, having so far tested only a Sopwith 1½ Strutter, which is a comparatively big two-seater, and a Camel. A Fokker Triplane will be next. Our
equipment consists of an Appareo GAU 1000, which stores a detailed history of flight attitudes and accelerations; a Futek stick-force sensor; and a data logger that stores
stick forces, control-surface positions and airspeed.

The Camel is a very small, light machine — 1,300 pounds as tested. It's roughly comparable to a Cessna 150 but with half again more wing area and who knows how much more
drag. On the other hand, with a 160 hp Gnôme engine swinging a 9-foot propeller at 1,200 rpm, the Camel climbs well: We recorded almost 1,700 fpm. The tests, Javier flying,
consisted of a series of turns, climbs, dives, and large, abrupt control movements. We investigated speeds down to 35 knots and up to 83, continuous bank angles over 70
degrees, and pitch and yaw rates of between 20 and 30 degrees a second.

The Camel's agility as a fighter was due in part to its notoriously weak stability in all axes. Its center of gravity was far aft, particularly with full fuel — the 30-
gallon tank sat behind the pilot, in lieu of armor. Nevertheless, it must have possessed some longitudinal stability because it had no trim and the pilot had to hold
forward stick after takeoff. If it had been neutrally stable or unstable, it would not have sought a preferred pitch attitude.
It had a vestigial fixed fin, an aerodynamically balanced rudder and a rather short aft fuselage. As a result, it didn't much care which way it pointed. It had no
inclinometer — the "ball" of a modern panel — so the pilot relied on the seat of his pants to stay coordinated. Most of the time, Camels were slipping or skidding; this was
a good thing, one veteran wrote, because an enemy trying to get a bead on you could not quite tell which way you were going. The ailerons, though large, seem to have been
rigged with reverse differential — more down travel than up — so that if you tried to turn with aileron alone, as you can in a modern airplane, you would see lots of yaw in
the wrong direction.

Our tests produced a lot of complicated-looking graphs. Some of the most striking are the time histories of steep turns. The airplane rolled into a 60-degree bank to the
left or right in about 2½ seconds, but the maximum roll rate, reached only momentarily, was about 40 degrees per second to the left and 30 degrees per second to the right.
No surprise there; left roll is torque aided. That the steady turn rate was about the same left and right was not surprising either: All airplanes turn at the same rate in
coordinated flight at a given speed and bank angle. The story about the Camel making a left 270 more quickly than a right 90 was evidently just a comical embellishment of
the fact that the Camel rolled into a left bank more easily than into a right one.
When a Camel pilot wanted to invert the airplane quickly, he would probably use a snap roll, not an aileron roll. It may in fact be true that the Camel snaps faster to the
left than to the right; this is a maneuver that Javier did not test.

To assess the gyroscopic moment, we measured the polar moment of inertia of a 160 Gnôme and its propeller. Putting that number together with rpm and a yaw or pitch rate,
it's easy to calculate the famous gyroscopic couple. For the turn rates we saw in our experiments — which were probably close to the greatest rate that a Camel could
maintain continuously for a long time — the moment was about 300 pound-feet. The tail moment arm is around 11 feet, so a tail force of 27 pounds or so would be needed to
balance it. That should be well within the capability of the tail at the 70-knot airspeed required to maintain, say, a 3G turn, but it might require significant control-
surface deflections.

It was said that a Camel could evade an attacker by maintaining a tight right turn until the opponent grew bored and went away or both airplanes reached the ground. That
makes some sense: Dogfights were typically fought while descending, and in a right turn, the gyroscopic couple pulls downward; the controls don't have to fight against it.

Calculated Sopwith Camel

 

[Shortround6]

Thank you

 

[soulezoo]

Thank you indeed... I was another who has believed that "myth" since childhood reading the books of the day.

 

[Sid327]

It is often remarked that each aircraft has it preferred direction of roll, which is in the opposite sense to the propeller's rotation.
Why should that matter ? While the aeroplane is in its trimmed condition there should be zero moment along the roll axis regardless of the engine torque.
To depart from this orientation force must be applied to the stick for rolling in either direction. Then why should it matter in which direction the plane is being rolled ? It does it from its state of equilibrium in both the cases.
I hope I got my question across.

In S&L flight the plane's ailerons will be trimmed to cancel out the amount of torque at that particular power setting, to enable hands free, stable (wings level) flight.