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Prop Torque Direction?
- Thread starter lionheart
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Bill, I'm pretty sure that's a conventional Lycoming style engine and it will go left...and require right rudder. I often read it’s a counter-clockwise rotation but I believe they mean facing the engine. The down blade, starboard-side in this case, has a higher AOA than the up-blade when aircraft has a positive AOA. So there’s a bit more thrust on that starboard-side…that’s a push left. The engine torque is opposite the prop, it will load the port-side…another pull left. The prop wash spins with the prop so it’s the port side v-stab that gets the force. And again a push to left.
There is a gyroscopic effect from the spinning prop too.
I'm not sure, but I believe this turns a pitch force into yaw force and vice versa.
I think a pitch up may actually yaw right...so what can you do
...three out of four forces prefer left.
There is a gyroscopic effect from the spinning prop too.
I'm not sure, but I believe this turns a pitch force into yaw force and vice versa.
I think a pitch up may actually yaw right...so what can you do
...three out of four forces prefer left.
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That's basically correct, precession means a force applied to a gyroscope will act at 90 degrees in the direction of rotation. So for a clockwise turning prop (from the pilot's seat) pitching up is effectively applying a forwards acting force to the bottom of the propeller disc, which 90 degrees later is a forwards acting force to the left hand side of the disc, yawing you to the right.
One thing about FSX at least is that there's an initial yaw in the wrong direction before torque and the p-factor* take over.
*This is the technical term for the asymmetry in lift caused by the downward going propeller having a greater angle of attack.
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Many thanks guys and thanks for the technical side of it as well.
It reminded me of some of the stories of early planes, WWI era, where the giant wood props acted as gyro's so moderately that they had to have tactical moves related to the direction of the gyro actions, like a right turn to dive, left turn to climb on some planes.
The little Demoiselle also had this issue.
I wonder if mass/weight of the prop adds to the gyroscopic properties? For instance, if a prop is all composite (Carbon fiber), and weighs nearly nothing, then wouldnt its gyroscopic effect be far less? For instance, if you had a iron gyro, about 5 LBS, and a gyro made of resinated cardboard, wouldnt the cardboard have almost no effect, where the iron disc would be a giant battery of gyroscopic energy?
Bill
It reminded me of some of the stories of early planes, WWI era, where the giant wood props acted as gyro's so moderately that they had to have tactical moves related to the direction of the gyro actions, like a right turn to dive, left turn to climb on some planes.
The little Demoiselle also had this issue.
I wonder if mass/weight of the prop adds to the gyroscopic properties? For instance, if a prop is all composite (Carbon fiber), and weighs nearly nothing, then wouldnt its gyroscopic effect be far less? For instance, if you had a iron gyro, about 5 LBS, and a gyro made of resinated cardboard, wouldnt the cardboard have almost no effect, where the iron disc would be a giant battery of gyroscopic energy?
Bill
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It wasn't the props that were the problem, it was the rotary engines, not to be confused with radial engines which look similar but have one crucial difference. Basically to aid air cooling of the early engines the whole thing spun round with the prop directly attached to it and the crankshaft attached to the aircraft. This helped with the cooling, but had the drawback that throttle control was basically on or off and most of the mass of the aircraft was spinning around at a few hundred RPM making it a massive gyroscope.
To confirm your musing, the 'strength' of a gyroscope is directly related to its mass and rate of rotation. Although a heavier prop would have greater rigidity* than a light one I don't think the difference would be noticeable in the controllability of an aircraft as the other forces acting on the airframe would mask it anyway.
*Rigidity is the tendency for a gyroscope to keep pointing in the same direction, the greater the rigidity the less susceptible it is to external forces acting on it. For avionics it's generally preferred to increase rigidity by spinning them faster, rather than making them heavier.
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For FSX it's a case of changing
rotation = 1
to
rotation = -1
in the [propeller] section.
I then had a few problems with the animation not liking what I was doing, but that was probably my fault and I got round it by rewriting that section of modeldef.xml Something to do with it making all the percentage rotation values negative when the prop goes anti-clockwise.
rotation = 1
to
rotation = -1
in the [propeller] section.
I then had a few problems with the animation not liking what I was doing, but that was probably my fault and I got round it by rewriting that section of modeldef.xml Something to do with it making all the percentage rotation values negative when the prop goes anti-clockwise.
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Well, that did change the direction of rotation of the prop, but not the direction of torque steer. I wonder if mine is already proper?
I need the prop to turn CW so rotation = 1 is fine. She pulls left. I'll have to see if I can dig something up that will pull her right. If I cant find anything, I'll just try to nullify it, negate the effect. The prop would be very lite, so I dont think it will have much gyro effect.
Bill
I need the prop to turn CW so rotation = 1 is fine. She pulls left. I'll have to see if I can dig something up that will pull her right. If I cant find anything, I'll just try to nullify it, negate the effect. The prop would be very lite, so I dont think it will have much gyro effect.
Bill
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Torque effect
Hi Bill,
I had the same challenge to reproduce the behavior of the F104. It has a noticeable right roll during engine deceleration and a left roll during acceleration. The reasons are a bit different to that experienced with a prop aircraft. I used the aileron trim to simulate the torque. For that I had to keep track for the raw aileron trim and the engine RPM change in a time. That works surprising fine.
What I do not understand from previous posts is the statement, that the down moving blade has a different AOA than the up moving blade. I know this only from helicopter blades. But even there, there is no change in AOA, provided, only a fixed rotor system is used. Only the speed of the air stream is different. At the forward moving blade, the TAS of the helicopter has to be added to the rotation speed and at the backwards moving blade it has to be subtracted. The result is are stronger lift force and drag at the forward moving blade. It has a funny effect if the TAS of the helicopter comes close to the rotation speed.
By my observations, the strongest force causing a prop torque is the blade drag caused by the blade AOA on today's aircraft. This is normally compensated with a static aileron trim. Another one is the torque caused by the engine rotation mass with the prop against the aircraft mass. The latter one is noticed only during power changes (the only torque with single engine jets). It even causes different torque directions during engine acceleration and deceleratin. The smallest influence on torque should be the gyroscope effect. It is only in the game if the lateral or vertical axis attitude is changed. I cannot state this for sure, it is only an observation and I would like to hear from the specialists, whether I am right or wrong .
To the main question, a clockwise rotating prop should cause a counter force in the opposite direction. So the torque moves the body counterclockwise.
Regards
Mike
Hi Bill,
I had the same challenge to reproduce the behavior of the F104. It has a noticeable right roll during engine deceleration and a left roll during acceleration. The reasons are a bit different to that experienced with a prop aircraft. I used the aileron trim to simulate the torque. For that I had to keep track for the raw aileron trim and the engine RPM change in a time. That works surprising fine.
What I do not understand from previous posts is the statement, that the down moving blade has a different AOA than the up moving blade. I know this only from helicopter blades. But even there, there is no change in AOA, provided, only a fixed rotor system is used. Only the speed of the air stream is different. At the forward moving blade, the TAS of the helicopter has to be added to the rotation speed and at the backwards moving blade it has to be subtracted. The result is are stronger lift force and drag at the forward moving blade. It has a funny effect if the TAS of the helicopter comes close to the rotation speed.
By my observations, the strongest force causing a prop torque is the blade drag caused by the blade AOA on today's aircraft. This is normally compensated with a static aileron trim. Another one is the torque caused by the engine rotation mass with the prop against the aircraft mass. The latter one is noticed only during power changes (the only torque with single engine jets). It even causes different torque directions during engine acceleration and deceleratin. The smallest influence on torque should be the gyroscope effect. It is only in the game if the lateral or vertical axis attitude is changed. I cannot state this for sure, it is only an observation and I would like to hear from the specialists, whether I am right or wrong .
To the main question, a clockwise rotating prop should cause a counter force in the opposite direction. So the torque moves the body counterclockwise.
Regards
Mike
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Sorry, should have added this applies to a tail dragger on the ground or any prop in a climb or flight condition where the aircraft angle of attack means the propeller disc isn't perpendicular to the air flow. If you're at slow speed level flight with a high angle of attack this can also be an issue, more so if you're in a twin which has just lost an engine.
In a helicopter the AoA is changed throughout a blades rotation to compensate for the change in relative airflow e.g. lower AoA on the advancing blade and higher on the retreating blade.
Bill, may be worth checking if the torque steer doesn't start to go in the right direction once the aircraft is moving down the runway/airborne. Seems to be a bug in FS that it initially goes the wrong way for reasons I haven't fathomed out. The Sea Fury I'm working on has a lot of torque, she can flip you on your back if you slam open the throttle at low speed which caused a few accidents in real life, some fatal. I've got that modelled in FS but she still pulls the wrong way for the first few seconds of ground run.
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Blade AOA
That is an interesting topic. I just imagine a situation where your working AOA at the blades is something around 4 degrees. The maximum blade AOA is assumed at 12 degrees. If the aircraft AOA is 9 degrees then the down moving blades would come into a stall situation with a power lost on the right side of the blade disc. The result would be even a negative AOA on the left side of the disk causing a CCW momentum around the vertical axis. I know this sounds very theoretical, but is this a known issue in flight dynamics?
Regards
Mike
That is an interesting topic. I just imagine a situation where your working AOA at the blades is something around 4 degrees. The maximum blade AOA is assumed at 12 degrees. If the aircraft AOA is 9 degrees then the down moving blades would come into a stall situation with a power lost on the right side of the blade disc. The result would be even a negative AOA on the left side of the disk causing a CCW momentum around the vertical axis. I know this sounds very theoretical, but is this a known issue in flight dynamics?
Regards
Mike
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This doesn't actually happen due to the flow of air through the disc reducing the effective angle of attack. Essentially the blades experience an airflow that is a combination of the vector from the airflow through the disc (including induced flow) and the vector from the rotation of the blades, this total airflow vector or relative airflow reduces the AoA quite considerably and keeps the blades well away from the stall.
The link below has a diagram about half way down the page which probably explains it better, in the section headed Airflow.
http://www.stefanv.com/rcstuff/qf200203.html
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Skippy,
It indeed does do that. It pulls left first, then pulls to the right.
Thanks.
Sort of like the Autopilot, when you switch it on, and the heading is aligned, and AP suddenly pulls right, then back to target.
That principle was used in Lockheed P-38 and some models of Piper Seneca. Could be more, just a couple that crossed my mind instantly.
Indeed, not at all uncommon in GA planes, including more Piper models, and some WWII vintage planes both German and Allied.
Unless I've missed them there are two other effects:
1 - If the propeller is rotating clockwise viewed from the cockpit then there will will a rolling moment on the aircraft equal to the propellor torque in an anti-clockwise direction (Newton's Second Law).
2 - There will be an imbalance of propellor forces due to angle of sideslip in the same way as there is due to angle of attack.
1 - If the propeller is rotating clockwise viewed from the cockpit then there will will a rolling moment on the aircraft equal to the propellor torque in an anti-clockwise direction (Newton's Second Law).
2 - There will be an imbalance of propellor forces due to angle of sideslip in the same way as there is due to angle of attack.
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