Please un-torque my neurons.... Prop effects et al...
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Flyer 101. Your entire reasoning dismissing p-factor is based on a faulty premise:
That a prop mounted in line with and aft of the wing, will encounter the air at the same angle as a prop conventionally mounted on the forward fuselage ahead of the wing, given the same conditions of pitch attitude, climb angle and prop shaft axis mounting angle.
That, as I said, is a badly flawed premise and completely negates the conclusions you've drawn. Here's why:
On a conventional airplane, if we assume that the disturbance of airflow ahead of the physical airplane is negligible *, the angle at which the prop encounters the airflow is going to be quite simply the attitude angle - the climb angle +/- the angle of the prop shaft to the longitudinal axis on the airplane. We're speaking of the angle of the airflow relative to the spin axis of the propeller, not the effective angle of attack of the prop blades, just the angle the air flows thru the prop disc.
The same is *not* true of an airplane with the prop mounted in line with and behind the wing, as in the photo you linked. On such a plane the prop is *not* encountering undisturbed, or minimally disturbed airflow. It is right in the air flowing off the training edge of the wing. And whatever your favorite explanation for how a wing generates lift, what is *not* in question is that the wing deflects air downward. So, if the axis of the prop is aligned with the chord of the wing, it will encounter descending air when the wing is generating lift. If it is aligned with the longitudinal axis of the aircraft, the air flowing thru it will be descending to an even greater degree. It appears from the picture (and common sense would seem to suggest) that the latter is the case rather then the former. In any case, the airflow thru the prop will be descending, relative to the rotational axis of the prop. Which is the opposite of the angle of airflow thru a conventionally mounted prop. Which according to the theory of P-factor, would predict that for a prop turning clockwise when viewed from behind, the p-factor would produce a *right* turning tendancy. Which is exactly what you report. QED.
There is one comment of yours which warrants a specific response:
Yes. We read that. The thing is, it is a completely unsupported assertion. The author just claims that it is true, without offering a shred of empirical evidence, anecdotal evidence, theoretical reasoning, or any suggestion of how he has concluded that is true, just that he states that it is true, and he expects it to be accepted as true because he states it is. Now it may be that he has what he believes are compelling reasons for stating that, but if he doesn't say what those reasons are, his claim has exactly zero value. He could have claimed with equal validity that left turning force was caused by gyroscopic precession or aliens.
* Objects moving through a fluid affect the fluid farther ahead of the object that many realize. Watch a video of an airfoil in a wind tunnel with smoke streams. When the airfoil is generating lift, the smoke streams are deflected upward well ahead of the airfoil. And yes, fluid is correct. At 172 speeds the air is a fluid, for all practical purposes. Many people visualize air as compressing as it flows around and airfoil. At those airspeeds (it's actually related to mach number, which isn't exactly airspeed, but we're drifting way off topic) it doesn't compress or squeeze or change density any significant amount, so it's acting as a fluid.
That a prop mounted in line with and aft of the wing, will encounter the air at the same angle as a prop conventionally mounted on the forward fuselage ahead of the wing, given the same conditions of pitch attitude, climb angle and prop shaft axis mounting angle.
That, as I said, is a badly flawed premise and completely negates the conclusions you've drawn. Here's why:
On a conventional airplane, if we assume that the disturbance of airflow ahead of the physical airplane is negligible *, the angle at which the prop encounters the airflow is going to be quite simply the attitude angle - the climb angle +/- the angle of the prop shaft to the longitudinal axis on the airplane. We're speaking of the angle of the airflow relative to the spin axis of the propeller, not the effective angle of attack of the prop blades, just the angle the air flows thru the prop disc.
The same is *not* true of an airplane with the prop mounted in line with and behind the wing, as in the photo you linked. On such a plane the prop is *not* encountering undisturbed, or minimally disturbed airflow. It is right in the air flowing off the training edge of the wing. And whatever your favorite explanation for how a wing generates lift, what is *not* in question is that the wing deflects air downward. So, if the axis of the prop is aligned with the chord of the wing, it will encounter descending air when the wing is generating lift. If it is aligned with the longitudinal axis of the aircraft, the air flowing thru it will be descending to an even greater degree. It appears from the picture (and common sense would seem to suggest) that the latter is the case rather then the former. In any case, the airflow thru the prop will be descending, relative to the rotational axis of the prop. Which is the opposite of the angle of airflow thru a conventionally mounted prop. Which according to the theory of P-factor, would predict that for a prop turning clockwise when viewed from behind, the p-factor would produce a *right* turning tendancy. Which is exactly what you report. QED.
There is one comment of yours which warrants a specific response:
PS near the end of section 8.5.2 of the same source, we read that the yaw effect of P-factor is SMALL compared to the yaw effect from the spiralling slipstream striking the vertical fin:
*8**Yaw-Wise Torque Budget
*8**Yaw-Wise Torque Budget
* Objects moving through a fluid affect the fluid farther ahead of the object that many realize. Watch a video of an airfoil in a wind tunnel with smoke streams. When the airfoil is generating lift, the smoke streams are deflected upward well ahead of the airfoil. And yes, fluid is correct. At 172 speeds the air is a fluid, for all practical purposes. Many people visualize air as compressing as it flows around and airfoil. At those airspeeds (it's actually related to mach number, which isn't exactly airspeed, but we're drifting way off topic) it doesn't compress or squeeze or change density any significant amount, so it's acting as a fluid.
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* The article about flying inverted was most interesting-- that's a good spin on these questions. It made a pretty good case that P-factor and spiralling slipstream are both important, with P-factor tending to dominate. I assume these aircraft had inverted skip-skid balls on the panel as well as upright ones.
* A Squared, that's a good point about the flow direction being influenced by the wing, in the case of a pusher prop located behind the wing. So P-factor might be minimized or might even operate in a reverse sense. I still think the yaw effect is primarily due to spiral slipstream in these cases, with the fin being so close behind the prop. I'll have to give some more thought as to how to explore this further experimentally.
* To those who are inclined to called the spiralling slipstream "nonsense"-- a few minutes of google searching on the terms "spiral slipstream tufts visualization fin" turned up this pdf http://www.lr.tudelft.nl/fileadmin/F.../2005_4_02.pdf entitled "Propeller Wing
Aerodynamic Interference"-- not a picture of tufts, but take a look at pages 16 (p.30 in the PDF) through 24 (38 in the PDF). The spiralling slipsteam is clearly described. Pay special attention to Fig 2.9 on page 23 (p. 37 in the PDF). The figure is showing a swirl angle of 3 degrees that appears to stay nearly constant as we move more distant from the prop.
Fig 2.25 on P. 43 (p.57 in the PDF) is kind of interesting -- how the prop slipstream changes the wing's angle-of-attack.
The experimental investigation section begins on p. 91 (p. 105 in the PDF).
See figure 5.43 on P. 138 (152 in the PDF) for a photo of deflected tufts due to spiral slipstream. This article focussed on prop-wing interference so it's not really what we want-- the tufts are on the wing-- we'd like to see a photo of tufts on the aft fuselage and tail-- but it's a start. A few more minutes of googling around would probably turn something up.
* A Squared, that's a good point about the flow direction being influenced by the wing, in the case of a pusher prop located behind the wing. So P-factor might be minimized or might even operate in a reverse sense. I still think the yaw effect is primarily due to spiral slipstream in these cases, with the fin being so close behind the prop. I'll have to give some more thought as to how to explore this further experimentally.
* To those who are inclined to called the spiralling slipstream "nonsense"-- a few minutes of google searching on the terms "spiral slipstream tufts visualization fin" turned up this pdf http://www.lr.tudelft.nl/fileadmin/F.../2005_4_02.pdf entitled "Propeller Wing
Aerodynamic Interference"-- not a picture of tufts, but take a look at pages 16 (p.30 in the PDF) through 24 (38 in the PDF). The spiralling slipsteam is clearly described. Pay special attention to Fig 2.9 on page 23 (p. 37 in the PDF). The figure is showing a swirl angle of 3 degrees that appears to stay nearly constant as we move more distant from the prop.
Fig 2.25 on P. 43 (p.57 in the PDF) is kind of interesting -- how the prop slipstream changes the wing's angle-of-attack.
The experimental investigation section begins on p. 91 (p. 105 in the PDF).
See figure 5.43 on P. 138 (152 in the PDF) for a photo of deflected tufts due to spiral slipstream. This article focussed on prop-wing interference so it's not really what we want-- the tufts are on the wing-- we'd like to see a photo of tufts on the aft fuselage and tail-- but it's a start. A few more minutes of googling around would probably turn something up.
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canard pusher prop observations?
It would be most interesting if someone w/ experience w/ canard pusher prop aircraft (Rutan etc) w/ no fin in the prop slipstream, would post re yawing and rolling tendencies under power. How do they vary w/ power and airspeed? What rudder input or trim is needed to center the ball, and what rolling tendency exists once the ball is centered? What direction does the prop turn (cw or ccw) as viewed from the rear of the aircraft?
W/ the aircraft in a constant pitch attitude, it would seem that P-factor would be the only thing making the aircraft yaw/ slip. W/ the ball centered, we'd expect some rolling tendency opposite the direction of prop rotation due to engine torque.
W/ the aircraft in a constant pitch attitude, it would seem that P-factor would be the only thing making the aircraft yaw/ slip. W/ the ball centered, we'd expect some rolling tendency opposite the direction of prop rotation due to engine torque.
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Thx for the link Brian!
I found the article about P-factor vs Slipstream very interesting, as well as this last article link by flyer101flyer, the observations by A Squared all of those who have posted their thoughts and explanations, and will try to "absorb" the contents as deep as my limited math, and physics in this area allow me to...
But the more I read across the thread, the more I am tempted to think that, while in most flight simulators the roll due to torque is probably the main effect being modelled, with some exceptions,
in real life it is compensated not only by the asymmetric slipstream hit of different aircraft surfaces, causing a left yaw and partially canceling the torque moment, while at the same time, minor pilot
inputs will overcome the rollig tendency, rigging will play it's role, and all together this results in, IRL, it being a lot easier to "fight" this rolling moment than in the sim, unless we're driving
a p51d, or another powerful prop aircraft ( without counter-rotating props ) and we push hard on the throttle, specially if at lower speed / higher AoA and find ourselves flipping upside down...
Another, somehow related question I have been asking myself is if there is a significative difference in the contribution to the net torque of a conventional reciprocating prop engine or non-free running turboprop,
as opposed to a free-running turbine, where there is no direct mechanical conection between the prop shaft and the engine case, other than the compressed air flow and the fixed slats on the
compressor... On such an engine / prop system, the torque comes mainly from the interaction of the prop with the airflow that it "cuts", causing the natural torque reaction, while in a convetional
reciprocating or non free turbine there are actually two cause for the torque, one inside of the engine, caused by the reaction to the force applied by the "crankshaft" and the other throught the
reaction of the airflow to the prop cutting through it. Or... am I completely missing something here ???
I found the article about P-factor vs Slipstream very interesting, as well as this last article link by flyer101flyer, the observations by A Squared all of those who have posted their thoughts and explanations, and will try to "absorb" the contents as deep as my limited math, and physics in this area allow me to...
But the more I read across the thread, the more I am tempted to think that, while in most flight simulators the roll due to torque is probably the main effect being modelled, with some exceptions,
in real life it is compensated not only by the asymmetric slipstream hit of different aircraft surfaces, causing a left yaw and partially canceling the torque moment, while at the same time, minor pilot
inputs will overcome the rollig tendency, rigging will play it's role, and all together this results in, IRL, it being a lot easier to "fight" this rolling moment than in the sim, unless we're driving
a p51d, or another powerful prop aircraft ( without counter-rotating props ) and we push hard on the throttle, specially if at lower speed / higher AoA and find ourselves flipping upside down...
Another, somehow related question I have been asking myself is if there is a significative difference in the contribution to the net torque of a conventional reciprocating prop engine or non-free running turboprop,
as opposed to a free-running turbine, where there is no direct mechanical conection between the prop shaft and the engine case, other than the compressed air flow and the fixed slats on the
compressor... On such an engine / prop system, the torque comes mainly from the interaction of the prop with the airflow that it "cuts", causing the natural torque reaction, while in a convetional
reciprocating or non free turbine there are actually two cause for the torque, one inside of the engine, caused by the reaction to the force applied by the "crankshaft" and the other throught the
reaction of the airflow to the prop cutting through it. Or... am I completely missing something here ???
Last edited by jcomm; 8th Mar 2014 at 21:52.
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Yeah, Newton's third law of motion. For every action there's an equal and opposite reaction.
So, if 15,000 inch pounds are being applied to the prop, than the prop has to be in turn applying 15,000 inch pounds to *something*. About the only thing that "something" could be is the engine. I suppose that you could make a case that in an straight thru axial flow turbine like the Allison 501, torque is applied to the engine and to the exhaust gasses flowing out the tailpipe which would result in rotational motion of the exhaust gas. I don't have any figures, but I suspect that the rotational momentum of exhaust gas exhausting from a turbine is pretty small, in no small part because the stators between the turbine stages are designed to reduce rotational flow. On second thought, the 15,000 inch pounds isn't being applied to the prop, that would be the torque the engine is applying to the reduction gearbox (RGB) as that is where tormenter measurements are taken on the 501. The engine-gearbox collectively would be applying about 17000 *foot* pounds (12 times larger than an inch-pound of course) of torque to the prop. (sorry, I'm still working on my first cup of coffee this morning) So if the rotational momentum of the exhaust gas stream form a small portion of the 15,000 inch pounds of turbine to RGB, it is a *very* small amount of the 17,000 foot-pounds of torque applied to the prop.
Oh, and re-reading you post, don't mix up the various torque action-reactions. You have to view them as individual systems: 1) Engine applies 17,000 ft-lb of torque to the prop, prop applies 17,000 ft-lb of torque to the engine. Now the prop applies some forces to the air which can't be as neatly summed up as torque on a shaft, and those forces are equal and opposite the 17,000 ft-lb of torque at 1021 rpm, but that's a separate interaction than the engine-prop interaction as far as Newton is concerned.
So, if 15,000 inch pounds are being applied to the prop, than the prop has to be in turn applying 15,000 inch pounds to *something*. About the only thing that "something" could be is the engine. I suppose that you could make a case that in an straight thru axial flow turbine like the Allison 501, torque is applied to the engine and to the exhaust gasses flowing out the tailpipe which would result in rotational motion of the exhaust gas. I don't have any figures, but I suspect that the rotational momentum of exhaust gas exhausting from a turbine is pretty small, in no small part because the stators between the turbine stages are designed to reduce rotational flow. On second thought, the 15,000 inch pounds isn't being applied to the prop, that would be the torque the engine is applying to the reduction gearbox (RGB) as that is where tormenter measurements are taken on the 501. The engine-gearbox collectively would be applying about 17000 *foot* pounds (12 times larger than an inch-pound of course) of torque to the prop. (sorry, I'm still working on my first cup of coffee this morning) So if the rotational momentum of the exhaust gas stream form a small portion of the 15,000 inch pounds of turbine to RGB, it is a *very* small amount of the 17,000 foot-pounds of torque applied to the prop.
Oh, and re-reading you post, don't mix up the various torque action-reactions. You have to view them as individual systems: 1) Engine applies 17,000 ft-lb of torque to the prop, prop applies 17,000 ft-lb of torque to the engine. Now the prop applies some forces to the air which can't be as neatly summed up as torque on a shaft, and those forces are equal and opposite the 17,000 ft-lb of torque at 1021 rpm, but that's a separate interaction than the engine-prop interaction as far as Newton is concerned.
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Again, thank you A Squared ! That really was an explanation :-)
Stators, and not slats, was what I wanted to know how to designate, but I'm Portuguese, and my English comes from high-school, long ago, so..., I wrote "slats" instead of "stators" as you correctly put it...
I will have to carefully read your post, make some sketches, and make sure I understand every piece of it, but I tend to have some difficulty in understanding that the torque resulting from the force applied by the engine to the prop, and by the prop to the airflow, aren't somehow scaled down on a free-running turbine because of there being no direct mechanical connection between the turbine and the prop shafts ( of course there are always bearings and the friction, but I assume that they have a very small impact...) other than the air leaving the tubine and entering / going through the stators and turbines of the compressor attached to the prop shaft.
At least a free-running turbine impacts the way FF reacts to prop RPM changes ( there being no FF variation along a long range of prop RPM variations ) contrarily to other types of turbines and reciprocating engines where there is a direct connection between the two...
At a fixed turbine regime, variations in prop RPM will not interact with the turbine rpm, and thus FF will not vary. So, I thought that somehow the same could happen to torque when we vary RPMs in a turboprop...
Ah! Been thinking about that inverted flight showing P-factor can be of more importance than the spiraling slipstream, but, I really think there should be some blending between the contribution of these two "prop effects".
On a taildragger, during the takeoff run P-factor will probably contribute more than the spiraling slipstream because the axis of the prop is at a considerable angle to the relative wind... But, an aircraft flying inverted is also at a higher attitude than it would normally be if flying straight, so, "the descending semi-disk" of the prop will also be at a higher AoA than usual, and that might account for the prevalence of P-factor under such occasions, while probably when flying straight, slipstream will have the higher contribution (?)
Stators, and not slats, was what I wanted to know how to designate, but I'm Portuguese, and my English comes from high-school, long ago, so..., I wrote "slats" instead of "stators" as you correctly put it...
I will have to carefully read your post, make some sketches, and make sure I understand every piece of it, but I tend to have some difficulty in understanding that the torque resulting from the force applied by the engine to the prop, and by the prop to the airflow, aren't somehow scaled down on a free-running turbine because of there being no direct mechanical connection between the turbine and the prop shafts ( of course there are always bearings and the friction, but I assume that they have a very small impact...) other than the air leaving the tubine and entering / going through the stators and turbines of the compressor attached to the prop shaft.
At least a free-running turbine impacts the way FF reacts to prop RPM changes ( there being no FF variation along a long range of prop RPM variations ) contrarily to other types of turbines and reciprocating engines where there is a direct connection between the two...
At a fixed turbine regime, variations in prop RPM will not interact with the turbine rpm, and thus FF will not vary. So, I thought that somehow the same could happen to torque when we vary RPMs in a turboprop...
Ah! Been thinking about that inverted flight showing P-factor can be of more importance than the spiraling slipstream, but, I really think there should be some blending between the contribution of these two "prop effects".
On a taildragger, during the takeoff run P-factor will probably contribute more than the spiraling slipstream because the axis of the prop is at a considerable angle to the relative wind... But, an aircraft flying inverted is also at a higher attitude than it would normally be if flying straight, so, "the descending semi-disk" of the prop will also be at a higher AoA than usual, and that might account for the prevalence of P-factor under such occasions, while probably when flying straight, slipstream will have the higher contribution (?)
Last edited by jcomm; 9th Mar 2014 at 09:25.
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Re the question about the free-running turboprop not directly connected to the propeller--
Torque is torque. When the prop speed is constant rather than changing, the ultimate reason the engine needs to exert torque, is to overcome the drag of the prop. It doesn't matter how the different bits are connected. If prop drag somehow vanished, the engine wouldn't need to make any more torque (except I guess to overcome mechanical friction-- but that torque is all "recovered" by the friction and no net torque is imparted to the airframe). Also if prop drag somehow vanished, the slipstream would no longer be spiraling, it would just go straight back. Prop drag causes the spiral. It's all connected.
I liked the way the author brought it around to an issue of conservation of momentum here *9**Roll-Wise Torque Budget :
"Using Newton’s law again, we see that if any air escapes while still rotating down to the right, the airplane will roll to the left."
Torque is torque. When the prop speed is constant rather than changing, the ultimate reason the engine needs to exert torque, is to overcome the drag of the prop. It doesn't matter how the different bits are connected. If prop drag somehow vanished, the engine wouldn't need to make any more torque (except I guess to overcome mechanical friction-- but that torque is all "recovered" by the friction and no net torque is imparted to the airframe). Also if prop drag somehow vanished, the slipstream would no longer be spiraling, it would just go straight back. Prop drag causes the spiral. It's all connected.
I liked the way the author brought it around to an issue of conservation of momentum here *9**Roll-Wise Torque Budget :
"Using Newton’s law again, we see that if any air escapes while still rotating down to the right, the airplane will roll to the left."
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quote... Re the question about the free-running turboprop not directly connected to the propeller--
Well it might not be connected via metal rods or gears, but the turbine gas forces are real enough, and for every action there is an equal and opposite re-action. So if the Prop is producing torque clockwise, the engine is producing torque anti-clockwise.
Well it might not be connected via metal rods or gears, but the turbine gas forces are real enough, and for every action there is an equal and opposite re-action. So if the Prop is producing torque clockwise, the engine is producing torque anti-clockwise.
