Please un-torque my neurons.... Prop effects et al...

[jcomm]

This very same questions have been asked certainly many times, on many forums, and I guess I even might have already raised the problem in other forms here at PPrune, but I really have to understand what's going on between Real Life, Theory and Flight Simulation....

I am a glider pilot since 1980. Have flown many times on prop aircraft, and even had the chance to "drive" a few, but I do not have a PPL for motorized aircraft other than SSG / SLG.

I have long been using flight simulators, and have found in almost all of them the prop effects: Torque, P-Factor, Gyroscopic Precession and Spiraling slipstream) translated mostly into roll instead of yaw, while, IRL, it is mostly yaw, and not bank, that results in the first place, on most GA aircraft ( naturally, a WW2 fighter is a different matter...) causing the nose of the aircraft to drift to the left ( on a CW rotating prop ) and the pilot to use right rudder to counter it. When he does it, the ball centers, there is no sideslip, and the wings are level.

Let's assume the aircraft in this post have all CW-rotating props.

Now, the books, and many texts for aviation name the torque effects as causing a rolling moment ( to the left ). The simulators I have used translate mostly into roll the sum of the 4 above mentioned prop effects, and you really have to use aileron, or aileron trim, because if you use only rudder, there will be a lot of it required to level the wings, and when they're level, "your ball" will be almost fully to the left (???) I always found this representation of the prop effects in most flight simulators not precise.

When I talk to fellow pilots and ask them about what they feel on their Cessnas, Pipers, Robins, etc... they all say that it's mostly yaw.

When I try to refute the arguments of flightsim designers they, most of the time, come up with a justification based on the fact that the pilot almost unconsciously adds yoke / manche inputs to counter the roll from torque, and levels the wings, but RL pilots tell me this is not the case, and that they can even let the hands off their yokes while they use pure rudder to counter the prop effects at high power / high AoA, keeping the ball centered and the wings level, without an hint of aileron or aileron trim use.

Also, most GA aircraft do not even have trimable rudder or ailerons and some have only a rudder trim. Yes I am aware that they have fixed trim tabs, some on the rudder others on the ailerons too.

Now, can you help me understanding why:

- In RL apparently the prop effects translate mostly into YAW, and of course roll will naturally be induced by if you do not take action to counter the yaw and stop the sideslip;

- In Theory, and many aviation syllabus the roll due to prop/engine torque effects is mentioned as existing and playing it's role;

Thanks in advance for any help/suggestions :-)

[A Squared]

Well, here's my thoughts on the matter:


First, gyroscopic precession.

There is no gyroscopic precession (acting in yaw) when there is no pitching motion. That's not to say that it doesn't exist; it certainly does when the tail comes up on a high powered taildragger during the takeoff roll. But in a steady state climb in a stable constant pitch attitude, there is no precession going on. So if we're talking about the forces in a constant climb, we can ignore precession.

Next, reactive torque. Sure there is some, and it does act around the longitudinal axis of the plane, but how big is it really?

well lets consider the C-172 engine which develops 150 hp at 2700 rpm. That equated to 291 ft-lbs of torque at full power and rpm. So how much is that in real terms? It's about the same as having a 200 lb passenger sitting in a seat 1.45 ft to one side of the aircraft center line. In other words if the rolling moment from engine torque was unopposed (more on that in a moment) it would be about the same force as the difference between having an adult male in the passenger's seat, and not having one. It's not non-existent, but it's also not very significant.

The thing is, it *isn't* unopposed, which brings us to spiraling slipstream. Personally, I suspect that the propeller slipstream doesn't "spiral" quite as much as it's made out to in the primary flying texts. But, no doubt it does some, and whatever spiraling it does will tend to roll the airplane as it impinges on the wings, fuselage and empennage. But it tends to roll the airplane *opposite* the reactive torque from the engine. So, it tends to reduce the already, "not very large" rolling moment from the engine torque.

So that leaves us with P factor, or asymmetrical propeller disc loading, which in a low airspeed, high power climb, is the predominant force and acts only in Yaw (or very nearly so)

so to summarize:

Precession: Absent in a constant climb

Engine torque: Produces relatively small rolling moment in a typical GA aircraft.

Spiraling slipstream: Serves to counteract rolling moment from engine torque.

P-factor: No significant rolling moment.


Now, I fly single engine GA aircraft (although regrettably not as much as I'd like to) and like the others you've talked to, I'm not aware of having to hold significant aileron input on a climb, but I am aware of the need to hold rudder. No doubt I do, subconsciously, hold a very small amount of aileron.

Now, I don't fly single engine flight simulators, but if the required aileron input in a *real* airplane is well below the consciously discernible level (and it is) , and the required aileron input in the simulator is well above it, (like you say) then it would seem to me that the simulator is not programmed to be an accurate representation of the real thing.

I'm not sure how you'd conclude anything else.

[jcomm]

A Squared,

thank you so much for having so clearly explained your oppinion on this subject.

I agree with every sentence you have written, and indeed I also use the same arguments next to those who keep finding that the excessive roll due to torque is correctly modeled on some flight simulators.

The spiraling slipstream hitting the vertical fin / rudder, which is usually above the CoG of the aircraft, as well as the down sides of left wing and left side of the horizontal stab also accounts for a moment that is opposed to that of reactive torque. Add to it the mild force that is actually produced by the engine/prop torque on your C172 example, and we have excellent reasons to understand why roll is far from being the main effect...

All summed up together will certainly account for very mild roll effects as opposed to yaw - which indeed should / is the prevalent effect.

[phiggsbroadband]

Hi, the problem is that all these unbalancing forces can be cancelled out by trim tabs and/or uniform loading (e.g. having two people in a twin seater.) However the balance only occurs for one set of conditions... Alter anything and the plane will no longer fly hands-off. The two examples I can quote are as follows...


During my QXC in a Tomahawk I was obviously flying solo and the weight of just me in the LH seat made the plane pull to the left quite a lot. After the first landing I moved my heavy pilots case from the back to the RH seat, and that made some improvement.


The second example is a C172, this is rigged to climb at full power without hardly any rudder requirement when two up. However in the cruise at the lower power setting, the rudder trim needs setting two notches to the left to centralise the ball... This in turn requires a slight amount of right aileron to stop the resulting turn.

One further factor is that fuel can migrate from one tank to the other if the fuel selector is set at 'Both'. If the wings are not level, the condition can get progressively worse, as fuel transfers to the lower tank, and it also does not help if you park with one wheel in a hollow. Also some aircraft without a 'Both Selector' only use fuel from just one tank at a time.

[hawk37]

Wow, A Squared, you should write a book.

I have a question for you on spiraling slip stream. Is there not also a significant yaw effect on the fuselage and vertical fin?

[A Squared]

Yes, there would be a yaw effect, I neglected to mention that as I was focusing on what did or did not create a rolling effect. But you're right that the spiralling slipstream would create both a roll (counter to engine torque) and a yaw. Whether it can be called *significant* or not, I don't know. I believe that it is substantially less that the P-factor yaw (but in the same direction.)

[dubbleyew eight]

the spiralling slipstream is largely a nonsense.
what happens is that the air flows straight back over the fuselage.
as the propellor blade scythes through the air there is a small sheet of vortices coming off the trailing edge. these are quite thin. these then move back in the slipstream and appear to spiral down the fuselage.

you can prove this for yourself with a piece of tape in the oil streaks on the underside of the fuselage. if you accidentally get it right you get a 2 per rev flap flap flap sound as the vortex sheet passes which is totally dependent on rpm.
look in the oil streaks after the flight and it will be apparent what is occurring.

a squared has given a pretty good accounting btw.

[A Squared]

Thanks. In an earlier post I said I was skeptical that there was much spiraling going on. My reason for questioning that like you describe. I used to fly the DC-6. The engine nacelles, especially the longer inboard ones were a fairly regular cylindrical shape until they met the wing. There was always a plentiful supply of engine oil running everywhere, and there were plenty of streamline streaks of dirty oil on the nacelles which pretty clearly indicated where the air was flowing (you could see how it flowed around the intake and oil cooler scoops, for example) but one thing that you couldn't see (and I studied it pretty closely) was *any* indication that the air flowing down the nacelle was spiraling. That's where I started to develop my skepticism about the "spiraling slipstream" theory.

[Meikleour]

Doubleyew eight: regarding your post, I have been flying a tail wheel single off rather muddy private strips and it is very obvious that more mud splats appear on the port side of the fin and tail plane. Why would that occur with symmetrical flow from the prop?

[dubbleyew eight]

vortex sheets have more energy than the straight back flow between them.

[jcomm]

@dubbleyew eight:

interesting your thoughts regarding the true importance of the slisptream itself, but then a question rises in my mind... Do you think that the force exerted by that spiraling flow, and the way it hits some aircraft surfaces asymmetrically, like the low section of the left wing, the tail fin and rudder, etc... should not be considered as opposing the rolling moment due to torque only ( this because there will always be yaw-induced roll if the pilot does not take action to center the ball and eliminate the sideslip... ) ?

Since I thought that that slipstream hitting the lower section of the left wing and also the vertical fin, above the CoG, would account for a right rolling moment, and thus oppose the torque-based left rolling moment, that would "kill" my theory :-/

[phiggsbroadband]

Does anyone know what angles are used to set the engine crankshaft in shall we say a Cessna 172. What Side and Down Thrust is engineered into the mounting cradle?

[flyer101flyer]

Here is a relevant point-- take an engine with clockclockwise-rotating prop (as viewed from rear) and mount it high up at the mid-fuselage like this http://blog.aopa.org/letsgoflying/wp-content/uploads/2010/09/My-New-Talon1.jpg, and what happens? Now you need to hold LEFT rudder, not right rudder, in a full-power climb at high angle-of-attack / low airspeed. Even at cruise power/ cruise airspeed, the aircraft will tend to need some left rudder, if there is no compensating trim tab.

Why? Surely because with the high engine/ prop location, the fin is primarily feeling the BOTTOM half of the spiralling slipstream, not the top half. So the aircraft tends to yaw right, not left.

Many, many ultralights and lightsport aircraft share this configuration and they all experience this effect. I have some firsthand experience with such aircraft. Take a look at the trim tabs on the rudders of such aircraft-- they are always set to cause the rudder to deflect to the left to help compensate for this effect. The OPPOSITE of what we typically see when we look at the trim tab on the rudder of a more "conventional" GA plane, with the SAME direction of prop rotation.

You CAN'T explain this with P-factor, engine torque, etc. It's got to be due to the yaw effect of the spiralling slipstream striking the vertical fin.

You can't tell me that the spiralling slipstream is not important! It certainly affects the balance of YAW torques. And through slip/ dihedral coupling, this has an influence on roll.

The balance of ROLL torques is another story. In the case of a clockwise-rotating prop, as the prop spins the air clockwise, the aircraft will tend to roll counterclockwise (left). Any ROLL torque directly created by the impact of the spiralling slipstream against the wings, fuselage, etc will take some of the "spin" out of the propwash, which will REDUCE BUT NOT ELIMINATE the net left roll torque created by the prop. It's an issue of conservation of rotational momentum. It is well explained here: section 9.5: *9**Roll-Wise Torque Budget (http://www.av8n.com/how/htm/roll.html#sec-propeller-drag)

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 (http://www.av8n.com/how/htm/yaw.html#sec-p-factor-angle)

We can deduce the same by noting that even with a clockwise-rotating prop, these aircraft with high-mounted engines tend to yaw right at high power, not left. P-factor would induce left yaw, just as with any more "conventional" aircraft with the same direction of spin of the prop.

We do all understand slip-roll coupling due to dihedral, right? If the ball is significantly off-center (say to the left), that generally means the aircraft is flying a bit sideways through the air (in this case a "yaw string" would blow toward the right). That sideways airflow interacts with dihedral to make a "downwind" roll torque (toward the right in this case.) To look at any roll effects from the prop that are NOT due to yaw/ sideslip, you'll need to first start by applying whatever rudder pressure is needed to center the ball, or more precisely, to achieve zero sideslip. (Yes there is a difference between these two things, as becomes apparent when we are dealing with very large rudder deflections-- eg single-engine flight in a twin-engined aircraft-- but that's a rather fine point in the context of a single-engine aircraft. Zero sideslip is achieved with slightly less rudder deflection than would be needed to fully center the ball.)

Re the original question-- let's say we have a prop effect-- which again I contend is mainly due to the spiralling slipstream-- that makes a left yaw torque. The nose will yaw some degrees to the left of the flight path, displacing the ball to the right and then the net yaw torque will be zero. Now the aircraft will fly along at a constant yaw/slip angle-- the ball is deflected to the right. The sideways flow over the aircraft will have a SMALL tendency to drive an unbanked left turn. But the sideways flow over the aircraft will also interact with dihedral to make the aircraft roll left. As the bank angle increases, this will create a much stronger left turn than would ever be created by the yawing/slipping condition when the wings were level. Sure, if you try to fix the problem by holding right aileron, the adverse yaw from the deflected ailerons will shift the ball further to the right. You can fly in a straight line this way, but not quite wings-level, and it's certainly not efficient-- the wind/ airflow is slamming into the right side of the fuselage. If you had fixed the problem by applying right rudder to center the ball, you would never have needed to apply the right aileron input. As a glider pilot you understand the yaw string. The ball is essentially the same, it just moves the opposite way. Center the ball with the rudder before you figure out what aileron input is needed. You might be pleasantly surprised. You are interpretting the fact that you are holding lots of aileron, as an indication that the prop is mainly making the aircraft roll, not yaw/slip . That's not a valid conclusion. The prop is making the aircraft yaw/slip , and because you aren't fixing that with the rudder, dihedral is then making the aircraft roll into a banked turn, unless you hold some aileron input.

PS Sorry-- I just re-read your question-- you are stating that on these sims, holding rudder to center the ball does not leave the aircraft balanced in roll, and holding rudder to stop the aircraft from turning does not leave the ball centered. It sounds like the sims are modelling a strong left roll torque from power, even with the ball centered or nearly so. In theory there ought to be SOME left roll torque from power, that should need some left aileron to counteract (w/ ball centered), but it sure sounds like these sims may be over-doing it. But maybe that is indeed realistic for prop-driven aircraft with very powerful engines-- that's outside my area of experience.

PPS re oil streaks on the fuse etc-- it really doesn't take that much spiral in the slipstream to have a strong effect on the vertical fin. A few degrees of change of angle-of-attack of the fin will surely have a significant yaw effect.

[jcomm]

Flyer101Flyer,

excellent explanation to add to the above contributions!

It's all been very important to try to organize my ideas on this tricky ( in the world of flight simulation... ) subject!

Thank you.

[chksix]

phiggsbroadband Does anyone know what angles are used to set the engine crankshaft in shall we say a Cessna 172. What Side and Down Thrust is engineered into the mounting cradle?

I'm interested in this as well. I've tried finding that info for a 152II but it seems it's unavailable.
The closest I've got is this image from the manual where I added "sight" lines to guesstimate the angle of the prop.
http://i58.tinypic.com/34ewc4m.jpg

[phiggsbroadband]

Hi chksix, just went for a flight in the C172 today, and took a plum-bob.


It showed that the lower tip of the prop is a good 4 inches in advance of the top blade, making an angle of about +4 degrees wrt the vertical with the nose-wheel suspension at its usual 3 inch extension.


I could not detect any significant side thrust, as the canopy is not too solid, and my measuring string was a bit elasticy.

[chksix]

Cheers! That confirms that the picture from the manual is accurate.

[Brian Abraham]

Might interest you jcomm

Is P-factor For Real? | Flying Magazine (http://www.flyingmag.com/technique/proficiency/p-factor-real)

[A Squared]

Might interest you jcomm

Is P-factor For Real? | Flying Magazine (http://www.flyingmag.com/technique/proficiency/p-factor-real)

Interesting article. The author talks about what happens when you climb inverted, which according to the respective theories behind P-factor and "spiraling slipstream", would cause them to negate each other, rather than reinforce each other.

But inverted flight required left rudder rather than right. The overriding force, therefore, was P-factor; slipstream rotation had no noticeable influence... Crow found the same in his Extra: right rudder upright, left rudder inverted and no apparent contribution from the slipstream.

[pattern_is_full]

One factor that no one here has mentioned (and Pete Garrison mentions only in passing in that Flying article) is that - if there is torque roll imparted to the aircraft while on the ground, counter to the (clockwise) prop rotation, it will tend to push the left tire down harder and increase tire drag, adding to whatever left-yaw tendency exists from other sources (p-factor, etc).

After take-off, of course, that extra yaw impulse disappears.

And, yes, I'd say desktop sims tend to overcook the apparent effects (along with other things, such as X-winds). They only have one dimension in which to replicate and "teach" the complexities of flight control (no 3D environment, no kinesthetic "seat of the pants" motion), so they tend to overdo what the computer can show, in order to make the sim more challenging.

[A Squared]

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:


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 (http://www.av8n.com/how/htm/yaw.html#sec-p-factor-angle)

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.

[flyer101flyer]

* 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/Faculteit/LR/Organisatie/Afdelingen_en_Leerstoelen/Afdeling_AEWE/Aerodynamics/Contributor_Area/Secretary/Publications/publications/doc/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.

[flyer101flyer]

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.

[jcomm]

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 ???

[A Squared]

Or... am I completely missing something here ???

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.

[jcomm]

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 (?)

[flyer101flyer]

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 (http://www.av8n.com/how/htm/roll.html#sec-propeller-drag) :
"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."

[phiggsbroadband]

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.