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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 1:28 AM   
pkoury


 

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Back to the original intent of my original post; It is quite evident from just these few posts that the hobby is full of a lot of theories and preconceptions that have nothing to do with the actual engineering or science involve. I guess I’m just beating my head against the wall hoping for some improvement in the accuracy of the sport.

Again.... Just grin and bear it!

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 3:47 AM   
fledermaus



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Status: offline 		Major Tom: you are correct, but I tried to indicate that I didn't think the spirals were reflective of the airflow, just the pattern of compression caused by the blades. If there had been significant tangential airflow it would not spiral, it would move at a tangent since once the air leaves the rotating blade it must continue in a straight line.

The images I posted were really with the intent to show where the concept of spiral flow might have come from - they are NOT flow patterns but are often mis-labelled as such because they fit with the preconception.

As noted, any propeller that put substantial tangential momentum into its slipstream would be a very lousy propeller (inefficient).

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 7:00 AM   
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Status: offline 		If I may offer a couple of observations...Major Tom, the Torque reaction of any engine/prop combination is always that provided by the engine, it cares not whether thrust is produced or not, if the stick in front is being rotated at an RPM then there will be torque, and it will be determined by the engine power and RPM. A glance at the engine torque/power curves will provide you with the torque at the RPM your engine is running. This does not change regardless of the prop, if the engine is running at an RPM, there will be an equivalent torque. As for the yaw associated with rapid power changes at low speeds, this also happens at any other speed, but is less noticeable due to the greater dynamic pressure on the flight surfaces. Consider the case of the F4U, it did not get its carrier wings due to the massive torque effect of the engine/prop at 'go-around', which could, and did ROLL the airplane into the ocean when a baulked landing was attempted. The only cure for this, at low speed, was the massive fin offset. Why, you ask. Basic aerodynamics. At low speed if you try to counter torque roll you risk stalling the wing on the down going aileron side. The drag increase and loss of lift if the wing stalled would dump you unceremoniously into the drink. Now as a full size pilot you all know that that at low speed, approaching the stall you maintain balance with the rudder, ie yaw. Model pilots with heavy models will tell just the same thing. I offer this explanation, to counter the roll caused by the engine torque, one side of the wing must be producing slightly more lift than the other, and therefore, just a bit more drag. This drag is countered by rudder as the drag would tend to manifest in a yaw toward the wing producing the extra lift. Low speeds and high AOA would simply magnify the effect until as the stall is approached, any rapid increase in torque (power) may result in the inability of the wing to provide sufficient lift to counter the effect, and the wing starts to drop on that side. As with the F4U, any addition of aileron will not help, and only application of rudder (in the slipstream too) can assist. I can think of no other likely explanation.
Evan, WB #12.

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 7:59 AM   
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quote:

ORIGINAL: fledermaus

I didn't think the spirals were reflective of the airflow, just the pattern of compression caused by the blades. If there had been significant tangential airflow it would not spiral, it would move at a tangent since once the air leaves the rotating blade it must continue in a straight line.





That makes sense, to me.

And yet, we know that the air can form helical patterns in the form of wing-tip vortices. (I'm gonna get shot down, now. I can feel it."

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 10:34 AM   
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Status: offline 		Not shot down, merely (again) the wrong terms. The vortex off a wing tip (including a prop tip) is more in the form of a rotating spiral, a helix is a specific geometric construction. Or, if you like, a spiral or vortex is a bit disordered, wheras a helix is specific. Think of it as a tornado verses a machine thread. You get the picture...
Evan, WB #12.

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 10:44 AM   
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Status: offline 		Well Evan, that kid's stuff has been proven full-size. You could try yourself with a single-engine Cessna (it helps if it's one with a big prop). Run it up statically, change flap deflection and watch the left-tendency changing as well. Precession of the helical propwash isn't limited to twins.

The first drawing from the book shows what happens pressure-wise and speed-wise and why the propwash is constricted. The "outside" pressure should make for constriction and centripetal force.

The second drawing shows what happens on a blade. Lower left shows axial (thrust) and tangential (spin) speed difference caused by the blade. I wouldn't expect that on the tips, though, and put streamers inside the propwash.

Third drawing shows maximum effect at 75-80% of radius, zero at tips.

How far behind the prop the slipstream vanishes is another question, as well as how it's spoiled by wing and fuselage. Anyway, I respectfully stay away behind an airplane running up its engines. By the way, the picture of the Hercules shows what happens to the propwash when it's "cut" and deflected by the wing.

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 11:43 AM   
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Status: offline 		To refresh... the following was in answer to why a 4 stroke (longer prop at less rpm) might have exhibited less yaw than a 2 stroke on a given model.

quote:

ORIGINAL: UStik

The following might explain that (more air accelerated less and thus rotating less).




Actually, your thought might have another tangent... that more air is compressed less. A prop is an air compressor creating high pressure aft of the prop and low pressure forward. Is it then that the compressed air pocket lies closer to the fuselage and is more compressed with the shorter prop at higher rpms and thus exerts a greater yaw force? Perhaps the yaw has less to do with any spiraling as it does from simple pressure differences.

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 12:23 PM   
AA5BY


 

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Status: offline 		Expanding a little more.... the plane illustrated is a small .20 size biplane having generous down thrust. Down thrust will cause the pockets of compressed air to be asymmetrical to the fuselage with the starboard side impacting the fuselage greater than the port side. Thus, down thrust while reducing p-factor, will increase the yaw force caused by the pressure zones because of the asymmetry of impact area.

I think.

< Message edited by AA5BY -- 11/19/2011 2:13 PM >

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 3:23 PM   
Jet_Plane


 

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Status: offline 		Tomski,

The idea that spiral slipstream is just something dreamed up by modellers and 'stick and rudder', is something that in reality you seem to have 'made-up'. Spiral slipstream effect is mentioned in many authoritative sources.

For instance Don Stackhouse used to design propellers for a living, including such famous props as that which powered Rutan's Voyager around the world. Here' a quote from him:
quote:

.....the torque of the motor, applied to the air by the propeller, is what makes the air in the slipstream swirl, and that swirl is ultimately responsible for many of the slipstream effects................................
Rolling and yawing slipstream effects are due to the helical swirl that the prop imparts to the slipstream interacting with the various parts of the airplane behind it. The classic example is the slipstream of a right-handed prop swirling around the fuselage and then striking the left side of the fin and rudder. This tends to shove the tail to the right, which therefore yaws the airplane to the left. Slipstream effects are influenced by power and airspeed (these influence how much swirl the prop imparts to the airflow)


Perhaps even more convincing; Dr Martin Hepperle, whose doctoral thesis was titled "A Contribution to the Aerodynamics of Propellers, taking a Free Wake into Account" and whose Senior Thesis (written in conjunction with Prof R. Eppler) was titled "A Computer Program for the Design and Analysis of Propellers" says the following:
quote:

Besides the contraction of the stream tube, a propeller also adds a swirl component to its outflow (wake).The amount of swirl depends on the rotational speed of the engine and eats up energy, which is not available for thrust anymore. Typical, well designed propellers loose about 1% to 5% of their power in the swirl of the propeller wake. The swirl angle (about 1°...10° ) may cause non symmetrical flow conditions on parts behind the propeller, e.g. at the tailplanes.



Readers of this thread can make up their own mind but personally I'll go with the learned opinion of those acknowledged as world experts in the field of propeller design, Plus of course my own practical experience (and to be honest basic common sense).
On the other hand we have your opinion which is backed up by....errrr...nothing

Steve

< Message edited by Jet_Plane -- 11/19/2011 5:12 PM >

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 3:50 PM   
Lnewqban



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quote:

ORIGINAL: pimmnz

If I may offer a couple of observations...Major Tom, the Torque reaction of any engine/prop combination is always that provided by the engine, it cares not whether thrust is produced or not, if the stick in front is being rotated at an RPM then there will be torque, and it will be determined by the engine power and RPM.


Just to clarify: No reactive force = no counter-torque

That reactive force can be coming form drag solely or from drag plus acceleration or deceleration of the rotating body (propeller in this case).

A rotating body, at constant rotational speed and in vacuum will produce no counter-torque.


quote:

ORIGINAL: UStik

Precession of the helical propwash...............?


From http://en.wikipedia.org/wiki/Precession:

"Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle (nutation) is constant. In physics, there are two types of precession: torque-free and torque-induced."

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 3:54 PM   
rgburrill



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Status: offline 		Isn't it amazing how "P-factor" affects dragsters too?

BTW, "aviation science" is a misnomer.  Certainly there is science and engineering involved in the initial designs of aircraft but testing always shows some anomolies.  Theories come and go, or are clarified by more theories.  (To Bernouili or not to Bernouili, that is the question.)  Aviation, or more accurately aeronautics, is an art - and there are so few real artists around. 

How about a good downwind turn argument?


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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 4:12 PM   
vasek



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quote:

ORIGINAL: MajorTomski

Back when my good professors at Parks were trying their best to beat an understanding of aerodynamics into my thick headed scull in the 1970's, they did succeed in making me understand that aviation science is one of precision and attention to detail.

Rush Limbaugh summarized this best in the 90’s: ''Words mean things!''

Why then do we, as a community, continually use the word ''torque'' when we mean ''P-factor''?

Specifically the application of right thrust and down thrust to mitigate ''torque effects''.

Torque is a rolling force about the Y axis of the airplane. It is the result of the engine trying to twist the propeller against the air loads that are applied to the propeller. Offsetting the thrust line in any direction; up, down right or left cannot and does not do anything to mitigate the magnitude or effects of this torque.

P factor is the yawing force caused by the differential thrust across the face of a propeller blade that is a combination of the difference in relative velocity and the angle of attack of the blade.

Down thrust contributes to minimizing the source of P-factor, right thrust moves the remaining P-factor yawing force closer to the CG and minimizes its effect.

Can we please use the proper terms for the forces in question?

Thank you rant over




That's all too complex for me. But you have a point of using the right word.

lets start by spelling correctly:

- useing
- rivits

I'm not asking much

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 4:26 PM   
bogbeagle


 

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Status: online 		It occurs to me that the designers of wind tunnels go to great lengths to "straighten" the airflow from the impellers.


I think they do, anyway.


That may not necessarily mean that the flow is rotating, though.

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 7:13 PM   
UStik


 

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quote:

ORIGINAL: Lnewqban

quote:

ORIGINAL: UStik

Precession of the helical propwash...............?


From http://en.wikipedia.org/wiki/Precession:

''Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle (nutation) is constant. In physics, there are two types of precession: torque-free and torque-induced.''


Yes, see post #17 above.

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RE: Torque and P factor, why do we continually confuse ... - 11/19/2011 11:39 PM   
pimmnz


 

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Status: offline 		I do not discount 'spiral airflow' (indeed, with the fan you can actually see it), I merely state, with model size engines and props, and doing a simple, practical test such as a streamer behind the prop, I cannot find enough to 'strike the fin and cause a yaw' from testing several types of model. With a much bigger prop, going not much faster than a model engine at idle, it MIGHT be possible, but even so, watching AT6's with smoke on, just behind the prop, in what must be the slipstream/'spiral flow' behind the prop, there is no evidence of it. A prop in the free stream might impart a spiral to the air, but if it is in front a big radial, fuselage and a couple of big wings effectivley acting as an airflow splitter, then I fear that the 'spiral' will be well modified before it reaches the tail. So far as models are concerned, I can find no experimental evidence for this spiral flow. So far as full size experience, then I can relate that rudder is used to counter torque, and it is not just used on the ground, at any high throttle setting some rudder must be used to 'keep the ball in the centre', especially in the climb, (low speed, high power). With a ground adjustable trim tab, set to maintain straight flight at cruise power, then you are always using a bit of rudder at most other times. Oh well, we shall just have to continue to discuss ad infinitum...
Evan, WB #12.

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 12:58 AM   
rmh



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Status: online 		For the guys who do not fly -just thumb thru old textbooks or Wilkapedia- try this
Borrow a good two-to eight ounce 3D foamy and have have a friend run the throttle whilst you hold it nose up vertically oriented
run the power up n down and slightly re-orient the model as you do this
You will feel the gyroscopic and the torque forces very easy- yo can also do tuft testing.
PS don't worry about the flat wing foil- it won't fly away .

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 1:03 AM   
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Status: offline 		I'm not sure how much it shows? But even with tail dragging Models (High Wing) for sure.Right rudder is needed on take-off.I know this from experiense.With a Model I have flown! It could be caused by the pitch of the prop wanting to push the nose to the left?Meaning a steeper pitch should make you need more right rudder on take-off?

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 2:42 AM   
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quote:

ORIGINAL: charlie111

I'm not sure how much it shows? But even with tail dragging Models (High Wing) for sure.Right rudder is needed on take-off.I know this from experiense.With a Model I have flown! It could be caused by the pitch of the prop wanting to push the nose to the left?Meaning a steeper pitch should make you need more right rudder on take-off?



Yup, you got it. When the prop is moving forward with a pitch up or pitch down on the airplane, there is an effect that makes it yaw the plane one side or the other. It's called p-effect and the steeper the pitch up or down the more it wants to yaw, and the more rudder you need to keep it from yawing.

Right on.

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 2:48 AM   
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Status: offline 		we are from all walks of life here, on all levels of understanding too, here to enjoy the hobby. i think we all know that if you worked for skunkworks you would not be saying elevator.

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 4:42 AM   
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quote:

ORIGINAL: charlie111

But even with tail dragging Models (High Wing) for sure.Right rudder is needed on take-off........................ It could be caused by the pitch of the prop wanting to push the nose to the left?

Yes, Charlie; the right half of the prop pulls harder than the left half while the tail is down and dragging and the plane is rolling for take-off.
Then, while the tail if lifting and trying to get in line with the nose, the pulling of both prop halves tend to equalize.
However, at the same time, there is another force named precession, which pushes the engine shaft directly across from right to left, and makes the plane pivot or yaw over the wheels.

If you are taking off in a crosswind from left to right, then another lateral force over the vertical fin adds to the equation of left yaw.
When the plane starts lifting the nose and taking-off, the direction of the precession force reverses and pushes the prop nose from left to right.

Since the tail and the nose of a trike stay at the same level during take-off roll, the left yaw caused by those two forces (P-factor and precession) is minimum or null.
That is why the easier trainers for take-off's are trikes.

Check this video:

http://www.youtube.com/watch?v=vWsuXNi_Vnw&feature=related

The precession force resists the wheel from returning (falling) to the horizontal position, while induces a rotation.

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 7:14 AM   
dabigboy


 

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Status: offline 		The problem with observing prop effects is that so many different things are in play at the same time. I'm not going to try to affirm or disprove the supposed effects of spiraling slipstream, though I will go as far as to say that certain aerodynamics texts for pilot training are less than accurate at times (like the illustration that claims tricycle gear aircraft will tend to nose up a little on landing because the mass of the tail "sinks" a bit when the wheels touch...ignoring the obvious fact that the CG is AHEAD OF the mains).

I've always been of the opinion that the exact "culprit" of why a plane tends to yaw left is not so important, so long as pilots are aware of all the POSSIBLE phenomena and what the net effect on the aircraft is. P-factor, for instance, is nice to know, but not a critical concern for a pilot who is on his toes and isn't afraid to use rudder. It's actually exactly the same thing helicopter pilots must correct for in forward flight, just in a much smaller way.

Gyroscopic precession is probably the most dynamic of all the various so-called "left turning tendencies", and is what I would stress the most. It's one of the things that gives taildraggers an extra-strong tendency to yaw left on takeoff (and a great argument for NOT popping the nose up quickly and trying to hurry into the air). And of course, it affects the plane when yawing too, not just when pitching up/down.......it is not an isolated event as some texts seem to treat it. For instance, one reason you will get "pitch coupling" when doing hammerheads, is because when making such an extreme yaw, the plane will pitch one way or the other (yaw/drop left = pitch up, yaw/drop to the right = pitch down). And of course, it *can* mess with your Cuban-eights under the right circumstances. This is something aerobatic pilots in particular should be aware of and compensate for.

I got a great demonstration of gyroscopic precession one day when practicing stalls in a Cessna Cutlass (172RG). I was pitching down through a power-off stall and had already run the power up to near full, I was almost at full RPM as I began dropping my nose. The plane yawed noticeably to the left...not something I was used to seeing in a trike!

Matt

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 1:11 PM   
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Status: offline 		I seem to recall reading in an old USN flight training manual of the effect of the decending blade of the propeller when the airplane was on the ground an accelerating mode. Seem to recall that the intent was to explain the effect of thedynamic situation between the propeller and the ground. Anybody out there with a better memory than I recall this? Maybe I've lost just too many grey cells!

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 2:09 PM   
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Status: offline 		If there were a powerful strong vortex produced by the prop that extended to the ground, it seems to me that friction with the ground would then cause the vortex to walk to starboard and carry the nose of the plane with it. That of course is not what is observed on a tail dragger. Perhaps it has some effect on a trike geared plane but would be compensated for by nose wheel correction and possibly not even noticed.

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 3:27 PM   
rmh



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Status: online 		For those with nothing else to do - try this
setup a motor and propeller FIXED to a mount
directly behind it , tether and suspend a nice flat foam model ( these do give very good simple results) positioned as if the motor was attached.
Now -when the prop is spinning -producing thrust - you can see if the resultant airstream is indead a spiral flow or simply pressurized air.
In doing the many foamie tests with my own stuff - I have noticed the deflection of the surfaces caused by the prop blast -
It appears that the flow is assymetrical (has some spiralling) but the torque is obvious
the gyro reaction is also easily felt in my fingertips as aI move the control surfaces and change the original spinning prop track-
P factor is not possible to feel .
Tho it obviously occurs with a moving model - fling these little models in calm air reveals all these factors
This beats the heck out of digging thru various textbooks
Plus, it's fun

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RE: Torque and P factor, why do we continually confuse ... - 11/20/2011 4:02 PM   
straitnickel


 

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Status: offline 		nicely said Matt

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