HiTech you were right. Spiralling slipstream is the problem area

[Citabria]

I've been researching this pretty hard (no I still can't tell you what the asymetrical thrust is for a 2750rpm f4u and the resultant calcualatable yawing moment that P-factor would supply)

but from what I read I can see why you are frustrated with my stupidity.  
you said yourself that you are researching the spiralling slipstream effects and they will perhaps change in future releases. will this restore the need for right rudder again?

from this source it makes it quite clear that the spiraling slipstream (helical propwash) is the major factor in the need for right rudder.

   

 

It would be nice if the propeller would just take the air and throw it straight backwards, but it doesn't. The propeller airfoil necessarily has some drag, so it drags the air in the direction of rotation to some extent. Therefore the slipstream follows a helical (corkscrew-like) trajectory, rotating as it flows back over the aircraft.
The next thing to notice is that on practically all aircraft, the vertical fin and rudder stick up, not down, projecting well above the centerline of the slipstream. That means the helical propwash will strike the left side of the tail, knocking it to the right, which makes the nose go to the left, which means you need right rudder to compensate.
You don't notice the effect of the helical propwash in cruise, because the aircraft designers have anticipated the situation. The vertical fin and rudder have been installed at a slight angle, so they are aligned with the actual airflow, not with the axis of the aircraft.
In a high-airspeed, low-power situation (such as a power-off descent) the built-in compensation is more than you need, so you need to apply explicit left rudder (or dial in left-rudder trim) to undo the compensation and get the tail lined up with the actual airflow.
Conversely, in a high-power, low-airspeed situation (such as initial takeoff roll, or slow flight) the helix is extra-tightly wound, so you have to apply explicit right rudder.

 http://www.monmouth.com/~jsd/how/
 http://www.monmouth.com/~jsd/how/htm/yaw.html#fig_p_factor_aoa

 

This angle-of-attack effect is of course zero when propeller axis is aligned with the direction of flight.5 The effect is never very large, because

1.
At low speeds, the airplane's forward velocity (as represented by the horizontal red arrow in figure 8.4 is so small that it can't have much effect on anything.
2.
At high speeds, the airplane has a low angle of attack, so the angle between the propeller disk and relative wind is necessarily small (except for helicopters, tilt-rotors, and such).

3.
At very high speeds, when you are going fast enough to over-run the geometric pitch of the propeller (so that the resultant coincides with the reference line in figure 8.4), you might think that a small difference in angle of attack would be a 100% effect. I suppose that's true, but in this case the total thrust is practically zero, and 100% of nothing is nothing.

This angle-of-attack effect is in addition to (and usually smaller than) the airspeed effect discussed previously. Both are small compared to the helical propwash effect.

   

[This message has been edited by Citabria (edited 01-13-2001).]

[Citabria]

p.s.

how do you feel about adverse yaw effects in A.H. ?

(inquisitive and most unconfrontational questioning mode engaged     )
 http://www.monmouth.com/~jsd/how/htm/yaw.html#sec_adverse_yaw

     

   

 

 

The basic rule is simple:

if you are rolling to the right, you must apply right rudder;
if you are rolling to the left, you must apply left rudder.
The amount of rudder will depend inversely on the airspeed.

Another version of the rule substitutes the word ``aileron' for ``roll':

right aileron requires right rudder;
left aileron requires left rudder;
In a steady roll, the two versions are more or less equivalent; at the beginning and end of a roll (when the roll rate does not match the aileron deflection) the truth lies somewhere in between. Split the difference.

[This message has been edited by Citabria (edited 01-13-2001).]

[Citabria]

back to the main subject: the author of
 http://www.monmouth.com/~jsd/how/

sumarizes the effects and their proportional importance relative to eachother

 

We have finally come to the end of this section, having covered the most important causes and effects of torques and motions around the yaw axis. There are quite a number of such processes:

The helical propwash effect is important, especially in high-power / low-airspeed situations.
Gyroscopic precession means that deflecting the flippers will cause a yawing motion (and deflecting the rudder will cause a pitching motion).
Adverse yaw means that deflecting the ailerons will cause a yawing moment.
The long-tail slip effect means that an inadvertent turn will be a slipping turn. This effect is very significant in gliders. It is much less noticeable in typical powered aircraft, but it has important implications for roll stability, as discussed in section 9.3.
P-factor exists in principle but is usually insignificant.

Actual motion around the yaw axis will create a yawing moment that tends to damp the motion.
Yawing the plane changes the direction it is pointing which does not automatically change the direction it is going; the ``boat turn'' effect exists but is feeble and inefficient.
The pilot can deflect the rudder to oppose the unwanted yawing effects and create the desired ones.

[Toad]

Great, informative illustrations, Cit.

Should help us all realize the difficulty of what HT has to do.

Modeling all the effects on an aircraft in flight has got to be one of the greatest challenges in computer programming.

[Downtown]

But us P-40 People knew all that already.

P-40 Tendancy to ground loop! P-40 Excessive Roll in a high Speed dive!

You could tell a P-40 Pilot because of his excessively muscled right leg, from standing on the right rudder.

Yes, the P-40, not yet modeled in AH.

(Curtiss extended the length of the plane by 20" and increased the size of the rudder to compensate.  At one point in time they had a raised spine that ran the length of the body to the vert stab, this also helped to compensate.)

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"Looks Mean as Hell! Clare Lee Chenault.
 
When?
"Downtown" Lincoln Brown.

[This message has been edited by Downtown (edited 01-14-2001).]

[Duckwing6]

Cit Adverse yaw is VERY present in AH

(much more than i'd expect from airplanes with the low aspect ratio than the fighterplanes at hand)

DW6
p.s. MAUAHAHAH learn to use the rudder TA152 folks

[hitech]

Citabria: Glad your getting into this stuff from a research level and not a feel level. Anyway, I blieve we are realy close on the adverse yaw at the transitional states of rolls. We might be off a little in steady state rolls but im not sure it's enof to realy change much. Also ive never seen much on steady state turning (i.e. a 60 deg bank level turn)on any WWII era planes how much rudder is required.

HiTech

[eagl]

Since the adverse yaw is present whenever the ailerons are deflected, it "should" be there during initial roll, steady state roll, and anytime the ailerons are not centered.

There is very little yaw associated with a stable BANK angle except in extreme conditions (high enough G that one wing traveling on the outside of the turn has a noticable effect, or low speeds with lots of rudder being used).  The exception to this rule of course is that many planes require opposite aileron to sustain a stable banked turn.  Above 60 degrees of bank, many planes tend to roll further into the turn all by themselves, requiring some aileron inputs to the outside of the turn.  This would in turn cause a small amount of adverse yaw pulling back on the low wing, causing the nose to dip down a bit.

How much adverse yaw is present depends on a bazillion things, including if differential aileron deflections are designed in (meaning that the aileron going UP deflects more than the aileron going DOWN), aileron size and aspect ratio, hinge type and location, etc etc.  Lots to think about for a relatively small overall input to the flight model.  Of course, when I was spending a lot of time building and modifying radio control aircraft, I spent a lot of time using control geometry tricks to minimize adverse yaw because it does have a huge impact on how small aircraft fly.  Hinging the top of the aileron instead of the center, sealing the aileron hinge gap, and using offset bellcranks and pushrods all can affect how much adverse yaw the aileron input has on the plane.  With the tricks applied, a trainer of mine would do consecutive axial rolls without getting out of shape.  In the stock, designed configuration, the plane would wiggle all over the place with only half aileron deflection, and consecutive full stick deflection aileron rolls were impossible due to yaw excursions.

It's not a prop plane, but the F-15E has a flight model "hole" the pilot must be aware of to avoid inadvertant departures.  If the stick is held with both pitch and roll deflection, then the stick is rapidly shoved forward, the ailerons suddenly get twice the allowed control deflection and tipstalls (uncommanded rolls in the wrong direction) coupled with yaw excursions are possible.  And that's in a modern plane where the designers thought a lot about it beforehand    The T-37 has cases of inadvertant spin entries when the plane isn't anywhere near the "normal" spin entry parameters (very slow with large rudder inputs), and some of those spin entries can be attributed to large aileron movements in accelerated stall (greater than 1 G) conditions.

In some planes, the adverse yaw can be enough to cause a high enough yaw rate in a stall situation to make the plane enter a spin.  The spin recovery technique in many jet fighters is aileron into the spin, because the drooped aileron on the outside wing gives enough adverse yaw drag to stop the rotation.  I could go on and on with other examples, but you guys probably don't want to hear about it unless I can produce charts...



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eagl <squealing Pigs> BYA
Oink Oink To War!!!

[Toad]

IIRC, the F-100 had a pretty well-deserved reputation with respect to adverse yaw turning final.

[hitech]

eagl: Said
Since the adverse yaw is present whenever the ailerons are deflected, it "should" be there during initial roll, steady state roll, and anytime the ailerons are not centered.

Not sure I agree with this eagl. If you view the adverse yaw do to the difference in induced drag between the wings, and that induced drag varies with the square of lift, if each wing is producing the same lift the drags should be the same. In a steady state roll both wings must be producing the same lift or the roll rate would be changing. This would be exactly the case if we were dealing with only a wing, and not the other pieces of the plane.


HiTech

[eagl]

HT,

You would be correct if the plane was in a vacuum, but the plane is rolling through air and it is the ailerons that cause that roll by creating differential lift between one wing and the other.  Otherwise once you started the roll with an aileron input, you could center the ailerons and the plane would keep rolling, right?

A continuous roll is maintained by the wing that has more lift continuing to create more lift as the plane rotates.  Without one wing creating more lift, the plane will shortly stop rolling, the time it takes to stop rolling depending on momentum and aerodynamic effects.

Just like a plane that is inverted at "one G" on the airframe will actually accelerate towards the ground at a net 2 G's and the plane that is upright at "one G" will simply fly level, the orientation of the plane away from horizontal doesn't mean much to the forces involved.  If you have one wing with a higher angle of attack and camber due to a drooped aileron, that wing will (up to the critical AOA) produce more lift than the opposite wing which has less AOA and less camber due to the aileron being deflected up.  It's not a matter of simply starting and stopping the roll, or the ailerons pushing the wind one way or another.  As long as there is a different amount of lift between the wings, there will be roll.  That different amount of lift is present regardless of the plane's orientation or roll rate, and therefore there will also be a different amount of induced drag.  As the roll rate increases, there will be differences in AOA and drag until the plane cannot accelerate it's roll rate any more due to the lift difference being balanced out, but the drag caused by the different amounts of lift ought to remain simply because one wing is still making more lift than the other.

Even with a fully symmetrical wing, you would still have a difference in lift unless the pilot reduced the AOA of the wing with pitch as he caused the roll.  In this special case (zero wing AOA and zero lift in a symmetrical wing before the ailerons are deflected), yes you would see the drag and lift balance itself out with no drag assymetries.  But our wings aren't symmetrical and we rarely see completely unloaded rolls.  This is why the fastest roll rate should be achieved in an unloaded state (bunt to zero G's then roll), and why we are taught to use unloaded rolls in Real Life when trying to get the maximum roll rate possible.  Remember, all that you do when you deflect the ailerons is alter the wing's camber and AOA.  More camber and AOA means more lift and more drag.  Putting the aileron up on one side decreases the AOA and in some cases causes negative camber, resulting in less drag.  Even when the plane is rolling, you still have one wing making more and one wing making less.

This is why a plane at zero G accelerates faster than a plane at 1 or more G - it's AOA has been reduced to where it's no longer making lift, and when it makes less lift it makes less drag.

Of course, this all changes when the AOA over the aileron altered portion of the wing exceeds the critical AOA...  Then it sorta works backwards    Oh yes, this also ignores the way some ailerons worked with less rigid wings (like the spitfire and hurricane) at very high speeds, where aerodynamic forces prevented the ailerons from deflecting resulting in the wing itself warping the other way, causing control reversal.  But that's an edge phenomena, and we're talking about basic aero stuff here  



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eagl <squealing Pigs> BYA
Oink Oink To War!!!

[eagl]

Just to make sure I'm not talking out of my bunghole, I pulled out my applied aero for pilots book.  No, it doesn't have a yellow cover or the word "dummies" on it...

It has simplified a lot of things, but it is pretty clear on one point -

"The increase in camber on one wing increases the drag, while the decrease in camber on the other wing decreases the drag on that wing.  This asymmetrical drag causes the yaw."

The AOA increase is due to the increased camber and the movement of the chord line based on the new camber, not the initial AOA through the air.  Even though the actual AOA is eventually reduced during the roll due to the rolling itself (like a fixed pitch propeller moving thru the air has less AOA than a stationary one), the camber remains altered until the ailerons are centered, and you still have the difference in camber between the two wings.  This effect is exaggerated with highly assymetrical airfoil wing designs, the more assymetric the airfoil the more you'll see a difference in camber since you're adding and subtracting camber from a wing that already has some built in.  Fortunately, adding camber to a wing can also increase it's critical AOA depending on the airfoil shape, otherwise we'd have more tipstalling going on.

This is also why when aircraft designers add proportional movement to the ailerons, they want the one moving up to move farther than the one moving down.  You get your lift differential at less of a drag penalty by decreasing the camber of one wing more than you increase the camber of the other wing.  This is what I was talking about with the model airplanes.  At the extreme, you remove the downward moving aileron entirely and just use spoilers on the wing you want to drop.  This reduces the drag, but at the cost of roll performance.

I sure wish I had my school aero textbooks, since they had a ton of diagrams showing lift/drag curves for various wing configurations.  They show a general trend towards more lift and more drag, and in some cases a higher critical AOA, as the camber increases, regardless of actual AOA.  Slotting the aileron further increases the critical AOA just like slotting flaps increases the critical AOA on the wing portion affected by the flaps.  More camber still means more drag though.

Again, I caveat this with the fact that this all changes at very high speeds...  Transsonic effects and the wing itself warping can cause odd results.


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eagl <squealing Pigs> BYA
Oink Oink To War!!!

[This message has been edited by eagl (edited 01-14-2001).]

[hitech]

Eagl:

If the total net torques in the roll axis are not = the plane must be changing its roll vel. The primary force to generate torque in this instance is change in camber v change in AOA do to the roll itself.

The question is how much does the induced drag to lift curve change do to camber. Because each wing must be generating the same amount of lift.

We are in agreement eagl, and why I stated in my description of adversers yaw the intial v steady state pieces of the model.

HiTech

[eagl]

HT,

I'm glad we agree    After the walls of text I put up, it boils down to my original point that there is still some drag differential during the steady state roll.  Exactly how much is your problem, not mine.  I think I just don't know how to express myself properly.  I'm still working on that.

I think I was caught up trying to explain that the AOA and lift increase was caused by the camber increase instead of any actual change in relative velocity, and started talking in circles.

Now if I can just remember to send you my "spins for dummies" book, we can make spin modeling much neater    As my flight standardization/evaluation liason officer, I've had to study up on spins and go through a few sorties dedicated to almost all the possible spin modes of the T-37.  This naturally makes me just a tad sensitive to how spins work in AH.  The modelling "feels" pretty realistic in my worthless opinion, but like we discussed, well, actually you admitted, over the 5 or so glasses of scotch I forced down your throat, it's a tad mechanical.  I think if/when you get the time to review it, it might be done a little better next time.  At the very least, I'll bring our spin pamphlets to the next con, and I'll bribe you with some more scotch to take a look at them.  Enough drinks and we can even fit in some demo spins if you like  

Thanks for replying in this thread HT, you da man.


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eagl <squealing Pigs> BYA
Oink Oink To War!!!