Turning Tendancies

[Terror]

There are 4 "turning tendencies" in a single engine aircraft.  (Left for your "standard" planes.  and Right for those foreign jobbies that have engines that turn in the wrong direction.)

The four forces are:

Torque (Left roll)
SlipStream (Left Yaw)
P-Factor (Left Yaw)
Gyroscopic Precession (Left Yaw)

Torque is the opposite reaction to the rotation of the engine, which causes a roll to the left.  Most prevalent during throttle changes, but present all the time.

SlipStream is the spiralling vortex down the airframe from the propeller.  The spiralling effect will push the "empennage" (fancy word for tail assembly) to the right causing a left yaw.  Most prevalent during slow flight, but present al the time.

P-Factor is caused by the descending prop blade taking a bigger "bite" of air than the ascending prop blade.  This causes a left yaw from a higher amount of thrust from the descending right side prop blade.  This is present during higher angle of attack situations, but is present during any increase in angle of attack.  (Negative G's can actually reverse this.)

(This next one is a bear for me to explain.)
Gyroscopic Precession is caused by the resistance of a "gyro" (the prop) to being forced out of the rotation plane.  A gyro lean 90 degrees in the direction of rotation when force is applied to move it out of plane.  So when a aircraft is pitched up, precession causes a left yaw. (Pitching down will cause a right yaw.)

Sorry for the book.  Thought people would be interested.

Terror

[GunnerCAF]

Thanks Terror!

Gunner <CAF>

[Seeker]

Thanks,
      I think I understand P-factor, but why is angle of attack so important a factor in it's generation? I would have thought that low airspeed/prop pitch would be more significant...

[Terror]

 

why is angle of attack so important a factor in it's generation? I would have thought that low airspeed/prop pitch would be more significant...

Angle of Attack is defined as the angle between the direction of relative wind and the chord of the wing.  So during high angle of attack manuevers, the descending blade takes an even bigger "bite" of air while the ascending blade takes a smaller "bite" of air and causes an even higher differential of thrust between the descending and ascending sides of the prop.  

Actually, you are right about the low airspeed and prop pitch.  During low speed flight you have to use higher angles of attack to maintain altitude.  This becomes very predominant during "slow flight" when you are "flying on the backside of the power curve".  During slow flight, more power and higher angles of attack are required to fly slower.  So P-Factor is a major player during this stage of flight.

Hope this helps...

Terror

PS.  Try "Slow Flight" in AH.  Pick any plane.  And see how slow you can make it fly.  You will find that to fly slower and maintain altitude, you will have to keep adding power and back pitch.  Using flaps will help you in this, but are not required.


[This message has been edited by Terror (edited 05-24-2001).]

[Terror]

Ooooops

[This message has been edited by Terror (edited 05-24-2001).]

[Terror]

OOPs

[This message has been edited by Terror (edited 05-24-2001).]

[Terror]

Sorry for the MultiPost.  I got a tad confused...

Terror

[Terror]

There are three other turning tendencies that I did not mention since they have nothing to do with the power plant.  But, after reading back, someone may be interested...

Adverse Yaw, OverBank, Yaw Induced Roll

Adverse Yaw is caused by the differential in drag between each wing when aileron control inputs are given.  This will cause a yaw in the opposite direction of the intended turn.
More lift causes more drag, so the "rising" wing will incur more drag than the "falling" wing.  Bank right, yaw left.  Bank left, yaw right.

OverBank is caused by the outside or "high" wing having to travel farther in a turn than the inside or "low" wing.  This means the outside wing will gain more lift by having to move through the air faster than the inside wing.  This will tend to make the aircraft continue rolling into the bank.  This is why in a "steep bank" (between 45-75deg) you will have to hold opposite aileron to hold the bank steady.

Yaw Induced Roll is two fold.  First during initial yawing, the forward moving wing will travel through the air faster than the rearward moving wing.  More lift is created on the forward moving wing.  Then if the yaw manuever is held into a "slip", the air flow on the leeward side of the airframe is disrupted causing partial loss of lift on the trailing wing.  So using left rudder causes left roll, etc etc....

Terror


[This message has been edited by Terror (edited 05-24-2001).]

[Lephturn]

Wow thanks terror, this is great stuff.

Hey, would you consent to let me collect this stuff together and put it up on my web site as an article?

Full credit will be given in as much detail as you wish of course.  

------------------
Sean "Lephturn" Conrad - Aces High Chief Trainer

A proud member of the mighty Flying Pigs
http://www.flyingpigs.com

Check out Lephturn's Aerodrome for AH articles and training info!

[HessuJ]

 

Originally posted by Terror:

Torque is the opposite reaction to the rotation of the engine, which causes a roll to the left.  Most prevalent during throttle changes, but present all the time.

-- clip --

Terror

Just some nitpicking...

Torque is not a reaction of engine spinning. Engine is very much stationary in the frame. (Hopefully...) Torque is caused by propeller spinning.

As for the other forces.

Adverse Yaw is not so much an issue in motored planes, but if you ever fly a glider you will definitely know this one.

Yaw induced roll is much more bigger factor in planes with swept-wings, because the yaw makes the airflow more disrupted in leeward side wing.
Most swept-wing fighters can be rolled 360 degrees using only rudder.

[This message has been edited by HessuJ (edited 05-25-2001).]

[Terror]

       

Torque is not a reaction of engine spinning. Engine is very much stationary in the frame. (Hopefully...) Torque is caused by propeller spinning.

Actually torque is the combined effect of the rotation of the engine (ok..ok the crankshaft) and the propeller spinning.  It is purely newtons law of equal and opposite reaction.  So even if there were no propeller, torque would still exist from the rotating crank in the engine.  Also, it's not alot of aerodynamics involved.  You use rudder or aileron to counter the effect, but it's an inertia problem as applied to the airframe as a whole.  True, air adds to the amount of torque due to the air resistance (drag) to propeller movement, but if the propeller were a solid disk, torque would still exist and almost to the same extent as if it were a normal prop (assuming the same mass).  The torque from air resistance is what is usually felt mostly during "static" throttle settings.  (To put it another way...Hypothetically...If the aircraft were in a vacuum with the engine running, torque and gyroscopic precession would still be there, but SlipStream and P-Factor would not.)

I'm not for sure I explained this right, so please jump in if I really screwed up...

       

Adverse Yaw is not so much an issue in motored planes, but if you ever fly a glider you will definitely know this one.

Adverse Yaw is a major factor in completing "coordinated" turns.  Which during normal flight is not a big concern(unless you want your passengers hurling on you), but in combat situations, making coordinated manuevers could save you that little bit of extra energy to make or break you.  Plus, making a hard roll in a slow/low plane without keeping coordinated could easily cause a snap roll into a lawn dart.

You are 100% correct about Yaw Induce Roll.  

Terror

PS.  I know..I know...an engine won't run in a vacuum....        


[This message has been edited by Terror (edited 05-25-2001).]

[Terror]

Lephturn,

You are very welcome to use whatever you would like from my posts.  I'm glad that people are finding the information useful!

Terror

PS.  If you have any questions...don't hesistate to ask.

[SpitLead]

of course, and then there's always your inadvertant pilot induced left turn due to pressing the left rudder key and forgetting to center the dang rudder cause there's no physical feedback like a rudder pedal deflection so you end up flying catywhampus for awhile until you figure it out and you feel like a dope...

been there and done that :-)

[Andy Bush]

One more minor point from the peanut gallery!

Regarding 'overbank', it's not the bank angle that is the culprit...it's the back pressure used to fly in the turn.

The airplane doesn't 'know' it's in a turn. Only the pilot does. He uses back pressure to maintain altitude.

I think this is a phenomenon more typical of light planes than fighters. As speed increases, this seems to be less a problem. I don't recall having to bank opposite the turn in any jet I have flown. The DC-9 that I fly now does a 45 degree 'steep turn' very nicely without noticeable opposite bank.

Andy

[bloom25]

If I haven't forgotten all my physics knowledge by now ( I know I've tried hard to do so   ), torque is the cross product of force and distance.  A cross product, in it's simplest form is f*D*sin(angle).  The distance is the length of the arm used to apply the force.  The cross product tells you the force is perpendicular to the direction of motion and the direction of the torque can be easily determined by the right hand rule.  (Curl your fingers in the direction of motion, your thumb will point in the direction of the torque force.)

LOL, I may have totally screwed that up, all I use cross products for anymore is to calculate electromagnetic field strengths.  I'll let the mechanical engineers mess with torque.  

Speaking of torque, how do you think satellites can rotate and hold their orientation in space?  They have a big gyroscope inside of them that by moving that moves the whole spacecraft.  I believe artificial horizon indicators work in a similar manner.



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bloom25
-MAW-
(Formerly of the)
THUNDERBIRDS