# Undesired Yaw

#### cesphil

##### Member
There have been many posts on this forum recently regarding yaw and torque effects on the stability and controllability of a gyroplane. In particular, due to several recent “Euro-Tub” crashes, posters have been trying to analyze and explain how torque and P-factor can generate an undesired roll or yaw in a gyroplane. Many of these posts have confused P-factor with gyroscopic precession. While both create a change in yaw due to a change in pitch (or vice versa), they are different dynamics and create opposite reactions for different reasons.

Let’s analyze what happens to a gyroplane when the nose is suddenly pitched up. For this example, we will use a pusher type gyroplane with the engine turning clockwise when viewed from the rear.

Most are familiar with engine torque and understand that it will cause a roll to the left so we won’t dwell on this. Do note that the torque is a product of engine power and is not a result of the upward pitch.

Now, let’s talk about gyroscopic precision. Remember in high school physics when the teacher handed you a spinning bicycle wheel with a handle on each end of the axle. When you tried to tilt the wheel, it reacted different to your expectation. The instructor was demonstrating “gyroscopic precession”. Gyroscopic precession causes a force to act at 90 degrees of rotation to the applied force. If the nose of the gyroplane is suddenly pitched upward, we are moving to top of the propeller rearward and the bottom of the propeller forward. With the propeller rotating clockwise (from the rear), gyroscopic precession will move the right side of the propeller rearward and the left side of the propeller forward which will yaw the aircraft to the right.

Now, we get to P-factor. I have seen posts in this forum where gyroscopic precession was incorrectly explained as P-factor. I have attached a publication by the FAA that explains; torque, gyroscopic precession and P-factor. I find their explanation of P-factor a little unclear so I have produced a drawing (PF-1) that I hope will help explain it.

When an aircraft is flying straight and level, the propeller is perpendicular to the direction of travel so the propeller blades see the same relative wind and have the same angle of attack in any location as they rotate around the crankshaft (or the propeller axis). As a result, the propeller is producing the same amount of forward lift (or thrust) at any point in its rotation, thus producing symmetrical thrust. If the aircraft were suddenly pitched upward while still traveling in a straight and level direction, the propeller axis is no longer parallel with the direction of travel (relative wind) so the angle of attack of a propeller blade varies with a position of the blade as it rotates about the axis. The downward blade will have a greater angle of attack while the upward blade will have a lesser angle of attack. As a result, the downward blade, which is on the right side of our clockwise rotating prop, will generate more push and yaw the aircraft to the left.

It is interesting to note that gyroscopic precession and P-factor counter act one another, so obviously one of the two will need to produce a more powerful action in order to generate a yaw. I will leave my explanation as such and let the debaters take it from here.

#### Attachments

• PF1.pdf
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• torque p factor gyro precision.pdf
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#### fara

##### AR-1 gyro manufacturer
Which "several" Eurotub crashes. Can you list them so I can study them that caused the accident.
Gyroscopic precession is temporary and is gone after the change in attitude is done. P factor remains longer as long as angle of attack is kept high. Or do you think differently.

Your explanation is adequate but people should note that it depends on your engine rotation and most Eurotubs as you call them have prop turning counter clockwise as viewed from the back and whether it's a pusher or tractor and also how your engine is angled. Not all gyroplanes create a perpendicular prop disc when flying straight and level.

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#### cesphil

##### Member
Which "several" Eurotub crashes. Can you list them so I can study them that caused the accident.

I am not talking specifically about Eurotubs. I only made note that others have made numerous posts about how P-factor may have played a role in some of the Eurotub accidents. I am not trying to understand or explain the cause for any specific accident.

My post was intended to clarify the difference between P-factor and gyroscopic precession as they relate to any aircraft. The dynamics of a propeller's reaction to a change in pitch (or yaw) are the same for any aircraft and understanding P-factor and gyroscopic precession should be part of any pilot's training.

I only used a clockwise turning prop as a specific example. (this is the configuration that I fly.) I don't think that the direction of prop rotation makes any difference with regards to what I am trying to explain. The point is that torque and P-factor will tilt or yaw the aircraft in one direction and gyroscopic precession will yaw in opposition to P-factor.

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#### Vance

##### Gyroplane CFI
Thank you for your thoughtful post.

Thank you for your thoughtful post.

That is a nice presentation Philp and nicely demonstrates an orderly mind.

I don't see much to debate.

Abid is correct in that the gyroscopic precision is transient and P factor is relatively constant as long as the pitch angle to the relative wind is constant.

When I was learning to fly an AAI modified RAF I often raised the nose too much on takeoff and experienced the effects of P factor making tracking the centerline difficult. Once I learned a better technique for taking off the centerline challenge went away.

The gyroplane I am flying (The Predator) doesn’t pitch up much on takeoff. The picture is one Wayne took of my departure from Santa Paula.

I don’t pitch up much on landing either as shown in the picture Ed took of an engine at idle landing at Hollister in The Predator. I might pitch up more if I was trying to stop quickly. In my opinion both P factor and gyroscopic precision matter less on landing because of the reduced thrust and propeller rpm so it is best to focus on the takeoff.

When I have flown two seat Dominators the takeoff roll was similar but I don’t have any pictures.

The Cavalon, MTO Sport and Calidus pretty much takeoff the same way.

With so little angle between the thrust line and the relative wind I feel there is not much P factor or gyroscopic precision on takeoff in a properly designed gyroplane flown well.

#### Attachments

• SZP lift off.JPG
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• HAS landing.jpg
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#### eddie

##### RAF, turbo subaru 230hp
Vance you just explained why I moved my main gear back 5",my takeoffs are now quite a

bit flatter without a nose high attitude,they are as you said a lot easier to handle ,and

more predicatable especially with the higher HP that I have.

Best regards,

#### WaspAir

##### Supreme Allied Gyro CFI
two names for the same thing

two names for the same thing

P-factor should not be difficult to understand for anybody who has trained in rotorcraft, because it is just the mildest form of dissymmetry of lift. If you could fly a helicopter with the mast pointed horizontally forward and a perfectly vertical disc (more on that below), there would be no dissymmetry, as the forward speed component is the same at every place on the disc. Start to tilt the rotor back a little, and now you have one blade advancing and one retreating, and dissymmetry appears. That's exactly what the propeller goes through when the nose is raised, giving rise to the so-called P-factor effect. Propellers don't have articulated or flapping blades, so there is no automatic compensation, and yaw force is the result. Continue to tilt back until the mast is nearly vertical, and you have the classic condition for helicopter flight, where substantial dissymmetry is compensated by flapping motion.

Of course, helicopters can only fly with the mast forward in James Bond movies (e.g., Tomorrow Never Dies, where a nose-down hovering helicopter menaces 007 by creeping slowly forward with its tail straight up in the air and rotor blades about to chop up the Brit -- check out :45 to :55 below for your amusement), but the idea shows the connection between P-factor and familiar rotor dynamics.

www.youtube.com/watch?v=u0N4nuBY2a4

As to precession, it can be a big deal on take-off for very powerful taildragger airplanes. Picking up the tailwheel (lowering the nose) abruptly while running full throttle down the runway can send an unwary pilot into the ditch. Most of us don't fly P-47 Thunderbolts, so we don't face the same level of concern.

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#### C. Beaty

##### Gold Supporter
Throttle-yaw coupling is a nuisance during the takeoff roll with the vertical tail dipping in the lower half of the whirlwind coming off the prop.

Most people do master the required toe dance but it also leads to rollovers when combined with a hard linked nosewheel.

Vance’s tall tail eliminates the rudder pedal toe dance.

#### eddie

##### RAF, turbo subaru 230hp
And I have eliminated the same problem with my hard linked hose wheel by moving my

mains back 5" there are several ways to solve the same problem!

Best regards,

#### VWPower

##### Newbie
How does it handle

How does it handle

How does it handle differently when the mains are moved backed 5".

#### fara

##### AR-1 gyro manufacturer
Which "several" Eurotub crashes. Can you list them so I can study them that caused the accident.

I am not talking specifically about Eurotubs. I only made note that others have made numerous posts about how P-factor may have played a role in some of the Eurotub accidents. I am not trying to understand or explain the cause for any specific accident.

My post was intended to clarify the difference between P-factor and gyroscopic precession as they relate to any aircraft. The dynamics of a propeller's reaction to a change in pitch (or yaw) are the same for any aircraft and understanding P-factor and gyroscopic precession should be part of any pilot's training.

I only used a clockwise turning prop as a specific example. (this is the configuration that I fly.) I don't think that the direction of prop rotation makes any difference with regards to what I am trying to explain. The point is that torque and P-factor will tilt or yaw the aircraft in one direction and gyroscopic precession will yaw in opposition to P-factor.

Oh I see.
I would really like to know if anyone does know of actual specific accidents that were caused by (not secondary events) as a primary cause torque rolling moment

#### eddie

##### RAF, turbo subaru 230hp
Danny, by moving the main gear back there is more weight on the nose,and it resists

raising up as I take off,by having the nose wheel stay in contact with the runway I can

maintain directional with the nose wheel and not the rudder when the nose wheel is raised

off the pavement. One of the several benefits is,postive directional control,and I can raise

the rotor to a better angle for a faster spin up to flying rotor RPM,its like getting a boat on

plan faster,I have reduced my takeoff distance by at least 40%,then there is the safety

factor,if you let the nose wheel back on the ground with a lot of rudder pushed for

directional control it will want to turn sideways if the nose wheel touchs the runway,

its not a happy place to be at 40-50 MPH.

Best regards,

#### VWPower

##### Newbie
RAF Modfide

RAF Modfide

Eddie.

I have a RAF modified and noticed that on take off I have to make sure that the nose wheel does not come in contact with the runway and that gets a little tricky with two folks aboard. Just wondering how much effort was it to move the main gear back five inches?

Thank Danny

#### eddie

##### RAF, turbo subaru 230hp
No big deal really, the new down tubes are located at the bottom of the mtr mount bracket on the mast,you will need a angle drill to drill the new holes.you will need longer down tubes that can be ordered from AC spruce,I got all of my info from AUSSIE PAUL he has a drawing of what to do.

.I like it because it will set on the nose gear when empty and it really makes takeoffs a cinch,and the landings are the same as before.

Best regards,

#### VWPower

##### Newbie
Contacting Aussie Paul

Contacting Aussie Paul

Eddie

I'm kind of new to the forum so I was wondering how I could contact Aussie Paul for the drawings to move the landing gear back on my RAF.

Thanks Danny

#### eddie

##### RAF, turbo subaru 230hp
I am sure he wouldnt mind and if you need help I could take a couple of pictures and send

them to you or you can call me if you want to,my phone number is listed at the bottom of

the page.

Best regards,

#### WaspAir

##### Supreme Allied Gyro CFI
I'm kind of new to the forum so I was wondering how I could contact Aussie Paul for the drawings to move the landing gear back on my RAF.

Try the "community" tab, then pick "member list", look him up by user name, go to his page, and try the "contact info" tab.

#### VWPower

##### Newbie
Thank you guys very much

Thank you guys very much

Thanks very much. I got in touch with Ausie Paul and he is going to give me the information on moving the landing gear on the RAF.

Thanks again.
Danny

#### NoWingsAttached

##### Unobtainium Member
Let me see...rough field takeoff you want the nose wheel off the ground sooner, that will get you off the ground faster but your climb out is extended as you build AS a few feet off the ground first. Short field takeoff you keep the nose wheel on the ground longer, which builds forward speed in a shorter distance between you and your obstruction (trees) and allows a steeper climb angle sooner.

These are CHOICES you will no longer have once to make this mod to your gyro, my friend. So what are you going to do when the time comes when you WANT that nose wheel to pop up? Nothing. You are going to be STUCK with your nose on the ground for ever.

Moving the mains back to force the nose wheel to stay on the ground for better ground handling? I once moved mine forward on a tandem CLT Air Command to get it to pop sooner, and in a little headwind with no passenger the gyro took off in twenty feet. THAT's Fun!

Why can't a pilot maintain control with or without the nose wheel on the ground? Beats the heck outta me. No matter, that's a LOT of trouble to go through (rebuilding the mains) just because you don't want to learn to maintain control of your ship with rudder and feel you need the nose wheel on the ground to steer the gyro going 45 MPH! I tell you this: I NEVER want to have my nose wheel on the ground in a such a short-coupled airframe as a side-by-side RAF or Sparrowhawk, that's for dang sure.

Good luck with that, and dont' listen to me - I'm just an idiot who doesn't have a clue as to what I'm talking about anyway. I just ramble, blurt out the first stupid thing that comes to mind and write it down here for everyone to pick at and scream about.

Enjoy.

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