Gyroscopic Precession

[stultus]

The topic has come up with my instructor a couple of times and I need some advice. We've gone over it a couple of times now and I'm pretty sure that he doesn't have his facts straight... When he asked me to explain the 4 left turning tendencies I left out gyroscopic precession and he told me I was forgetting one. So I explained to him that GP is only a left turning tendency when the nose has a downward moment because during liftoff the impressed force would be on the prop at the 6 o'clock position (from pilot's vantage point) in an upward motion. 90 degrees in the direction of rotation would be at the 9 o'clock postion and the resulting force, now on the left side of the prop, would pull the nose to the RIGHT.

He looked at me like I had a third eye in my forehead, and asked me where I had EVER (his emphasis) heard that gyroscopic precession was a right turning tendency. Then he tried to explain it to me and basically said: yes the resulting force is 90 degrees in the direction of rotation, but the force being applied to the prop is actually the wind pushing on the bottom of the prop causing the nose of the plane to turn left.

Which left me kinda confused. I told him I don't think I understood it the way he explained it. He seemed kinda frustrated--annoyed with me is probably better. He shook his head and told me he couldn't teach me if I didn't want to learn, and that I better not explain it as a right turning tendency to the DPE on my oral. The way he acted left me feeling uncomfortable---he seemed offended that I didn't believe him. We've been flying together for 6 months now and this is the first time we've had some sort of conflict and I left our lesson feeling kinda bummed.

Basically I'm asking: what is gyroscopic precession? Where can I find out more about it?

 

[braidkid]

You are 100% correct and your instructor is wrong. Grab a copy of the Pilot's Handbook of Aeronautical Knowledge and look
it up. Then show this to your instructor. After your instructor seeing that he was wrong he might get over his frustration.

When there is an upward force on the prop, there is an opposing force 90 degrees in the direction of rotation, hence to the right.
And vice versa for a downward force....

Gyroscopic Precession is mostly prevelant in taildraggers during slow flight and high power settings such as after takeoff.

 

[sbav8r]

[ QUOTE ]
Gyroscopic Precession is mostly prevelant in taildraggers during slow flight and high power settings such as after takeoff.

[/ QUOTE ]
Gyroscopic precession is not prevelant. It is a constant force that acts on all rotating objects such as a propeller or a wheel. It has the same effect on all aircraft no matter what wheel configuration they may have.

Gyroscopic precission occurs when force is applied to a spinning object. The result is that the force is displaced by 90 degrees in the direction of rotation of that object.

The initial lift off of the tail in taildragger pushes the prop forward at 12 o'clock which results in the force acting at 3 o'clock in the same direction. (pushing the nose to the left) If you ignore this effect you will yaw to the left immediately after the tail comes up. Proper counter measures are to put right rudder in at the same time you push forward on the stick to raise the tail off the runway.

You are correct that when you pull back on the stick to pitch the aircraft up you are creating a right turning tendency. This is true for all clock wise rotating prop driven aircraft, tail dragger or tri wheel. Remember that this force only lasts for the period of time that the prop is being displaced from its plane of rotation. Once you have established proper attitude this force will be gone. The same happens for an aircraft being pitched down, the force will result in a yaw to the left because the pushing force at the 12 o'clock acts 90 degrees in the direction of rotation, which is at the 3 o'clock just like in the intial lift off of the tail in a dragger.

Any time the plane of rotation is displaced the force will act 90 degrees from which it was applied. Therefore, applying rudder control left or right will result in a gyroscopic precession that pitches the aircraft up or down depending on which input you use.

With all that said, I will tell you that you do have a good understanding of how gyroscopic precession effects the aircraft. However, I have heard that the relative wind creates a pushing force at the six o'clock position(from in front of the aircraft) when flying at a high angles of attack just like your instructor explained to you. This would, as you know, result in a yaw to the left because the force would be directed 90 degrees in the direction of rotation.

I personally don't fully understand that theory and have my doubts to its' existance. Since the propeller is pulling itself through the air I fail to see how the relative wind could create a pushing force on the lower arch of the prop. But I will say, your instructor is not alone in his explanation. Keep in mind this does not make him correct, I have heard instructors use totally outrageous theories to explain all kinds of aerodynamic effects just because that was the way they were taught.

I have yet to see any reliable literature concerning this and suggest you consult the "Aerodynamics for Naval Aviators" text published by ASA, which is more likely to explain this than anything else. Most texts only describe the effects of gryoscopic precession in terms of its effect on taildraggers like I just explained and you already seem to have a good understanding of the principles involved.

Don't let this incident bother you, it's a good thing to question your instructor. Most of the time he will be right, but every once in awhile the student may correct him.

 

[braidkid]

Great post sbav8r.
I was quoting straight out of the ASA CFI Oral Exam Guide about gyroscopic precession being most noticeable on takeoff's
in taildraggers.

 

[E_Dawg]

OK I have a few questions here as well:

#1) When you pitch *down*, forward of the CG you'd have a more downward relative wind, and aft of it you'd have a more upward relative wind. Since the prop is fore of the CG, wouldn't this lower relative wind cause a force applied from below?

If so, the result would be a yaw to the right!

I know that's incorrect, but it does make more sense, so if someone could explain it that'd be awesome.


#2) What exactly is the *force*? I know it's because the plane pitched down, but what acts on the prop? When you pitch down you are not 'pushing' downward on the prop, you are pushing upwards on the tail and the prop moves downward as a result.


#2) Why does the resulting force always act 90 degrees from the direction of the force applied? Is 90 degrees some magic number that always works or what? It seems to me that if I were to whack a spinning gyro on the side with a newspaper, that gyro would go flying in the direction of the hit, not off to the side somewhere.

 

[sbav8r]

[ QUOTE ]
#1) When you pitch *down*, forward of the CG you'd have a more downward relative wind, and aft of it you'd have a more upward relative wind. Since the prop is fore of the CG, wouldn't this lower relative wind cause a force applied from below?

[/ QUOTE ]

I don't quite follow that question.

[ QUOTE ]
#2) What exactly is the *force*? I know it's because the plane pitched down, but what acts on the prop? When you pitch down you are not 'pushing' downward on the prop, you are pushing upwards on the tail and the prop moves downward as a result.

[/ QUOTE ]

Euler's 2nd law explains the relationship between inertia and the angular velocity.
SMcx = SMcz = 0
SMcy = Id/dt (Ic wc)

This probably doesn't look like anything to you. It sure doesn't to me, but to a physics prfessor this is the short answer to your question.

Try to think of it this way. Get a wheel from a bike and spin it. You will notice that you will have a difficult time trying to turn that wheel over on its' side. As the wheel is spinning it will resist any attempts that you have at moving it and kinda of twist in your arms. This is gryscopic precession at work. If you hold it in one hand in front of you and try to pitch it down or up (like an airplane being pitched up or down) than you will notice that not only does it resist this motion it will push against you towards the right or left. The rotating body (wheel) acts as though the attempt to move it out of its current postion is a force just like if you were to push it with your hand. With a rotating object though, the force is applied 90 degrees in the direction of rotation. Holding that wheel in your hand, if you pitch it down it will try to pull you to the left. If you pitch it up it will pull you to the right. If you leave it alone it will spin and have no force acting upon you. Go get your bike and try it.

What makes all this happen is like I said before the spinning objects resistance to movement, or rigity in space. When you begin work on you IFR rating you will learn about how vacum driven instruments use this same principle to give us information to keep us upright.

 

[Systems_Operator]

The way I understand it is as follows:

Because the the longitudinal axis of the airplane is "pitched up", the angle of attack on the propeller is also changed, lower angle of attack at the 12 o'clock and higher angle of attack at the 6 o'clock position relative to the airstream in a climb. The propeller at the 6 o'clock position takes more of a "bite" of the air than when the propeller is at the 12 o'clock position. This would cause more lift at the 6 o'clock position (the "force" that SkyGuyEd was asking about). Precess the force at the "bottom" of the rotating propeller towards the "back" of the airplane, and the resultant force will be at the 9 o'clock, "forwards", causing a RIGHT turning tendency.

The opposite applies when pitching "down".

As to why 90 degrees? I'm sure there's a huge formula somewhere to prove this, I'll see if i can look it up. why doesn't the rest of my post show up?

 

[sbav8r]

[ QUOTE ]
#1) When you pitch *down*, forward of the CG you'd have a more downward relative wind, and aft of it you'd have a more upward relative wind. Since the prop is fore of the CG, wouldn't this lower relative wind cause a force applied from below?

[/ QUOTE ]

Ok, let me give this a try. I'm not sure exactly what your asking, but I think I have an idea.

A plane traveling through the air is going to have relative wind right. the angle between the relative wind (air the plane is traveling through) and the chord line of the wing is the angle of attack. The relative wind stays the same when you raise or lower the angle of attack.

What I referenced earlier was the theory that at a high angle of attack (picture a plane in a high angle of attack with a straight line coming at it that represents the relative wind) the prop is going to be pitched up. As the relative wind strikes this prop it will have a greater effect on the bottom half of the rotation than the top half right. So using the principles that I explained in the last post this would result in the net force actually being acted upon the left side of the prop. (90 degrees from the bottom) So the prop is going to act as though something was pushing on the left side of the prop (facing the prop from the front of the aircraft) and want to yaw the aircraft in that direction.

Take everything I just said with a grain of salt. As I stated earlier I have only heard this and have yet to see it in publication. I have my doubts that the relative wind will have any "pushing" effect on the prop. To me this theory undermines P-factor. (which is a topic for another discussion)

All you need to know for the sake of an examiner is how gyroscopic precession works. If you take off in tail dragger the force of pushing that prop forward as the tail comes up yaws the aircraft left, which requires right rudder. When you lift off by pulling back on the yoke and raise the nose you will experience a right yaw and when applying forward pressure on the yoke(lowering the nose) to gain airspeed as you leave ground effect again you will experience left yaw.(turning tendency) This is true for any wheel configuration.

If you really want to experience a weird sensation fly a russian aircraft with counter clockwise rotating blades and all forces are reversed, so you need strong left rudder at high angles of attack.

 

[sbav8r]

[ QUOTE ]
Because the the longitudinal axis of the airplane is "pitched up", the angle of attack on the propeller is also changed, lower angle of attack at the 12 o'clock and higher angle of attack at the 6 o'clock position relative to the airstream in a climb. The propeller at the 6 o'clock position takes more of a "bite" of the air than when the propeller is at the 12 o'clock position. This would cause more lift at the 6 o'clock position (the "force" that SkyGuyEd was asking about). Precess the force at the "bottom" of the rotating propeller towards the "back" of the airplane, and the resultant force will be at the 9 o'clock, "forwards", causing a RIGHT turning tendency.

[/ QUOTE ]

What you just explained is P-factor and it's the downward moving propeller that has a higher angle of attack (right side of prop when looking from cockpit) then the upward moving prop. The result is a left turning tendency not a right.
AC 61-23C (1-10)

These are two different principles.

 

[Systems_Operator]

Ahh....yes....thanks for correcting me on that.

 

[E_Dawg]

First off, a huge thanks to the people who spent the time explaining this!

[ QUOTE ]
What I referenced earlier was the theory that at a high angle of attack (picture a plane in a high angle of attack with a straight line coming at it that represents the relative wind) the prop is going to be pitched up. As the relative wind strikes this prop it will have a greater effect on the bottom half of the rotation than the top half right.

[/ QUOTE ]

OK that's understood, however as I understand, gyroscopic precession really is only a factor during pitch changes, not during steady state flight. Think of an inverted aircraft; you are flying at a negative angle in steady state flight. Gyro precession will have no affect on the prop, yet as soon as you push forward on the yoke (thereby pointing the nose skyward), the airplane will go left in relation to the pilot.

[ QUOTE ]
When you begin work on you IFR rating you will learn about how vacum driven instruments use this same principle to give us information to keep us upright.


[/ QUOTE ]

Well I have the rating and I understand the results of all of this; I'm just having a hard time understanding the whys.

I'm sure we could work it out in person; but it's kind of hard to explain / understand this stuff in words alone. I have Kershner's Advanced Flight Manual so I'll just have to look it up there sometime! Thanks again.

 

[stultus]

[ QUOTE ]
However, I have heard that the relative wind creates a pushing force at the six o'clock position(from in front of the aircraft) when flying at a high angles of attack just like your instructor explained to you. This would, as you know, result in a yaw to the left because the force would be directed 90 degrees in the direction of rotation.

I personally don't fully understand that theory and have my doubts to its' existance. Since the propeller is pulling itself through the air I fail to see how the relative wind could create a pushing force on the lower arch of the prop. But I will say, your instructor is not alone in his explanation. Keep in mind this does not make him correct, I have heard instructors use totally outrageous theories to explain all kinds of aerodynamic effects just because that was the way they were taught.

I have yet to see any reliable literature concerning this and suggest you consult the "Aerodynamics for Naval Aviators" text published by ASA, which is more likely to explain this than anything else. Most texts only describe the effects of gryoscopic precession in terms of its effect on taildraggers like I just explained and you already seem to have a good understanding of the principles involved.


[/ QUOTE ]

Hey, man I share your doubts. I'm not a physics/mechanics guru, but I can't really figure out a logical explaination for that "relative wind" force at 6 o'clock. It definitely aint gyroscopic precession since the force is not strong enough to overcome the upward pitch of the prop--i.e., if there is a motive implied force, it's irrelevant to GP since it doens't disturb the gyro out of it's plane of rotation. And like you said, what happens then to P-factor? You gotta throw that out, or at least make some clarifications.

Thanks everyone for the replies. Aerodynamics has really fascinated me since I started flying last December. And a couple people have suggested I read "Aerodynamics for Naval Aviators" though between working full time and getting ready for my pvt checkride (it was scheduled for today) etc., there hasn't been a lot of time for "fun" reading. I should have time to start it before the month is over. I really wish the Jep. textbook explained the mechanics behind some of this stuff a WHOLE lot better. Most of the concepts aren't THAT difficult though I do see that the book would be twice as big (therefore 3x as expensive too), and lots of equations like SMcx = SMcz = 0 and SMcy = Id/dt (Ic wc) are usually kinda off putting to non-geniuses like myself.

There's no way I'd let this incident bother me too much--even though I've tried to kill him multiple times, I REALLY dig flying with my instructor

. I think I'll try to get some published info that explains the theory a little better than the Jep. manual and slip it in his flight bag.

 

[sbav8r]

[ QUOTE ]
I understand, gyroscopic precession really is only a factor during pitch changes, not during steady state flight.

[/ QUOTE ]

Sorry if I did not make that clear. You are absolutely correct that when the aircraft is in steady state flight there is no gyroscopic precession. It is only a reaction to pitch changes, just like you said.

 

[E_Dawg]

Ah ha! I got it... listen up I'm gonna lay it down

:

First let me say three things:

The gyro is rigid in space.
Relative wind means NOTHING to the gyro.
Every force is reacted to 90* from the application.

OK: the 'force' applied is actually just the act of moving the gyro's axis of rotation in space. The prop would be happiest staying in the same direction the entire flight, but every time you change pitch, you are forcing the prop to move it's rotational axis.

This 'force' is actually acting on all areas of the prop simultaneously; an example is pitching down (like you would in a takeoff in a tailwheel airplane). This is in relation to the prop:

The top 'feels' a force from behind pushing forward (because you forced it to pitch down).
The bottom 'feels' a force from the front pushing forward (because you forced it to pitch down).

Think of a soda can: if you push on the top in one direction, and push on the bottom from the opposite direction, the thing will 'pitch down', or if you push hard enough it will topple.


Now you add in the fact that all forces are felt 90* from the point of application:

-The top part from behind is reacted to at the right side, pushing it forward from the pilot's perspective.

-The bottom part from the front is felt from the left side, pushing it back from the pilot's perspective.

The result is the airplane yaws to the left.

Phew! I hope that made some sense; I am a huge dork!

 

[seagull]

As stated above, GP is independent of any aerodynamic forces. It may help to visualize it by considering a weight at the end of a rope that is swinging around in a circular motion. Imagine that it is swinging in front of you like a prop would be. Now, imagine that someone is standing with a baseball bat and hits it as it passes the 9 o:clock position, hitting it away from you. Now, the arc that it is following is different. It was moving just vertically at the 9 o:clock position, but now it is moving vertically and horizontally away from you. It starts moving out that way, but it is still tethered to the center, so it can only move away from you until is hits 12:00 than it is pulled back in.

Now, with that in mind, picture the new arc it is travelling in, it is now canted with the top away from you. In other words, it has shifted 90 degrees to the original plane of rotation in the direction of motion. A disc works the same way, it is just an infinite series of single points that are acted in in the same way.

So, a force from the bottom (rotation on takeoff) will have the net effect of rotating the arc to the right.

 

[MikeD]

bumped for interest