Why no gyroscopic precession on Asymetric thrust.

[iflyforpie]

For asymmetric, I assume you are talking about P-factor and not an engine failure on a multi-engined aircraft with engines mounted off the centerline. Asymmetric thrust and P-factor are the same in that case.

First, even though this is how it is taught, P-factor is not from the increased angle of attack of the down-going blade. It is from the increased velocity in comparison to the relative airflow for the down-going blade (and vice versa for the up-going blade). The angle of attack is equal in each and is directly related to dissymmetry of lift of a helicopter's main rotor.

Usually when this is demo'd, instructors use a model propeller and tilt it upward to show the bigger 'bite' the blade takes, but neglect the fact the plane of rotation has also changed. Using the helicopter analogy (rotating the propeller about a vertical axis) as you move it forward helps the student to visualize the difference in velocity. Then take it down to a more normal (but still exaggerated) axis to represent a nose high attitude.

Gyroscopic precession only happens when we change the axis of rotation of the propeller. P-factor, torque, and spiraling slipstream are all present without changing the axis. Gyroscopic precession introduces a left-turning tendency (on right turning engines) when lowering the nose.

Typically, gyroscopic precession is only a big factor on taildraggers when bringing the tail up, and float planes when getting on the step. That's when you need that little bit of extra rudder. Once you are on the mains or the step, you can ease off a bit.

The absolute best demonstration for this is an old bicycle tire spun and held by the axles. Only when the axis is moved do we feel that precession. Even better is sitting in an office chair with some good bearings and using the tire to 'steer' yourself around.

[rob-air]

For asymmetric, I assume you are talking about P-factor and not an engine failure on a multi-engined aircraft with engines mounted off the centerline. Asymmetric thrust and P-factor are the same in that case.

Yes sorry got lost in translation..

First, even though this is how it is taught, P-factor is not from the increased angle of attack of the down-going blade. It is from the increased velocity in comparison to the relative airflow for the down-going blade (and vice versa for the up-going blade). The angle of attack is equal in each and is directly related to dissymmetry of lift of a helicopter's main rotor.
Usually when this is demo'd, instructors use a model propeller and tilt it upward to show the bigger 'bite' the blade takes, but neglect the fact the plane of rotation has also changed. Using the helicopter analogy (rotating the propeller about a vertical axis) as you move it forward helps the student to visualize the difference in velocity. Then take it down to a more normal (but still exaggerated) axis to represent a nose high attitude.

Perfect thats what i needed. I always hated the bigger bite thing.

Gyroscopic precession only happens when we change the axis of rotation of the propeller. P-factor, torque, and spiraling slipstream are all present without changing the axis. Gyroscopic precession introduces a left-turning tendency (on right turning engines) when lowering the nose.

Typically, gyroscopic precession is only a big factor on taildraggers when bringing the tail up, and float planes when getting on the step. That's when you need that little bit of extra rudder. Once you are on the mains or the step, you can ease off a bit.


The absolute best demonstration for this is an old bicycle tire spun and held by the axles. Only when the axis is moved do we feel that precession. Even better is sitting in an office chair with some good bearings and using the tire to 'steer' yourself around.



Ok so if i get this right even if a force is applied on the rotating object but not throug its axis there will be no force felt 90 deg. further in the rotation

[iflyforpie]

Ok so if i get this right even if a force is applied on the rotating object but not throug its axis there will be no force felt 90 deg. further in the rotation

Not quite.

We will get gyroscopic precession only if that force--wherever it is applied--changes the orientation of the axis of the propeller.

So I suppose that if you have a pilot with lazy feet and allows the aircraft to turn left by the P-factor and spiralling slipstream, that gyroscopic precession would be present and give a 'nose up' tendency to the aircraft. But if that is the case it will be the least of your worries!

Such a tendency would be extremely small (especially on tricycle gear aircraft) and easily and naturally countered by the elevators.

Where you would see it the most would be if your tail swung while on the main wheels of a taildragger at the beginning of a ground loop. But again, the force would be inconsequential to the force your elevator should be putting down on the tailwheel.


Once the aircraft is in the air, generally gyroscopic precession is pretty minimal owing to the increased stability of the aircraft and generally low pitch/yaw rates. Maybe somebody with a bit more aerobatic experience can relate to gyroscopic forces in a hammerhead or tumble.

[Hedley]

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[Posthumane]

It would be quite a workout trying to do aerobatics or tactical/evasive maneuvers in an aircraft like a Camel with a rotary engine. Even level left turns were an issue in those from what I hear. Have you ever flown one Hedley?

It's kinda funny thinking about it now, but in my younger, dumber, pre-flying days I used to think that hammerheads and the like were simple maneuvers. Just point it at the sky, and when it slows down step on the rudder pedal, right? At that time I would not have thought about things like elevator for procession, opposite aileron to account for yaw/bank coupling, power settings, etc. I find it amazing how little I know every time I learn something new.

[Cat Driver]

It didn't make much difference, really. Even if I had been able to find anyone competent to check me out on the Beech 18 - they all old, retired and dead now - I doubt any of them would have explained the incredible importance of gyroscopic precession to me.

You forgot I'm still out here Hedley, and I could have advised you on how to get rid of the right swing due to gyroscopic precession during the lowering of the tail on landing the B18

Feather both engines before landing...problem solved.

[Hedley]

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[Hedley]

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[Hedley]

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[Shiny Side Up]

It didn't make much difference, really. Even if I had been able to find anyone competent to check me out on the Beech 18 - they all old, retired and dead now - I doubt any of them would have explained the incredible importance of gyroscopic precession to me.

You forgot I'm still out here Hedley, and I could have advised you on how to get rid of the right swing due to gyroscopic precession during the lowering of the tail on landing the B18

You do fall into the old and retired category Cat. I think he was insinuating that your age would impact your ability to impart this knowledge. You really shouldn't let this insult stand.

Camel with a rotary engine. Even level left turns were an issue in those from what I hear. Have you ever flown one Hedley?

AFAIK there are no currently flying examples with rotary engines. Even the ones I've seen, replicas and restorations have had their rotarys replaced with a similar sized radial. Since there are no more veterans around from the Great War, I doubt there's many who live with such experience anymore. A little piece of aviation knowledge which has, or is shortly to become extinct.

It's all about the prop. Metal prop, lots of gyro effect. Composite prop, nearly none.

It's all about polar moment of inertia, which is defined as the integral of radius squared dm which I don't think I'm allowed to mention here, despite that many people here think I am a moron compared to them.

You could bring a formula into the discussion, but all that would really get across is that you know a formula and have a degree and possibly the rest do not. In the case of our opening poster they're wondering how to present this as an instructor:

2-And is a simple explaination like this is enough to fool an examiner on a instructor ride because to me its seems like it's a bit more complex than that.

First, never attempt to fool the examiner - while it might be possible to do, if you get caught in the attempt to spread some bullshit, your goose is cooked. Secondly though, you don't need a university degree to give an explanation, but it will help if you have some good metaphors to use with the student. The essence of Gyroscopic theory really falls back to Newton's laws for your starting point. Figuring out a good way to get the message across to your students of how it works also transfers to a lot of other areas you'll probably need to be able to explain as an instructor. How the turn coordinator, directional gyro and artificial horizon work as the most immediate examples.

[Hedley]

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[Hedley]

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[Shiny Side Up]

Wrong. Never been to Old Rhinebeck? I'm sure there are others in the USA, as well.

Hence why I preluded the statement with the AFAIK, which I don't always assume to be everything, nor does anyone else here. I stand corrected. How many working, flying rotary engined airplanes does this constitute world-wide then?

Sigh. Integral radius squared dm tells you that the effect of moving mass outwards is horrendous, even if you don't understand the all the details. A prop with the same mass but concentrated at the center will fly completely differently.

Missing the point. While I understand you view that Integral radius squared dm is a reasonable starting point for the discussion, I don't think it is a good one for any student who doesn't hold an engineering degree, keeping in mind that a good portion of students will have math abilities of High school quality or less. What's the simplest way this principle can be explained to a layman?

I didn't think this was a flight test - this is an internet forum. See the difference? In any case, now that I have been so informed, I shall remember it!

The original question though was relating the principles discussed above to possibly explaining them to an examiner pretending to be a student on a flight test. Maybe I'm just an idealist but I would like to see an effort made so that said forum tries to be somewhat of a useful resourse for people to ask questions on, but then I'm pretty insane and unreasonable that way. No one has to play on the internet the way I wish after all, so feel free to ignore it. Too bad though since I sense that this particular forum is drying up, no one wants to weather a barrage to ask a question. Unfortunate.

I have personally seen this LeRhone rotary engine aircraft fly, blipping it's engine on short final.

Next time I talk to Bill Gordon I will be sure to inform him that he is extinct.

Would it be fair then to say his species is endangered? Should something happen to that airframe/engine combo, how many left will there be? If something happens to Bill, how many others with said knowledge will be left?

[rob-air]

First, never attempt to fool the examiner - while it might be possible to do, if you get caught in the attempt to spread some bullshit, your goose is cooked. Secondly though, you don't need a university degree to give an explanation, but it will help if you have some good metaphors to use with the student. The essence of Gyroscopic theory really falls back to Newton's laws for your starting point. Figuring out a good way to get the message across to your students of how it works also transfers to a lot of other areas you'll probably need to be able to explain as an instructor. How the turn coordinator, directional gyro and artificial horizon work as the most immediate examples.

Not that i want to fool the examiner, But most instructors teach the p-factor with the big bite small bite thing, so i figured that one of them tried it on the examiner at one point. The downgoing blade having a bigger AOA never made sense to me i knew it was more of a airspeed thing so i was looking for a better way to teach it the heli thing seems to be it.

My first question was why is the p-factor is causing yaw and not pitch, since the force should be felt 90 deg. further in the rotation but i guess that the p factor is causing yaw and yaw will move the axis and will then produce a pitching movement....would that makes sense.

anyway thanx for the help and the great stories

[Sulako]

I have removed a bunch of off-topic posts and edited some others. It's not cool to pick on other forum members, and next time I see that, it'll be an extended vacation for the bully involved.. If I were posting things like that, I'd be asking myself right about now whether the enjoyment I get from being mean is worth the sadness I'd feel to have my account wiped out. This is not negotiable, and I'm not kidding.

Anyway, where were we...

[Beefitarian]

How is a threat like that any different?

The so called "bully" seems to be projecting quite a bit also.

[Shiny Side Up]

Not that i want to fool the examiner, But most instructors teach the p-factor with the big bite small bite thing, so i figured that one of them tried it on the examiner at one point. The downgoing blade having a bigger AOA never made sense to me i knew it was more of a airspeed thing so i was looking for a better way to teach it the heli thing seems to be it.

Really depends on the test and the examiner. The "big bite, small bite" explanation of p-factor will do when making the explanation to RPP/PPL level students and would probably suffice for a class 4 test. An instructor, even of class 4 level, should have a better understanding of what's at play though and be prepared to delve further into it depending on the student. Definitely a detailed explanation could be expected of on a class 2 ride. I would concur that the helicopter analogy is a good path to follow.

[warbirdpilot7]

Rotary engine equipped aircraft are a good example of gyroscopic precession. You have a huge, spinning mass of metal on a very light airframe(Camel, Pup, DR1, Nieuport, etc....) Not to mention that most of the weight of these WW1 scouts were concentrated within the first 4 feet of the aircraft(Engine, fuel, guns, pilot).

The camel required very coarse use of rudder in turns to compensate. Even today, the majority of WW1 replicas have radials installed, so comparing these to the airplanes of the day is not valid.

I think that most instructors are weak on knowledge because the "P" factor is just not as pronounced on todays training platforms, the Tri-Gear stuff.

I guess the only way to teach this is to not correct for the yaw on climb-out and point ot the ball position on the T/C.