Gyroscopic effects on a stopped propeller!

[Maverick Su-35S]

Hi,

 

 

As I've been playing with the K4, I've started testing the gyro effects driven by the spinning propeller on the BF-109K4 and found out that when it's completely stopped or stuck, the gyro effect becomes tremendous, where in fact it should be null.

 

Now as most of the prop plane pilots might know, when you have a yaw or pitch rate and especially when you reverse it you should notice that a commanded pitching effect will unavoidably induce a yawing effect and vice-versa. This effect, combined with the P-factor effect (which is purely aerodynamic) will tend to counter each-other (i'm not going through these details..., who has the knowledge or likes to understand it, can tell) when pulling-pushing the stick or snapping the rudder, but the gyro effect is mostly dominant above the P-factor in all cases. So, in reality, if there would be no P-factor, then the gyro effect would have an increased amount, but only when the propeller spins...!

 

There should be no gyro effect at all (cause there's nothing to create it) and even more no word of P-factor effect (this one tends to become infinitely low when the prop's pitch is low and should be non-noticeable even with the prop in high pitch). With the prop fully stopped, the whole plane starts swirling in pitch and yaw just the way both the P-51 and FW-190 do as well when the prop doesn't spin.

 

Please verify this cause this has nothing to do with reality.

Here's a track regarding all that I'm saying:

 

[ATTACH]120616[/ATTACH]

[Yo-Yo]

Hi,

 

 

As I've been playing with the K4, I've started testing the gyro effects driven by the spinning propeller on the BF-109K4 and found out that when it's completely stopped or stuck, the gyro effect becomes tremendous, where in fact it should be null.

 

Now as most of the prop plane pilots might know, when you have a yaw or pitch rate and especially when you reverse it you should notice that a commanded pitching effect will unavoidably induce a yawing effect and vice-versa. This effect, combined with the P-factor effect (which is purely aerodynamic) will tend to counter each-other (i'm not going through these details..., who has the knowledge or likes to understand it, can tell) when pulling-pushing the stick or snapping the rudder, but the gyro effect is mostly dominant above the P-factor in all cases. So, in reality, if there would be no P-factor, then the gyro effect would have an increased amount, but only when the propeller spins...!

 

There should be no gyro effect at all (cause there's nothing to create it) and even more no word of P-factor effect (this one tends to become infinitely low when the prop's pitch is low and should be non-noticeable even with the prop in high pitch). With the prop fully stopped, the whole plane starts swirling in pitch and yaw just the way both the P-51 and FW-190 do as well when the prop doesn't spin.

 

Please verify this cause this has nothing to do with reality.

Here's a track regarding all that I'm saying:

 

[ATTACH]120616[/ATTACH]

 

 

It's not gyro. Just draw two horisontal wings presenting horisontal blades of stopped wings (I describe four-blade prop in ortogonal position, but it works in any position and for any number of blades). As the AoA is changed both blades change their AoA but in opposite directions, so their lift and drag will have different additions. So, AoA changes produce yaw moments. The same is for AoS changes.

[Maverick Su-35S]

It's not gyro. Just draw two horisontal wings presenting horisontal blades of stopped wings (I describe four-blade prop in ortogonal position, but it works in any position and for any number of blades). As the AoA is changed both blades change their AoA but in opposite directions, so their lift and drag will have different additions. So, AoA changes produce yaw moments. The same is for AoS changes.

 

Hi Yo-Yo,

 

 

I know what you're saying, and that's the P-factor effect and if I remember correctly, a similar discussion had place for the P-51's behavior regarding the same aspect. I'm not telling that I don't believe you, but all I'm saying is that the P-factor, especially when the blades are in low pitch (or lowest anyway), the difference in AoA felt by each blade (as the plane's AoA and beta angles (side slip) vary) has VERY little affect (almost insignificant) on the CL and CD (or lift and drag) of each blade as they are all stalled anyway, and I believe you know it too...!

 

For me it's impossible to accept that the variation of lift and drag on each blade, which indeed vary with AoA and beta, can have such a huge amount of effect when the blades are stalled. Even if the blades would be in high pitch (or highest possible) and not stalled yet, the yawing and pitching moments created on the tons of metal (aircraft) can't be as high as when the prop is spinning and in fact they are even greater than if it was spinning and you can also check that out. Check the gyro and P-factor when the prop is spinning at high RPM and when it's stuck and you'll see the same thing that I'm talking about!

 

Just please..., show us a real life footage or some wind tunnel test data or any kind of real life proof that an aircraft with stopped props would behave like this and swirl so much in pitch and yaw and I'll believe you!

 

Please don't get me wrong, I have at least 10 years of experience in aviation and I'm a pilot, I have seen many and learned many things, yet if there's something that might seem impossible from my perspective and in fact it's true, please be the one to show it and I'll happily respect it.

 

Thank you Yo-Yo!

Edited August 18, 2015 by Maverick Su-35S
2nd paragraph idea completed

[Yo-Yo]

Hi Yo-Yo,

 

 

I know what you're saying, and that's the P-factor effect and if I remember correctly, a similar discussion had place for the P-51's behavior regarding the same aspect. I'm not telling that I don't believe you, but all I'm saying is that the P-factor, especially when the blades are in low pitch (or lowest anyway), the difference in AoA felt by each blade (as the plane's AoA and beta angles (side slip) vary) has VERY little affect (almost insignificant) on the CL and CD (or lift and drag) of each blade as they are all stalled anyway, and I believe you know it too...!

 

For me it's impossible to accept that the variation of lift and drag on each blade, which indeed vary with AoA and beta, can have such a huge amount of effect when the blades are stalled. Even if the blades would be in high pitch (or highest possible) and not stalled yet, the yawing and pitching moments created on the tons of metal (aircraft) can't be as high as when the prop is spinning and in fact they are even greater than if it was spinning and you can also check that out. Check the gyro and P-factor when the prop is spinning at high RPM and when it's stuck and you'll see the same thing that I'm talking about!

 

Just please..., show us a real life footage or some wind tunnel test data or any kind of real life proof that an aircraft with stopped props would behave like this and swirl so much in pitch and yaw and I'll believe you!

 

Please don't get me wrong, I have at least 10 years of experience in aviation and I'm a pilot, I have seen many and learned many things, yet if there's something that might seem impossible from my perspective and in fact it's true, please be the one to show it and I'll happily respect it.

 

Thank you Yo-Yo!

 

Sometimes common sense speculations give wrong conclusions. I bet that you never encountered stopped prop diving in a plane having the prop blades area to wing area ratio like powerful fighters. Try to use blade element method just to blade tip using the data of 360 degree airfoil lift/drag and you will see the effect.

[Anatoli-Kagari9]

Regarding the OP,

 

there are even instances of the p-factor effect, in the K4 and the other DCS prop modules, which can become somehow unexpected, but, if we try to "see" it in 3d will have, at least mathematically, a possible explanation, assuming other moments wouldn't compensate for it...

 

One such situation is flying in a sideslip, climbing or level. If we do not apply right rudder, and just use aileron to try to keep the wings level, a pronounced sideslip will develop.

 

Now, if we want to become coordinated, and start applying right rudder, gyroscopic effects could account for a tendency of the nose to drop when you initially apply rudder, but instead you will observe that the K4 pitches up, and if you do not use proper countering fwd stick, you will end up in a vertical climb....

 

What is playing it's effect here ? Apparently 2 factors:

 

1) If I'm not wrong, P-Factor again, as modeled in the DCS K4 ( and I'm NOT suggesting this is wrong !!! ).

 

Look at it in 3d and go figure, trying to, by rotating the plane / referential 90º clockwise, see this corresponds to lifting the tail on a taildragger reducing the P-factor effect...

 

2) The K4 tendency to pitch up... which was being partially countered, while in sideslip state, by P-factor.

Edited August 20, 2015 by jcomm

[Maverick Su-35S]

Sometimes common sense speculations give wrong conclusions. I bet that you never encountered stopped prop diving in a plane having the prop blades area to wing area ratio like powerful fighters. Try to use blade element method just to blade tip using the data of 360 degree airfoil lift/drag and you will see the effect.

 

Right Yo-yo, indeed I haven't flown nor seen a RL footage of a blades stopped aircraft gliding where the pilot would pitch or yaw the plane in that condition to encounter the kind of swirl effect that we have, and after all that I've said and told that there can't be such high variation of lift/drag ratio or of either of them separately when the blades are stalled (especially in low blade pitch), when the AoA and/or Beta of them varies, I'll take your word as it is even if I'm not fully convinced (based on what I've mentioned) that even for a prop aircraft with quite high sum of blades area to wings area ratio it would be plausible to get such swirling effects when you vary the AoA and/or sideslip. All I'm saying (being skeptical) is that the swirl effect developed in such conditions should be smaller than it is, but this is only how I see it.

 

Thanks!

[Maverick Su-35S]

...

One such situation is flying in a sideslip, climbing or level. If we do not apply right rudder, and just use aileron to try to keep the wings level, a pronounced sideslip will develop.

 

Hi jcomm,

 

That's adverse yaw due to roll, and although you didn't mention the blades spin direction, by what you say, they should be spinning to the right and if you were telling about P-factor, it develops from the beta (sideslip) and AoA only (the roll alone does not affect P-factor) and thus judging by this it results that the P-factor would tend to pitch the aircraft down (as the 12 o clock blade would have more lift than the 6 o clock one) and it would later counter with the gyro effect which would tend to yaw the aircraft to the right and I've already mentioned before that the P-factor and gyro effects are countering each-other (each having it's particular magnitude). The P-factor's magnitude is affected by a law described as the derivative of the P-factor to the AoA + a derivative of it to the beta angle, while the gyro effect's magnitude is affected by the derivative to AoA rate + the derivative to beta angle rate, at least the theory tells it, so one depends on the static angles, while the other depends on their rate of variation.

 

No need for further explanations cause I know these things well too, it's the magnitude of the overall effect that bothered me from the very beginning if you understand...! They are being simulated, and they had been so from the first prop plane that appeared, the P-51D, but the overall values just seem out of the ordinary, but I take Yo-Yo's word for it and try to believe that this is realistic.

 

 

Cheers jcomm!

[Anatoli-Kagari9]

Hi Maverik,

 

I believe you didn't understand my point...

 

The sideslip inflight, if not caused by other reasons including pilot-induced, results from the prop effects at speeds and power regimes not matching those for which the aircraft trim was adjusted.

 

In my description, I meant flying at power settings / AoA where the prop effects, mainly asymmetric slipstream and torque, create a yaw ( and consequent induced roll ) to the left, asking for right rudder.

 

If instead of using right rudder the pilot uses only roll input ( to the right ) to keep the wings level, the aircraft will keep flying in a sideslip, right ? The ball will show a deflection to the right, asking for right rudder, which is not being used.

 

Now, as we kick on the right rudder pedal to coordinate, the gyro precession would cause a pitching down moment but p-factor would be reduced ( look at it rotated 90º clockwise ) and that accounts most probably for the fact that the aircraft starts pitching up. Yes there is also less drag once we start flying coordinated and the aircraft becomes more effective aerodynamically I know...

Edited August 31, 2015 by jcomm

[Maverick Su-35S]

Hi Maverik,

 

I believe you didn't understand my point...

 

The sideslip inflight, if not caused by other reasons including pilot-induced, results from the prop effects at speeds and power regimes not matching those for which the aircraft trim was adjusted.

 

In my description, I meant flying at power settings / AoA where the prop effects, mainly asymmetric slipstream and torque, create a yaw ( and consequent induced roll ) to the left, asking for right rudder.

 

Roger jcomm, that's simply the prop's effect induced yaw as I told and as I repeat it only affects the P-factor if there's no rate of change in AoA or sideslip and I didn't say anything contrary to this!

 

If instead of using right rudder the pilot uses only roll input ( to the right ) to keep the wings level, the aircraft will keep flying in a sideslip, right ? The ball will show a deflection to the right, asking for right rudder, which is not being used.

 

Again, this is yaw induced roll, which mainly generates a P-factor that depends on angle of sideslip and dynamic pressure and as the ball stands deflected (meaning that there are sideforces, lateral accelerations or the existence of a yaw rate that would create them) there's also a small gyro effect (precession) that takes place. I also know this and there was no need to mention it again. I feel like we're not going anywhere with this!

 

If you feel that I don't understand something else, I'm pleased to listen, otherwise I know how these effects occur and repeat myself, the only thing that bothered me was some particular effect's magnitude when the prop is not spinning anymore (is stopped) and for instance that can only be the P-factor (cause there's no talk about gyro effects or gyro precession if there's no angular momentum)

 

Now, as we kick on the right rudder pedal to coordinate, the gyro precession would cause a pitching down moment but p-factor would be reduced ( look at it rotated 90º clockwise ) and that accounts most probably for the fact that the aircraft starts pitching up. Yes there is also less drag once we start flying coordinated and the aircraft becomes more effective aerodynamically I know...

 

Again, this is the case when a prop is still spinning with some given rpm and I've also confirmed/told that the gyro effect (precession) is somewhat countered with the P-factor, so we're again saying the same thing and neither of us understands what does the other one try to say different;)!

 

Thx. jcomm, cheers!

Edited September 4, 2015 by Maverick Su-35S