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Gyroscopic motion after engine failure


Lixma 06

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There should be some initial movement - there was a torque, then it’s gone, and after that the aircraft should be out of trim, but after that - in the absence of pilot induced oscillations - it should damp out...

Are your controls steady ?

Cheers.

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It can’t be “supposed to be” p-factor from stopped blades - stopped blades don’t precess.

It could be precession doesn’t get turned off.

Does it yaw one way if you pull the stick back and the opposite way if you push it forward ?

Cheers.

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It can’t be “supposed to be” p-factor from stopped blades - stopped blades don’t precess.

It could be precession doesn’t get turned off.

Does it yaw one way if you pull the stick back and the opposite way if you push it forward ?

 

 

P-Factor has nothing to do with gyroscopic effects.

 

 

Simply think of each stopped blade as a wing, generating lift ( and drag ) and try to figure out why that "swirling" movement is there...

 

 

Is it right ? I don't think so - I think it's overdone.

Flight Simulation is the Virtual Materialization of a Dream...

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There should be some initial movement - there was a torque, then it’s gone, and after that the aircraft should be out of trim, but after that - in the absence of pilot induced oscillations - it should damp out...

Are your controls steady ?

 

Steady - I was gliding looking for a place to ditch so I kept movement to a minimum.

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P-Factor has nothing to do with gyroscopic effects.

 

 

Simply think of each stopped blade as a wing, generating lift ( and drag ) and try to figure out why that "swirling" movement is there...

 

 

Is it right ? I don't think so - I think it's overdone.

 

Bf109 does not exhibit any gyrating behavior with a seized engine. Only yaw/roll to the right that needs to be countered. Once appropriate stick and rudder input is applied it stays steady. P-51 should be the same, no?

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P-Factor has nothing to do with gyroscopic effects.

 

 

Simply think of each stopped blade as a wing, generating lift ( and drag ) and try to figure out why that "swirling" movement is there...

 

 

Is it right ? I don't think so - I think it's overdone.

 

You're right, the p doesn't stand for precession, but p-factor still doesn't happen with a stopped prop:

 

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller,[1] that is responsible for the asymmetrical relocation of the propeller's center of thrust when an aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.

Cheers.

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It's a "generalized" form of P-Factor.

 

 

P-Factor is due to asymmetric lift / thrust due to the difference in prop blade relative AoA between the two ( left / right ) sectors of the imaginary circle created by the prop rotation, and differs, of course, depending on it being CW or CCW... particularly felt in taildraggers.

 

 

This is present at any flight situation actually, whenever the relative wind is not aligned with the prop rotation axis, so the generalized case doesn't limit to the left / right sectors, but to any possible opposing sectors of the imaginary circle. A prop aircraft flying level in a sideslip will create P-Factor. Combinations of sideslip and "longitudinal AoA" will create even more interesting / complex variations on P-Factor.

 

 

DCS Flight dynamics model does account for these effects, although I always tended to feel that it's somehow overdone...

 

 

 

Now, in a stopped / "braked" prop, the kind of situation that easily becomes possible ( contrarily to real life ) with DCS prop warbirds, each blade is at it's own, fixed, angle to the relative wind, thus creating it's own lift not in phase with the other 3 ( there are 4 in this case ).

 

 

If you think about it, starting at one of the blades, and admiting the huge prop in front of the P51d could actually play such a role, as DCS's flight dynamics model of that aircraft assumes to be possible, you'll find the reason for the swirl :-)

 

 

Pick a pencil and a paper and try it :-)

 

 

Ah! And there's yet another "asymmetry" in the P51... Even assuming your aileron and rudder trim tabs are set to neutral, the tail fin is canted 2º to port-side... There being enough stream / dynamic pressure, this will induce a sideslip, even without thrust...

 

 

P.S.: Hey !!! You're a Lucky Guy living where you live! I want to go there soaring !!!!!!!


Edited by jcomm

Flight Simulation is the Virtual Materialization of a Dream...

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It's P-Factor Lixma, imo overdone in it's effects...

 

It's just calculated or modeled, if you wish. In both cases. As a prop downwash with X-wind...

 

 

For those who do not understand why stopped 4-bladed prop can produce Lissajois-like movement:

 

the P-51 prop blades have total area about 1 sq. m and they are not less than 4 wings. Let's say that blades stopped as a straight cross (the actual position does not matter much, but it will be easier to understand the reason). Let's imagine that the plane got small AoS increment. The angles of attack for horisontal pair of blades changes are insufficient, but for vertical pair the changes are close to the plane AoS changes. One of them, so, increases drag and the opposite - decreases. Thus, the plane gets LONGITUDINAL moment, and it begins to change AoA. As AoA starts to change, yawing moment appears.

So, there is 90 deg shifted constant destabilising pair of moments.

 

The plane has damping moments, so, from the point of view of classical stability theory the system is nonlinear with positive feedback, and we can see typical oscillation.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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It's just calculated or modeled, if you wish. In both cases. As a prop downwash with X-wind...

 

 

Touché :-)

 

Then, I believe it can actually be realistic, although not easy to replicate IRL because certainly few pilots, alive, have been able to experience flying under such circumstances and living to tell about the effect.

 

It makes sense, as my previous posts suggest - trying to explain it as a "generalized form of P-Factor", but I was somehow renitent to accept it could make a heavy 51d move that way… Probably it would.

Flight Simulation is the Virtual Materialization of a Dream...

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Touché :-)

 

Then, I believe it can actually be realistic, although not easy to replicate IRL because certainly few pilots, alive, have been able to experience flying under such circumstances and living to tell about the effect.

 

It makes sense, as my previous posts suggest - trying to explain it as a "generalized form of P-Factor", but I was somehow renitent to accept it could make a heavy 51d move that way… Probably it would.

 

1 sq m at the end of the plane... it's a good reason to swing the plane :)

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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