acceleration question

[Ardy123]

I'm not as knowledgeable as I wish I were when it comes to the physics/mathematics of flight and prop wash dynamics, so I'm hoping that some of you can shed some light on this. This discussion isn't intended to be in reference to the game.

My understanding is that as the prop pitch increases (angle of attack of propeller blades), it takes a larger bite out of the air.
1) Would this decrease the angle of the at which the air spirals around the airplane (I know its probably poorly worded, I posted a pictures to help)?
2) Would this effect the amount of 'roll' induced by the slip stream (because the angle at witch the air is hitting the wings is different)?
3) Depending on the current speed of the aircraft, I'm assuming there is an optimal pitch for acceleration, is this correct?
4) In systems where you manage the pitch indirectly via RPMs, is the 'highest-safe' RPMs always the best for acceleration, or would 'taking a larger bite' out of the air at a lower RPM sometimes be more 'optimal' than spinning faster and taking smaller 'bites' out of the air?

Thanks

[FLS]

I think you just need to look at which settings give you maximum thrust and you trim for a combination of thrust and aircraft speed. I'm guessing the helical angle is an unnecessary distraction.  Of course the experts may disagree. 

[mtnman]

I'm not as knowledgeable as I wish I were when it comes to the physics/mathematics of flight and prop wash dynamics, so I'm hoping that some of you can shed some light on this. This discussion isn't intended to be in reference to the game.

My understanding is that as the prop pitch increases (angle of attack of propeller blades), it takes a larger bite out of the air.
1) Would this decrease the angle of the at which the air spirals around the airplane (I know its probably poorly worded, I posted a pictures to help)?
2) Would this effect the amount of 'roll' induced by the slip stream (because the angle at witch the air is hitting the wings is different)?
3) Depending on the current speed of the aircraft, I'm assuming there is an optimal pitch for acceleration, is this correct?
4) In systems where you manage the pitch indirectly via RPMs, is the 'highest-safe' RPMs always the best for acceleration, or would 'taking a larger bite' out of the air at a lower RPM sometimes be more 'optimal' than spinning faster and taking smaller 'bites' out of the air?

Thanks
(Image removed from quote.)
(Image removed from quote.)

When it comes to boat props and RC aircraft props, the lower-pitch prop gives faster acceleration, but lower max speed.  Higher pitch gives slower acceleration, but higher max speed.

[Tupac]

When it comes to boat props and RC aircraft props, the lower-pitch prop gives faster acceleration, but lower max speed.  Higher pitch gives slower acceleration, but higher max speed.

I fly my skyraider with an 11x4 and it is the fastest plane at the field besides some of the bobcats

[FLS]

I believe Ardy123 is more interested in the performance of variable pitch propellers. Generally this is a constant speed propeller where the pitch angle is controlled automatically and is not set by the pilot.

[Ardy123]

I believe Ardy123 is more interested in the performance of variable pitch propellers. Generally this is a constant speed propeller where the pitch angle is controlled automatically and is not set by the pilot.

Ok so the thoughts behind the questions...(in red)

1) Would this decrease the angle of the at which the air spirals around the airplane (I know its probably poorly worded, I posted a pictures to help)?
2) Would this effect the amount of 'roll' induced by the slip stream (because the angle at witch the air is hitting the wings is different)?
If the airspeed isn't very high, this could mean that the angle at which the air is hitting the wing roots is well above the AoA and thus at low speeds the wing roots are in a stalled state, so decreasing the angle of the helix, could reduce the stalling and increase lift.
3) Depending on the current speed of the aircraft, I'm assuming there is an optimal pitch for acceleration, is this correct?
the 'bite' of the prop probably has a max efficiency at a particular angle of the prop blade, which I is depending on the speed, which would mean if you min-bite for acceleration, you could actually be wasting power instead of accelerating efficiently.
4) In systems where you manage the pitch indirectly via RPMs, is the 'highest-safe' RPMs always the best for acceleration, or would 'taking a larger bite' out of the air at a lower RPM sometimes be more 'optimal' than spinning faster and taking smaller 'bites' out of the air?
In game max throttle/ max RPM appears to be max thrust, but that would seem to imply that max rpm is smallest 'bite' of the prop, but is that really the case?

[hitech]

For all practical purposes max rpm is always delivers the most power thrust HP.  Very simply the prop must convert all HP from the engine to thrust and (drag around the prop) it will increase pitch until all HP is used.

The only way I can see that this would not be the case is if the engine HP curve started dropping above a certain RPM and if this were the case the max RPM Governor setting would not allow you to go above that rpm.

3) Depending on the current speed of the aircraft, I'm assuming there is an optimal pitch for acceleration, is this correct?

You can not just change a pitch and have everything else the same. If you flatten pitch then the engine will speed up. On a fixed pitch prop you then must reduce throttle to not over rev an engine.


For any throttle setting the best (thrust/acceleration) pitch will be the pitch that holds the prop at max rpm.

HiTech

[Ardy123]

For any throttle setting the best (thrust/acceleration) pitch will be the pitch that holds the prop at max rpm.

Thank you for the response Hitech.

Given an airplane that cannot feather its prop. As the speed increases, in order to keep the same RPM, the pitch increases, correct? If so, there should be a point at which the engine begins to overspeed, because the prop can't increase its pitch further, for example in a very steep dive for a prolonged period of time correct?

Also, at slow speeds, would the helictical airflow caused by the propwash, cause the wing roots to be in a stalled state because the airflow across them would be above the AoA? (thus reducing the angle of the helictical spiral would lower the aoa and increase lift?)?

Thanks

[FLS]

I believe a constant speed propeller will create enough drag to avoid overspeed. You'll have other problems before you have propeller problems.

As to airflow from propwash, remember that it's opposite on one wing from the other so that would create a rotating force from any wing lift it creates and you already have a rotating force from the propwash itself. I don't know how you would separate those effects to isolate any lift differences from pitch changes.

[Ardy123]

I believe a constant speed propeller will create enough drag to avoid overspeed. You'll have other problems before you have propeller problems.

As to airflow from propwash, remember that it's opposite on one wing from the other so that would create a rotating force from any wing lift it creates and you already have a rotating force from the propwash itself. I don't know how you would separate those effects to isolate any lift differences from pitch changes.

Well, I'm assuming that when slow, the propwash destroys lift, because the angle that the air is hitting the wing root is way beyond the angle of attack. I guess if the angle of the prop-wash helix could be less than the angle of attack, then the wing roots wont stall and your wing would be more 'efficient'.

Also, as in regards to hitechs comment, I was under the assumption that props did not produce constant thrust for a given throttle setting, but rather thrust changed depending on the speed of the aircraft (I'm guessing due to the inefficiency of the prop?). At increased speed, the pitch will change to maintain the same RPMs... so a double whammy happens, the helix angle is flattens out because both the prop pitch has changed and the the fact that the aircraft is moving with some speed. Therefore, there must be a point at which it is within the angle of attack and more of a rotating from lift( as you pointed out), than a just shear pushdown from the wind pushing down on one wing and up on the other, yet it appears that 'torque' from the engine becomes less as the airplane flys faster?

[FLS]

When you fly slow you are nose high to create a high AOA but the propwash is still almost straight on relative to the fuselage. Lift comes from the aircraft moving through the air so the propwash isn't going to be significant in regards to lift.

Aircraft need to be trimmed for whatever speed they are flying and for high speeds you want less drag from control surface deflection so you design the aircraft to need less manual trim correction at higher speeds. The torque is the same.

Aerodynamics is very complicated. I frequently get confused thinking about one aspect out of context. 

[kvuo75]

I believe a constant speed propeller will create enough drag to avoid overspeed. You'll have other problems before you have propeller problems.

the constant speed prop will adjust pitch to maintain the rpm, nothing more.. it has mechanical limits though, coarse and fine. if it can't fully feather, theoretically you could get fast enough that it will overspeed.

the drag the propeller is creating is what's creating the rpm in the windmilling situation, thats why you feather a dead engine's prop.. dead engine with windmilling prop is drag central!

[Ardy123]

When you fly slow you are nose high to create a high AOA but the propwash is still almost straight on relative to the fuselage. Lift comes from the aircraft moving through the air so the propwash isn't going to be significant in regards to lift.

My understanding is that AoA is the angle of the air as it passes over the wing. That means if in the most extreme case, if you not moving, but your propeller is blowing, then only the spiraling air will hit your wings correct? If so, then depending on the angle of the helix, it will hit your wing well above the AoA for non-stall conditions, thus the wing roots where the air is spiraling will be in-effect 'stalling'. or am I just way off on all of it, if so, please correct me with a better description, I'm trying to lean.

[FLS]

If you are not moving you are not creating lift so you can't stall.

[Ardy123]

If you are not moving you are not creating lift so you can't stall.

Maybe my terminology is incorrect, by stalling I mean the the wing is/would be in a state of stall, aka, the wind(from the prop, in this case) is hitting the wing at very steep AoA.