acceleration question

[FLS]

It seems like you're thinking of the propwash in isolation without regard to the rest of the air you're flying in. Think of the propeller as creating a pressure difference with lower pressure in front and higher pressure behind it and think of the wings as creating lower pressure on the top and higher pressure below the wings.

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.

The wind that creates lift and allows flight doesn't come from the propeller, it comes from the aircraft moving forward in the air. The propeller creates thrust that moves the aircraft forward. The significance of the helicoil propwash is that it creates a rotational reaction and also that it pushes your tail to one side.
In your example of an aircraft that was not moving, there is no lift so there is no stall since a stall is a disruption of lift.

[RTHolmes]

vastly overestimating the spiralling of the prop wash too - the drawings show >360deg spiral before hitting the wing leading edge.

I'd be amazed if the prop wash spirals more than a few degrees over the whole length of the fighter. my gut feeling is that there might be a small, possibly noticable effect (ie enough to need trimming out) on yaw as the flow hits the tail a few degrees off the thrust line as a large part of the fin is in the wash. the effect on roll at the wing should be alot less as a much smaller part of the wing is in the wash than the tail, although the angle off the thrust line will be greater than at the tail because the wing is closer to the prop.

 

[hitech]

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.

First taking the extream case when plane is static does not really have a lot of meaning when speaking of stall. I have to do some researching to find if the helix angle to the wing with no forward motion is greater the max aoa or not. My guess is it would not be.

Around stall speed the helix can make a 1 or 2 degrees differences at what AOA the roots of the wing will stall.

HiTech

[Ardy123]

First taking the extream case when plane is static does not really have a lot of meaning when speaking of stall. I have to do some researching to find if the helix angle to the wing with no forward motion is greater the max aoa or not. My guess is it would not be.

Around stall speed the helix can make a 1 or 2 degrees differences at what AOA the roots of the wing will stall.

HiTech

Exactly where I was going, this means that a stall could be prevented by changing the pitch of the propeller so that the AoA is within the envelope, if the helix angle is greater or near the max aoa.
hmmmmmm.

[FLS]

Seems more likely you'd just see a minor difference in the wing dropping at the stall.

[Ardy123]

Seems more likely you'd just see a minor difference in the wing dropping at the stall.

but that could be huge in a fight!

[FLS]

I think that reducing throttle would be faster, easier, and more effective than adjusting RPM but it was an interesting question.

[Ardy123]

I think that reducing throttle would be faster, easier, and more effective than adjusting RPM but it was an interesting question.

Adjusting throttle would reduce the torque, but it would probably increase the helix angle too.. Go take a plane up in the game, and reduce throttle, there is a band where you can reduce the throttle yet the rmps don't change, which implies the prop pitch is decreasing to maintain rpms. I guess if the helix angle is always within the AoA, its irrelevant, but, that hasn't been confirmed yet (hitech said he would have to look it up).

[FLS]

I expect the helix angle effect on lift and roll is inconsequential regardless of AOA compared to thrust differences.

[RTHolmes]

Exactly where I was going, this means that a stall could be prevented by changing the pitch of the propeller so that the AoA is within the envelope, if the helix angle is greater or near the max aoa.
hmmmmmm.

you'll just bring the stall on sooner this way - reducing the rpm control reduces engine power and hence thrust.

[Ardy123]

you'll just bring the stall on sooner this way - reducing the rpm control reduces engine power and hence thrust.

This would be true if there was no torque, but often I find that throttling off as you get near stall speed, actually allows  you to hold the nose up longer because the engine torque doesn't drag and role your ride. Try it, do a power on stall, then powered off stall, you'll see you can get your nose to hang there below stall speed with a power off stall.

Now, if you could preserve more thrust by instead of powering off, lowering the rpms, maybe you could get a way with throttling off less while still having some output thrust from the engines.  I'm guessing the torque is from the slipstream created by the prop.

[hitech]

Exactly where I was going, this means that a stall could be prevented by changing the pitch of the propeller so that the AoA is within the envelope, if the helix angle is greater or near the max aoa.
hmmmmmm.

Your forgetting the other wing will stall sooner. Prop increases aoa 1 side decreases other side.

2nd your just completely mixing definitions.

For the ENTIRE plane you should define stall as the greatest velocity AOA (I.E. cord line to velocity vector not air stream) for a given speed and throttle setting that will generate the most lift.

This will almost always be that some PIECES of the wing is stalled and some not. So now you are into more complex questions some of which are.

How much washout is in the wing?
How wide is the slip stream?
How long is the cord line at the root vs tip.
What speed is the plane traveling.
Is the slope of the back side of the lift curve steeper the the front side.
Will the loss in thrust require more AOA for same lift.

These are just some of the factors needed to be known to answer the question you are posing, I'm sure I've missed some.

HiTech

[Ardy123]

Your forgetting the other wing will stall sooner. Prop increases aoa 1 side decreases other side.

2nd your just completely mixing definitions.

For the ENTIRE plane you should define stall as the greatest velocity AOA (I.E. cord line to velocity vector not air stream) for a given speed and throttle setting that will generate the most lift.

This will almost always be that some PIECES of the wing is stalled and some not. So now you are into more complex questions some of which are.

How much washout is in the wing?
How wide is the slip stream?
How long is the cord line at the root vs tip.
What speed is the plane traveling.
Is the slope of the back side of the lift curve steeper the the front side.
Will the loss in thrust require more AOA for same lift.

These are just some of the factors needed to be known to answer the question you are posing, I'm sure I've missed some.

HiTech

Hmmm, I didn't think of all of that, nice points thanks  

Your forgetting the other wing will stall sooner. Prop increases aoa 1 side decreases other

I believe most ww2 airplanes had NACA2300 series airfoils were not symmetrical (cross section of the wing) thus the upper part of the wing generated more lift then the lower part, and thus, you would have asymmetrical lift, where the slip stream passed over the wing depending on the side of the aircraft the wing was on, correct? If so, then a little trim could correct for only the one section on one side stalling where the other was not, as supposed to having it stall on both sides where the slip stream went over the wing.

For the other items you mentioned, ok, here is an example... just to see how the math fits...
washout - none (keep it simple)
width of slip stream(8 feet, I know in real life it would probably be more of a cone shape)
chord: simple square wing (30 feet long 10 feet wide, 300 feet wing area, chord 10 feet)
speed: 120 IAS
Is the slope of the back side of the lift curve steeper the the front side: don't quite understand perfectly, back side being under side of the wing?
Will the loss in thrust require more AOA for same lift: no idea, how would I calculate this?

I'm assuming Id have to do some iterative calculation, but I don't know the formulas, please inform.

[smoe]

Have to remember that the ultimate measurement for propeller lift power is torque and not necessarily just horsepower. Maximum torque does not always mean maximum horsepower. Every engine is different, one would need to study the horse power/torque curves of any engine. The propeller size and pitch angle are a whole different story. I would believe an airplane engine may be designed for maximum power at maximum rpm because that would make the most sense. Spinning an engine faster than its peak horsepower/torque rpm speed curve would very likely reduce lift thrust/efficiency on the propeller.

Horsepower is more of a controversial measurement. Ask one person and it is a calculable area displacement of the pistons. Another will say it is the amount of transferred power to the crank shaft.

PS. If someone has never taken a thermodynamics college class please don’t try to contradict me (too much), but you are free to do so:)

[hitech]

Have to remember that the ultimate measurement for propeller lift power is torque and not necessarily just horsepower.

You are completely mixing terms and there by are confusing the discussion. By definition " measurement for propeller lift power" is horse power (I.E. power = power Horse just is a unit of measurement)

Torque is a force measurement not a power measurement. And you say things like (Spinning an engine faster then its peak HP/Torque) again mixing terms.

Spinning faster then peak HP would make no sense. Spinning faster then peak torque would make lots of sense.

Ardy123:

thus the upper part of the wing generated more lift then the lower part,

Is not what a non symmetric airfoil does. It simply means that top and bottom of the wing are shaped differently. But the change in AOA still produces a linear lift curve. I.E. a +1 and -1 deg change in AOA will have the same effect on both sides as long as you are not on the edge of stall.



HiTech