Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Ardy123 on November 04, 2010, 05:00:40 PM
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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
(http://i55.tinypic.com/255474j.png)
(http://i54.tinypic.com/vfjdvs.png)
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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. :D
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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
(http://i55.tinypic.com/255474j.png)
(http://i54.tinypic.com/vfjdvs.png)
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.
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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
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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.
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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?
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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
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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
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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.
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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? :headscratch:
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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. :D
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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!
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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.
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If you are not moving you are not creating lift so you can't stall.
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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.
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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.
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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.
:headscratch:
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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
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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.
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Seems more likely you'd just see a minor difference in the wing dropping at the stall.
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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!
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I think that reducing throttle would be faster, easier, and more effective than adjusting RPM but it was an interesting question.
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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).
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I expect the helix angle effect on lift and roll is inconsequential regardless of AOA compared to thrust differences.
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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.
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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.
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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
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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 :aok
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.
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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:)
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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
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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.
Now I'm really confused, I always thought that the thickness and curve of an airfoil changed the cl factor of the wing for a given AoA, so if it was non-symmetrical it would have different cl depending if the wing was right side up, or up side down?
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Yes the CL will change but the cl curve (is really a straight line) threw most of the non stall angles.
The big difference is the with a non symmetric wing the curve shifts. So that at 0 AOA you have a positive CL instead of 0 with a symmetric foil. And at -1 or -2 deg aoa you will have 0 lift.
so if the plane is flying with 5 Degrees AOA for instance if the prop is adding 2 and subtracting 2, (3 and 7 degrees net) the cl would change the same amount but just in different directions.
HiTech