Prop Pitch and Angle of Attack?

[Luffberry]

I’ve been puzzling over something recently, maybe you guys can help me out.

You want thrust to be equal to drag so the plane will fly at a constant speed; but what do you want the AoA of the prop to be doing at this speed?

I would think that you would want the prop AoA to be 0 degrees, so that it could throttle up or down without effort. And pitch is also what ties rpm (and thus thrust) to airspeed (and thus drag). And so thrust will either be always greater than drag, always equal to drag, or always less than drag (meaning a no-fly plane).

How closely should we be matching prop thrust to the drag of our plane, and is it pointless to consider the AoA of the prop, as long as it is generating enough thrust?

Also, is it just my imagination, or is the static thrust constant equal to the in-flight thrust constant when the prop is at the same AoA to the air?

[adam_one]

...I would think that you would want the prop AoA to be 0 degrees, ...?
That may happen in a dive when the prop pitch speed may equal the airspeed (prop thrust becomes zero).
The prop AoA is max when the plane is standing still and decreases as the airspeed increases.

[Luffberry]

Not exactly a detailed reply to my post, adam one, but thanks for posting.

By the way, prop thrust only becomes zero at negative AoA, which would happen in a dive.

Any more detailed answers to my other questions?

[hul]

have a look at Martin Hepperle's site. There is a section on prop theory and a prop simulation (Java prop):
http://www.mh-aerotools.de
You want thrust to be equal to drag so the plane will fly at a constant speed; but what do you want the AoA of the prop to be doing at this speed?you probably want maximum prop efficiency, try Java prop for an answer.
It's possibly related to what AoA gives best glide in a glider. I'll have to think about that.
I would think that you would want the prop AoA to be 0 degrees, so that it could throttle up or down without effort. And pitch is also what ties rpm (and thus thrust) to airspeed (and thus drag). And so thrust will either be always greater than drag, always equal to drag, or always less than drag (meaning a no-fly plane).drag and thrust change with flight speed (thrust goes down, drag up with speed). A change will just change the point where thrust and drag are equal again. Don't think you want 0 degrees AoA.
How closely should we be matching prop thrust to the drag of our plane, and is it pointless to consider the AoA of the prop, as long as it is generating enough thrust?personally I think that static thrust is misleading and therefore useless for normal planes (those that need forward speed to fly). Power/weight and prop efficiency (the more diameter the better) are more important.
Excessive pitch will cause excessive AoA and stall the prop if there isn't enough forward speed, causing low thrust until sufficient forward speed is reached. Depending on drag and thrust, enough forward speed may not be reached at all.
Also, is it just my imagination, or is the static thrust constant equal to the in-flight thrust constant when the prop is at the same AoA to the air? you may be asking the wrong question. AoA decreases with speed. That will change thrust, increase it if the prop unstalls because of it, otherwise decrease it.

hope that helps

Hans

[Luffberry]

you probably want maximum prop efficiency, try Java prop for an answer.

No, I’m clearly asking for prop theory here.

The prop’s AoA seems dependent on the speed of the prop and the airspeed at that time. If you increase a prop’s speed but the airspeed stays the same, the AoA of the prop increases.

One thing this could allow you to do is to match a static thrust constant to a specific motor RPM at a specific airspeed; and so anywhere along that specific prop angle of attack, you can know FOR SURE what the thrust will be. Now granted, the plane will probably accelerate way earlier with such an increase in prop speed. But that just means the expected thrust is doing its job. :)

drag and thrust change with flight speed (thrust goes down, drag up with speed).

This is not quite correct. Drag increases with the square of the airspeed velocity; static thrust increases with the square of the RPM; and airspeed velocity increases with the motor RPM because of the pitch (but it has to accelerate first). However, the prop has to match the same AoA at all times for thrust to be equal to the square of the velocity; which might mean that the thrust equation I’ve copied from other sites needs some work. Otherwise, the only thing that brings the thrust down to match the drag is that we run out of motor. And it is trying to match the prop to us running out of motor (and other points in the power curve) that I am trying to explore here.

Thank you for posting Hans; although I think you misunderstood the depth of my question. There is a reason I posted this to the “Modeling Science” section of the forum. ;)

I’d like to see some variable pitch propeller guys speak up; they are used to matching the AoA of the prop to airspeed for efficiency and maximum power. Maybe they have some insight on this fixed propeller problem.

[hul]

No, I’m clearly asking for prop theory here.
follow the link in my earlier post or see the link below.

This is not quite correct. Drag increases with the square of the airspeed velocity; static thrust increases with the square of the RPM
prop thrust typically decreases with air speed (exceptions see my earlier post). Static thrust is not relevant once you have forward speed.

However, the prop has to match the same AoA at all times for thrust to be equal to the square of the velocity
you lost me. Why is AoA constant?

which might mean that the thrust equation I’ve copied from other sites needs some work.
what thrust equation is this?

Otherwise, the only thing that brings the thrust down to match the drag is that we run out of motor.
I think, running out of pitch speed brings down thrust. No more thrust once air speed and pitch speed are equal.

There is a reason I posted this to the “Modeling Science” section of the forum.
the link in my earlier post is to a scientific site. Follow it and read the prop theory. This page in particular:
http://www.mh-aerotools.de/airfoils/pylonprops_2.htm#F3DOptimumPropeller
look at the table with definitions for thrust, power, advance ratio and efficiency. Thrust and power coefficients are a function of advance ratio. And follow the step by step instructions in section "all together now!".
This theory has been around for a while (80+ years, see bottom of that page) and seems to work quite well.

I’d like to see some variable pitch propeller guys speak up; they are used to matching the AoA of the prop to airspeed for efficiency and maximum power.
full size varioprops adjust pitch to control rpm. Power and therefore flight speed is controlled by the throttle, I believe.

Hans

[Luffberry]

follow the link in my earlier post or see the link below.

prop thrust typically decreases with air speed (exceptions see my earlier post). Static thrust is not relevant once you have forward speed.

A prop is a bit like a wing, only slightly more complicated due to airspeed messing with the angle of attack of the propeller. So, just as the lift of a wing at the same angle of attack is proportional to the square of the airspeed, the thrust of a prop at the same angle of attack is proportional to the square of the RPM.

The equation I was using before I posted was failing to account for angle of attack, which probably makes quite a bit of difference on the thrust. I’m going to have to be more careful when I look at modeling sites, I guess.

But whatever the equations are, ultimately it is the motor that holds back the thrust (and beyond that, speed of sound). A motor can only turn a prop so fast, and eventually the airspeed catches up, reducing the AoA, until equilibrium is reached.

you lost me. Why is AoA constant?

My point is, you can calculate a formula that will tell you the angle of attack of a prop at a given RPM and airspeed. And you can do the inverse; calculate a function of RPM vs. airspeed where the angle of attack is the same. And because the angle of attack is the same, you can use a thrust constant times the square of the RPM to get an accurate thrust value at an RPM and airspeed. And this way, you can use the thrust constant of the static thrust in this formula, because you can set the formula to use the angle of attack of the static thrust.

I think, running out of pitch speed brings down thrust. No more thrust once air speed and pitch speed are equal.

This is where the AoA of the prop approaches 0. So, theoretically, there is still some thrust here; just not enough to overcome drag. If you sped up the prop again, the thrust would increase, and the AoA of the prop would increase until the airspeed increases. And it would come to equilibrium somewhere else. But, if there isn’t enough thrust to match drag at 0 AoA, air speed is going to fall behind pitch speed. And that means that the prop will have more drag on it, because of the higher AoA. If it’s possible, it would probably be best to have the prop AoA be 0 here, wouldn’t it?

the link in my earlier post is to a scientific site. Follow it and read the prop theory. This page in particular:
http://www.mh-aerotools.de/airfoils/pylonprops_2.htm#F3DOptimumPropeller
look at the table with definitions for thrust, power, advance ratio and efficiency. Thrust and power coefficients are a function of advance ratio. And follow the step by step instructions in section "all together now!".
This theory has been around for a while (80+ years, see bottom of that page) and seems to work quite well.

Haven’t gotten to it yet. I’ll post my thoughts after reading it.

[hul]

what you're saying is not wrong. But it's simplified, doesn't take into account a lot of odd effects that are going on (which I don't know either, I take smarter people's word for it). You can't even treat a wing as an airfoil, pretty certain you can't treat a prop as an airfoil.
I would assume that the guys who came up with the prop theory 80 years ago started at the same point as you but then found discrepancies between this simplified theory and actual prop behaviour. I think they then went and invented their theory, which I believe works ok for reasonable props (extreme cases may be off again).
BTW, ct and cp are functions of advance ratio and those functions are fix for a given prop.

Hans

[Luffberry]

I just read the article you pointed me to. He’s using the same over-simplified equations for thrust, power, pitch, and airspeed from pitch that led me to ask these questions. I am beginning to feel, after some very careful thought, that his equations are wrong (even though they were lifted from a reasonably authentic source), even for the limited testing they were designed for.

But this is besides the point quite a bit.

Hul, I’m not afraid to bring out the calculus on problems like this, especially if I can get a more detailed estimate on propeller performance. Once you know how to apply calculus to a problem, it’s no biggie. The question is, what kind of qualitative goals are we trying to achieve? Do we want 0 AoA at cruise so we can increase or decrease the prop speed quickly? Do we want 0 AoA at maximum efficiency, to make sure our flight lasts longest and at the best performance? Is there even enough thrust from the prop at 0 AoA? Is maintaining thrust ultimately more important than anything else, even if it means reving the prop faster to get a higher AoA, and more thrust?

This is what I’m asking. :D

[Sparky Paul]

As a prop is just a rotating wing, i'm surprised you haven't found the prop airfoil that will give you the least drag, the most lift (thrust), and all at 0 AOA.

[Luffberry]

Sparky Paul, the issue at hand is whether the thrust generated from a prop at 0 AoA is enough to match the drag of the plane at the airspeed where the plane is in equilibrium. At a higher AoA, the prop will generate more thrust.

If the drag is too much for the prop at 0 AoA, the airspeed will drop until the prop is at enough AoA to support the drag again.

It is this problem that I need some input about.

[adam_one]

Assuming continuous full-throttle:
The prop produces max thrust and has max slippage when the forward speed is zero.
As the forward speed increases the thrust and slippage decreases until the thrust equals the drag and max forward speed is reached.
So, I think that the ideal prop in terms of efficiency and flight duration is the one that has the least slippage during cruise speed (for sport planes) or least slippage at max speed in case of pylon racers.
Prop slippage is the difference between pitch speed and forward speed.

Also, for the same power available, the prop that gives max level flight speed is less efficient during take-off, acceleration and climb, whereas the prop that accelerates the plane quickly from standstill will give lower top speed.
That's why full size planes have often props with variable pitch.

[biber]

Adam-one, if there's no prop slippage there won't be thrust either.
Same thing for the effective AoA of a propeller blade section. I do say effective AoA since the geometrical AoA isn't very meaningfull for aerodynamic calculations, it merely is easier to measure but depends on the actual airfoil chosen for that particular section of that particular prop.
So, what I don't seem to get at all is Luffberry's global zero AoA obsession.

Speaking for prop's effectiveness aswell as for the wing/plane's effectiveness, the zero (effective!) AoA will make the thrust/lift also to zero, and thus makes also the effectiveness (for a wing/plane known as L/D ratio) to zero. Not good.

OTOH, geometrically zero AoA is just a positive effective AoA for any common asymmetrical airfoil (wich luffberry seems to be assuming anywhere anytime ;) ) depending on its actual amount and distribution of camber and the RE-number it is currently running at. In general the geometric zero AoA has nothing more than randomly to do with any optimum point.

biber

[adam_one]

Adam-one, if there's no prop slippage there won't be thrust either.
I agree, so as long as the prop produces thrust it has also some slippage (forward speed lower than pitch speed).
But since the higher slippage (higher AoA) the lower prop's efficiency is, the most efficiency for a certain speed should be when one gets the least AoA, which not necessarily means zero AoA.

The full size plane's pilot chooses a fine pitch during the take-off and increases the pitch as the forward speed increases, like changing the gears on a car.
For the fixed pitch props (as most models have) one has got only one gear, so it's about a compromise, depending on the plane's type and on the kind of flying.

[biber]

But since the higher slippage (higher AoA) the lower prop's efficiency is, the most efficiency for a certain speed should be when one gets the least AoA, which not necessarily means zero AoA.
The props's efficiency is (thrust*airspeed)/(torque*rpm*2*pi) if im correct. So, if the torque and rpm can be assumed roughly as constant (of course they aren't really) and thrust will go to zero when reaching zero AoA the efficiency of the prop will also drop to zero then! Just like it is zero when airspeed is zero. Somewhere in between the efficiency will max out.

biber

[adam_one]

The props's efficiency is (thrust*airspeed)/(torque*rpm*2*pi) if im correct. So, if the torque and rpm can be assumed roughly as constant (of course they aren't really) and thrust will go to zero when reaching zero AoA the efficiency of the prop will also drop to zero then! Just like it is zero when airspeed is zero.

biber
Yes, that's true if you get zero thrust, which only may occur in a dive.
But I've got another formula for efficiency ;) :
Prop efficiency = (thrust*airspeed)/(input power)
As the airspeed increases the thrust decreases and so (thrust*airspeed) may roughly be kept the same while the input power needed decreases (the motor unloads), which increases the efficiency...

[biber]

What i wanted to say is that zero AoA means zero efficiency. So, maximum efficiency is certainly a bit off from the zero AoA.

biber

[adam_one]

What i wanted to say is that zero AoA means zero efficiency. So, maximum efficiency is certainly a bit off from the zero AoA.

biber
It might be, but at zero AoA the prop still may produce some thrust, like an airfoil at zero AoA still may produce some lift...
The prop's blades have often cambered airfoils, which may produce some thrust at zero AoA.

[biber]

It's better to use the effective AoA in aerodynamical discussions than the geometrical AoA since the differential angle depends on the particular airfoil used in every special case. For instance generally a geometric zero AoA dosn't say anything but aerodynamic zero AoA definately means zero lift.

biber

[adam_one]

It's better to use the effective AoA in aerodynamical discussions than the geometrical AoA since the differential angle depends on the particular airfoil used in every special case. For instance generally a geometric zero AoA dosn't say anything but aerodynamic zero AoA definately means zero lift.

biber
Biber,
I don't think I've got it, I only know one definition of AoA:
It is the angle between the airfoil's chord line and the direction of flight.
You might be interested in reading following thread:
How to determine airfoil AOA (http://www.rcgroups.com/forums/showthread.php?t=390899)
One may find some food for thoughts there.

[biber]

Maybe it would have been more appropiate to use the term lift coefficient to get away from the AoA thing. The prob I have with the usage of AoA on different airfoils is their different specific zero lift angle that leads to an uncertainty in talking about the lift at zero AoA.

biber

[vintage1]

yeah, AofA is a ternm that means many things to many people.

What is 'the' angle of a curved surface?

[biber]

And what is an East Anglia? :D

biber

[vintage1]

It's a bit like a Bavaria, only more civilized :D

[adam_one]

It's a bit like a Bavaria, only more civilized :D
Hmm, that sounds rather contemptuous... but means "more civilized" that there are no hooligans there? :D

By the way, the airfoil's AoA has the same definition in every aerodynamics book I've read so far... :)

What is 'the' angle of a curved surface?
All surfaces are curved.
What you call a straight line is in fact just an arc from a circle with a radius close to the infinitum...

[biber]

So, forget about my rambling about AoA. I'll prefer using cl next time to avoid that. As for the common definition of AoA you are correct. For aeroscience things I still find it more easy to deal with it after idealising it by replacing airfoils by their zero lift angles since that is exactly what is counting for cl and thus lift/thrust.
But I guess I can't expect the rest of the world to either do so :rolleyes: .

Vint, I just tried to make a silly joke on your location sounding like some weird old fashioned kind of angle unit.
A big difference beetwen East Anglia and Bavaria certainly is that in case of Bavaria people of big parts of the world seem to think that all the country is like that :eek: .

biber

[adam_one]

Don't worry Biber, I promise to forget...
I think that your joke was rather harmless, e.g. the french call them "les anglais" so the resemblance can be further confusing. :)

I've been travelling through Bavaria once and saw many jolly good fellows gracefully flying their model airplanes over the green fields there.

[Luffberry]

yeah, AofA is a ternm that means many things to many people.

What is 'the' angle of a curved surface?

Airfoils are designed from a specific reference airfoil with a specific chord line to define AoA from. Propeller airfoils are (almost always) created as asymmetrical with positive camber so that they generate lift, and thus thrust, efficiently; just like an asymmetrical wing.

Trying to redefine the chord line of a specified airfoil, and thus the angle of attack, simply to conform to the currently popular view on aerodynamics is rather silly. Yes, you can define an airfoil, and its chord line, multiple ways; but it is how the airfoil designer defined it that really matters. Really, we’re still using “Clark Y”s and NACA airfoils. Until we are talking about a series of airfoils defined some other way, just stick to the published chord lines to determine angle of attack publicly. Do whatever you want, privately.

Personally, I think it would be better to define the chord line in terms of minimum drag; that would really highlight the difference between a lifting wing and a symmetrical wing. But, for most airfoils in use, the designed chord line is substantially close to that anyway.