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pitch angle vs. angle of attack


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#1 rjl2001

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Posted 25 February 2008 - 13:11

I was reading the FAA Rotorcraft Flying Handbook, and got a little confused about blade pitch angle and angle of attack. My understanding was pitch angle = angle between chord line and a reference plane; AOA = angle between chord line and relative wind.
So either I'm misunderstanding this or maybe it's a typo because the book says, "Angle of attack should not be confused with pitch angle. Pitch angle is determined by the direction of the relative wind."
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#2 southernweyr

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Posted 25 February 2008 - 13:43

Yeah, it must be a typo. Good catch.

#3 Kelly N.

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Posted 25 February 2008 - 13:57

Yeah, it must be a typo. Good catch.



Isn't the pitch angle (the mechanical angle of the blade) also in part influenced by the relative wind? For example, the relative wind is always parallel to and opposite of the direction of flight, but if the relative wind changes (due to induced wind, turbulence, etc) wouldn't you have to potentially modify the mechanical angle (pitch angle) in order to assume the needed adjustment to the angle of attack?

Regardless, even if that is the case and what was meant, I thought the statement was confusing when I came across it as well. Could definitely use some re-wording at best.

Kelly

#4 anaik1

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Posted 25 February 2008 - 14:18

There is no "typo" just to let you know. It says "not to be confused with" meaning the AoA is not the same a the pitch angle. AoA=cord line and resultant relative wind. Pitch angle, also known as Angle of Incidents (AoI)= Cord line and rotational relative wind. In powered flight AoI is larger than AoA...In an autorotation AoA is larger than AoI. Its all in the rotorcraft flying handbook. Cheers!

#5 Linc

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Posted 25 February 2008 - 14:36

PITCH ANGLE—The angle between the chord line of the rotor blade and the reference plane of the main rotor hub or the rotor plane of rotation.

ANGLE OF ATTACK—The angle between the airfoil’s chord line and the relative wind.



Pitch angle - The angle between the chord line of a main or tail rotor blade and the plane of rotation (tip path plane). It is usually referred to as the blade pitch angle.

Angle of attack - The angle measured between the resultant relative wind and the chord.

Relative wind (RW) - The air in motion that is equal to and opposite the flight-path velocity (FPV) of the airfoil. Because FPV may be modified by an induced flow of air, the RW experienced by the airfoil may not be exactly opposite its direction of travel. In this manual, this modified relative wind is referred to as resultant relative wind.


Edited by Linc, 25 February 2008 - 14:39.

Linc
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#6 Fastlane

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Posted 25 February 2008 - 17:04

Pitch Angle is the angle of the chord line in relation to the rotor blades rotational plane. Angle of attack is the relation to the actual angle of the rotor blade to the relative wind.

Plain and simple. You're on the right track.
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#7 southernweyr

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Posted 27 February 2008 - 20:02

There is no "typo" just to let you know. It says "not to be confused with" meaning the AoA is not the same a the pitch angle. AoA=cord line and resultant relative wind. Pitch angle, also known as Angle of Incidents (AoI)= Cord line and rotational relative wind. In powered flight AoI is larger than AoA...In an autorotation AoA is larger than AoI. Its all in the rotorcraft flying handbook. Cheers!


AoA.jpg
It says, in the rotorcraft flying handbook, "Pitch angle is determined by the direction of the relative wind." Are you saying that that statement is correct?

In the picture it is showing a blue arrow indicating the relative wind and a reference plane that is used for pitch angle. I don't see how that statement could be correct if they are defining the relative wind as what is indicated in the picture.

#8 Linc

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Posted 28 February 2008 - 03:20

I agree it is a mistake. In the section above, where the manual discusses pitch angle it says, "The pitch angle of a rotor blade is the angle between its chord line and the reference plane containing the rotor hub." No mention of relative wind there, and there shouldn't be. The diagrams also show the AoA in relation to the relative wind and pitch angle relative to the reference plane.

Comments regarding this handbook should be sent to U.S. Department of Transportation, Federal Aviation Administration, Airman Testing Standards Branch, AFS-630, P.O. Box 25082, Oklahoma City, OK 73125.

The only question is how long will it take to make the correction, or is there already a notice out regarding this point?
Linc
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For this gull, though, it was not eating that mattered, but flight. More than anything else. Jonathan Livingston Seagull loved to fly.

#9 helonorth

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Posted 28 February 2008 - 12:17

The statement that "pitch angle is determined by the direction of the relative wind" is, at best,
misleading. Pitch angle, as I understand it, is the angle between the chord line and the vertical
axis of the rotor plane (not necessarily the mast). "Angle of incidence" is the angle between the
chord line and the longitudinal axis of an airplane. That angle changes as the disk or "wing" changes
(on a helicopter) with cyclic inputs.

#10 joker

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Posted 02 March 2008 - 11:27

I don't think it is a mistake, just a little slack on their use of terms.

The paragraph explains pitch angle as the angle between the reference plane and the chord line. As in diagram 2.4

That same 'reference plane' can also be called the 'plane of rotation'. It is the direction the blade is moving in space! It is also referred to in basic aerodynamic terms as the 'relative wind' as that is the direction the air meets the blades.

This is the case for a helicopter sitting on the ground with a little collective pulled in. The pitch angle is the angle between the chordline and the relative wind (aka plane of rotation or reference plane). It is the same value as the angle of attack.

That's about as far as fixed-wing pilots need to go, because where they point their nose is generally where they are going. Their nose pitch is the same as their wing pitch..so their pitch angle is always the same as their angle of attack.

However, we helicopter pilots must go one step further. For example, although the blades might be going round and round horizontally, sometimes the wind might not be meeting the blade horizontally. This could is due to turbulence, climbing or decending flight regieme, or at the smallest level - blade flapping and induced flow.

Now we helicopter pilots have a problem. The direction of the 'relative wind' we talked about above (the plane of rotation) is not exactly the same direction the wind hits the blade from.

Take a helicopter falling through the sky downwards, but still level with a level disc, and the same amount of collective as the one sitting on the ground. The plane of rotation hasn't changed, nor has pitch angle.

Due to the falling, the wind hits the blade from below. At the same time, due to the RPM, the wind hits the blade horizontally from the front. If we add these two effects, we get a new wind that we have to think about.

We need a name for this! Many call it 'Resultant Relative Wind'. This is what really counts when we think about lift. This helps us know the 'angle of attack'. The angle of attack is between the chord line and the 'Resultant Relative Wind'.

Back to the FAA book.

So they are correct, but lazy.

They say that AoA and pitch angle are different and should not be confused. That's correct.

Their diagram (the one above), shows AoA as the angle between the chord line and the 'relative wind'. This is a diagram showing ANY blade...not necessarily a helicopter blade. So forget planes of rotation or planes of reference. This is where the confusion sets in. The diagram is saying..."Forget what wind vectors made this relative wind be this direction, we're not interested at the moment. We're just talking about the direction the air meets the blades, so we can explain AoA and show how it can be different to pitch angle."

Put another way, if you are not interested in what wind vectors make up a relative wind, then you may as well call it just 'relative wind'. On the other hand, if you are going to look at the ingredients of a 'relative wind' you call the result the 'resultant' relative wind, to make it clear what the line is on the diagram!

Hence they are correct, but it is a little lazy too.

Well, I hope that rambling explanation helps someone!

Joker

#11 Wannabe1

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Posted 02 March 2008 - 12:54

Good Morning Joker...

Thanks. I had asked my CFI about this back in mid Feb so was interested when this thread came up. We went around on it (and several real MCCoy typos I have found in other ASA books) trying to make sense of the paragragh. Your explaination has been the best so far.

Tx,
Tom


I don't think it is a mistake, just a little slack on their use of terms.

Well, I hope that rambling explanation helps someone!

Joker



#12 Linc

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Posted 02 March 2008 - 14:13

To say it is correct but lazy seems more like you don't want to say Flight Standards missed something in their copyediting and got it wrong. If the pitch angle was measured from the relative wind, then the pitch angle paragraph would say "relative wind" instead of rotational plane or plane of reference.

If a blade flaps up or down, the pitch angle wouldn't change, but the angle of attack would because the relative wind changed. And that's without accounting for induced flow, which would further modify the relative wind to the aforementioned resultant relative wind. They're not equal and that's why the attempt was made poorly to disambiguate the two different angles relative to the blade.

Insert "Angle of attack" into the sentence in question instead of "Pitch angle" and the paragraph makes much more sense and then tends to agree with everything the manual said before that paragraph.
Linc
OH-58D KW pilot

For this gull, though, it was not eating that mattered, but flight. More than anything else. Jonathan Livingston Seagull loved to fly.

#13 RockyMountainPilot

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Posted 02 March 2008 - 14:46

I don't think it is a mistake, just a little slack on their use of terms.


No, its just plain incorrect.

The paragraph explains pitch angle as the angle between the reference plane and the chord line. As in diagram 2.4

That same 'reference plane' can also be called the 'plane of rotation'. It is the direction the blade is moving in space! It is also referred to in basic aerodynamic terms as the 'relative wind' as that is the direction the air meets the blades.

This is the case for a helicopter sitting on the ground with a little collective pulled in. The pitch angle is the angle between the chordline and the relative wind (aka plane of rotation or reference plane). It is the same value as the angle of attack.


This is incorrect. As Linc mentioned, the only time the pitch angle and AOA are if the blades were at 0 pitch and AOA which is impossible due to blade twist. But let's assume no blade twist and 0 pitch and AOA. When pitch increases, AOA increases and then induced flow immediately decreases AOA.

That's about as far as fixed-wing pilots need to go, because where they point their nose is generally where they are going. Their nose pitch is the same as their wing pitch..so their pitch angle is always the same as their angle of attack.


Nope. An airplane can descend with a negative pitch and a positive AOA. If I am on approach with power at idle, I am pitched down to maintain airspeed. However, because the airplane is descending and the RW is coming from below my AOA is still positive. Conversely, I can climb out vertically with enough power and need no lift from the wings and be at a 0 AOA with a 90 degree pitch.

However, we helicopter pilots must go one step further. For example, although the blades might be going round and round horizontally, sometimes the wind might not be m
eeting the blade horizontally. This could is due to turbulence, climbing or decending flight regieme, or at the smallest level - blade flapping and induced flow.

Now we helicopter pilots have a problem. The direction of the 'relative wind' we talked about above (the plane of rotation) is not exactly the same direction the wind hits the blade from.


No, the relative wind comes from exactly the same direction the wind hits the blade. That is why it is called relative wind. It may not be the opposite direction from which the blades are traveling however.

Take a helicopter falling through the sky downwards, but still level with a level disc, and the same amount of collective as the one sitting on the ground. The plane of rotation hasn't changed, nor has pitch angle.

Due to the falling, the wind hits the blade from below. At the same time, due to the RPM, the wind hits the blade horizontally from the front. If we add these two effects, we get a new wind that we have to think about.

We need a name for this! Many call it 'Resultant Relative Wind'. This is what really counts when we think about lift. This helps us know the 'angle of attack'. The angle of attack is between the chord line and the 'Resultant Relative Wind'.


The relative wind is the resultant of ALL wind components. Forward flight, blade rotation, induced flow, etc etc. There is no such thing as the resultant relative wind as the RW is already the resultant of all wind components. This is explained in many texts and is actually on the same page of the error in question.

"RELATIVE WIND
Relative wind is created by the motion of an airfoil
through the air, by the motion of air past an airfoil, or by
a combination of the two. Relative wind may be
affected by several factors, including the rotation of the
rotor blades, horizontal movement of the helicopter,
flapping of the rotor blades, and wind speed and direction."

#14 joker

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Posted 03 March 2008 - 11:51

OK, RMP and Linc,

I will concede that this is a mistake in the book. I had a long explanation typed, but will not post it, as it seems that AoA would be more appropriate in that paragraph.

That is not to say that I agree with all of RMPs rebuttals. Particularly his statement: "There is no such thing as the resultant relative wind..." That is just like saying there is no such thing as centrifugal force!

From the point of view of a blade, then you're right - see diagram 2.6 in your book. From the point of view of looking at the ingredients of that wind, it is useful to name the resulting vector. Particularly, as the name given to the wind created by the movement of the blade in its plane of rotation (along the reference plane) is also commonly referred to as 'relative wind'.

Sometimes when instructing, I find its necessary to use a little leeway to explain a point. Often when describing physics we have to change our frame of reference in order to discuss a concept. Do what is necessary to get a point across, I guess.

So to those astrophysicists, apologies for a few technical inaccuracies - to the students, well maybe I might have helped a little.

Joker

#15 RockyMountainPilot

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Posted 03 March 2008 - 12:37

OK, RMP and Linc,

I will concede that this is a mistake in the book. I had a long explanation typed, but will not post it, as it seems that AoA would be more appropriate in that paragraph.

That is not to say that I agree with all of RMPs rebuttals. Particularly his statement: "There is no such thing as the resultant relative wind..." That is just like saying there is no such thing as centrifugal force!

From the point of view of a blade, then you're right - see diagram 2.6 in your book. From the point of view of looking at the ingredients of that wind, it is useful to name the resulting vector. Particularly, as the name given to the wind created by the movement of the blade in its plane of rotation (along the reference plane) is also commonly referred to as 'relative wind'.

Sometimes when instructing, I find its necessary to use a little leeway to explain a point. Often when describing physics we have to change our frame of reference in order to discuss a concept. Do what is necessary to get a point across, I guess.

So to those astrophysicists, apologies for a few technical inaccuracies - to the students, well maybe I might have helped a little.

Joker


Resultant relative wind is a term created by someone I know, because he had the same misunderstanding about relative wind you do. Relative Wind IS a resultant. That is why it is relative. The term resultant relative wind is like saying the rear trailing edge, or swinging pendular action. The addition of the term resultant is superfluous.

The RFH is the most poorly written flying book I have ever had the displeasure of reviewing. There are hundreds upon hundreds of technical errors. Most of the people who wrote the book had not ever flown a helicopter, and the editor hadn't flown one in decades.

I won't get into the centrifugal force discussion. LOL

#16 joker

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Posted 03 March 2008 - 12:43

OK - whatever.

Just one thing...do you need to quote my whole text. That is supurfulous! Use the 'Add Reply' at the bottom, rather than the 'Reply' at the foot of my post!

Joker

Just out of interest, what do you call the wind caused by the rotation of the blade in the plane of rotation?

Edited by joker, 03 March 2008 - 12:46.


#17 RockyMountainPilot

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Posted 03 March 2008 - 12:52

OK - whatever.

Just one thing...do you need to quote my whole text. That is supurfulous! Use the 'Add Reply' at the bottom, rather than the 'Reply' at the foot of my post!

Joker

Just out of interest, what do you call the wind caused by the rotation of the blade in the plane of rotation?


Yes, I do need to quote your whole text.

I call the "wind" caused by the rotation of the blades a component of relative wind.

#18 SquirrelFlight

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Posted 03 March 2008 - 13:56

Yes, I do need to quote your whole text.

I call the "wind" caused by the rotation of the blades a component of relative wind.


Basic fluid mechanics was a long time ago, but I believe that what you're referring to is the pressure head: the apparent increase in pressure of a fluid increases due to the motion of a solid object within the fluid.
No matter where you go, there you are

#19 helonorth

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Posted 03 March 2008 - 15:22

We always called it the resultant relative wind.

#20 Eric Hunt

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Posted 04 March 2008 - 19:32

"Joker

Just out of interest, what do you call the wind caused by the rotation of the blade in the plane of rotation? "

It is called the Rotational Flow. Add to it the flow from induced flow, or from an autorotational descent, or anything else, and you get the Relative Air Flow. The RAF is the vector sum of all airflows, to give one final resultant.




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