Constant speed propeller angle of attack and thrust?

[coriolis]

I find the aerodynamics of fixed pitch props relatively easy to understand -
the angle of attack varies with both forward speed and RPM.

However, something puzzles me about constant speed prop thrust. I realise
that the blades twist to give a different pitch, and therefore when you
increase MP to maintain the same RPM the blades will be at a higher helix
angle. However, won't the angle of attack (calculated by the relative wind)
by the same? Isn't that the idea? If the prop is at the same angle of
attack, why is the thrust higher? Once again, I know the pitch is higher,
but isn't the angle of attack the important thing in generating lift (which
is what thrust is) rather than simply the pitch angle? I've seen texts that
say you get a bigger bite, but that doesn't seem to explain why that matters
since lift is predicated on angle of attack.

Anyone have a good explanation?

TIA

[[email protected]]

On Aug 18, 3:03*pm, "coriolis" > wrote:
> I find the aerodynamics of fixed pitch props relatively easy to understand -
> the angle of attack varies with both forward speed and RPM.
>
> However, something puzzles me about constant speed prop thrust. I realise
> that the blades twist to give a different pitch, and therefore when you
> increase MP to maintain the same RPM the blades will be at a higher helix
> angle. However, won't the angle of attack (calculated by the relative wind)
> by the same? Isn't that the idea? If the prop is at the same angle of
> attack, why is the thrust higher? Once again, I know the pitch is higher,
> but isn't the angle of attack the important thing in generating lift (which
> is what thrust is) rather than simply the pitch angle? I've seen texts that
> say you get a bigger bite, but that doesn't seem to explain why that matters
> since lift is predicated on angle of attack.
>
> Anyone have a good explanation?
>
> TIA

If you increase MP you will increase the prop's angle of
attack. The governor is set by the prop control, and if it senses any
RPM increase as power is increased it will increase the pitch of the
blades. AoA therefore increases, thrust increases, speed increases; as
speed increases the AoA drops back some but not to the former value,
since the higher aircraft drag at the higher speed needs more thrust
to maintain and so the prop's AoA will need to be higher to create
that thrust. If it didn't change, the higher power (torque) would
increase the prop RPM, as with a fixed-pitch prop.

AoA on a prop can vary a lot. Common wisdom is that useful AoA will be
at least 2 degrees, varying with RPM and speed up to the stall angle
of the prop blade, which might be 15 to 18 degrees or so.

Dan

[Matt Barrow[_8_]]

"coriolis" > wrote in message ...
>I find the aerodynamics of fixed pitch props relatively easy to understand -
>the angle of attack varies with both forward speed and RPM.
>
> However, something puzzles me about constant speed prop thrust. I realise that
> the blades twist to give a different pitch, and therefore when you increase MP
> to maintain the same RPM the blades will be at a higher helix angle. However,
> won't the angle of attack (calculated by the relative wind) by the same? Isn't
> that the idea? If the prop is at the same angle of attack, why is the thrust
> higher? Once again, I know the pitch is higher, but isn't the angle of attack
> the important thing in generating lift (which is what thrust is) rather than
> simply the pitch angle? I've seen texts that say you get a bigger bite, but
> that doesn't seem to explain why that matters since lift is predicated on
> angle of attack.
>
> Anyone have a good explanation?
>

http://www.avweb.com/news/pelican/182082-1.html

Best one I've seen.

Matt

[Orval Fairbairn[_2_]]

In article >, "coriolis" > wrote:

> I find the aerodynamics of fixed pitch props relatively easy to understand -
> the angle of attack varies with both forward speed and RPM.
>
> However, something puzzles me about constant speed prop thrust. I realise
> that the blades twist to give a different pitch, and therefore when you
> increase MP to maintain the same RPM the blades will be at a higher helix
> angle. However, won't the angle of attack (calculated by the relative wind)
> by the same? Isn't that the idea? If the prop is at the same angle of
> attack, why is the thrust higher? Once again, I know the pitch is higher,
> but isn't the angle of attack the important thing in generating lift (which
> is what thrust is) rather than simply the pitch angle? I've seen texts that
> say you get a bigger bite, but that doesn't seem to explain why that matters
> since lift is predicated on angle of attack.
>
> Anyone have a good explanation?
>
> TIA

On a fixed-pitch prop AOA reduces as airspeed increases. RPM has to
increase in order to maintain a constant AOA until it reaches redline.

Essentially, the variable-pitch propeller increases angle-of-attack
(AOA) as speed or RPM decreases and MP remains the same.

Those propellers are also a set of compromises, since their twist is set
up to give maximum efficiency at a given airspeed/RPM. Deviations from
those conditions reduce efficiency.

You can make a simple program to map airspeed, RPM, AOA at various
stations along the prop span.

--
Remove _'s from email address to talk to me.

[gpsman]

On Aug 18, 5:03*pm, "coriolis" > wrote:
> isn't the angle of attack the important thing in generating lift (which
> is what thrust is)

http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html
-----

- gpsman

[coriolis]

>>>If you increase MP you will increase the prop's angle of
attack.<<<

Ok, this is where I was going wrong. Most probably by my misreading of some
texts, I was under the impression that the blade would seek one particular
AoA (regardless of the pitch) throughout its operating range until it hit
the stops.

Thanks to all who replied.

[Stealth Pilot[_2_]]

On Wed, 19 Aug 2009 07:03:55 +1000, "coriolis" >
wrote:

>I find the aerodynamics of fixed pitch props relatively easy to understand -
>the angle of attack varies with both forward speed and RPM.
>
>However, something puzzles me about constant speed prop thrust. I realise
>that the blades twist to give a different pitch, and therefore when you
>increase MP to maintain the same RPM the blades will be at a higher helix
>angle.

you are one confused puppy. let me correct some of that text.

the blade is twisted to give the same pitch at different radiuseseses.
the twist is something fixed in the shape of the prop blade, for
aluminium, during the forging process used to make the blade.
the pitch angle is a function of the circumfrence of the circle
prescribed by the radius point and the forward advance in one
revolution. the pitch angle = arctan(pitch advance/circumference)
typically the units are in feet. if you have the same pitch along the
blade then you have a helical twist in the blade.

in an inflight adjustable prop the blades rotate in the hub to change
the pitch as the hub is spun through the air by the engine.

you'll find it a lot easier to undersand if you use words more
precisely.

> However, won't the angle of attack (calculated by the relative wind)
>by the same? Isn't that the idea? If the prop is at the same angle of
>attack, why is the thrust higher? Once again, I know the pitch is higher,
>but isn't the angle of attack the important thing in generating lift (which
>is what thrust is) rather than simply the pitch angle? I've seen texts that
>say you get a bigger bite, but that doesn't seem to explain why that matters
>since lift is predicated on angle of attack.
>
now, what dan said in another post.

Stealth Pilot

[Brian Whatcott]

coriolis wrote:
> I find the aerodynamics of fixed pitch props relatively easy to
> understand - the angle of attack varies with both forward speed and RPM.
>
> However, something puzzles me about constant speed prop thrust. I
> realise that the blades twist to give a different pitch, and therefore
> when you increase MP to maintain the same RPM the blades will be at a
> higher helix angle. However, won't the angle of attack (calculated by
> the relative wind) by the same? Isn't that the idea? If the prop is at
> the same angle of attack, why is the thrust higher? Once again, I know
> the pitch is higher, but isn't the angle of attack the important thing
> in generating lift (which is what thrust is) rather than simply the
> pitch angle? I've seen texts that say you get a bigger bite, but that
> doesn't seem to explain why that matters since lift is predicated on
> angle of attack.
>
> Anyone have a good explanation?
>
> TIA
I'll give this a shot:
Fixed Pitch Prop:
prop AoA inversely proportional to IAS
prop AoA proportional to prop RPM

Const Speed Prop:
prop AoA inversely proportional to IAS
prop AoA proportional to throttle setting
& therefor shaft HP

Brian W

[Franklin[_7_]]

On Wed, 19 Aug 2009 07:03:55 +1000, coriolis wrote:

> Once again, I know the pitch is higher,
> but isn't the angle of attack the important thing in generating lift (which
> is what thrust is)

Thrust isn't lift. Lift is opposite weight. The forces are not with you.

[Franklin[_7_]]

On Wed, 19 Aug 2009 15:24:04 GMT, Stealth Pilot wrote:

> you are one confused puppy. let me correct some of that text.
>
> the blade is twisted to give the same pitch at different radiuseseses.

He may be confused but *he* is not a misspelling moron.

[coriolis]

>>>the blade is twisted to give the same pitch at different radiuseseses.
the twist is something fixed in the shape of the prop blade,<<<

I'm aware of that - by "twist" in this circumstance I mean the total pitch
changes due to the constant speed unit. Rotate - as you have used - would be
a better choice.

What I am confused about is why some texts talk about "taking a bigger bite
of air" as if the PITCH angle is what causes thrust rather than the ANGLE OF
ATTACK. I was under the (mistaken) impression that the AoA was kept the same
over the operating range of the CSU, but have been told this is not the
case.

[Orval Fairbairn[_2_]]

In article >, "coriolis" > wrote:

> >>>the blade is twisted to give the same pitch at different radiuseseses.
> the twist is something fixed in the shape of the prop blade,<<<
>
> I'm aware of that - by "twist" in this circumstance I mean the total pitch
> changes due to the constant speed unit. Rotate - as you have used - would be
> a better choice.
>
> What I am confused about is why some texts talk about "taking a bigger bite
> of air" as if the PITCH angle is what causes thrust rather than the ANGLE OF
> ATTACK. I was under the (mistaken) impression that the AoA was kept the same
> over the operating range of the CSU, but have been told this is not the
> case.

As you go from the center of the prop, spanwise toward the tips, the net
airspeed of the prop increases. Ideally, you wish to maintain the same
AOA of prop blade to air at any given station, with some other, minor
adjustments. Twist accomplishes this criterion.

Most props have a variation of the old Clark Y airfoil section; more
sophisticated props should change airfoil section (and sweep) spanwise,
in order to gain efficiency. Desired airspeed and RPM determine the
blade twist.

Add a means of changing blade angle and you have a controllable pitch
propeller. Add a governor and you now have a constant speed prop.

The desired speed/RPM range still determine the twist of the blades.

--
Remove _'s from email address to talk to me.

[Mike Ash]

In article >, "coriolis" > wrote:

> What I am confused about is why some texts talk about "taking a bigger bite
> of air" as if the PITCH angle is what causes thrust rather than the ANGLE OF
> ATTACK. I was under the (mistaken) impression that the AoA was kept the same
> over the operating range of the CSU, but have been told this is not the
> case.

"Taking a bigger bite of air" is just a colloquial way of saying "higher
angle of attack".

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

[coriolis]

Yup, understand all that.

My confusion - since cleared up - was what the resulting AoA was at
different manifold pressures. I had mistakenly thought that the forces would
resolve to make the AoA the same, and therefore couldn't figure out how
increased thrust would result from an increase in MP while leaving the prop
lever alone. I'm pretty sure I've got it now that I realise that AoA does
not end up the same.

[coriolis]

>>>"Taking a bigger bite of air" is just a colloquial way of saying "higher
angle of attack".<<<

Some texts seem a bit ambiguous - they refer to the "bite" and AoA as a kind
of separate thing.

For example:-

"For any single revolution of the propeller, the amount of air handled
depends on the blade angle, which determines how big a "bite" of air the
propeller takes. Thus, the blade angle is an excellent means of adjusting
the load on the propeller to control the engine rpm.

The blade angle is also an excellent method of adjusting the AOA of the
propeller."

This is from the FAA's Pilot's Handbook of Aeronautical Knowledge.

[[email protected]]

On Aug 19, 9:24 am, Stealth Pilot >
wrote:

> the blade is twisted to give the same pitch at different radiuseseses.
> the twist is something fixed in the shape of the prop blade, for
> aluminium, during the forging process used to make the blade.
> the pitch angle is a function of the circumfrence of the circle
> prescribed by the radius point and the forward advance in one
> revolution. the pitch angle = arctan(pitch advance/circumference)
> typically the units are in feet. if you have the same pitch along the
> blade then you have a helical twist in the blade.
>
> in an inflight adjustable prop the blades rotate in the hub to change
> the pitch as the hub is spun through the air by the engine.

It's still a compromise. The pitch distribution over the blade span
is designed to be most efficient at one particular blade angle
setting. When the blade angle increases, the whole blade rotates; the
pitch on the outer blade increases the same as that on the area at the
hub, which is NOT ideal. It puts the outer blade at a higher AoA than
the inner blade, screwing up the distribution of the load. A perfect
prop would increase the pitch at the hub more than at the tip. If you
do the trig you'll see it.

Suppose a six-foot prop is set for 60" pitch. That will be 15.5
degrees at the tip and 38.5 degrees a foot from the crank center. If
we rotate the blade to a coarser setting so that the tip angle is now
20 degrees, the pitch at the tip will be 77.4" but at the one-foot
station the pitch, now at 43.5 degrees, has a pitch of 71.6". The
inboard area is being lazy, see, and forcing more of the load outward.

Repitching of fixed-pitch props is limited to a couple of inches or
so because of this and because the inboard section won't twist at all.
It's too thick.

Dan

[Stealth Pilot[_2_]]

On Wed, 19 Aug 2009 14:13:58 -0400, Franklin <"Franklin
>> wrote:

>On Wed, 19 Aug 2009 15:24:04 GMT, Stealth Pilot wrote:
>
>> you are one confused puppy. let me correct some of that text.
>>
>> the blade is twisted to give the same pitch at different radiuseseses.
>
>He may be confused but *he* is not a misspelling moron.

you say radii do you? good for you. pom pom girl.

[Peter Dohm]

"Mike Ash" > wrote in message
...
> In article >, "coriolis" > wrote:
>
>> What I am confused about is why some texts talk about "taking a bigger
>> bite
>> of air" as if the PITCH angle is what causes thrust rather than the ANGLE
>> OF
>> ATTACK. I was under the (mistaken) impression that the AoA was kept the
>> same
>> over the operating range of the CSU, but have been told this is not the
>> case.
>
> "Taking a bigger bite of air" is just a colloquial way of saying "higher
> angle of attack".
>
> --
> Mike Ash
> Radio Free Earth
> Broadcasting from our climate-controlled studios deep inside the Moon

I believe that you might mean angle of incidence.

Peter

[Mike Ash]

In article >,
"Peter Dohm" > wrote:

> "Mike Ash" > wrote in message
> ...
> > In article >, "coriolis" > wrote:
> >
> >> What I am confused about is why some texts talk about "taking a bigger
> >> bite
> >> of air" as if the PITCH angle is what causes thrust rather than the ANGLE
> >> OF
> >> ATTACK. I was under the (mistaken) impression that the AoA was kept the
> >> same
> >> over the operating range of the CSU, but have been told this is not the
> >> case.
> >
> > "Taking a bigger bite of air" is just a colloquial way of saying "higher
> > angle of attack".
>
> I believe that you might mean angle of incidence.

I don't think so. If I'm going fast and suddenly pull up, increasing AoA
and causing the wings to produce more lift, I think I could reasonably
describe this as "taking a bigger bite of air". On the other hand, if I
somehow increased the angle of incidence of my wings (ignoring the
difficulty in actually doing this) then went flying, it wouldn't make
sense to describe that as "taking a bigger bite of air", not to me. It
seems that the same would hold true of propellors.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

[[email protected]]

On Aug 20, 1:54 pm, Mike Ash > wrote:

> > > "Taking a bigger bite of air" is just a colloquial way of saying "higher
> > > angle of attack".
>
> > I believe that you might mean angle of incidence.
>
> I don't think so. If I'm going fast and suddenly pull up, increasing AoA
> and causing the wings to produce more lift, I think I could reasonably
> describe this as "taking a bigger bite of air". On the other hand, if I
> somehow increased the angle of incidence of my wings (ignoring the
> difficulty in actually doing this) then went flying, it wouldn't make
> sense to describe that as "taking a bigger bite of air", not to me. It
> seems that the same would hold true of propellors.

Exactly. The angle of incidence of the prop might be fixed in
relation to its hub as with a fixed-pitch prop, but its AoA changes
with RPM and forward speed. The constant-speed prop's "angle of
incidence," more correctly known as pitch, changes in order to achieve
more efficient AoAs throughout the performance envelope.

Dan

[Franklin[_7_]]

On Thu, 20 Aug 2009 14:04:07 GMT, Stealth Pilot wrote:

> On Wed, 19 Aug 2009 14:13:58 -0400, Franklin <"Franklin
> >> wrote:
>
>>On Wed, 19 Aug 2009 15:24:04 GMT, Stealth Pilot wrote:
>>
>>> you are one confused puppy. let me correct some of that text.
>>>
>>> the blade is twisted to give the same pitch at different radiuseseses.
>>
>>He may be confused but *he* is not a misspelling moron.
>
> you say radii do you? good for you. pom pom girl.

No but *you* wish I did so that you could stick your tiny wee dick in my
pom pom pussy, Old Lecher.

[Franklin[_13_]]

Franklin <"Franklin wrote:

> On Thu, 20 Aug 2009 14:04:07 GMT, Stealth Pilot wrote:
>
>> On Wed, 19 Aug 2009 14:13:58 -0400, Franklin <"Franklin
>> >> wrote:
>>
>>>On Wed, 19 Aug 2009 15:24:04 GMT, Stealth Pilot wrote:
>>>
>>>> you are one confused puppy. let me correct some of that text.
>>>>
>>>> the blade is twisted to give the same pitch at different radiuseseses.
>>>
>>>He may be confused but *he* is not a misspelling moron.
>>
>> you say radii do you? good for you. pom pom girl.
>
> No but *you* wish I did so that you could stick your tiny wee dick in my
> pom pom pussy, Old Lecher.

That's a poor attempt at a troll. Score two points out of a possible ten.