Constant speed propeller angle of attack and thrust?

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 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

"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

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.

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

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.

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

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

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.

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.