Constant speed propeller angle of attack and thrust?

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.

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.

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

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.

"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.

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

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.

"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

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

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