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#11
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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. |
#12
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Constant speed propeller angle of attack and thrust?
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. |
#13
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Constant speed propeller angle of attack and thrust?
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 |
#14
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Constant speed propeller angle of attack and thrust?
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. |
#15
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Constant speed propeller angle of attack and thrust?
"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. |
#16
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Constant speed propeller angle of attack and thrust?
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 |
#17
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Constant speed propeller angle of attack and thrust?
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. |
#18
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Constant speed propeller angle of attack and thrust?
"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 |
#19
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Constant speed propeller angle of attack and thrust?
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 |
#20
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Constant speed propeller angle of attack and thrust?
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 |
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