Prop theory/Blade Theory question

[BGRing]

MYTH: AT POSITIVE ANGLE OF ATTACK, THE DESCENDING PROP BLADE IS AT A HIGHER ANGLE OF ATTACK, SO IT PUTS OUT MORE LIFT, PULLING THE PLANE LEFT.

NOT THE WHOLE PICTURE!!! You only THINK this because you stand on the ramp beside your airplane on the ground, looking at the prop hub, imagining the nose pointing up and therefore the right-hand blade seeming to take a greater "bite" out of the air. Here is where your visualization is WRONG: If the nose is pointed up with respect to the air stream, then that right-hand prop blade is actually NOT DESCENDING STRAIGHT DOWN LIKE YOU ARE VISUALIZING, BUT IS INSTEAD MOVING FORWARDS A BIT AS IT DESCENDS, SO THE ANGLE OF ATTACK IS NOT INCREASED LIKE YOU THINK ON THE DESCENDING BLADE!! So why DOES the plane pull left in a climb even though the descending blade is NOT at a higher angle of attack? Look to helicopters for the answer. Their ADVANCING blade (the main rotor blade that is coming FORWARD) wants to put out a LOT more lift since it is moving at its rotational speed PLUS the speed of the aircraft. The RETREATING rotor is traveling a lot SLOWER!! It is traveling at its rotation speed MINUS the speed of the craft. So there tries to be a lot more lift on the right side of the rotor system, and the pilot has to enter a lot of correction in blade pitch to fly straight! A climbing airplane is the same: The DESCENDING blade is actually ADVANCING INTO THE AIR A LITTLE BIT (THUS MOVING FASTER) SINCE THE PLANE IS TILTED UP! The CLIMBING blade is retreating away from the onrushing air a bit for the same reason! Thus, the DESCENDING blade puts out more lift from its HIGHER SPEED when the airplane is at high angle of attack.
I have to admit that it will not kill you not to know this (;-)) but the aviation MYTH says otherwise so I want to correct it... also the angle of attack does change a bit as well since the air is not rushing head-on into the prop but instead coming up from underneath a bit, but it is the "advancing/retreating" speed change that you do not know about.
This is a quote I came across from what I thought would be a Reputable source...
I am having trouble understanding how he (Lets call him Mr X) How Mr X is trying to disspell the P Factor theory.
To me, It appears he has a slight mix-up with heli theory similarities, and is trying to use the heli flap back theory and also the RBS theory to add to the Other forces at play on a rigid prop. when I think all he has done is not realized that they are one in the same (Without the mix-up)

I posted this hear. because I know most of you are Reputable. and I have seen a few posters here that (If still Active on the forum) will probably see the same slight Misconception Mr X has Mixed up...


Any takers..

[NickLappos]

The velocity argument holds water, IMHO. If you assume a prop shaft angle of 12 degrees relative to the velocity, that gives an in-plane velocity component of 21% of the free stream (cosine of 12 degrees), a big delta. The down-swinging blade sees +21% and the up-swinging blade sees -21% of the free stream. If one assumes the prop tip speed is 400 knots, and the airplane is flying at 65knots, this makes the down blade see 413 knots and the up blade see 386 knots. The difference in dynamic pressure is 14%, so is the difference in lift.

For those who don't yet see the picture, imagine the prop at an angle of attack of 90 degrees. Then one blade sees all the forward speed and the other subtracts it.

Interesting!

[BGRing]

Wow nick. You did the math on that one. Good....

I would also agree that the advancing blade(Down swinging blade) is a little faster through the Relative air flow.. very small percent, but it is there. (And only at a higher speed).

But the down blade theory is not just for High speed.

Yes there are different stages through out the variables. Speed/AoA Relative AoA and Prop pitch. Plus the fix differences with different craft. Plow wash out, Tail dragger or level carriage, plane-ing angel of aircraft during different weight loads.

But I still think there are flaws and Contradictions in Mr X's Quote.
I'm thinking. No flap back in a Fixed Prop. Totally rigid.


I am also trying to think about the difference in the AoA of the prop from center and at the tip.
Think of how a change in the change in relative airflow (That +- 21% you mentioned)would be more of an Affect on the tip of a Prop where the AoA is made minimal due to its higher air speed (Wash out). making it even More Prominent.


Nick, thanks for your feed back. Look forward to more discussion if there is more...

[brett s]

I can recognize the author pretty easily, given the liberal use of caps & his phrasing - Austin Meyer, creator of the flight sim X-Plane :)

[Dave_Jackson]

For the fun of it, I take the other side and suggest that the velocity argument is a minor one.

IMHO, the Angle of Attack argument may only be relevant to a tail-dragger, since it requires more yaw correction during the initial take-off roll, before the tail is lifted off of the ground.

Climb is instigated by increasing the power from the engine. Initially, this will result in an increased airspeed (and lower AoA). Then the increased airspeed will increase the wing's lift. Eventually, the craft will settle at a slightly faster airspeed and the desired greater rate of climb. The resulting change in the AoA is probably very small.

It appears that primary cause of yaw, due to a change in the power setting, is the result of the upper portion of the prop-wash spiral striking the vertical stabilizer. A strong argument supporting this position is the fact that the vertical stabilizer is not located in the stream-tubes of airplanes with twin engines. Numerous people have commented that on a twin-rotor craft it does not matter if the engine+propellers rotate in the same direction or are counter-rotating.

Other factors are the P-factor and the torque of the engine. Opposing these three is a comment that the aerodynamic forces on a blade want to twist the blade so as to reduce its a AoA (similar to delta3).

From a practical point of view, I found the pedals to be little more than foot-rests, except for; cross-wind landings, playing with stalls, esthetically looking aerobatics and tail-rotor take-offs.


This might be, or perhaps has been, a interesting discussion for the gyrocopter community. This is because the gyro fuselage is very loosely coupled to the lift device (2-blade teetering rotor) and the tail- feathers have very short moment arms.

Just an opinion.

Dave

[BGRing]

So why DOES the plane pull left in a climb even though the descending blade is NOT at a higher angle of attack?
It was just this one part of the quote that has me.
He goes on to explain how a heli has the Higher speed on the advancing side (True) but the thing is a Heli dose also have a lower pitch on that side to bring it into balance.
Now if the heli blade he uses as an Example was Fixed in pitch angle, like a typical Prop (Just imagine for now), Then that advancing blade would have both the Added aircraft speed and The increased AoA on that side (Relative to airflow from transition and beyond), so the statement even though the descending blade is NOT at a higher angle of attack is incorrect.
Am I wrong or just getting confused with is example

PS Good call brett s That is Mr X.:D

David. thanks for you input, I do understand your HO and Value it. thanks again. (Auto gyros do have cyclic eh, thus able to affect blade pitch during the blade rotation)

I will say that. if one understands heli blade dynamics they will most certainly understand Prop Dynamics. that is to say. heli have a lot more to learn eh.
So, Mr X was correct and wise to turn to heli for his example.


David. check out this link for an example of why multy engine don't feel P Factor so much.http://en.wikipedia.org/wiki/P-factor

I will go so far as to say that excessive yaw will cause a double effect (Two engines) on p-factor and sudden yaw will give a Gyro kick. the effects of excessive Pitch and sudden pitch will have a lesser effect on P-Factor and Gyro kick (On same twin engine example).
Thanks again people

[Dave_Jackson]

Thanks for the information about a failed engine on a twin-prop craft.
_________________

No one, including Mr. X, appears to dispute that the descending blade creates a greater aerodynamic force. The discussion appears to be more about the order of importance of the various couplings.

What is difficult to understand is Mr. X's myth.MYTH: AT POSITIVE ANGLE OF ATTACK, THE DESCENDING PROP BLADE IS AT A HIGHER ANGLE OF ATTACK, SO IT PUTS OUT MORE LIFT, PULLING THE PLANE LEFT.

Is he talking about LIFT, and if so, is he referring to a PULLING THE PLANE LEFT by the increased roll inducing counter-torque of the rotating propeller; which will be compensated for by the ailerons.

~or~

Is he talking about THRUST, and if so, is he referring to a PULLING THE PLANE LEFT by yaw; which will be compensated for by the rudder.

:confused::confused:

[BGRing]

No one, including Mr. X, appears to dispute that the descending blade creates a greater aerodynamic force. The discussion appears to be more about the order of importance of the various couplings.
This is true.

But as for your last two Guess's I thought something different.
He is not mentioning any of the other three prop Effects
Gyro forces, Torque, or Spiral wash. and as far as I knew. Torque dose not induce a Yaw.

See. I thought he was talking about something different.
I thought he was not disputing that the down blade dose produce more force.(as you said)
Rather, I though he was disputing the cause of that extra force on the down swinging blade. (Just the P Factor part)
First he acknowledges the prop is at a greater AoA with his "NOT THE WHOLE PICTURE!!!" comment.
Then he dismisses it with the "the descending blade is NOT at a higher angle of attack" comment.

What is greater by the way(In the P Factor part). Is it the AoA difference or the Speed difference ?
My guess would be the AoA.
As for which of the Prop effects cause more yaw. it would vary on the individual plane and the Attitude and Speed and Prop type and....... so on.

What say I put this out there for the Fixed wing Prop guys to have a look at?

[rotorque]

BGRing,

Just for info, torque can induce a yaw during the ground roll.

If I remember correctly it is caused by drag/friction on one of the main wheels being 'loaded' by the torque reaction, usually during takeoff/go around's etc...

[BGRing]

rotorque; Thanks for the info.
I never thought of that.:O
I can see how that could happen.
I also now imagine the amount would be different for different gear and their individual castor and camber etc etc?
I imagine it be even more pronounced if you are using Ski's ?

David; I stand Corrected :)(But happy to have a New piece of info in my noggin)

[Graviman]

Hmmm, propwash vorticity acting on tailplane would outweight any torque generated by the prop since rudder has a much larger moment arm. Don't forget prop angle will be designed to be level at nominal cruise speed, so this "effect" would not occur. Effect could only become apparent at stalling and near Vne when wings operating away from cruise AOA - again unlikely to be the dominant force.

If you have an enjoyable conversation with the pilots that flew classic fighters like the Hurricane, Spit and original Typhoon they will tell you that the biggest thing to learn was control cross coupling. That big gyro on the front meant that pulling back the stick would produce left yaw etc. That is definately where heli & fixed wing dynamics meet.