PROPWASH THEORETICAL QUESTION

[emessys]

I posted this question on another forum, and havent gotten any replies, but Im guessing some maven here will have the answer to my question.

Assuming a 100% efficient prop with a 10" pitch turning 6300 RPM, the
propwash behind the propeller should theoretically be moving at approx 60MPH (10 X 6300 X 60 /12 X 5280) assuming the prop is held static.

Now, assume the prop is propelling a "dragless aircraft", the prop/plane
should be moving forward at 60MPH.

So, is the propwash behind the propeller now 60MPH or is it 120MPH.

I think still 60MPH.

And if that is the case, assuming a plane that is not dragless......would
the plane move forward at maybe 45MPH and the prop wash relative to the plane
still be 60MPH with a contributing additional factor from the prop (say
15MPH) added to the forward airspeed?

Am I missing something, or am I crazy or stupid?

Ira

[Al Lewis]

First questions:
In that scenario think of the air as a solid and the prop slicing through it. For each rotation of the prop the plane, and prop, move forward through the solid 10". Like a screw, it would have no choice but to move that complete 10" each revolution regardless of the speed at which it's turned. The plane is moving through the solid at 60 mph and the "prop wash" is 0. Kinda like a hot air balloon in a 60 mile per hour wind. Speed is 60, wind in basket is 0. Prop wash, as you are relating to it, will only occur if the plane is restrained in some manner. Then the air off of the prop is moving greater then the airspeed of the plane, the difference being prop wash.

Last question: Yes to all three

LOL Just kidding.

[emessys]

	Quote: Originally Posted by Al Lewis
	First question: In that scenario think of the air as a solid and the prop slicing through it. For each rotation of the prop the plane, and prop, move forward through the solid 10". Like a screw, it would have no choice but to move that complete 10" each revolution regardless of the speed at which it's turned. Second question: Yes to all three LOL Just kidding.

Yes but you didnt answer the first question. I think that the air will move past the fuse at RPM X PITCH whether on the ground or in the air (not counting inefficiency, prop unloading etc).

The second question was obviously a setup. By the way, wasnt the OTHER Al Lewis also a comedian.

I think I need Robert Vess to spot this thread.

[Al Lewis]

In a drag-less plane with a 100% efficient prop the air doesn't move past the plane. The plane moves through the air. If the plane is held static then the air moved by the prop, disregarding friction, would move at 60 mph.

Yes, and a good one too!

This may explain it clearer for you.

http://home.wanadoo.nl/pereivers/prop-power.xls

[brycesteenburg]

Here ya go. Can the prop you stated, still being 100% efficient at the rpm you stated, able to displace a medium at 60mph, mounted to a test stand with 100% drag be moved?
Even though the medium is being displaced at a certain level, it still can't overcome the drag holding it back. You still can't move your stand because of the high drag. Your prop now becomes only efficient enough to move an object at 45mph in that medium with the drag excerted against it through that medium. 75% efficiency. If I remember correctly. A lot more to this than meets the eye but this should give you the idea.

Bryce

[Al Lewis]

Also remember that air is a compressible medium, unlike water, which adds another variable to the equation. The air will first compress behind the blade and then release that force. One of the many things that makes props not 100% efficient.

[sprice50]

60mph.

[exeter_acres]

depends on who makes the treadmill.........

[knighttwister]

	Quote: Originally Posted by Al Lewis
	First questions: In that scenario think of the air as a solid and the prop slicing through it. For each rotation of the prop the plane, and prop, move forward through the solid 10". Like a screw, it would have no choice but to move that complete 10" each revolution regardless of the speed at which it's turned. The plane is moving through the solid at 60 mph and the "prop wash" is 0. Kinda like a hot air balloon in a 60 mile per hour wind. Speed is 60, wind in basket is 0. Prop wash, as you are relating to it, will only occur if the plane is restrained in some manner. Then the air off of the prop is moving greater then the airspeed of the plane, the difference being prop wash. Last question: Yes to all three LOL Just kidding.

yes, air density is the key factor here. also the gasses that make up the air. if the air is volatile enough, you may end up igniting it through compression and lose the plane in a fireball! theoretically that is.

[TheTank]

Physics is our friend!

Al is correct. Think of the air as medium through which the propeller is moving through (in a 100% efficient environment). The propellor is not pushing anything, it is screwing itself through the air in this efficient, dragless environment.

Al, the hot air balloon example was great as well. A lot of people really have a hard time grasping that concept, a la the treadmill controversy.

[emessys]

Somehow I knew the treadmill was going to come up when I first posted this.

So to sum up, if I am a wingwalker on a biplane with a 100% efficient prop and a dragless airframe on a day that the air doesnt compress...as I walk from the wingtip to the center of the fuse behind the prop, I will feel no change in airflow.

And actually, the original question had a purpose, which had to do with fuel siphoning from the carb throat on a Yellow P47. But that is a whole other story.

Thanks for confirming my assumptions.

Ira

[Al Lewis]

Only if the wingwalker was completely dragless. Normally a wind equal to the air speed of the plane would push against them.

[TheTank]

	Quote: Originally Posted by emessys
	So to sum up, if I am a wingwalker on a biplane with a 100% efficient prop and a dragless airframe on a day that the air doesnt compress...as I walk from the wingtip to the center of the fuse behind the prop, I will feel no change in airflow. Ira

Correct, with 100 percent efficiency, there would be no difference. You could almost test this in flowing water, with a biplane, where the propellor would be windmilling in the flow stream, and you measured the flow at the wing tips, as well as the center of the wing... Only difference would be that you would see the effect of turbulance off the prop in the water test as their would be no way to eliminate the drag.

[emessys]

	Quote: Originally Posted by Al Lewis
	Only if the wingwalker was completely dragless. Normally a wind equal to the air speed of the plane would push against them.

No, I meant that the airspeed wind would be the same at the wingtip as behind the prop. Not that he would feel no wind.

[TheTank]

	Quote: Originally Posted by emessys
	And actually, the original question had a purpose, which had to do with fuel siphoning from the carb throat on a Yellow P47. But that is a whole other story. Ira

I guess it could happen... but, the lower pressure area created outside the carb would have to overcome the low pressure area of that inside the carb, created in the crankcase from piston movement. That said, you could get a little spray being blown out at low RPMs right before the reeds fully close as the piston travels down.