Downward air resistance on helicopter

[DaveC426913]

I have no idea what you mean by all this.

I see what he's saying.

Right. The air entering the blades draws on a larger area. Therefore a slower velocity. Therefore less drag on an element above than below the rotor. The airststream below the rotor tends to draw inward, to something less than the diameter of the rotors--the opposite of the airstream above. Air drag tends to run proportional to velocity squared.

Pardon my primitive lingo:

The funnel of air entering a fan is larger (greater in cross-section) than the funnel of air (in cross section) exiting the fan.

So if I hold up an 8x10 piece of paper one foot behind a fan it will slightly reduce flow, but if I hold up an 8x10 piece of paper one foot in front of a fan, it will greatly affect its performance.

Same with a heli's rotor. Putting the fuselage in the (small, fast) exit airstream causes a bigger performance hit than putting the fusleage in the (large, slow) intake stream.


And it's directly applicable to the OP's question.

 

[Gmanme]

I would venture a guess of probably 80-90%.

Well, since in most helicopters there is no fuselage under the outer half of the blades, then there should be no resistance on the 80-90% of downward air thrust. Right?

I have also noticed that some blades don't even flatten out into a fan for the first few feet from the center, mybe to avoid pushing air onto the aircraft.

 

[DaveC426913]

I have also noticed that some blades don't even flatten out into a fan for the first few feet from the center, mybe to avoid pushing air onto the aircraft.

Or perhaps because they are moving so slow at that at point as to provide no appeciable lift.

 

[Cyrus]

The funnel of air entering a fan is larger (greater in cross-section) than the funnel of air (in cross section) exiting the fan.

If you look at figure 2.19, you will see that the change in wake contraction above and below the rotor over the distances we're talking about here don't have anything to do with the topic at hand. Sure, the air entering the fan is larger, *if you go really far away from the rotor. Is the fuselauge *really far away from the rotor? -no.

So if I hold up an 8x10 piece of paper one foot behind a fan it will slightly reduce flow, but if I hold up an 8x10 piece of paper one foot in front of a fan, it will greatly affect its performance.

I think you said this backwards.

Same with a heli's rotor. Putting the fuselage in the (small, fast) exit airstream causes a bigger performance hit than putting the fusleage in the (large, slow) intake stream.

Again, what do you mean by 'small' - it isn't small. In fact, its very nearly the same as just above the rotor disk plane. What we care about here is the blockage of air, but mostly the imparted momentum of the air from the blades hitting the fuselauge.

 

[Cyrus]

Well, since in most helicopters there is no fuselage under the outer half of the blades, then there should be no resistance on the 80-90% of downward air thrust. Right?

I have also noticed that some blades don't even flatten out into a fan for the first few feet from the center, mybe to avoid pushing air onto the aircraft.

"Fan"? Do you mean a prop? Turbo-Fan (type of jet engine)? There is variable twist in the geometery to account for the variation of angle of attack of the blades at each station as the tip speeds increase from the root to the tip.

 

[Cyrus]

Or perhaps because they are moving so slow at that at point as to provide no appeciable lift.

This is correct - hence why helicopters have root cut outs.

 

[DaveC426913]

If you look at figure 2.19, you will see that the change in wake contraction above and below the rotor over the distances we're talking about here don't have anything to do with the topic at hand. Sure, the air entering the fan is larger, *if you go really far away from the rotor. Is the fuselauge *really far away from the rotor? -no.

Where will I find this figure "2.19"?

I think you said this backwards.

No. I was talking about a household fan, where airflow is in at the back and out at the front. Sorry, I did not spot the ambiguity.

Again, what do you mean by 'small' - it isn't small. In fact, its very nearly the same as just above the rotor disk plane. What we care about here is the blockage of air, but mostly the imparted momentum of the air from the blades hitting the fuselauge.

OK, small-er.

I'm not saying it's true, I'm just clarifying Phrak. He's simply saying the intake area is larger than the output area, thus output area will be proportianally more affected by blockage from an object of fixed size.

 

[Gmanme]

"Fan"? Do you mean a prop? Turbo-Fan (type of jet engine)? There is variable twist in the geometery to account for the variation of angle of attack of the blades at each station as the tip speeds increase from the root to the tip.

I mean when the prop changes from the round bar at its root, to a flatened blade for pushing air at its ends.

My point was if the inner half of the blade only makes 10-20% of the thrust then the amount of air resistance on the fuselage is very little.
And if the inner 1/4th of the blade isn't shaped to push any air then there would be no air produced down on the fuselage.
There would still be a small amount of resistance from the helicopter tail since its outside the inner part of the blade.

To try and simplify what Phrak said, Air can be sucked into a fan from many directions on one side, but is only pushed out of a fan in one direction (downward) on the other side. This makes the area it can pull air from larger than the area it can push air to.

 

[Cyrus]

I mean when the prop changes from the round bar at its root, to a flatened blade for pushing air at its ends.

A propeller isn't a round bar anywhere. I don't know what you're talking about, and I am starting to think you don't either.

http://www.spotmatic.org/images/airplane_propeller.jpg​

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I don't see "round bars" anywhere.

My point was if the inner half of the blade only makes 10-20% of the thrust then the amount of air resistance on the fuselage is very little.

And it is. It increase the power in hover by 5-7%, i.e. "very little" - but not none.

And if the inner 1/4th of the blade isn't shaped to push any air then there would be no air produced down on the fuselage.

What are you basing this on? Air swrills on the way down. It isn't a perfect uniform column of streamlines going literally straight down from where it was entrained into the blades. And to be clear, what you have stated is wrong. The blades "ARE" shaped to push air EVERYWHERE.
Because of the low dynamic pressure near the hub, it simply *cant* push the air (and create lift) as well as it can at the tips (which have high dynamic pressures).

There would still be a small amount of resistance from the helicopter tail since its outside the inner part of the blade.

Please do not describe things as the "outside part of the inner part of the blade" I have no idea what this ambiguity means.

To try and simplify what Phrak said, Air can be sucked into a fan from many directions on one side, but is only pushed out of a fan in one direction (downward) on the other side. This makes the area it can pull air from larger than the area it can push air to.

I would generally agree with that statement.

 

[FredGarvin]

A propeller isn't a round bar anywhere. I don't know what you're talking about, and I am starting to think you don't either.

I don't see "round bars" anywhere.

I believe this is what they are referring to (which happens to be about 24" IIRC from the blade retaining pin to the initiation of the airfoil):

 

[Cyrus]

I believe this is what they are referring to (which happens to be about 24" IIRC from the blade retaining pin to the initiation of the airfoil):

I should have figured as much. In which case the "bar" is called a spar.

 

[DaveC426913]

A propeller isn't a round bar anywhere.

On some props, the base of the blade has so broad a camber as to appear almost circular in cross-section.

http://i.pbase.com/v3/69/534869/2/45881749.gosh4.jpg
http://upload.wikimedia.org/wikipedia/commons/f/ff/F4U-4_Bu97388_front_view.jpg

 

[Cyrus]

On some props, the base of the blade has so broad a camber as to appear almost circular in cross-section.

I'd like to see a picture of that if you could find one.

 

[Cyrus]

On some props, the base of the blade has so broad a camber as to appear almost circular in cross-section.

Can you please resize your image dave, it is absurdly large. I would like to point out that for the prop you show (on an F4U corsair), the part near the hub isn't aerodynamic. The airfoil sections stop about 4" out. The "round" part you're seeing is actually that - a long round tubular section for the specific purpose of bolting a round shaft into a round mounting hole on the hub connected to the piston engine. That's all it is. It was probably easier to just manufacture it that way. It serves no purpose aerodynamically and it isn't 'highly cambered'.

 

[DaveC426913]

Can you please resize your image dave, it is absurdly large. I would like to point out that for the prop you show (on an F4U corsair), the part near the hub isn't aerodynamic. The airfoil sections stop about 4" out. The "round" part you're seeing is actually that - a long round tubular section for the specific purpose of bolting a round shaft into a round mounting hole on the hub connected to the piston engine. That's all it is. It was probably easier to just manufacture it that way. It serves no purpose aerodynamically and it isn't 'highly cambered'.

Thank you to whomever reset my images. I can't control their size, but links are good.


I'm merely defending Gmanme's comment:

... when the prop changes from the round bar at its root, to a flatened blade for pushing air at its ends...

You accused him of not knowing what he's talking about. Considering I've just demonstrated exactly what he described (excepting that it's a prop, not a rotor), I think you owe it to him to retract that.


(The fact that I messed it up by implying it's cambered - is my fault, not his.)

 

[Cyrus]

Thank you to whomever reset my images. I can't control their size, but links are good.


I'm merely defending Gmanme's comment:

You accused him of not knowing what he's talking about. Considering I've just demonstrated exactly what he described (excepting that it's a prop, not a rotor), I think you owe it to him to retract that.


(The fact that I messed it up by implying it's cambered - is my fault, not his.)

Ok, fair enough.