- #36
Cyrus
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DaveC426913 said: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 said:Or perhaps because they are moving so slow at that at point as to provide no appeciable lift.
Where will I find this figure "2.19"?Cyrus said: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.
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.Cyrus said:I think you said this backwards.
OK, small-er.Cyrus said: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 said:"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.
Gmanme said: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.
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 said: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.
FredGarvin said: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 said:A propeller isn't a round bar anywhere.
DaveC426913 said:On some props, the base of the blade has so broad a camber as to appear almost circular in cross-section.
DaveC426913 said:On some props, the base of the blade has so broad a camber as to appear almost circular in cross-section.
Thank you to whomever reset my images. I can't control their size, but links are good.Cyrus said: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'.
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.... when the prop changes from the round bar at its root, to a flatened blade for pushing air at its ends...
DaveC426913 said: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.)