Downward air resistance on helicopter

[Gmanme]

Hello,

Why are helicopter blades put on top of the aircraft? Doesnt the air pushed down from the blades push down on the helicopter? Wouldnt it make more sense to have the blades on the bottom?

 

[DaveC426913]

Hello,

Why are helicopter blades put on top of the aircraft? Doesnt the air pushed down from the blades push down on the helicopter? Wouldnt it make more sense to have the blades on the bottom?

Bottom or top, the body of the craft is still in the way.

Think about a fan. Cover the front of it, you stop the flow. Cover the back of it ... and you stop the flow.

 

[Gmanme]

So the air above of the aircraft is sucked downward into the blades, and around the helicopter as resistance, and is equal to the force of the air being pushed downward below the blades?

 

[nucleus]

Here is a picture of a helicopter:
http://www.1000pictures.com/view.htm?caircraft/helos+f050823-n-0733p-005_1024x768.jpg+x1024+y768
Yu will notice that it has skids on the bottom the aircraft. It could also have wheels or floats, but it doesn’t matter because their purpose is to allow the aircraft to land on ship decks, runways and other surfaces.

Don’t you think it would be hard to land when you have blades whirling around below you?

 

[minger]

There is also the stability factor. If the upward force is applied below the center of gravity, then the system is inherently unstable and requires constant adjustments to be made. However, if the force is above, then any disruptions in body roll will be self-adjusted.

 

[DaveC426913]

There are certainly many other reasons why one might not want whirling blades of death on the underside of a vehicle, but in the spirit of the OP's question, he's interested primarily in the efficiency aspect.

Consider: if he were right, and it were more efficient that way, it would be worth trying to solve these other problems for the sake of efficiency. I'm trying to show him that, in principle, there's no reason to put them on the bottom.

So the air above of the aircraft is sucked downward into the blades, and around the helicopter as resistance, and is equal to the force of the air being pushed downward below the blades?

Yes.

 

[Cyrus]

It's more for safety than anything else.

Blades on the bottom are just 100% IMPRACTICAL.

 

[FredGarvin]

I'd like to know if I had to go in hard, momentum wouldn't be making me go into the rotor disc.

 

[Cyrus]

I'd like to know if I had to go in hard, momentum wouldn't be making me go into the rotor disc.

Not only that, rotors on the bottom are just FUNDAMENTALLY wrong on almost every level.

Q: What is the POINT of a helicopter?

A: To do what an airplane cannot do.

Are you going to rescue someone by winch by lowering a basket *through* a rotor disk?

How will you get *into or *out of the helicopter? Wait for a three minutes until the blades stop spinning?

How will you put wheels or landing skids on it?

The list goes on.....




...and on...






...and on.

 

[DaveC426913]

Guys, there are lots of solutions to rotors on aircraft, some have them on top, some have them in the fuselage, some have them on the wings, or even in the wings.

If one wanted to design a better rotor craft, one would look at each of these challenges you mention (rotors being in in the way) and determine whether they were something that could be worked around or not with different configurations.


The OP is proposing a modification that when isolated as a design factor i.e. all other factors being equal or ignored he thinks should make a more efficient craft.

I'm trying to show that in principle this not going to achieve the desired improvement.

 

[Cyrus]

Guys, there are lots of solutions to rotors on aircraft, some have them on top, some have them in the fuselage, some have them on the wings, or even in the wings.

If one wanted to design a better rotor craft, one would look at each of these challenges you mention (rotors being in in the way) and determine whether they were something that could be worked around or not with different configurations.


The OP is proposing a modification that when isolated as a design factor i.e. all other factors being equal or ignored he thinks should make a more efficient craft.

I'm trying to show that in principle this not going to achieve the desired improvement.

We'll, we are answering his question. In principle, it would be better to have to rotors on the bottom. This is exactly why the Chinook has the front rotor at a lower elevation that the aft rotor. This way, the air entering the rear rotor is realtively unaffected by the downwash of the front rotor.

Also, downwash of the rotor on the fuselauge increases the power requirements by approx. 5-7%. Putting the rotors below the fuselauge would theoretically make things better.

But then you would have a useless helicopter with no utility. And a helicopter is a utility vehicle. Not a point A to point B vehicle. That's the job of an airplane.

 

[DaveC426913]

Also, downwash of the rotor on the fuselauge increases the power requirements by approx. 5-7%. Putting the rotors below the fuselauge would theoretically make things better.

Really? Oh. Then I am wrong, and the OP is correct in his assertion.

 

[Cyrus]

Really? Oh. Then I am wrong, and the OP is correct in his assertion.

It makes sense when you think about it. The rotor is entraining air from above the rotor and imparting momentum on the air down below it. When its being accelerated downwards, it literally hits the fuselauge, creating a downforce on it. This is exactly the design tradeoff you pay when using a compound helicopter (one with wings on it to increase lift). If the rotor is at the bottom of the helicopter, there is nothing for the air to "run into".

But then again, such a helicopter is purely an academic exercise and of no value.

Example of a compound helicopter:

 

[DaveC426913]

It makes sense when you think about it. The rotor is entraining air from above the rotor and imparting momentum on the air down below it. When its being accelerated downwards, it literally hits the fuselauge, creating a downforce on it. This is exactly the design tradeoff you pay when using a compound helicopter (one with wings on it to increase lift). If the rotor is at the bottom of the helicopter, there is nothing for the air to "run into".

Right, but that should have the same effect if the fuselage and winglets are above the rotor; it just reduces the inflow rather than the outflow.


No, I guess not... It's much harder on a vacuum or fan's efficiency if you block the outflow rather than blocking the inflow...

 

[FredGarvin]

You would, undoubtedly, have to increase the rotor disc size because you would block a larger area towards the center of the hub. That in itself would be a design killer since tip speed for rotors dominates a design.

 

[Cyrus]

Right, but that should have the same effect if the fuselage and winglets are above the rotor; it just reduces the inflow rather than the outflow.


No, I guess not... It's much harder on a vacuum or fan's efficiency if you block the outflow rather than blocking the inflow...

You would, undoubtedly, have to increase the rotor disc size because you would block a larger area towards the center of the hub. That in itself would be a design killer since tip speed for rotors dominates a design.

Those are both true. The question is which would increase power more. Since no one has ever made a rotor below the fuselauge, my opinion* is that it would be slightly better for the same reason you put a pusher prop behind an airplane to avoid downwash onto the fuselauge.

 

[Gmanme]

So from what I gather it might be slightly more efficient?

 

[Cyrus]

You would, undoubtedly, have to increase the rotor disc size because you would block a larger area towards the center of the hub. That in itself would be a design killer since tip speed for rotors dominates a design.

But who cares, the center of the hub contributes very little to the overall lift! You care about the outter 1/3 of the blade.

To the OP: I'm going to send an email posing this question to my professor of helicopter aerodynamics. He is a leading world expert.

 

[Phrak]

The outer half of the blade produces _____% of the lift. (?)

 

[Phrak]

Right, but that should have the same effect if the fuselage and winglets are above the rotor; it just reduces the inflow rather than the outflow.


No, I guess not... It's much harder on a vacuum or fan's efficiency if you block the outflow rather than blocking the inflow...

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.

 

[Cyrus]

The outer half of the blade produces _____% of the lift. (?)

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

 

[Cyrus]

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.

I have no idea what you mean by all this.

 

[Phrak]

I have no idea what you mean by all this.

Which part don't you understand? The area of the airsteam normal to the airsteam is larger above the rotor than below.

 

[Cyrus]

Which part don't you understand? The area of the airsteam normal to the airsteam is larger above the rotor than below.

That's true. How does this answer the OP though?

PS: Using words like 'area of the airstream normal to the airstream' makes no sense. How can something be normal to itself? Try to be more precise in your language when describing things.

 

[Phrak]

That's true. How does this answer the OP though?

The issue of drag came up. You just like to give me a hard time.

PS: Using words like 'area of the airstream normal to the airstream' makes no sense. How can something be normal to itself? Try to be more precise in your language when describing things.

Fine. The area normals that are parallel to the airsteam.

 

[Cyrus]

The issue of drag came up. You just like to give me a hard time.

I honestly have no idea on what you posted has to do with drag.

Fine. The area normals that are parallel to the airsteam.

This is no better. Simply stated, the rotor wake expands and contracts in the far upstream and fully developed slipstream of 'far wake'.

 

[Phrak]

I honestly have no idea on what you posted has to do with drag.



This is no better. Simply stated, the rotor wake expands and contracts in the far upstream and fully developed slipstream of 'far wake'.

None of this has anything to do with airsteams, now does it?

 

[Cyrus]

None of this has anything to do with airsteams, now does it?

What on Earth are you talking about?

 

[DaveC426913]

What on Earth are you talking about?

He may be suggesting that you're bifurcating bunnies, not to get clarity, but to give him a hard time.

 

[Cyrus]

He may be suggesting that you're bifurcating bunnies, not to get clarity, but to give him a hard time.

I told him I don't understand what he's saying, and instead of clearing up what he's talking about he's claiming I am giving him a hard time. To reiterate: how does what he posted have to do with rotor induced drag?

 

[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.