# Downward air resistance on helicopter

• Gmanme
In summary: For example, if the craft was designed to land on a ship deck, having the rotors on the bottom would make it difficult to do so. There are also stability issues with having rotors on the bottom--if the upward force is applied below the center of gravity, then the system is inherently unstable and requires constant adjustments to be made.
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?

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Gmanme said:
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.

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?

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.

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.

It's more for safety than anything else.

Blades on the bottom are just 100% IMPRACTICAL.

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

FredGarvin said:
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.

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.

DaveC426913 said:
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.

Cyrus said:
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.

DaveC426913 said:
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:

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Cyrus said:
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...

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.

DaveC426913 said:
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 said:
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.

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

FredGarvin said:
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.

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

DaveC426913 said:
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.

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

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

Phrak said:
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.

Cyrus said:
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.

Phrak said:
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.

Cyrus said:
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.

Phrak said:
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'.

Cyrus said:
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?

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

What on Earth are you talking about?

Cyrus said:
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.

DaveC426913 said:
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?

Cyrus said:
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.

Cyrus said:
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.

Gmanme said:
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.

DaveC426913 said:
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.

Gmanme said:
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.

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