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Author Topic: Is a chopper different from a VTOL?  (Read 10683 times)

Offline Geezer

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Is a chopper different from a VTOL?
« on: 27/01/2010 01:27:33 »
Vertical takeoff and landing aircraft achieve lift by accelerating a mass of air towards the Earth. Now, because F=ma yada yada yada, the aircraft is able to lift off.

Helicopters, on the other hand, don't appear to accelerate a large mass of air towards the Earth, or do they? When you boil it all down, are there really two different effects, or only one?


 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #1 on: 27/01/2010 12:34:29 »
Ha! What a good question.

I suppose helicopters are akin to fixed wing aircraft in that the engine power is used in some way to pass an aerofoil through air to create low pressure above and high pressure below to create lift.

I would suppose that at the level of the air molecules it is similar in some respects. The wing/aerofoils are large areas so there are many molecules being deflected at lowish downward velocities where a VTOL jet would be fewer but at very high velocities. The view of looking at the lift being due to pressure being just a better way to calculate the effects of the complex interactions with the surrounding air.

However, there may be an additional component (I think) from the air's behaviour as a fluid. The pressure changes around a wing are the effect of fluid resisting the disturbance. Maybe the wings give you more lift than simple conservation of momentum (as with a downward jet). Certainly you can gain from ground effect. Someone must know!

Wings are real crap in a vacuum though :-) A bet some pedant will say so is a jet, so I should say rocket instead, but you know what I mean.
 

Offline Bored chemist

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Is a chopper different from a VTOL?
« Reply #2 on: 27/01/2010 19:01:56 »
"Helicopters, on the other hand, don't appear to accelerate a large mass of air towards the Earth, or do they? "
I always assumed that was exactly what the big fan on the roof was for.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #3 on: 27/01/2010 19:40:01 »
"Helicopters, on the other hand, don't appear to accelerate a large mass of air towards the Earth, or do they? "
I always assumed that was exactly what the big fan on the roof was for.

LOL. Spoken like a true chemist!

See, that's the question BC. The "big fan" is really a rotating wing, but does that really make any difference to the fizics?
 

Offline Bored chemist

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Is a chopper different from a VTOL?
« Reply #4 on: 27/01/2010 22:13:12 »
The usual explanation of how wings work ( Bernoulli's equation- curved upper surface .. yadda yadda) doesn't work, since you can fly planes upside down until the pilot gets a nosebleed.
I have yet to see any "wing" that isn't used with a significant angle of attack. I still think it's a big fan on the roof.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #5 on: 28/01/2010 01:20:03 »
The usual explanation of how wings work ( Bernoulli's equation- curved upper surface .. yadda yadda) doesn't work, since you can fly planes upside down until the pilot gets a nosebleed.
I have yet to see any "wing" that isn't used with a significant angle of attack. I still think it's a big fan on the roof.

That was my initial thought too (that it's simply a question of accelerated air mass) but, as I really don't lnow much about it, I thought it safer to ask. If it's true for helicopters, then the same is likely true for fixed wing aircraft - which seems to be the point you are making.
 

Offline Madidus_Scientia

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Is a chopper different from a VTOL?
« Reply #6 on: 29/01/2010 07:57:12 »
If you ever see a heavy helicopter hovering low over water you'll see that they do indeed accelerate a significant mass of air downward
 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #7 on: 29/01/2010 10:29:30 »
How about this reasoning that would suggest that you gain a lot from use of a wing in a fluid?

A glider towed to 1000 feet and released has a certain potential energy with which it can use to descend over many minutes, with no aid from thermals or unusual updrafts. A jet engine powered VTOL vehicle, even of similar weight, would certainly have to use considerable energy to maintain such a slow descent. At least there can be no argument it would have to use some extra energy whereas the glider uses none.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #8 on: 29/01/2010 23:16:06 »
I'm beginning to wish I had not asked the question  ;D

There must be a vertical component that balances gravitational force (assuming gravity produces a force, but I'd rather not get into that here).

Is there another mechanism to support the glider other than accelerated air mass? What happens to a wing in, say, water? I think I'm going to lie down now.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #9 on: 30/01/2010 07:51:53 »
The VTOL can only climb because of the air mass it accelerates.

A wing moving in air creates a pressure difference between the top and bottom of the wing so that a force is applied to the underside of the wing. But I would think that means the air on the underside of the wing must be compressed, which means a mass of air actually is being accelerated. An aerofoil might just be an efficient way of accelerating the air mass.

I going to have to lie down again.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #10 on: 30/01/2010 18:18:30 »
DD: When I said VTOL I was thinking of the Hawker Harrier that uses vectored thrust from a jet engine. The Osprey(?) is a variation on the helicopter although it is a VTOL. I should have been more specific.
 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #11 on: 30/01/2010 18:39:44 »
You're going to need another lie down now, Geezer...

What about turbine style thrusters where the driving is a rotating turbine inside a cylinder. The only output is really a mass of air at velocity, so akin to a jet or rocket, even though driven by a rotating set of blades. Is this in some way different from a helicopter?

Actually, I think I need a lie down too.
 

Offline LeeE

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Is a chopper different from a VTOL?
« Reply #12 on: 30/01/2010 22:40:24 »
The usual explanation of how wings work ( Bernoulli's equation- curved upper surface .. yadda yadda) doesn't work, since you can fly planes upside down until the pilot gets a nosebleed.
I have yet to see any "wing" that isn't used with a significant angle of attack. I still think it's a big fan on the roof.

You're right; it's possible to fly a plank of wood, at a suitable AoA.  The Bernoulli principle just makes wings much more efficient than planks.  Very early wings, before the Wright Brothers, were generally just flat surfaces but because they were usually cloth covered to save weight they would naturally acquire a suitable curve.  Infact, very early on in the history of powered flight the term 'planes' referred just to the wings and empennage and did not refer to the entire aircraft, as it does now.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #13 on: 30/01/2010 23:24:09 »
At the risk of answering my own question, it's beginning to sound like there really is bu**er all difference between a helicopter and, say, a Harrier. They both have to accelerate a sufficient mass of air straight down to maintain their elevation with no forward velocity. So, in a very gross sense, as far as Physics is concerned, they really are no different. Or did I miss something?
 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #14 on: 31/01/2010 00:13:03 »
Well explain my first point regarding a glider. I can't decide.
 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #15 on: 31/01/2010 10:18:24 »
I think it is certain that for hovering above the ground (high enough to avoid ground effect) there is no essential difference between a helicopter (or other rotating blade driven device) and a jet powered VTOL in how they achieve this. There are no other dynamics involved other than pushing large quantities of gas downwards (hmm, reminds me I should not have had those beans with the beer last night).

The interesting question that arises is what makes planes (however powered) and helicopters more efficient when they are moving forward. Propelling a plane forward uses a lot less energy than trying to maintain a hover, but does it still stay in the sky by accelerating a mass of air downwards?

And I've had a lie down now too.
 

Offline LeeE

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Is a chopper different from a VTOL?
« Reply #16 on: 31/01/2010 13:36:40 »
Helicopter rotor blades operate in the same way as wings i.e. by creating a low pressure region above the wing.  In fact, heli rotor blades are extremely high-aspect ratio wings, with an aspect ratio far higher even than gliders.  While the heli is in ground effect the downwash will increase the pressure of the region of air below the rotor and make it more effective, but as I understand it, most of the lift still comes from the production of the low pressure region and not directly from thrust due to downwash.  Were it to be so, then standing beneath the rotor blades while they're spinning fast enough to maintain lift would crush you, yet it's relatively common for military folk to disembark from a heli while it's still hovering; they don't get crushed.
 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #17 on: 31/01/2010 13:55:30 »
You are right that the total area swept by a wing (per second) is higher, but the total mass/second you have to accelerate downward (to some velocity) to maintain altitude, if that is the only mechanism, is still high. Why does it take less power to maintain altitude when moving forward unless there is another mechanism, other than conservation of momentum, coming into play?

To put it another way, if wings were simply air-diverters that turned the oncoming air stream at right angles (downwards) the force of the air you would have to drive into would be the same as the gravity you are trying to overcome. It would use as much energy as a VTOL arrangement.

At the end of the day it has to be down to conservation of momentum because the only medium for lift are the bouncing around of air molecules, but I am having difficulty understanding the detail.

Another lie down I think.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #18 on: 01/02/2010 06:18:54 »
Yes, but :D an aircraft must generate sufficient lift (vertical force up) to counteract gravity, and the only means at its disposal is the acceleration of air mass. Conservation of momentum does not help. Rip the wings off a plane; it makes no difference whether it's travelling at 300 MPH or 0 MPH, it will still "land" at the same time, just in different places.

I think the subtlety may lie in the way the wing compresses the air underneath it. At 300 mph, a wing does displace a huge volume of air vertically in time. The air must be compressed because it can't immediately vacate the space that the wing wants to occupy, and compression is just a form of acceleration. The aerofoil section of the wing reduces the compression effect on the top side of the wing, (presumably because it accelerates the air less) so the net effect is a force to counteract gravity.

I suppose we could do some research to find out how it really works, but what would be the fun in that  ;D
 

Offline LeeE

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Is a chopper different from a VTOL?
« Reply #19 on: 01/02/2010 20:43:06 »
Wings do not support an aircraft primarily by compressing the air beneath it.  Were it to be so then the pressure exerted beneath the aircraft would be equal to its weight and so any one disembarking from a hovering helicopter would be crushed: as I mentioned earlier, this does not happen.  It doesn't seem to be bother the chaps hanging onto the early Bell heli in the pic I've attached at the bottom either (is it possible to inline attachments or can the only appear at the end of a posting - I can't see any suitable tags in editing mode?).


Then there's this pic too:
http://www.earlyaeroplanes.com/archive/image3/pup_3.jpg

Neither does a lot of air need to be displaced by moving through it at relatively high speeds; have a watch of this video...

http://www.youtube.com/watch?v=WkkvpVPwvRw

...the aircraft is clearly flying and thus being supported in the air, but at the very low speed it's flying at the downwash will be negligable


Furthermore, the downwash, where air is deflected downwards by the wing, occurs behind the wing and not directly beneath it: if you think of a large delta-winged aircraft, such as the Vulcan bomber, you can see how this isn't going to work as all of the lift would be acting right at the rear of the aircraft and not near its CoG.
« Last Edit: 01/02/2010 20:45:11 by LeeE »
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #20 on: 02/02/2010 17:30:24 »
Wings do not support an aircraft primarily by compressing the air beneath it.  Were it to be so then the pressure exerted beneath the aircraft would be equal to its weight and so any one disembarking from a hovering helicopter would be crushed: as

OK. So, if there is no force acting on the lower surface of the wings, why does a plane fly?  I always thought the wings were more than just decorative ;D
 

Offline graham.d

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Is a chopper different from a VTOL?
« Reply #21 on: 02/02/2010 17:46:53 »
Wings produce most lift from the low pressure above rather than high pressure below. I expect that is what Lee is referring to.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #22 on: 02/02/2010 19:28:48 »
I agree. But even if there was a total vacuum above the wing, the wing will only lift by virtue of the air pressure on the bottom side of the wing, and if there is pressure exerted by air on the bottom side of the wing, that air is being compressed.

 

Offline LeeE

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Is a chopper different from a VTOL?
« Reply #23 on: 02/02/2010 22:35:26 »
Geezer; you seem to be thinking that if you reduce the pressure on one side of something it automatically results in an increase of pressure on the other side; this is not the case.

Consider a sealed box incorporating an air-tight but freely moving wall that divides the box into two compartments.  Start with both compartments at the same air pressure and then start evacuating the air from just one of the compartments, let's say the left one, lowering its pressure, whilst the other compartment on the right remains totally sealed.  As you start to remove the air from the left compartment, the air-tight wall will move towards left, pushed by the expanding air in the sealed right hand compartment.  However, far from being compressed above its original starting pressure, the air in the sealed right hand compartment will have dropped in pressure.

An even simpler example is drinking something through a straw; when you suck on the end of the straw to draw the drink up it, you are not increasing the pressure on the surrounding air to force the drink up the straw.
 

Offline Geezer

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Is a chopper different from a VTOL?
« Reply #24 on: 02/02/2010 22:58:25 »
Lee: I understand your analogy, but I'm not sure it is good anaolgy for a wing.

By point is that the top of the wing is not "sucked", so the bottom of the wing must be "blown". Therefore, the air under the wing is supporting the weight of the aircraft as opposed to the absence of air above the wing. And since the wing moves rapidly into a volume that was previously occupied by air, the air has to be compressed in the process of getting out of the way of the wing.
 

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Is a chopper different from a VTOL?
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