Tall Paul

Bernoulli.. the rotor creates lift
Newton. The down going air creates lift
http://rcuvideos.com/item/93H7TBCBZTZ1TS31

da Rock

So the force of the downgoing air on the board can hold the helicopter down? What's that got to do with Bernoulli or Newton? Or lift generated by the rotor?

HighPlains

My Feedback: (1)

looks like Bernoulli theory in tank.

B.L.E.

I doubt that Daniel Benoulli ever made a claim that airplanes fly only because of his principles, which were derived from Newton's laws of motion by the way, because the airplane wasn't invented (sucessfully flown) until 1903, which was 121 years after his death.

Red B.

Here we go again...

FWIW, from a physics point of view the whole Newton vs. Bernoulli debate is rather absurd. There is no conflict between Newtons's laws and Bernoulli's law. Newton's laws explain on a microscopic level how a wing produces lift by deflecting air molecules downward causing a reaction force that we on a macroscopic level observe as lift and drag. What Bernoulli's law does is simply to keep track of the pressure and velocity changes that will occur in the vicinity of the wing as a result of the airstream being deflected.

da Rock

Actually, with the air blowing down on what amounts to a BAFFLE, there isn't much about Bernoulli or Newton demonstrated in that video.

What it proves is that you can keep a helicopter from lifting off by harnessing the downblast with a big sheet of foam board.

The lift that was created either by Bernoulli or Newton was cancelled out by the force on the board. Cut the board down a bit at a time and what will happen? At some point the lift from the blades will be greater than the downforce on the board and the sucker will barely lift. What does that prove? You've gotten just beyond the balance of two forces. One created by the blades and the other by the blast.

rmh

drunk cowboys have tried to lift themselves by their own bootstraps -
I am amazed at the ongoing nitpicking about what creates lift-
I feel that simply knowing that a pressure differential will create lift - is enough for almost any purpose. .
The long winded theories which went on ad nausium here,about air moving downward from the trailing edge etc., always made my eyes glaze over
Mother nature shows many examples of lift at play - and they all demonstrate pressure differences .
Once pressure is equal - nuthin happens- if pressure is unequal, mama nature will try to help rebalance things
On the helicoper demo the board created a closed enviroment -(same as a helicopter in a box with the skids nailed to the floor)
sort of like the plane on the conveyer belt thing -it is no t a lesson in aerodynamics - it is just an example of logical outcome..

Nathan King

AMEN!! Aircraft fly using BOTH principles. I do agree, however, that the video didn't really show either.

Tall Paul

I consider Bernoulli, who explains how the pressure differential is created, to be a subset of Newton, who explains why the pressure does anything.
The "feud" between advocates of only one or the other is just silly!
Looking at the edges of the original square plate, that's a lot of air bending those edges down.
But, it's like blowing the fan into the sail. If the fan is attached to the boat, the boat can't move.
And the heli can't lift.

drela

Amen. Not only is there no conflict between Bernoulli and Newton, Bernoulli's relation IS Newton's Law integrated along a streamline. Physically they are in fact the same thing, and it takes only one straightforward calculus step to go from one to the other. For those that remember Calculus 101:

Integral [ Newton's Law ] ds -> Bernoulli
d[Bernoulli] / ds -> Newton's Law

rmh

Shakspeare said it best -

Tim Green

To say that a discussion is silly, when people are still compelled to write books on the subject - is - well - it's silly. These discussions are the soul of aerodynamics, and just because some of the old dogs on the forum don't like em - that's no reason to quit wondering why, sharing, and discussing.

Red B.

Probably an "old dog" I feel compelled to answer:
Aerodynamics is an engineering subject that is based on physical principles. Aerodynamic research has progressed significantly during the last couple of decades, mostly due to advancements in computational fluid dynamics (CFD), but also due to experimental aerodynamics. None of the findings, neither theoretical nor experimental that have been published in scientific litterature have challenged the basic Newtonian physical principles that are used in modern CFD in the form of the Navier-Stokes equations, or if viscosity effects are ignored, the Euler equations. IMHO, arguing whether Newton or Bernoulli were right is futile simply because Bernoullis law is a concequence of Newtonian physics.

Mr Cox

Well said, I don't understand what the fuss is all about and I certainly don't understand what that video is meant to show....

Paper Cub

This explains all.

http://www.av8n.com/how/htm/airfoils.html

Tall Paul

Does it ever!

CrateCruncher

My Feedback: (1)

Here's my take on the video: The rotor creates thrust downward. The cardboard attached to the skids acts as a "deflector" changing the thrust direction to a radial one. Thus, left cancels right, forward cancels back, etc. I also seem to recall the Harrier and other VTOL aircraft were prone to "recirculation" near the ground. This occurred when the thrust was taken up into the intake in what amounts to a nested loop. With the thrust being directed so close to the blades I'm compelled to think that what little pressure difference is being generated between the top and bottom of the rotor is being undermined by this very same recirculation effect.

BTW, I found this video much more interesting than the airplane on the treadmill which seemed to generate endless discussion. I admire this authors knack for experimentation. (and I also enjoyed that link Paper Cub!)

Crate

Allfat

My Feedback: (1)

It has nothing to do with the recirculation of air back onto the rotors. The problem that a lot of VTOL aircraft had was that the exhaust circulated around and was taken into the engine causing it to choke.

Tall Paul

The plate attached to the landing skids prevents the lift of the rotors from making the heli climb.
The flow at the plate is no different than the flow without the plate, when the plane is on the ground.
It's just action.. the downward moving air's lift force preventing the plane from climbing because the air hits the plate fixed to the machine... reaction.
It's the old bootstrap situation.

BMatthews

Talking to a buddy that was in the full size heli industry for a while I found out that the size of the fuselage does make a difference. They were modifying some sort of Sikorsky for the heli-logging industry by shortening the fuselage by something like 4 to 6 feet. It made a difference to the load capacity being hoisted of a good few hundred more lbs of lift.

da Rock

There is every reason to believe that the drag created by airflow is created whether the airflow is horizontal to the earth or perpendicular. And that drag certainly has a direction. With fixed wings, they mostly see only horizontal because that's what they see. And increase the airflow that creates lift/drag and they see more drag for sure. And we've all gotten used to and understand that drag certainly has a direction. So with a chopper, the airflow that runs straight into the fuselage on it's way down is going to create a force that lines up with the load the chopper is lifting.

Frontal area has always played a part in the speed of fixed wings. I guess "downal area" is what we'd call the helicopter's planform view.

rmh

looks like everyone has ferrerted out the obvious

CrateCruncher

My Feedback: (1)

Then if the plate had three side walls with one side open the helicopter wouldn't be forced (albeit very gently) in the opposite direction? Answer: On a frictionless surface the helicopter would creep in the opposite direction.

Rock,
I agree drag would occur in the vertical direction and could rob thrust but I'd bet the logging mod was to trim tare weight for more payload. Green wood is extremely heavy and non-essential weight in a commercial transport application can make a huge difference in total profit over the useful life of the equipment.

Shoe

This is an interesting illustration. I think you can take it a few steps farther with a simple thought experiment. If you were to keep making the "ground plate" smaller, I think most would agree that you would reach a critical size below which the helicopter would be able to lift off. What if instead, you increased the distance between the rotor disk and the ground plate (with fixed plate size)? I think most would also agree that beyond a certain critical rotor-to-plate separation, the helicopter would also be able to lift off.

For any given rotor-plate separation, can you always make the plate big enough to keep the helicopter on the ground (assuming the plate doesn't get heavier as it gets bigger)? I think most would say "yes you can".

Suppose that there were a scale under the foam plate in the video. Would the scale reading go down as you applied collective? If so, how would the size of the plate effect this? Could you make the plate big enough so that the scale reading wouldn't change at all as you applied collective?

What does this have to do with Bernoulli and Newton? Well, suppose that you COULD make the plate big enough so that the scale reading wouldn't change with collective. And suppose further that no matter how much you separated the rotor from the ground plate, you could always make the the plate big enough to keep the scale from changing with collective. This raises an interesting question about momentum. If the scale reading doesn't change, that suggest that the air exerts no NET vertical force on the helicopter-ground plate system. By Newton's third law, if the air exerts no net force on the helicopter-ground-plate system, then the helicopter-ground-plate system exerts no net force on the air. Newton's second law then leads us to the inescapable conclusion that the helicopter ground plate system can't be changing the NET vertical momentum in the air.

This should be troubling if you think that a helicopter flies by transferring momentum to the air. If the above is true, then if you consider a big enough piece of the earth's surface, the net downward force exerted by the helicopter on that surface is equal to the upward force on the helicopter (at any altitude). If that's the case, then the helicopter can't possibly be changing the momentum in the air. Yikes, that would certainly challenge my notions of how a helicopter flies!

Tim Green

IMHumblerO, the formula for gravitational acceleration doesn't explain what causes gravity, what gravity is. The formula merely allows us to mathematically model gravity. So, do Newton's forumlas, or Bernoulli's, explain lift? Or do they merely allow us to mathematically model certain observed phenomena when air moves over a wing? Without explaining the cause(s) of lift?