Nathan King

For those of you that don't fly full scale: Get an instructor to take you up on an introductory flight. You'll really feel the similarities between water and air.

Tim Green

Kind of leaving out the details, aren't you? There are always pressure differences when fluids are in motion relative to each other, or to a fixed object. Whether lift is occurring, or not.

rmh

I was simplifying - not trying to obfuscate.
Note I did not say "All pressure differences cause lift "-I simply noted " LIFT is the result of pressure difference."

Tim Green

Really - perhaps pressure differences are the by-product of lift? And the lift is the direct result of the wing's reaction to the air being thrown downward by the wing?

da Rock

Dick,
I'd say you did simplify perfectly.

Couldn't have put it more simply myself, and lord knows I'm simpler than most, so ought to be able to do that.

Sport_Pilot

Yes that is true. Newton said that for every action there is an equal reaction, so lift means some air must be moved down to equal that force. Both Bernoulli and Newton are correct and both complement each other. Why is this even an arguement?

BMatthews

I was wondering when this discussion would roll around to this arm wrestling match between pressure and downwash....

There's a hugely long thread or two from a few years back about this. Some pretty bright minds were arguing about the true nature of lift. It really came down to a chicken or the egg situation where you can't separate one from the other because they both exist and support the other.

Case A) if you study the pressure distribution and magnitudes you can resolve the pressure curves of the upper and lower surfaces and it'll end up as a force equal to the lift of the wing. AKA Bernoulli.

Case B) If you examine the mass of air and the acceleration downwards of that mass due to the wing passing through it or by forceing air to pass over a stationary wing you can determine that the acceration x mass added to the air to push it down is equal to the lift of the wing. For each force there is... AKA Newton.

BUT... now we have two amounts of lift from one wing. That can't be or we'd be able to lift twice as much. So what it washes down to is that there is one measure of lift and it is being generated by this combination of pressure variation AND mass accelleration.

So which one causes the other? If you look at the pressure curves you can show how the lowering of pressure will bend the air to flow down. But if you look at how the air is forced to flow around the airfoil you also show that this acceration itself causes the pressure differential. The two effects are tied together just as solidly as the chicken and egg deal You get one from the other and the other from the one. They are tied together and are components of the same reaction. It seems that which you believe in depends on what sort of instruments you use to measure them. It's also quite comparable to the old parable of the two blind men feeling an elephant and trying to describe which is the front and which is the back.

Tim Green

The experiment shows quite clearly that the cause of lift is 100% Newton. Blocking the downward moving air stops the lift. Not blocking it allows lift.

That's 100% Newton.

So what's all this about lift is due to Newton and Bernoulli?

Nathan King

victorzamora

I believe that that experiment does NOT block the air from going downward. It does NOT prove that by the cardboard "blocking" the air it stops lift. It DOES show that not blocking it allows lift, but we all knew that (regardless of Newton or Bernoulli, you must have air moving downward). The force of the downward moving air (something that EVERYONE knows exists in the chopper) hits the cardboard, acting like the sail on a boat with the wind directly at its back. Its surface area absorbs some of the energy of the air blowing downwards. Since it is attached to the skids of the chopper, it transfers that downward force into the chopper. Once the board is folded, the sail is "put up" and there is no more downwards force.

Saying that it would be possible for the heli to take-off under those circumstances if only Bernoulli was accurate when it came to lift is like saying I should be able to pick myself up, completely suspended (kind of like in the cartoons). Obviously the board doesn't absorb all of the energy, but it absorbs enough to keep the heli from lifting off. I'd be willing to bet BIG money that if it were on a scale, the heli would get a little lighter....simply because the board doesn't absorb 100% of the energy. I can see how the Newton/Bernoulli fight has been going on...but I'm depressed that the experiment hasn't been mentioned yet!!

As far as Newton vs Bernoulli, I doubt that either one describes fully what's going on. I'm sure that using either set of formulas you can get the total lift, like BMatthews said. Also, like he said, Newton-lift (NL) influences Bernoulli-lift (BL). I'd bet that if you subtracted what NL added to BL from the total BL calculations (V1), and subtracted what BL added to NL from the total NL calculations (V2), and added V1 & V2 that you would get the total lift. I realize that all of that is next to impossible to attain, but that's the only way to look at lift:

victorzamora

I forgot to subscribe. I do that a LOT [:@]

Tim Green

Formula's describing what happens during lift have little to do with understanding the causes of lift (look at the formula for gravity, for instance). That goes for Bernoulli and Newton.

However, the experiment does contribute to understanding lift. And it's clearly shows that it's due to the downward momentum of the air.

Which leaves us with working out the primary explanations for the net downward change to the air's momentum. Along with working to understand the reasons airfoils work so much better than flat plates.

But the plane or helicopter stays up, because of the air being thrust down.

rmh

I love this stuff- let's se - the wing thrusts the air down
so above the wing the upper surface sucks up?
Much ado about nothing

victorzamora

How does that experiment prove anything?? C'mon Dick, throw your .02 in here.
I've read countless posts from you, so I feel like I know you . Boy would I love to have almost 10k posts under my belt!

I agree with you Tim....right up until you said that the chopper experiment proved something. Oh, and I think the reason that flat plates aren't as good as airfoils is that you don't have Bernoullian lift helping out Newtonian lift. The airfoil causes a bigger pressure differential than the flat plate.

Tim Green

Speaking of much ado about nothing - did you make a technical point? Because if you did, I'm missing it.

My point was that the experiment clearly shows it's the air thrust providing the lift. And the reasons behind this are simple - if the momentum of the air didn't matter, than that plate wouldn't affect the lift. Therefore, the momentum of the air is the lift.

rmh

Just pressure differential it's so simple -why all the fuss?
I will slooowwly exxxplain:
the whirly things create a lower pressure above the blades - the pressure below the blades increase.
In this goofy experiment however the ENVIROMENT is not the same above and below the blades so the pressure difference is cancelled out
HOW ?
the increased pressure (what you called momentum of air) is directed 90 degrees to the original pathYour momentum (?) is now off course by 90 degrees . To get back on course
The same experiment can be done by simply trying to lift one's self by grasping one's bootstraps.
all of the downward air travel etc., just a red herring.
.

Tall Paul

Dick, I do believe that if you put yourself on a scale, and pulled on your bootstraps, the indicated weight wouldn't change.
However, I did get a change of about 3 oz. with the chopper/plate, so there is -some- lift from the rotor not cancelled by the accelerated air impacting the plate. It would take 16 oz. of lift to move the whole assemblage, as seen with the plate folded under the chopper and not impinging on the plate.
Without the plate, the chopper will take off regardless of the initial condition with the downward air being forced out to the sides, as it is with the plate.
It's not the -deflection- of the air by the plate that prevents upward motion, it's the -reaction- on the plate to the downgoing air that does.

Allfat

My Feedback: (1)

You can't have a reaction without a deflection. They are one in the same.

This whole thing is a cyclical argument and the fact that you guys are arguing about it is pretty funny to me, because you are all in agreement with each other, you just don't realize it.

onewasp

The REAL laugh is that YOU don't know who you're dealing with here.
Better do some research on who 'THEY' are ____ 'cause the joke's on you.

rmh

You know that any difference was due to a slight downward vector in the total airstream -creating the 3 ounces .
on the full scale Harrier - the nozzles swing to do exactly the same thing - the NET force difference determines which way the craft moves .
It sucks in air accelerates it and supplies pressure in the selected direction.
It is a summed arrangement - low pressure and high presure and mass to be moved
The "downward momentum of air" argument makes me shake my head . It is simply a result .
Oh well - I am stuck with actually using the forces -I don't just recite old text book larnin (for what it is worth - text book larnin is good properly utilized) .

Tim Green

What in this experiment, would change the pressure gradients within a couple of centimeters of the blades? I don't see anything that would - the plate's simply too far from the blades. ie Bernoulli's unaffected by the plate.

The plate simply converts the energy from the air's downward momentum into a force opposing the lift. Therefore, that downward momemtum of the air has enough energy to lift the chopper. Therefore, the downwash is the lift.

rmh

what lift?
the piddling spill-the 3 ounces?
now then -if you made a nice tapered "cone " from the base of the blades to the extreme outer ends of the attached board - the lift would improve - the vector is changed.

Nathan King

Tim Green

[/quote]
what lift?
the piddling spill-the 3 ounces?
now then -if you made a nice tapered "cone " from the base of the blades to the extreme outer ends of the attached board - the lift would improve - the vector is changed.
[/quote]

Dick,

The lift I was referring to was "THE LIFT". Not the 3 ounces Paul noticed on the scale.

Since the plate negated "THE LIFT" by intersecting the airflow, then the airflow is all there is to "THE LIFT". Otherwise, the chopper would have taken off, with the plate attached.

rmh

No the plate changed the environment
In this case , the pressure beneath the blades is acting on the HELICOPTER- because the plate is effectively part of the helicopter.
- there is NO differential pressure to make the helicopter lift. (look under the cardboard for the correct answer-)
It was a trick question
much like those which change baseline in math.
EX "if you add 10% to 100 then remove 10% - the original number is re established .
Yes?
No?