victorzamora

Paper Cub, I know that, I'm saying that at with a plane at 40,000ft AGL, there's NO way of telling that it deflects air downwards regardless of how precise your equipment is. I meant relative to the 40,000 feet difference he was talking about.

I think ground effect is one wingspan of the plane.....and beyond that the air deflected downwards does NOT hit the ground. In helis it does seem like it's 5-6 rotor-lengths up, but still...not 40,000ft!

BMatthews

Sorry victor but at one wingspan or a thousand a wing (or rotor which is just a rotating wing) will still deflect the air downwars just as Paper Cub says. We don't need any instruments to know that. It happens.

victorzamora

I know it deflects....it's just NOT going to be noticeable on the ground no matter how sensitive your equipment is, even if you're doing a thought experiment where ANY effect is noticed.

Tall Paul

If it's still possible to stand on the extended centerline of a runway that handles large jets... a mile or so back from the threshold on the approach end..
If the plane passing over is say a 747 at 250 feet.. when it is touching down, the wake vortex it created will be touching the ground about where you are standing.
You'll hear it coming, feel it, and see the disturbance on the ground as it passes.
The plane moves a LOT of air in flight.

Sport_Pilot

Why does this matter? If a tree falls in the woods and nobody hears it, is there sound? I don't get this?

OH wait, this is like the fly in the cockpit or the birds in a truck myth? That is if a plane is flying in the air and a fly is flying in the plane, then the fly does not add any weight while its flying. Or if a truck is full of birds and they are flying you cannot weigh the birds. I think Mythbusters busted this one a while back. If not, I appologize, I am not trying to be smart.

CrateCruncher

My Feedback: (1)

Judging from the popularity of this great thread I would figure there is a keen interest in the subject. Being the oldest and best understood of the branches of physics, classical mechanics is the way the world around us works. I searched the internet looking for a non-math tutorial on the subject and didn't find anything. There are 100's of books on the subject but all expect at least some understanding of calculus from page 1. (I guess this is understandable being that Mr. Newton invented calculus in order to prove his kinetic theories to the nay-sayers.)

If you do want to delve further, I highly recommend Engineering Mechanics Vol. 1&2, by J.L. Meriam. Another common textbook is Engineering Mechanics by R.C. Hibbeler. There are at least 5 editions of either book now and obsolete editions can be had at any used book store near an engineering college for about $5-10.

If your calculus is rusty like mine or you want an intro to the basics I can strongly recommend "Calculus Made Easy" by Silvanus Thompson. It was first published in 1910 and today is more popular than ever. It's a $4 paperback.

If you aren't blessed with a cheapskate bookstore in the neighborhood theres always Amazon or Abebooks on the internet.

Tim Green

I think the great thing about this experiment, is that it allows us to draw conclusions, without calculus. Thus logic is sufficient in this case, to prove that it's the air's momentum that provides the lift.

Tim Green

And one of the good books on the subject is "Understanding Flight" by David F. Anderson and Scott Eberhardt (McGraw Hill publishers).

BMatthews

Ah... SORRY! I was thinking that you were going the OTHER way with your previous post. You're totally right of course. The deflection motion will be totally absorbed before it can be felt even a fraction of the way to the ground.


An interesting point. At our local airport there's a spot out by the fence where you can stand in the open and look directly overhead at the huge airliners on their glide paths. I've had 747's fly over me about 2 to 3 wingspans high and didn't feel a thing. Yet it was close enough to easily see all the hydrualic lines and other details in the landing gear wells. Pretty amazing when you think about it. The ONLY sign of their passing overhead was a fluffy swishing sound that I assume was their wing tip vortices ripping by overhead and colliding with each other. And I couldn't feel those either. Just heard them.

victorzamora

HOW?? The chopper is trying to lift itself PLUS a plate that weighs as much as the total energy of the air it's deflecting! I'm using "weight" not as in Kg, but as in Newtons....the way "weight" really is, not mass.


BMatthews: It's all good, I'm just glad I was able to FINALLY express that point well.
Tall Paul: a 747's wingspan is ~225ft....so at about 250ft AGL, the plane is essentially one wingspan above the ground. I'm NOT saying that it stops at 1 wingspan, I'm just saying at around one wingspan. As BMatthews said, at 2 or 3 you cease to feel the wind. I know from personal experience.

Tim Green

??? The weight of the plate is negligible, since the chopper lifted the folded plate, but not the unfolded plate. Therefore, the plate deflected the air when it's unfolded, but not when it's folded. Therefore, it's the air causing the lift, since if it's deflected, the chopper won't rise. And if it's not deflected, and allowed to continue it's downward motion, the chopper lifts.

victorzamora

You didn't read what I said. Its weight is essentially only the amount of air it deflects. Put a plate NOT attached to the chopper 1" under the rotor blades! That'll deflect the air. It'll also allow the heli to lift.

Sport_Pilot

Which allows the mass to trasfer from the chopper to the enviornment. If the plate was attached then the reaction of air on the plate cancels the reaction of the air leaving the blades, just as an astronaut in space throwing a rock at his feet, if he hits his foot he doesn't move, if he misses then he will move the opposite direcetion as the rock. Your example only proves this point.

BTW the air pressure above the blades will be lower than the air under it, so both work, they just use differant ways to come up with the same answer.

Tall Paul

More grist for the over-analytical...
Can a fan blowing into a sail move a sailboat?
It works in the cartoons...
http://rcuvideos.com/item/WD64D0YB9Z95CSWT

CrateCruncher

My Feedback: (1)

Its a lot like the Heli but I see one major difference. The air can't be deflected into the ground so I would think the normal force increases (car pushes on the earth more) when the prop runs-up. It could be verified by placing the car on a bathroom scale. I would also think the reason the car creeps forward a bit is because the deflector bent under the force of the prop blast lending a very slight horizontal component to the deflected thrust (sorta F cos89.5). If that joint is stiffened up the car shouldn't want to creep anymore. Neat vid!

victorzamora

The reason the car moved the first time is because the plate ISN'T 100% EFFICIENT!

Tim Green

Neat [sm=thumbup.gif] experiment - similar to your original experiment that started this thread. And demonstrating clearly that lift occurs due to the thrust provided by the momentum of the air.

If the air's blocked or deflected in all directions - no thrust, resulting in no motion (no lift).

If the air's deflected in one direction or not blocked at all, thrust occurs, resulting in motion (or lift).

Tall Paul

As with the helicopter on the scale, there was just enough residual thrust in the first condition to allow the vehicle to creep forward.
The amount of power required for the vehicle to move when the motor was reversed was much less than the full power needed for the first condition, which in essence resembles that of a turbo-jet using reversed thrust to stop.
But as a propulsive scheme, as way too many people wish to use for pushing a sailboat, it's as impractical as anything could be.

captinjohn

HA! Boy if B_ _ L - S_ _ T was music...you guys would have a BRASS BAND!!!! no I did not say that!

Tim Green

Yes - you're absolutely right - Newton, not Bernoulli.

B.L.E.

I would say Bernoulli on the left would most likely kick Newton's butt on the right. Bernoulli looks like he has at least 80 pounds on Newton, who, quite frankly looks thin and sickly, almost anorexic. Maybe it's all the court partying or his lifelong feud with that other charlatan who claimed to have invented calculus first, Gottfried Leibniz.