Tim Green

And it is. Why do you deny,what's in front of you?

Tim Green

Choppers won't fly, unless they're throwing heaps of air down. Everyone knows this.

Does Bernoulli contribute to this downward direction of the air? In a way, but it really doesn't matter. The air's moving down, and that's the direct cause of the lift.

Tim Green

Nope - helicopter was bolted to plate, not to the box.
Also, air doesn't need to push on the earth to cause reaction in the object generating the air stream. Haven't you ever pointed a water hose, or an air hose, at nothing - and noticed that it pushes against your hand anyway?

rmh

Shakespeare said it best

victorzamora

Do you mean "Much ado about nothing" ?? Possibly. Well, probably is more like it. This thread is reminiscent of the Conveyor Belt Thread, except there IS a right answer...mine!

Tim Green

Except I think Dick's venting at me, when he quotes the Bard. And he did it not once, but twice, he likes it so much.

rmh

Vent?
no
just chiding

victorzamora

First quoting Shakespeare, now using words not used SINCE Shakespeare. I think Dick is showing his true age now!!!

mjfrederick

Your response proves my point... Like Dick continues to point out, the discussion is pointless because all that really matters to most of us is that it works... I went back and re-read the first page of posts on this topic and remembered why I didn't post for so long: the people who responded initially had already perfectly explained my position... OK, I'm done now...

Nathan King

Jesus, guys. Aerodynamics isn't exactly a black art here. I suppose it's human nature to try and quantify something complex and varying into a single "absolute" explanation that can be readily understood by the layman and attempts to accurately portray exactly what's happening through every single phase of flight. It's futile; Any explanation is generic at best. This is why I posted the popcorn smiley a couple of times.

The truly interesting part of all this is that neither Bernoulli or Newton ever thought up a theory about airfoils, yet their work continues to shape how we think about something that didn't exist until almost 200 years after these men lived.

Tim Green

Let's be clear here - the reason Dick's complaining that the discussion's pointless is because he's found that some people disagree with him. Apparently you are of the same school.

mjfrederick

If I'm of the same school as Dick Hanson, I'll take that as a compliment. Then again, I know that he and I do disagree on some finer points, but it's good company to be in.

CrateCruncher

My Feedback: (1)

Now thats interesting. What would happen if we hooked up a digital fish scale to a helicopter with a flat airfoil and measured the lift force as a function of pitch collective? What if we measured airspeed in the rotor wash using a wind gauge? Any takers?

victorzamora

If I could fly a chopper, I'd be more than willing to...but that's why I'm SELLING my Blade CP+. If anyone wants it, lemme know.

da Rock

Gentlemen,
Please discuss the topic, not each other.

A couple of posts that were nothing more than the poster's opinion of another poster were just deleted. Please be so kind as to remember that the topics at RCU are about RC and flying, not about other posters.

Please resist the urge to curse, flame, degrade, insult or embarrass someone in your post. We encourage the free flow of your ideas, but believe that they can be communicated (and received) much more effectively if you keep things civil. If you have to vent, take it offline. We carefully monitor posts and will ban individuals who engage in offensive conduct within the forums. Thanks. (RCU Policies)

Tall Paul

.
The field is basically unexplored... add to the information.

Lnewqban

Gentlemen:

I believe the thread is still interesting; and we all can learn if we think about the points of view expressed by others here.

Following is a description of my simplified understanding of the physical process of lifting, for what it may help this sharing of good thoughts.

When I try to understand these complicated phenomena’s, I have to make it microscopically simpler, to try to see it.

I like to imagine all airfoils of helicopter rotors, and fixed wings, behaving like a thin surface moving fast through the atmosphere in a direction perpendicular to the gravitational force.

No force will develop when that surface (let’s call it blade (a flat thin blade of a helicopter rotor) from this point on) and the direction of the movement are the same.

As soon as the position of the blade tilts and creates an angle with the direction of the movement, the blade is forced to follow (engine force combined with actitude of the helicopter), it begins to disturb the molecules of air that are in its way.

What happens on the lower surface?

Molecules hit by the lower surface of the blade, receive energy directly from the blade (and get compressed into a volume of air at higher pressure/density under the blade).

The increment of energy of the molecule results in an increment of its velocity, in the direction dictated by the angle of the blade (molecules are driven from a high pressure volume of air to the volume of air at atmospheric pressure below).

This, happening to a bunch of molecules at once, results in a stream of air moving downwards from the blade at certain angle.

What happens on the upper surface?

Molecules hit by the vacuum created by the sweeping effect of the upper surface of the blade, receive energy from the blade as well (and get expanded into a volume of air of lower pressure above the blade) (first, the blade uses energy to create vacuum, then the molecules absorb that energy from the vacuum, which results in an increment of its velocity, and move down to fill the empty space up) (molecules are driven from a volume of air at atmospheric pressure above towards the low pressure volume of air below).

This, happening to a bunch of molecules at once, results in a stream of air moving downwards toward the blade at certain angle.

What happens on the flying machine?

As a reaction of what is happening above and below, the blade is forced to move upwards.

The blade, and the helicopter attached to it, will move upwards exactly as much as the molecules of air move downwards, since the transferred energy is the same in both directions.

The amount of that energy can only be one, and it is the product of the mass and the velocity of each (helicopter and stream of air).

The velocity of the helicopter moving up is as smaller than the velocity of the air moving down as the mass (weight) of the first is bigger than the mass of the second (Mheli x Vheli = Mairstream x Vairstream).

At the same time, the force pushing up can only be one, and it is the product of the differential pressure created by the engine-blade and the area of the circular rotor that is projected perpendicular to the gravitational force (Lift = DP x Arotor).

Notice that the energy supplied by the engine is used to create the differential pressure that moves the air stream downwards, and also to compensate for the losses caused by drag and turbulence.

Collective and cyclic controls set to zero:

When the plate of the experiment blocks the whole area of the air stream, then Vairstream = 0, it is killed, from the point of view of the atmosphere; as a consequence Vheli = 0.

The same result would be achieved by applying collective control to set angle of attack of my hypothetical flat thing blade to zero.

What if the size of the plate decreases to 2x2?
What if the shape of the plate would have been a cone with the vertex pointing up?
Not all, but some of the volume of air moving down would have been received by the atmosphere, and some lifting force would have developed.
May it be not enough to fully counteract the weight of the model; same as with light collective applied.

Notice that the helicopter of the experiment tends to yaw or rotate, since the effect of the tail rotor is present, but not the counterpart twisting effect of the air stream hitting the atmosphere below. Only friction between the plate and the pedestal prevent the yaw.

Collective and cyclic controls applied:

If the plate of the experiment would only block a portion of the area of the air stream, then Vairstream increases, from the point of view of the atmosphere; as a consequence Vheli increases.

The same result would be achieved by applying collective control to set certain possitive angle of attack of my hypothetical flat thin blade.

The higher the angle of attack, the higher the value of Vairstream, and the stronger the lifting effect (the limits are the muscle of the engine and the stall angle of the blade).

Now, if only one half of the plate would have been folded down, what would have happened?
What if the helicopter is located off center the plate?
Not all, but some of the volume of air moving down would have been received by the atmosphere, and some lifting force would have developed; only that it would have been pushing up off the center of gravity of the helicopter, causing a tipping moment to develop.
May be not enough to fully tip the rotor and the model; same as with light cyclic applied.


If you did not fall to sleep at this point, I thank you, and ask you what principle do you believe applies better to the description above:

Newton:

First law: A particle (the molecule of air) will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force (the blade hitting the molecule at an angle, changing the direction of its velocity from horizontal to near vertical).

Second law: The net force on an object is equal to the mass of the object multiplied by its acceleration (The pushing force of the molecule of air comes from its mass and the change of the direction of its velocity, which equals acceleration, and is forced by the action of the rotor over the molecule).

Third law: Every action has an equal and opposite reaction (The harder the molecule of air is hit and deviated from its original velocity, the harder it pushes (lower surface of the blade) and pulls (upper surface) the blade).

Bernoulli:
For a non-viscous fluid flow (air is viscous and compressible), an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's gravitational potential energy (only because the internal energy of the fluid within an isolated enclosure is conserved, and it moves among three types of energy).

For lifting effect to develop, energy must be added to the air surrounding the aircraft, which is not within an isolated enclosure, but the open atmosphere; I believe.

[sm=idea.gif]

Shoe

When one object pushes against another, the other object will push back with an equal and opposite force (Newton's 3rd Law). If the force that one object exerts on another is unbalanced, then the object experiencing the unbalanced force will experience a change in it's momentum (Newton's 2nd Law). The helicopter in the video is pushing against the air and the air is pushing back. What seems to be the fundamental question here is: is the force that the helicopter exerts on the air balanced or unbalanced? If the force is unbalanced the air will experience a change in its momentum. When I say "the air" in this context, I am referring to ALL of the air, not just the stuff in the vicinity of the helicopter's rotor. If the force on the air is balanced, the air will not experience any net change in its momentum. No wacky aerodynamic principles here, just high school Physics.

The force on the plate in the video is clearly a balanced force because the momentum of plate does not change (the plate remains stationary in the "inertial" reference frame of the room where the video was shot). "Looking" at the air, it is apparent that there are two things pushing on it: the helicopter and the plate. The helicopter is pushing down and the plate is pushing up. If the magnitude of these push forces is the same, then the air can't be experiencing any change in its momentum (just physics). Soooo... are the magnitude of the forces the same? The video with the helicopter on the scale shows that the answer to this question is clearly "no". The helicopter is pushing down 2-1/2 ounces harder on the air than the plate is pushing up on the air. Unless something else is pushing on the air, the momentum of the air must be experiencing a rate of change.

But what happens if you make the plate REALLY large? I think it is quite plausible (although there appears to be some disagreement here) that with a very large plate, the reading on the scale wouldn't change even at full collective. In other words, with a very large plate, the forces on the air might be balanced and the air would therefore NOT experience any change in momentum.

The reality is that helicopters always fly over a very large plate (known as the earth). Imagine a helicopter hovering at the highest altitude that a helicopter has ever flown (just over 40,000 feet if you trust Google). In this case the helicopter is "tethered" to the Earth by gravity. In order for the proportions to be the same as in the video, the plate for a helicopter hovering at 40,000 feet would need to be about 45 miles on a side. Given that the Earth curves only by about 500 feet in 45 miles, it's pretty safe to consider it a plate. Even when you consider a patch of Earth about 150 miles on a side, the variation in its elevation (due to curvature) is only about 4,000 feet, or 10% of the highest altitude where a helicopter has ever flown. So here's my challenge... repeat the experiment with the helicopter on the scale, but with a stiff plate that is 72" x 72". If the reading of the scale doesn't change noticeably when you apply full collective, then the rotor can't be changing the net momentum in the air (at least not by much). Because you can scale this up to real life, this would suggest to me that there would be no way that a helicopter could ever change the net momentum in the air. Instead, the force the helicopter exerts on the air would always end up balanced by the force the ground exerts on the air.

The mechanisms at work here are fundamentally different than for a rocket in a vacuum. In that case, the rocket pushes against its propellant and the propellant pushes back. Both the rocket and the propellant experience unbalanced forces and they accelerate away from each other. In this case it's straightforward: a rocket in a vacuum gets its thrust by imparting momentum to its propellant. I think it's interesting that helicopters don't appear to work in quite the same way. The air doesn't act like the propellant does. Rather than absorb momentum from the helicopter, it appears instead to serve as a medium for the helicopter and the ground to exchange forces "at a distance". I think this says something very interesting about the nature incompressible flow, but it appears I may be alone.

CrateCruncher

My Feedback: (1)

The atmosphere is a viscous fluid. Viscosity is friction and friction dissipates energy. At 40,000 ft the helicopters rotor wash can't possibly be felt on the surface of the earth (as someone has already mentioned). The helicopter compensates for acceleration of gravity by accelerating air (sucking it or blowing it, take your pick) toward the center of the earth. Newtons Third holds.

(Which makes me realize I made an error in my Lunar Lander post. In the atmosphere of earth, the deflector plate becomes increasingly irrelevant as the distance increases from the rotor due to the dissipation of energy through friction. However in the vacuum of space, according to Newton's First Law, the accelerated propellant will continue to stream toward the plate at constant velocity even if the distance is 100 miles! If the landing gear are 100 miles in length the propellant will eventually hit the plate, change direction 90 degrees, and prevent the lander from moving.)

There seems to be a bit of confusion about acceleration in our discussion. Acceleration is a "change in velocity". The definition of velocity is "speed with a direction". Therefore a change in direction is also an acceleration even though the speed perhaps remains the same. F = ma, Newton's Second law, says that any time a velocity changes (in other words, an acceleration occurs) there has to be a force applied. So there has to be force applied in order to change the direction of any mass moving at speed. Thats the reason there is a balanced force acting on the deflector plate preventing the heli from lifting off in Pauls experiment. When you stick your hand out the window of a car moving at 60mph you can feel the same force exerted on your palm. The force is created as the mass of air is being accelerated out of the way of your hand by rapidly changing its direction 90 degrees relative to your hand.

Shoe, I assumed that Pauls helicopter rotor imparts an acceleration to the air in the vicinity of the blades that is straight down but I'm not a heli guy. If that assumption holds true then increasing the plate size won't effect the system once it is large enough to cover the radius of the rotor. If the rotor wash is cone shaped it is reasonable that increasing the plate size relative to the wash angle would have the effect you mention.

Newtons laws apply everywhere in the universe, not just in space. It's that pesky atmosphere that complicates things enough to make grown men cry.

Tim Green

Right -

No flat plate attached to helicopter skids - air goes down, heli goes up.
Flat plate attached to helicopter skids - air goes sideways in all directions, heli goes nowhere.

You could put an L shaped chute under the chopper, that redirects air behind chopper, and if chopper was on floats, it would move forward, like a Florida everglades airboat. Of course the airboats don't mount their props like this, because forcing the air around a corner in an L shaped chute causes energy loss.

victorzamora

Shoe, your whole thing made NO sense to me. I mean, I understood what you were saying....I just saw no logic in it. Care to try it again?

Shoe

Even at 40,000 ft, the rotor wash HAS to be felt on the surface of the earth (viscous or inviscid atmosphere). The "overpressure" would be exceedingly small because it would be spread over such a big "footprint", but if you were to sum up the pressure times area, you would get the weight of the helicopter. Consider the consequences if this weren't the case...

victorzamora

Are you saying that if space were filled with air (instead of being a vacuum) and we put the chopper in space, it wouldn't fly?? Regardless of whether you believe Newton or Bernoulli....the prop wash off the rotors do NOT have to interfere with any surface (other than the rotor blades, obviously)!! When a plane flies, the flow of air around the wings is returned to nearly the spot where it would've been regardless of the wing's presence.

BMatthews

Actually they are very similar. It's just that the rocket takes the reaction mass with it while a helicopter flies through its reaction mass.

Actually the air DOES act like the propellant in that the heli causes it to accelerate downwards. That's the same as the rocket does but as mentioned in the first sentance above because the rocket is in a vacuum it has to bring it's reaction mass with it. Meanwhile the helicopter if flying through a medium (air) that can be forced to become a reaction mass. The heli accelerates this mass from stationary to downwards and due to the reactions of this acceleration a force is imparted upwards on the heli. The exact same reaction.

Your "earth as a flat plate" idea is faulty because it ignores the fact that the air itself has mass and can be accelerated. You're thinking of it as a medium for transfer rather than a "soft mass" that can be acted upon on it's own. The earth sees nothing of the helicopter's mass once the heli has risen above a height where the air motion is capable of reaching the earth AND returning to the heli.

So if the air is accelerated downwards by a passing rotor or wing it must have energy. Why yes it does! It has kinetic energy (motion) imparted to it by the rotor or wing as it passes through the stationary air and causes it to accellerate in a downwards direction. So where does this energy go if it doesn't reach the ground? It's converted to heat. As this accellerated air moves down it encounters non moving air and friction between the moving and stationary air drags the moving air to a halt. You see there's a huge amount more non moving air than there is moving air as caused by the passage of our insignificant little heli or airplane. And as that air is slowed down it converts the kinetic energy of motion into frictional losses. And what happens when we have friction? Heat. The air mass warms up by an amount equal to the kinetic energy originally induced to it and none of the kinetic energy reaches the earth even from a relatively minor alititude. Such is the end of any fuel burned by any aircraft. It either comes out the end of the engine as heat or kinetic energy. In truth it's a mix of both. But the heated air forced out by a jet engine actually warms the air MORE than suggested by the temperature of the exhaust. The kinetic energy of the rearward blast passes its heat energy to the surrounding air but at the same time the kinetic energy of it's speed is absorbed by the air and turned into more heat. All the fuel put into any vehicle's fuel tank is destined to be converted to some amount of other chemicals and heat.

If you look up information about helicopter ground effect I think you'll find that it's considered to be an insignificant effect once the heli is 4 to 6 or so rotor spans above the ground. Certainly by the time it's 10 rotor spans high any effects would be extremly minor.

Paper Cub

Dead wrong - it is deflected downward.