mjfrederick

This statement implies that when lift is being generated it ALWAYS results in upward movement. The problem with this "experiment" is that it shows nothing. We can easily say that the downward flow of the air is an "equal and opposite reaction" to the lift force being generated by the blades as a result of Bernoulli's principles. Subsequently, the force of lift generated gets negated through another Newtonian act on the surface of the plate (the helicopter not lifting up is an equal and opposite reaction to the force exerted on the plate by the downward-flowing air). All that was done here is a system was generated that uses energy with a net output of 0 (theoretical) work. In other words, the most inefficient machine that can be made.

This whole argument is a chicken and egg scenario. Newton's theories have been tested and proven. Bernoulli's theories have been tested and proven (remember, Bernoulli's original experiment had nothing to do with a wing generating lift, it had to do with constricting airflow in a pipe resulting in a reduced air pressure through the restriction). Airfoils are merely an application of this principle.

Now, please, I beg you, let this one go...

Oooh... I just had a thought, I'd like to see this supposed experiment being recreated in this manner: hold the helicopter in your hand about 5 feet off the ground (with the plate attached), run up to full throttle, quickly remove your hand. It won't prove anything, I'd just like to see if you bust your helicopter up...

Tim Green

First of all - my statement implies nothing of the sort.

Secondly - I'm going to try this one more time ...

Take for instance - you - you're floating in space. You take a rock, and heave it down, towards your feet, missing your feet. What happens to you? We'll, you'll move in a direction opposite of the rock. The momentum of the rock provides the lift, so to speak.

Now - instead of missing your feet, throw that rock into your feet. What happens? For a split second, you'll go in the opposite direction from the rock - until it hits your feet. Then your motion will stop, as the force of the rock hitting your feet counteracts the reaction of your body to the action of throwing the rock.

This is the same as rotor blades throwing down air - if the air misses the plate (like throwing the rock but missing your feet) the chopper lifts. But if the air hits the plate (like the rock hitting your feet) the chopper won't lift.

This ain't rocket science.

mjfrederick

You're assuming the blades are throwing the air down as opposed to the pressure differential causing a downward airflow. A flat-bottomed airfoil at 0-degrees angle of attack produces lift. There will be no appreciable downward flow of air, yet the force of lift is still present. The reason the downward flow of air is not appreciable is that the airfoil is well passed the area where the downward force is exerted and the air mass in that area is able to easily absorb most of the flow by... what other than an increase in pressure (thanks for explaining that, Bernoulli). The problem with a helicopter that is trying to lift off is that the blades are rotating in the same space over and over again, concentrating and changing that negligible downward flow of air into a noticable downward flow of air, in fact the force generated by the downward flow of air would, in a perfect system (due to the law of conservation of energy), be equal to the force of lift generated. By attaching the plate to the skids, that force generated is held within the system of the helicopter, negating any upward motion that would be caused by the lift. The problem with your argument is that it assumes Bernoulli was wrong... he wasn't... Neither was Newton. It has been pointed out several times, the theories of both work hand-in-hand, not against each other. It is not a matter of one versus the other, Newton's laws explain why Bernoulli's theories result in the lifting force generated by an airfoil. Oh... one more thing just for fun: think about why the downward force is actually less than the force of the lift. Sorry, felt like throwing in something to show that Newton was not always right either.

I'm voting for the chicken coming before the egg... I have my reasons...

Lnewqban

Tim:

You are right.
Nobody exposing here is wrong, I believe.

Just relocate your helicopter 1000 feet up, and the result will be the same: the plate blocks the mass of air being pushed downwards, killing the velocity of the mass of air; hence, killing the force upwards.

Your assembly of helicopter-plate has the same flying capacity than a closed cardboard box.
The atmosphere doesn't feel any stream of air pushing on it (because the entire storm is happening inside the closed box), and it does not see the need to react to non-action.

All the forces generated by the rotor of your helicopter are internal to the assembly, and do not affect the surroundings.

Sir Newton stated three laws, one of which dictates that only an external force can modify the physical state of an object, regarding speed.

Bernoulli calculated the jig-saw of the energy of an ideal and non-compressible fluid: the amount of energy remains the same within the fluid, becoming kinetic (fluid with velocity), potential (fluid with pressure), or a combination of both.

Pressure above the rotor and under the plate is the atmospheric pressure.
There, the velocity of the fluid is zero.
Remember that nothing is transferring energy to the atmosphere.

Between the rotor and the plate, the velocity and the pressure of the fluid is high, because it is receiving the energy from the burning fuel.
But unfortunately, because your box is closed, that energy is not transferred to the atmosphere.
All you have is a furious blender inside a cardboard box, which is falling like a stone.
In addition, your assembly will not have any stability while falling down.
You will have the same effect as if a strong spring is compressed inside the box.

The rotor of your helicopter and the propeller of my model airplane produce a force in direct proportion to the mass of air that they can move times the velocity that they can make that mass of air move, in reference to the velocity of the surrounding air.
The EXTERNAL reaction to that EXTERNAL force makes the models move in the opposite direction, or just hover in the air.

Same applies to any bullet or rocket: action and reaction of external forces.

Regardless of the lifting process or method, part of the chemical energy (mentioned by Bernoulli) contained in the fuel, is transferred (as explained by Sir Isaac Newton) to the atmosphere.

Regards!!!

victorzamora

Tim, you eloquently explained exactly what I've been trying to say!

The astronaut in space throwing a rock is EXACTLY how I saw it.

victorzamora

....and STILL see it.

Tim Green

You got that right. And the experiment proves it.

rmh

This analogy is not related -
c'mon

mjfrederick

No it doesn't, all it proves is that the downward flow of air can be used to negate the force of lift generated by the blades... Your experiment confirms portions of Newton's laws, but proves nothing regarding Bernoulli.

OK, I finally thought of a way to show you why your experiment is invalid. Take that helicopter up to, say, 50 feet. Hover it. Stand directly under it. Do you honestly think that you're going to feel that downward flow of air? For strictly Newtonian laws to be applied, the downward flow of air would have to constantly be pushing on the earth to maintain flight. It's not. Instead, it is the pressure differential created that is allowing the helicopter to stay aloft. The air pressure is lower above the blades than it is below the blades. That is all that Bernoulli's experiments showed. All your experiment showed was that a helicopter can't lift off if you bolt it to the surface it is sitting on... I coulda saved you a lot of time.

Sport_Pilot

Actuall Newtonial laws do not requrie that the earth be there. It only calls for a mass to be ejected at a velocity, with an equal reaction.

mjfrederick

I made that statement to show the fallacy in his argument because his argument is assuming that the downward flow of air pushing on the surface that the helicopter is sitting on is what allows it to lift off of that surface.

victorzamora

Dick, in what way is the analogy NOT related? Oh, wait. To relate it, you have to strap a huge plate to your feet and hit the plate with the rock....and then fold the plate in half and miss hitting the plate. THAT makes is the analogy EXACTLY related to the experiment. The experiment doesn't show anything related to how lift is created.

rmh

You are comparing - action /reaction-
the goofy experiment was about lift -
which is and was and always will be - a unit of work derived from differing pressures above and below the prop (in this case).
but the gag experiment (?) does not permit lift as the pressure differential relative to mamma earth is short circuited , (the attached cordboard.)
All the work done by the blades is directed to 90 degrees from required flow .

Now then if you blocked off one side of the board the chopper would go sideways. Just like a Harrier.
Like a leaf blower
maybe this should be a discussion on leaf blowers -

CrateCruncher

My Feedback: (1)

Darn, you beat me to it Sport_Pilot! That is precisely what Newton's Third Law is about. Maybe a thought experiment of the Shoe variety would help.

Instead of a helicopter resting on a planet in an atmosphere we have a lunar lander with one big rocket motor underneath resting still in interplanetary space. When the motor fires the propellant is ejected out the nozzle and the lander moves in the opposite direction. The equation Force = mass X acceleration must balance so, m(lander) x a(lander) = m(propellant) x a(propellant) tells us the lander will accelerate only slightly even though the propellant accelerates dramatically because of the large difference between the masses of the two objects.

Next we fix a plate to the bottom of the landing gear of the lander so that when the propellant is ejected from the nozzle it hits the plate and changes direction 90 degrees and radiates in all directions. The landing gear legs will be under tension holding the plate but the entire lunar lander will remain stationary because all forces are perfectly balanced (i.e. all propellant is accelerating away from the lander equally 360 degrees around the plate like the flame of a blow torch).

What happens if the plate slips or becomes detached from one of the legs? Now the propellant is shooting off at an angle and is unbalanced. The lander moves away in the opposite direction AND will begin to spin about its cg because the thrust vector does not pass directly through the cg.

That is why I've said that the distance of the plate is the critical dimension. As the distance increases it plays less of a role in deflecting thrust until it is totally irrelevant.

If we move our experiment back to earth's surface everything is the same only we are subjected to a constant force field in the direction of the center of the earth with a value of a = -32.2 ft/secsec. Now the lander must accelerate more than that value (+32.21) in the other direction to begin to move.

If we replace the solid propellant rocket motor with a mechanical rotor accelerating air from the earths atmosphere we are back to the actual experiment witnessed in Tall Pauls video without ever mentioning Bernoulli or his fluid models. Incidentally, I think one reason the value on the scale changed slightly is because the plate began to fold along its crease creating a small vertical component to the direction of flow.

Jetdesign

My Feedback: (8)

I haven't read all the pages of this post, but I was thinking about something last night: people say that bernoulli's equation works when the air on top of the wing moves faster by traveling a greater distance in the same amount of time as the air on the bottom. We all know that the air doesn't have to end up at the same point. Bernoulli, though, says that as long as the air on top is actually moving around it will create a low pressure zone, so if its turbulent, swirling, moving, whatever, when velocity increases, pressure decreases.

rmh

yes a leaf blower -

Lnewqban

Gentlemen:

I hope the following explanation will liberate the Swiss mathematician and inventor Daniel Bernoulli from some of the blame regarding flying machines.

Notice that he only experimented with liquids, long before anybody had attempted to fly; hence, he only could be blamed in relation to swimming machines.

Please pay close attention to his assumptions, which are listed at the end.

Taken from Wikipedia.org:

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

Bernoulli's principle

In fluid dynamics, Bernoulli's principle states that for an inviscid flow (a fluid flow where viscous (friction) forces are small in comparison to inertial forces) , 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.

Bernoulli's principle can be applied to various types of fluid flow, resulting in what is loosely denoted as Bernoulli's equation. But in fact there are different forms of the Bernoulli equation for different types of flow. The simple form of Bernoulli's principle is valid for incompressible flows (e.g. most liquid flows) and also for compressible flows (e.g. gases) moving at low Mach numbers. More advanced forms may in some cases be applied to compressible flows at higher Mach numbers.

Bernoulli's principle is equivalent to the principle of conservation of energy. This states that in a steady flow the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. If the fluid is flowing out of a reservoir the sum of all forms of energy is the same on all streamlines because in a reservoir the energy per unit mass (the sum of pressure and gravitational potential �gh) is the same everywhere.

Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.

In most flows of liquids, and of gases at low Mach number, the mass density of a fluid parcel can be considered to be constant, regardless of pressure variations in the flow. For this reason the fluid in such flows can be considered to be incompressible and these flows can be described as incompressible flow.

Bernoulli performed his experiments on liquids and his equation in its original form is valid only for incompressible flow.
The original form of Bernoulli's equation is:

(vxv/2)+(gxh)+(P/d)=constant

where:
v is the fluid velocity at a point on a streamline
g is the acceleration due to gravity
h is the height of the point above a reference plane
P is the pressure at the point
d is the density of the fluid at all points in the fluid

The following assumptions must be met for the equation to apply:
• The fluid must be incompressible - even though pressure varies, the density must remain constant.
• The streamline must not enter the boundary layer. (Bernoulli's equation is not applicable where there are viscous forces, such as in the boundary layer.)

victorzamora

Actually, the blades are pushing the air downwards regardless of the plate at the bottom of the chopper. The plate is pushing the air in a different direction (as you say, at 90*)...so the force of the air going down is acting upwards upon the helicopter's blades. The plate, stopping the vertical flow of the air and forcing it in a direction perpendicular to original flow of the air, is acting DOWNWARDS upon the plate, which is attached to the helicopter. Since it's blowing the air outwards, in a circle, the lateral forces cancel out. So, if the air is being blown horizontally....then all of the energy created by the blades is immediately canceled out when it hits the plate. Makes sense?

CrateCruncher

My Feedback: (1)

Bingo!

mjfrederick

Again, you say tomato, I say the air flow is a result of the pressure differences created as a result of Bernoulli principles. You make the assumption that the blades are "pushing" the air. Instead, why don't you PROVE the blades are pushing the air. To attempt this would not be a problem. Set some flat-bottomed blades to 0 degrees incidence (angle of attack) and remove collective pitch. Spin up the blades. If there's no air flow below the blades, then you're right, the blades are "pushing" the air down. I promise that won't be the case, though.

And I still say the chicken came before the egg... doesn't anyone want to argue that?

Lnewqban

Yes, Victor, it makes perfect sense.

The plate acts like a deflector; only that it doesn't produce a nice distribution of air leaving the assembly at 90 degrees.

Once the mass of air hits the plate, it goes crazy trying to find a way to escape, being pushed by the high pressure and more air coming down; hence, it goes in many micro tornadoes that eat a lot of energy, and eventually it finds its way to the lower (atmospheric) pressure outside the stream of air forced by the rotor.

If you watch the video of the experiment close enough, you will see that one of the attachments is loose, and the gear of the model lifts a little; also the edge of the plate bends downward, allowing some of the mass of air flow downwards, creating some reaction up or lifting force.

This force is smaller than the weight of the assembly, which remains on the pedestal.

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?
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?

I hope we will discuss created pressure differential instead of Bernoulli principle from now on!!!!!!!!!!!

mjfrederick

Sorry, I had already written my post prior to reading this. I am aware that Bernoulli's Principle does not directly apply, but much of his work in fluid dynamics led to development of the first flat-bottomed airfoils. I still say that it's a pressure differential that results in lift, and the downward flow of air is an "equal" and opposite reaction to that. Equal is in quotes because we all know it is not truly equal.

Tall Paul

.
These discussions are sometimes more amusing to observe than participate in.

victorzamora

lnewqban, I was using a 90* example for two reasons. 1) That's what everyone else was quoting. and 2) So that the cancellation will be 100%. Obviously it's not going to work with 100% efficiency! That's why the chopper looked to get a little lighter on the skids. That's also why, as you pointed out, when one skid became detached it started lifting up.

mjfrederick

I was trying to just watch... couldn't hold back any longer... To tell the truth, I just want the thread to die... Guess posting this doesn't really help in achieving that goal.

Edit: And pass the popcorn!