Mike SVOR

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[sm=confused_smile.gif]

Your homework tonight is to go home and place a running shop vac hose on the back side of your hand.
Let us know how little sensation you feel.

Then determine how many shop vac hoses on the back side of your hand would be needed to keep your hand up.

Rocketmagnet

I'm assuming you mean that you'd need lots of these to hold your hand up?

I just tried this. I would coincidentally need about one vacuum cleaner to hold up my arm. (I'm not sure what a shop vac hose is, so I'm using a vacuum cleaner). The surface area
touching my hand is about 2-3 square inches. To hold up a plane, which has a much larger wing area than that, you'd need a lot less suck.

Hugo

rmh

For you younger guys the vacuum sales guys would lift a bowling ball with a vacuum (and a cup attachment).
I once knew a girl who could -
never mind.
anyway the experiment is not relevant to lift developed in an ambient surrounding.

mjfrederick

I think Inewqban's explanations have probably been the best I've seen so far. His near scientific explanation also validates what Dick over-simplified earlier and got so many people heated. The whole "hand out the car window" experiment is invalid. Have you ever seen an airfoil that was shaped anything like your hand and arm? Furthermore, you can't feel the pressure differential being created in this situation anyway because the area on the back of your hand has a constant flow of higher-pressure air moving in to fill the area from all sides not just from your fingertips (of course this depends on how you're holding your hand as well), hence you feel "wind". The whole shop vac experiment is invalid too. In this situation you're not dealing with a pressure differential, you're applying a suction directly to your skin, and not allowing airflow to occur in relation to the "vacuum" created. Lift the hose an inch or two from your hand, and you'll just feel airflow not a "vacuum", which is all you feel when you stick your hand out the window. Bottom line, it's not a vacuum on top of a wing and it's not high pressure on the bottom of a wing, it is the difference in pressure that matters, everything else is pointless to discuss (as Dick has tried to point out many times).

Now, as for the arguments that the only forces acting on an airplane are lift and drag. Where did gravity go? Are we to understand that the argument is gravity is just a form of drag? Drag is an aerodynamic force that depends on many variables (shape, surface area, surface textures, etc.), and it always acts against thrust. As Dick has pointed out, in some cases it can be directed to be useful drag, but it always has a mostly rearward vector. For our purposes gravity is not variable, and its direction cannot be changed (like Dick pointed out, you can't have lift in space). Are we to understand that thrust is simply a form of lift? What about turbines (no, I am not referring to turboprops either) and rockets? Yes, they create a great amount of pressure behind them, but to call what they do generating lift requires way too loose of a definition of lift. Even with propellers, the force generated should not be considered as lift when dealing in relation to the entire airplane, even though they work on some of the same principles as the wing. The reason for this is that the propeller is what helps define the direction of airflow for the airplane, rather than being dependent upon that direction to do its job. Notice, I said it helps, I'm well aware it does not actually define the direction of airflow. Also, the propeller blade is usually moving way too fast for a stall to occur, usually the closest you get is a differential in thrust between two sides during a radius (a part of the always fun-to-discuss P Factor, which it seems most people are aware of, but few understand... please don't respond about P Factor, that's a whole other thread).

There, Dick, someone mentioned gravity.

mjfrederick

Visions of a golf ball and a rubber hose come to mind.

Mike SVOR

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Where is the face-palm smiley???

CrateCruncher

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So far I've tried to stay out of this one but now that it's 8 pages long.........

What makes an object in a airstream accelerate is not "Lift" but the "Aerodynamic Force". The Aerodynamic Force is a summation of {A: change in pressure integrals (Bernoulli) or B: change in velocity integrals (Newton)} around the entire surface of the object.

The "Aerodynamic Force" is perpendicular to the "Induced Downwash Angle", not the chord line.

"Lift" is the vector component of the aerodynamic force that is perpendicular to the undisturbed airstream. The portion of "Total Drag" called the "Vortex Drag" or "Induced Drag" is the vector component of the "Aerodynamic Force" which is parallel to the undisturbed airstream. "Profile Drag" is by definition, also parallel to the direction of the undisturbed airstream.

With respect to high-speed applications, Induced Drag + Profile Drag is often refered to as a "Total Drag".

Rocketmagnet

Bingo!

Hugo

ArCeeFlyer

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If you research a bit about epidermal nerve endings, you will find that our palms are highly sensitive to pressure, much more so than the backs of our hands. Attaching a vacuum hose to the back of your hand is only going to pinch up your skin and of course your going to feel that, especially when nothing is going on underneath your palm. That's a totally invalid comparison. I was simply tying to point out that when holding your hand in an airflow and trying to simulate a wing, that the reason you "feel" the pressure on your palm much more than you feel a reduction of pressure on the back of your hand, is because of the nature of nerve cells in the palm. So the "hand out the car window" test is perceptually misleading.

da Rock

So put your hand out the window upside down.

B.L.E.

No need for that, just hold it at a negative angle of attack. Who says lift has to be "up"?

I think that astronaut's space suits are only pressurized to 4.8 psi. Do they feel like they are being sucked apart living in an environment that is about 10 psi lower than the pressure at sea level? Can you buy a vacuum cleaner that has that much suction?

rmh

I was doing that test and a carload of #%&%# gave me the finger and some other weird signs- Cheeeee

ArCeeFlyer

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Of course pressure is felt on any part of skin, but the palms and finger tips are much more sensitive to it. You'll feel the pressure on the back of your hand too if turned to the bottom against the oncoming air, but not as sensitive. I guess the simpler point is that skin, especially palms and finger tips, senses pressure far more than it senses a reduction of air pressure around it. Thus, making you feel like it's all push and no pull. A better test than the "hand out the car window" would be to put pressure sensors on the top and bottom of the wing and take actual unbiased readings. If somebody would have the time and resources, it would be interesting to see the results. Of course, I'm sure NASA, and all the big aviation companies have already done that long ago.

LOL!!!

Mike SVOR

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Ok, then stick your head out the window. LOL

Notice how your lips are blown open to the shape of a doughnut? Feel your eyes getting pushed to the back of your skull?
But the back of your hair stays nice and manageable?

ArCeeFlyer

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I used to do that all the time until I got a June bug rammed down my throat.

Lnewqban

This forum started in half September, and we have reached October already.
We have discussed airfoils, doors, vectors, vacuum, sails, and now, car windows and hands!!
.......But we all have learned!!!

Like Mike, I like to see and feel what I don't understand, and I have been thinking of an experiment that could respond his original question.

Since he started using the poor man's wind tunnel, I suggest building a small airfoil that could be tested in a car.

AOA has to be fixed and moderate, around 10 degrees or so.
The speed of the car must be consistent as well.
The top of the wing should be able to have spoilers.
Properly installed spoilers destroy the air flow on the top surface of an airfoil, and the low pressure (and lift) that the air flow would create.
Just compare the lift force generated by the wing with and without spoilers deployed.

If the lift force created by the normal wing is more than double of the lift generated with spoilers; then, the theory discussed here could be verified.

Attached here is a reference picture of a test station assembled by Mr. Rutan in the last century:

Regards!!

Rocketmagnet

Mike,

I'm not exactly sure which bit you don't understand/believe. Is it:

A. That the drop in pressure on top is greater than the increase in pressure on the bottom?
B. That the drop in pressure on top makes any difference?

Hugo

da Rock

Al Rabe did airfoil testing down in Texas 20 or more years ago and did quite a good job of it. He built a rig he placed on the front of his truck. He solved a lot of the problems such a rig might cause quite cleverly. And he sorted through a bunch of airfoil ideas. He used what he'd found to create competitive models that were very innovative. And he kicked butt in Precision Aerobatics with those models. That competition happens to be quite a rigorous test environment of its own. They were semi-scale models of planes like the Bearcat when that sport was dominated by model airplanes that'd evolved to look very little like any full scale airplane. And he published magazine articles about it to share his findings with the modeling world. He was quite an innovator.

BTW, the AOA can be whatever it'd be in a wind tunnel. And the speed of the car.......... Yeah, lots of things to consider. And the details he found worked in application. Pretty good proof of method.

Moreover, he happened to be a commercial pilot who also delivered refurbed P51s for fun. Now there's a hobby.

rmh

One article he published -showed a short wing section -mounted above the hood (engine compartment bonnet)- and the subject was a full tapered symm airfoil inc flap- OR same sized foil- but ending with the flap section being flat.
For non fliers here (ahem), the coupled flap on a control line model is used for increased lift at very low angles of attack -
anyway - the flat section of flap showed to be the most effective.
His deduction was that the upper surface- when flap went down-deflected- created a smooth line
My take - not the same - I saw the flat flap as providing EFFECTIVELY more difference in shape with less deflection.
You wind tunnel experts can ponder this - I am just a plain ol country boy-.who doesn't know lift from drag .
The test vehicle , as I recall, was a 52 Fjord sedan.and easily ran at 50-60 mph.
my mom had a bran new one them***good thing he wasn't after 100 mph tests.

Lou Crane

CratreCruncher, to your post #178...

Thanks! Clear and simply stated.

Your summary includes the idea that the effective Aerodynamic Force could be called the summation of the forces on points along the chord, above and below the airfoil, if one chooses to see the force in terms of those points. (It is simpler to test the end result over the entire section than at each imaginary point.)

Mike SVOR

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I guess just a different thinking around the way a wing works.

A wing's top surface is designed to bend the air direction from the leading edge to the trailing edge WITHOUT a drop in pressure. Once the air becomes detached from the top surface, flow over the top stops and becomes 'stalled'. (to different degrees, so don't kill me on the definition)
The whole curved top area is there to create a smooth transition (of air flow) to the trailing edge. NOT create a vacuum. If the tops of wings were only meant to create a low pressure area, they would look very different (think of an upside down parachute falling into the wind)
The top curve aids the oncoming air travel when high AoA is stressing the stall envelope.
The bottom of the wing is definetly a high pressure area. Maybe not 1 nanometer from the surface of the bottom wing, but under it.
The bottom wing stalls out also if air isn't forced across it fast enough. Know where that stalled out air goes first? Over the top of the wing. That signifies a high pressure zone.

banktoturn

Mike,

Perhaps we are at the root of the misunderstanding. The top surface is specifically designed to cause a drop in pressure. If you look at the cp plot that I posted the other day, you will see this very clearly. The shape of the top of a typical wing is exactly what one would expect if the goal were to generate a low pressure region ('vacuum' is not the ideal term here). You seem to have exactly the opposite impression about the shape of the top surface and its effect on pressure. How did you come to this view? Do you disagree with the conclusion from the cp plot?

The bottom of a lifting wing will experience higher pressure than the top, but not necessarily higher than the ambient pressure. A cp plot of a fairly thick wing operating at a small angle of attack would show pressure drops on both the top and bottom surfaces, but a somewhat lower pressure on the top. This wing would still generate lift. I haven't found a cp plot showing this, but if I track one down I'll post it.

banktoturn

Rocketmagnet

I am also very interested to know where you heard this.

Have the diagrams and pressure plots not convinced you otherwise?

Why on earth would somebody design a wing to not cause a drop in pressure?

Hugo

B.L.E.

Perhaps the statement was poorly phrased. Ideally, the pressure should be recovered by the time the air gets to the trailing edge of the wing. Potential energy gets converted into kinetic energy and then the kinetic energy gets turned back into potential energy.

Mike SVOR

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No, they have not.
I believe that NASA needs to find a better wing simulator.
This one seems to be a little biased toward making it look like the convex side of a wing does more than it does.

I like how the graph shows air from way above the wing, coming down, to go under it.
Yea, sorry, that's not gona happen.