Airplane Mechanics Help Please: Understanding Lift

[JKFlyguy]

Airplane Mechanics Help Please!

Okay so I have a question about lift.

I've heard different explanations of lift from various sources. I first thought that the SHAPE of an airplane wing was the reason that lift occured.

I was taught that since the surface area on the top of the wing is larger than the surface area on the bottom, the wind on the top of the wing is moving faster than the wind on the bottom of the wing to cover a larger distance over the same amount of time.

Because the wind above the wing is moving faster, the molecules occurring to the top surface of the wing are more spread out compared to the molecules occurring to the bottom surface of the wing, thus the pressure of fast wind on top is less than the pressure of slower wind on bottom.

Then the wind with higher pressure on the bottom would push towards the wind of lower pressure on the top, creating the force, lift.

Then I was told something else. I was recently informed that one can make an airplane with flat boards of wood for wings. I questioned how this was when the air traveled the same distance over the same amount of time both over and under the wing.

The person replied that when the board is placed at an angle and moved forward through the air as to direct the air downwards with its slope in horizontal flight, the wind that would have been above and behind the board is pushed down below and in front of it and a vacuum is left behind in the board's wake where the wind would have been had it not been directed downwards by the angle of the board, causing the pressure of the air beneath the board to push towards the vacuum.


For example, a flat board moved through air at a 90 degree angle to its motion would have a vacuum wake following immediately afterwards as the air scrambles to refill the place that the board just was.

I mean of course in an airplane you wouldn't have a wing fly 90 degrees to (flat against) its motion. I'm just exaggerating the effect of tilting the board by showing what happens in an extreme circumstance.

My question is this: is there any validity to the first explanation of lift, and if not, why is NASA teaching this explanation? Both make sense to me and I'd really like to know the real definition.

If the first explanation does hold truth, then couldn't you just take a board, and, like blowing over a piece of paper to make it rise, attach large enough fans to its top and blow the air across its surface to make the air above the board less dense so the pressure from the bottom would push it up?

 

[mjsd]

this first explanation does work... one of my friend did an experiment on that a while back...and it lifted ok when the right side was up, and if the wrong side was up, it didn't lift. besides that it is the standard way we explain the phenomenon at the 1st year uni level.
mind you in real planes, the aerofoils are usually attached in an angle not perfectly horizontal (c.f. your second explanation).. but too big an angle may result in turbulence; also, there are flaps near the end of the aerofoil for the pilot to adjust the relative "surface area"... but as far as the theory behind the aerofoil, what you have mentioned seems fine to me. But I would certainly love to hear what our resident aerospace experts on this forum have to say about this

 

[FredGarvin]

No. The first explanation does not work. That is referred to as the "equal transit time" theory and it is simply not true.

The fact of the matter is is that lift is not a simple issue to explain. Here is a good quick read on this, and it is from NASA, so I'm not sure where you've read about the improper teachings coming from NASA:

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

The best thing one can say is that lift generation is due to a combination of pressure differences and momentum transfer.

 

[LURCH]

It should be mentioned that this question comes up now and then in these Forums, and it always starts a very lively discussion.

My question is this: is there any validity to the first explanation of lift, and if not, why is NASA teaching this explanation? Both make sense to me and I'd really like to know the real definition.

If the first explanation does hold truth, then couldn't you just take a board, and, like blowing over a piece of paper to make it rise, attach large enough fans to its top and blow the air across its surface to make the air above the board less dense so the pressure from the bottom would push it up?

My answer is going to be exactly opposite of Fred's. Not attempting to confuse, but I just want to make it clear that there is disagreement on this subject.

1) Yes, the first method does generate lift, and
2) Don't know if it would work with the fans attached, but yes, if you blow a strong enough wind over the top of a board while keeping the air underneath it stationary, you can lift the board just like a piece of paper. Of course, it would take a much stronger wind.

Now of course, the second method will also generate lift, strictly using angle of attack. But there's a reason commercial airlines don't put flat pieces of board on passenger jets as wings. When I was taking lessons, we used to have to practice flying around at "minimum safe airspeed", the slowest speed at which the aircraft could remain airborne be controlled. For the Cessna 150, that's about 55mph. At this speed, the plane flies around with its nose high in the air, and the wings are generating nearly all of their lift through angle-of-attack. This generates a lot of drag and the plane performs like a pig wallowing through mud. In fact, once I've slowed down to that speed and gotten the nose up, I have to add throttle until the power plant is running at nearly the same throttle setting it uses at regular cruise speed. In other words, I'm going 55-60mph, while conuming fuel at the same rate as I do when traveling 150, and I've barely got control of the aircraft.

Anyhow, that's the differing view, you'll probably have to figure out for yourself which answer you think is correct. There are plenty of resources for both.

 

[JKFlyguy]

"No. The first explanation does not work. That is referred to as the "equal transit time" theory and it is simply not true."

Fred, it seems that out of all the people responding to this question, you have the most expertise to answer (at least according to this website). What would you say about the phenomenon of blowing over a piece of paper to make it rise? It is true that if you hold a limp piece of paper from the bottom of your lips and blow over it, the paper will rise to meet the airstream. If pressure is not affected by airspeed how is this possible?

 

[JKFlyguy]

From the NASA website that you gave me -

"Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas. "

So in other words, the velocity of wind above a wing is greater and has less pressure compared to the slower air beneath it, but that is not due to the different-distance-over-the-same-amount-of-time. I've read in other places that the wind on the top of the wing actually gets to the back edge of the wing faster than the air on the bottom does.

Is this true?

If it is, why does it add to the lift and not take away?

Also, I was wondering if flight could also be inspired by extreme heat directly above the wings. Great heat would expand a volume of air, making it less dense. Please let me know what you think about this idea...

 

[mjsd]

"No. The first explanation does not work. That is referred to as the "equal transit time" theory and it is simply not true."

Note: when I said "the first explanation does work..." I was referring to
your comment

I first thought that the SHAPE of an airplane wing was the reason that lift occured.

and my friend did an experiment to verify that it did lift better if you have the right side up.

as far as the actual physics behind that phenomenon, there is perhaps no doubt that the "usual" explanations that we hear are somewhat over-simplified...
but judging by your above comment, I thought, when I first replied, that you doubted whether the SHAPE has anything to do with it at all...

 

[FredGarvin]

Just to keep the reading going, here are a few more links:
http://www.straightdope.com/mailbag/mairplanesfly.html
http://en.wikipedia.org/wiki/Lift_(force )
http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm
http://www.grc.nasa.gov/WWW/K-12/airplane/right2.htmlFirst, I have to reiterate that I do completely agree that the pressure differences between the top side and the bottom side of the airfoil do exist and are partially responsible for lift. That pressure distribution is a measurable quantity and has been measured for quite some time. Also, there is a very real difference in velocities between the bottom and top surfaces.

So in other words, the velocity of wind above a wing is greater and has less pressure compared to the slower air beneath it, but that is not due to the different-distance-over-the-same-amount-of-time. I've read in other places that the wind on the top of the wing actually gets to the back edge of the wing faster than the air on the bottom does.

Yes. That is true. The third link above has two very nice flow pictures where it is very easy to see the streamlines of a flow past an airfoil and just how much faster the top surface's speed is (note the puffs of smoke and their relative locations to each other).

I think I like the layman's explanation from the aerodave writer from the StraightDope article:

The next step is realizing that when a fluid is made to curve, its pressure is "thrown" to the outside by centrifugal force (that's the simplest way of describing it). As the air crests the hill at the top of the airfoil, it pulls away from the wing surface, creating a slight vacuum next to the metal. The pressure differential helps push the wing up. In addition, the wing causes the flow behind it to move downward. Some of the downward flow is caused by air deflected by the lower surface, and some comes from air that is forced down as it rounds the top. (That's where the missing lift we mentioned in connection with the AOA explanation comes from!) In short, an airfoil generates lift up by deflecting the air flow down, a clear example of Newton's Third Law.

This explanation does a good job at trying to describe what the shape of an airfoil has to do with the whole process (even though I don't like some of his terminolgy). This also goes hand in hand with the basic Newtonian description for the forces on a particle in a streamline. I found a nice excerpt from one of my fluid mechanics books discussing Newton's second law along a streamline:

The physical interpretation of Eq. 3.4 $$-\gamma (sin \theta)-\frac{\partial p}{\partial s} = \rho V \frac{\partial V}{\partial s}$$ is that a change in fluid particle speed is accomplished by the appropriate combination of pressure gradient and particle weight along the streamline.

Ref Munson, Young & OkiishiHowever, if you think that's the end of it, consider the one thing that negates the theory that only the Bernoulli effect produces lift...the symmetric airfoil. In that case, the only way to produce lift is to introduce an angle of attack.

 

[FredGarvin]

Also, I was wondering if flight could also be inspired by extreme heat directly above the wings. Great heat would expand a volume of air, making it less dense. Please let me know what you think about this idea...

Honestly, I have never thought about that idea before so I have no real numbers to offer up right now. I can't see this working with current technology. There would have to be a huge temperature difference between the two. Plus, the hard part would be to keep that temperature difference local to the two surfaces. Heat will want to travel to try to reach an equilibrium. The higher the temperature difference, the greater the heat flow. I don't see how the heat would be insulated to prevent that from happening. Also, I would think it would take a lot of energy to create that temperature difference based on flow and size of the wing. It doesn't seem to be a terribly efficient way to go based on the energy requirements to get a normal aircraft to operate.

 

[JKFlyguy]

Then what you are suggesting is that the Bernoulli Effect only serves to help lift, but most of it comes from angle of attack. Seeing as air pushes at 14.7 pounds per square inch I'd think that we'd be using that more to our advantage. Perhaps we just haven't found the right energy source. Another thing I'd like to ask is - would ionizing the air directly above a wing cause the molecules in the air to repel each other statically because of the excess of electrons, creating an area of low pressure? How effective would this be and at what voltages would the ionization have to take place at in order for a pressure difference to actually occur?

 

[JKFlyguy]

Very strange to think that because of the bernoulli effect all the wind above you is going faster than all the wind below you.