# how lift

#### seagull

##### Well-Known Member
In reference to my previous post, I ended up not having time last night to write much. I only have a couple of minutes now, but thought I'd get the ball rolling here.

First, there is NO "Bernoulli vs Newton" debate. Period. They are not competing theories. There is one very particular aspect that might be construed as "competing" (but it's not, really), and that is "impact lift" which is only a factor at extremely high altitudes and mach numbers, where the effect of high velocity air hitting the bottom of the wing starts to be significant compared to more conventional lift, due to the extremely thin air and very high kinetic energy of the air molecules.

Second, there is no truth to the notion that air molecules that go over the top of the wing reach trailing edge at the same time as air that goes underneath the wing. In fact, the air that goes over the top of the wing actually will generally reach the trailing edge BEFORE the air that goes under it!

This whole notion that the air that is deflected downwards creates an equal and opposite reaction that pushes the wing upwards is a misunderstanding of Newton's laws. It is similar to people saying that the air rushing out of a balloon is creating an equal an opposite reaction so the balloon moves. No, the balloon moves because it is pushed! The truth is that the place where the opening is on the balloon is the only area that is NOT pushing, so the balloon moves the opposite direction.

I think some of the confusion comes from the fact that all of the Bernoulli equations are derived from Newton's laws. Really just extrapolations of them.

I am out of time at the moment, but the next thing to consider is a discussion of potential and kinetic energy, and how that relates to a fluid and the terms used for a fluid vs. a solid object. We will then look at rotational flow and finally how to achieve that flow.

Thank you!

Looking forward to going through the other topics you mentioned.

I've always understood that Bernoulli's (in the case of an airfoil) describes the interaction of a solid body with the fluid (airflow.) While Newtons law (action, = &amp; opposite reaction) is born out in the downwash that is a result of the the airfoil/airflow interaction.

Does that seem accurate?

What you describe is the popular myth, but no, it is not accurate.

I have a minute or so, so will continue a bit.

There are two types of energy we will discuss. Potential energy and kinetic energy. In an object, we can most easily understand this by looking at a roller coaster. Imagine zero friction/drag. The car at the top of the hill has almost zero forward speed, but a lot of altitude. The elevation is potential energy. As it goes down the hill, it trades the potential energy for kinetic energy, the forward speed. No energy is created or lost in this ideal example. Incidentally, all of flying is energy management in this same way, trading kinetic for potential, back and forth, and adding chemical energy to supplement as required, but I digress...

In air, the potential energy is pressure. The more pressure, the more the _potential_ to do work. Kinetic energy is due to the forward velocity. Just as in the rollercoaster, they trade back and forth. More pressure, less kinetic, and vice versa. This is how Bernoulli came up with the continuity equation, which is what most people think of when they talk about Bernoulli -- as the speed goes up the pressure goes down, and vice versa. It is literally no different than the rollercoaster example above.

So, if we know that pressure goes down as we increase speed, we can also consider what makes an object move. Simply, it is a differential in pressure from one side to the other -- i.e., it's pushed. So, we know that if we can an object moving in the air to move in a direction, we need to find a way to get the air to push it in that direction. We also know that higher speed = lower pressure.

Now, consider a rotating cycliner in an airstream. Look on it from the end, and imagine the rotation is clockwise, with airflow coming from left to right. The rotation of the cylinder will drag the air along the top, due to friction, and slow it on the bottom. This rotation will lead to faster moving air on the top thant he bottom, and, lift, or more accurately, the higher pressure on the bottom will push the object upwards.

The key here is that the air is rotating around the object. Obviously, it would be difficult to build a wing like this, but imagine a cyclinder in the airflow that is not rotating. The air moves above and below it at the same rate. If we stick a fin on the cyclinder pointed down and to the right, the air now follows that new path -- it is a rotational flow. The air will move around the obect and create lift the same way that a rotating cylinder does. Difficult to explain in a few words, which is again why I recommend Chris Carpenter's Flightwise series (not that he had the corner on it, but I think he does a better job explaining the theory than any other publication I've seen).

Back to Newton a bit, without getting too deep in it, many are familiar with the lift equation, L=Cl S rho/2 V^2, being Coefficient of LIft, Surface area, Velocity, and rho being the density. The first part deals with the particulars of that wing, i.e, the Cl and S, the second with the kinetic energy of the air, also known as q-factor. Do you know where the KE comes from? Newton derive kinetic energy as KE= 1/2 mv^2. In air, we replace the mass with rho/2, that's it!

Apologize for errors, running short of time and doing all of this off memory, no time to drag out any texts!

What happens when I fly a kite? There's no rotational flow introduced, however lots of lift being produced. I can feel it when I hold the string. The pressure on one side of the kite is greater than on the other side but the pressure (lift) is not being produced by imparting rotation to the slipstream.

It's my understanding that this form of lift development is the basis of "ground effect" where an airplane will develop lift (Newtonian) before its capable of producing lift (Bernoulli).

Thanks for the further discussion.

I'm still stuck on where the 'myth' is in what I'm saying, and I fear that I may not be expressing what I'm thinking. Let me give it another go:

In simple, general terms (for my sake), an aircraft in flight is a (somewhat) rigid body passing through a fluid. Away from the A/C, the fluid fluid is a constant velocity, constant static pressure situation. Near the A/C the air is accelerated as it passes around the shape of the A/C. Varying air velocities correspond with varying static pressure. These pressures act to 'push' or lift the A/C.

At the same time, the 'action' of the A/C remaining in the air must correspond to an equal and opposite reaction in the air that it is passing through. As expected we observe a downwash as the A/C passes.

As you say, these two items are not competing.

Am I still off-base here?

Thanks again for laying out the explanations so far!

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At the same time, the 'action' of the A/C remaining in the air must correspond to an equal and opposite reaction in the air that it is passing through. As expected we observe a downwash as the A/C passes.

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The downwash you describe here does not contribute to lift...it is actually drag. It is a by-product of lift commonly called induced drag. It is an entirely different force than the downward force produced during ground effect.

You are correct in saying that the wing downwash has an equal and opposite reaction, however the sum of the vector analysis of the wing downwash is a rearward acting component force, not a downward acting force.

The term "downwash" is probably a bit misleading.

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At the same time, the 'action' of the A/C remaining in the air must correspond to an equal and opposite reaction in the air that it is passing through. As expected we observe a downwash as the A/C passes.

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The downwash you describe here does not contribute to lift...it is actually drag. It is a by-product of lift commonly called induced drag. It is an entirely different force than the downward force produced during ground effect.

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But, I didn't say that it 'contributed to lift' just that it is a consistent with the Newtonian world we live in. IE: Every action has a reaction.

My understanding is that while the wingtip vortices create a local downwash, the entire wingspan produces a rotation of the air flow, or downwash.

Rugby,

I added to my post above after you posted. Check that out and see if that makes sense.

MD88-

I think I'm following what your saying.

This is all stuff i used to have down pat in college, but through ten years of non-use, it has grown hazy.

My real goal in this discussion is to jog the ol' memory and come up with a version of the topic that is digestable to the average PPL student.

Thanks for the conversation!

MD88, actually, there IS rotational flow around the kite, same as any other object. I'm not sure why you don't think there is?

Another aspect that I left out above is that as the air pressure decreases due to the rotational flow, that lower pressure leads to the air in front of the wing being accelerated (pushed by higher pressure air behind it) towards the top of the wing. Thus, the air is flowing even faster, sort of self-perpetuating the lift force, creating lower pressure, etc.

Rugby, I think you are not offbase if I'm understanding you correctly. That is a way to model the situation, but it is not really telling you much of interest in itself. It is just a useful tool to solve problems mathematically. It is similar to the one of using the flow component that is perpendicular to the leading edge to calculate the lower relative mach number due to sweepback. It tells you nothing about WHY it works, but it is useful for the engineers....

I see what you're saying.

Going back to the number two installment, the potential/kinetic aspect was helpful for me. I tend to just think in terms of PStatic + PDynamic = PTotal, which is kind of a mathmatical thing, where the Potenetial/Kinetic seems more palpable. I can feel the rust breaking loose!

Thanks!

Rugby

I think you should really take a look at Carpenter's books. He was inspired to write them when he discovered that many of the graduate aero students really didn't have a grasp on the actual concepts of the field. Sure, they could calculate anything, but they couldn't actually explain what was happening. All of this can be understandable if you look at it the right way.

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First, there is NO "Bernoulli vs Newton" debate. Period.

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Yeah, sorry, I didn't mean imply you were going to pass a load on us. I was refering to an a.net thread I saw a while back.

You're absolutely right that the whole "vs." thing is misguided and that they're different aspects of why a wing works.

I think of lift in terms of an exchange of momentum (mass x velocity). The wing effectively exchanges energy with the freestream by changing the magnitude and direction of the airstream's momentum. At first glance that sounds like there's free energy in the air (maybe for gliders), but its all relative. The airplane isn't decelerating a raging airstream to produce a lifting force, but rather is accelerating some mass of air downwards.

Minute

Bernoulli and Newton aren't "different aspects". See my post above.

Seagul,

Sorry, I did not realize that a kite produced a rotational flow. I thought imparting a rotation to the airflow was due to the camber introduced on the airfoil. Maybe a kite has some camber, but it sure seems that in many phases of flight the relative wind never flows over it.

Also, can you produce rotation without a curved surface? Can a flat plate imparted to the relative wind produce the rotation required to provide lift?

Now for ground effect...what is providing the lifting force, while the aircraft is not yet capable of flight? Wouldn't this be Newton's Third Law of Motion?

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Also, can you produce rotation without a curved surface? Can a flat plate imparted to the relative wind produce the rotation required to provide lift?

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Yup! A flat plate at a positive AOA produces an flow pattern similar to a cambered airfoil. Seagull touched on it earlier when he said that high pressure air underneath forces more air up over the leading edge and over the top surface.

As an example similar to the flat plate problem, if my source (a T-38 pilot) was correct, the T-38 has a symetrical (non-cambered) airfoil.

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I was refering to an a.net thread I saw a while back.

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Hey, if were talkin' A.net style debate here, then I just have to say.....

Kutta Rocks! Without him, airplanes would be droppin' outta the sky! I say Kutta/Bernoulli/Newton is 95%/2.5%/2.5%.

Kidding, of course.

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Minute

Bernoulli and Newton aren't "different aspects". See my post above.

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Perhaps I didn't state my claims clearly. I'll refer to a very good online book, See How It Flies, that offers an overview on the mechanisms at work to produce lift, and this excerpt in particular (with coincidently similar verbiage):

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The wing produces lift "because'' it is flying at an angle of attack.
The wing produces lift "because'' of circulation.
The wing produces lift "because'' of Bernoulli's principle.
The wing produces lift "because'' of Newton's law of action and reaction.
...
There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli's principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton's laws

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I have read the "see how it flies" website. The guy is ok, but he's not accurate in many areas. I don't have the time to go through his book again, but I recall finding several errors at the time. In any event, while I suppose the portion you quoted isn't incorrect, I do think it can be misleading.

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