Siefring
Posts: 152
Joined: 10/22/2003
From: Owings,
MD, USA
Status: offline
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quote:
Carl, Draw a picture that shows where and how a force is applied to the wing due to the downwash. To keep the wing up against gravity there must be a force applied somewhere. Start with the downwash and show how it is down. Start with the momentum changes just above the wing and show were the vertical force comes from.
Ben, I think the problem is that this debate is about more than one thing and some of it is very ill posed. It's Bernoulli vs. Newton, when really Bernoulli is just an extension of Newton. I now realize the point you were trying to make about the downwash. We were both trying to make the same point with a different approach. I thought you were crazy trying to talk about a force in the downwash, because there is no force in the downwash. One problem is some have said the downwash is totally inconsistent with Newton, even though that is not true. The downwash is just the mass flowing (in motion) on the streamline. Newton says things in motion stay in motion unless acted on by a force. Absolutely consistent with Newton.
I can see now, you were talking about forces in the downwash trying to get people to think about what is going on -- not because you believe there are forces in the downwash. I’ve been trying to get people to realize that momentum is not proportional to force, only change in momentum is proportional to force. Really the same concept.
Ben, I know you understand this, but for others. If you look at the 1-dimensional flow Bernoulli/Euler equation it is exactly F=ma. It is just you apply the force perpendicular to the direction that the motion finally occurs. Since the fluid can't compress it squirts out the sides. It's more complicated, but Newton's laws are still followed. Same for the wing. At every point on the wing, above the wing and around the wing Newton's laws are obeyed. For a wing there is a cascade of force actions and reactions. In the end, there is steady state flow and a pressure change perpendicular to the flow. That is all Bernoulli. However at every point in the space Newton’s laws (F=ma, action and reaction, things in motion stay in motion unless acted on by a force is obeyed). It is not an either/or question, it is both.
I’d like to change the tone of the debate and see how much a physicist and AE can agree.
There is no force in the downwash. I think we agree.
If you followed the change in momentum (action/reaction, F=ma) at every location you could derive the lift on the airfoil. This is exactly what is done when Naviar-Stokes Equations are solved on a grid, isn't it?
For reasonably well behaved flow the Bernoulli formulation blows the Newton formulation out of the air! We definitely agree.
Why is it that the Bernoulli formulation is better?
The forces lifting the wing are fairly simply related, pressure, relative wind, AOA etc. You can measure pressure only near the wing surface and get the right answer. Probably you can add some more.
I think we can agree that the downwash, implies a vertical momentum change in the air, and therefore the wing lifted. After all there are only two thing to push on in the system, the air and the wing. From a systems engineering point of view I would think we agree. We can’t easily tell how much lift, but we can infer lift.
To get forces by keeping track of momentum you need to keep track of it everywhere. Not only that, you have to keep track of how quickly it is changing (F*deltaT=deltaMomentum) everywhere also. Too many things to keep track of.
You wrote: quote:
That the wing in the process of lifting has produced conditions that allow downwash to be created yes, but downwash does not create lift.
And I now understand why you phrase it this way.
However, will you go one step further. The forces that CREATED the downwash are the reaction forces to the lift force. (I know, I’m pushing it  )
It is not Newton vs Bernoulli. Fluid flow is just Newtonian Physics, where the laws are reformulated to account for force balance on a bazillion little tiny cubes of air.
Only high pressure matters. This is an unneeded complication. It is just as correct to say only pressure differences matter, at least that’s the way I look at it. If you talk about adiabatic expansion of gas into a vacuum, your statement is probably the only way to look at it. In aerodynamics there is an assumed static pressure in the fluid and the airflow causes differences in pressure. Only the differential pressure matters. Pressure is higher in the ram direction, but always lower in the direction perpendicular to the flow.
The process of keeping track of the momentum of all of the bits can be useful. With viscosity added to the problem, the flow along the streamlines is modified. The simple Bernoulli picture breaks down if more than a small amount of energy is sucked up in the viscous effects. To do the dragonfly shedding vortices picture, keeping track of momentum is the only way.
Anyway, in my opinion it is just plane  wrong to phrase the debate in terms of Newton vs Bernoulli, because they both are valid approaches to understanding the physics.
Carl
< Message edited by Siefring -- 2/20/2005 4:28:17 AM >
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All Forums >> RC Airplanes >> Aerodynamics >> RE: Jef Raskin article on aerodynamics in Fly RC Magazine 
RE: Jef Raskin article on aerodynamics in Fly RC Magazine - 
