Ben Lanterman
Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline
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Well anytime there is a dynamic process going on you can pretty well freeze it and do a force and moment diagram. We did that all the time before computers. Doing a dynamic analysis with a slide rule just gets you a smoky sliderule and another migraine headache.
Ok let us buy the assumption the air has to go down for the wing to go up. The only way that works is for there to be a transfer methodology involved. Air going down doesn't make the wing go up unless there are thousands of little force vectors pushing on the wing or maybe one big one. The CG of the wing must have a force on it. Right?
The Mv=mv and ma=ma is a great shorthand for eliminating the F inbetween. But indeed the F has to be there in one form or another. MwAw=Fw=Fdw=MdwAdw > mass wing x acc wing = force wing = force downwash = mass downwash acc downwash.
Rockets, guys throwing bricks on ice skates, etc. Sometime in that dynamic process is a force located for a time period and a change in mass. (The wing doesn't change mass so it makes it easier) Keeping it simple the force accelerates the brick as you throw it and the same force the opposite direction accelerates you because it is located in your palm. Similarly the pressure in the rocket chamber accelerates the exhaust one way and the rocket the other, etc.
There is a force interface in every condition you can suggest. We talk about it as if it weren't there, wing to go up requires air to go down, but we conveinently have forgotten to mention the force interface. But it is indeed there. Where there is motion there is force being applied. Right??
That means you must have a mechanism for the downwash to be converted into a force somewhere on the wing. In general the force wouldn't hard to find and normally the force is found where the CG is being accelerated. We have the wing being accelerated against 1g. We have the downwash being downward accelerated into the low pressure. So we have two masses being accelerated. Wing and downwash and we have corresponding forces.
Where is the force interface? The wing says the force on it is up at the 1/4 chord and is due to pressure. This is what all instruments that can be put on a wing's surface and their resultant measurements indicate (certainly close enought for government work most of the time).
Now - how does the downwash get the momentum, ma, etc. force to the wing? You must be able to find the force interface. Unless you can answer that - the downwash answer to lift is bogus.
If there is a big hand grabbing chunks of air and sweeping them downward you have the answer. It is the hand. If you are like the Harrier, it is the nozzles. It is very easy to recognize and find. Where is the interface when the wing is flinging the downwash down?
Just saying, "In order for the wing to stay up there, it has to send air down." isn't an answer. It must have a physical mechanism to resolve the forces involved. It isn't metaphysical or magic and angles are no longer carrying wings for us.
Any takers? Do you understand what I am asking?
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Ben Lanterman
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 
