Posts: 1284
Joined: 10/27/2002 From: St. Charles, MO, USA Status: offline
Tim, you said, "What I like most about these discussions, is the quick bypassing of an essential issue."
No bypassing intended, all of us know what is happening. I graduated from Purdue in 1965 as an aero eng and worked in the field until I retired a couple of years ago. All of understand the essential factors involved in lift production. I was merely restating what happens. The Canola effect is one of many fluid flow effects at work all of which are rigously mathematically definable (also totally boreingly to watch and read). But it is not THE producer of lift. What do you supposed causes the spoon in your example to move into the flowing water. It is the PRESSURE of the air on the other non water side of the spoon. Canola is an isolated look at one part of the lift production process.
"If the object moving throught the air is your fist, the air filling the vacuum behind your hand will be quite turbulent. Doesn't do much. But if the object moving through the air is an airfoil with a normal AOA, then the air moving behind it does wonderful things for us - as it attempts to fill the vacuum created by the positive AOA. Downwash, and a pressure differential. The pressure differential can be measured, and used to predict lift (downwash). "
The interesting thing about F=ma is that it applies to masses and forces and relates the accelerations resulting. The wing attached to an airplane (with mass) is moving at one g (gravity opposite is the other g making level flight). What is the force on the wing. Pretty simple - the force is the ONLY force that can be delivered by air - it is pressure.
Those pressures come from somewhere. The pressures on the wing are a result of the total lifting process. Modify any part of the process including the flat plate example I gave above and you modify the lift, but you don't eliminate it. The pressure field existing at all points on the wing and at the trailing edge of the wing is a result of the wing acting on the mass of the air as it moves through it. The pressure variations finally cause accelerations on the air at the around the wing and trailing edge resulting in mass flows we call downwash.
Show me where the downwash is creating a direct action on the wing to satisfy F=ma for the mass of the wing. It doesn't. The pressure field is the F in F=ma for the wing and the F in F=ma for the mass of the air and the flow field developed we call downwash. Each phenomena is a response to the pressure field.
Follow me a little longer. What the folks who say that downwash is creating lift have done is the following (with appropriate conversion factors).
MassWing X AccelerationWing = WingForcePressureField = FlowForcePressureField = MassFlow X AccelerationFlow
They remove the two identies in the middle and get
MassWing X AccelerationWing = MassFlow X AccelerationFlow
Mathematically it is OK but it bypasses the actual physical existance of the pressure field doesn't it? It messes up the logic - resulting in a statement like "lift is resulting from downwash". It just simply is not true.
The same thing occurs when I am sitting on an asteroid in space and fling a rock away from me. The asteroid and rock both move in reaction to what happened. So we have RockF x Rockm = AsteroidF X Asteroidm. A mathematically correct equation. It is the result of removing the force terms which are equal. However ask a bug between the rock and my hand of the force is important. Yes he would say.
The real answer to the motion of the rock and asteroid is F=ma. The force over a time exerted on my hand gives the mass of the asteroid an acceleration. As long as that force exits on my hand it doesn't matter if it is the rock being accelerated, a small rockek or the hand of God pushing on my hand. The asteroid is responding to F=ma, not RockF x Rockm = AsteroidF X Asteroidm. The later is just a mathematical regiously correct simplification. Don't let that lead you to assuming that is what happens in reality.
Posts: 804
Joined: 1/1/2003 From: Moreland, GA, USA Status: offline
quote:
It is incidently nice to know downwash values to compute horizontal tail effectiveness. If we could measure the strength of the downwash and take care of the bookkeeping of all of the energy inputs to the system we would find that the downwash is probably equal to the lifting process. I don't think that is surprising, the downwash is a result of the lifting process occurring in over around, etc. of the wing.
I think we are near a breakthrough. I feel kinda like a psychiatrist whose patient has just admitted he hates his father. There is still a lot to do, but it’s a start. I proceed.
For others who may not like reading without pictures, I have included some illustrations this time. Please refer to them for reference.
quote:
We both have done the smoke trails in wind tunnels, ink flow studies, tufting studies in flight and so on. We have measured the effects with pressure rakes and probes. It all matches within experimental accuracy to theory - as it should.
I agree completely. I have never said that flow analysis is in any sense wrong. Figure 1 is such a picture of smoke trails around an actual wing. (Principles of Aerodynamics, by James H. Dwinell) I have chosen several points in the stream to discuss and have noted them in figure 2.
I will discuss things from the point of view of a stationary observer, not because it really makes any difference but because certain things are easier to see from that viewpoint. First consider the stagnation point A. The wing is pushing air ahead of it such that at point A, the air (resisting motion by its inertia) has piled up and is pressurized to dynamic pressure q. The blob at 1 is a unit mass of air, above the stagnation streamline, that the wing will pass beneath. We will follow that blob as the wing passes. As the wing moves through the air, an area of negative pressure has formed and is moving with the wing. Fig 2(b) is the pressure distribution at the upper surface of the wing. The zero ordinate is atmospheric pressure remote from the wing (free stream static pressure). Fig 2(c) is the distribution of the accelerating forces acting on the blob due to the pressure forces in 2(b). And finally, Fig 2(d) is the instantaneous velocity of the blob.
Beginning at A, pressure is positive and as the wing passes, the pressure moves toward zero at B, and continues in a negative direction until it reaches a maximum negative value, then moves toward zero at the trailing edge. Our blob is forced (accelerated) upward by the physical presence of the wing and the positive pressure between A and B. At point B the blob has some upward velocity but the force accelerating it upward is no longer acting. The wing continues to move past, and the negative pressure acting on our blob begins to accelerate it downward. It continues to move upward expending the energy imparted between A and B, but slowing until its upward velocity reaches zero just past 3. From this point it continues to accelerate (continuously being pulled down by the negative pressure above the wing) until at the trailing edge the acceleration ceases and the velocity is a maximum. Beyond the trailing edge, the blob encounters the higher pressure emerging from the under surface of the wing and begins to slow down.
Our stationary observer will have seen the blob first pushed up then accelerated downward having some downward velocity remaining after the wing has past which will begin to slow down. (The motion would also have included a slight forward component, which accounts for the drag of the wing.)
During the passage of the wing the mass (inertia) of the blob is resisting the force acting upon it with a corresponding upward force. You said “…..we would find that the downwash is probably equal to the lifting process.”. That is not just a probability, it is a certainty. The overall lift is equal to the summation of the distribution of negative pressure over the effective wing area. It is obvious that the resisting force (inertia) is also equal to the sum of all the acceleration forces (which are a mirror image of this same pressure) over the same area. Furthermore it is obvious that these forces (pulling the air downward) cease as soon as the trailing edge has past. It’s not the downwash behind the wing that produces lift; it is the deflection (acceleration) of the air that occurs during the wings passage.
quote:
Do a wind tunnel test. Put a horizontal flat plate aligned with the tunnel floor and located at the trailing edge of a wing's airfoil (the airfoil being at angle of attack) and eliminated the downwash as a free flowing thing and caused the velocities and pressures involved in making the downwash to impact on the plate - the wing would still produce lift.
As I mentioned in a previous post, if a bullet strikes a barrier once it has left the barrel, it doesn’t affect the guns recoil in the least (nor does it alter the fact that the recoil is a product of the bullets mass times its acceleration). The same is true once the deflection has taken place over the wing. In that sense, After the wing is past, it does in fact become “poop”. That doesn’t alter the fact that the inertia of the mass of air above the wing resisting being pulled downward is as much the source of lift as the pressure pulling upward on the wing. Neither can exist without the other.
quote:
I think that when you say the ultimate source of lift is the acceleration of a mass of air (downwash) it is doing the listener a disservice. The pressures impacting the wing are doing the lifting (pressures developed as a result of air flow around the wing). The downwash is a major result of creating lift, yes, but downwash is not creating the lift.
When the crane is lifting your new Mercedes to the deck of the ship for delivery to your dealer, What is holding up the car? It is equally valid to say that it is the cable, or to say that it is the crane. The crane is not just a byproduct of the cable, but is an essential component of the car-lifting system. For purpose of analysis it is perfectly acceptable to mentally cut the cable and deal with the car-cable as a free body. But if you actually cut the cable the results could be disastrous. What is the ultimate and which is the proximate support for the car? When a lifting wing moves through the air, pressure is reduced over the wing’s surface, and air is deflected downward. Both actions must and do occur together, they are inextricably joined in the production of lift.
Again, for purpose of analysis, the pressure distribution around a wing is typically separated from the deflection of the air above. But in actual fact it is just as valid to say that lift results from deflection of the air as it is to say that it is due to pressure differences. How is that a disservice?
Posts: 177
Joined: 9/28/2003 From: Ridgecrest,
CA, USA Status: offline
OK here it is...
These plots show what happens to a mass of air as a lifting wing passes by. At time t=0, the mass is 1 span in front of the wing, 1/2 span outboard of the left wingtip, and at the same altitude as the wing. At time t=2, the wing has passed by so the mass is 1 span behind the wing. These plots clearly show that, as the wing passes, it provides an upward impulse to the mass of air. The upward impulse experienced by this mass of air must be accompanied by a downward impulse on the wing. It's tough to conclude that lift results from momentum transfer when the momentum transfer is in the wrong direction.
This analysis shows clearly that you have to be very careful about the mass that you choose to analyze, or you get the wrong answer. What is the right mass to choose if you want to draw a connection between lift and momentum transfer? It has to be all of it.
(there is a picture attached, you have to "click for full size" to see it)
Posts: 177
Joined: 9/28/2003 From: Ridgecrest,
CA, USA Status: offline
LouW,
What I am trying to show is... For every unit of mass that you point to and say: "As the wing went by, this mass was deflected downward, thereby exerting an upward force on the wing", I can point to another and say: "As the wing went by, this mass was deflected upward, thereby exerting a downward force on the wing". Which observation is correct? Both. In order to know what the resultant force on the wing is, you have to carefully add up all the contributions from all the bits of mass that were deflected.
To consider all the bits of mass, you have to "let the boundaries of your control volume go to infinity". When you do that, you get a curious answer. You find that when the control volume is tall and skinny (viewed from behind the wing), the net force on the wing from vertical momentum transfer to the air is equal to the lift (I think you would be comfortable with that result). However, when the control volume is short and fat, the net force on the wing from vertical momentum transfer to the air is zero (the upwardly deflected bits of mass precisely cancel the downwardly deflected ones). The wing still exerts a force on the air, but in this case it is exactly balanced by the sum of the pressures exerted at the boundary of the control volume.
Believe it or not, I am with you. For me, momentum transfer is by far the most intuitive mechanism by which to understand a lifting wing. That said, I have carefully gone about calculating the rate at which a wing transfers momentum to the air. I am troubled by the fact that I can (sometimes) add up all the momentum that a lifting wing is imparting in the air and get zero (and I am not ignoring the downward "torrent" just behind the wing).
If you read my post above comparing the two Harriers side-by-side, it brings up an interesting question (I know you don't think it's interesting Ben and Dick, but that doesn't hurt my feelings)... If the rate of vertical momentum transfer from the engine-borne Harrier is always equal to the weight, how is the wing-borne Harrier special such that the rate of vertical momentum transfer is only sometimes equal to the weight?
The best "reason" I can come up with is that the sum of momentum induced by a lifting wing is not a "Lebesque integral". Try to explain that to a smart high school student.
Posts: 10026
Joined: 12/12/2001 From: slc, UT, USA Status: offline
Ya got me -- I was trying to figure out how one could determine how the Osprey (the big twin rotor deathtrap ) could be made more reliable . The real question: How do you take a machine - which upon takeoff, relies on a perfectly balanced load to the rotors - then shifts the load partially to the wings (new balance(s) req'd?) as the rotors re-orient both thrust and lifting surfaces for forward motion. I see only one answer - MORE thrust -a hell of lot more . This leads me back to my basic thought on powered aircraft . power and weight relationships mean everything. All of the work with airfoils is really just necessary "patchwork" to cover inadequate power and overweight conditions.
Posts: 122
Joined: 12/3/2002 From: Dana point,
CA, USA Status: offline
During aircraft design you first determine power required and then choose a powerplant... the difference between power required and power available does determine top speed and rate of climb two of the most important performance specifications. There are two ways to look at this, you have a given airframe... it has a given power required curve... it is obvious that the more power available the higer the top speed and the higher the climb rate. But you could fix the power available and tweak the airframe to lower the power required curve and guess what you would get a higher top speed and more climb rate.... what I mean by tweaking is increasing the L/D ratio by decreasing drag, power required is equal to drag. We spend lots of time perfecting the wing design to get the lowest posible drag/power required... you ask why? when all you have to do to increase performance is put a bigger engine on it. The awnser is simple.... money!!! A bigger engine costs more money to purchase, uses more gas (again more money), all that extra gas means less payload (less people paying me to take them someplace) The result is that when you take the simplistic more power is more better approach you go bankrupt... aircraft design is all about cutting costs and the best way to do that is by optimising the aerodynamics. Oops, I left out structures... those guys are important too they decrease power required by lowering weight, which means smaller wings, smaller engines, less fuel ect....
Posts: 10026
Joined: 12/12/2001 From: slc, UT, USA Status: offline
Understood - -but for fighter performance -or racing -or aerobatics - power changes make more difference than all th the fine tuning of airframe ever did. The best of the new aerobats have one thing only in common - highest power to weight. The P51 (for you boys in the armchairs) was a dud -till they did what? wanna guess?
Posts: 95
Joined: 4/10/2004 From: Madison, OH, USA Status: offline
quote:
What do you supposed causes the spoon in your example to move into the flowing water. It is the PRESSURE of the air on the other non water side of the spoon. Canola is an isolated look at one part of the lift production process.
Ben,
I've already done tests of your hypothesis, using a bent straw to redirect the water flow. It also moves in an opposite direction of the water it "ejects". If the water streams are identical for both spoon and straw, they both move the same distance - if the spoon is as light as the straw.
For the straw I think you'd have a hard time postulating that Bernoulli's involved.
I'm a big adherent of the scientific method - observe, create hypothesis, then test - and someone else already presented me with your hypothesis a couple of years ago. So I'd already done this test. Try it yourself. It's quite revealing. The Coanda effect (where the flow of a fluid follows a surface) only serves to cause action/reaction. (I wonder if the force is exerted where the fluid changes direction, or where it's expelled?) At any rate, experimentation proves the straw moves due to Newton, and since the water streams and subsequent displacements are identical, so therefore does the spoon.
< Message edited by Tim Green -- 5/9/2004 5:44:44 PM >
Posts: 10026
Joined: 12/12/2001 From: slc, UT, USA Status: offline
If you ever worked with fluidic valves - you would see how the Coanda effect really does nifty things. There are valves which -upon simply puttin a puff of air on the junction of a Y , will cause the air to release from one path and attach to the other one . I really had fun learning these little buggers.
Posts: 2040
Joined: 7/17/2003 From: Greensburg,
LA, USA Status: offline
what does all of the above have to do with Model Airplanes, any size. this foreum is supposed to be about the hobby Not a foreum for some aircraft manufacturing co. in competition for a Gov contract. I am not trying to put any one person down, but lets talk MODEL AIRCRAFT/PLANES. dick
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Not bypassing, assuming some facts in evidence - 
