Posts: 94
Joined: 4/10/2004 From: Madison, OH, USA Status: offline
quote:
do not want to start a war-- BUT What does your posts have to do with Model Airplanes. we are not in a College Class. dick
This is an aerodynamics forum, and this thread is titled "Bernoulli's Equation".
I'm sorry you don't enjoy this particular thread - but it's not like this is the only thread in the aerodynamics forum. You might want to consider one of those, rather than suggesting that those of us enjoying this one are out of place. I actually find your postings more disquieting than the ones by those who disagree with my theories on what causes lift. At least they are on topic, and they usually help me to elevate my knowledge - whether they agree with me or not. Your posting though, haven't increased my knowledge. At least not yet. Why not join in with your thoughts on lift - you certainly can't be any crazier than the rest of us.
Posts: 2033
Joined: 7/17/2003 From: Greensburg,
LA, USA Status: offline
sorry about my post. get alittle carried away when things drift from balsa shaving. have tried to forget the all the mathematics when I retired from it. new ventures and models have taken its place. Retirement is nice. dick
Posts: 804
Joined: 1/1/2003 From: Moreland, GA, USA Status: offline
(Dick, consider yourself forewarned. This is a long ramble on the esoteric field of aerodynamics, and if you have no interest, just don’t read it.)
Shoe, I can identify with Ken who is tired of this thread, however you raise some good questions and they merit a thoughtful response. We may be on the verge of writing a sequel to “The Never-Ending Story” but here goes.
quote:
It's clear that we will never agree on this subject and that is OK.
I certainly hope that that is not the case. It’s not my purpose to “win” some argument or push some point of view. Rather it’s my intent to help expand our understanding of what goes on in the physical world in the fascinating area of heavier-than-air flight. Before getting into my response, I will revisit the term “significant” that we discussed before.
Many people will stoop to pick up a penny from the sidewalk. Even if that person has an income well below the poverty line most would not consider that penny as having a significant effect on their financial condition. The fact that something is real and measurable does not necessarily mean that it is significant. To be significant something should be important, or consequential. In practical engineering, it is perfectly legitimate to ignore the “penny” stuff.
As a wing moves (at subsonic velocities) through an unbounded volume of air it affects the air in its vicinity in several ways. It creates disturbances that move before it telegraphing its approach. Like the heralds before the king’s carriage, it says, “Get out of the way. A wing is coming through.” As these signals press ahead, some of the air molecules panic and pile up at the stagnation point (resulting in a rise in pressure) before being pressed on around. Others are more cooperative moving up or down, following paths that have been abundantly described in previous posts. Near the wing tips still others sweep sideways and up to get out of the way. How far these signals go before they are no longer effective depends on several factors such as the speed of the wing and how streamlined or blunt it is. Recognizing that theory may see some “pennies” to the limit of the volume, the significant effect is limited to the immediate vicinity of the wing. Several chord lengths forward and about a semi-span above, below and sideways are the generally observed limits.
Now to address your first comment regarding our last exchange, you said,
quote:
“All of the air outboard of the wingtips is deflected upward, and for a finite wing there is much more air outboard of the wingtips than inboard.”
It is only true that some of the air is deflected upwards and that only within the immediate vicinity of the wing. That upward deflection is the beginning of the trailing vortex and it is in the moment of the wings’ passage that the swirl is begun. Effect on the air beyond that is “pennies”, if it exists at all. Within the wings’ area of influence, there is a net downward deflection as described before.
True, the theoretician is free to choose any control volume he may desire, but if it’s other than the volume actually affected, the results are not meaningful.
Now let’s consider the volume of air after the wing has past. The wing sent ahead disturbances warning of its imminent arrival. As the wing passes it splits the air by its physical presence pulling and twisting it, starting motions that remain well after the wing has past. However once the wing has past, there remains no mechanism wherewith it can still move the air around the wing. The air simply proceeds to move toward the equilibrium it had before the wing so rudely interrupted its peace. The motion behind the wing, though it may be furious just after the wing has past, is in the process of dissipating, and is no longer capable of interacting with the wing.
The photos clearly show the extent of the area affected by a wings passage, and shows the air behind already moving back into the space it originally occupied. (By the way, the picture was not just a fortunate opportunity, but was part of a systematic study of downwash by Cessna, and used a B-25 for a camera plane.)
Now I will address your comments concerning ground effect. When a fixed boundary is introduced within the vicinity of the wing, it greatly alters the way in which the wing disturbs the air. Other than the flight of the droid fighters in the crevices of the death star, I can’t imagine a situation where a solid surface intrudes from the sides, and only in some wind tunnel setups could one be above the wing. But in the process of take off and landing, the earth forms a solid boundary well within the area of influence of the wing. Before going into a discussion of ground effect, consider an aircraft toward the end of a takeoff roll. There is a point where much of the weight is supported by the wing, but a little is still being supported by the landing gear. I’ll come back to this concept.
Theoreticians postulate a “mirror image” of the horseshoe vortex bound to the wing. Combining the actual velocities with the reflected velocities from the mirror vortex gives good correlation with actual data. However it does little to describe the physical changes in motion that cause the differences. It is obvious that the mirror vortex is just a mathematical fiction since it can’t really exist under the ground. I will attempt to describe how it actually takes place.
When operating in ground effect, there are two primary changes in the wings effect on the air. The motion at the tips is affected such that the air that would be moving mostly downward in an unbounded volume is forced to move more outboard. This has the effect of moving the tip vortices outboard increasing the effective span of the wing. The wider effective span effects a larger mass of air so it doesn’t have to deflect it as much to produce the same lift.
The disturbances sent out from below the wing, announcing its arrival, instead of dissipating as usual are reflected back by the surface of the ground causing the pressure to increase between the lower surface of the wing and the ground. The air is “squeezed” in this area forming a “cushion”. Since the ground is a solid unmovable surface against which the pressure can react, this cushion supports a little of the weight much like the landing gear above. These two phenomena describe the physical basis for ground effect.
Significantly it all takes place before and during the wings passage. Once the wing has past it no longer effects the air. Of course the air continues to move as it returns to a state of equilibrium, much like a windup toy runs down after it is set in motion.
quote:
One feature of subsonic flow is that the conditions at one location are connected to the conditions at all other locations.
This is a little misleading. In a wing centric view (during steady state flow) it may appear that it is true. And for steady state flow it is useful to assume that it is so for ease of handling computations. However it takes a finite time for a wing to pass, and different points in the “flow” are separated by time. What is seen as flow ahead of the wing is happening before the wing arrives, and what is seen as flow after the wing passes is the result of forces applied earlier and is not what is happening now. Subsonic or sonic, there is just no mechanism by which the motion of air after a wing has past can influence the forces on the now gone wing.
If Harry Potter could work some of his wizard stuff and point his wand and cause the Citation to suddenly vanish, the path through the clouds would remain and the air would still move toward the stillness that was its original condition. It’s merely fulfilling the destiny that was established with the passing of the wing. In order for the wake to affect the wing, it would somehow have to travel back in time and change what the wing has already done. (The gun analogy is still valid.)
< Message edited by LouW -- 5/22/2004 1:55:11 PM >
Posts: 174
Joined: 9/28/2003 From: Ridgecrest,
CA, USA Status: offline
I'll say it again... the gun analogy just plain does not hold in subsonic flow. Although I don't necessarily agree with the mechanisms you describe, you say:
quote:
ORIGINAL: LouW When operating in ground effect, there are two primary changes in the wings effect on the air. The motion at the tips is affected such that the air that would be moving mostly downward in an unbounded volume is forced to move more outboard. This has the effect of moving the tip vortices outboard increasing the effective span of the wing. The wider effective span effects a larger mass of air so it doesn’t have to deflect it as much to produce the same lift.
I think you are suggesting that the ground modifies the shape of the wake (vortices), and therefore the forces on the wing. How can moving the tip vortices outboard affect the the forces on the wing if the gun-bullet analogy holds? The tip vortices don't form until the air has left the wing.
Can you provide any analysis (other than qualitative) to show that the lift on the wing is balanced by momentum transfer? I'll make the same request you made of me a few posts back: "Do it."
Actually calculate the rate at which a wing is transferring momentum to the air and show that is equal to the lift.
I am not sure if this has been mentioned before but the wing tip vortices will cease to exist when an airplane touches down on the runway or will not form until the a/c has left the runway.
Posts: 1284
Joined: 10/27/2002 From: St. Charles, MO, USA Status: offline
Lou, you once said, "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. "
Perhaps this is where I come to a halt when I read what you write. You say the pressure is pulling upward on the wing. It doesn't pull. The summation of pressures gives a net positive on the underside of the wing which is pushing up on the wing. The negative pressure on top of the wing does not pull. It is an important difference I would think.
When considering air, pressure can only push, not pull or suck. The mass of air above the wing is not resisting being pulled downward. It is resisting being pushed downward by the higher pressure in the far distance above the wing. The push downward on the air from far above is what is causing the downwash flow. Not the suction of the negative pressure above the wing.
If the air can only push then the only action on the aircraft wing or body is a push. Downwash is not causing a push on the wing or body.
I truly believe your thinking about the negative air space sucking the air downward and sucking the wing upward is biasing your outlook on this issue.
Dick back when you were talking about wind tunnels being different whether being pushed or pulled - in a continuous flow tunnel the fans are turned on. The inertia of the air makes the increase in speed of the circulating air slowly increase. Eventually you get the whole mass moving at the velocity you desire. The fans are adjusted to maintain this velocity. Typically you would like the fans downstream of the test section with enough tunnel distance in the remainder of the big rectangle of duct to have time to remove rotation and cross section velocities to the point that for all useful purposes the flow is smooth and steady. When the test section is finally reached you hope to have the flow very nice. At that point in time there is basically no difference in pushing or pulling on the air as it were. Actually pulling on the air is a misnamed action. You create a negative pressure with the fans and the air flows into the section as a result of the pressure differential.
Even in this case the high pressure upstream is pushing on the air into the low pressure regions.
Hal, are you sure about this? A wing tip vortex is caused by high pressure air flowing into the low pressure air on top of the wing (assuming some lifting action is taking place). As such it wouldn't matter if the airplane was on the ground or not, just whether or not the wing was producing lift. Now generally the wing is producing very little lift as it starts down the runway and is rotated to an angle of attack just prior to takeoff. That would cause the vortex formation to really take place at the rotation point which just happens to coincide with leaving the runway. Think about it.
Posts: 9931
Joined: 12/12/2001 From: slc, UT, USA Status: offline
Ben - I really did not state my "what if" clearly ,re: pushing or pulling air over the wing. I was not assuming a proper wind tunnel- Rather, simply an open tube -fan at one end - atmosphere at the other. It seems to me that creating a low pressure AFT the test section, would creat a more coherant flow. On an unrelated matter --I was always very impressed with the design of the jet engine used in the V-1 and then modeled as the Dynajet. This always seems to be genious at its best . Who came up with the concept?
Posts: 174
Joined: 9/28/2003 From: Ridgecrest,
CA, USA Status: offline
Ben,
To echo what you are saying, I think it's important to clarify some terminology. There's really no such thing as "negative pressure" - gas can only push against a surface. In the limit where the density of the gas is reduced to vacuum, the pushing goes to zero, but it never pulls. If you define a pressure coefficient: (local static pressure - free stream static pressure)/(dynamic pressure), then you can have "negative pressure coefficients", but not "negative pressures". Even pressure coefficients reach a lower limit when the local static pressure is zero. Under certain conditions, old-fashioned high altitude airplanes like the SR-71 actually relied on compression on the lower surface of the wing for lift.
Posts: 804
Joined: 1/1/2003 From: Moreland, GA, USA Status: offline
You and shoe are right of course. Everyone knows that air can't "pull" and The only "scientific" reference datum for pressure is the hard vacuum of interstellar space. It is obvious that since I choose to write in common English that anyone can understand, I couldn't possibly have anything important to say. There are religious persons that will only discuss religion with a lot of "thees" and "thous", lawyers that sprinkle "wherefores" and "party of the first part" throughout their writing, and engineers who insist on the stilted technical report writing style. In each case such jargon makes one appear above the great unwashed masses. I happen to think that most ideas can be expressed using standard English that is well understood by most English speaking people.
By the way, since we are addressing imprecise language you might consider dropping the description of flow, as in air flowing over a wing. Everyone knows that except in a wind tunnel air doesn't flow at all. It is stationary and the wing passes through it. But of course that's different............really?
You guys tickle me.
< Message edited by LouW -- 5/23/2004 7:14:51 PM >
Posts: 174
Joined: 9/28/2003 From: Ridgecrest,
CA, USA Status: offline
LouW,
No offense was intended, but I think the terminology issue here goes beyond mere semantics. In common English, pressure is understood to mean force per unit area. The term "negative pressure" implies force in the direction of the surface normal (a pull force). I was only trying to reinforce Ben's suggestion that most gasses cannot support a force in the direction of the surface normal. The fact that engineers use pressure coefficient (which can be negative under normal conditions) clouds the issue. You are welcome to the opinion that the clarification was offered for the purpose of assuming a superior appearance. I give everyone who cares to read these ramblings credit for being able to separate meritorious content from superfluous vocabulary. I apologize if using the term "flow" was causing confusion.
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