RE: But Lou

RE: But Lou
View related threads: (in this forum | in all forums)

Logged in as Guest

Users viewing this topic: none

Printable Version

All Forums >> RC Airplanes >> Aerodynamics >> RE: But Lou

Page: << < prev 2 3 [4] 5 6 7 8 9 10 11 next > >>

Login

Message

<< Older Topic Newer Topic >>

RE: But Lou - 2/25/2004 10:24:46 PM

Ben Lanterman

Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline

Comparing airplanes to women is fine as long as they don't find out we are doing it.

But a question is in order. What accelerates air? How can you move a volume of air? How do you accelerate anything that has mass?

Answer - apply a force to that mass.

How can you apply a force to a volume of air?

Answer - pressure differentials across the volume.

If you can accelerate a volume of air with pressure differentials, then why not accelerate a wing the same way?

Seems pretty simple. Simons in his book Model Aircraft Aerodynamics (a good simple book) talks about heavier airplanes having larger masses of air deflected, the same with higher airspeeds. Is is all consistent with conservation of mass and energy laws. Of course, anywhere in the process those laws must be obeyed. You get upwash, downwash, circulation, etc. All can be determined and analyzed. But it is interesting that he goes on to say on Page 13 while talking about an airfoil,

"On both sides there is still acceleration away from the stagnation point but on the underside the peak velocity is less than on the upper side. Pressure is therefore higher below than that above the wing and lift is produced."
Regardless what you explained to your students, what we are talking about is, hopefully, exact science. What happens when you put an air flow over a control surface? You get pressure differentials caused by some fundamental aero principles. Those pressures put forces on the surfaces. The direction of the forces can be easily evaluated by looking at the direction they are pointing. Like camber effects. Interesting how all of the forces on an airplane are a result of the same process.

The bottom line is - what are the forces on the airframe that actually move the airframe, not the downwash, air, or anything else. What are the forces on the airframe for F=ma to occur for the airpalne? - The answer is pressures on the airframe.

It is amazing how many text books on my shelves talk a lot about pressures on a wing and practically nothing about downwash. But I found page after page of pressure work, Why?, you must ask. Are they all wrong? Why not chaper after chapter about downwash instead of pressures?

Lou you are simply putting the emphasis on the wrong thing.

_____________________________

Ben Lanterman

(in reply to LouW)

Post #: 76

RE: But Lou - 2/26/2004 3:03:26 PM

LouW

Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline

Ben I think it’s obvious by now that I’m not going to change your opinion and you’re not going to change mine. I’ve enjoyed the discussion and I respect your ideas. Now you can go back to your books, and I’m going flying (and push some air around).

(in reply to Ben Lanterman)

Post #: 77

RE: Bernoulli's equation - 2/26/2004 3:13:26 PM

LouW

Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline

elmerfud, please accept my apology. It must be obvious to you by now that we engineers are characteristically unable to respond to a simple question with a direct answer. We are taught from the beginning to "complexify" stuff. We must maintain the mystery, otherwise how could we justify making the big bucks?

(in reply to elmerfud)

Post #: 78

One last question then. - 2/26/2004 4:52:13 PM

Ben Lanterman

Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline

If the only thing creating lift is downwash, which is what you say is necessary for heavier than air craft to fly, then how is that lifting mechanism transferred to the surface of the wing???

It is not a matter of opinion, and never was and it is not just my ideas, it is what I have understood after a lifetime of working as an aero engineer. It is a matter of science and fact. It is also what is taught in every book on the subject that has been written and in every college course. Recently there is a tendency to avoid looking at the wing and look at the downwash as the main emphasis in the process. That has become trendy to teach as a mechanism of lift. It is only a part. It is not necessary to dumb down (probably a bad choice of words but they come to mind) the way an airplane flys.

You and I both know that it is entirely possible to enjoy flying and understand all of the bits of the mechanism too. One does not entirely preclude the other. I know several very good aero professors that enjoy flying a lot.

I have always presented what I hope to be a balanced view with appropriate force transmission mechanisms in place and understanding the resulting motions of the air and airplane. You seem to be locked into the "downwash only" explaniation to the point that you are putting blinders on the fuller more complete description of the mechanism of lift. It is not that difficult to do.

_____________________________

Ben Lanterman

(in reply to LouW)

Post #: 79

one last reply - 2/26/2004 5:51:56 PM

LouW

Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline

Ben, you continue to misunderstand my comments. The “what” is downwash, the “how” is pressure. I studied the same texts, and through my years in industry probably did a lot of the same stuff as you. Everything you have said about pressures is true. I am the first to agree that when designing wings ‘n things study of pressures is paramount. I am only puzzled at your reluctance to acknowledge “the rest of the story”.

(in reply to Ben Lanterman)

Post #: 80

RE: one last reply - 2/26/2004 9:42:00 PM

KenLitko

Posts: 243
Joined: 1/3/2003
From: Crown Point, IN,
Status: offline

I think you guys may be missing some facts with the downwash explanation of lift.

1) There is an upwash in front of the wing. (This is subsonic flow after all). I don't think anybody has been accounting for the "negative lift" that this creates.

2) An airfoil has NO net downwash, yet generates lift. Fact: A wing has more downwash than an airfoil because it is inefficient at creating lift.

Downwash does not create lift.... the pressure does... as many of you have pointed out. Any ADDITIONAL downwash created by a wing (this is, downwash created due to the fact that air spills over and around the tips of a real wing) is actually an inefficiency of a wing.... you're getting more drag for the lift that you are creating.

I'm giving this explanation because someone compared the lift on a wing to the pressure in a rocket. I did a paper on this a while back. Back to the explanation....

Lift can be created in two ways.... pressure lift and mass lift. This is similar to the way that a rocket/jet creates thrust... there are two kinds... pressure thrust and mass thrust. In a jet or rocket, the primary mechanism for creating thrust is moving mass at high speed out the back of the engine. You do not want a pressure difference between the exit plane and the free stream. If you do have a pressure difference, you CAN get more thrust, but at the expense of efficiency (not good if you are trying to conserve fuel).

Now... think the exact opposite for wings. As Ben has pointed out... the only thing that the wings "feels" is the pressure. All it needs is a pressure differential to create lift. Upwash.... wing... downwash. The upwash and downwash is an effect of the wing moving through the air. More net downwash (as in the case of a real wing) means that there is mass moving downwards. Does this create lift... yes... but only a small portion of it... it DOES NOT account for the majority of lift.

Here's the kicker...

YOU DO NOT NEED A MASS OF AIR MOVING DOWNWARD TO CREATE LIFT... END OF DISCUSSION. All you need is energy to create that pressure differential... where does it come from.... the thrust from the engine.

Please please... no more explanations of lift as a mass of air forced downward by the wing... it just doesn't work that way.

_____________________________

Ken - http://www.litkoaero.com/
R/C Aircraft Laser Cutting and Design

(in reply to LouW)

Post #: 81

RE: one last reply - 2/27/2004 2:07:51 AM

LouW

Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline

quote:

YOU DO NOT NEED A MASS OF AIR MOVING DOWNWARD TO CREATE LIFT... END OF DISCUSSION.

Maybe in some other universe. On this planet , this is absolutely wrong.

(in reply to KenLitko)

Post #: 82

RE: RE: A bone of contention = - 2/27/2004 3:35:22 AM

JimTrainor

Posts: 1308
Joined: 10/25/2002
From: Ontario, ON, CANADA
Status: offline

Why is it called a foil?

(in reply to LouW)

Post #: 83

RE: one last reply - 2/27/2004 3:37:35 AM

KenLitko

Posts: 243
Joined: 1/3/2003
From: Crown Point, IN,
Status: offline

An efficient wing (high AR) pulls air upward as much as it pushes it downward... with no -net- downwash. It bends the air. This is how lift is created. Look at circulation theory.... it's in the equations.

The airfoil explanation is relatively simple... an airfoil is an infinite wing. While we cannot build an infinite wing, we can simulate one. When we test this airfoil, if there is any net change in momentum around an airfoil... upwards or downwards... it logically follows that because an airfoil is of infinite length... we have just created an infinite force (change in momentum). There is no infinite force, yet we still get lift.

Are you going to argue that there is no upwash in front of the wing? Or that that same upwash creates a negative lifting effect? I don't think i've seen anything in this thread to account for what happens in front of the wing... only behind. It's like trying to explain road apples without seeing a horse eat... sorry that was the best analogy I could come up with! LOL

Remember a wing does not simply push on the air... it itself is pushed -through- the air... if it isn't, it falls out of the sky.

_____________________________

Ken - http://www.litkoaero.com/
R/C Aircraft Laser Cutting and Design

(in reply to LouW)

Post #: 84

RE: one last reply - 2/27/2004 4:41:13 AM

FHHuber

Posts: 4931
Joined: 1/1/2003
From: gone, , USA
Status: offline

OK.. make a home-brew wind-tunnel... and we can settle if a wing HAS to produce a downwash to create lift. The crudest of wind tunnels will work well enough.

Here's the wind tunnel plan: get a refrigerator box and a box fan. Cut the side of the rifrigerator box to insert a window of some sort. (you have to be able to see whats happening..) The box fan is set to PULL air out of the refrigerator box.
(The crude wind tunnel will only give about 5 to 7 mph equiv... so you aren't going to do a lot of tests with it...)

Build a rig to hold a flat bottom wing at ZERO AOA.

Build another little stand to hold a cigarette appx 4 inches in front of the LE. (need a smoke trail to observe...)

Light the cigarette and turn on the fan. If you have it all set up correctly... the airflow will pull the smoke right along the upper srface of the wing... and you wll see it droop below the bottom of the wing as it goes off the TE.

Now how the heck did that happen if there is no downwash?

Try again at -2 deg AOA. You STILL get the downwash. (+2 to -2 deg AOA is about all this setup is good to test. You'll stall the wing with much more than that. Airflow is too slow.)

(in reply to KenLitko)

Post #: 85

RE: one last reply, really - 2/27/2004 3:38:28 PM

LouW

Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline

I will explain one more time (maybe if I speak slowly and distinctly……).

First let me clarify a definition. The term “downwash” is technically defined as the angle through which air is deflected behind a wing and has units of radians. The only time I used the term earlier in this discussion I was referring to the downward acceleration of a mass of air by the passage of a wing, which units would be slugs/sec/sec. Even though they refer to the same phenomenon, they are not interchangeable. I will try to be more precise.

There is a tendency of technical folk to sometimes confuse the mathematical model representing reality with reality itself. There are two concepts that make higher math possible. Without them we could only count and measure. They are the abstract concepts of zero and infinity. As absolutely indispensable as they are, if they are not carefully considered, results can be obtained that are at best misleading.

The lift of an infinite span wing is one such example. From Newtons second law,

F=ma

Or transposing,

a= F/m

For a hypothetical wing of infinite span, the mass of air affected by its’ passage becomes infinite. Dividing F by infinity results in acceleration being zero (for any finite value of F). This is all very interesting, however it is also obvious, that for a real wing with a finite span affecting a finite mass of air, there must be a downward acceleration for any real value of F (lift). A high AR wing effects a large mass of air so the acceleration is small. A low AR wing affects a lessor quantity so the acceleration is more.

I could go on with the development of circulation theory which is again the combination of several mathematical constructs to make the mathematical model represent more closely actual observed phenomenon. But I move on.

All that has been said about the pressure being the only thing the wing “sees” is correct. The pressure distribution around the wing is the only direct force acting on the wing and is the proximate cause of lift. The argument comes with the analysis of a wing as a “free” body. Now this is a perfectly valid engineering technique for force analysis. It is used to isolate joints in trusses, beams, and other structural elements, as well as all manner of static and dynamic problems where things can be represented by vectors to simplify analysis. Lets look at an example of an airplane not moving but suspended from a rope. If the rope is represented by a vector acting on the airplane as a free body, it is easy to see that the weight is balanced by the tension in the rope, and the rope may be said to be the source of “lift”. However, a rope pulls two ways. (See Newton Law 3.) It is entirely appropriate to ask, what is supporting the other end of the rope? It could be a crane, a parachute, or a rafter in the Smithsonian museum. To fully understand what is supporting the airplane it is important to know what is happening on the other end of the rope.

Back to the airplane flying through the air, there is an area of low pressure just above the wing, and its net effect on the wing is the lifting force supporting the wing. The net effect of that low pressure on the air is to cause the air above to flow downward toward the wing. The air thus set in motion continues to move downward after the wing (and its area of low pressure) has past (Newton 1.). The resulting downward flow is not some arbitrary side effect, but the direct result of the pressure (force) supporting the wing. In fact one simply cannot exist without the other. (Newton law 3.) It is the other end of the rope.

In the above example it is OK to say that the rope is supporting the airplane but it is a more complete understanding to say that it is supported by the crane, or rafter, or whatever is on the other end of the rope. I agree completely that the net pressure distribution is the proximate cause of lift, but in a fluid there is simply no way to create a force on a body other than by accelerating a mass of the fluid. In the final analysis, when you cut through all the details, a wing is simply an air deflector.

(in reply to KenLitko)

Post #: 86

RE: one last reply, really - 2/27/2004 4:23:46 PM

KenLitko

Posts: 243
Joined: 1/3/2003
From: Crown Point, IN,
Status: offline

quote:

ORIGINAL: LouW
in a fluid there is simply no way to create a force on a body other than by accelerating a mass of the fluid. In the final analysis, when you cut through all the details, a wing is simply an air deflector.

Sure there is a way to create a force without moving mass.... F=ma can be re-written as F=PA... Force is equal to pressure times an area.... no mass movement necessary. At least not with that equation.

The difficult thing to understand is how you get a pressure field set up in a fluid without moving it. After all... a fluid cannot sustain a shear force... right? Any pressure field in a free fluid will quickly dissipate because there is no way to confine it by using the fluid itself. Think of an explosion.... a quick, intense, pressure field, then nothing.... because it dissapates.

Like i mentioned before, a wing does not simply move through the air... it is pushed through the air. This "pushing" on the air creates this pressure field.

Don't be so quick to dismiss the theory as just a bunch of equations. That bunch of equations is pretty good at predicting the performance of a real wing.

BTW... you still haven't argued anything about the upwash. In order for this "mass of air moving downward", as you say, to create lift... it has to overcome the "mass of air moving upward" in front of the wing.

Another BTW... i goofed that explanation of an infinite force created by a airfoil... it's not infinite because it occurs along the length of the airfoil... it's infinite because each molecule of air moving downwards behind an airfoil would need to be replaced by a molecule of air above it... in an infinite stack... this infinite stack of molecules accelerated downward by an airfoil is what would cause an infinite lifting force to be developed. This is simple continuity.

This web page has a surprisingly good explanation of the concepts i've described:
http://regenpress.com/circlatn.htm

_____________________________

Ken - http://www.litkoaero.com/
R/C Aircraft Laser Cutting and Design

(in reply to LouW)

Post #: 87

RE: RE: A bone of contention = - 2/27/2004 4:51:56 PM

JimTrainor

Posts: 1308
Joined: 10/25/2002
From: Ontario, ON, CANADA
Status: offline