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Ben will hate this : But I think it is true Anytime you move something thru air , you change the pressure all around the object. The lowered pressure , can be lift or drag- (a rose is a rose). You need both - but when/how and whare is where the learnin comes in. Look at a parachute - It's primary job is to add drag (low pressure directly opposite the direction it is heading)-which reduces it's speed . If we redesign the parachute to make it steerable - we can change the direction of the heading. If we really get clever with it - we can increase speed and actually get lift. How does that happen?? We moved the low pressure -relative to the direction the parachute is headed and turned it into lift. (way over simplified). So what you call "lift " -----------------------is low pressure workin for you . What you call "drag"----------------------- is usually low pressure workin against you -not always just usually.
OK -is this explaination badly flawed?
It is not complete - but let's hear another BASIC explanation .
< Message edited by dick Hanson -- 2/15/2005 5:21:45 PM >
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Ben, Well, tossing the balsa sheet is an elegant method of proving what you say. Mind if I use that in my arguments at the field? I just wonder why this argument keeps cropping up among the professionals? It was explained to me and I understood it easily... and I didn't even graduate from high school! There must be something to the downwash arguement that I'm missing. Wouldn't that only come into play during ground effect?
Man, I can't wait to use the balsa sheet method. My AE friend will think I'm smart! (unless he's lurking around here...)
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Referring to Martin Simons series in "R/c Soaring Digest, June '97 thur Oct '97, drag for model sections is measured using a multi-ported pressure rake which moves across the tunnel behind the model. The presssure ports on the tubes on the rake face upstream, and measure the variation of pressure from above to below the test model. The change in flow momentum is integrated to compute drag. Lift is measured by a force balance.
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Which implies one could measure change in flow momentum on the lift (vertical, perpendicular) axis and compute lift, and it should be pretty darn close to the number reported by the balance?
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quote:
Never said downwash didn't have anything to do with lift. Re-read the thread, as Bruce so eloquently put it - you can't have one without the other.
Yes, but you keep making statements that don’t make physical sense. A lot of people here respect your comments and opinions., I certainly do and I’ve only been reading this forum for a few months.
quote:
If it were downwash coming off the back of the balsa then it would seem that it should rotate nose down.
Wrong. Look the streamline plots below. Where are the vertical forces acting on the air. It is where the airflow is changing direction. Seems to me it’s mostly centered right around 25% of the cord. There is only a vertical force acting where the flow changes direction (or changes velocity) in the vertical coordinate. There is almost no force at the trailing edge. You even argued that there is no force involved in the actual downwash in an earlier post. If there is no force why would it push the nose down.
If you add flaps and have a change in the flow direction near the trailing edge you do get an additional pitching moment along with additional lift.
quote:
Drive down the road in my long nosed 1968 Pontaic. Release all of the hood catches. The hood lifts up and over the car. Why. Certainly not downwash. It's differential pressures on the front of the hood.
Again, it’s both. You’ve only considered what the air does to the hood, not what the hood does to the air.
quote:
It can't be work with in any practical way. It is there but totally useless in life.
I don’t agree. If you want a good intuitive felling for something you have to try to understand all that is going on -- not just half the picture. Not only that but Conservation laws (momentum, energy, particles etc.) are at the core of fluid theory. Are you saying that fluid theory has added nothing to our understanding of aerodynamics? I seems to me that, if you want to understand aerodynamics when things get really messy (turbulent drag, bumblebees flying, etc.) you need to keep track of the momentum of all the fluid chunks.
quote:
These guys did a heck of a lot of work if pressures are meaningless.
Maybe Jef Raskin said this, but I never said pressure is meaningless. I even argued that all the aero-engineer needs to describe lift is the pressure perpendicular to the flow. Anyone who says aerodynamics is all wrong because an aero-engineer does not care how the aircraft effects the air, is just full of …
quote:
Note that they believe they can measure drag with the probe just aft of the wing.
Can they do this with, one probe and turbulent flow? You say the momentum characteristics of the air are not important, but you would gladly adjust drag terms if you start uselessly stirring the air in circles, instead of creating downwash.
Carl
Oh, here is something else that I completely agree with.
quote:
I have found that attempting to reduce any complex subject, such as aerodynamics, to a two or three page article is at best a waste of time. At worst, it can lead to protracted arguments between individuals who are otherwise rational and even friendly.
< Message edited by Siefring -- 2/15/2005 6:49:45 PM >
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DanSavage, no one answered your question (what is the D/P between the upper & lower surfaces of a wing in flight). Assuming that you are discussing models, if you take a standard trainer with 600 sq in & 6 lb mass, the D/P needed to sustain flight would be .01 psi. For a warbird it would be ~ 50% higher. Not much, but it works.
WRT the general discussion in this thread, I'm a former military pilot & I still occasionally bumble around the sky in light aircraft. I'm also a graduate engineer (U of Western Ontario) & a fluid dynamics specialist with 37 yrs of experience, twenty of them managing a fluid dynamics engineering research lab -- I absolutely agree with Ben Lanterman.
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Justin - very interesting, I had no idea it was that way.
True Dick except unfortunately things cannot suck, they only blow. That is a basic fact of physics. Vacuum is a space where there are no molecules to bounce off a surface. Less pressure and more pressure across the parachute is just the difference in pressures, big guy against little guy in a pushing match (not a rope pulling match by the way, but pushing) . little guy loses because he can't push as hard.
Lift is indeed low pressure working for you, the closer to pure vacuum you can get the better the lift. Hence all the work on wing surfaces and contours.
Jim - if we could measure the momentum flow and apply it properly it would work - yes - but the process is not easy. It is much easier to put a force balance into the wing, model, whatever and directly read out the forces.
As a good exercise figure out how to do it and report back. I throw it out to anyoneto work out as a matter of fact. I showed lift process calculations with the F=ma in just a few steps but had no idea how to work the momentum results into lift. I would enjoy seening it done.
Ptulmer - I have no idea why a real engineer can't grab onto the notion that lift is pressure on the wing. It is what they were taught and worked with. The momentum is there but is not a good system to work with.
Bobber, there is a simple explaination, just read my comments in the thread. It is explained in one paragarph. Jeff dispels one myth and then instead of presenting good balanced information presents his view of the world which is not at all what the rest of the world presents.
Carl - My comments mostly make physical sense I think. Jeff said that the change of momentum of the air as evidenced in the downwash (look at the drawings in his article) is lift. It surely implies that the downwash is pushing on the wing. He uses the rocket anology. Downwash down and wing up. You are saying what I was trying to say. Indeed there is no pressure differential at the TE worth mentioning but a heck of a lot of downwash.
We don't care what the hood does to the air just as we aren't too bothered about where the downwash goes. Interest is put on where the work is being done, as you note on the wing and up forward.
You don't agree... (rest of statement) Have I ever said that fluid flow theory has nothing to do with aerodyanmics - lordy no. We have to understand everything and momentum is a part of the whole. If you read some past comments I have written about insect flight (the clap and flap and clap and fling are vortex methods of lift) and I embrace the whole theoretical aerodynamics field - good stuff - have never said different. But what pushes on a wing to keep it up - pressure.
Beats the heck out of me if they can do it with one probe. They seem to think they can. Paul gives the answer. We arn't taking about stirring the air in circles, we are talking about nicely working wings making lift. Nothing more.
Don't forget - the only way to move an object in the most basic way is through a Force. Then ask - where does the force come from - molecular impact? Then ask - what do we call that process - pressure.
Paul, we always measured drag on our models with a force balance too. Granted they were expensive pieces of equipment but the only way of getting really accurate data - sufficient to pickup small configuration changes. Perhaps if the tennel test rig can only measure lift and moments you have to revert to the change in flow momentum approach. Is there any chance you could copy the article and present it here. Sounds like very interesting stuff. You do have to wonder how the glider guys are getting the data from just one rake setup. Might be a lot of assumptions applied to the data.
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About the drag rake, Simons says ( ) that due to the full tunnel width of the models, eliminating any tip flow, creating a balance that can measure drag with this setup is expensive. Strain gauges can measure the lifting force. As the rake has many pressure pickups, each feeding an indicator, traversing the rake across the tunnel gives a good presentation of the forces above and below the wing, to the point that open structures can be seen to have different flow characteristics where the covering sags between the ribs. The process is detailed in the Soartech volumes from UIUC. A balance couldn't detect the between-the-ribs variations, it would see only the overall effect.
< Message edited by Tall Paul -- 2/15/2005 8:57:52 PM >
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The only thing I was describing - was that lift - is not necessarily the exclusive property of a wing. I think that many people get caught on "what makes a plane fly "-- with that old picture of a Clark Y airfoil and some talk about a guy named Giribaldi Brinelli or whatever. ( Take a sidewalk survey if you don't believe me .) I was trying to (apparantly badly) illustrate that a difference in pressure - irrespective of how it occurs - CAN produce lift-----or drag.
Oh my gosh there is all kinds of mistakes in this document.
Fig3 shows a wing that will lift but the wrong streamlines are drawn. Somehow this is supposed to make a point!
"The lift of a wing is proportional to the amount of air diverted down times the downward velocity of that air."
This above statement is just plan wrong. If there were no other forces active you could invoke momentum balance to calculate the upward velocity of the wing. Set MassPlane*VelocityPlane=MassAir*DeltaVelocityAir and get the new wing velocity.
However, you do not get the lift force. Force=delta_momentum/delta_time. To get the lift force you need keep track of how the air momentum is changing over time. Or as Ben says just integrate the pressure over the wing.
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Dang, how much misinformation is out there?!
There is a pressure increase under the wing, right?(because the wing is displacing air and moving it due to the AOA) And this is what is being called "downwash", right? And the only time this effects ANYTHING is in ground effect, right?
Wait, let me try again. The pressure I'm speaking of is after the lift is generated by the pressure on the wing surface. The movement of the air is redirected down causing a pressure "bubble" in ground effect.
Some of this stuff is starting to confuse me. I thought I had a pretty good grasp on this. The 3hr round trip to the hobby shop is usually centered around politics because my friend thought I had my head on straight...little does anyone know...
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From the pilot's point of view, the wing as a device to accelerate air downward is just fine. At any given speed, you need so much angle to keep the wing supporting the aircraft. If you go too slow, you can't move enough air no matter what the angle. If you have too much angle to the incoming air, your wing can't deflect it downward, no matter what the speed. In flight, when the wing can't deflect enough air to support the aircraft, we call it "stalled".
This idea originated with Wolfgang Langeweiche's book "Stick and Rudder", which is a practical discussion of how an airplane flies. It starts out by stating that aerodynamics is much more complex than he'll be discussing, and that he's 'way off base as far as what the air is actually doing. He's trying to help the pilot FLY the aircraft, and the visual images help the pilot a whole lot more than a knowledge of the actual dynamics involved.
As Ben states, if one's trying to teach the science of aerodynamics, then there's a whole lot more than most people imagine. And if you get the science wrong, those who DO know it will call you out.
If you're trying to teach the ART of flying, then you can simplify the science needed down to the practical visualizations needed to control the aircraft. But the visualizations now need to actually help the pilot effectively control the aircraft.
Don't think the article did either task.
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Well shoot - Ulmer - I had not heard those explanations before. But you are right misinformation is rampant! (Not you Bax -I was typing at the same time as you) edited In really basic English-- lift is produced when there is a pressure difference. The shape of the wing confuses lots of people . 99% of the shapes were done to work in a specific application -be it speed / weigh lifting -etc.. For generating lift - a flat board will work (among many other shapes) You have to move it at an angle which will get the pressure differences (top to bottom)correct. Not enough angle or too slow = no lift. Too much angle for the speed - = ALSO no lift. Everybody and their dog has produced some "magic " airfoil". The full scale pros have worked this to a science -to get the best lift for a specific job. On models - such as we fly - you are still subject to the basic rules - but because of the size - speed and typically low weights we play with. Well-- the finely tuned shape
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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 - 
I just wonder why this argument keeps cropping up among the professionals? It was explained to me and I understood it easily... and I didn't even graduate from high school! There must be something to the downwash arguement that I'm missing. Wouldn't that only come into play during ground effect? 
) that due to the full tunnel width of the models, eliminating any tip flow, creating a balance that can measure drag with this setup is expensive. 
