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IF- an airfoil was round (might be) the lift would be very small as you smash it flatter - the lift increases if it is completely flat - then lift occurs over most of the surface It appears to me - that lift ( a pressure differential) occurs anywhere the pressure is different from one side to the other As you change the AOA -the lift changes -a little then a lot . How do you know where along the chord the pressure differential is occuring? Looks like a moving target to me . and if you do an outside loop with that airfoil - what do you plot?
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Bruce apparently removed the big hairy explaination about wind tunnel and force and moment testing that I wrote that answered all of your questions. Come on Bruce, I was being nice if wordy.
-----Edited------
< Message edited by BMatthews -- 3/24/2005 8:56:48 PM >
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Here are some of my findings: In all wind tunnels, the drag of the walls, floor and ceiling tends to slow the stream down at the edges and the walls have also their own boundary layer characteristics introducing errors into measurements taken near them. Another measurement of particular importance for low Re numbers is the turbulence factor of the tunnel. Any small, microscopic turbulance in the tunnel will have a disproportionately large influence on the drag of the small airfoils under test. For some years ago NACA has carried out an extensive series of tests down to Re 42,000. The results published in their report 586 are still often quoted in modelling magazines and surely used in designing of models. However, as the NACA authors reported at the time, the turbulence factor was 2.64, which implies that the stated Re should be multiplied by this figure for a better but still very crude approximation to the truth. Other tunnel tests have also suffered from the same difficulties, but when published the turbulence factor was not stated, so that not even the crudest correction can be attempted. Modellers should not take seriously any wind tunnel results published if the turbulence factor is not known.
Bruce apparently removed the big hairy explaination about wind tunnel and force and moment testing that I wrote that answered all of your questions. Come on Bruce, I was being nice if wordy.....
Yes you were Ben but I commited to deleting any posts not concerned with reflex airfoils as stated in my earlier post on page 4. Sorry.
This new wind tunnel stuff is also a bit too related to the vector discussion so I've edited or deleted a couple of those as well as you can see. I'll have another look and perhaps it'll come back. But I edited your last post and can't restore that anymore.
PS: Ben's wind tunnel post has been restored and slightly edited.
Please all take a moment to review the rules that can help keep things respectful and friendly.-
It's OK to rebuttal back and forth all day long as long as it's data, facts, theories or other material being used. But when it comes down to expressing frustration in inflamatory or derogetory terms be it subtle or overt or demeaning a person's schooling or career background that's where I have to draw the line. If you guys think of yourselve sitting around a big table having the same discussion I think you'll all agree that we would not say some of the things that have been said here and in a couple of past threads. Fair enough?
Carry on.....
< Message edited by BMatthews -- 3/24/2005 9:16:02 PM >
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Joined: 10/27/2002 From: St. Charles, MO, USA Status: offline
I'll attach some photos and I don't think they are in order.
One is a model of the F-15 (and not me) in the Boeing PSWT. Notice the tunnel walls are pourous. The wall boundary layer is removed even in this small size tunnel. Early in the tunnel's existance a standardized model was put in the tunnel and the maximum size determined to allow very accurate measurements. The model is rotated about the rough CG to keep it in the center of the airflow. No one can afford to do much wind tunnel work on airfoils today so most is done with CFD and the best results are tested.
One is a crop of the data reduction options. Notice they list body axis and stability axis systems along with a host of others. These are stricty aerodynamic parameters, no thrust or real airplane weight included. My earlier diagram illustrates the aerodynamic forces you can measure in a wind tunnel.
One is the equations from a old MCAIR report that give the conversion factors.
One is a rather old drawing that is a little more complex that the one I had drawn but shows the relavance between wind, body, and stability axes systems.
When dealing with pure aerodynamic data (no thrust or mass) the figure only has the lift, drag, and pitching moment summaries on it. All aero forces from what ever source are measured by the sting supported "strain guage balance". The photo with the cute lady engineer in it shows the sting inserted in the aft of the model between the engine nozzles for minimal distortion and messing up the data. The actual balance is at the fuselage station where the wing intersects the fuselage. This allows measurements of the aero forces.
If you were going to do a free body diagram (assume no yaw) of the aluminum and steel model including part of the sting you would have the lift, drag and pitching moments, the weight of the actual model and sting, the force supporting the sting along it's horizontal axis, the vertical shear at the freebody interface and the moment at that same interface.
To think of it in terms of an airplane flying the lift, drag, and pitching moments would be the same, the weight of the airplane, and the thrust of the engine would be the horizontal force of the sting support.
All aero forces, no matter where they come from (wing, body, tail, missile, flaps, etc, ) are resolved into the basic aero forces along each axis - x,y,z and moments about those axes - mx, my, mz. There are no other aero forces that are measured by the balance. You can then calculate the various coefficients.
The key is that in the wind tunnel with a complete model ALL forces and moments on the body are accounted for. There are even additional tests ran to account for the sting presense/distortion effects. It is possible to get the exact full scale aerodynamics pretty accurately.
The complete rendering of the types of figures we put on this forum is a complex subject. Most of the time the simple renderings are more than sufficient to explain what is happening but at times those of us used to working in the field forget that not everyone knows everything needed to interpret what we present. In the case of my drawing it is showing only the pertient aero forces as they went from body to stability axis systems, which is what your drawing seemed to represent.
Remember that when only the term CL or lift is put on a diagram of a wing or wing-body-tail (such as the ones shown below) it generally means ALL aero forces up and down on the model have been summed (no matter the source of the lift)
Embarrasing spelling mistake corrected.... -bcm
< Message edited by BMatthews -- 3/24/2005 9:17:55 PM >
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Thanks for the detailed description. As I can see the data you get from the wing tunnel is mainly coefficients (non-dimensional) values based on pressure measurements at different locations, such as drag coefficient Cd and lift coefficient Cl plotted against AOA, lift to drag ratio L/D and pitching moment coefficient Cm.
quote:
In the case of my drawing it is showing only the pertient aero forces as they went from body to stability axis systems, which is what your drawing seemed to represent.
My drawing shows the net forces acting longitudinally only (in pitch) assuming a steady flight where their sum are equal to zero (all pitching moments equal to zero).
Following is my further research on tunnels:
The forces on model wings at low speeds are so small that very sensitive instruments are required to measure them with accuracy. In all cases the raw force measurements coming from the instruments have to be corrected to allow for various defects which cannot be entirely removed from any tunnel. If there are any supporting struts or wires an estimate of their effect on the figures has to be made. When the model is fixed to the wall (giving the effect of infinite AR) there are problems caused by interference of the flow in the corner. The corrections applied may be carefully worked out but they are always somewhat approximate. For these reasons the results reported from one wind tunnel always differ to some extent from those originating elsewhere. Even in full-size practice, wing tunnel results are not applied directly. A suitable wing profile may be chosen in the first place on the basis of its comparative success against other profiles in the tunnel, but when final calculations are made, it is assumed as a rule that the wing in service will not be accurate enough to give the same performance. Corrections have to be applied to reduce the tunnel results to those expected in reality. So, wings are built and tested in service and experience is thus accumulated.
< Message edited by adam_one -- 3/24/2005 11:45:05 PM >
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Joined: 10/27/2002 From: St. Charles, MO, USA Status: offline
It does sound as if you have been investigating the wind tunnel, there is hope. Most of what you have presented is the type of section testing that is done. Applying that to a configuration is indeed a large amount of work. When a airplane is designed the designer picks an airfoil and planform, get a fuselage shape and away he goes to the wing tunnel (if he can show his initial predictions of performance are reasonable and good).
The results of a configuration in actual WT testing is excellent. Going back into the good ole days when they were testing full span sections in old creaky wind tunnes - the data was iffy but no more. The roll, lift and yaw data for the rigid airframe may be used straight out of the tunnel. Corrections are made for sting interference on drag and those are pretty accurate - usually within a drag count or two.
The calculations that are made for performance have to have the rigid wind tunnel data corrected for flexibility effects (altitude, maneuver and mach number). There is nothing else to do about that, the guys are stuck with it, and that is normal practice - but it doesn't mean the basic data is iffy. For instance the F-15 configuration was developed in the wind tunnel (granted a lot of changes were made during the wind tunnel testing - it is a great tool) and except for clipping a bit of the tip off the wing was used as developed.
However what I was trying to show you was that the aero forces only are what are measured in the wind tunnel (the list of the items that are available for print out during the test) and that is why I used the notation that I did in the drawing. Notice in the axis systems notation figures that they are all aero forces and moments. Discussions involving aerodynamics, lift and drag, etc can be carried on without bringing into thrust and weight of the airplane as it keeps the discussion to a less complex level. It is not necessary to always talk about the airplane in flight.
Without getting Bruce mad at me again, let me say that the longitudinal aerodynamics can be discussed sufficiently well to know where the forces and moments come. Thrust and weight as vectors in a diagram don't bring any additional information to the aerodynamic balance of the airplane. When you are finally discussing performance and flight stability characteristics the the calculations will need force and weight included obviously. But generally a diagram of CL, Cm, CD, etc. drawn on an airfoil or airplane and without showing thrust and weight assumes that it is a stability diagram, in essence you can think of it as if in a wind tunnel.
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Joined: 10/27/2002 From: St. Charles, MO, USA Status: offline
quote:
ORIGINAL: dick Hanson
IF- an airfoil was round (might be) the lift would be very small as you smash it flatter - the lift increases if it is completely flat - then lift occurs over most of the surface It appears to me - that lift ( a pressure differential) occurs anywhere the pressure is different from one side to the other As you change the AOA -the lift changes -a little then a lot . How do you know where along the chord the pressure differential is occuring? Looks like a moving target to me .
and if you do an outside loop with that airfoil - what do you plot?
Posts: 10204
Joined: 12/12/2001 From: slc, UT, USA Status: offline
I have been doing that - trying to find the link between all of this tech talk and models . To be serious tho - This forum has all but been abandoned by model builders looking for info or simply bouncing ideas about. It is a model forum and I think that unless the general tone of threads relates to models - the "model" surfer will simply look elsewhere . Not that the history of tunnel testing etc., is not interesting - Even the flights of fancy are interesting -- But related to model interests? I am a firm believer in results. In this case the forum has increasingly become a smaller group . Is that the intention? Or is there any validity to my thought.
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Joined: 2/26/2003 From: Stockholm, SWEDEN Status: offline
You are right Dick. I don't think wing tunnel data is so much relevant for the large majority of model builders who scarcely design their models by testing them in the tunnels, especially at such low Re's. Besides, one will tend to believe that lift is only about pressure differentials forgetting how those pressure differences actually are created or just considering this action as a secondary effect.
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 
-bcm 
