Bill Freeman wrote:
Paul,
Here is a discussion of flat plate drag, and they reference
Hoerner on flat plate drag coefficient. Scroll down until you
see the flat plate stuff, the reference is a "72", which is a link to
the Hoerner reference.
http://www.hq.nasa.gov/office/pao/History/SP-468/ch1.htm
Bill
Bill there are some real idiot's out there. A flat plate has a
drag coeff. of ONE by DEFINITION!!!!!!.
Yes you can put some restrictions on it. Only round flat
plates one square foot in area 1/8 of inch thick and flat to 10 micro inches
per inch are to be used. Did they take the flat plate out of the Wright brothers
wind tunnel and compare it to their flat plate and found that the Wright
Brother's flat plate came up deficient?
Is there a size effect? Yes perhaps there might be. At a given speed a 100
square foot plate might have a different C sub d than a one square foot plate.
Nobody is saying that. If that is indeed the case then there would be a frontal
area size correction factor. I think some thing like the Reynolds number
would handle that.
What some idiot at NASA or anywhere else thinks otherwise matters not.
Just because it is on a web site or printed in a book does not change a thing.
A round one square foot flat plate of specified characteristics is here and
forever after DEFINED as having a drag coefficient ONE. Pure and simple.
No if ands or buts!!!!! I don't care if some silly dilettante Ph.D. is trying
to make a name for himself by saying otherwise.
Sheesssss!!!!!
Paul Lamar
Paul - A flat plate DOES NOT HAVE A DRAG COEFF OF ONE BY DEFINATION. An equivalent
flat plate is an imaginary reference to the drag of an imaginary area which would
have that much drag, if
the surface had the dynamic pressure evenly distributede over the face with no
effects of friction or
reduced base pressure. And in no way is a real flat plate eve used to calibrate a
wind tunnel. When peoplew state an equivalent flat plate area for an airplane, they
usually reference the drag at top speed, where
the induced drag is minimized.
According to Bill Freeman, and I have heard nobody dispute this so far, the
Wright Brothers were the first persons to build a wind tunnel and they used a flat plate
as the reference. Therefor the Wright Brothers flat plate was by definition
one. If you can prove otherwise I am all ears. It is like going back and measuring
the dead Kings foot years later and saying; "Oh it is not 12 inches it is really
13 inches."
A very clean F1 racer can get slightly below one
square foot of
effective flate plate drag (Nemisis probly significantly less).
Yes of course. That does not change anything. Say the Nemisis frontal area
is 4 square feet and the Nemisis C sub d is .001 the drag at 100 MPH
is then the drag is .104 pounds what is wrong with that?
A very clean two
place fixed gear plan will
be close to two. Some people calculate out the induced drag which is still there for
a "better" number,
but this does not represent the relationship between power and speed, which i think
is the best
reason to compute this number. Current teaching among the aero heads totally ignores
frontal area
in the belief that it is all skin friction, but they still quote effective flat
plate drag area.
Nahthey are just trying to make a name for themselves. Historical revisionists!
Technically politically correct :-) >
I am convinced
that there are at least 3 kinds of drag in the real total - form drag (how much you
torture the air going
around corners), skin friction, and of course induced drag from lift.
Absolutely and I totally agree with you here. In fact I am willing to
lay down my life to defend your position on this. Well almost :-)>
The three kinds are form drag, skin friction drag and induced drag due to lift.
A flat plate has no skin friction or induced drag due to lift therefore it is
near pure form drag. It is not generating any lift either. The reason it has minimal
skin friction drag is because of external diffusion. Yes I agree that the
Equivalent Flat Plate Area term does not take this into consideration and
lumps it all in to one Cd x S factor but nobody ever mentions or explains that
including NASA. The books like Hoerner don't break it down either.
That is why the drag coeff. of a wing is based on the surface area
of the wing and not the frontal area when it is generating no lift. Skin
friction drag is the primary component in that case. If this were not
the case people would then find it extremely difficult to separate
the skin friction drag from the form drag. For example; where does
separation actually take place along the cord of the wing?
What IS done is the wing C sub d is given on the basis of the surface
area of the wing and a lot of measurements are made at different
Reynolds numbers to sort out the skin friction problem.
As per the curve you printed - Hoerner indicates that a real flat plate has a Cd of
as much as 1.8
at low Reynolds numbers, and appears to flatten out near one at high Reynolds
numbers with
the base vortices doing all sorts of things in between. Hoerner does not spoon the
information
out like one would like to see it in a text book, but reports from a lot of sources
with little
judging of the meaning or validity of the data - A good source tho - Vance
Yes I agree about the low reynolds numbers as well. If you test a flat plate
in molasses the skin friction drag would be very high at low speeds :-)>
BTW Vance you get a C + or maybe a B if I were grading on a curve :-)>
Paul Lamar
Sorry paul - if you still think that somewhere there is an SAE standard "flat plate"
for calibration I would have to give you a D minus :;>) The fact that it is an
imaginary flat plate allows it to work. It is like that weightless, none elastic,
flexible cord that is so common in Physics. and it is a drag value that
would exist if it were represented by a flat area with "Q" uniformly distributed
over its surface.
Incidently, airfoil drag coefficients are really not skin friction either - since they
are not taken against the real surface area which has two sides, but the
projected area, which helps it work with the lift coefficient for simplicity.
It is really a combination of skin and form.
Vance
How do they know that a flat plate does not have drag coefficient of one?
What DID they use as a standard? What is the drag coeff. of a round flat
plate as opposed to a square flat? Each has "Q" differently distributed
yet the majority of data in Hoerner says the drag coeff. is nearly
the same.
I agree with you one the second point but I used that as an illustration
of an area chosen as a reference area. It is mostly skin friction drag at zero
angles of attack.
Paul Lamar
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