equivalent flat plate area ??
METM
(or similar phrases) in comparisons of light plane and
automobile drag. My aero- texts shed no light on what
this means exactly.
I can think of a number of possibilities, but if anyone
knows a definition, I'd be pleased to hear it. thanks.
john kallend
Analytical Methods {NWNet}
that while:
Drag Where: Drag = resistance force
CD = ------------------ rho = density
.5 rho Vel^2 Sref Vel = Velocity
Sref = Ref Area
is nondimensional, the Sref is awkward, as how the reference area is
chosen can differ. So if we instead define:
Drag
f = -------------
.5 rho Vel^2
the Sref no longer appears, but f has the units of length squared. If
English units are used, you get the equivalent flate plate area (f) in
units of square feet. Note, this does not mean that a flate plate of
the same area as f would have the same drag - a flate plate has a CD of
roughly 1.17 (in 3-D flow, according to Hoerner and 1.98 in 2-D flow),
not 1 as implied in the equation! We can see this from:
Drag = CD .5 rho Vel^2 Sref
where Sref is the frontal area of the plate. Hence, the equivalent
flate plate area is:
CD .5 rho Vel^2 Sref
f = ----------------------
.5 rho Vel^2
So you can see that the flate plate of area Sref has an equivalent flat
plate area 1.17 times its true area (in 3-D flow)!
-Dave Lednicer
Analytical Methods, Inc.
da...@amiwest.com
Ed Falk
>I have seen references to equivalent flat plate area
>(or similar phrases) in comparisons of light plane and
>automobile drag. My aero- texts shed no light on what
>this means exactly.
It's the coefficient of drag times the surface area of the object.
The name is slightly misleading. An actual flat plate has Cd = 1.17
--
-ed falk, sun microsystems
sun!falk, fa...@sun.com
He who dies with the most friends, wins.
Henry W. Woolard
area" I realized that I had another book that might address the
subject. See Ref 6 below.
McCormick, in Ref 6, states that the quantity that I (and others) have
defined as the "equivalent parasite area" may be called *EITHER* the
"equivalent flat-plate area" OR the "parasite area ". He makes no
distinction between the two terms as I and others have. He does admit
that the connotation "flat plate" is misleading in his useage. As Dave
Lednicer mentioned, this leads to the result that the "equivalent flat
plate area" of a flat plate is 1.17 times its true area.
It has been many years since I been have involved in drag estimates,
so on the basis of Dave Lednicer's posting and McCormick's book I can
only presume that in recent years there has been a change in
terminology from that which was known to me in earlier years. The
exact terminology used is not critical in itself as long as the
participants clearly understand the meaning of the terms. It seems to
me, however, that the ambiguous terminology applied to the relation
given by McCormick just leads to more confusion. I do feel that the
older terminology was more logical.
Summarizing (main points only, details,invariants,etc. not mentioned):
From the older literature:
The "equivalent flat-plate area" is the area of a flat plate having
the same drag as the subject body.
The "equivalent parasite area" is the area of a fictitous body with
unity drag coefficient and having the same drag as the subject body.
From Dave Lednicer (my words, not his):
The "equivalent flat-plate area" is the area of a fictitous body with
unity drag coefficient and having the same drag as the subject body.
From Barnes McCormick (Ref 6):
The "equivalent flat-plate area" is the area of a fictitous body with
unity drag coefficient and having the same drag as the subject body.
The "parasite area" is the area of a fictitous body with unity drag
coefficient and having the same drag as the subject body.
*****??????????????????????****
References
1.-5. See previous posting.
6. McCormick, Barnes, "Aerodynamics, Aeronautics, and Flight Mechanics",
John Wiley & Sons, 1979, p196.
Henry Woolard
Aerospace Engineer, Retired
Fresno, CA
hwo...@eis.calsate.edu
Henry W. Woolard
> I have seen references to equivalent flat plate area
> (or similar phrases) in comparisons of light plane and
> automobile drag. My aero- texts shed no light on what
> this means exactly.
In presenting the value of the parasite drag for a body it is
sometimes useful to compare the drag of the subject body to the drag
of some reference body. Let "CD" and "A" be respectively the drag
coefficient and frontal or cross-sectional area of the subject body,
and "k" and "a" be respectively the drag coefficient and frontal or
cross-sectional area of the reference body. Then EQUATING THE DRAGS of
the two bodies and solving for "a", we obtain
a=CD*A/k (1)
Von Mises, in Ref 3, calls "a" the equivalent frontal area. If "k" is
taken as the flat-plate drag coefficient, variously as 1.17 and 1.28
in the literature, then "a" is called the "equivalent flat-plate
area". If "k" is taken as unity, then "a" is called the "equivalent
parasite area".
The "equivalent flat-plate area" is defined in this manner in Refs 1,
2, and 4 below. The "equivalent parasite area" is defined in this
manner in Refs 1, 2, 3, and 5.
Sorry about the antique references, but that's all I have around.
References
1. Dwinnell, James, H., "Principles of Aerodynamics", McGraw Hill,
1949, pp 248-251.
2. Dommansch, D.O., Sherby, S.S., and Connolly, T.F., "Airplane
Aerodynamics", 4th edition, Pitman, 1967, pp 192-198.
3. von Mises, Richard, "Theory of Flight", McGraw Hill, 1945,
pp95-98. (There is a Dover reprint)
4. Durand, W.F.,editor, "Aerodynamic Theory", Vol. V, Div. O,
"Airplane Performance" by L.V. Kerber, p 237, Julius Springer,
1935. (Reprinted by Dover, Durand Reprinting, and others.)
5. Perkins, C.D. and Hage, R.E., "Airplane Performance Stability and
Control", John Wiley & Sons, 1949, pp 94-97.
Henry Woolard
Aerospace Engineer, Retired
Fresno, CA
Richard Shevell
METMK...@minna.acc.iit.edu (METM) wrote:
> I have seen references to equivalent flat plate area
> (or similar phrases) in comparisons of light plane and
> automobile drag. My aero- texts shed no light on what
> this means exactly.
The equivalent flat plate area of an airplane, also called the
equivalent parasite drag area, is the area of a theoretical flat
plate, perpendicular to the direction of airflow, which has a drag
coefficient of 1.0, and has the same drag as the zero lift drag of the
airplane. It is equal to the Cdp ( zero-lift drag coefficient of the
airplane ) times the reference area (usually the wing planform area).
For further information see "Fundamentals of Flight", R.S. Shevell,
Prentice-Hall,1983,1989. In the second edition it is on page 185.
Richard Shevell
(Ed Falk) wrote:
> In article <1994Aug26.1...@iitmax.iit.edu> METMK...@minna.acc.iit.edu (METM) writes:
> >I have seen references to equivalent flat plate area
> >(or similar phrases) in comparisons of light plane and
> >automobile drag. My aero- texts shed no light on what
> >this means exactly.
>
> It's the coefficient of drag times the surface area of the object.
>
> The name is slightly misleading. An actual flat plate has Cd = 1.17
Small clarification: The area used is the reference area upon which the Cd
is based. For airplanes this is usually the wing planform area. For
bodies, like fuselages, nacelles etc., it would be the maximum frontal
area. If the drag coefficient is based on wetted, or surface area, then
surface area would be correct.
--
Richard Shevell
Email: shevell.leland.stanford.edu