Aerodynamic Drag

"Ernest Christley" wrote in message
om...
Kyle Boatright wrote:
Help me figure out how much speed my RV can gain by fairing the Pitot
Tube.

The pitot is made of 3/8" OD aluminium tube, with a 6" length
perpendicular to the airstream. That gives a surface area of 3/8" x 6" =
2 1/4 sq inches. My understanding is that the Cd of a cylinder is .5 and
the Cd of a faired shape can be as low as 0.005, but let's assume 0.05
for the faired shape.

The effective area is .5 x 2 1/4 square inches = 1.125 square inches for
the cylinder.

and

0.05 x 2 1/4 = 0.1125 square inches for the streamlined shape.

The difference is 1.125-.1125 = 1.06875 square inches. Let's call it 1
square inch...

Assuming I'm in the ballpark so far, how much power can I "save" at 175
mph by streamlining the pitot, which eliminates essentially 1 square inch
from the aircraft's effective frontal area?

KB





Kyle, any answer you get will be further complicated by the modifications
in intersection drag. But ignoring that...

From http://142.26.194.131/aerodynamics1/Drag/Page4.html , the equation
for parasitic drag is:

Dp = CDp x S x ½ r V^2

Essentially, once you remove the drag of the unfair pitot tube, and then
accellerate, Dp will come out the same. You're using the all the horses
the engine has to make the plane go faster, vs dragging a pitot tube
around. So:

CDp x S1 x ½ r V1^2 = Dp = CDp x S2 x ½ r V2^2

I only marked S and V with ones and two's, because everything else will
drop out of the equation (making a BIG and unlikely assumption that the
drag coefficient doesn't change), leaving:

S1xV1^2 = S2xV2^2

But the useful form of this for your purposes is:

V2^2 = S1 x V1^2 / S2

Now, your fairing the pitot is only one part of the entire airplane, and
the entire airplance has to be considered to determine the increase in
speed. I have no clue what the surface area of an RV of any sort is (but
a Delta is 183ft^2 8*) Assuming your RV has the surface area of a C-172,
174ft^2, you've just dropped 0.007ft^2. Plugging the numbers:

V2^2 = 174 x 175^2 / 173.993
V2^2 = 30,626
V2 = 175.00352

Are we ready for Reno yet 8*) You'd probably double this increase by
cleaning up the intersection drag. If the tube is located in a position
that will cause it to produce rough air for everything behind it, there
would also be some (insignificant) benefit to be gained there.


Fortunately (unfortunately?) the pitot tube is probably the easiest "big"
thing on the airframe I can streamline without hacking up the cowl to reduce
cooling drag or taking some other *extreme* measure. I had considered
fairing the pitot, the two fuel vents, the two fuel strainers, and the 4
aileron brackets that protrude into the airstream. My guess is that all of
these items together only amount to maybe 3 to 5 times the drag of the pitot
tube. Bottom line, if the combination of our math skills is correct, I'm
lookin' at less than a 1/2 mph improvement? If that's the case, I've got
MUCH better ways to spend my time. ;-)

As to your assumptions, I'd guess the RV has half the surface area of a
C-172. A couple of sources indicate the RV's flat plate area is ~2.3ft^2.

KB