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.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."