Aerodynamic Drag

[Kyle Boatright]

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

[Ernest Christley]

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)."

[Kyle Boatright]

"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

[Mike Rapoport]

Eanest:

How about a related question? On my Helio Courier the fusilage/wing
intersection is slightly less than 90deg. The intersection looks like an
aerodynamic disaster. How much drag would be eliminated with a fairing than
can only come down the fusilage three inches? How about a two part fairing
(one part would be on the door) that could come down 6 inches?

Mike


"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.
>
> --
> 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)."

[Kyle Boatright]

"Richard Riley" > wrote in message
...
> On Fri, 23 Sep 2005 21:57:55 -0400, "Kyle Boatright"
> > wrote:
>
> :
> :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. ;-)
>
> Are your gear legs faired? I assume you have a good set of wheel
> pants.

Yes on both counts. My airplane is pretty slick. The 175 mph figure I used
in my original note is my typical cruise in the 60- 65% power range on an
O-320 fixed pitch RV-6. I'd love to find another few mph without doing
major surgery on the airplane. Sounds like that ain't gonna happen.

KB

[Matt Whiting]

Kyle Boatright wrote:
> "Richard Riley" > wrote in message
> ...
>
>>On Fri, 23 Sep 2005 21:57:55 -0400, "Kyle Boatright"
> wrote:
>>
>>:
>>: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. ;-)
>>
>>Are your gear legs faired? I assume you have a good set of wheel
>>pants.
>
>
> Yes on both counts. My airplane is pretty slick. The 175 mph figure I used
> in my original note is my typical cruise in the 60- 65% power range on an
> O-320 fixed pitch RV-6. I'd love to find another few mph without doing
> major surgery on the airplane. Sounds like that ain't gonna happen.

Probably not. If it was easy, Van's would likely have included it in
the design.


Matt

[Ernest Christley]

Mike Rapoport wrote:
> Eanest:
>
> How about a related question? On my Helio Courier the fusilage/wing
> intersection is slightly less than 90deg. The intersection looks like an
> aerodynamic disaster. How much drag would be eliminated with a fairing than
> can only come down the fusilage three inches? How about a two part fairing
> (one part would be on the door) that could come down 6 inches?
>

Mike, I think you are mistaking me for someone who knows what they are
talking about.

Seriously, though, I read a NACA report from the LARC site a while back
describing an experiment with fairings on a high wing aircraft. I can't
remember all the particulars, and a quick search just now proved
fruitless, but I remember that they realized significant speed gains
with increased fairing sizes. In my *opinion*, the more fairing you can
get the better.

Consider the right wing. The problem with intersection drag is that you
have the air being compressed both from the left off of the fuselage and
up from the wing. Down there in the corner, the poor air doesn't know
which way to go. So like FEMA or a southern city mayor, it chooses to
spin around in circles. We call these vortices, and they eat up a lot
of energy without producing anything. A fairing, even a small one, will
help direct the air, and stop the spinning, at least partly.

Tell you what, Mike, why don't you try and experiment? Roll up a few
sheets of newspaper for filler, and temporarily duct tape in a 3"
fairing. Fly the plane and measure the before and after speeds. It
would make a nice Sport Aviation article.

--
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)."

[Mike Rapoport]

Thanks. I will try the experiment this winter (or at least I intend to try
it ). In addition to the Helio, I have a MU-2 which has large radius
fairings between the wing and fusilage. On the right side of the Helio
there is plenty of room for the fairing to come down the side of the
fusilage before it reaches the window but on the left side the door
restricts the fairing size. It is surprising to me that Helio left the area
unfaired.

Mike

"Ernest Christley" > wrote in message
om...
> Mike Rapoport wrote:
>> Eanest:
>>
>> How about a related question? On my Helio Courier the fusilage/wing
>> intersection is slightly less than 90deg. The intersection looks like an
>> aerodynamic disaster. How much drag would be eliminated with a fairing
>> than can only come down the fusilage three inches? How about a two part
>> fairing (one part would be on the door) that could come down 6 inches?
>>
>
> Mike, I think you are mistaking me for someone who knows what they are
> talking about.
>
> Seriously, though, I read a NACA report from the LARC site a while back
> describing an experiment with fairings on a high wing aircraft. I can't
> remember all the particulars, and a quick search just now proved
> fruitless, but I remember that they realized significant speed gains with
> increased fairing sizes. In my *opinion*, the more fairing you can get
> the better.
>
> Consider the right wing. The problem with intersection drag is that you
> have the air being compressed both from the left off of the fuselage and
> up from the wing. Down there in the corner, the poor air doesn't know
> which way to go. So like FEMA or a southern city mayor, it chooses to
> spin around in circles. We call these vortices, and they eat up a lot of
> energy without producing anything. A fairing, even a small one, will help
> direct the air, and stop the spinning, at least partly.
>
> Tell you what, Mike, why don't you try and experiment? Roll up a few
> sheets of newspaper for filler, and temporarily duct tape in a 3" fairing.
> Fly the plane and measure the before and after speeds. It would make a
> nice Sport Aviation article.
>
> --
> 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)."