SNIP
In general, I have a lot of work to do on drag reduction. I was talking
to
a local RV-6 builder who has the second fastest RV-6 at 238 MPH (behind
Tracy Sailor) and got some interesting insights. When I asked him how
he
learned the best way to improve drag, he told me the best thing he
learned
was to ignore the aeronautical sacred cows and start looking at the
airplanes that go fast (he goes to all the Reno races). As an example,
he
pointed out his wing intersection fairings. He doesn't have any, just a
flush fit with a tiny rubber gap seal. My plane has large wing
intersection
fairings designed using the results of some NACA study on intersection
drag.
They look cool but he said if I wanted to go fast I should remove them.
I'm
going to try this and a bunch of other things he pointed out.
Tracy Crook
rws@altavista.net
www.rotaryaviation.com
Come on Tracy quit slandering the engineers. You are one :-)
Some guy told me those root fairing work better at high altitude
hence higher angles of attack hence higher lift coefs. More
skin friction at high speed lower angles of attack.
They are a trade off. I buy that. Why do you think heavily loaded
747 have them and jet fighters don't?
Paul Lamar
Tracy Crook wrote:
Not trying to slander engineers, just saying that they need to re-think
their theories in the face of contrary evidence. Don't know if the 747 /jet
fighter analogy is applicable. We don't fly anywhere near the speeds where
compressibility is factored in.
The picture of the wing root fairing you sent out is very instructive. Note
that it does not go forward of the mean aerodynamic chord. My NACA
designed fairing goes all the way around the leading edge with a fairly
large radius. The RV-6 (180 HP Lyc) builder I mentioned tried this same
fairing and found it cut 7 MPH off his top speed. When he suggested I
remove mine he did say that the fairing around the trailing edge is
beneficial in some phases of flight. The rest of it was nothing but drag
during any part of the flight envelope. Hard to argue with him. Unlike any
of the other RVs I've flown with, he goes significantly faster, stalls
slower, and burns less fuel at any given airspeed. For that kind of result,
I'll trash the NACA study and never look back.
Tracy Crook
rws@altavista.net
www.rotaryaviation.com
It is not about compressibility Tracy. The theory works fine. Nothing wrong
with the theory. The simple fact of the matter is at 200 MPH
the dynamic pressure is about 104 pounds per square foot at sea level. That means with
a 100 square foot of wing area and a gross weight of 1500 pounds the Cl is only
about .15 at sea level. That also means the angle of attack is only about 2 degrees
or so depending on the camber of the airfoil used. The effectiveness of the
fillet is probably a strong function of the angle of attack so at high
altitude where the air density is half that the angle of attack will be
near double and the wing fillet may then show an advantage over no fillets.
Therefor when you talk about how well a modification of this nature works
or does not work you must also talk about a lot of other factors such
as airfoil used, wing aspect ratio, fuselage configuration, wing loading
and last but by no means least... altitude. In the case of one RV verses
another all of the first things mentioned are the same so altitude becomes
the main variable.
Anybody that claims the engineers are wrong about wing fillets does not
have or understand the whole story on the NACA study. What were the factors
such as airfoil used, wing aspect ratio, fuselage configuration, wing loading,
etc., etc, and altitude?
Get a copy of the classic book Theory of Wing Sections by Von Doenhoff or
Horners Fluid Dynamic Drag and read up on it. Paul Lamar
Paul Lamar
I reviewed Hoerner on this subject and he states on page 8-11; "At any rate,
the interference drag [between wing and fuselage] approximately increases
as the square of the lift coefficient." Running the above numbers .15 C sub L times
.15 is .0225 and .3 times .3 is .09. That is .0225/.09 or four times more
interference drag when the air density at high altitude is halved and the
lift coefficient is doubled. In a case like that the increased surface area
of root fillets could be offset by a reduction in interference drag with
a net reduction in drag.
Again all this this is highly configuration sensitive.
It is well known that a mid wing on a round fuselage has less interference
drag than a low wing on a slab sided fuselage. As I recall a high wing
on a slab sided fuselage is better than a low or mid wing.
Paul Lamar
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