On 2/27/07, Bob White <rlwhite@comcast.net>
wrote:
I haven't said much about ducts as I
don't have any experience except
that my first installation doesn't work very well. They have been on
my mind quite a bit lately though as I'm going to have to redo the
system. My take from K&N was similar to yours Thomas. But maybe Ed
has made an advancement.
I'm thinking about putting the rad and oil cooler horizontally next to
each other with the wedge duct as shown on Paul's typical cooling
layout picture. Except I think my radiator/oil cooler will be closer
to the front than shown. So i was just going to run the wedge opening
straight out to just behind the prop.
I'm also not fond of the exit as shown, and plan on something similar
to the wedge on the bottom but with more room.
Anyone want to speculate on my chance of success.
Bob W.
Bob,
I give you a very high chance of success. The wedge inlet is
almost as efficient as the K&N and is a heck of a lot easier to
install (much of the reason I chose it for my latest iteration).
Paul's website shows the wedge being less efficient as an exit,
but I had a modified wedge exit and it worked fine..
On Tue, 27 Feb 2007 13:52:26 -0500
"Thomas y Reina Jakits" <
rijakits@cwpanama.net> wrote:
> Hmm,
>
>
if I remember right from reading what I have from K&N - if you do
not have the optimal lenght available you start to cut it from the
intake - take radiator and apply the optimum duct, measure from the
radiator towards the intake whatever distance you have available and
cut the duct there....
>
Thomas, I agree with you and Ed on this point. That is the
optimal way for pressure recovery and airflow, but will be high drag if
not all that cooling is needed.
> Dave,
> are you a Navy-flyer or a medic?? :)
>
Ex Navy Flight Surgeon.
>
> Hummm, Dave, perhaps my understanding of what it takes to keep
the boundary layer attached to the duct wall is flawed.
>
> From what I believe I understood regarding airflow
in a duct is that the pressure recovery both aids and hinders the
boundary layer's attachment to the duct wall. The pressure build up
(area of slower molecules) tend to push and keep the boundary layer
pushed against the wall of the duct as it curves out - at the same time
it is slowing down the boundary layer. So its the point of separation
is (at least in part) contingent on how much speed the boundary layer
has enabling it to push how far into the pressure recovery area -
before it ultimately separates. The further the better is my
understanding.
I agree with everything except the pressure causing the
separation. The pressure aids in boundary layer adhesion except for
possibly some small effects on viscosity and Reynolds number.
>
> My understanding is
that in a duct - it is the recovery pressure which builds in the
expansion area just before the core. This "high" pressure area will
"push" back on the boundary layer causing it slow and eventually to
separate from the wall. . However, if you keep the boundary layer
speed up it pushes further into the pressure recovery area following
the duct curve before the "back pressure" slows it enough to cause it
to separate.
>
I still don't get why slowing the boundary layer would make it
want to separate. This is a laminar vs. turbulent issue. Slower is
better for laminar every time.
> Also the speed of a molecule
in
all random directions is much, much higher than the component imparted
by the airspeed - about 1100 ft/sec at sea level as I recall compared
to about 40 ft/sec in the duct.
True, but it is still subject to fluid flow dynamics and the
speed of the air (airplane) does make a difference.
So my interpretation is that (at
least
in a duct) its the back pressure of the recovered pressure that causes
the separation - not necessary the curve of the duct alone although
that certainly contributes to the pressure recovery.
Not buying it yet..
That being said,its clear that the
three factors (duct curve, expansion area and separation) really go
hand in hand.
Agree totally
The greater the curve the more
pressure recovery occurs and the greater the tendency for separation.
The higher the velocity of the boundary layer the further it can
penetrate into the higher pressure area before being slowed and
separation occurs.
>
greater curve = more separation = LESS pressure recovery
> There is NO doubt that having
a
longer duct would improve the situation. However given I only had 3
-6" my take was that speeding up the air (and boundary layer energy)
would ensure it penetrated deeper into the bell shape before the
pressure recovery caused separation. But, as I have often stated - I
could be completely wrong about what I think I understand.
You don't want boundary layer to penetrate, you want laminar
flow
to penetrate. Boundary layer is turbulent and slow (yielding less
pressure recovery). The central laminar high velocity is what you are
trying to keep as long as possible. It has to terminate sometime
before going through the rad. So the idea is to minimize separation
and boundary thickness as long as possible, keeping the high velocity
non-boundary air.
>
> You are after-all the
Navy flyer and I know they cram a lot of areo into Navy pilot's
heads. Me- I'm a electrical engineer, so what I know about
aerodynamics is what I have read (and think I understand).
I'm not claiming to be educated on the subject. And Navy flight
school was certainly no help. They are still teaching that lift is
derived from air flowing faster over the top of the wing. I get
my gestalt from my undergraduate degree, Atmospheric Sciences (way too
much fluid dynamics and stuff that I was not that good at anyway).
More importantly though, I think there is no real way to test
WHY
one duct works and one does not, you can only test WHICH ones work.
The theories are just conjecture on anyone's part. Measurements of
say, pressures at every point in the duct, will of course help refine
the theories but they are still just theories - no matter where you
hear them.
That being said, someone must have hit upon a theory that is
closest to accurate (mine appears to be second best... according to
anyone I happen to be discussing the issue with at the time :-)
And someone else must have hit upon the best duct design given
the
conditions required to cool a rotary via the stock inlet of an
RV-6, and that appears to be you!