----- Original Message -----
Sent: Saturday, October 04, 2003 9:48
PM
Subject: [FlyRotary] Re: Thick Vs Thin
Radiators - NASCAR
<... The key appears to be
the dynamic pressure available to produce the Mass flow through
the radiator ....>
I was
talking for a while at the Rough River fly-in last week with an engineer who
argued pretty forcefully that it's the STATIC pressure that forces the air
through the radiator core. He said if you want to succeed, you need to
have a big plenum. The inlet needs to expand into the plenum so the
kinetic energy of the air is converted to pressure which then forces the air
through the core. He was adamant that your ramp from the intake
expanding into the plenum can't exceed 7 deg until the x-section of the plenum
is twice, and preferably three times the inlet area. This is so the flow
will stay attached to the ramp and expand (kind of like an airfoil - too much
curvature (as in chamber) and the flow separates (stalls) and you have a great
big eddy of dead air behind the too-radical curve. If your ramp
"stalls", the flow pretty much stops. If you ramp up "gently" the flow
stays attached and expands uniformly and totally into high pressure air in the
plenum. I read stuff that sounded like this from PL's College or
Convoluted Rocket Science a few years ago. As I recall, the P-51 scoop
on the P-51 ramped relatively slowly up into a largish plenum and a very thick
radiator. So the effectiveness of 7" thick radiators would seem to turn
on the internal aerodynamics of the plenum.
But I've already told you more than I know
.... Jim S..
Hi Jim,
Thanks for bring this up because I think the issue
does confuse folks.
I really think we are talking "Apples and Apples"
here. Static pressure for us fliers amounts to the ambient atmospheric
pressure. The equation Pt = Pa+1/2pV^2 gives the total pressure
which is a combination of ambient pressure (the Pa term) and dynamic pressure
(the 1/2pV^2 term) IF there is any. The contribution from the
second term will only exist if there is moving air (dynamic) otherwise it is
ZERO leaving only the ambient pressure term Pa The second term is
composed of the density of the air (p) and the velocity squared
of the air (V^2)
What the second term really means is that
- it is the contribution to total pressure Pt (which is static
or not moving) that moving air makes when its kinetic energy is converted to
static pressure. However, if you do not have any dynamic pressure then
the pressure is the same on both sides of your radiator (Pa). So technically
the pressure that remains once the energy of the moving area is converted to
pressure is static. However, the pressure differential or increased
pressure on the face of your radiator depends totally on this
contribution to this total pressure (Pt) by the dynamic factor
(1/2pV^2). If you plane is sitting still there is no pressure
differential (unless your prop is blowing it into your duct) because there is
no dynamic pressure.
Therefore most folks refer to the pressure build up
(which in the end is static) caused by the kinetic energy of the moving air
(which is dynamic) as the Dynamic Pressure. Technically, I guess we
should call it the "Dynamic Pressure Contribution to the Total Pressure
Equation", but most folks just refer to it as "Dynamic Pressure"
The engineer is quite correct about the purpose of
the plenum. It's purpose is to convert the dynamic kinetic energy of the
air into an localized pressure increase in front of the face of the
radiator. Studies have indeed show that (depending on the type of duct)
that a maximum 7 degree divergence is the optimum to preclude separation and
turbulence in the plenum.
My most recent experimentation was to take my old
plenum inlet which diverged very sharply at many/most points from the
optimum. Basically the inlet simply opened into a box without any
attempt at making the transition smooth. Yet, despite these deficiencies
it provided adequate cooling. However, once I filled in the plenum with
foam and shaped this foam to provide smooth curves from the inlet to radiator
face - trying but certainly failing to maintain anywhere close to the 7 Deg
optimum (the distance from radiator to inlet is simply too short to achieve
that optimum) I found I could reduce the inlet from 24 square inches to 8-9
square inches (a 33 % reduction in my overall radiator intake area - both
combined of 48 square inches) and still had adequate cooling. So
the smoothing of the transition did have a benefital effect. I actually
reduced the plenum volume by over 50 percent (filling it with expanding foam
and shaping it after it dried)
So regarding the engineer's statement.
1. Technically it is static pressure -
however, it results from the dynamic airflow conversion to pressure and is
commonly (if perhaps inaccurately) referred to as Dynamic Pressure. No
dynamic air = No moving air = no pressure increase = no pressure differential
= no cooling (well maybe some radiant and convection cooling, but nothing
close to the cooling provided by the airflow).
2. The K&M duct flow studies, do indicate
that 7Deg is optimum, however, they also have two different duct types and it
dependents on which duct type you use. But, the point here, is while 7
degs appears to be the OPTIMUM, that does not in any way mean that there is no
benefit to pressure recovery by smoothing the airflow even if less than
optimum. Besides who is going to tell your duct its not 7 Deg
{:>)
So I do not see any significant difference between
my statement and what the engineer told you, however you want to refer to the
pressure increase due to the moving (dynamic) air, that is the part that
pushes air through your radiator. Since it is the contribution caused by
dynamic (or moving) air, I will (as do most folks) continue to refer to it as
the dynamic pressure.
Did this help or did I just make it more
confusing?
Best Regards
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com