Hi
George,
As you
know, taking heat away from your radiator cores requires sufficient air mass
flow - a number of factors affect this - one of the principle factors is
pressure differential across your core. No pressure differential = no
flow. The primary positive pressure on the front side of the core
comes from converting dynamic energy of the moving air into a local static
pressure increase in front of the core. This is basically limited by
your airspeed and efficiency of your duct/diffuser. The back side of
your core air flow (in most installations) exits inside the cowl.
Therefore any positive pressure above ambient under the cowl is going to
reduce the pressure differential across your core. So once you have
the best duct/diffuser you can achieve on the front side of the core - the
only thing left to increase the pressure differential is to reduce the
pressure under the cowl.
An extreme
example is someone who flies with an opening (such as one of the typical
inlet holes beside the prop) exposed to the air flow. In effect this
hole with little/no resistance to airflow can "pressurize" the cowl and
raise the air pressure behind the radiator cores reducing the pressure
differential and therefore the cooling. Exhaust augmentation is
theoretically a way to reduce the under the cowl pressure by using the
exhaust pulse to "pump" air from under the cowl, thereby improving the
Dp across
the core and therefore your cooling.
While
exhaust augmentation can apparently work - there was a KITPLANE issue back
several years ago on the topic showing several installations where this was
used. However, from what I read (and think I understand), it takes
some carefully planning to get an installation to work correct and the
effort is not trivial. Give the challenges you may encounter (such as
motor mount struts, etc), fabrication of the augmentation exit, the
need to have the exhaust pulse exit at or inside the cowl (or construct an
extended bottom cowl tunnel) means you would have the bark of a rotary in
front of your feet. Also, to gain maximum advantage of these
techniques, it is desirable to have the exhaust velocity at the maximum -
which implies little/no muffling. Having had my muffler back out one
time (at the cowl exit), I can tell you that you do not want to position the
pilot behind the exhaust outlet (in my opinion). It is much quieter
when you have the exhaust exit behind the position of the pilot
{:>).
Some few
people seem to have been able to achieve some degree of success, but even in
aircraft where you have an engine without the aggressive bark of the rotary,
you seldom see it used. The basic reason (in my opinion), is that it
offers few advantages (cooling wise) that can not be achieved easier and
more reliability by other methods. For an all out racer where noise
and discomfort is secondary, it may have some benefit.
Having said
that, it's clear that in some installations it appears to work well (see
KITPLANE issue), but if it were the magic solution, I think many more folks
would be employing it - but, again, just my opinion.
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On Behalf Of George Lendich
Sent: Tuesday, April 27, 2010 9:41
PM
To: Rotary motors in
aircraft
Subject:
[FlyRotary] Re: 20B RV-8 cooling results
Can't
say as I understand Tracy's set- up completely, other than it's toward the
lower end of Rad sizes. I was thinking to myself how I could create a
-ve pressure in the rad outlet to create a suction on the Rad. We all know
how the exhaust augmentation works and I was wondering why we can't do the
same thing with the rad outlets by running the rad outlets inro a larger
outlet fed by outside air. At idle the air is fed by the prop air stream and
at level fight it is fed by outside air stream.
The
outside air could be could controlled by a butterfly - simple enough. I
know there emphasis on using shutter /flaps to control the cowl outlet and I
believe their good at restricting air flow, but I don't know if this equates
to a good -ve pressure behind the Rad. This presupposes the Rads are
completely enclosed for both inlet and outlet air.
75% of my cooling
problems were solved with the oil cooler change I did but still needed
more margin for hot weather climbs. Made the decision to not change
or enlarge the cooling outlet (that adds drag) so went ahead and
butchered the pretty inlets I made.
Ed Anderson's spreadsheet on
BTUs & CFM cooling air required was instrumental in deciding to go
this way. It showed that without negative pressure on the back
side of the rads, there would never be enough cfm to do the job during
climb at full throttle. Negative pressure is what I had when I flew
without the cowl on but oh what a draggy condition that was.
The
old inlets were 4.5" diameter for the radiator and 4.125" diameter for oil
cooler.
New inlets are 5.190"
for the rad, and 4.875" dia for the oil.
This may not
sound like a lot but it represents a 36% increase in inlet
area.
Results were excellent. Oil temp went down 19 degrees
at the test speed (130) and water temp dropped 9 degrees. On 80
degree day and 500 ft msl the oil temp maxed out at 194F at 210 mph which
is way faster than I would normally go at this altitude. Temp was
around 175 at 130. Oil Temp in climb remained below redline
(210) but the temperature lapse rate today made results not very
meaningful. OAT was dropping 14 degrees a minute at 3000 fpm climb
rate.
now back to that nasty composite work to pretty up the
inlets again. They look like large stubby pitot tubes now.
I
hadn't thought of a good name for the RV-8 but a friend in California
recently came up with the winning idea which fit it well.
"Euphoriac" It's a term from a Sci Fi book (Vintage
Season) meaning something which induces euphoria.