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.