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.