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.