Subject: [FlyRotary]
radiator
Hi, fellow rotary
enthusiasts.....I'm getting ready to order a radiator for my 13b powered SQ2000
canard pusher. I talked to Al Wick, and he has a 160 hp Subaru powered
Cozy and is cooling it very well with a radiator that is 17" x 7.25" x
3" deep. Any idea if that size might also cool a rotary?
We all know that the cooling system is
the most frequent problem area in custom engine installations. One reason
is that too often it is done on the basis of ‘what worked for Joe Blow
should work for me’. It’s not that I argue with what works;
it’s just that configurations are very different, and the applications
(air speeds) are different. Ducting and air flow are probably the most
variable. Right off hand, Paul, I’d guess that a 17” x
7.25” x 3” would work for roughly a continuous 100 hp climb on a 90
degree day.
The cooling system can be
‘engineered’ without too much complex math to give you pretty good
odds on having it work when you get in the air. Ed has taken a good step
to putting together some analysis. When I get a chance I’ll write
up how I engineered my system; although it would be safer to talk about that
after I get it proven in flight, wouldn’t it. But maybe some
guidelines (and opinions) could help.
We know how much heat we have to reject
from the rotary engine coolant, and from the oil. We know about what the
flow rates are, we know the specific heat of water (and 50/50 eg &water)
and we know the specific heat of air. Radiator designs have lots of
variables, but there is plenty of experience to tell us pretty well what is
optimum; so we can compute what air flow we need for a given power and air
temp.
Without going too much into numbers,
I’ll say that the stock Mazda water pumps are fine for a NA 13B. You need
about 0.2 gpm per hp at 6000 rpm. Full power climbout is the limiting
condition; and my ‘opinion’ is that we should design for at least 80%
power on a 90 degree day. We have to compromise if we don’t have
adjustable ducting because we don’t want the drag associated with full
power capability for our cruise flight.
Interestingly, with the above
assumptions, the air flow you need in cubic ft/min (cfm) is very close to the
heat rejected from the coolant in Btu/min. And that’s about 27/hp
for the coolant and about 11/hp for the oil. And if you know how fast
you’re going you can calculate the inlet area needed. Take the
required cfm, divide by the forward speed in ft/min, divide by a scoop
efficiency of about 85% (.85, maybe less if you are using a NACA scoop), and
there you have it – inlet area in sq. ft. Multiply by 144 gives you
sq. in. Yes, you need to know how get from there to dimensions of
whatever inlet shape you’re using.
Inlet diffusers (expanding duct) are
important to convert the dynamic pressure into static pressure that can force
the air through the radiator. This depends on a whole bunch of factors,
but for a tractor plane you get some external diffusion up there on the front
of the cowling, and on pushers with the scoop toward the back you
don’t. So on a pusher the duct needs to slow the air by at least
about a factor of 3-5, that is expand it to 3-5 times the area (. On a
tractor type, maybe 2-3 will work. So now you know, roughly , the cross
sectional area of your radiator. Using something close to the K-M duct
expansion profile (attached) is good.
The specifics of rad design are
complex. You know about the old thick vs thin argument, as though that
were the only variable; but it isn’t. You can have thick if the
core density (fins/in, tube spacing, etc.) is low and you have a high diffuser
ratio. But my impression is that with the right amount of coolant flow and the
right amount of air, there is a fair amount of latitude. There is plenty
of data from the racing world that suggests that a rad thickness of about 2.5”
may be optimum; 3 will be fine. The rad area times thickness gives you
volume.
Somewhere around 2.5 – 3.0 cu. in./hp
will be fine for cruise. But for that climbout on a hot day, you’re
going to need more; probably around 4.
And don’t forget; to keep the drag
down you’d like to liberate that cooling air back into the free stream at
something like free stream velocity. So some of that static pressure you
recovered in the inlet duct can be used to accelerate the air in the outlet.
Outlet area should be on the order of 1.5 times inlet area, and try to let the
air out in a low pressure area.
So that ought to get you into the
ballpark, and if you do things right it will work just fine. You can do
some tuning after you get the thing flying.
Al