Lot's of true statements. But
there's one vital component that's overlooked. If you look at crash history, you
may notice that one of the main causes is marginal design. Designing systems on
the edge of failure.
We have excellent example from
forced landing just a few months ago. The guy that experienced overheating after
changing to Evans coolant. If he had robust cooling system, he would not have
considered changing coolant mid journey. Marginal cooling contributed to
this decision. Forced landing resulted.
I require myself to have
compelling reasons to make a design change to any engine system. So if I
minimize the hose diameter, minimize the radiator surface area. What do I
gain? Likely I save a half lb. So for that trivial weight advantage, I
add risk every flight. I lug along a spray bar to compensate. I
would have to watch climb rate, make sure I don't fly on hot
days. This is the perfect setup for failure.
Strongly encourage robust cooling
design. Don't let perceived value of weight reduction lead to risky
decision.
The cause of the problem you mention is the change
to Evans coolant without a proper test program to see if it was adequate. Being
on a journey with a marginal cooling system in the first place. And not fixing
the cooling system during the flight test program.
The key word here is "marginal". I am not
suggesting a marginal design, I am suggesting a properly engineered and
tested one. The thing I did not talk about was the aerodynamic design
of the cooling system, only the heat exchanger sizing. Even if you
use 3in hoses and a great big thin radiator but a
poor aerodynamic design, you will still have an inadequate cooling system.
This is where most of the installations drop the ball. There is no magic
size of this or that to give optimum performance. It must
be properly engineered, for each specific situation with all components
designed to work together. The math is not that hard. Even the aerodynamic
design can be done with little more than:
P = 1/2 rho x Vo^2
Where:
P is the available dynamic pressure
rho is the density of air at your
altitude
Vo is the TAS of the aircraft
You also need:
Mdot = rho x Vi x Ai
Where:
Mdot = mass flow of air
rho = density of air
Vi = Velocity into the inlet which is usually not
equal to Vo
Ai = area of the inlet
This is all you need.
To find the heat rejected by the engine look up the
drag curve for the airplane and see what Hp you need at a
given speed. Multiply this by 1.2 to allow for 80% prop efficiency. For the
rotary you will need to reject about 2/3 of this value in the water and 1/3 in
the oil. The Renesis may need a little more on the water side, so add 20% on the
water just to be safe.
Thanks to Tracy, we now have some idea of the delta
T for air using the thick radiators. The one remaining assumption is the ratio
Vi/Vo. It would be great if Tracy could get some data on this. Even without the
data you can look at a conservative range...say .5 to .8 so you at least
know where you will start to have trouble. Adjust
accordingly.
It is simple math. How many people do this? How
many people actually look at what the pressure drop in a given length of hose is
at a certain mass flow of water? This is engineering design.
Everything else is just guessing.
If you do this at various flight conditions and
size for the worst (reasonable) case. Then properly test and evaluate the
system, You will not have a "marginal" design.
Using some simple math will get you in
the ball park, to see if what you are considering is even reasonable. Ground
testing can then be used to quantify things like pressure drop across the
radiator for a given flow of air. Part of your go no go list on test
flight take-off is: have I got the required pressure ahead of the radiator?
No....then abort the take off and regroup. Don't just take off and pray. Do your
flight test on a cold day. Have a spray bar just in case. Expand your flight
envelope into higher power, higher OAT and lower speed. Fix problems as
necessary. Don't conclude flight test until you have explored the
whole flight/operational envelope and are satisfied with the
results.
What is the worst reasonable case?
Some would say it is full power climb at VX on a
hot day.
I don't feel that way. I prefer a spray bar and a
properly sized radiator for where the airplane spends it's time.
The spray bar is only used in full throttle climb
at Vx on a very hot day with an already hot engine (a safety feature to get you
out of trouble which should be avoided operationally). Or while idling on the
ground for extended periods of time at 100F OAT. How often do you do these
things? What percentage of flying is this? If you design a properly sized system
for these conditions it will be 3X larger than necessary for normal
operations. The type of heat exchanger needed is entirely different than
the dense high pressure drop type needed for cruise. It is a large area thin
radiator with low pressure drop. You will need an electric fan to make even this
work for extended periods on the ground, especially downwind taxi.
During cruise you will have to accept a massive
drag penalty for perceived safety. I say perceived, because a properly sized and
ducted thick radiator will work just fine once you are over 100 mph or so. You
just can't climb at full throttle at 60 mph all day and expect it to work.
The large thin radiator greatly complicates the aerodynamic ducting, and
packaging design. You can't escape the Q=mdotCpDT equation. You just went
from DT=125 for the thick rad to DT=50 or less for the thin one. Now
you need a much greater Mdot air for the same Q. So you need a huge inlet, an
even larger exit. If you use the thin giant radiator and an inlet that
is too small, it will not cool as well as the thick radiator with the same
inlet. Assuming you do get the inlet and outlet sized properly,
you are flying around with a barn door attached to your airplane all so
you can amaze the crowds by flying around in slow flight at full
throttle on a 100 deg day without the use of a spray bar. No real gain in
safety.
We are all design engineers now. At some
point we must decide what we are designing, an airplane or a tow
truck.
Do whatever makes you happy.
Monty