Mark wrote>
Not really what Tracy proposed. The Fuel/oil heat exchanger is
dumping
engine oil heat into the fuel, but the fuel is not exchanging this
heat to
the air as Tracy proposes, rather just relies on the the huge flow
rate (A
GE90 usually exceeds 10 tons/hour fuel flow during take off) and
dumps this
heat with the fuel into the combustion chamber.
As others have pointed out, Tracy's plan sounds like airliners
submerged
hydraulic system heat exchangers. Using this to add the extra
cooling
capacity for the hot day climbs sounds good. How do you intend to
regulate
this so you are not increasing the vapor lock risk with nearly
empty tanks?
(Although looking at normal flight profiles - whenever you need
power, you
normally have either lots of fuel, and /or lots of airflow over
the wings).
What is the worst case?
Long taxi to a parking place at Oshkosh after a long x-country -
no safety
concern here?
Missed approach on hot day at high altitude airfield after long
X-xountry -
will this be an IFR RV-8?
Tracy, have you tried putting preheated or cooled fuel in you RV-4
and
getting some data on heat rejection rates? Would be interesting to
know how
much heat you can reject per gallon of fuel in your tank
configuration per
minute.
Do you intend using the stock RV cowl? When you add the weight and
complexity of oil lines to the tank plus the heat exchanger, it
would seem
easier to just use a larger radiator. The RV-8 cowl can't be that
lmited in
volume.
Mark Wrathall.
The vapor lock issue could probably be countered by using submerged
automobile
type fuel pumps. The fuel is certainly a large heat sink, and the
surface has
high scrubbing velocity from the air.
Not in the cards for us composte guys - Vance
Ok I can't hold back any longer ;-)
I used to own a ford tempo. The "geniuses" at ford ran the fuel return line
and the exhaust in the same tunnel in the floor pan. They also used an in
tank submerged pump. No problem normally. Any day where the temp was over 95
and the trip was longer than a couple hours and you had serious problems!
The fuel would get hotter and hotter until it finally began to boil. When
this happened the car would buck and surge like a rodeo bull. At first I
thought I would be smart and stop and put some cooler fuel in the car. This
worked for a while until I had a tank FULL of boiling fuel. Then I stopped
at a car wash and tried to cool the tank with water. That got me about 30
miles down the road. The next thing that happens in this wonderful chain of
events, under the blazing sun, is the pump, which is cooled and lubricated
by the fuel, dies. Not a pretty picture.
Most aircraft which use fuel for a coolant (including the SR71) do so with
the fuel before it is injected into the engine. This actually helps increase
the efficiency of the engine (marginally). In our case, hot fuel into the
engine does not help since it goes in before the mixture is compressed, and
therefore reduces volumetric and thermodynamic efficiency.
Now, all this being said, if you are cruising at 14500 with nice cold air
flowing over the tanks, it is a very different situation than the one in my
bedraggled ford tempo cruising along in the blistering summer heat, but
something to keep in mind none the less. A few gallons of hot fuel left in
the tanks at low altitude in summer could pose a problem with vapor lock.
The other thing that makes me nervous is the possibility of a tank
explosion. In tank pumps are tested rigorously by the auto guys to eliminate
this problem. I'm not sure that we have the resources to make sure that our
installations are safe in this regard. A loose wire due to fatigue or
chaffing, and a tank full of volatile hot fuel vapor and air could ruin your
day. Remember Flight 800? (conspiracy theories aside, there have been
documented fuel tank explosions due to everything from lightning to ground
fueling accidents)
Monty
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