“Has
anyone done any work with cooling drag?”
Walter and the GAMI boys have done nice experiments and
learned a lot, particularly about reversed flow in the cowl, leakage out from
inlets and other neat stuff. I have spent a lot of time over the last few
years trying to learn more since my original training included a lot of heat
transfer, internal fluid flow and such, and I think cooling drag is about the
last place to reduce drag on a Lancair IV in cruise. (Reno racing with high G turns creates
different conditions and requires more mods, but I digress.)
I have corresponded off line with several of our members on
this topic, and a lot of good work is being done, much of it covertly to gain
some benefits in racing.
Cooling drag is at times a counter intuitive business, and
like much in aviation, there is a lot of mis-information because it can be a
bit complex.
Observation: if you want to wring the most out of the
airplane, it will take a lot of work. I am into cowl rebuilds, cowl
flaps, cooling plenums, discharge nozzles and such for more than 300 hours and
I am not done. I have had to make a lot of tooling and scratch build a
lot of composite parts, all time consuming. So this is a business
for fanatics only. Benefits? On a Lancair IV, my
current guess is plus 10-12 knots.
Perspective: One an aspirated C210 or Bonanza class airplane
at 170 knot cruise, cooling drag is reported at about 7%. These planes
have a flat plate drag area of about 4.5 square feet, more or less. A
Lancair IV has a flat plate drag area of about 2.1-2.2 square feet, about
half. I think the Legacy comes in at about 1.7-1.8 square feet. Therefore,
if you use the same method of cooling as for the spam cans, cooling drag will
be approximately 14-15% of total drag.
If you are turbocharged, pressurized, and at 25,000 feet, it
is probably more like 20-25%, maybe more because of cool air and added flow
from intercoolers. And it is harder to minimize.
Rule of thumb for small changes: a 2% reduction in
drag will give about a 1% increase in speed. So if you can get that 14%
of drag down to, say, 4% (probably not possible, but reach for the stars!) then
the 10% drag reduction gives a 5% speed increase. For the Legacy and
aspirated LIV guys (like me) who are in the 200-240 knot range at maximum
cruise, 8000 feet, you could get 10-12 knots. So that brackets
expectations.
Some other rules of thumb.
1)
Flow
only the air the engine needs to stay cool. That means no leakage (and
there are leaks everywhere, big and small in a stock installations) and restricting
the airflow to the necessary amount in cruise. The easy and less
effective way favored by most is to choke down the inlets. The more
effective but much more complicated way is to use cowl flaps to throttle the
exits. Also, this means don’t overcool the engine. I will let
Walter speak, here, but I think that the choke in the cylinders is sized
assuming that the CHT for cruise is around 350-400F. I think 380F is a
good trade off number, but I may be wrong (since it happens all the
time). J
2)
No
leakage means a top plenum, attention to leakage detail, and in particular
extreme attention to detail eliminating leakage out front to behind the spinner,
usually one of the largest offender areas.
3)
Hard
core fanatics will do the following:
- Reposition the inlets outward from the spinner to get
out of “fouled” air coming from behind the prop shanks and spinner
boundary layers which are thick because of centrifugal and shear
effects. Legacy does this. LIV does not. Get the inner
edge of the inlet at least 1.0-1.5 inches away from the spinner.
- Reposition the inlets upward about 1.5 inches above the
crankshaft centreline to straighten the S bend for the flow as it enters
then moves up above the engine. Columbia does this as well as moving the inlets outward.
- Size the inlets so that the inlet velocity is about 0.4
of the free stream velocity to get good pressure recovery IN FRONT of the
inlet where it is frictionless. This velocity ratio will get you 84%
of the total ram pressure before the flow enters, and then the diffuser
and flow inside is less important. This means about 6 inch diameter
for aspirated 550s, and 7 inches for turbo 550s.
- Streamline the first few inches behind the inlet to reduce
residual losses.
- Use a plenum. Make its volume large to minimize
internal velocity above the engine and thus minimize losses and help make the pressure distribution
more uniform.
- Don’t worry about the flow below the engine UNTIL
you approach the exits, then worry a lot.
- Have nicely contoured exhaust nozzles to accelerate the
flow aft and get a bit of thrust recover to offset the momentum loss that occurred
when the flow slowed in the first place.
- Watch for leaks out nose gear doors, hinges, and other
places where the flow can squirt out and now backward.
- If you are a fanatic and use cowl flaps to control the
cooling flow, you will pressurize the cowl a LOT (necessary to accelerate
the flow backward) and it will try to get round like a balloon. On a
Lancair IV, this means the top of the cowl will rise a lot, enough to let
conventional cooling baffles above the engine to flop back under the
influence of air pressure, and let air escape over the top of the baffling.
This creates flapping rubber baffles against the cowl. I am told
that the resulting sound is never forgotten causing immediate secretion of
large amounts of bodily fluids.
If folks want to learn more, I am willing to share what I
know, but like Brent, keep in mind I am an idiot, and your mileage may vary.
Fred Moreno