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Wonderful stuff on Lancair putting a Walter 601 in a Lancair IV. There are
two little things that we should be aware of before we all rush off and toss
our piston engines in the trash. The first is SFC, the second is power at
altitude.
Turbines are great things: air goes in one end, gets compressed, fuel is
added and burned, pressure and heat turn turbine blades which give us HP and
keep everything running. Heck, the thing even takes in excess air to cool
itself. What could be better?
Efficiency in all heat and combustion engines is limited by the maximum heat
and pressure differential that can be developed in the working gas,
multiplied by the expansion ratio in the power producing areas, minus all
losses and accessory loads. Looking at turbines we find that compression and
expansion in the turbine engine is limited by the number of "stages" and the
compression/recovery of each stage. Stages are made of airfoils, airfoils
cost money, cheap turbines don't have many stages, cheap turbines have poor
efficiency. Big airliner turbines are among the most efficient engines yet
made. They have 5 to 12 compressor stages and 4 to 8 turbine stages. Small
turbines usually have only 2 or 3 compressor stages and 1 to 2 turbine
stages. More would cost too much money, so efficiency is sacrificed for
lower cost. Most sub 1000 HP (sea level) turbines have SFC in the range of
.55 to .8 lb./HP/hr. (lb.fuel per HP per hour) compared to a TSIO-550 at .42
to .46 and a modern piston engine at .35 to .41. Triple turbo compounded
piston engines have achieved SFC below .30. This is a far cry from a
Walter's .65.
So what happens when we replace a 350 HP takeoff - 263 HP cruise - .44 SFC
TSIO-550 with a 700 HP takeoff - 315 HP cruise - .65 SFC Walter? We go 6%
faster in cruise at 25,000 ft. and burn 77% more fuel doing it!!! What a
GREAT trade off.
Why the low power at altitude? Well, the only way to vary compression and
expansion on turbines is to use axial stages and variable vane compressors.
These were pioneered on the J-79. But varying the stators can only affect
compression so far and small, low cost turbines usually don't have this and
rely on at least one centrifugal stage (which can't use variable stators...).
So compression is fixed. This means that power falls off with atmospheric
pressure. This means that the power of a 700 HP (sea level) Walter is only
315 HP at 25,000 ft. Ooops.
Turbocharged piston engines have wastegates. They run nearly normally
aspirated at sea level (as long as we are talking about GOOD piston engines
and not over boosted certificated bombs). As you climb, the wastegate closes
and the turbocharger picks up the loss in atmospheric pressure. Back
pressure in the exhaust is less than people think since the backside of the
turbo is seeing the lower than sea level atmosphere. At altitude, a turbo'd
piston engine can achieve many times the compression and expansion ratios of
a turbine. With them comes higher power at altitude and lower SFC.
Oh, yeah. Don't forget that because the cooling air is being pulled in the
front of a turbine engine with the working gas that max power becomes
temperature limited in hot or high conditions. Lots of modern turbines are
engine control protected from meltdown. Older ones just let the pilot look
at the TIT as the throttle is advanced. Of course, there's a little lag in
there that can allow you to push up the throttle and melt your turbine stages
before you catch it, but that's only a $100,000+ mistake for a whole new hot
section. We may get cooling limited with our piston engines, but the lag is
longer and the consequences of a few seconds of overtemp is usually not loss
of the whole mess.
The Walters that are coming over here for bargain prices ($45k to $80k) are
run out from low altitude commuter airline and cargo service. They are not
protected from throttle transient induced meltdowns.
Now I hear all sorts of wonderful things about the new Williams FJ-X.
Williams has historically achieved better SFC than average at the expense of
even less power at altitude. At least his engine controls protect against
meltdowns. Having seen it, I have to ask how Williams expects this to be a
useful aircraft engine without any accessory drives. The only accessory
power on the whole engine comes from one puny starter/generator. No
compressor bleed air, no hydraulic pump, no vacuum pump, no dual alternators,
no de-ice system, no air conditioning, etc. Worst of all, no provision for
ever adding them! Everything gets driven electrically and is counted
separate from the engine weight. Not really apples to apples, and it gets
worse when you find out that the rating on the one starter/generator can't
run all the accessory loads of a TSIO-550.
As a professional aerospace engineer and aircraft designer I cringe every
time I hear about how great small turbines would be for aircraft like the
Lancair IV. While I rail against the lack of progress in certificated piston
engines, I am working with a manufacturer of modern technology engines that
achieve low enough SFC and high enough power at altitude to dust the utility
of the turbines in light aircraft. TCM's efforts in EFI are at least a step
in the right direction. Look at what you REALLY get from a small turbine
with accessory loads at altitude vs. what you can get from a turbocharged
piston engine and it will surprise you. The small power gain vs. the fuel
cost is rarely worth it below 500 HP at 25,000+ ft.
Eric Ahlstrom
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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