...snip,
Paul,
While on the supercharger subject: a fellow on a Yahoo list is
experimenting with a 1/2 turbocharger. He removed the broken
exhaust side, and added a cog belt drive -stepup 1:10 ratio-
in it`s place. He
only wants 2-5 psi, so the compressor speed is adequate.
A really cheap supercharger.
Do you think such a concept will be (remotely) efficient and
workable?
-
Erik Snyman
Messina
South Africa.
You pay a fuel penalty with all super chargers as opposed
to turbo chargers. All the HP to drive the supercharger
comes from the net mechanical HP of the engine. In other words
if your super charged engine is generating 200 HP net at the
e-shaft the engine is burning 220 HP worth of fuel.
The BSFC does not change. At a BSFC of .5 the fuel burn
is 110 pounds an hour or 18.33 GPH.
If your turbo charged engine is making the same net 200 HP
at the shaft the engine is burning only 205 HP worth of
fuel or 102.5 pounds or 17.08 GPH. The supercharger is
costing you over 1.3 GPH in this case. The reason is the
turbo charger is extracting otherwise waste
HP from the exhaust. A step towards turbo compounding.
It gets even better at altitude for the turbo charger.
Note the efficiency of just the compressor is the same
in both cases because he is using the same compressor.
Centrifugal compressor efficiencies are the highest in
the compressor industry. Also the mechanical drive for the
super charger is not 100% efficient. If the compressor is 80%
efficient and the drive 98% efficient the overall super charger
is only .98 times .8 or 78.4 % efficient.
Paul Lamar ...No rotor no motor.
Paul,
A turbocharged engine is more fuel efficient than a supercharged
engine, as you said.
However, that's not the whole story for airplane efficiency. A
supercharged airplane can be more fuel efficient than a normally
aspirated plane -- although not quite as efficient as a turbo
plane.
The reason is that a supercharged engine can let us to fly at
higher altitudes where dynamic pressure is lower, so we can
fly faster at
lower power level. This results in more range for the amount
of fuel
burned, or better miles per gallon.
Let's take a Cessna 182 for example, which has a 230 hp
engine and
cruises at about 145 knots at 8000 feet at 75 percent engine
power.
Dynamic pressure at that speed and altitude is 55 pounds per
square foot. The 182 has an equivalent flat plate area of
about 5.5 sq-ft,
so total drag is 302.5 lbs.
With a supercharged engine we can cruise easily at 16,000 where
dynamic pressure is only 42 psf at the same 145 knots. Our total
drag is only 231 pounds. We can reduce power to about 55
percent to
fly the same speed.
Even though it is costing us a little bit of power to run the
supercharger, the reduced power setting more than makes up
for it.
We can run through the numbers quickly to see just how much:
At 8000 ft, our drag is 302.5 lbs. Since thrust must equal
drag in
straight and level unaccelerated flight, our engine and prop
must be
making 302.5 pounds of thrust.
Thrust is simply power available divided by velocity. So we can
calculate power available by multiplying our thrust times our
velocity:
Power available = 302.5 * 245 fps / 550 = 135 thrust horsepower
(We divided by a constant of 550 because one hp equals 550
lbs. * 1
ft.; or 1lb * 550 ft).
The actual engine horsepower is just thrust horsepower
divided by
our propeller efficiency so if we assume prop efficiency of
0.8 we
get 135 THP / 0.8 = 168. This is 73 percent of the 230 hp
engine's power. Assuming a fuel burn of 0.45 pounds per
horsepower hour, we
find that we are burning 75.6 pounds per hour, or 12.6
gallons per hour.
Now flying at 16,000 feet with our supercharged engine, we
have a
drag of only 231 pounds.
This requires only 102 THP. ( 231 * 245 fps / 550 = 102)
102 THP divided by our prop efficiency of 0.8 equals 128 engine
horsepower. This is 55 percent power. Assuming a higher fuel
burn of
0.5 pounds per horsepower hour for our supercharged engine,
we get a
total fuel burn of 64 pounds per hour or 10.7 gph.
We are flying the same speed, 145 knots in both cases but our
supercharged engine flying at 16,000 feet is burning nearly two
gallons less fuel per hour. over a four hour trip we can increase
our endurance by three quarters of an hour and increase our
range by
more than 100 nautical miles.
A turbocharged engine would be even more efficient, but a
supercharged engine is still better than NA. The supercharged WW2
fighters got pretty good range and endurance by cruising up
high at
low power levels and could circle for quite a while like this
until they spotted bogies. Only when they engaged did they go
to high
power and this usually did not last more than a few minutes.
Supercharged engines have the advantage of easier boost boost and
you don't have the heat issues of a turbo.
Regards,
Gordon.
All very true. No doubt about it.
You need oxygen at 16,000 feet and IFR in most places in the US :)
I don't know about Canada. I think 12,000 feet is a more practical
altitude limit for most of us VFR pilots in the US.
BTW As I recall VFR on top is illegal in Canada as I recall. Every
time I fly in Canada there always appears to be a top :)
Paul Lamar
VFR on top is legal in Canada, I can't remember what the requirements
are but something like, 2000ft between layers VFR destination w/ only a
certain 'octa' sky coverage and a few other nikk-nakk's but it is do-able.
[ I think you have to be current for night flight as well??] If your really want
to know I can go look it up for ya. I don't use it ever as I file IFR 95% of the
time that I fly.. the other 5% is usually 20-50 mile hop's where its just too
big a pain to file for..
Also IFR clearances are only required over FL180, you need a CVFR
clearance over 12500 for flight up to 17,999ft or in Class B. That is unless
your in the Northern Domestic Airspace.. but then.. that only takes
effect.. like.. North of Anchorage [lattitude] some place.. I used to fly
into the old N.W.T. and didn't need to concern myself about it then
as it wasn't far enough north.. Some of that stuff has changed a
bit, but haven't needed to use it so I never boned up on it too much..
Fwiw..
Jarrett
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