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BSFC vs gas mileage, 2 stroke vs 4 stroke



 
 
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  #1  
Old August 22nd 04, 07:14 PM
Jay
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Default BSFC vs gas mileage, 2 stroke vs 4 stroke

I've been under the impression that 4 stroke engines get better gas
mileage in aircraft due to their higher BSFC numbers. But I've
recently realized that since the 4 stroke engines are so much heavier
than a same horse powered 2 stroke you end up having to design a
larger aircraft (and engine) to carry the original intended payload
plus the additional weight of the engine. So in the end, a purpose
built 2 stroke airplane will be smaller, lighter may even get better
gas mileage. Unfortunetly, most designs that use 2 stroke engines are
high drag ultra lights so going distances isn't really what they do,
but it would seem to me there is some potential for a low weight, low
drag, in the neighborhood of 500lb empty, 2 seat aircraft designed for
one or two 2-stroke power plants. The Pulsar XP seems close because
I've seen some with Rotax 582's. The aicraft that attempt to exploit
these engines in this way always look like the wings are too small
compared to the proportions we're all used to seeing on C172 and other
similar formula aircraft. Examples are the cri-cri, BD-5, AR-5
http://www.ar-5.com/sportav93.html
  #2  
Old August 22nd 04, 08:00 PM
Vaughn
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"Jay" wrote in message
om...
but it would seem to me there is some potential for a low weight, low
drag, in the neighborhood of 500lb empty, 2 seat aircraft designed for
one or two 2-stroke power plants. The Pulsar XP seems close because
I've seen some with Rotax 582's. The aicraft that attempt to exploit
these engines in this way always look like the wings are too small
compared to the proportions we're all used to seeing on C172 and other
similar formula aircraft. Examples are the cri-cri, BD-5, AR-5


Yes, the cri-cri immediately cames to mind, it is a high wing load design
that seems to try to minimise drag by minimising the airplane.

But you left out the other end of the wing loading scale, where you will
find many self-launching motorgliders, some have fixed engines in pusher or
tractor configurations, but some of the neatest ones have retractable engines.
With their very low drag in the glider mode, they raise the possibility of a
"sawtooth" cruise mode where the engine is used for climb only, and is shut down
for long periods in between climbs.

Vaughn


  #3  
Old August 22nd 04, 08:22 PM
Richard Isakson
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"Jay" wrote ...
I've been under the impression that 4 stroke engines get better gas
mileage in aircraft due to their higher BSFC numbers. But I've
recently realized that since the 4 stroke engines are so much heavier
than a same horse powered 2 stroke you end up having to design a
larger aircraft (and engine) to carry the original intended payload
plus the additional weight of the engine. So in the end, a purpose
built 2 stroke airplane will be smaller, lighter may even get better
gas mileage.


Not so, Jay. The two stroke burns fifty percent more fuel then the four
stroke so you have to carry fifty percent more fuel weight. For an airplane
with fuel for a four hour cruise, both airplanes weigh about the same.

Rich


  #4  
Old August 23rd 04, 04:07 AM
Mark Smith
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Richard Isakson wrote:

"Jay" wrote ...
I've been under the impression that 4 stroke engines get better gas
mileage in aircraft due to their higher BSFC numbers. But I've
recently realized that since the 4 stroke engines are so much heavier
than a same horse powered 2 stroke you end up having to design a
larger aircraft (and engine) to carry the original intended payload
plus the additional weight of the engine. So in the end, a purpose
built 2 stroke airplane will be smaller, lighter may even get better
gas mileage.


Not so, Jay. The two stroke burns fifty percent more fuel then the four
stroke so you have to carry fifty percent more fuel weight. For an airplane
with fuel for a four hour cruise, both airplanes weigh about the same.

Rich



pretty generalstatement,

have you been around many two strokes ?

the reason i ask is my 618 GT 500 burns right at 3 1/4 GPH, as does a
frineds 912 powered GT 500,

and mine will double his climb rate !
--


Mark Smith
Tri-State Kite Sales http://www.trikite.com
1121 N Locust St
Mt Vernon, IN 47620
1-812-838-6351
  #5  
Old August 23rd 04, 06:39 AM
Jay
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Yes, the 2 stroke burns 50% more fuel for the same HP, but the point I
was trying to make was that since the 2 stroke powered aircraft can be
built lighter all around (on account of its lighter engine), you can
use a smaller engine (which consumes less fuel) and still fly the same
payload. This relationship is unique to aircraft since weight means
SO much.


"Richard Isakson" wrote in message ...
"Jay" wrote ...
I've been under the impression that 4 stroke engines get better gas
mileage in aircraft due to their higher BSFC numbers. But I've
recently realized that since the 4 stroke engines are so much heavier
than a same horse powered 2 stroke you end up having to design a
larger aircraft (and engine) to carry the original intended payload
plus the additional weight of the engine. So in the end, a purpose
built 2 stroke airplane will be smaller, lighter may even get better
gas mileage.


Not so, Jay. The two stroke burns fifty percent more fuel then the four
stroke so you have to carry fifty percent more fuel weight. For an airplane
with fuel for a four hour cruise, both airplanes weigh about the same.

Rich

  #6  
Old August 23rd 04, 03:08 PM
Matt Whiting
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Jay wrote:

Yes, the 2 stroke burns 50% more fuel for the same HP, but the point I
was trying to make was that since the 2 stroke powered aircraft can be
built lighter all around (on account of its lighter engine), you can
use a smaller engine (which consumes less fuel) and still fly the same
payload. This relationship is unique to aircraft since weight means
SO much.


Except that they don't burn 50% more fuel for the same horsepower.

Matt

  #7  
Old August 23rd 04, 03:46 PM
BllFs6
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Hi guys

In the past year or 2 I've done a fair bit of back of envelope cals involving
gyro's and small GA aircraft...

And as one poster mentioned.....4 hour flight endurance seemed to be the
breaking point.....

Less than 4 hours, and your payload (and therefore possibly to some extent the
actual aircraft structure) was less for a 2 stroke than a 4 stroke....

More than 4 hours flight time and the 4 strokes were overall lighter load....

It just kinda stood out to me that the guy said 4 hours, because thats the
number I usually came out with...

Now, I was typically comparing the lower end (HP wise, with 80 HP or so
generally being the max) 2 strokes vs 4 strokes, using published/internet
values (but not neccessarily from one source)...

FWIW (probably not much in reality

take care

Blll
  #8  
Old August 23rd 04, 10:01 PM
Jay
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Are you saying they're better or worse? The numbers I've been using
for my calcs are 2 stroke (BSFC .6-.65) versus 4 stroke (BSFC .4-.5).
Thats a worst case of 62% more fuel and a best case of 20% more
ignoring my point that a lesser horsepower engine is needed to fly the
same load. If you could fly the same payload with a 50hp 2-stroke
powered purpose built plane as an 80hp 4-stroke powered plane, you'd
be even on fuel consumption.

Going on the other poster's 4 hour number assuming same plane weight,
just different engine weight, the advantage of carrying fuel weight
versus engine weight are several:
1) Flexibility of changing fuel load depending on flight plan (no
option of carrying half an engine for 4 stroke)
2) Fuel load can be bore in wings, doesn't increase bending loads and
require stronger/heavier structure.
3) Could theoretically be jetisoned in the event of a forced landing.
4) Lower average and landing weight as fuel is depleted during course
of trip.
5) Lower initial investment (smaller engine plus fuel later vs. larger
engine now, less fuel later)

Matt Whiting wrote in message ...
Jay wrote:

Yes, the 2 stroke burns 50% more fuel for the same HP, but the point I
was trying to make was that since the 2 stroke powered aircraft can be
built lighter all around (on account of its lighter engine), you can
use a smaller engine (which consumes less fuel) and still fly the same
payload. This relationship is unique to aircraft since weight means
SO much.


Except that they don't burn 50% more fuel for the same horsepower.

Matt

  #9  
Old August 23rd 04, 10:08 PM
Jay
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Great anecdotal evidence Mark, thats what I'm talking about! Same
fuel burn, double the climb. The key of course is that less work
(useless work) is being done because your kite has less mass. You're
moving the same payload, just less airplane. I'm sure you can land
way slower at the end of the day.

Mark Smith wrote in message
the reason i ask is my 618 GT 500 burns right at 3 1/4 GPH, as does a
frineds 912 powered GT 500,

and mine will double his climb rate !

  #10  
Old August 24th 04, 01:26 AM
Dave Hyde
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Jay wrote...

The key of course is that less work
(useless work) is being done because your kite has less mass.


All other things being equal in a *stabilized climb*
to get double the climb rate you need to halve the weight.
(Ps = ROC = speed*(thrust-drag)/weight)

There's more to this story than a weight delta.

Dave 'fundamentals' Hyde



 




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