BSFC vs gas mileage, 2 stroke vs 4 stroke

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

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

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