Model Forum / Radio Controlled / Air Models / September 2006

The short answer is your calculations are entirely correct.

At identical RPM a 2- stroke is firing twice as often and producing
twice the power. And probably 4 times the gunk on the model.

The simplicity of its porting and valve timing means it can probably rev
even higher and deliver even more power too.I am fairly sure that when I
checked a racing 36 2-stroke with a tuned pipe would deliver something
like 2-3bhp at 18K or more..thats really pushing it Maybe 600bhp per
litre. GP motorcyles on 50cc 2-strokes rev up there too, and similar
high specific powers are available. On normally aspirated 4 strokes the
best is probably an F1 engine producing - at similar RPM - around 900bhp
from a 3 liter engine - again about 300 bhp per liter.

However all this comes at a price, and that is flexibility. Its hard to
get decent idling at those sorts of valve timings needed to do that, and
4 strokes are used because they throttle better, run cleaner, use less
fuel and generally maintain torque at lower RPM better. This means that
they can swing BIG props slower, and thats better for most airframes in
terms of prop efficiency. Since isit is not common to put reduction
gears on model engines.

You have also noted that the best power figures happen at the sort of
RPM that no one in practice uses...once down to practical RPM ranges -
and quiet props are sub 10K RPM props - 2 strokes lose much of theor
advantage, and teh flexibility of te 4 strokes tend to favor them - not
because they are powerful, but because they behave well and don't die on
you.

I fly mainly electric and its an observed fact that we can fly with
equivalent performance to - say - a 40 two stroke, producing maybe 1500
watts, or a 60 4 stroke, similar power, with an electric motor whose
INPUT is less than 400W.

Why? because we gear or use motors that turn 13x8" props at 5-7000RPM,
not motors that turn 10x6" props at 11k RPM..

This works for bigger models that fly slow. It doesn't work for hot
ships that are pushing 100-120mph. Here the RPM/props size of a tuned
.40 2 stroke is optimal for the airframe, and to dupe that we need
something like 2Kw 3Kw - 3-5bhp.

Nope,  the 4-stroke YS .63 is rated at 1.6PS by the maker.  Weighs 498 grammes
including silencer.

It depends on what the model is,  a pylon racer will probably go faster with a
screaming 2 stroke on a high pitch/small diameter prop, but a more "normal"
model will do better with a bigger prop at lower rpm.  4-stroke engines excel at
that, but most rc 2-strokers are also optimised for the kind of props people are
likely to actually use.  The powers quoted by 2-stroke engine mfrs are likely to
have been obtained at higher revs than practical and may also have been taken
open exhaust which is not feasible for most people either.

The upshot is that if you pick a 4 stroke engine which weighs the same as the
2-stroker (Note : including silencer - most important) then they will likely
have similar power at practical rpms and the 4-stroker will deliver better
static thrust because of the bigger prop it swings.  Choosing engines on a
comparable weight basis will generally result in a bigger capacity 4-stroke
engine because their silencers are lighter.  Unless you are caught by a
competition rule then capacity means very little and weight is the thing to watch.

Hth,