SELECTING THE PROP
Start your quest for the perfect prop by choosing a propeller within the engine
manufacturer’s recommended range. Balance it, bolt it to your engine, and use a
tachometer to see how many rpm you get from the engine. I like to set up a simple engine
test rig so that I can do this without having to assemble the entire model. Once I find a
promising candidate (or two), I bolt the engine to the model and go flying to see how it
As an example, my most recent project, a ¼-scale Sopwith Camel (also in this issue),
is powered by a Zenoah G- 38 and originally propped with a Xoar 18 x 8. With this
combination, the Camel had good power and took off easily and flew nicely, but the engine
would over-rev, producing high rpm but not a lot of thrust. After talking with several of
my World War I flying buddies, I switched to a 20 x 8 propeller. This dropped the rpm, but
the model’s overall climb rate improved noticeably. For a W W I airplane with a fair amount
of drag from an extra set of wings and flying wires, the G- 38 provided better performance
with a slower turning propeller with a bigger disc area. The difference between the
18-inch prop and the 20-inch prop is close to 60 square inches in disc area, and the
difference in flight performance was noticeable.
You will discover that all props are not created equal. Prop shape and design have a big
effect on efficiency, so size and pitch changes don’t always equate to linear rpm changes.
One brand of 18 x 6 prop will turn more or fewer rpm than another 18 x 6. This is why you
have to experiment.
For my Sopwith Camel, increasing the propeller one size in diameter lowered the engine rpm and increased the
propeller’s disc area to produce better overall performance.
When it comes to picking prop and engine combinations, don’t lock yourself in to what
other people tell you. Do a little experimenting yourself, and take your time to find the
best combination for your model. Take notes, then apply the data to your next modeling
project. Maximizing performance is a lot of fun! J
ENGINE DISPLACEMENT: RECOMMENDED PROP SIZES
■ Maxx Products
■ Bob’s Hobby Center