Model Forum / Radio Controlled / Air Models / November 2006

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An SC engine is just a Sanye engine that is made in China. I like them.

In the US, they used to be known as the ASP brand of engines. Indy R/C used
to import them into the US.

Global Hobbies (Hobby People) are now importing them into the USA as Magnum
engines.

England (and maybe parts of Europe - I don't know) import the same engines
with SC cast into the bypass port side of the engine as their brand name
(Super Custom).

They are, for all practical purposes, the same engines. All good.

Pitch in a propeller is analagous to the gears in our car transmissions. The
lower the pitch number, the lower the gear we are using in the car. We all
know that if we start in first gear in a car, we will accelerate faster, but
have a lower top speed than if we used a higher gear. This analog works very
well for explaining how pitch affects the pull and thrust of a model
airplane propeller. In a sense, the diameter of the prop can be viewed as
the size of the rear tires (or front, if you please) that are receiving the
power output of the engine. Larger tires put my power on the ground. Let's
assume for this argument that the word "large" is being applied to the width
of the tires. Not the diameter.

Higher pitch and higher diameter increase the load upon the engines we use,
whether two-stroke, four-stroke or electric. Just as the timing of an
electric motor can be changed in order to optimize the most power output to
power consumed ratio, our IC engines are "tuned" to operate best within
certain rpm ranges. Most sport two-stroke engines are tuned to operate over
10,000 rpm. Some much higher. Our engines should be allowed to rev into
their power band in order to extract the most power and to help them create
less heat. Heat is the killer for all IC reciprocating style engines.

Today we are beginning to see a migration of two-stroke engine tuning toward
lower rpm operation. Why? It is quieter and it optimizes the engine's tuning
for turning larger, higher load propellers, such as those being used in 3D
flying. Fitting a Webra .50 GT with a 12" prop for 3D flying has proven not
to be the best use for this engine. It was designed to scream. Let it
scream.

I bought a Rossi .45 3D engine a while back. What's different? It's
tuning/porting/timing is optimized for swinging the aforementioned larger 3D
props. Everyone associates the Rossi name with high rpm engines. I predict
that this new Rossi is just the beginning of a new trend in lowering the
operating power band to optimize the two-stroke's performance for 3D flying.

Cooler operation is better. That's why I say to let your high rpm designed
two-stroke rev up. It is quite possible that these engines will become
extinct in the not to distant future.

All of this is just my personal opinion and is not presented as gospel. I'll
be happy to listen to any arguments, as long as they remain civil.

Ed Cregger

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This is an addendum to my just sent previous response to your questions.

Our glow engines have somewhat fixed timing. Running a larger prop with
timing that is optimized for a smaller prop (glow plug, fuel, mixture,
compression) will advance the timing. This makes the engine run hotter if
the larger prop presents more of a load to the engine. Hot is bad.

One can run the engine a bit richer to retard the timing, if the jump in
size is not too great. Or, one can change glow plugs to something more
suitable for the given load. Or one can add a head shim to decrease the
compression. Decreasing the compression effectively retards the timing and
lowers the operating temperature. Good. Lowering the nitro content of the
fuel will also retard the timing, but you will lose a little power. The head
shim or lower heat range glow plug, while maintaing the nitro content is the
best choice in most instances.

Ed Cregger

     The angle of attack on the propeller's blades are the highest at
the start of the takeoff roll. A high angle of attack equals a higher
drag figure, so that a prop with a higher pitch won't allow the engine
to run up closer to its rated RPM and horsepower output will be lower
until the forward speed is much higher and the prop's AOA drops.
     The prop's blade length determines, to a large extent, its
efficiency at lower airspeeds. The longer prop, with a lower pitch,
will generate more thrust at a given RPM than a shorter one having a
higher pitch, since tip vortices affect a smaller percentage of the
span and the resulting drag is less. It's much more efficient to move a
large column of air at a lower velocity than a small one at a high
velocity. The designers of seaplanes, helicopters, turboprop airliners
and high-bypass turbofan engines are well aware of this.

       Dan

Thanks to Ed and Dan, I think I'm beginning to get a handle on the propping.
I also found an article in "Getting Started in Radio Control Airplanes" that
might help me understand this issue better. Still have to read it, but
comments like yours will help me to grasp it easier.

Thanks,
Harlan

Diameter = Thrust
Pitch = speed