Simon,

The best speed to climb depends on a balance between airplane and propeller
efficiency. Also, the rate of climb is proportional to the thrust power
produced by the prop (thrust x speed) minus the power used by the airplane
to fly (drag x speed). Excess power produces climb rate. For example if
you have 0.10 excess horsepower (74.6 watts) you have 55 foot-pounds per
second of excess power. This will climb a 10 pound model at 5.5 feet per
second.

If propeller efficiency is fixed, the airplane climbs best at "minimum sink"
speed, which is the glide speed at which the airplane sinks the slowest. At
this speed, the value of (lift coefficient to the 3/2 power) divided by the
drag coefficient is maximized. This is somewhat slower that best L/D speed
and on most gliders the best minimum sink rate is obtained with a few
degrees of flap. Note that at this speed, the product of drag x speed is
minimized.

But:

Propeller efficiency is dependent on airspeed, especially for smaller,
ungeared props. The efficiency of a prop when static is zero. As the
airplane speeds up, its efficiency gradually improves until it reaches a
maximum value. At this speed the product of thrust x speed is maximized
(assuming constant motor power (not a great assumption, but a reasonable one
for the purpose of this discussion)). This optimum airspeed is dependent on
the motor RPM and the prop pitch. It is likely that in the climb mode,
especially at minimum sink speed, the airplane is going slower than the
optimum speed for the prop. This means that if you speed up you will obtain
some increase in prop efficiency in exchange for a small reduction in
airplane power demand, and a better climb rate will result. This is because
at minimum sink speed, the power required is minimum. This means that the
slope of the power required versus speed curve is flat. If you increase the
speed a little, the power required by the airplane is increased only a tiny
bit whereas the prop efficiency is likely to improve more.

Where this may not work out as described is for an airplane with an
extraordinarily large prop, geared way down. This prop could be designed to
give best efficiency at minimum sink speed, but for most airplanes this
could be a pretty big prop!

Another time when it might not work out is when you have so much power that
you go straight up, fast. In this case you want to clean up the airplane
since it is making no lift.

Example: Some years ago I helped to design contest rules for a high school
competition. The object was to climb for 30 seconds (including a takeoff
roll) and then glide for two minutes. The team that could do this with the
greatest payload was the winner. All of the teams used flapped airfoils.
All climbed with zero flap and glided with flaps down a bit. These models
were not highly powered, and you could see clearly the benefit of flying
faster during the climb. If the pilot pitched up and slowed down the
reduction in climb rate that ensued was clear-cut. The motors were not
geared because they were mild two-strokes! The supporters of the program
were coming mostly from the gas model world. The contest worked out pretty
well, by the way.

Conclusion: I bet your model will climb best without flap and at somewhat
elevated speed. Electronic measuring devices could be very handy in
figuring out exactly what that best speed turns out to be. Be sure that you
make measurements in very early morning calm air. My experience in making
measurements is that the differences you are trying to measure are small,
and the air can move around quite a bit.

Hope this information is helpful.

Regards,

Blaine Beron-Rawdon
Envision Design
San Pedro, CA
http://members.cox.net/evdesign


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