Great Planes Sportster Bipe 40 Kit - GPMA0510 - Manual (Page 29)

IF MWNT WIDTH EXCEEDS T H E SPACE AVAILABLE
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Figure 5
GENERAL INFORMATION
Center of gravity:A nose heavy airplane can be a problem! Make sure you check the CG location during
construction with the radio components installed. You may need to put the radio as far back in the compartment
as possible. This will avoid having to carry tail weight.
Props:Always start with the manufacturer’s instructions (especially for break in) for prop selection. Read their
instructions carefully because 4 cycles use entirely different diameter and pitch propellers because they operate
at a lower RPM and produce greater torque than equivalent displacement 2-cycles.
You may find that after operating your sportster with recommended props that you’ll want to experiment
with other props. It is amazing how much you can change flight characteristics of a model by changing props.
If you are looking for higher flight speeds from your Bipe than you are getting from the average recom-
mended prop, consider reducing the diameter and/or blade area slightly and increasing the prop pitch to bring the
RPM back to the normal range. For example,with our Enya .46 equipped Sportster Bipe, the instructions recommend
13
x 5, 12 x 6, 11 x 7, 11 x 6, 10 x 6, 10 x 7 or 10 x 8 props. We started with an 11 x 7 propeller which gave us fair
performance. Larger props will provide slower speeds
...
fine for big biplanes ... but not what we wanted from our
racy Sportster. For faster speed, we found that a 10-1/2 x 7-1/2 propeller added approximately 10 mph to the
straight and level flight which help make vertical maneuvers crisper due to a faster entry speed. There is a trade
off, however, as the engine idle was somewhat higher due to the lower mass of the wood 10-1/2
x 7-1/2 propeller.
Four-cycles like flywheel weight from large diameter props. This made the plane land a little faster than with the
11
x 6. The prop we used was a prop designed for .60 size pattern engines, a Max Daily (Radio South) propeller.
A similar prop is also available from DW Products. Their addresses are: .
Radio South DW Products
180 E. Burgess
Pensacola, FL 32503
5634 Crystal Ct.
Santa Rosa, CA 95404
Fuel Tank: For simplicity, we recommend a 2 line tank for your Sportster Bipe. If you can’t easily reach the fuel
line for fueling purposes, you might consider using one of the Dubro fueling valves. This will save extra plumbing
that could leak or cause problems. Most 4 cycles have a small enough venturi that muffler pressure is unnecessary.
Don’t forget the crankcase drain line. A lot of excess fuel comes out of the pressure fitting. Make sure this drainage
exits your fuselage. Treat it as though it were a separate exhaust.
Flying:Other than the obvious difference, you’ll notice one other big difference from your first take off
with a four cycle equipped Sportster Bipe: You’ll need to add more right rudder than normal to counter the increased
torque on acceleration during takeoff. Also, you may be surprised how big an effect the larger propeller may have
on abrupt change maneuvers such as a square loop. Because there is more mass to the propeller, it has a larger
gyroscopic force. When you change directions in maneuvers like a square loop, you may find that the plane changes
heading. This may be a result of gyroscopic precession and/or P-factor.These factors become more prevalent with
larger diameter
4 cycle props and with slower speeds generally encountered with these engines.
Also don’t forget to have a slight amount of toe-in with your main wheels. Toe out and lot’s of torque can
cause problems with ground handling. Use some toe-in, add some rudder and your takeoffs will be in a straight line.
GOOD LUCK AND GREAT FLYING!
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