2-Cycle prop selection

[Jim_Woodward at beaerospace.com]

Hi All,

I'm going to branch off a little from the OS160 setup thread because it 
brings up some good points.  Here are some of my thoughts on prop 
selection.  With the total power available with the 2-cycles, you do not 
necessarily need to use the prop that gives the 
most-ridiculous-ultmate-power.  In fact too easily, the horizontal & 
vertical speeds can just look ridiculous fast.  Unlimited power is easily 
achievable given a variety of setups. 
1.  The power "setup" or "delivery" is what we should really be after here 
as the main goal. 
2.  The overall speed envelope is critical to achieve a consistent looking 
pattern flight to flight, day-to-day. 

Setup:  Throttle servo:  start with the throttle servo in the 1/2 stick 
position with a medium or long servo arm.  On the carb, I use the longest 
position available on the arm.  Adjust the pushrod to ensure the throttle 
barrel is 1/2 or a little less open when the throttle servo is at 1/2 
throttle.  I use ball-links to ensure positive control of the throttle 
arm/servo connection.  Use the throttle ATV to adjust the high and low 
points.  You may have low ATV numbers, but I don't think this is 
necessarily bad.  The longer servo arm will make for more linear movement 
of the throttle pushrod.  (this will affect the Throttle curve later). 

Throttle curve: This is speed-maneagements best friend!  Lean to use this 
function.  You do not want a power setup where the engine is producing 75% 
of the power when the throttle stick is at 1/2 position.  Once the engine 
is adjusted to run well in the midrange and top end, use the throttle 
curve to adjust the engine rpm in the range you normally fly.  This is the 
easiest way to slow the plane down for horizontal lines, if you are 
uncomfortable flying maneuvers with the throttle stick in the 1/4 stick 
range.  I find that I need a steeper line in the beginning to have prop 
wash over the rudder for stall turns, then a flatter section for a while, 
then a steeper section so that the power comes on and off during the last 
3-4 throttle stick positions.  I find I'm more comfortable flying at 
around 5/8" to 3/4" throttle stick position, so I use the curve to adjust 
the power available at that setting.  You can do this with any engine 
(even the YS need a throttle curve).  What we are talking about here is 
tailoring the speed of the plane to the position you are comfortable 
flying throttle stick in.

Prop:  Of course, the prop has a huge impact on overall speed.  The prop 
and throttle curve go hand-in-hand to control the speed of the plane.  I 
do not want a large speed envelope "gap" between low throttle speed, 
horizontal maneuver speed, and full throttle speed.  Nor do I want a prop 
that exaggerates the velocity of the plane or any on-off-pipe issues. 
Generally, I think more diameter and less pitch is the way to go (but 
that's just me).  Nor do I want to run a prop that 'overloads' the engine 
just to make it slow down.  I want to run a prop that is easy on the 
engine, has good breaking, and has good acceleration after the upline 
snap.  Use the throttle curve to adjust where the power comes on/off to 
suit your flying style. 

In 25 words or less: 1.  use the throttle curve, 2. don't run a prop which 
allows the plane to really accelerate away from you or has a large 
velocity gap. IMHO.
Thanks,
Jim W.

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