[rebel-builders] Prop and Gear Reduction Calculator
Posted: Sun Feb 19, 2012 12:20 pm
A larger diameter prop results in more thrust per hp at low to moderate
speeds. Less energy wasted by giving a larger mass of air a smaller
acceleration. Think helicopter. A high reduction ratio lets you run a
larger diameter prop without necessarily getting a high tip speed. I
believe it is prudent to design for lower tip speed both for noise and
energy efficiency. ie I don't believe it is fair to claim that max
thrust per hp. requires such high tip speed for most prop designs. .8M
might be much more reasonable than .9 for under 200 mph airplanes and I
run even slower.
I also believe that generally better all around results are achieved by
running a converted engine closer to max torque rpm than max power rpm.
The power output does not usually drop off quickly if you ease back a
bit from max hp rpm. You are matching the engine to a prop that has a
steeply rising torque demand with rpm. ie the torque demand of the prop
rises with rpm faster than the engine torque rises. My full power
static rpm is essentially my max torque rpm and also where I cruise with
my fixed pitch prop.
Before you change anything why not fly as is. On an SR you can likely
consider a larger diameter prop if needed. It is wrong to assume that a
high tip speed equates to efficiency IMO. Larger diameter yes for our
airspeeds, higher tip speed not necessarily. There are way more guys
wishing for and buying higher ratios for better performance than you
will every find wishing for a lower ratio. But neither the gear ratio
nor the prop diameter is as critical as some make it out to be.
Ken
Mike Kimball wrote:
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speeds. Less energy wasted by giving a larger mass of air a smaller
acceleration. Think helicopter. A high reduction ratio lets you run a
larger diameter prop without necessarily getting a high tip speed. I
believe it is prudent to design for lower tip speed both for noise and
energy efficiency. ie I don't believe it is fair to claim that max
thrust per hp. requires such high tip speed for most prop designs. .8M
might be much more reasonable than .9 for under 200 mph airplanes and I
run even slower.
I also believe that generally better all around results are achieved by
running a converted engine closer to max torque rpm than max power rpm.
The power output does not usually drop off quickly if you ease back a
bit from max hp rpm. You are matching the engine to a prop that has a
steeply rising torque demand with rpm. ie the torque demand of the prop
rises with rpm faster than the engine torque rises. My full power
static rpm is essentially my max torque rpm and also where I cruise with
my fixed pitch prop.
Before you change anything why not fly as is. On an SR you can likely
consider a larger diameter prop if needed. It is wrong to assume that a
high tip speed equates to efficiency IMO. Larger diameter yes for our
airspeeds, higher tip speed not necessarily. There are way more guys
wishing for and buying higher ratios for better performance than you
will every find wishing for a lower ratio. But neither the gear ratio
nor the prop diameter is as critical as some make it out to be.
Ken
Mike Kimball wrote:
I just uploaded a spreadsheet to the "Software" section that may be helpful
in particular to those of you planning to use an engine that requires a gear
reduction. I have recently discovered that my decision to use a 2.11 to 1
ratio was a bad idea. Based on the max engine RPM I desire I should have
used a lower ratio. I am now faced with trying to modify my gear box or
look at other options. Even for those not using a gear reduction you might
find it useful if you are wondering what diameter prop to use. If you know
your max engine RPM enter that and just enter one (1) for the reduction
ratio and play with the prop diameter to find your optimum prop diameter.
Don't forget to take into account ground clearance. I think the FAA
mandated minimum clearance in a level attitude on the ground is 7 inches.
Mike
044SR
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