Propeller Selection (TurboProp)
The Xicoy TurboProp unit is useable to power in a wide variety of aircraft. Being available in two ratios
enables users to select the most appropriate for the intended application. The turboprop unit has been
tested in a range of situations and loads. The main indicator of the performance of the unit from the
users point of view, is the rpm that can be achieved with a given propeller load. Larger propellers and/or
more blades produce higher torque figures but not always the highest thrust figures.
We always recommend good quality wood props. Carbon are very nice but very unforgiving in a crash or
nose-over. Make sure the propeller is well balanced and the tips are visible (paint a strong colour) when
running, they can bite hard.
As the Turboprop has two reduction options there are optimum propeller sizes for each ratio.
High reduction approx maximum, 6000rpm, ideal props 0.61-0.71m / 24-28”.
Low reduction approx maximum, 8000rpm, ideal props 0.51-0.61m / 20-24”.
Higher rpms are achieved with smaller diameter propellers and / or shallow pitches. It is most important
that propellers significantly smaller than listed are not used or if they are, the maximum rpm limiter
should be preset to prevent overspeeding and possibly of overstressing (the propeller). In all cases
follow the propeller manufacturer’s recommendations for preparation and use.
Tipspeed
Most propellers are designed for a maximum tipspeed around 200m/s or 655ft/sec. Above this the tips
start to make that familiar howling noise. This can be just in a dive or most of the time if a prop with
shallow pitch is fitted to a fast airframe with a powerful engine.
A rough guide to optimum tipspeed is calculated by 3820 / prop dia in mtrs.
So for example a 610mm prop, max rpm would be 3820 / 0.61 = 6262rpm
For imperial measures 12,510 / prop dia in feet, or 150,100 / prop size in inches.
So for example a 26” prop max rpm would be 150,100 / 26” = 5773 rpm.
In practice the best performance comes from
slightly
overloading the engine with a slightly over-sized
propeller. This gives lower rpm on the ground but this is recovered as soon as the plane has forward
airspeed.
General notes on propellers
All sizes are 2 blades. If you want to use 3 or 4 blades, reduce by 50mm/2” on diameter:
Slow planes (Pilatus PC6 / Cessna highwing types) should ideally have a 26x10 or 28x8 pitch with 2
blades, and high ratio. Slow flying aerobatic planes can use the same.
Medium sports and scale plane like Pilatus PC7 / PC21 / Tucano needs a smaller diameter but larger
pitch, a 22x14 or 24x12 with high ratio, or 20x12 or 22x12 on low ratio. The larger diameter and pitch
enables plenty of thrust for good forward speed with a quiet operation and scale appearance.
Fast sleek plane needs a 20x14/16 or 22x14 pitch to get good forward airspeed. The larger pitch and
smaller dia allows a higher propeller rpm and corresponding high forward airspeed.
Prop-hanging requires large diameter and small pitch for high static thrust. Around 28x8 pitch is about
right with high ratio gearbox but the forward speed will not be high but takeoffs will be brisk.
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