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I am currently getting an X4 cylinder, 150cc, 4 stroke engine 3D CAD Modelled, this engine is initially being designed for Powered Paragliding (PPG), classified as experimental Aircraft, so no certification is required!

I have read several discussions here, that largely relate to large ridged winged Aeroplanes travelling at high ground speed.

PPG is a Low & Slow (22~75kph) Aviation sport, there are obvious differences with this project being;

Most PPG engines max. rev are from 7'000~12'000rpm. Then a Re-drive is used to slow Prop down to approx. .88~.90 mach.

With a PPG, it is the Wing that sets the Speed in flight, not the engine! A larger more powerful engine only provides a higher climb rate or maintain lever flight easier.

Most engines of similar capacity swing a 125cm (49/28") Prop at around 2'700rpm to under 3'000rpm.

The engine we are designing uses a unique engine that has replaced the 100yr old conventional crankshaft with a patented Contra-rotating Trilobe centre drive shaft (CDS) technology, that uses two trilobe cams, that strokes each Piston 6 times on One revolution, thus providing 3 combustions in two revolutions of CDS, this engine will not be revving over 3'500rpm maximum.

Engine is currently 12.5kg and 41cm width x 37cm high x 20cm deep.

I am planing on using Two 100cm (40") Contra-Rotating propellers!

Raptor X4-150

I would appreciate it if anyone could assist/help me if this is an appropriate Sized Prop & what Pitch would be best ?

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    $\begingroup$ "With a PPG, it is the Wing that sets the Speed in flight, not the engine!" This is true of all aircraft. $\endgroup$ – Steve Mar 14 '17 at 12:28
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    $\begingroup$ @Steve Not exactly. In a conventional aircraft, once you select a certain power setting and adjust stab/elevator trim until you are in level flight, the resulting speed is dependent on the power setting. In a PPG, there is no stabilizer, so level flight will always be at a fixed speed. $\endgroup$ – Sanchises Mar 14 '17 at 12:54
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    $\begingroup$ @Sanchises In a conventional aircraft, in straight and level flight, if the power setting is increased (and only this is changed), the aircraft will initially speed up, but then it will start climbing. It should stabilize back at the original speed, which is set by the trim. $\endgroup$ – Steve Mar 14 '17 at 13:48
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    $\begingroup$ Why do you think you need contra rotating propellors for an engine predicted to produce only 20hp? $\endgroup$ – KuangGradeMark11 Mar 14 '17 at 14:00
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    $\begingroup$ A contra-rotating prop is not needed, it is about making PPG flight a lot Safer, as it will eliminate Torque twist ...caused by one prop!as this engine As this engine does not have a normal crankshaft we can do this, unlike any other engine. $\endgroup$ – The Raptor Mar 14 '17 at 21:30
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Contra-rotating props correctly address the problem of how torque complicates the steering of a PPG. (To solve this, others have even powered PPGs with large "quadcopters" where two of the four props are counter-rotating, despite the screaming of four small fast props.)

In conventional aviation two contra-rotating props typically have the same diameter and pitch and speed. For a PPG, keeping them the same size is particularly useful to cancel torques.

To replace one prop with two, and otherwise keep everything the same, we should not change the pitch speed or RPM, only the diameter, to spread the thrust load evenly across both props.

The OP says that a similar engine powers a 49x28" prop at 2700 rpm. The OP's engine redlines not much past that, so call the RPMs the same. Then the prop pitch also remains the same, so prop diameter is the only thing left to vary.

Coaxial contra-rotating props are roughly 10% more efficient, compared to non-coaxial props as on a conventional twin-engine airplane. So each one of the pair has 45% of the load of the original single prop.

So each prop provides 45% of the thrust of the nominal 49x28" prop.
If all else is equal, thrust varies with the fourth power of diameter.
So we should scale the diameter of the 49x28" prop by 0.451/4 = 0.82.
So the engine should use a pair of 40x28" props.
Exactly the diameter that was guessed.

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