Standard 125cm prop or larger?

 

Quick and immediate answer> go for the large if your hands are long enough.

Given the same engine, large prop will have>

– slightly higher maximum thrust
– a lot more thrust in midrange thus better fuel  economy at cruise
– less torque.

Ok, but is an increase from 125 to say 132 cm relevant? It only 5%!

Let us do the math and physics. I will do some simplifications to make it easy.

adjustable propeller

On every prop, the middle part of about 40 cm is not effective because>

– the profile close to the center is determined by the locking mechanism and is far from optimal
– the speed of rotation is very low. The prop tip moves very fast but the center not.
– it is hidden behind the pilot

On every prop the very end of the prop blade (say 10) cm is not effective.
This is the place where vortices are created around propeller tips. They create most of the noise and drag but no lift.

why-large-propeller-is-better-for-paramotors

 

Increase in prop diameter by 5% will lead to increase in thrust by around 10%.

Conclusion> Have the paramotor cage larger by 3cm (1.2 inch) on each side and gain 10%. Come on, that’s a good deal.

Next step is to decide if you need a clutch or not. To find out click here>