Props and gear ratio's

[Guest GraemeM]

I need some help here please people, is there a site somewhere or some info that somebody has on props and gear ratio's?

 

For example, if I had a 74" three blade prop what would be the ideal gear ratio for a 582 rotax?

 

Or a 70" three blade , or if I had a 3.4:1 or 3:1 ratio box what would be the ideal prop diameter for this engine?

 

Thanking you all.

 

Graeme.

 

 

[Guernsey]

I presume that this is for a powered parachute so I would recommend you contact Bolly Props in SA as they have fitted many props to PPC's and done a lot of field testing on these flying machines.

 

Alan.

 

 

[Guest GraemeM]

Thanks, maybe I should add that I play with gyro's and fixedwing ultralights also, not just PPC's.

 

Graeme.

 

 

[farri]

Hi Graeme, You need to be more specific! Based on the information you`ve given, there is no correct answer. Gearbox ratio can`t be determined solely on the number of blades and diameter of the prop, nor can the number of blades and diameter be determined solely on the gearbox ratio.

 

Example A: We`ll assume that the engine is developing full power and we have the Rotax E type gearbox with 2.62:1 reduction. Our prop has three blades and a 74 inch diameter, now we need to find the correct blade pitch that will allow the engine to develop the required RPM and performance.

 

Example B: We have the Rotax E type gearbox 2.62:1 ratio. We want a prop with three blades and we want to determine the prop diameter . As props are not all designed the same we need to know the design of the prop then the diameter will be determined by the amount of pitch on the blades....To achieve the required RPM and performance, the coarser the pitch the smaller the diameter needs to be,or, the finer the pitch the larger the diameter can be.

 

There is no easy answer!

 

Frank.

 

 

[facthunter]

You've got considerations like tip clearance from the ground or structure.

 

Draggy planes like slower larger props also, (as a general principle). This is an advantage of having a gearbox. Nev