The Culver Props calculator will give you a theoretical pitch...as if

the air were a solid and the prop is screwing itself through the air

without slip. Pitch(inches) x RPM = Speed(inches per minute) would be

the equation for this simplified view of the situation. However, if

you think about it for a moment, you'll see that if an airplane were

actually flying at this speed with this pitch, there would be no prop

wake or acceleration of the air rearward by the prop as the plane

passes through it. So the pitch must be a bit more than the simple

theoretical no slip equation above to account for both slip and the

increase in air velocity as it passes through the prop disk in order to

make thrust. I guess what I'm trying to say is, don't just use the

number from the culver props calculator. It probably isn't enough

pitch. Better to consider the force your prop is actually applying to

the airframe at the 70mph cruise and calculate mass of air the prop has

to push rearward in order to create this thrust at 70 mph. That's why

larger diameter props work well at low airspeeds. In order to create

the necessary thrust, they accelerate a larger mass of air to a lower

final velocity thereby creating less drag on the fuselage and other

airplane parts that must be dragged through this somewhat higher

velocity air. A smaller diameter prop would have to accelerate the

smaller volume of air to a higher velocity in order to create the same

thrust. I don't have time to go through all the equations and math at

the moment but that's basically what's involved. Probably better to

find a flying example of what you want to build and see what they're

using and if the pilot's satisfied with it, use it as an idea for what

you'll do.

--- In AirVW@yahoogroups.com, Robert Richtsmeier <cmpuflite@...> wrote:

>

> Mark Smith wrote:

> >

> > What pitch would folks suggest for a 58 inch prop doing 70mph. The

> > tables seem to indicate a higher pitch than I see folks actually

using.

> > Mark, *try this calculator it looks as though 23 should do it. *

> >

>

>

> http://www.culverprops.com/pitchselection.htm

> rich

>