Blade area RATIO.......

[Tevens]

When sizing a propellor and a known Blade area ratio is required (from powering program) but a CPP is going to be fitted. At higher speeds to keep efficiency of the propellor the pitch is changed, does this decrease the BAR, which would be problematic as a higher BAR is required at higher speeds.

[Tevens]

sorry i don't sound very coherent.

basically does the BAR of a CPP change as it pitch changes?

[Eric Sponberg]

No, the Blade Area does not change in a controllable pitch propeller, only the pitch changes. The Blade Area is the actual area of the blades in ratio to the area of the circumscribing circle of the propeller. Both areas are constanst even as pitch changes, so the Blade Area Ratio is constant.

Eric

[Tevens]

thanks, so does that mean what does change is the disc area ratio

[Eric Sponberg]

I think you mean Developed Area Ratio. No, that stays the same as well. Same with Expanded Area Ratio, it is constant. All of these relate to the actual areas of the blades as they are manufactured. Once cast and machined, these areas do not change--they are fixed. The area ratios are constant.

Eric

[gonzo]

The Disc Area Ratio is what the view perpendicular to the propeller would be. As the pitch changes, it would change. However, it doesn't affect the calculations. CPPs only work efficiently at a modest range of pitches.

[DMacPherson]

Tevens, this may help.
Report 135 - Blade Area Ratio Defined, at: www.hydrocompinc.com/knowledge/library.htm

Don MacPherson