Kiwiprops™

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Disk Area Ratio

Potential customers often question the Disk Area Ratio or DAR of Kiwiprop units.

The DAR ( sometimes referred to as Blade Area Ratio or BAR ) is the projected area of the blades expressed as a ratio over the area of a circle that the blades sweep or  P(i) x Radius^2.  Larger blades for the same  diameter will thus have a larger DAR, but this does not mean the propeller will be more efficient as is often assumed.  Interference between the blades actually reduces efficiency as the number of blades increases. Extra blades will however have extra thrust and absorb extra power.

Dave Gerr's definitive Propeller Handbook  page 76 gives a table listed below showing the multiplier to calculate changes in efficiency with changes in DAR.

To calculate the efficiency of a propeller with a different DAR - but same Pitch Ratio, multiply by the appropriate factor from the table.  For example if we know a 4 bladed propeller of Pitch Ratio 1.2  and a DAR of 0.50 has an efficiency of (n) = 0.55, to find the efficiency of the same propeller with a DAR of 0.90 - we multiply by the variable from the table which equals 0.900 so the efficiency of this unit with wider blades will be  0.55 x 0.900 = 0.49 - slightly lower than the previous 0.55.

It can be seen from this data that moderate changes in blade width and thus DAR make relatively small changes in efficiency - and that higher DARs reduce efficiency.  Kiwiprop units have relatively narrow blades and consequently lower DARs which enhances efficiency. The raked tips also increase the diameter of the units - we measure diameter at the center of the blade tip and this has the effect of apparently increasing  DAR versus measuring at the actual tip which increases diameter and thus lowers the DAR.

Any comparisions or analysis of DARs must be undertaken with great care and a detailed knowledge of propeller design variables. Taken in isolation the DAR of a propeller can provide little useful information. Using this one design variable in isolation can lead to quite erroneous conclusions.


PITCH       DAR = 0.30      0.50      0.65      0.80       0.90

1.4                      1.01         1.00      0.97      0.95       0.92

1.2                      1.150       1.00      0.965    0.93       0.90

1.0                      1.02         1.00      0.96      0.92       0.88

0.8                      1.025       1.00      0.95      0.90       0.96

0.6                      1.03         1.00      0.94      0.88       0.84

 

RULE OF THUMB: The combination of pressure force and blade thickness, gives the rule that the blade area should be at least 6 to 8 cm2 per kW propeller output

The graph below from a Volvo publication shows the trade-offs between blade area and the number of blades and efficiency. While you should always target higher efficiencies - as the power increases this will require higher bade areas which tends to lower efficiency.

These types of trade-offs highlight the multiple and complex trade-offs involved in any propeller optimisation calculation.