4 blade prop on BN2
Joined: Jul 2004
Posts: 513
Likes: 0
From: Sydney NSW
not a BN2 expert but...
This example shows the very small differences that accrue at low Reynolds Numbers with well-designed props. I have assumed conditions to be low-speed on the grounds that complainants are at ground level with bags of thrust applied. I chose a 9ft prop because the Hartzell manual was the first one I saw on the shelf. If you can up the Advance Ratio "J" to around 2.0 then for a power coefficient of around 0.30 then the four blader comes more into its own. But that would require 460kW [620hp] and a speed of 215kts!
Anyway sample figures to show 3 blades vs 4 blades, all else being equal
the four-blader at low Reynolds Numbers is slightly more efficient. This may result in lower noise levels. The example here in metric units is also a 9ft dia prop at 400hp, 1000lbf thrust and 97kts or thereabouts.
Three Blades
Diameter D 2.74m
Speed of Rotation n 1200rpm
Velocity v 50 metres/sec
J=v/(nD) 0.912
Efficiency η 73.40%
Thrust T 4.412kN Ct 0.1603
Power P 300kW Cp 0.1992
β at 75%R 28.2°
Four Blades
Diameter D 2.74m
Speed of Rotation n 1200rpm
Velocity v 50 metres/sec
J=v/(nD) 0.912
Efficiency η 74.20%
Thrust T 4.450kN Ct 0.1616
Power P 300kW Cp 0.1989
β at 75%R 27.9°
Anyway sample figures to show 3 blades vs 4 blades, all else being equal
the four-blader at low Reynolds Numbers is slightly more efficient. This may result in lower noise levels. The example here in metric units is also a 9ft dia prop at 400hp, 1000lbf thrust and 97kts or thereabouts.
Three Blades
Diameter D 2.74m
Speed of Rotation n 1200rpm
Velocity v 50 metres/sec
J=v/(nD) 0.912
Efficiency η 73.40%
Thrust T 4.412kN Ct 0.1603
Power P 300kW Cp 0.1992
β at 75%R 28.2°
Four Blades
Diameter D 2.74m
Speed of Rotation n 1200rpm
Velocity v 50 metres/sec
J=v/(nD) 0.912
Efficiency η 74.20%
Thrust T 4.450kN Ct 0.1616
Power P 300kW Cp 0.1989
β at 75%R 27.9°
