But, wait isn’t that also where Thrust = Drag? Does that mean if thrust = zero, then my drag is then also zero (not likely {:>). Sure enough if you look at the formula for Thrust T if the factor DV goes to zero then the whole equation T = 0 because every factor is multiplied by Dv (zero in this case). So I must be missing something because clearly my drag at cruise is not = 0. Arrrggg! Never have felt that I understood this prop stuff.
Ed
Fixed pitch propellers designed to fly, just don't work well on the ground. Airflow over the front of the blade exceeds the speed of sound long before the prop tips reach that speed. Prop drag goes up sharply before the tips get close to that speed. Light plane propellers perform better once some speed has developed.
Over powering a prop makes it cavitate. That is the blade stalls for lack of adequate air inflow. Once the plane is moving, relative air speed over the blades, Alters the angle of attack and the stall is defeated and RPM drops as more work is accomplished by the prop.
The cruise prop is more likely to stall as the blade is formed with a high angle of attack. A climb prop is less likely to stall as the blade is formed with a shallow angle of attack.
Propellers designed for static and low speed thrust have wide blades with thick high lift airfoils. They have many blades 5 to 7 blades. In some cases two propellers in a counter rotating assembly will be used. The leading propeller may have fewer blades than the trailing propeller. The leading propeller may be turning at a higher or lower RPM than the trailing propeller. The two propellers will have different angles of attack.
You can fire up your fully loaded airboat and drive it across 100 yards of dry grass to the water if your 550 is running well.
The airboat prop might not get your plane above its stall speed. So, no need to go there.
Measuring the static thrust of a plane with a spring scale or similar, will get you the static thrust for that minute of that day.........and not much else.
Lynn E. Hanover
|