symmetrical vs asymmetrical
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symmetrical vs asymmetrical
i have been led to beleive that if you beep the rrpm down (to about 98%) on a machine that has symmetrical blades you can get more lift for the same amount of torque. is this true and does it have the same effect on a machine that has asymmetrical blades a la as350? (not that the 350 has a beep range but rrpm can be adjusted).
It was certainly the case on the Bell 205 (symmetrical), we would beep down from 6600 to 6400 N2 and pick up a few more miles per tank of fuel.
No accurate knowledge of the aerodynamic considerations, but I have had discussions on this in the past along the lines of:
- Normal operating rpm better for vibration reduction and best tail rotor authority, but main blades running at something less than most efficient angle of attack.
- slow blade down a bit in the cruise, need to pull more pitch to make same lift, same induced flow so angle of attack increased to most efficient, aircraft a bit more bouncy but getting better lift/drag and therefore more fuel efficiency.
I guess it depends whether a non-symmetrical rotor at normal rpm is at its most efficient angle of attack or not.
Also, I'm pretty sure the AS350B has symmetrical blades but the BA doesn't.
No accurate knowledge of the aerodynamic considerations, but I have had discussions on this in the past along the lines of:
- Normal operating rpm better for vibration reduction and best tail rotor authority, but main blades running at something less than most efficient angle of attack.
- slow blade down a bit in the cruise, need to pull more pitch to make same lift, same induced flow so angle of attack increased to most efficient, aircraft a bit more bouncy but getting better lift/drag and therefore more fuel efficiency.
I guess it depends whether a non-symmetrical rotor at normal rpm is at its most efficient angle of attack or not.
Also, I'm pretty sure the AS350B has symmetrical blades but the BA doesn't.
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The effect has nothing to do with symmetrical airfoils, it has everything to do with the fact that most helos are a compromise between hover and high speed, and one rotor must take them both places. In short, most helos have a bit too much blade area to be fully efficient in a hover, since that same rotor must be taken to high speed cruise.
To make the rotor seem to have less blade area, you can slow down the tip speed, which effectively makes the rotor fly closer to stall, at a higher and usually more efficient average angle of attack. This lower rotor speed allows you to hover with less engine power for your gross weight. But the resulting effect on tail rotor pedal margins could ruin your day.
At high speed, faster rotors are nice, because the extra lift potential is necessary for near-stall maneuvering.
To make the rotor seem to have less blade area, you can slow down the tip speed, which effectively makes the rotor fly closer to stall, at a higher and usually more efficient average angle of attack. This lower rotor speed allows you to hover with less engine power for your gross weight. But the resulting effect on tail rotor pedal margins could ruin your day.
At high speed, faster rotors are nice, because the extra lift potential is necessary for near-stall maneuvering.
Nick, what's the answer for the cruise then - I think most posters here are talking about beeping down the Nr in the cruise rather than the hover. I guess the improved lift/drag ratio is still the answer as long as you accept the reduced manoeverability/stall margin.
For a change, I'm with Crab. On the 412, beeping down to 97% in the cruise improves range (and reduces noise signature), but the RFM requires 100% for take-off and landing.
