View Full Version : Advancing Blade Compressibility
masimkhanniazi
11th May 2020, 18:36
I have two questions :-
1. How does advancing blade compressibility effects stability of a helicopter?
2. As blade reached close to speed of sound, drag increases substantially! Which type of drag is it (Induced/Form/Rotor Profile) ? And what type of power is needed to counter that (Induced power/ Rotor Profile power etc)?
Thank you
Jimmy.
11th May 2020, 22:43
I haven't found in my books the answer for effects on stability, but have found that rotors are designed in a way that retreating blade stall tend to occur before compressibility stall. So, the increased profile (question 2) drag is its first manifestation. That means more power required. As the source of power is just one (the powerplant, 1, 2 or 3 engines), the definition of profile drag/power (induced, parasite...) is just theoretical.
As the beginning of the retreating blade stall is indicated by vibration on the cyclic, than in the airframe and possibly an uncommanded pitch up (if the servos are overpowered), I think the compressibility stall may (and I'd like to be corrected) be similar, but with a pitch down tendency, due the side of the rotor the stall occurs.
The shock wave caused by the blade hitting MCrit (critical Mach Number) causes density changes in the air along with disruption to the laminar flow and resulting changes in lift and drag on the advancing side. As for the type of drag - parasite drag would be my guess but I stand to be corrected.
Jimmy.
12th May 2020, 10:34
Crab,
Ray Prouty, on a chapter about forward performance, discusses the profile power (used to overcome friction drag on the blades) and give 15% to 40% of the main rotor power in hover values as a reference. He also states that this value is "fairly constant" as the airspeed increases "until at high speed when blade stall or compressibility, or both, rear their ugly heads. At this point, the profile power inceases and usually becomes the dominant component." (Helicopter Aerodynamics, vol 1, p. 111)
Crab,
Ray Prouty, on a chapter about forward performance, discusses the profile power (used to overcome friction drag on the blades) and give 15% to 40% of the main rotor power in hover values as a reference. He also states that this value is "fairly constant" as the airspeed increases "until at high speed when blade stall or compressibility, or both, rear their ugly heads. At this point, the profile power inceases and usually becomes the dominant component." (Helicopter Aerodynamics, vol 1, p. 111)
Sounds good to me Jimmy - if Ray says it is so then it is so:ok: