Aviator168
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I don't get this. How can the drag be lower when speed is higher?
Rotor blade chord
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XXavier
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Sounds strange, yes, but those are figures supplied by Vance. Anyway, 26 and 56 knots might be, in Vance's gyro, 'at the back of the power curve...'. In that region, total drag decreases with airspeed. The minimum could be between 56 and 87 knots. It's not impossible...
JEFF TIPTON
Senior Member
To me hard part to grasp is to imagine the rotor as a disc. It you tilt the disk to forty five degrees to the relative wind it will have more exposed area than when the disc has an angle of ten degrees. This change in frontal area will result in less drag and this change in angle is happening on the gyro has speed increases assuming level flight.
XXavier
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Not essentially different from a fixed wing, either. The angle of attack of the wing decreases with the airspeed and result in less frontal wing area and less wing drag...
Aviator168
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Yeah. Except the wing area is the entire disk and that's huge and you have to have some kind of positive aoa for it to auto rotate. If you can help it a bit with engine power, the aoa can be reduced and you can go faster.
XXavier
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Well, yes, the parallel between a rotor disk and a fixed wing is only 'essential'. There may be important differences, as JC has pointed out in this thread. Another difference is that the rotor 'disk' is a wing that does never stall, however large the AoA may be...
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This is only the rotor drag because that is what you asked about.
It appears to me the profile drag of the rotor remains the same throughout the airspeed range of the gyroplane because the rotor rpm is relatively constant (rotor rpm does go up slightly with an increase in indicated air speed).
It appears to me the induced drag goes down as the speed increases because the angle of attack of the rotor disk is reduced as the speed increases.
It appears to me the parasitic drag of the aircraft goes up by the square of the increase in the speed.
According to my testing The Predator’s Vx (best angle of climb) is 46kts; Vy (best rate of climb) is 49kts.
These numbers are typical of most gyroplanes and part of what you should know for your FAA knowledge test and the oral part of your proficiency check ride. It is also likely included on your pre-solo test.
The Predator has a very large frontal area because Mark Givans who designed The Predator did not make drag reduction a priority.
Consequently her top level speed is 90kts at altitude (7,500 msl) and 85kts closer to sea level. I have her over propped for cruise so her 160 horsepower becomes more like 144 horsepower (less at altitude).
Aerodynamics of the Helicopter by Gessow and Meyers would give you a lot of information about rotary wing aerodynamics and gyroplanes and has a focus on the basics.
There are lots of NACA reports on gyroplane aerodynamics and there are other books and articles in the Rotary Wing Forum “Technical papers, Books, and publications.
I recommend that you learn to fly a gyroplane before you attempt to design one because you will likely learn a lot from the process and your priorities will probably change as you define you mission.
Aviator168
Member
I just had my 10th hour of gyro instruction. Half way to solo by my instructor. I came from 3 axis.
