Aerodynamic Design of Modern Gyroplane Main Rotors

kolibri282

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title: Aerodynamic Design of Modern Gyroplane Main Rotors
author: Stalewski, Wienczyslaw
comment: The popular 8H12 airfoil is perhaps not the last word in autogyro rotor design. Here is an investigation of other profiles, using (of course) computational fluid dynamics. The chaps who do that stuff in my company come up with some really amazing results. This group from Poland has also conducted flight tests which showed a considerable improvement. Further tests are planned to take place soon. There is a very nice plot showing the driving and driven regions of a blade around the disk.
http://ilot.edu.pl/prace_ilot/public...242_2016/6.pdf
 

XXavier

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kolibri282;n1137886 said:
(...)
There is a very nice plot showing the driving and driven regions of a blade around the disk.
(...)

True... We are all used to the concentric diagram of driving/driven rotor regions, that refers to the special case of a vertical autorotation, while in the more usual case of the gyro moving at speed, the configuration is different, the rotor being clearly driven 'from one side'...

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Jean Claude

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Thank you very much, Juergen, for sharing with us the publications you find on the theory of gyroplanes.

Concerning this one, I have some remarks to make:

- A good point is that the author estimates the minimum drag coefficient of profile NACA 9H12 close to 0.011 in fig 12, instead of 0.06 obtained in wind tunnel to low turbulence. This seems to me more realistic in real working conditions of a rotor.

- Less relevant, however, is that it assigns the new profile family the lower Cd [SUB]min[/SUB] than the 9H12 reference profile.
i.e Cd [SUB]min[/SUB] = 0.0111 for Naca 9H12 and Cd [SUB]min[/SUB] = 0.0104 for ILW LT 09. It just seems to me due to the lower relative thickness. (See NACA report 610, fig 53)

- The author finds 16 rpm lower than with the 9H12 profile. However, a lower drag coefficient normally leads to a slight increase of the autorotation rpm. Therefore this suggests that aerodynamic collective pitch has inadvertently increased in flight. So, the profile power was artificially reduced, and the comparison of drag with respect to the reference rotor is distorted.

- The author says to have seen an increase of 10% maximum fligh speed, thanks to this only change of blade profile. But this seems simply impossible to me: Since parasitic power increases like the cube of forward speed, then it increases by 30% for 10% forward speed increase. However, at 180 - 200 km/h, the parasitic drag of the airframe is roughly 2/3 of the total drag. To increase the maximum forward speed by 10%, it would then be necessary that the rotor drag has practically disappeared relatively to reference rotor!

The author considers that the "optimized" rotor should have blades with variable chord, from 0.2 m at the root until reaching the maximum of 0.32 m at 80% of the radius of 5 m
I wonder if its root 0.022 m thick and 0.2 m of chord can withstand the bending at rest of blades of 28 kg each. And if so, then a rotor of the same collective pitch and constant thickness and chord giving the same bending strain at the root (diameter 11 m) seems to me more effective.
 
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C. Beaty

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Here are some rotor drag measurements that I made in the 1970s, showing the influence of airfoil thickness. Measurements were made via a pressure transducer acting along the rotorhead roll pivot axis with an inclinometer mounted on the keel to determine inclination of the roll pivot.

The “skinny” blades were a set using an NACA 0006 thickness distribution but cambered for a flat lower surface and with enough reflex to eliminate the pitching moment. The spars were milled from 2024 bar stock with bonded, lap jointed 0.020” thick 2024 skins.

The relevant comparison was between the “skinny” blades and the Hughes blades which used the NACA 0015 airfoil.

Testing was performed at an airspeed of 50 mph using a propeller type airspeed indicator which indicated true airspeed rather than density based airspeed. Correction for air density would have lowered airspeed to ~40 mph which would have increased calculated values of induced drag and decreased profile drag.

The Bensen blades, of NACA 8H12 airfoil, have high values of profile drag because of air leakage from gaps between upper skin segments. The Bensen type wood blades pay a drag penalty for the external nose weights.
chart.JPG
 

Jean Claude

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I fear that in his fig 16, WS forgets that forward speed is limited by the stall of the retreating blade, as I show below.
Sans titre.png
 
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