DARPA DiskRotor

AeroFranz said:
You'd think DARPA was burnt enough with the Heliplane and CRW. But it is an important mission to fulfill...maybe better luck this time?
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The Heliplane was more a factor of the contractors running out of money. First Adam Aircraft (provider of the airframe) went under then Groen Brothers (rotor system) pretty much did the same.
 
In 1962, Lockheed proposed a VTOL variant of the F-104 Starfighter. It had a small, triangular disc/rotor mounted over the center fuselage. Control was by tilting the triangular disc tips (new word?).

During the 1970s, a variety of manufacturers proposed various Stealth helicopters, though I doubt if any flew or was made public. In 1989, Testors released plastic model kits of a trio of fictional, stealthy aircraft. Stingbat was the fictional stealthy helicopter with scimitar (constant radius) rotor blade that could retract into the large disc.
 
RanulfC said:
Ok, question? While Boeing 'admits' that the disk-rotor would be "very difficult" but "possible" I seem to undertand that a majority of this is the aerodynamics of the "disk-wing" itself, is that correct?

So, why not a "traingle" or "hexagon" type wing instead, especially if you're going to have to have fairly large wings in addition to the rotor housing itself?

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Randy
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A triangular disc might provide some lift. See Lockheed's 1962 proposal for an F-104 Starfighter with a triangular disc/rotor rotating above the center fuselage. It was controlled by tilting disc tips.
OTOH a hexagonal disc is a totally different concept from an aerodynamic perspective. Since a hexagon is almost circular, it lacks leading edges. In comparison, regular rotor blades have very high aspect ratios and generate lift similar to the wings on competition sailplanes. IOW rotors need lots of "leading edge" to generate enough lift.
Helicopters can only use those extremely high aspect ratios because rotor blades are stiffened by centrifugal forces.
And to answer some one else's question: first, second and third generation helicopters (e.g. Sikorsky S-61 Sea King) have constant chord, constant angle of attack rotor blades. Constant chord rotor blades are easy to "extrude" in wood or metal. They have constant angle of attack because that is the easiest to manufacture. Individual rotor blades maintain pitch stability with airfoils that curve up at the rear (see Wortmann and Fauvel) .
It was only circa 1980 when Westland Lynx flew with BERPs (sp?) with fancy rotor tips. It is possible to build a wide variety of rotor blade planforms, but no one seems to have done it YET. Emphasis on YET.
 
The shift of the center of pressure is also what leads you to have as much as suitable a constant chord blade: at every rotation each blades change pitch constantly from low to high in the retreating blade sector. With a varying center of pressure (and aerodymynamic point oscillation for the entire blade along the 360 rotation), you have a fluctuation that could affect stability and structural integrity.
Many blades are in effect symmetrical (and even NACA 4 symmetrical) where the center of pressure is only slightly affected by pitch angle. That way, center of pressure and blade aerodynamic center are somewhat constant). What you loose in theorical performances is re-gained by the power range available with a simpler system (less stability equal more power losses through collective, flapping (stability is gained via blade loading that increases the angle of flapping) and anti-torque).

For fixed pitch rotor application (quad rotors and other drones), you can introduce a varying aspect ratio along the blade length and some twist with fairly good results.
 
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