Rotor Disc Loading

Disc loading is the rotor's version of wing loading, and it does the same job: one number, thrust divided by the area carrying it, and from it follows almost everything about how the machine behaves. $DL = T/A$, where $A$ is the area of the circle the blade tips trace and not the area of the blades themselves. On a slender modern rotor the blades occupy perhaps eight percent of that circle, and it is the circle that matters, because the circle is what the air passes through.

The units to think in are pounds per square foot. That is not parochialism, it is simply where the whole vocabulary of rotorcraft was built, and 1 lb/ft² is 47.88 Pa exactly. Read a number against the families it separates. A light piston helicopter sits around 3 to 5 lb/ft². A medium or large turbine machine sits around 8 to 12. A tilt-rotor, whose proprotors have to work as propellers in cruise and are therefore much too small for the hover, runs near 20. A jet-lift fighter hovering on its nozzles is in the hundreds, which is why it needs a prepared pad and why the exhaust will strip the surface off anything less.

The commonest mistake on this page is entering a mass where a thrust belongs. In a steady hover the thrust equals the weight, and weight is a force. A 1 200 kg helicopter weighs about 11 770 N. Enter 1 200 and the disc loading comes out 9.8 times too small, which will make every downstream number — downwash, hover power, figure of merit — wrong in a way that looks entirely plausible until someone checks it against a real aircraft.

What disc loading actually buys is the subject of the induced-velocity and hover-power pages, and the short version is worth having in advance: downwash goes as the square root of disc loading, and so does power per unit of thrust. Low disc loading means a gentle downwash, a cheap hover, a quiet aircraft and a survivable autorotation, because a big slow-turning rotor stores more energy and descends more slowly with the engine off. Every one of those benefits is bought with rotor diameter, and rotor diameter is bought with hangar space, blade cost, tail boom length and the ground clearance the tips need. That trade is the reason helicopters look the way they do.

Two bookkeeping notes. If the aircraft has more than one lifting rotor, the thrust splits between the discs before it is divided by area — a tandem, a coaxial and a quadcopter each carry a fraction of the weight on each disc. And overlapping discs, as on a tandem with intermeshing rotors, share part of their inflow, so their combined effective area is somewhat less than the sum of the two circles.