Disk Loading Calculator

Disc loading indicates how hard the rotor must push air down to keep the aircraft in the air. It is a single value calculated by dividing weight by the rotor disc area. This value distinguishes a helicopter that can hover for hours from a jumpjet that can only remain at one position for a few minutes.

If you enter two of three values (weight, rotor diameter or disc loading), this tool will calculate the third in either metric or imperial units. Selecting "hover power adjustment" also estimates the downwash velocity and power required by the rotor to maintain hover.

What is disc loading?

The disc loading (DL) is the value obtained when the weight of the aircraft is divided by the area swept out by the rotor disk. This refers to the full circular area that the rotor blades describe as they spin. Although only the thin rotor blades are present in this circle, aerodynamically speaking the entire disk is pushing air down.

where: W = weight of the craft, A = area of the disc, D = diameter of rotor. The unit for the result is a pressure unit, usually pounds per square foot or Newtons per square meter. A lower value means that the rotor is pushing a large amount of air down at a reasonable speed. A higher value means that only a small amount of air is accelerated to very high speeds, which requires much more power. If hover performance is important, then a lower value is better.

Here's how to use this calculator tool:

Enter two of the three values (aircraft weight, rotor diameter or disk loading) and the tool will calculate the missing value and display it next to the other values. Often you know the weight and size of the rotor and want to determine the disk loading, so enter those two values and read off the result. Select the appropriate units for each field. Both metric and imperial units can be used, with automatic conversion on entry. For multi-rotor drones, specify the number of rotors to ensure that the weight is distributed across all rotor disks. By activating the "Hover Load Adjustment" option, you can also display the induced velocity, hover power and power density.

Explanation of formula:

The mathematical principles are based on a combination of geometry and Newtonian mechanics. The area of the rotor circle is a circle, so it increases in proportion to the square of its diameter.

For example, the UH-60 Black Hawk has a maximum gross weight of about 20,000 lb and a main rotor diameter of 53.7 ft. Since the rotor disc area is approximately 2,265 square feet, this gives an approximate loading per unit area:

For a general helicopter this is in the middle range and significantly higher than for light helicopters (2 to 4 lb/ft2), but much lower than for tiltrotor aircraft (20 to 25 lb/ft2, e.g. V-22). The Harrier Jump Jet's load per area unit of over 100 lb/ft2 is in a completely different category.

Load per area unit, downflow and performance:

The loading per unit area is not only a measure of the rotor itself; according to rotor aerodynamic theory, it determines the downwash velocity and power required for hover. The air flows out of the rotor disk with an induced velocity.

The Greek letter rho represents the air density, which is about 1.225 kg/m3 at sea level. The ideal lift force is given by the product of thrust and this induced velocity; however real rotors require significantly more power, which must also be divided by the rotor quality factor (typically between 0.6 and 0.8).

The specific power, or thrust per unit of power, can also be directly calculated. As it depends on the square root of disc loading, the specific power improves by about 40% if the disc loading is halved. So large rotors with low disc loading need little fuel to hover while small rotors with high disc loading use a lot of fuel.

The calculation turns out as follows to result the Rotor Disk Loading:

Typical values for disc load:

The range of values for different rotorcraft is very large and often spans two orders of magnitude. The following table shows some representative values to compare your own results with.

Why disc load is important in design:

Rotor designers consider disc loading to be one of the most important criteria in determining main rotor size. Larger rotors reduce disc loading and improve hover performance at high altitude in particular but also increase weight and cost and require more space which can prevent them from being accommodated on ship flight decks. The value chosen is typically a function of mission requirements; helicopters required to hover over mountainous terrain at high altitudes will have low values while naval helicopters operating out of frigate hangars will accept higher values. Another important aspect is that the higher the disc loading, the faster the downwash velocity which can lead to brownout effects in dusty landings and also determines how far back ground personnel must stand back.

Tips for accurate results:

Use the total weight or maximum takeoff weight instead of the empty weight. The load on the rotor at an empty weight represents a configuration that is not flown in reality. Measure the rotor diameter from tip to tip so it covers the entire circumference. If only the radius is given in the specifications, double it. Be careful when balancing the weight between kilograms and Newtons. A helicopter with 4,500 kg weighs about 44,145 N, and confusing the two values will result in results that are off by a factor of ten. If the "weight" field is given in kgf, you can enter the mass value directly.