The MacDonald-Douglas MD 520N helicopter is part of a family of light utility helicopters with a revolutionary design. The dispensed with a conventional anti-torque tail rotor in favor of the Hughes/McDonnell Douglas developed NOTAR system. This helicopter was spotted in the Turks and Caicos Islands and the photos are posted here for your delight.
The MacDonald-Douglas MD 520N helicopter is part of a family of light utility helicopters which includes the MD 500E, and MD 530F. One was spotted in Providenciales for the first time, and as usual, I worked my camera and did some digging on the principle of operation.
The MD 520N was launched in January 1989, first flew May 1, 1990 and was awarded certification on September 13, 1991.
The MD 520N introduced a revolutionary advance in helicopter design being dispensed with a conventional anti-torque tail rotor in favor of the NOTAR system. I’ve reproduced Wikipedia’s explanation of the principle of operation.
How NOTAR System Works
First of all, the name is an acronym derived from the phrase no tail rotor. The system uses a fan inside the tailboom to build a high volume of low-pressure air, which exits through two slots and creates a boundary layer flow of air along the tailboom utilizing the Coandă effect. The boundary layer changes the direction of airflow around the tailboom, creating thrust opposite the motion imparted to the fuselage by the torque effect of the main rotor. Directional yaw control is gained through a vented, rotating drum at the end of the tailboom, called the direct jet thruster. Advocates believe the system offers quieter and safer operation.
The Coandă effect is a result of entrainment of ambient fluid around the fluid jet. When a nearby wall does not allow the surrounding fluid to be pulled inwards towards the jet (i.e. to be entrained), the jet moves towards the wall instead. The fluid of the jet and the surrounding fluid should be essentially the same substance (a gas jet into a body of gas or a liquid jet into a body of liquid). In one application, a jet of air is blown over the upper surface of an airfoil, which can have a strong influence on the overall lift, especially at high angles of attack when the flow would otherwise separate (stall).
- Air intake
- Variable pitch fan
- Tail boom with Coanda Slots
- Vertical stabilizers
- Direct jet thruster
- Downwash
- Circulation control tailboom cross-section
- Anti-torque lift
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