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Skywriting rotorblades

Spartan RC demonstrated the concept of Skywriting rotor blades at 3D Masters 2005. This amazing blade design relies on persistence of vision which involves the perceptual processes of the brain and the retina of the human eye to retain an image for a split second. On a typical helicopter with a rotor speed of 1600RPM or higher, the image is refreshed more than 26 times per second; therefore by using persistence of vision solid flicker free images can be produced in thin air.

Background

When a bright light is moved quickly across a human’s field of vision, a line of light appears to stay for a split second. This is because the human brain and eyes are slow to interpret fast changes in light intensity. This phenomenon, known as persistence of vision, enables us see a continuous illuminated circle in the dark sky when night blades are used. In a similar manner, if a row of LED’s are mounted on a spinning rotorblade they would generate several concentric circles. But, what would happen if you had the ability to precisely switch on and off each individual LED at specific points of every rotation? You could, for example, generate half circles, quarters, even small individual dots. Using a miniature microcomputer one can achieve the required precision for the LED switching and therefore it is possible to fix the position of these dots which can then form text or images in a very similar way as a computer or mobile phone display.

Implementation

To generate such skywriting images, a small microcomputer is fitted on the root of each rotorblade. This microcomputer is responsible for rapidly switching on and off each LED with very high precision as the rotor blade spins around. To do this the microcomputer needs to be constantly aware of the rotorblade position. A simple, yet very effective way to accomplish this is by using an optical or magnetic reference. In our implementation this reference is simply a narrow beam LED mounted on top of the tailboom and emitting an infrared light towards the blade.

As the rotorblade passes over the infrared reference a sensor on the blade sees the infrared beam and enables the microcomputer to collect two very important pieces of information. First, the infrared reference indicates to the microcomputer that this is the starting point of a new revolution. Secondly, the microcomputer has already been measuring the time since the reference was last seen. This time, known as revolution period, makes the microcomputer aware of the rotor RPM.


Knowing the starting point, elapsed time and assuming that the measured RPM will not significantly vary between two consecutive revolutions, the microcomputer can work out the blade position and LED switching points simply by knowing how fast the blade is travelling. When this is done repeatedly, the same area is scanned several times showing the same dot pattern each time. Done at high speed it generates an excellent virtual display in thin air.


Using the display

Once such a display is implemented the possibilities are endless. The microcomputer can display any static text and simple graphics from a pre-programmed playback memory, it could make it scroll or even display the rotor RPM and battery level. Understandably in most cases someone would like to display his own message and having to keep up with modern computer developments USB was our obvious pick for reprogramming the displayed messages.

Also interesting is the ability to switch on the display in flight. Using an electronic RC switch and a spare receiver channel it is possible to control the reference LED. When the reference is off, the microcomputer switches off the display.

(Written by: Angelos Gonias)




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