How fast is the rotor of a Helicopter

With an exposure time of 1/100s and a roto diameter of 11m one estimates about 220m/s for the speed of the blade tips.

One often reads again that the blade tip of a helicopter reaches supersonic speed (over 340m /s). In hover, this is not the case for this helicopter. The helicopter, Eurocopter BK117-C1 has a rotor diameter of 11m. During the exposure time of 1/100s the tips moved about 2.2 meters. The rotor moves at approximately 220m /s and turns nearly 7 times per second.

Defraction limit using a digital camera

The resolution of a lens is limited due to diffraction in dependence of the aperature.

Die Auflösung eines Objektivs ist beubungsbegrenzt von der Blende abhängig.

In the picture you can see small sections (about 100 pixels wide) of a photograph that was taken with a 50mm lens from a distance of 5m from a laptop screen. We see significant uncertainty in the larger aperture numbers (22). The lower quality in Aperture 1.8 is due to other aberrations.

Due to the diffraction limit the diameter of a pixel is d = 2.44 · λ · B, where F-number B and the wavelength λ determined. With a typical value of λ = 555nm we obtain d = B · 1.3μm. The typical pixel size of today’s digital SLR cameras is about 5μm, so that we reached at f / 4, the theoretical limit. Digital Cameras have in some cases significantly smaller pixel sizes because they work with smaller image sensors.