Why do the tips of chopper rotors light up in infrared images while the rest of the blades are regular? I have seen this in all of the infrared videos played by the news networks during the war.
The tips of helicopter rotors approach the speed of sound, and compressive heating of the air in front of them warms them up a bit, evidently enough to show up on infrared images.
I think they also generate “St. Elmo’s Fire” type effects under certain conditions. These, while too faint to be seen with the naked eye, are visible with a starlight scope.
GaryM is right, it’s St. Elmo’s fire. The blades spinning in dry, dusty air generate a tremendous amount of static electricity, which is visible in night vision scopes as it leaks off the blades when they approach the ground.
BTW, the green night images you see on TV aren’t infrared. They are based on light amplification, not heat.
Those green nightscope images that are typically shown are just light amplification. In the second round of images of the PFC Lynch rescue they showed images from the helicopter that looked like B/W images shot in daylight. The latest-greatest in military night-vision likely uses ambient and enhanced light, plus infra-red and maybe UV and X-rays and creates a 3-D image.
I thought they only turned about 300 or 400 mph.
For the Blackhawk with a 16.4 meter dia rotor spinning at 400 rpm gives a tip speed of 343 m/sec. speed of sound is 331 m/sec.
I found a spec for a military chopper that calls out a blade tip speed of 707 ft/s or 482 mph at hover. Add in the max cruise speed of 175 knots or 201 mph, and the max tip speed works out to 683 mph. That’s getting there. I don’t think any rotor tip has ever exceeded the speed of sound, but I could be wrong.
That can’t be right. Not your math, that’s dead on, but the spec of 400 rpm doesn’t sound right. If this were accurate the thing would be very loud.
The nominal Blackhawk rotor speed is about 260 rpm.
I was using the cited rpm, didn’t have that info. But the UH-1 Huey did go supersonic at the tips, thus the distinctive sound, you could hear those things coming for miles. Probably is electrostatic corona showing up in night vision.