# Chopper rotors in Infrared **URL:** \ **Category:** Factual Questions\ **Created:** [April 13, 2003, 12:26am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354 "2003-04-13T00:26:09Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Longhorn49](https://avatars.discourse-cdn.com/v4/letter/l/50afbb/32.png) [@Longhorn49](https://boards.straightdope.com/u/Longhorn49)\ **Post date:** [April 13, 2003, 12:26am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/1 "2003-04-13T00:26:09Z")
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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 13, 2003, 12:36am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/2 "2003-04-13T00:36:34Z")
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. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [April 13, 2003, 1:04am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/3 "2003-04-13T01:04:30Z")
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. ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [April 13, 2003, 1:18am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/4 "2003-04-13T01:18:16Z")
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. ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [April 13, 2003, 1:22am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/5 "2003-04-13T01:22:43Z")
BTW, the green night images you see on TV aren’t infrared. They are based on light amplification, not heat. ---
**Author:** ![Coriolanus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/coriolanus/32/18769_2.png) [@Coriolanus](https://boards.straightdope.com/u/Coriolanus)\ **Post date:** [April 13, 2003, 2:13am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/6 "2003-04-13T02:13:41Z")
> [@](#): > > \*Originally posted by yoyodyne \* > \*\*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. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [April 13, 2003, 2:15am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/7 "2003-04-13T02:15:07Z")
> [@](#): > > The tips of helicopter rotors approach the speed of sound I thought they only turned about 300 or 400 mph. ---
**Author:** ![3.885AM](https://avatars.discourse-cdn.com/v4/letter/3/58956e/32.png) [@3.885AM](https://boards.straightdope.com/u/3.885AM)\ **Post date:** [April 13, 2003, 2:36am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/8 "2003-04-13T02:36:46Z")
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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 13, 2003, 2:39am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/9 "2003-04-13T02:39:44Z")
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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 13, 2003, 2:47am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/10 "2003-04-13T02:47:12Z")
> [@](#): > > \*Originally posted by bbbill \* > \*\*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. \*\* 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. ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [April 13, 2003, 3:09am UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/11 "2003-04-13T03:09:56Z")
The nominal Blackhawk rotor speed is about 260 rpm. ---
**Author:** ![3.885AM](https://avatars.discourse-cdn.com/v4/letter/3/58956e/32.png) [@3.885AM](https://boards.straightdope.com/u/3.885AM)\ **Post date:** [April 13, 2003, 5:15pm UTC](https://boards.straightdope.com/t/chopper-rotors-in-infrared/168354/12 "2003-04-13T17:15:39Z")
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.