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• Helicopters have a speed limit that has nothing to do with laws.

• Well, unless you count the laws of physics.

• Hey it's me Destin.

• Welcome back to Smarter Every Day. The show where we do science. So today

• I'm gonna explain to you something pretty interesting about the dissymmetry of rotor flight.

• But before we get too serious, let's just have a little fun.

• Check this out. Nighttime flying with Carl.

• ( rotor noise ) Sick!

• ( rotor noise )

• ( excited shouts )

• ( laughing )

• Right there.

• 'kay we're going to do some light painting with a helicopter at night.

• ( rotor noise )

• ( music )

• ( music )

• Did you see that? Look real close. Go back to the image.

• If you look on one side you see this really tight radius of curvature.

• But on the other side you see a much larger radius of curvature. What's going on there?

• OK so to explain the effects of this unsymmetric travel of the blades

• I've rigged one up on a stick here, and I'm gonna try, let's see if this works.

• it's moving faster relative to the air because the helicopter's moving so you

• add those two values together, but the retreating blade, you subtract

• away the velocity of the blade from the forward air speed and that's the total relative

• velocity of the blade through the air. This causes some funny things.

• OK Let's start the blades and check things out. As the chopper flies forward, the air

• flows over both sides of the helicopter. The advancing blade is also

• travelling forward so this adds to the air velocity of the rotor on that side.

• Now as long as the air speed of the rotor stays under the sound barrier you're OK.

• But if the helicopter goes too fast you'll create shock waves and start

• to damage things. The retreating blade sees the same airflow of the vehicle movement

• but because the blade is travelling in the opposite direction from that movement the

• actual air speed of that rotor is less. This creates something called

• Dissymmetry of Lift, and to counterract this the rotor on the retreating

• side is given more pitch to produce more lift. This works up

• to a point, but if the helicopter goes too fast, the pitch becomes too great and

• you lose lift creating what's called a retreating blade stall.

• The cool thing about a retreating blade stall is that it is a self correcting problem. If you think

• about it due to gyroscopic procession, if you have a dissymmetry of lift between a left

• and right side of the helicopter, it won't roll the helicopter like you think it would, it

• actually pitches it. That's good news because as you're flying along if you get too fast

• and you get a retreating blade stall, it'll just slow the helicopter down, automatically.

• OK There's a lot of things I did not cover in this video series, but for the most part you should be

• way smarter than when we started on helicopters. Smart enough in fact where you can make an educated

• guess as to which of these three helicopters is the fastest in the US Army inventory.

• While you're thinking about that please consider going to the Facebook page. I put all the photos from the

• night flights on there. Go download them, use them as your desktop background, stuff like that. While you're there

• please Like the Facebook page. Also if you have ideas for future Smarter Every Day episodes

• It's been about a year since we started Smarter Every Day. If you have ideas for a one year episode I'm all ears.

• OK. Enough babbling. The answer is the Chinook. It is the fastest in the US Army

• inventory. I'm Destin. You're getting Smarter Every Day. Have a good one.

• ( music )

• [ Captions by Andrew Jackson ]

Helicopters have a speed limit that has nothing to do with laws.

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# The Helicopter Speed Limit - Helicopter Physics Series - #7 - Smarter Every Day 51

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ricky posted on 2014/09/17
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