Name: The Helicopter Speed Limit - Helicopter Physics Series - #7 - Smarter Every Day 51
Uploaded: 2014-09-17T09:33:37.000Z
Duration: 5 min 40 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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

Well, unless you count the laws of physics.

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.

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

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.

There we go.  So as the advancing blade comes around,

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

please Tweet me, I'd appreciate that, at SmarterYoutube.

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.

Subtitles ListPlay Video

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

pitch

side

advance

lift