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Hey it's me Destin. Welcome to Smarter Every Day. So you know you're in trouble
when you have to break out the tinker toys to explain a concept. What are you gonna build?
(son) Tinker toy ducks, scrod and rolls over your ham. [??]
Good idea. What are you gonna do? (Daughter) The sunset. The sunset.
OK. So we're gonna start building. Go.
Alright here at Smarter Every Day we're right in the middle of a series on how helicopters work, and if you
recall, I told you at the beginning that helicopters are very very complicated.
So here's the tinker toy helicopter that I just made, and if you recall from the other videos,
I told you that as the blades on the helicopter spin around
they have the ability to change pitch as they go around
in the rotor disc. Now this is called cyclic pitch, and if you don't
understand this concept you need to go watch this remedial video so you can remind yourself
about what I'm talking about. So let's assume that we fully understand how that works.
Here's the question. If I have a helicopter and I
simply want to make a manouver and I want to tilt the helicopter up and forward just like
this, how do we change the pitch in the rotor disc. So
to me it's logical that I would want to increase the lift on the back of the
rotor disc, so what that would do is that would cause more lift here which would
cause it to tilt forward. This makes sense to me. Does it make sense to you?
Well here's the deal. You're absolutely wrong if it does. This is
why. You actually provide more lift on the side
of the helicopter and that will tilt the helicopter forward.
When I first figured this out, it blew my mind because it just did not make
intuitive sense, but it has something to do with this little gadget right here.
You may have seen one before. It's called a gyroscope.
So what does this have to do with helicopters. If you think about it,
it's a big mass spinning very fast. Look at
a helicopter. What do we have on top? It's a big mass,
spinning very fast. So when the rotors are aligned with the
helicopter body, if I wanted to pitch the helicopter body
forward like so, I would expect us to
be in phase right here, and I would expect to take less of a bite with this rotor and and more of a
bite with this rotor to rock it over is that what?.. (Carl) That's not the case.
It does seem like that would be, but due to gyroscopic
precession, any force on a spinning disc, which these blades
do act as a disc, takes effect over a 90 degree
phase. So if we give it a force here, to push down
or up to roll the helicopter forward, it'll actually take effect 90 degrees
later, and roll the helicoper sideways. So in order to roll it forward
we give the pitch when it's 90 degrees away
from .. (Destin) Oh, so it's like.. It's almost like predicting the future
or something like that. (Carl) Something like that. (Destin) So, if I wanted to rock the helicopter
forward, I would take less of a bite when I'm 90 degrees out of phase
and more of a bite over there, and that would do it? (Carl) Yep, so the
blade here pulls up. This one pushes down, and it takes effect 90 degrees
later when it's parallel with the machine, and the machine will rock, like so.
Yeah. I'm not getting it either.
In fact, I got a one-on-one explanation from an ex-pilot at the
Smithsonian and I still didn't get it. To control to
90 degrees in front, on the swashplate. (Destin) Like everything on
Smarter Every Day, I finally understood this when I made an experiment for myself.
Alright so Carl and I have setup a really super high tech experiment involving
bicycle wheels. Hey the Wright brothers did it. It's good enough for me if it's good enough for them.
And we have a camera aligned along a force
application device, which is a metal strip,
and do you want to explain what we've got going on here? (Carl) Alright, we're going to (Destin) Wait!
I'm better, go ahead. (Carl) We're going
to apply a force, straight up, and as you can see here
the tyre rotates in the same plane as we're moving this bar.
But, when it's spinning, it's going to be different. (Destin) Let's..Let's just check it out.
Here we go. I used to play with my mom's exercise
machine when I was like 5, so I'm highly qualified to apply angular momentum
Angular momentum applied! Hit the brakes.
(Carl) Trying to control this thing. (Destin)Alright. (Carl) So now we're
gonna do the same test again. We're gonna apply force straight up, here.
And.. it rotates, 90 degrees from where we apply the force.
(Destin) Alright this principle is called gyros.. [cough] gyroscopic
precession, and that's basically the forces applied
orthogonal to the plane of rotation, it acts 90 degrees out of phase to that
applied force. I think it's pretty interesting. So, I have a plane of rotation
here of the force, but it actually acts in this plane.
And so if you look at the horizon that the camera is looking at as he pushes up, it rotates
opposite of that. It's pretty cool! Anyway
that's it. That's why helicopter blades operate 90 degrees out of phase.
Anything you want to add? Besides the fact that I was kneeling in a horse biscuit
the entire time? Hey horse. What do you know about
gyroscopic precession? [silence] It's what I thought. [laugh]
So I realise this was one of the more complicated videos and I hope you got it. If you would, leave me some
comments and let me know, so I can figure out how to best explain things in the future.
Also, if you're interested, subscribe because next week we're gonna talk about the
helicopter speed limit, and it's not because of the FAA. It's physics.
I'm Destin. You're getting Smarter Every Day.
[ Captions by Andrew Jackson ]
Captioning in different languages welcome. Please contact Destin if you can help.
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Helicopter Physics Series - #4 They're Gyroscopes - Smarter Every Day 48

3278 Folder Collection
Furong Lai published on December 17, 2012
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