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Astronaut Ed Lu's second letter from the ISS

04 June 2003

Flying

Part 2 of 2

In reality, there are lots of handrails and other objects to grab
onto so you can actually just move your way slowly along them making
continuous adjustments to your speed and direction. Most of the time
your initial push is enough to give you enough speed to get where you
are going. The handrails along the way end up being used to make
those midcourse corrections to your trajectory and to control which
way you are facing. If you get your initial push-off about right then
you shouldn't need to push or pull very hard on any of the handrails
from there to your destination (unless of course you have to turn a
corner). I think the closest thing on the ground to this is swimming,
but it is very different since in the water if you don't keep pushing
yourself along you stop pretty quickly. 

The next problem when flying is to keep yourself pointed the way you
want and to control your rotation. If the force you impart from a
push-off point is directed through your center of mass you will not
spin yourself up. But if you don't push through that point, you end
up making your body rotate around your center of mass which can mean
doing flips or rolls across the room. So if it turns out that a
straight line from your push-off point through your center of mass is
exactly where you want to fly, then great - all is well and you will
fly there without rotating. If not, think of rowing a boat with one
paddle - it is hard to both go straight and keep the boat pointed
where you want. In space what you have to do is apply a torque to any
handrail you push off by twisting it as you push. This counteracts
the twisting moment you get from simply pushing off the handrail.
This is the equivalent, for you pilots, of flying a twin-engine
airplane with one engine out and having to kick in a whole lot of
rudder. Of course by using two hands on separate handrails you can
make this much easier because you can apply a lot bigger torque than
you can by twisting one handrail with a single hand.

Which gets me back to chasing Sox down to the node with all that
equipment in the one hand. I had the equipment in my left hand and
was holding it close to my chest, and my right hand was outstretched
in front of me grabbing onto and pushing off of handrails. The
problem with that is that with the big lever arm (distance between my
right hand and my belt buckle), you have to torque the handrail
really hard to control your motion. Which is why I had problems the
harder I pulled to try to catch up to Sox. One way to minimize this
problem I've found is to keep your pushing hand closer to your waist
(this makes sense since it is then closer to your center of mass).

I've since been experimenting with different ways to fly around while
carrying equipment. If the object is big and bulky, the easiest thing
to do is to simply let it fly on its own. Push it in the direction
you want to go, and follow along with it while not actually carrying
it, giving it a nudge here and there to keep it on track. Another way
to carry something like a bag is to simply hold it in your legs while
you use both hands to fly around. Yuri and I have decided that a
monkey with a prehensile tail would do very well up here since it can
grasp handrails with not only his hands, but with his feet and tail!

Lately, I've been trying to perfect my technique flying across the
node. When flying from the FGB there is a handrail right at the
entrance to the node. If you grab that handrail for a split second as
you pass and immediately tuck, then you will do somersaults across
the node into the lab. If you time it just right you can grab a
handrail right at the entrance to the lab to stop your rotation. If
you time it wrong, you crash into the wall or go careening into the
lab. Single flips are pretty easy now, but when doing double flips I
still end up hitting something about half the time. I guess I need
more practice. 

In the end, you don't even think much about the mechanics of flying
any more than you do about the mechanics of walking on the ground. It
is pretty intuitive, so you don't need to be a physicist to figure it
all out. But I am still having fun thinking about the physics of
flying! 

-Ed Lu-

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