04 May 2005

Helicopters and tornadoes

One of the things I love about flight instructing is using examples of life experiences to reinforce a lesson I'm trying to teach.

I try to individualize instruction to the recipient. Midwestern students generally are familiar with the destruction caused by tornadoes. Believe it or not, there is a comparison between helicopters and tornadoes that helps students understand both.

Helicopter rotors rotate. DUH!
Ahh, but there are subleties that go with that rotation that are amazing! Gyroscopic precession. Coriolis Effect. No need to go look those up for now, unless I have "tweaked" ya! I'll get into them in detail later.

I need you to get into visualization mode again. Think of looking down at a helicopter in flight, moving at 100 miles per hour.

Now, think of the main rotor. For the sake of simplicity, let's assume a rotor blade is turning at 500 miles per hour at its' tip, (and actually, that's pretty close!)

Back to the view from the top.........when the rotor crosses the tail of the helicopter, the only wind it senses is its' rotational speed.......500 m.p.h.. But as it starts its' journey forward into the wind, the "relative wind" going across this airfoil increases until it reaches the 90 degree point...
directly abeam the helicopter. What is the total wind it is experiencing at this point?

500 m.p.h. tip speed
+100 m.p.h. helicopter speed
=600 m.p.h.

Now, the rotor continues its' journey forward to the point where it is exactly in front, and again, the only wind it feels is its' own speed........500 m.p.h..

From this point in the journey, our rotor feels continuing diminished wind, because it is running away from the wind of the helicopter's forward speed! At the "abeam" point, it senses:

500 m.p.h. tip speed
- 100 m.p.h. helicopter speed
= 400 m.p.h.!

Part of what produces "lift" and makes flying machines fly, is the amount of air flowing over the airfoil. The "advancing blade" on a helicopter has an easier job of producing lift than does the "retreating blade"! This can cause some interesting problems for helicopter pilots......which we'll get into in another lesson.

A tornado rotates. A tornado moves across the ground. If the wind at the edge of a tornado is 200 miles per hour, and the storm is moving across the ground at 70 m.p.h., the wind on the advancing side of the tornado is:

+ 70

The wind on the opposite side is:

- 70

If you get hit by a tornado, do you want your home to be on the advancing side of this storm, (270 m.p.h.), or the retreating side, (130)?
DUH again!

Now you know why some homes hit by tornadoes are totally destroyed, and some, just a few yards away, are relatively undamaged!

This applies to Hurricanes too, by the way, only on a much bigger scale, obviously!

Neat stuff, huh?

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