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Saturday, September 21, 2013

Altitude and Airspeed: Money in the bank

I often describe the relationship between airspeed and altitude as like having a checking account: if you have extra cash, you can deposit it in your account; if you need cash, you can write a check. In the video below, I demonstrate an extreme version of this:

In this case, let's say that altitude is the bank account and airspeed is cash. I start off with a lot of money in my altitude account, but no cash. Immediately, I make a large withdrawal from the account to spend on a large purchase of airspeed. I then spend this money on getting me to my destination, which is the patch of trees on the beach. Just to show I'm not getting any funds from outside, I cut the mixture to kill the engine just as I'm coming up to the cliff.

The outside view also demonstrates something about precision landings: when you're trying to hit an exact spot, you can't worry too much about trying to make a greaser. Just plunk it down where you want it. If you had enough energy to make it a beautifully smooth landing, then you probably had too much energy. I plan to devote another post and another video to this topic in the near future.

The cockpit view lets you see how you hit the spot you're aiming for. Watch the patch of trees and note how they stay roughly stationary. (I'm actually aiming for a spot just before them, since I know I'll have to flare and roll out, but that's close enough to the trees to be good enough.) The goal is to get them to neither go up or down in the windscreen, but just get bigger and bigger. If they start going up, I'm getting a little low, so I trade some airspeed for altitude. If they start going down, I'm getting a little high, so I convert some altitude into airspeed. The goal is to run out of airspeed and altitude right on my selected mark.

Below is a sequence of images with the tree highlighted with a red rectangle to make it easier to visualize this concept. I put a blue line below the rectangle so you can also see how I'm trying to keep the point at the same distance above my sighting point.

Touchdown spot

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