Physics for Pilots

I've heard some pilots remark on how much they hate physics. I find this strange coming from people whose very lives depend on the proper application of fluid dynamics...

This is the place where I'll collect a few principles of physics and relate them to everyday aspects of aviation.

Kinetic Energy and how it relates to landing distance

The formula for kinetic energy is (m * v²)/2. For the purposes of this discussion we'll use the metric system because it doesn't require memorizing all sorts of arbitrary and stupid unit conversions.

Take a fully-loaded Skyhawk on approach to land at maximum weight (2450lb or 1111kg) and at a normal approach speed (62kts or 31.895 m/s). The kinetic energy of this airplane is 56.51x10⁴ joules. A quick glance at the short field landing distance chart tells us that it takes 525 feet of ground roll to get rid of this energy, for an average of 1.07 kJ of energy lost per foot.

Now let's take that same airplane and have it fly the approach at 65kts (33.439 m/s). Its new kinetic energy is 62.11x10⁴ Joules. Assuming the same energy loss to braking per foot that we had before, the new landing distance is around 577 feet. This is a 10% increase in landing distance for only a 5% increase in approach speed.

It's also interesting to note that this change in landing distance is about the same as a change in the field elevation from sea level to 2,500 MSL. Something to keep in mind the next time you obsess over the landing performance calculations on preflight.