April 15, 2015

Ps =(T-D)V/W

In aircraft design and analysis, there is an equation called specific excess thrust. This equation describes how fast an aircraft can climb given its current state, making it an important indicator of maneuverability and performance. The equation for specific excess thrust (Ps) is as follows

Ps =(T-D)V/W

where T is thrust, D is drag, V is velocity and W is weight.

This equation, beautifully crafted in its simplicity, invites a great analogy for life—as nature tends to do. I’ve thought a lot about this equation. On my senior aircraft design team in college, I was the self-designated aircraft performance lead, wanting to better understand what made one aircraft better than another in the rawest and purest form. This equation was a great tool to get a baseline understanding of an aircraft’s capabilities. It also has great insight into life, and how balance, efficiency, and awareness affect your ability to navigate it.

When looking at the variables of this metric, Ps, you can begin to create a more natural and intuitive understanding of the term by dissecting its parts.

First there is velocity—your raw forward movement—how fast you cut through your environment. The greater this value, the more energy you have to put towards making your ascent into the clouds. This value has its drawbacks though as you can see in the next term.

Thrust minus drag is the struggle between forward motive power and the resisting force to that motion. Thrust is the forward force that propels you. Given your fuel and the way you turn that fuel into motion, thrust is the driving force that makes lift possible. However, at some point increasing thrust just makes you burn fuel faster, continuing the climb but potentially sacrificing the mission.

Drag, on the other hand, reveals where motion is met with resistance. Drag is a function of velocity whereby the faster you go, the more resistance you experience. The shape of your craft, the curves and features that exist, all work against forward motion. Even the wings that provide lift and make it possible to climb create drag as you push forward.

And finally weight. Your ability to climb is directly influenced by it. The more you have, obviously, the more difficult it is for you to climb. But consider this--fuel has weight. Without that energy an airplane couldn’t move. Wings have weight, but without wings a plane wouldn’t produce enough lift to fly.

The point here isn’t to just cut more weight or find a way to eliminate drag altogether; it’s finding what is necessary for you to reach new heights while still meeting your needs. It’s about understanding what resistance strengthens you and what is just holding you back. These forces live in balance, affecting performance in all aspects. As a designer, you must choose the parameters that fit your mission. As much as anything else, aircraft design is about harmony, balance and self-awareness.

As you hustle, check your fuel gauge. You might be climbing, but are you in danger of burning out? Are the features that give your life shape ones you want to hold on to or do they create too much drag or weight without contributing to the climb?

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