Tuesday, May 25, 2010
The Myth (Almost) Of Tailwinds
Logic suggests you should have tailwinds roughly 50% of the time. Logic is wrong.
Even if that were true (which it isn’t), simple math suggests that headwinds act on an airplane longer than tailwinds, so you’ll always lose time in any consistent headwind/tailwind situation. A 100-knot airplane flying into a 20-knot headwind will require 2+30 to complete a 200 nm trip. Turn the airplane around, and the return trip will demand 1+40. That’s 4+10 for the round-trip. In no-wind conditions, the same airplane will demand only 4+00 for the round-trip, so you’re better off with no wind at all than with a tailwind. In order to break even on the return flight, you’d need to increase the tailwind to 33 knots. (The situation deteriorates even further as the wind increases in strength. With the scenario above, a 50-knot, direct head-/tailwind results in a total flight time of 5+20 compared to 4+00 in no-wind conditions.)
One standard trick to avoid losing speed to winds is to fly the tailwind leg as high as possible, where the wind is strongest with minimal influence from the topography below, then make the return trip as low as feasible to operate in the least headwind.
Then, too, pilots who’ve been flying for a few years know that any consistent wind from any direction on a round-trip always costs you speed. A direct, 90-degree crosswind of any velocity will always subtract a knot or two from groundspeed, and you won’t begin to realize any advantage from a tailwind until it’s at least five degrees behind you. That means you start off with a 10-degree disadvantage. Score: wind 190, pilot 170.
Similarly, strong winds often make the ride rougher, and that means less efficient. While it’s certainly possible to find tailwinds that will whisk you to your destination with a ride as smooth as a pool table, the cobblestone road is more often the case when the wind is brisk. Turbulence of any kind always costs you speed, more on some airfoils than on others, so smooth air with minimal wind allows a wing to operate with maximum efficiency.
In an age of XM weather and GPS direct flight plans, the tendency is to punch up the destination identifier and fly the resulting course line. That will work, but don’t be too quick to blindly follow the GPS mantra of direct-to.
For some pilots, there may be a better method, and it’s called pressure-pattern flying. This can be a partial solution to unfavorable winds, but it’s usually reserved only for high-Mach aircraft flying long distances. It doesn’t work well on short hops, because weather systems are too large and far apart to impart any benefits over short legs.
If you’re traveling coast to coast in a modestly fast airplane, you might consider selecting a route that logs more miles but provides you with better winds; then, pull the same trick and choose a more favorable route for the return. Airlines and other operators of long-range turbine equipment sometimes employ this technique to reduce en route time. After all, time, not distance, is all that matters, and the airlines are masters of that equation. (Remember that the next time you fly from San Francisco to Orlando by way of Boston.)
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