SANSA Monitors Space Weather

a “soft field takeoff,” the effect on distance is again small, except to the extent that holding the nose up tends to get you airborne at the earliest possible moment.

The thing to bear in mind about getting airborne at a very low speed is that you may be unable to climb until you have gained more speed in ground effect. The takeoff roll may be shorter, but the distance to clear an obstacle is no different.

The big factors in takeoff distance are the ones you’d expect: density altitude, weight, wind, surface condition and gradient.

For unturbocharged aeroplanes, density altitude reduces the power and thrust available and therefore the rate at which the aeroplane accelerates. According to the program – your results may vary – takeoff distance increases by about 8 or 9 percent for every 1,000 feet of density – not pressure! – altitude. You can find presumably exact information about density altitude and weight in the POH, so it’s not necessary to memorise any rules of thumb. Weight is a critical factor because it affects both acceleration and liftoff speed. The Skylane POH, for example, shows the aeroplane rotating 5 knots slower at 2,400 pounds than at 2,950 and getting off the ground, at sea level, in 440 feet rather than 705.

The Skylane POH says that every 9 knots of headwind reduced takeoff distance by 10 percent. That’s a useful rule of thumb, but couldn’t they both be 10? Either way, the rule goes astray in really strong winds, since in a 55-knot wind a Skylane’s takeoff distance would be zero, not two or three hundred feet.

Tailwinds are a more sensitive matter, since they increase the liftoff groundspeed and so make the aeroplane eat up more runway at the departure end of its roll. The Skylane book says to add 10 percent for every two knots of tailwind, which seems extreme; a Mooney 231 chart shows a 26% increase for a 10-knot tailwind. My program predicts a 34 percent increase for a 10-knot tailwind for a hypothetical Skylane-like aeroplane. So you see we’re all over the map.

Various kinds of runway surfaces present different resistances to the rolling aeroplane, but it’s impossible to categorise surfaces in any precise way. My program, whose surface

Downhill and downwind - or uphill and into wind?

options extend from concrete to quicksand, shows a 50% increase in takeoff distance for a “soft” field, and 20 percent for “grass” – not to be confused with “turf” (around 5 percent). But how soft is soft, and how tall is grass? (In quicksand, the aeroplane does not move at all.)

Unlike surface condition, runway gradient can be specified very precisely. Hanselman has personally measured the slopes of all sorts of godforsaken landing strips, some no more than paler places in the sagebrush, and reports them to two decimal places. But then what? And, more troublingly, what if the wind is blowing downhill?

Well, here’s what the computer has to say. If you’re taking off with a 5-knot wind at your back, you’ll need a 5-percent downslope to get off the ground in the same distance as you would from a level runway with no wind. Oddly enough, it works the other way around too: a 5-knot headwind cancels the effect of a 5-percent upslope. This can’t be a rule of thumb, though; it’s too simple. When you take off with a tailwind your climb gradient will be unexpectedly shallow. Be aware of obstacles ahead that you would normally expect to clear; a tailwind may carry you into them.

A simple rule to remember – call it the two-thirds rule – is that the second half of your takeoff distance will take half as long to cover as the first, but you’ll travel twice as far gaining the second half of your takeoff speed as you did the first half. For example, if your takeoff roll is 600 feet and you lift off at 50 knots, you will have reached 25 knots in only 200 feet; but if it takes you 8 seconds to reach the 300-foot halfway mark, it will take only another four seconds to cover the remaining 300 feet.

Admittedly, that is hard to understand and even harder to remember. That is why some pilots use a simple go/no go rule for tight takeoffs. If you don’t already have more than two-thirds of your liftoff speed when you reach the midpoint of the runway, abort the takeoff. At that point, there’s not enough time left for thinking it over. j

how soft is soft, and how tall is grass?

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