Peter Garrison - TABULATING TAKEOFF

TABULATING TAKEOFF

If the runway’s too short, at least it’s virtual.

GALEN HANSELMAN, who has published several guidebooks and charts for pilots interested in landing on something other than 5,000-foot paved runways, sent me his twovolume flyer's guide to Utah and the associated "Supplemental World Aeronautical Chart," which resembles a WAC chart but includes a slew of backcountry airstrips not on the WAC.

The quality of his productions and the amount of effort that he has put into them are impressive. One volume of each set covers the airstrips themselves in great detail, with frank and often rather disconcerting comment about their condition and risks; the other concerns why you would want to go there – history, lore, hikes, scenery, fishing and so on.

I exchanged a few emails with Hanselman, and at one point he commented that he wished he had something more to tell his readers about effects on takeoff performance of wind, altitude, runway gradient and surface and so on than just the rules of thumb found in the late Sparky Imeson's Mountain Flying Bible, which is, if you’ll pardon the redundancy, the bible of mountain flyers.

I responded that I had a computer program for calculating takeoff distance that incorporated those variables and more, and I ought to be able to come up with something.

Ah, the vanity of human wishes! It turns out that takeoff distance is very difficult to calculate with anything like precision. And, for that matter, how would you know that you had the answer right? The best you can hope for is to simulate a few existing aeroplanes and see how well your results match the POHs.

Here’s how the program works. At intervals of a tenth of a second it calculates thrust from a few known – or guessed – propeller characteristics. From this it subtracts resistance due to friction, drag and runway slope, and applies what’s left to the mass of the aeroplane. This results in an acceleration and a certain distance travelled. The process repeats itself, reporting speed, distance, acceleration,

Running up to full power against the brakes.

lift, and so on at one-second intervals – that is, every ten cycles – until the virtual aeroplane reaches its takeoff speed.

The procedure is pure physics and ought to work fine. What goes wrong is the data. Garbage in, garbage out. Thrust and friction cannot be known with precision; each type of propeller has its own quirks, each backcountry airstrip surface is unique, each pilot handles his aeroplane in his own way, and many of the doubtful variables influence one another.

It often happens, however, that even when a computer simulation gets the wrong answer it may still teach us something new or call our attention to something we’ve overlooked.

For example, if I vary the time to full power between zero and, say, seven seconds, I see very little difference in takeoff distance. Why? Well, it’s common sense, really: The aeroplane is moving very slowly at the start of the takeoff roll, and so it doesn’t roll very far in the first few seconds no matter what you do with the throttle. A rolling takeoff may help you avoid getting bogged down in soft soil or sucking up pebbles into your prop, but it won’t make a noticeable difference in where you leave the ground. Running up to full power while standing on the brakes allows you to adjust the mixture for best power at high altitude, but it does not materially affect the length of the takeoff roll.

Since the aeroplane is moving fastest at the end of the takeoff roll, the takeoff distance is going to depend heavily on the liftoff speed. For instance, rotating a Skylane at 60 knots rather than 55 (those are calibrated airspeeds; the Skylane’s ASI is way off) lengthens the takeoff roll by 25 percent.

The influence of aerodynamic drag is relatively small. Thus, delaying flaps until late in the takeoff roll doesn’t make much difference. Surprisingly, too, although there is an “optimum” compromise between increased drag and reduced friction when you hold the nose up for

Knowing how much runway you will need can be a life saver.