General Principles - Take-off — Page 160, Lesson 186

Let’s pick up right where the take-off distance story gets interesting — the runway surface itself. Even on a perfectly smooth, dry runway, the wheels don’t roll for free. There’s rolling resistance, and that comes from two sources: bearing friction inside the wheel bearings, and tyre distortion — the tyre squashing and flexing as it rolls. So even in ideal conditions, some of your thrust is being used just to overcome that rolling resistance. Now, if the runway is contaminated — by snow, slush, or standing water — you add two more drag components on top of that. First, fluid resistance: the wheels and tyres have to push through the liquid. Second, impingement: the liquid actually strikes and splashes against the aircraft structure. Both of these add drag. And here’s the key behaviour: this contamination drag increases with speed, but only up to a point. That point is called the critical speed — specifically, the hydroplaning speed. Above that speed, the drag starts to decrease. Why? Because once you’re hydroplaning, the tyre is riding on a film of water rather than ploughing through it, so the fluid resistance drops off. But don’t take comfort from that — any contamination at all increases drag, and increased drag means increased take-off distance. The hydroplaning speed is where the drag peaks, not where the problem disappears. Now, what if you reject the take-off and need to brake? This is where contamination really hurts. On a wet, icy, or snow- or slush-covered runway, the coefficient of braking friction is severely reduced. That’s the measure of how much grip the tyres have for braking. With less grip, you must severely reduce brake pressure to prevent skidding — if you slam the brakes on, the wheels lock and you slide. The consequence is that the stopping distance is greatly increased. So contamination lengthens both the accelerate phase and the stop phase of a rejected take-off. Let me show you the effect of slush density on that slush drag — . That figure plots slush drag against speed for different slush densities, and you can see how the drag rises with speed up to the hydroplaning speed, and how denser slush gives more drag. Now, the airframe itself. All the performance data we use assumes the aircraft is not contaminated by frost, ice, or snow at take-off. In fact, it’s a regulatory requirement that at the commencement of take-off, the aeroplane must be free of ice or snow. Why so strict? Because snow and ice on the airframe do three harmful things: they increase drag, they reduce lift, and they increase the weight of the aeroplane. Any one of those alone hurts performance; together, they reduce aircraft performance and increase the take-off distance. So a contaminated airframe is not just a nuisance — it’s a direct violation of the take-off requirement and a serious performance penalty. Finally, flap setting. Flaps affect two things: the maximum lift coefficient of the wing — that’s CLMAX — and the drag. Let me walk through both effects, because they pull in opposite directions. Increasing flap angle increases CLMAX. A higher maximum lift coefficient means the wing can generate the same lift at a lower speed, so the stalling speed reduces, and therefore the take-off speed reduces. A lower take-off speed means you need less distance to get airborne — so this effect reduces take-off distance. But increasing flap angle also increases drag. More drag means less acceleration, which means you take longer to reach take-off speed — so this effect increases take-off distance. The net effect is a balance. Take-off distance will decrease as you increase flap angle from zero, but only up to a certain flap angle. Above that certain angle, the drag penalty starts to dominate, and the take-off distance increases again. So there’s an optimum flap setting for each type of aircraft — a sweet spot where take-off distance is minimised. Any deviation from that optimum setting, whether you use more flap or less flap, will give an increase in take-off distance. Let me show you that relationship — . The graph plots take-off distance required against flap angle, and you can see the curve dip down to a minimum at the optimum flap setting, then rise again on either side. And just to tie the runway slope into this — shows that on a downslope, a proportion of the aircraft’s weight acts in the direction of thrust. That downhill component adds to the accelerating force, so a downhill slope increases the accelerating force and helps you get airborne sooner. It’s the same principle as the flap drag trade-off — every factor either helps or hinders the acceleration, and the take-off distance is the sum of all those effects.