Stability and Control — Page 286, Lesson 340

We're starting a new topic now: Stability and Control, and I want to walk you through the take-off control requirement. This is the first real constraint we look at in this chapter, and it's a practical one. At take-off, the aeroplane must have sufficient elevator control power to assume the take-off attitude prior to reaching take-off speed. Let me unpack that. The elevator is the movable surface on the horizontal tail that pitches the nose up or down. Control power means the moment—the rotational force—it can generate. And the take-off attitude is the nose-up pitch angle the aeroplane needs to rotate into, to get airborne. So the rule is: before the aeroplane actually reaches the speed at which it can fly, the elevator must be strong enough to rotate the nose up into that attitude. Now, why is this a critical condition? Let's look at the forces during the take-off roll. Figure 10.44 shows the principal forces acting on the aeroplane during the take-off roll: lift, weight, tail load, and rolling friction. When the aeroplane is in the three-point attitude—that's the attitude on all three wheels, nose wheel and both main wheels—at some speed less than the stall speed, the wing lift will be less than the weight of the aeroplane. So the wings aren't yet carrying the full weight. Here's the key point. The elevators must be capable of rotating the aeroplane to the take-off attitude. The critical condition will be with zero load on the nose wheel, and the net of lift and weight supported on the main gear. So the moment of rotation is judged at the instant the nose wheel is just about to leave the ground—that's when the elevator has to do its work, with the aeroplane pivoting about the main gear. Now let's look at what's fighting that rotation. Rolling friction, resulting from the normal force on the main gear, creates an adverse nose-down moment. Adverse means it opposes what we want—it pushes the nose down. Also, the CG, the centre of gravity, being ahead of the main gear contributes a nose-down moment. So we have two nose-down moments working against us: one from rolling friction at the main gear, and one from the weight acting through a CG that sits ahead of the main gear. To balance these two nose-down moments, the horizontal tail must be capable of producing a nose-up moment big enough to attain the take-off attitude at the specified speed. So the tail has to generate an upward load—that's the tail load in the figure—creating a moment that overcomes both nose-down moments and pitches the nose up. There's one more interesting contrast here, between propeller and jet aeroplanes. The propeller aeroplane at take-off power may induce considerable slipstream velocity at the horizontal tail, which can provide an increase in the efficiency of the surface. The slipstream is the accelerated airflow behind the propeller, and when it washes over the tail, it increases the tail's effectiveness. The jet aeroplane does not experience a similar magnitude of this effect, since the induced velocities from the jet are relatively small compared to the slipstream velocities from a propeller. So the propeller-driven aeroplane gets a boost in tail efficiency at take-off power that the jet simply doesn't get to the same degree. So the whole picture is: at take-off, the elevator must be powerful enough to rotate the aeroplane to the take-off attitude before take-off speed is reached, overcoming the nose-down moments from rolling friction and the CG ahead of the main gear, with the critical case being zero nose-wheel load. And the propeller aeroplane has an advantage here because its slipstream energises the tail, while the jet does not.