Pitch follows airspeed, not power
I’VE BEEN WORKING with some students on two or three manoeuvres that all begin the same way:
- reduce the throttle to idle, and raise the nose as the aircraft slows, in order to maintain a constant altitude.
That’s an effective way to conduct an entry to a power-off stall and to commence a demonstration of “slow flight” – also known as flight at minimum controllable airspeed. It’s also the first half of one of my favourite teaching exercises which I’ve come to call the waterskier.
In the waterskier the pilot smoothly reduces power to idle, and slows the airplane while keeping altitude unchanged, then, when the airspeed has decayed to some nominal value (say 65 knots) and before the onset of aerodynamic stall, full power is smoothly added and the aircraft returns to cruise, the exercise to be conducted at constant altitude – without either climbing or descending.
The second half of the waterskier also matches the return to cruise flight from slow flight – the aircraft accelerates and the nose is lowered.
The common thread here is to learn how the pitch of an airplane in level flight is connected to its airspeed: as the plane flies slower the nose needs to be raised. When the airspeed increases the nose must come down. Why should this be?
Consider how the airplane generates lift, and what that lift depends upon. The two factors under the pilot’s direct control that affect lift are airspeed, and Angle of Attack. While the wing is flying, that is, not close to being stalled, we can say:
- Lift = some stuff x Angle of Attack x Airspeed2
If your goal is to stay in level flight you must arrange for the lift to remain constant (and equal to the weight of the aircraft, very closely.)
So as airspeed decreases, the Angle of Attack has to increase; and it must increase at just the right rate, so that at no time is there a change in the overall lift that the wing creates. If the lift were to increase, the aircraft would start to climb. If the lift were to decrease the aircraft would sink. We want neither.
How does the pilot sense Angle of Attack? While the aircraft is flying horizontally (level flight) the pitch attitude more-or-less is the Angle of Attack. Raising the Angle of Attack is done by raising the pitch angle, that is, by raising the nose.
Every student comes to an intuitive understanding of this relationship: that when the aircraft slows, to maintain your altitude you have to raise the nose. But the common error in, say, slowing down, is to reduce the power then immediately raise the nose. Reducing power is correct – but the airplane has momentum and takes time to start to slow significantly. The nose has to be raised only at a rate that matches the decay in speed, not quickly to match the quick reduction in RPM. If the nose is raised too quickly – let’s say, immediately after the power is reduced, the aircraft will balloon upwards in height.
Similarly, during the second half of the waterskier, or when exiting from slow flight and returning to cruise speed, the nose has to be lowered. But it must come down only as the airspeed increases again. If the pitch attitude is adjusted downwards quicky at the same time as the power is added, then the airplane will sink.
Are there complications? Yes, of course, which makes these exercises both worthwhile and somewhat tricky.
Amongst others, there are adverse yaw effects when reducing and (mainly) when adding power at low airspeeds: controlling the ywa requires rudder input. And, at least the way I teach it, it’s helpful to use nose up trim in the entry to slow flight, and nose down trim again in the return to cruise flight. So the trim may have to be managed too.
But foremost, we have the pilot watching the pitch of the aircraft by looking at the horizon yet controlling the pitch by applying force to the yoke. The yoke force is affected by the propellor wash, very strongly in some airplanes. So when you reduce power there is an instant tendency for the nose to go down. The pilot has to pull against this change in force in order to keep the horizon fixed at first – but not pull too much. And then increase the pull and raise the nose as the speed decays.
The opposite is true when adding power: the change in rpm creates a strong nose up trim change, which the pilot will have to override by pushing forward, when adding power at a low airspeed. And then, at just the right time, more nose down force must be applied to have the nose go down not to soon but at just the appropriate rate.
But the most important lesson is that in all these exercises you are balancing airspeed against pitch angle – or Angle of Attack, and therefore pitch follows airspeed, not power.