Whether you are a pre-solo student pilot or you have several hundred hours in your logbook, developing a conceptual flying style can fundamentally change the way you pilot an aircraft. Instead of forcing the plane to do your bidding, the conceptual approach involves developing kinesthesia, the ability to visualize, and an appreciation of your aircraft’s stability characteristics to establish a plane/pilot partnership. You can begin this partnership with a gaining a deeper understanding of your aircraft's pitch stability at different airspeeds and in different configurations, but first you have to let go.
Don’t Do Something, Just Sit There
A clear understanding of your aircraft’s pitch stability will pay big dividends, from how promptly you can trim the aircraft to how accurately you can hand-fly a predetermined altitude. It can also determine whether your passengers think you are a smooth operator or a ham-handed G-junkie. Most aircraft are inherently stable: When the flight path is disturbed, most aircraft tend to re-establish equilibrium by themselves. Since longitudinal (pitch) stability is closely linked with the wing’s angle of attack, the amount of lift being generated, and the resultant drag, the first experiment in pitch stability flying involves hands off flying.
Establish most any single-engine training aircraft in a level cruise attitude, start a timer, and then momentarily apply forward or backward pressure on the yoke. Using rudder inputs to keep the wings level, keep you hands off the yoke (or stick) and observe what the aircraft does. The aircraft’s initial response to being upset is referred to as static stability. Most GA aircraft are designed with positive static stability - the aircraft will tend to recover by pitching a direction opposite from which it was disturbed. Once the aircraft has made it’s initial response to the disturbance, continue watching with minimal control inputs and see what happens next.
|Aircraft's initial response = Static Stability|
How an aircraft responds over time to an upset is referred to as dynamic stability, which explains the suggestion of using a timer. In addition to positive longitudinal static stability, most GA aircraft have positive longitudinal dynamic stability: They will slowly oscillate, pitching up and down, gradually reducing amplitude until equilibrium is restored.
|Positive Static and Dynamic Longitudinal Stability|
Using a timer helps you discover the length of time between each pitch-up and pitch-down oscillation (sometimes called a half wave). Cessna single-engine aircraft generally have positive static and positive dynamic longitudinal stability with a 5 to 7 second interval between the initial half-wave oscillation. Most Piper singles also have positive static stability, but a weaker positive dynamic stability that includes an initial half-wave oscillation of around 10 seconds. Once you’ve figured this out, you can apply it to normal flight procedures. A good resource for visualizing these oscillations is the UND Aerodynamic Training software, specifically the stability app (used to create the screen shots above).
Once you understand your aircraft’s static and dynamic stability characteristics, it should be clear why trying to fly an altitude using just trim may not be a useful strategy in some aircraft (there are exceptions). Let’s say you feel back pressure is required on the yoke and decide to immediately add nose-up trim. If the aircraft was in the middle of a pitch oscillation, it was eventually going to recover on its own: Your pitch trim input just made things worse. If you find yourself becoming a trim-junkie, there is a solution.
- Hold the aircraft in the desired pitch attitude using the primary flight controls for the number of seconds equal to the aircraft half wave oscillation
- Briefly let go of the primary flight controls and see what the aircraft wants to do
- Re-stabilize the desired pitch attitude using the primary flight controls
- Make a trim adjustment, return to step 1.
But don't stop there. Experiment with how the addition or removal of power affects pitch stability. Try extending flaps or landing gear and see how the aircraft behaves. By all means maintain positive aircraft control, but try to make minimal flight control inputs so you can observe your aircraft’s natural pitch stability tendencies. Some aircraft will pitch up when flaps are configured, others will pitch down, while some will not change pitch attitude at all. Knowing how the aircraft responds to these changes will allow you to develop successful strategies for configuring your aircraft so that it naturally tends to do what you want it to do. Cessna singles tend to pitch up when flaps are extended and they tend to pitch down when power is removed which is why a good strategy during approach to landing or setting up for slow flight is to reduce the throttle first and then add flaps. Keep in mind that the strategies you develop will depend on your particular aircraft.
In the next installment, I’ll talk about how visualization and kinesthesia can help you avoid common errors made during takeoff.