What is a Phugoid?
Not bad for a dictionary, but let’s put it in pilot talk.
If, for example, you are flying straight and level, constant airspeed, and altitude, trimmed out meaning hands-off and the airplane doesn’t move, and you introduce a small pitch up input and release the controls, what will happen to the airplane?
If it is longitudinally stable, the airplane will decrease in airspeed as it climbs from the pitch input, causing the total lift to decrease and then decreasing lift and airspeed will cause the airplane to nose over during which time the airplane will pick up speed equating into more lift generation, causing the airplane change direction again and begin to pitch back up. You are trading energy and lift in a sinusoidal motion. If the airplane is stable longitudinally, this sinusoidal or up and down motion will decrease over time and the airplane will return to a trimmed level state. If the aircraft is unstable in longitudinal stability, the pitching up and down and the altitude loss will get worse or become divergent. This would not be ideal! A phugoid is typically characterized as a long period event with no real change in angle of attack, but a change in altitude and airspeed.
Go try it:
A simple Cessna 172 is a good airplane to demonstrate the phugoid, assuming no bags or passengers, just two folks upfront. Trim the 172 out (90-100kts is fine), take your hands off the yolk and if the airplane is flying straight level then pull the nose up to about 5-8 degrees and release. Fly airplane with rudders to keep the wings level and watch. Observe how high it climbs and the airspeed at the top of the phugoid. Then observe how far it pitches down and the maximum airspeed and the lowest altitude. Observe over several iterations and what you’ll see is that the airplane will reach smaller and smaller deviations in airspeed and altitude and eventually begin flying level and trimmed again. Your starting and ending altitude is usually only 100-200 feet or so. Try this for 2-3 minutes at a time.
Congrats, on your first test piloting tools.
Advanced explanation:
There are two main longitudinal modes in aircraft stability to consider: short period and long period excited by some input or disturbance (like a gust of wind). Short-period oscillations result in a rapid angle of attack changes, but small changes in airspeed. The event is simply short-lived. But, this is an important phenomenon to ensure is properly dampened or your flying qualities and predictability will be questionable. The long period phugoid is more analogous to the slow rocking of a large ship in a sea state that has huge rollers. Fortunately for the phugoid characteristic, since the duration of the event is longer (25 seconds to several minutes depending on the airplane) it is more predictable to respond to if the airplane is not properly damped or stable. Regardless, it is a characteristic that dictates aircraft behavior.
Life and the phugoid:
Demonstrating the phugoid to pilots or flight students is entertaining and helps to build confidence in understanding aircraft stability and control, especially in light GA aircraft. In the flight test world, it is an easy cursory check during flight test to see how the aircraft behaves with simple pitch changes from a trimmed condition. It is part of the crawl, walk, run the process of flight test where we start with lower risk testing to discover the characteristics and behavior of an airplane. Then from there, we begin to walk and then run toward the higher-risk flight test activities that are necessary to fully understand the air vehicle. Life is like a phugoid, where peaks and valleys occur but with proper tools, planning, and good headwork, we can dampen out those peaks and valleys making the life ride smoother and enjoyable.
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