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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Wednesday, December 1, 2010

The Myth of Centrifugal "Force"

As a follow-up to a previous post on Equilibrium, I thought I'd talk a little bit about a common non-equilibrium maneuver—the steady turn. Much of what is said here can also be applied to pitching maneuvers—in fact, any maneuver that involves a curved flight path and the associated accelerations.

Our interest in turns, for today, stems from the often-debated "Centrifugal Force". Centrifugal Force is "often-debated" because there are those who claim that it doesn't exist, while other insist that it must because you can "feel" it in a turn. From the title of this post, you can probably guess which camp I fall into. I would argue that centrifugal force does not exist. However, in fairness tho those who disagree, it really does depend on your perspective (I'll clarify that shortly). And we'll see that centrifugal force is still a useful concept when considering objects that are moving in a circle.

Centrifugal force fits into a category of forces that physicists call "pseudo-forces", and engineers call "inertial forces". these forces exist—in that they can be both calculated and measured—only if you are using an accelerating object as your reference (a "non-inertial reference frame" in technical terms). Normally, we measure position, velocity, and acceleration from a position of equilibrium. In this case, the position and velocity of an object might vary depending on our reference, but the acceleration will always be the same.

This changes if we measure or observe from an accelerating reference. Measuring the acceleration of an object from an accelerating reference means that we will get a different value than someone else measuring the same acceleration from a different reference (either in equilibrium or accelerating differently). Inertial forces, including centrifugal force, ultimately stem from this discrepancy—recall (from Newton's Second Law of Motion) that acceleration is closely related to forces.

Let's consider what happens in a car going around a turn (we'll use a car here instead of an airplane in order to eliminate the small complication that comes from considering bank angle). When you're sitting in a moving car and the car is taken around a turn, you feel yourself being "pulled" to the outside of the turn. That sensation is very real. The problem is that your frame of reference—the car—is not in equilibrium. The physical sensation of centrifugal force is in fact the sensation of the car pushing you into the turn. An observer outside, sitting on the curb and in equilibrium, sees the friction on the tires pushing the car into the turn, and sees the car seat (and associated hardware) pushing you into the turn. No forces are present directed to the outside of the turn.

The same principle applies in an airplane, except that we turn in a banked attitude, so the "force" that we feel is directed straight down through the floor of the aircraft, not to the side (assuming the turn is coordinated—a topic for another day). Ditto for wings-level pitching maneuvers—such as entering or exiting climbs or descents, or aerobatic maneuvers such as loops.

In practice, the importance of Centrifugal "Force" is in it's relationship to load factor. Load factor is a measure of the aircraft's "normal" acceleration—where "normal" means perpendicular to the flight path. There's a limit to how much load factor we can take. At low speeds, we eventually reach the stall limit. At high speeds, we eventually reach the structural limit. At any speed we might reach our own physiological limit (grey-out – black-out – G-LOC), which can vary significantly from person to person. Load factor is controlled by controlling lift. At any given airspeed, lift—and therefore load factor—is controlled by Angle of Attack.

Is Centrifugal Force real or isn't it? Well, when you're sitting in an aircraft pulling 6 g's it certainly feels real! The structural loads are real, the physiological effects are real. So at the very least, the concept is useful. Perceiving the aircraft as if it were in equilibrium with a new "gravity" force may be easier on the brain than keeping track of all the physics of flight—especially while simultaneously trying to stay oriented and maintain control!

So the bottom line here is that Centrifugal Force is not truly a force, but it's still a useful concept. It simplifies our picture of the world and helps reduce the brain power dedicated to things other than flying the airplane. But if you want to hangar fly and argue the nitty-gritty of flight theory, it really doesn't exist!

Happy Flying!

2 comments:

Anonymous said...

Centrifugl compressors in turbine engines?

Steve Pomroy said...

Centrifugal compressors add energy to the airstream by rotating it. When the rotation is stopped (or reduced) in the diffuser, the velocity of the air is converted to pressure. Centrifugal force in this compressor is still a pseudo-force. To an outside observer, the air "wants" to continue in a straight line. To an observer riding the rotor, that straight line looks curved outward.

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