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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.

Tuesday, September 11, 2012

Step on the Ball!

I recently wrote a post at Ask-a-CFI on a thread about stalling while turning. The post touched on the accuracy (or lack thereof) of the "ball" when it comes to indicating coordination. So I thought I should write a post here as well. The blog format allows me to expand on the inclinometer's strengths and weaknesses more than I could on a forum thread about a different topic.

The inclinometer (AKA - the "ball") is used to indicate coordination of an aircraft. Coordination is a term we use to reference the slip angle. In coordinated flight, the slip angle should be zero. With any non-zero slip angle, we are in uncoordinated flight.

The best way to measure and indicate coordination (or slip) is with a direct measurement of the airflow angle. Gliders use a yaw string to do exactly this. The string aligns with the relative airflow, so if the string is pointing straight back, the glider is coordinated. The rule here is "step on the space".

In a powered aircraft (at least in single engine aircraft with the engine on the nose), a yaw string won't work because the airflow is disrupted by the propeller slipstream. Since this slipstream effect will vary with airspeed, power setting, angle of attack, and slip angle, there is no easy correction or compensation. So a yaw string is not a practical option in this case. As a result, the inclinometer (the ball) is used to provide coordination information.

The problem is that the inclinometer doesn't actually measure slip angle. It measures lateral acceleration and compares it to bank angle (in other words, it compares centrifugal "force" to gravity). This balance between lateral acceleration and bank angle is then used as a proxy indication of coordination (slip). In a glider, this would actually work quite well—except that it would require a look inside the cockpit to confirm coordination, whereas the yaw string allows for continuous outside reference.

In coordinated straight and level flight, the wings are level and there are no lateral accelerations. The ball is held in the low point of the glass tube by gravity. If we roll the aircraft but don't turn (requiring the use of opposite rudder), the low point moves, and gravity pulls the ball off-center. On the other hand, if we yaw the aircraft while keeping the wings level (requiring the use of opposite aileron), the air loads on the fuselage cause a lateral acceleration, and the ball gets pulled off center. If we roll the aircraft and allow it to turn, gravity and the lateral acceleration balance and keep the ball centered.

Again, though, the ball is measuring lateral acceleration, not slip angle. So even in a perfect world, the ball becomes less sensitive at lower speeds—meaning that a given slip angle will be indicated by smaller and smaller ball deflections as speed is reduced (lower speeds mean less lateral drag force at a given slip angle). So at lower speeds, where slipping becomes more and more important from a safety standpoint, the ball gives us less information.

But wait! It gets worse. The aircraft can be flying in a slip with no lateral accelerations (and therefore no ball indications). This is because yawing moments (for example from the engine) are balanced by a force applied at the rudder. This force doesn't just yaw the aircraft, but also pushes it laterally. This lateral push is cancelled by an equal and opposite lateral push due to the slip. Multi-engine pilots will be familiar with this, as it is an important consideration after an engine failure (it is, in fact, the reason why we "raise the dead").

In a single-engine aircraft at low speeds and high power settings, there is lots of yaw induced by the engine, so lots of rudder force is required. At the same time, the lateral drag caused by a slip is reduced by the low speed. So balancing these two (and thus centering the ball) can require a large slip angle. This is one (of several) of the reasons why an aircraft is more prone to spinning when it stalls power-on.

All of the above notes refer to the ball during "slow" maneuvers—when the point we're rotating around is well outside the aircraft itself. But what about rapid yawing? In this case, the ball will tend to move away from the center of rotation. For example, in the Grob G-120A, we have 2 inclinometers installed—one for the student and one for the instructor. In a spin, they often indicate in opposite directions—left inclinometer goes left, and right inclinometer goes right. These conflicting indications occur regardless of the spin direction. This is why, in a spin, we don't use the ball to get oriented. Instead, we use the needle, which is a gyroscope and indicates yaw direction/rate with no reference to lateral accelerations.

The bottom line here is that the ball is not very accurate at low speed, high power, and/or high yaw rate. But it still serves us well under most normal flight conditions. Understanding it's limitations can help us use it more effectively.

Happy Flying!

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