No dumb questions

Q. Although I carefully maintain my final approach airspeed during sideslips, when I take the airplane out of the slip the airspeed instantly changes a few knots. Why?

A. What can be happening is that the airspeed indicator is displaying erroneous information while you are in the sideslip. Remember, the airspeed indicator determines airspeed by mechanically subtracting the static pressure, sensed through the static port. from the total pressure, sensed through the pitot tube. A change in pressure through either source causes a change in indicated airspeed.

Imagine an airplane with a single static source on the right side of the fuselage. If that airplane is in a right sideslip, the relative wind is coming from the right. The air is no longer flowing directly past the static port ? some of it is being forced into the port. This ram effect increases the static pressure the port senses.

If the pitot tube doesn?t sense a change in the total pressure, the erroneously higher "static" pressure results in an indicated airspeed slower than the airplane is actually flying.

When you take the airplane out of the slip, you remove the ram effect and the air again flows directly past the static port. The port senses an immediate drop in pressure, and the airspeed needle jumps to a faster indication than during the slip.

When you enter the slip you don?t notice the indicated airspeed decrease because you automatically adjust the airplane?s pitch attitude to maintain the original airspeed indication. After all, the most compelling reason to use a forward slip is to increase your descent rate without increasing your airspeed on final approach.

In our scenario, if the single static port is on the left side of the fuselage, airspeed during the slip might read erroneously high. In a right slip (right wing down and left pedal), with the relative wind from the right, the pressure at the static port might be lower than ambient static pressure. This is one reason many aircraft have two static ports ? one on each side of the fuselage.

It?s impossible to predict the airspeed errors that can occur with either of these configurations. Because such errors are possible, airplane designers take great care in locating the static ports. The goal is to eliminate or minimize pitot-static errors for all likely flight conditions and airplane configurations.

The pitot system can display similar errors. Total pressure is best measured when the pitot tube is pointed directly into the relative wind. This only occurs at one angle of attack. Any other angle of attack means the tube is at an angle to the relative wind. The same argument applies to a slip, which is nothing more than angle of attack from the side. The errors caused by pitot tube orientation are typically small for reasonable angles of attack and sideslip when compared with the potential static error.

Even if you have no pitot or static errors associated with the sideslip, you may still observe a change in indicated airspeed upon releasing the pro-slip flight controls. Picture an airplane with a pitot tube near the left wing tip, and let?s put the plane in a right sideslip. During the slip the pilot holds left pedal, right yoke, and whatever forward or aft yoke is necessary to maintain the desired indicated airspeed. The relative wind is coming from the right of the plane?s nose.

When the pilot centers the yoke and pedals, the plane yaws nose-right, reorienting itself into the relative wind. During this yawing motion the left wing tip briefly moves faster through the air than any other part of the airplane. The pitot tube at the left wingtip encounters the total pressure from the airplane?s forward speed, plus the extra speed of the wingtip swinging through the air as the plane yaws.

The airspeed needle indicates the speed of the pitot tube, which is momentarily faster than the airplane during the nose-right yaw rate. We?re assuming the static pressure is unchanged during the slip. If the slip were in the other direction, the pilot would observe a momentary decrease in airspeed indication after releasing the pro-slip controls, because the wing tip with the pitot tube would swing backward relative to the rest of the airplane.

You can most easily see the airspeed indication variances caused by yaw rate, while you?re in slow-speed flight in an airplane with a long wingspan and the pitot tube near the wingtip. The longer wing span means the wingtip travels faster through the air for a given yaw rate. The slower the airplane is traveling, the greater the influence of the yaw-rate-caused airspeed indication change because it?s a bigger fraction of the airplane?s forward speed.

Most modern airplanes have very little airspeed error associated with sideslip, but if you decide to experiment with your airplane, consider a few safety precautions. Stalling a cross-controlled airplane may result in a violent departure with extreme attitude changes and possible spin entry. Some airplanes have limitations on sideslip in certain flap configurations.

If a sideslip airspeed error occurs, you won?t know it until after you return to coordinated flight. If the airspeed indication in the slip is faster than the airplane is actually flying, you?d be flying slower than you think and possibly closer to stall than you want to be.

If the indication in the slip is slower than the airplane is actually flying, you risk over-speeding flaps or landing gear, or even exceeding a structural limit of the vertical fin or rudder. Before venturing into any unknown flight regime it?s always a good idea to study the pilot?s operating handbook and consult a flight instructor. Finally, never experiment with a new maneuver at low altitude, and always have a plan for the unexpected.