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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, January 1, 2013

Slip Slip Slipping Away

After my last—very long ago—post about being coordinated, and more specifically, measuring coordination, I though I might follow up with a post about being uncoordinated. So today I'm going to ramble about the use of a slip in flight operations.

What is a slip?
A slip is a condition of flight where the relative airflow is crossing over the fuselage with a side-ward flow. Slips are generally considered to be undesirable. They produce drag, they produce a rolling response, and they can result in all kinds of terrible things in the vicinity of a stall. Inadvertent slips can be caused by the yaw associated with the engine, by turbulence, or by aileron drag / adverse yaw.

Occasionally, we want to slip. There are really two scenarios that lead to this: the desire to produce drag (forward slip and slipping turn), and the need to correct for a crosswind during landing (sideslip). It's important to note that, aerodynamically, the forward slip and the sideslip are exactly the same thing. They differ in application and visual cues (due to the effect of wind).

Forward Slip
The purpose of a forward slip is to produce drag so the descent rate can be increased without a corresponding increase in airspeed. This is accomplished by creating a side-on airflow to the fuselage, thus reducing the streamlining of the aircraft and increasing drag. The drag being produced is parasite drag, so the forward slip tends to be more effective at higher speeds. However, as a practical matter, forward slips are normally done during an approach, so our ability to increase our speed without de-stabilizing the approach may be limited. A notable exception to this is the emergency descent.

Slipping Turn
A slipping turn is really just a forward slip with an increased bank angle or reduced rudder deflection. The effect is that the aircraft will turn while slipping (hence the name!). This allows us, for example, to start a slip to lose altitude on the base leg of an approach, and continue the slip through the turn to final. This can be especially useful in a forced approach scenario, where we have only one shot to get it right. But, as with the straight-ahead forward slip, it can be used during normal approaches to bleed off excess energy.

Side Slip
The sideslip is used to align both the aircraft and the flight path with the runway for an intended landing. Under crosswind conditions, coordinated flight demands that our heading and track are mismatched by the wind-correction-angle. This is not acceptable for landing since it will result in side-loading the landing gear, and the production of rolling and yawing moments on touchdown (in a taildragger, the side-loading of the gear can quickly evolve into a ground-loop). In order to align the heading with the flightpath, we must have some component of slip (i.e. - airflow across the fuselage).

Entering a Slip
The entry into a slip is where one of my pet peeves shows up—and is really what this post is about. Common wisdom (for example: the Wikipedia entry, the Wikihow entry, and a Flying Magazine article) has it that to enter a slip, you lower one wing and then apply opposite rudder to prevent a turn. Aerodynamically, this will work. But in flight operations, it has questionable usefulness. This is because in practical scenarios, we normally want to change our heading on entering the slip.

In a forward slip to lose altitude, we are initially established in coordinated flight on approach. This means that we are heading and tracking toward the runway. We would like to keep tracking toward the runway while changing our heading so that we are effectively flying sideways. Lowering the wing and then preventing a turn with rudder will not accomplish this. Instead, it will put us in a slip with a heading toward the runway, but a track that takes us off the approach. This is no good. If instead we enter the slip with rudder first to yaw the aircraft, and aileron second to roll into the slip and prevent further yaw, we will find ourselves still tracking toward the runway, but pointing elsewhere (which is exactly what we want!).

Similar logic can be applied to a sideslip to correct for a crosswind. In the initial approach, we are in coordinated flight with a wind-correction-angle (AKA - crab angle). So our track is toward the runway, but our heading is angled into wind. As with the forward slip entry, we would like to maintain our present track, but change our heading. So the entry is the same—rudder first and aileron second.
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So really there are two way to enter a slip: rudder-then-aileron, and aileron-then-rudder. Both control actions will result in a slip. However, in most practical applications, the rudder-first method is what we actually use. Note that by "most" practical applications, I mean to leave room for the slipping turn, which can indeed be entered aileron-first in practical applications.

Happy Flying!

1 comment:

Mike Zias said...

Thankyou. Makes good sense.

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