Side Slips vs. Forward Slips
by Jamie Beckett
Published
Updated
Before there were flaps. Before pilots knew how to recover from a spin. From the very early days of heavier than air machines taking flight, there were slips.
What is a slip?
The term “slip” is generic. It simply means the airplane is uncoordinated and the tail is not tracking the nose. The pilot has deflected the ailerons one way while going in the opposite direction with the rudder. In our early flight lessons we’re encouraged to avoid slips, because they’re not a desirable part of covering the four fundamentals of flight (straight-and-level, turns, climbs, and descents).
Slips also can exist during turns, but that's not in the scope of this article. Here, we're talking about slipping the airplane while traveling in a straight line.
The two versions we'll focus on are known as side slips, and forward slips. And that’s where the whole topic starts to become confusing.
Let’s simplify.
The Side Slip
A side slip is generally used to keep the airplane aligned with the runway when landing in a crosswind, keeping the relative wind on the nose. A sharp pilot will lower the upwind wing slightly (the side the wind is coming from) while putting in just enough opposite rudder to keep the airplane headed straight for the centerline.
The side slip is often a fairly benign maneuver. The pilot only puts in enough of a bank to counteract the crosswind. Since the goal is simply to hold a straight line on approach there is no need to get extreme with the side slip.
The Forward Slip
A forward slip is the same, but different. It is typically used to lose altitude without sacrificing airspeed or deploying flaps. The pilot still lowers one wing while deflecting the rudder in the opposite direction. This causes the relative wind to hit off center, on the fuselage, significantly increasing drag.
When performing a forward slip the increased drag on the airframe does the work. A higher bank angle requires more rudder which results in more drag and a faster descent.
By using the forward slip to lose altitude, a pilot can come down slowly, or quickly, while maintaining a given airspeed and remaining in complete control of the airplane.
Why Learn About Slips at All?
In the early days of flight, the flaps we often use for landing today weren’t part of the standard equipment. It took more than a decade from the time the Wright Brothers first flew until the most basic flap assembly was tested in flight. The Fowler Flap that we’re all so familiar with today wasn’t introduced to the marketplace until the 1930s. Lindbergh had already flown New York to Paris by then. Aviation had entered its Golden Age.
But what goes up must come down. Power makes it possible for us to climb. But it is the slip that allows us to control our rate of descent on a moment-by-moment basis, even without the use of flaps. Knowing when to employ a slip, which slip to use, and how to slip safely can be a great asset to any pilot.
There are risks
You’ll notice that both the side slip (which is used for landing) and the forward slip (which is used for descents) both happen while the airplane is moving in a straight line. That’s both intentional and extremely important. Because both also have the unfortunate characteristic of putting the airplane into an uncoordinated state.
Should the airplane stall while uncoordinated there is a chance the result may be an unexpected loss of control. This is a major part of the reason we include cross-control stalls in the Risk Management section of the Private Pilot Airman Certification Standards.
Slips with flaps deployed may be prohibited by the airplane's POH/AFM, so always become familiar with the slip requirements for your particular airplane.
All Airplanes Are Not Created Equal
In flight it is imperative that any pilot have accurate information to base their decisions on. When initiating a side slip or forward slip the really good pilot will know enough about their airplane to be able to correct for any airspeed indication irregularities they may encounter.
Consider the flight training champion of the 20th Century, the Cessna 150 and 152. They are wonderful trainers and perfectly respectable machines for personal transportation. But they have a design quirk that should be taken into consideration when slipping the airplane.
There is only one static port on the Cessna 150 and 152. It’s on the left side of the cowl, just in front of the pilot’s door.
When slipping with the left wing low and the rudder deflected to the right, the airspeed indication in the 150 series may read artificially low. The airplane is traveling faster through the air than the airspeed indicator suggests.
The ambient pressure is higher on the left side of the nose due to it being shifted slightly into the relative wind. With an artificially high reference pressure, the airspeed indicator compares that value to the pressure provided by the pitot tube and displays a speed that is less than the airplane’s actual airspeed.
When slipping with the right wing low, the opposite is the case. An artificially low static pressure when compared to the pressure from the pitot causes the airspeed indicator to read higher than the airplane’s actual airspeed.
This is where the risk of an inadvertent cross-controlled stall can become a real issue.
Know your airplane and its systems.
Slip well, and slip with confidence
The side slip (when landing) or the forward slip (when descending) both represent an important tool in any pilot’s bag of tricks. But like so many maneuvers we initiate in the air, they have to be well understood and respected for us to use them safely over the course of our life in the air.
About the Author
Jamie Beckett came to aviation as a second career after taking a handful of flight lessons on a whim. He’s made general aviation his focus for more than 30 years serving as an instructor pilot, restoring WWII aircraft, writing for a variety of aviation publications, and flying just for the joy of it. Visit Jamie's channel at www.youtube.com/@MadPropsAero
