Slips and Skids...the truth -

The following article is one I pulled off of Facebook and was written by a local pilot: Jon "Jughead" Counsell, Master CFI - Aerobatic, EAA #1018257, JugheadF15@yahoo.com.  It is by far the best description of why a slip is safe and a skid is dangerous.  It takes some time to read but is well worth the read and just might save your life.


There are really only two things that slips and skids have in common. One is that there is often a lot of confusion by pilots about them and the other is that they are both cross-controlled maneuvers. Everything else about them is pretty much opposites. We all know how slips are useful to help us lose altitude and also to allow us to land in a crosswind correctly. There are only a few specialized areas of flight where skids are appropriate. Some of these include aerial application/firefighting, aerial gunnery, aerobatics, low level-low energy turns where dragging a wingtip is a concern and formation flying to name a few.

So let's look at the aerodynamics associated with each of these maneuvers. One thing that occurs in both is we change the airflow characteristics of each wing while performing them. One wing assumes sweptback characteristics and the other assumes forward swept properties. Why is this important? Because the stall properties are opposites! Let's take a look at wing shapes and how they behave as they reach Critical AOA or AOAcrit, or what you all know as a STALL. If we remember back to our aerodynamics studies we recall that a rectangular wing like we find on most GA aircraft stalls along the wing root and the trailing edge first. This is designed into the wing through twist, stall strips, differing airfoil shapes etc. As these wings start to approach AOAcrit, the airflow starts to separate and becomes turbulent. It often passes over the tail, producing a nice buffet that can be felt in the airframe and flight controls. The separation then progresses forward and outward. These features allow the ailerons to remain effective well into the stall on most GA aircraft. A sweptwing airfoil will experience span wise flow and the initial separation of airflow will occur near the outer and aft portion of the wing near the wingtip. This means the aileron is affected very early in the stall progression. A forward swept wing experiences the opposite. It's stall begins at the root and concentrated more forward than outward compared to a straight wing.

To help visualize this wing sweep concept for either a slip or a skid, take a model airplane to rotate it along it's appropriates axis's to produce a slip and then a skid. In a left turning slip, the outer or upper wing is swept back and the inner or lower wing is swept forward in relation to the relative wind. The opposite is true in a left skidding turn. To really understand why this is important we need to understand what the Center of Pressure or Lift, COP, is doing on each of these wings as a stall progresses. In review, the center of pressure is that point where the average of all the lift vectors being created along the wing's surface would be located. As a wing approaches AOAcrit and airflow separation begins to occur, the amount of lift in the separation areas is decreasing. That results in the COP moving away from the stalled region. So a straight wing stall causes the COP to move outward and forward. A sweptwing COP moves forward and inward along the wing. And a forward swept COP is moving forward and outward.

When the airplane is flown in a coordinated state without a rolling motion, the relative wind strikes each wing the same. That means as the wing progresses towards AOAcrit, the COP travels the same distance at the same rate across the wing. This prevents any rolling motion from dissimilar COPs from being imposed on the aircraft as AOAcrit is reached. However when in a slip or a skid, there exists a large dissimilarity between the COPs in direct relation to the severity of the slid or skid. What is vital to understand is how the differences impact the airplane.

Lets look at a slip first. In a slip the stall speed associated with AOAcrit is actually reduced. I will explain this in a moment. In a slip, the aircraft provides lots of warning about an impending stall. The amount of buffet often begins earlier and progresses longer with a greater intensity. The airplane screams a warning to the pilot. “Do you really want to pull anymore AOA? If you do, I’m gonna stall!” Teaching this hundreds of times, the students were always amazed how much warning and how obvious it was as they were approaching a stall. Now remember, the COPs are in different locations! This means they are applying a different ARM or rolling motion to the airplane! The low wing in the slip is swept forward and has a larger ARM associated with it’s COP. When the high wing stalls, and it will stall first, the natural tendency of the aircraft is to roll towards a WINGS LEVEL attitude! As soon as the aircraft stalls, the pilot simply has to slightly reduce the AOA and the aircraft instantly stops rolling due to Positive Dynamic Stability (that’s another two plus page description). A quick reduction of AOA and neutralizing the rudder will result in a nearly wing’s level attitude with an aircraft that is safely above stalled AOA. If the slip was properly flown, the nose will still be above the horizon throughout the process. A slip is a very benign and safe maneuver that really has to be forced to reached a stalled AOA and then it is easily and safely recoverable. The skidded stall is everything the slipping stall is not!

But first, why does the stall speed go down during a slip? Airplanes fly due to the laws of physics associated with Bernoulli’s Principle and Newton’s 3rd Law. We all know how air flowing over a curved wing accelerates, reducing the pressure, leading to a differential of pressure and viola, we have lift. But a lot of people forget that Newton’s 3rd Law of Equal and Opposite Actions also applies.

Take a model airplane and watch what really happens when you apply slipping control inputs. We will execute them one at a time for ease of discussion. Point the airplane directly away from you and visualize the relative wind coming straight at you. Holding the airplane, apply a right rudder input. Notice how that simply yaws the nose to the right. Now apply left aileron. This lowers the left wing, leaving the nose above the horizon, assuming you didn't reduce your AOA with a forward elevator input. Now look at the aircraft and how the relative wind is striking the side of the fuselage. The air striking the side of the fuselage is being directed downward with a given force. The opposite and equal force is being applied to the side surface area of the fuselage, pushing it upward! We have given up some vertical lift component due to the bank angle, but we have gained lift from newton's 3rd law. Don't buy it? Think about this. At any airshow you will see aerobatic aircraft or military jets perform knife-edge flight. Their wings are perpendicular to the horizon, so they are not contributing any vertical lift component. So how does the aircraft maintain level flight? Because of Newton's 3rd law is providing the necessary force to offset gravity. Yes, there is a small vertical component to the thrust line of the engine, but believe me, it's not enough to keep the airplane level. This increase in lift is what causes the stall speed of a slipping aircraft to be reduced. The wing doesn't have to support as much weight therefore stall speed is lower. And as we begin to descend, even more of the relative wind strikes the side of the fuselage. We will also look at airspeed indicator issues and common mistakes. As soon as we apply control inputs for a slip we disturb the smooth airflow into our pitot tube and we disrupt the airflow over our static ports. This leads to an erroneous airspeed indication. What type and magnitude of error is completely dependent on each and every aircraft. It also will vary between similar types based on their pitot static systems. It really doesn’t matter though. We already discussed how hard it was to actually stall the airplane in a slip. I personally teach the technique to leave the nose attitude alone. Don’t add AOA or reduce AOA when you enter into a slip. Looking out the front, leave the nose sight picture the same. It should be slightly above the horizon. A very common error I witness almost every day is young pilots and young CFI’s lower a wing, put in a little bit of rudder and immediately lower the nose. You can actually see the airplane accelerate as it’s nose drops and the aircraft descends. The goal of a slip is to lose altitude without, gaining airspeed. It does no good to arrive over the numbers at 5 feet and 20 knots fast! In modern GA aircraft if you need to slip, the limited rudder input is usually the limiting factor. Therefore, full rudder, aileron to maintain directional heading and aft stick to maintain nose attitude/AOA. Slip into the wind and don’t change slipping direction! Go practice at altitude until you are comfortable with this.

Now what about when we skid. Let's take that same airplane and start over, holding it straight and level, pointed into your imaginary relative wind. Skids usually begin with the airplane already in a banked turn. The pilot is not producing the turn rate or radius that he was anticipating leading to an overshoot situation, so he starts to step on inside rudder to point his nose into the turn. Bank your model airplane to the left. Now apply left rudder, rotating the aircraft around its vertical access. The rudder simply causes the nose to swing left and DOWN! In the pattern where inappropriate skids are usually performed this also causes the aircraft to start to accelerate and descend at an increased rate. Both of these conditions are opposite of the pilot’s desired aircraft state. So he applies back stick pressure to bring the nose up. But bringing the stick back increases the AOA and causes the aircraft to further roll to the left. This is due to increased rudder effectiveness at rolling the aircraft at high AOA. To counter this, right aileron is added to maintain the bank angle. All of these actions actually decrease the desired turn performance, only making the overshoot situation worse. We are in a HUGE CROSS CONTROL AND DANGEROUS SITUATION! Notice that as this is happening the RIGHT side of the fuselage is exposed to the relative wind. Newton's third law is now pushing the aircraft downward! We have the reduced vertical lift component and downward airflow from the fuselage and thrust line have both increased! During these flight conditions you get very little to NO warning before the stall occurs. The Buffett may be non-existent, stall horns may not sound and when the aircraft stalls and breaks we still have the asymmetrical COPs. The low wing is the swept back wing with its COP moving inward and forward. The high wing COP is moving outward and forward. We know have a large moment arm on the high wing that is producing more lift than the lower swept back wing with the smaller moment arm. Remember, that sweptback wing WILL stall first! This is what causes the airplane to quickly snap roll onto its back, and if the AOA isn't instantly reduced, quickly lead towards a spin. Even in the Extra 300, when I knew what was going to happen, I would end up with at least 135 degrees of bank before you could reduce the AOA enough to stop the rolling motion. In the traffic pattern, base to final turn, it would be extremely difficult to recover from this before hitting the ground. You were too nose low and too low on energy even in the 300HP Extra 300 to safely recover.

So where does this leave us? First let me say that I don’t blame any of our CFI’s or other pilots for not knowing this stuff. I feel very fortunate to have had the opportunity to teach Upset Prevention Recovery Skills to professional pilots of all walks of aviation. It was working in that industry where I fully developed my skills and knowledge. Even my F-15 and T-38 military experience didn’t fully prepare me for the knowledge base to effectively teach this subject. It has been over 50 years since most flight instructors were war veteran pilots and/or cropdusters that had a much high level of understanding of these principles. I firmly believe that because of FAA guidance over the last several decades we don’t teach enough hands on flying skills. I’m on old school guy that believes that ALL pilots should learn in basic taildraggers and be exposed to spins and basic aerobatics. Then they should learn the new high-tech capabilities that can tremendously add to our aviation SA and safety. I have no doubt that this document will produce many questions amongst our members. If you have questions, please call or message me. I will work with you to answer your questions. If you don’t agree with what I have said, and want to have a civil discussion of our differing views, that is also welcome. One can ALWAYS learn from others, no matter what their experience or skill levels.