In the illustration, the side slip is shown as having a ground track angle---for the purpose of dealing with a crosswind, imagine that the air that surrounds the plane is moving sideways, so the "correction" compensates for the sideways movement of the air, resulting in a staright ahead ground track. But, as is obvious, a side slip can also be used to simply move the plane sideways to better align it with the runway centerline.
Other than these useful functions, generally, pilots are taught to "coordinate" the rudder and ailerons to avoid a slip as well as it's relative, the skid. The indicator for this "coordinated" turn is a slimple level or inclinometer--a ball inside a tube.
Generally, we are taught to "step on the ball" to keep it in the center.
The plane is in a slip when the ball is on the inside or downhill side of a banked turn. The plane is in a skid when the ball is on the outside or uphill side of the banked turn. A simple way to think is that a slip generally is caused by too little rudder in the direction of the turn and a skid is generally caused by too much rudder in the direction of the turn. When you "step on the ball" you are inputting rudder to correct the slip or spin. Another technique is to change the aileron input--"pull the ball to the center". In a slip, in a right turn, you can increase right rudder (step on the ball) OR move the stick left (pull the ball) which reduces the aileron and bank angle. In a skid, in a left turn, you can decrease left rudder (by pushing on the right rudder pedal (step on the ball) or you can increase bank with more left aileron (pulling the ball). Note below that the problems related to skids are generally when rudder is added or aileron is decreased--either to turn sharper or to decrease bank angle--or both.
We are taught that skids are dangerous as they easily lead to a spin and these spins often occur when turning to final on approach at low altitude--often resulting in a fatal crash. But, slips can be dangerous as well. A skid AND a slip can result in a "surprise" roll to inverted flight and a spin--as this excellent video teaches.
Another way to understand slips and skids is to think if the stick (ailerons) and the rudder as represented by the front and rear wheels of a race car with four wheel steering. (I am a race car driver, so this is what made sense to me.)
A slip can be thought of as an understeering condition. The plane is slipping to the outside of the turn--the nose is pointed outward from the actual track of the plane.
A skid can be thought of as an oversteering condition. The rear of the plane is skidding to the outside of the turn--the nose is pointed inward from the actual track of the plane.
Understeering---skids are generally defined as "stable". The driver/pilot does not lose control unexpectedly. Oversteering (loose in NASCAR terms) is dangerous as the plane can "spin out" with the driver/pilot losing control.
In a turn, the plane is banked with the stick (ailerons)--think of this as steering the front tires of a car. Think of the rudder as steering the back tires of the car. Too little "rear steering" and the car understeers--the plane skids. Too much "rear steering" and the car oversteers--the plane skids. A "neutral" or "coordinated" turn requires just the right amount of front and rear steering---the ball in the center tells you that it is coordinated.
So, other than using a slip during final approach, the safest flight requires "coordinated" turns--perhaps with a margin of error in favor of a slight skid. For this reason, depending on the plane, it is generally best to begin the turn with aileron input just before rudder.
But slips during the final approach and landing can be very useful if not necessary. They require what is called "cross controlling"---the rudder is moved in the opposite to the normal coordinated turn direction.
To steepen descent, add aileron to drop a wing and add opposite rudder to stop the turn. (Some planes do not recommend this if you are using flaps to steepen descent.) Essentially, this adds drag and slows the plane's forward speed. There is a bit of controversy as to which way you should move the ailerons--into the wind, or opposite the wind. (Opposite the wind produces more drag as the plane is more sideways to the wind.)
Go back and review the video and notice the rollover that occured when too much OPPOSITE rudder was introduced. This is the same "cross controlled" attitude that you are in when doing a forward slip. The rollover occured when the pilot pulled the stick back---so be very careful to keep the nose down, in descent, with a safe margin above stalling speed when doing a forward slip!
And---when in a slow climbing turn, be very careful when/if using opposite rudder!
To manage crosswind landings--add aileron to drop the wing into the wind. Then add opposite rudder to keep the plane aligned with the runway. The amount of aileron added is that which keeps the plane from being pushed to the side.
The trickly part of this "sideslip" is when the crosswind is gusty. Now, the pilot is having to change the amount of aileron to hold the plane's ground track and the rudder to control the plane's alignment. The biggest problem is generally "overcontrolling" ---to respond to a gust, the pilot increases inputs, then the gust stops and the increased inputs are adjusted to a lesser amount--sometimes even to neutral---so that when the next gust appears, the plane is completely unprepared and is the upwind wing is lifted. If this happens when flying a taildragger at the same time the wheels touch down, a ground loop is very possible.
So far, we have ignored the important effect of the elevator. The elevator is just as important as ailerons and rudder in managing a coordinated turn. Since the plane is banked in a turn, the wings are producing both lift and turning forces...and the tail is made to follow the nose both by the vertical stabilizer and rudder along with the horizontal stabilizer and elevator.
Stick back tightens the turn and increases the bank angle. It reduces a slip or increases a skid. it makes the ball in the inclinometer "move uphill". When the plane is turning the stick back up elevator also does two other things simultaneously---it causes a lowering of speed and an increase in stall speed because of the steeper bank--which can become dangerous unless power is added.
Again, gusty crosswinds makes things more complicated, as changes in elevator will affect the correct combination of aileron, rudder and elevator.
The "correct" or "necessary" inputs of aileron, rudder and elevator depend a great deal on the plane's design. For example, when ailerons are used to turn/bank the plane, there can be a dramatic difference in drag---the high wing having more---which causes the nose to turn in the opposite or adverse direction of the turn--this is called adverse yaw. Because vintage aircraft had a great amount of this adverse yaw, you will read often than the primary purpose of the rudder is to correct this adverse yaw. More modern planes have aileron designs that minimize adverse yaw---but rudder input is still required to achieve a "coordinated" turn.
Some planes are designed so that once the bank is established with the ailerons, the bank angle is maintained when this initial input is removed. Other planes, when in a shallow bank (less than 20 degrees) will tend to return to level flight and therefore require a more constant aileron input. Know your airplane and practice!
The following video from Rich Stowell, CFI and Aerobatic Guri will make you think. Ailerons bank the wings, rudder moves the tail side to side and elevator becomes the primary (but not the only) "control" for the turn.
The best advice I received from a CFI so far is from Jim Alsip:
- Don’t stall
- Control yaw.
Controlling yaw is the essence of coordinated flight and is the essence of using slips to your advantage during approach and landings. Slips and skids by themselves are not dangerous, but they do increase the severity of consequences in the event of a stall. You can generally recover from a stall when in coordinated flight, but often, when uncoordinated, when close to the ground, a stall is deadly. (Skids make a spin and inversion rollover more likley and more sudden when a stall occurs.)
Review:
Slip=(ball downhill) not enough rudder, too much aileron, not enough elevator
Skid=(ball uphill) too much rudder, not enough aileron, too much elevator
Stall=not enough speed for a given attitude.
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