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Side-slips
Excerpted from The Art of Avoiding Ground Loops
A side-slip is a cross-controlled condition that faces the broad side of the airplane fuselage partially into the relative wind to create more drag. It is commonly used to increase rate of descent in order to steepen the glide path.
In a slip to the left, left stick is applied to lower the left wing just enough to stay on course while right rudder is applied to swing the nose to the right to increase drag. Elevator is used to control airspeed.
A slip to the right is just the opposite: the right wing is lowered as left rudder is applied to swing the nose to the left, and of course, elevator controls the airspeed.
Although aileron and opposite rudder are applied nearly simultaneously, it is important to remember that the amount of rudder deflection actually governs the steepness of the slip, and the angle of bank required to stay on course follows from that.
Side-slips can produce a high rate and a steep angle of descent which makes them great for getting into short fields over tall obstacles, or for bleeding off extra altitude to avoid an overshoot during an emergency landing—or during any other landing in which you have chosen to stay a little high to be assured of reaching your chosen landing spot without needing power. Side-slips are also great fun and are very entertaining to watch, although they can frighten passengers if you don’t warn them ahead of time.
You can control the rate of descent by varying the amount of outside rudder you use. Simply stated, the more sideways you put the plane into the relative wind, the more drag it generates and the faster it comes down. Of course the angle of descent, which is your primary concern, depends on airspeed as well as rate of descent, so speed control via the elevator is also important.
Since a slip produces lift off the side of the fuselage (pulling the airplane to the outside of the slip), you must also control your direction of travel to stay lined up with the runway by varying the bank angle. Use a steeper bank to move the airplane to the low wing side and use a shallower bank to move it to the high wing side.
When trying to slip do not use inside (downside) rudder. Uncoordinated flight with excess inside (downside) rudder is a skid, not a slip. An inadvertent stall during a skid will flip the airplane upside down as it enters a spin—not at all what you want to happen on short final.
A variation on the side-slip is the slipping turn, in which you bleed off excess altitude by slipping through the 180 degree turn from downwind to final on a short approach. A short approach is required because slipping all the way through an otherwise normal approach pattern would result in a loss of way too much altitude.
The goal is to blend varying rudder deflection, bank angle, and back pressure on the stick as required to guide the slipping airplane around the 180 degree turn and come out of the turn properly aligned with the runway and at the proper altitude and airspeed to just reach the planned touchdown point. It may help to first practice 90 degree slipping turns from base to final before attempting the full 180 degree slipping turn.
It may also help to practice non-slipping 180 degree short approaches first if you have not already done so. This means cutting power to idle and beginning your descent on downwind, possibly slowing to below best glide speed to increase rate of descent, setting flaps to further increase rate of descent, and keeping the approach in close enough to preclude the need to add power.
As in any side-slip, assuming constant airspeed, you control rate of descent primarily with rudder. More outside (top side) rudder deflection produces more drag and a higher rate of descent. Feeling a little too high? Just increase outside rudder pressure to create more drag and use aileron to steepen the bank as required to maintain the desired horizontal turning radius/ground track. Feeling a little too low? Just reduce outside rudder pressure to reduce drag and use aileron to reduce the bank angle as required to maintain the desired horizontal turning radius/ground track.
Alternatively, if you are losing altitude too quickly, neutralize the rudder and reduce the bank angle to transition into a normal non-slipping turn. If you later decide you need a steeper descent angle, you can transition back into a slipping turn by adding outside rudder and increasing the bank angle. Undershooting your final? Just decrease your bank angle and release some of the back pressure to widen the turn. Overshooting your final? Increase your bank angle and add some back pressure to tighten the turn, but do not slow the airplane below your desired approach speed and do not pull enough additional g-load to cause an accelerated stall.
Do not ever use inside or bottom rudder to try to correct an overshoot of your final, because a skidding turn to final is extremely dangerous. If you are near your stall speed for the current g-loading, application of the inside rudder is likely to cause the inside wing to stall, just as it does in any other pilot-commanded spin. The airplane will suddenly flip upside down as it obediently enters the spin, this time with insufficient altitude for recovery. Avoid this fatal error at all costs. If you can’t salvage the approach without skidding, return to coordinated flight and go around.
Mastering the slipping turn makes you realize that in your straight-in side-slips you have been doing the same thing but with a different focus. In the straight-in side-slip your focus is on altitude loss and you vary bank angle and pitch (back pressure on the stick) as required to stay on course and at approach speed. In the slipping turn your focus is on staying on course (a curved course this time) and you vary rudder pressure as required to control rate of descent.
In both cases you are controlling your horizontal path (ground track) with bank angle and the g-loading imposed by the elevator (back pressure on the stick). You are controlling your rate of descent (drag) with rudder, and your airspeed with elevator. Note that this dual use of elevator to control both airspeed and g-load can require a compromise between airspeed and g-load in cases where you need more g-load to tighten your turn but also more airspeed to avoid a stall under the higher g-load. In this case it is always better to avoid the stall. Maintaining a safe airspeed and/or unloading the wing are your priorities whenever you are near a stall, especially when near the ground.
In either a straight-in side-slip or a slipping turn, you can go in and out of the slip as required to maintain the desired glide angle. If you have been slipping at the last minute, just be sure to round out and transition back to coordinated flight—or to a forward slip if required by wind conditions—before touching down.
This may be a good time to stress the main differences between side-slips and forward slips. In a side-slip, your goal is to lose altitude. You don’t particularly care where the nose is pointed. You can point it as far from your direction of travel as you wish, depending on how much extra drag you want to generate. This is quite different from a forward slip, in which your goal is to arrest any crosswind drift with bank while using rudder to keep the nose precisely aligned with your direction of travel (or slightly downwind), maintaining altitude an inch above the runway with elevator, and maintaining airspeed with power.
Note: Details on how to safely learn and practice side-slips, slipping turns, stalls during side-slips and during slipping turns, and forward slips for crosswind landings are found in The Art of Avoiding Ground Loops, by George Bachich.
