Types of Slips
Of the two different slips, the forward slip is used to augment any approach path available through the use of flaps. The sideslip is used to cancel out any crosswind effects to achieve and maintain runway alignment. Both of these slips cause the aircraft to descend more rapidly. The slip is a variable descent control.
Forward slips: There are two slips. The forward slip is used to lose altitude. It is used when the wind is nearly down the runway. Slipping uses the side of the fuselage to drag the plane into a steeper descent. The more the side is exposed to the relative wind by rudder application the steeper the descent angle. Smooth, simultaneous control inputs are the key to successful slips.
Most every aircraft will slip but some slip better than others do. This is one area in aviation where older tends to be better than younger. The best slip speed is critical and difficult to determine since in the slip the air strikes the pitot tube on an angle. At altitude, you should practice both right and left slips with full rudder applied and develop a chart of airspeed to rate of descent for use closer to the surface.
The slipping process requires a smooth application aileron followed by sufficient opposite rudder to keep the aircraft from turning. Continue the process until you have the descent rate desired or you run out of rudder authority. Like the entry, a smooth removal of aileron and rudder pressures will accomplish the slip recovery. Properly controlled the slip will transition directly into the landing flare at the appropriate altitude with required crosswind correction. The properly controlled slip to a landing is a thing of beauty.
The forward slip for purposes of losing altitude into a short field or over an obstruction is pretty much a lost art due to the existence of flaps. As later models of Cessna decrease their flap settings the forward slip once again becomes useful. Especially so if the windshield is obscured by ice or oil.
My students and I find great satisfaction in the controllability of a smoothly performed slip. I personally prefer to slip all the way from abeam the numbers around to the landing. Aircraft can be slipped nicely in a turn. However, some examiners frown on such slips so I additionally instruct slips that are performed only on final. Slips give beautiful visibility and control down to the landing. It does take some experience and practice to time the flare. When slips are performed without flaps as is often required by the POH there may be considerable float. Timing this float for a precision touchdown is not easy. The greater the headwind the better the forward slip in losing altitude. The cross-controls are applied but the nose is held at an angle to the runway for maximum altitude loss. The greater the control deflection the greater the slip effect or descent rate. When the runway is made, cross control application is removed to allow the aircraft to straighten to the runway alignment. This sideslip is used in crosswind landings to align the aircraft with the runway. It also continues the descent to landing.
Sideslip
The sideslip is used for runway alignment. The sideslip consists of half a Dutch roll. The rudder holds the nose straight while the wing is lowered to slide the aircraft to one side or the other to center on the runway center line. Skill in the sideslip is very important since most student pilots tend to land off the runway centerline.
The sideslip can be practiced by practice descents along straight lines. They can be practiced on long straight-in landing approaches. The most important phase of the sideslip can only be practiced during actual landings. Getting aligned with the centerline often requires that sideslips be performed close to the runway even when the runway is not in sight. It takes a special wide peripheral view over the nose to maintain alignment by watching the nose's relationship with the visible horizon to each side.
POH Advice
When an aircraft has a POH or a placard that says that slips with flaps should be avoided this means the same as prohibited to the FAA. If you are in a situation where a slip with flaps is required for a safe landing, do so only as an emergency procedure. Go-around is your best option and you need the practice. A well-controlled slip is a maneuver of joy. The sink rate is controlled at will. The transition from one slip to another requires different control pressures to assure that the nose attitude will give the required indicated airspeed for approach. The difference between the slips is the control pressures applied to achieve the intended purposes. A full forward slip will require application of controls at an indicated speed in the forties to achieve a maximum descent rate for ground covered. The rudder is used to hold the fuselage sideways while the aileron controls the flight direction. The most effective forward slip is with the nose at 90-degrees to the wind direction.
Slips and Dihedral
When we first read about dihedral we learn of the geometric kind where the tips of a flying surface is higher in a horizontal plane than the base. There is another kind of dihedral known as effective dihedral. Effective dihedral occurs when an aircraft is slipped with one wing low and opposite rudder. Effective dihedral differs between high and low wing aircraft by its effect on the roll response of the aircraft. Dihedral is said to have either a positive or negative effect on the roll of an aircraft. Positive dihedral effect causes an aircraft to want to level off out of the roll. Negative dihedral effect causes it to increase the roll. High winged aircraft are positive while low winged aircraft are negative. This is caused by the flow of the relative wind over the wings of the different types. Since the positive effect is more to be desired you will find that geometric dihedral occurs in more low-winged aircraft to offset its inherent negative dihedral effect.
A pilot in making a slip is faced with dihedral effect from all the flying surfaces. He uses only the accumulative effect to maintain a specific slipping condition. There is always sufficient elevator authority to over-control the rolling effect of dihedral.
Use of Slips
A slip is a turn that is stopped by use of the rudder. The forward slip has the aircraft moving toward the runway with the nose pointed at an angle to the runway. The side slip is used in crosswinds. It has the aileron holding a wing low while the rudder keeps the nose aligned parallel to the runway center line Any increase or decrease in aileron will cause the aircraft to slide to one side or the other of the center line.
The forward slip is used to lose altitude; the side slip is used for runway alignment. Both slips can be varied at will. The forward slip is best performed without power since it is using the fuselage to control the descent rate. Aircraft tend to be stable in the slip and easy to control as to airspeed. Inexperienced slippers tend to allow an increase in airspeed.
The slip entry need not be abrupt. Lead with the aileron and use opposite rudder to prevent a turn. Incremental additions of aileron and rudder can increase the descent rate. The slip is very easy to control and can be gradually reduced to give a smooth transition into the flare.
The Why of "Slips with Flaps"
The 'Slips with Flaps' controversy stems for POHs that have a warning statement such as, "Steep slips should be avoided with flaps settings greater than 20 degrees due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings."
This warning cannot be considered a prohibition else a pilot would not be allowed to make conventional crosswind landings with flaps extended. Earlier C-172s with 40-degrees flap extension tend to be more subject to this warning that do later models with the extended rudder faring. A pilot is well advised to used an aircraft specific operations manual. It is not a dangerous condition and is easily stopped by relaxing yoke pressure.
I and several other experienced pilots have experienced the nose down pitch that occurs when the flaps blank out the airflow over the horizontal tailplane. This is abrupt but easily recovered from in a couple of hundred feet. Over the years of Cessna aircraft production the Pilot Operating Handbook has grown more and more extensive in advising on allowable flight procedures. This has been a direct result of lawsuits and insurance settlements. One of these additions to the POH has been regarding the use of flaps.
Prior to the invention of flaps, all aircraft would control their descent, along with the use of power and airspeed, through the use of slips. A slip is a cross-controlled right-left or left-right application of rudder and aileron that through its lack of coordination uses the side of the aircraft as an air brake. As the slip decreases forward motion, the aircraft's rate of descent can be increased dramatically. A slip is most often applied without flaps, but not necessarily so.
It was discovered that an occasional abrupt application of a slip when a high-wing Cessna had full flaps would cause a bobble or wavering of the nose. There has never been, as far as I can discover, an accident from this event. Never the less, Cessna Corporation in an effort to reduce the potential financial impact of an accident, appended to their POH a remark to the effect, "Slips with flaps not recommended."
Over the years a broad series of interpretations of this mild warning have led to exaggerated statements and even prohibitions. These have been spread by the uninformed, misinformed, deluded, and imaginative until it has become a matter of aviation folklore. The misconceptions arising from this folklore has led to a failure of many pilots to learn and use what can be a very useful aspect of aircraft control. The "Slips with Flaps' T-shirt is a small effort at facing up to the teaching and learning problem derived from the lawyer instigated statement. The number of my aircraft as it appears on the shirt is in recognition of my contribution to the teaching and learning of flying. I am honored.
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