It's kind of a joke that most pilots don't know how to use their feet, especially jet jocks whose feet are flat on the floor most of the time they're flying. Fair enough. Even pilots who started flying in a J-3 Cub off a grass strip need to practice their stick-and-rudder skills once in a while. Some pilots never have the chance learn how to let their feet dance on the rudder pedals. Tailwheel flying is becoming more of a lost art and a specialized skill. And now that Part 141 Commercial Pilot applicants are only required to have around 100 hours total time, a pilot's knowledge of basic stick-and-rudder skills is going to become an arcane skill.
For a taildragger pilot, constant use of rudder is a fact of life and necessary to taxi, take off and land without an embarrassing incident, but it doesn't take a tailwheel for a pilot to get the best performance out of their airplane. All conventional airplanes respond to good use of rudder, and proficient pilots know that by keeping the airplane in trim, they'll get the best cruise and climb performance. Every pilot can learn the stick-and-rudder skills necessary to slip an airplane to lose altitude while keeping an airplane in a steady descent, and if necessary, to slip to a precise point on the ground.
Slips are pretty simple. If you aren't in coordinated flight with your butt centered in your seat, then you're in a slip. If you're in a slip, your airplane isn't in trim and isn't getting best cruise speed or best rate of climb. Sure, you can fly with a wing down and out of trim all day, but you won't go as fast as you would if the airplane were in a cleaner configuration. When you fly out of trim, you're giving away your time and money. Next time you're in a climb, check it out. Purposely climb in an out-of-trim situation, with the ball out of center, and watch your VSI or altimeter. Then center the rudder and climb in trim, and watch the difference in rate of climb. Sometimes, the difference is dramatic.
An airplane flying with the ball centered will get its best cruise and climb performance, but putting it out of trim, in a slip, can be useful too. Slipping or cross-controlling an airplane creates drag and acts like a speedbrake that slows an airplane down. Slipping also enables a pilot to increase their descent rate without gaining speed (and without flaps), and it gives pilots in airplanes with less forward visibility a way to see in front of them. For taildragger pilots in particular, slipping is one of the important tools in their bag of tricks.
The light bulb came on for me early in my career, courtesy of pioneer Alaska bush pilot, Alden Williams. Alden was the real deal. He was in his 70s and retired, but used to do a little flight instruction at Merrill Field in Anchorage, where I was working on my ratings. I asked him to give me some "advanced" (or so I thought) tailwheel training and rented a pretty beat-up Taylorcraft for the mission. Of course, the lesson started before takeoff. I was ready to jump in and crank the engine, but Alden gave it a very thorough preflight, frowning at some of the dents in the leading edge before he agreed to get in the thing at all.
At a nearby airport, Alden asked me to land the T-Craft on a particular spot on the runway, and I blew right past it; we went around and tried it again. This time, he told me to land on the gravel short of the runway, and I blew past it again. Alden took the controls and said something like, "I know what your problem is—you don't know how to slip!" He expertly demonstrated to me how to put a little rudder and opposite aileron to control my descent at a slower speed, enabling me to make the precise point of touchdown every time. What a revelation! So this is how the bush pilots did it! This is how they landed on gravel bars and tiny spots on the tundra. Now I knew the secret: Real aviators knew they could "slip" their airplanes down final approach, controlling their rate of descent without increasing their speed, cutting the power just before the touchdown point.
Slipping has served me well over the years in Barons and Bonanzas, turboprops and jets, but most of all in aerobatic airplanes that have very little forward visi-bility at low airspeeds. Slipping is our drag, our rate-of-descent control, our speed brake. We don't have flaps or speed brakes, but who needs them when you can put the airplane into a slip and slide right down to your touchdown point. If someone can show me a better way to see the runway when landing an Extra or a Pitts than using a slip, please let me know. The alternative of a turning descent that cuts the pattern short for every landing or "dragging" the airplane in with power, blind to what's in front of you and me, isn't a very appealing one.
Slipping is intentionally cross-controlling the rudder and ailerons. You can do this while flying straight ahead, as in a forward slip, or in a turn, as in a side slip. Whether the pilot is turning or not, cross-controlling the airplane will put it in a slip and increase the descent rate. Common mistakes students make when learning slips is to lower the nose as soon as they start to cross-control, which, in my opinion, defeats the purpose of the slip (which is to increase but keep the descent rate constant). I was flying in the back seat of a friend's 172 recently, and we were slightly high on approach. I heard the instructor say, "Just add a little rudder." She did, and we made our touchdown spot. There are lots of times on approach when you can "add a little rudder."
Like anything, it takes a little practice to get the technique. Go up high to practice. Pick a long, straight road or a section line, find an intended touchdown point in the distance, cross-control the airplane slightly and then experiment. Some airplanes are prohibited from slipping in certain flap configurations, so before experimenting at altitude with slips, be sure to discuss with your flight instructor and check your flight manual before attempting them.
Arguably, the best reason to become proficient in slips is Newton's Law of Gravity—what goes up must come down. You can get up in the air, but there might be a time when your airplane decides to land even if you're not ready; engine failure is rare, but possible. Gravity sucks. If the rare but unexpected happens, you might as well be ready for it. Knowing how to control your descent and put your airplane on a precise preselected spot might just save your and your passengers' lives, and that technique might be dependent on knowing how to slip your airplane to that spot.
It could happen to you. In 1983, it happened to Capt. Bob Pearson in an Air Canada 767. On a flight from Montreal to Edmonton, both engines shut down due to fuel starvation (due to a problem computing fuel from litres to gallons), and Capt. Pearson, an experienced glider and light-airplane pilot, was able to dead-stick his airplane, slipping it into the Gimli, Manitoba, Airport. It's a great story and shows how important stick-and-rudder skills are at all levels of being pilot in command.
Slips are useful in formation flying where the lead airplane is always in front, leading their wingman. Sometimes when wing is joining on lead and they're about to overtake them, they can add a little rudder and opposite aileron to slow down while still flying straight ahead. It's a great use of the slip. I asked Debbie Gary, formation guru, about landing in the four-ship formation team she was a part of, and she said she learned she had to slip almost all the way to touchdown in the team's Pitts S-2As in order to see the other three airplanes during their formation landings.
At our aerobatic school in St. Augustine, we spend as much time developing airmanship stick-and-rudder skills as we do in teaching aerobatics, and a good part of that training includes forward and side slipping. Good airmanship is about controlling your airplane in all attitudes, and one of those skills is the ability to slip the airplane on an exact spot on a road, or in a field, in the event of an engine failure.
I have a picture that I'm going to blow up and hang in our school of a Pitts upside-down on a runway with its nose embedded in a bloody cow. I felt sorry for the cow, but I felt sorry for the pilot, too. He had just picked up his airplane from the factory and hadn't secured insurance yet. And to make things worse for him, he later got sued by the farmer to pay for his prize cow.
I don't like to second-guess other pilots (you know the saying, "There but for the grace of God, go I."), but if he had been slipping the airplane to touchdown, I imagine this situation might have been avoided. Of course, you have to level the wings sometime before touchdown (I like to leave it to the last few feet above the runway), so sooner or later, you'll be blind to what's in front of you, and you just have to hope it's not a cow.