How much slipping and sliding go on during short final?

Question

NOTE: This is not a question about coordinated flight in the pattern. There is no disagreement that hasty skids to turn the plane faster from base to final can and do kill. The question is about what happens once the plane is aligned on final and now continuous micro- and small-adjustments are necessary to precisely land the plane on the centerline.

While flying Condor the other day, I noticed that when I depend on automated turn coordination it is exceedingly difficult to align the (simulated) glider with the runway. The reason is readily apparent in that a typical glider has a bank rate of less than 30deg/sec, and thus bank corrections happen far too slowly to effectively steer the plane. The world is simply going by too fast for the slow rate to yield the precision required for landing. And from a theoretical controls perspective, this is a poorly damped oscillatory system, which are famous for being touchy to control.

And yet, when actually flying for real, a century of anemically maneuverable aircraft have demonstrated that high roll rate is not a requirement for safe landing. What then is going on?

I hypothesize that while on short final we use the rudder extensively for fine control, while keeping the wings appropriately level for the cross-wind conditions. In other words, we intentionally and unconsciously slip and slide the plane in small amounts, because coordinated turns in this phase of flight are slow and even dangerous.

However, this is merely an uninformed hypothesis, and there is certainly a large body of evidence which can definitely answer this one way or the other. In particular, this must be well understood for autolanding systems.

Can anyone point to studies which looked at the rudder movement on final approach vs. in standard flight?

P.S. Anecdotes are interesting, but I think these should be confined to the comment section. I think in order to really answer this question we need results from empirical studies.

Answer 1

It is fine to slip and slide through a turn when you have enough altitude; doing so while being slow and close to the ground is much less advisable. Coordinating controls in turns is especially important when being slow and low.

The key to a good approach is space: Fly a large pattern so you have enough time to adjust heading and sink rate during the final. It is amazing how well the human mind can interpret small changes in the viewing angle - fly coordinated turns until your flight path is parallel to the runway and adjust flaps and flight speed so the glide path will intersect with the ground maybe 50 m (150 ft) ahead of the point where you intend to touch down. If you are too high, increase speed and shift that imaginary intersection closer to you, and vice versa. When all is adjusted, you should have used up maybe half of the final approach and can enjoy the nice, stabilized approach and prepare for the touchdown. Round out when maybe 3 m (10 ft) high and kick the rudder just when you think the wheel will touch the ground in order to align the fuselage with the direction of movement. Open the speed brakes fully only when the wheel has started rolling. This will produce a great landing.

Use side slipping if you are too high and make sure that your speed is at least 30% higher than stall speed. The direction of the sideslip should be with the nose pointing away from the crosswind so the misalignment between fuselage axis and glide path is small. If this produces a well stabilized approach, do as I said above and end the slip just before the beginning of the flare. But train sideslipping at altitude before doing this for real!

The scale of gusts gets smaller as you approach the ground, so the large ones which can shift the flight path and roll angle considerably should die down as the glider sinks lower. Also, gusts tend to cancel out over time, so you shouldn't be too upset about single ones. In gusty weather you need to fly your approach at higher speed so you have some margin against wind shear close to the ground. Because roll damping decreases and aileron authority increases with increasing speed, being thrown around by gusts indicates that your approach speed is too low.

The glider's inertia is already reason enough that you cannot fly in a coordinated way while being blown around by gusts, so short-term deviations of the yaw string can be disregarded. Your reactions, however, should be coordinated. When flying faster, adverse yaw is lower and it will be easier to keep the controls coordinated.

And if there are still gusts, a glider will find thermals and should not land yet.

Answer 2

You could be drifting into dangerous territory (pun intended...hahahah). There should be NO slipping and sliding going on in any airplane or glider on final except for side slips induced on purpose to lose altitude, or for crosswind compensation during the landing flare.

All maneuvering turns in flight must be coordinated. Cross control or skidding maneuvers, close to the ground, except for deliberately induced side slips, invite a spin if the plane gets too slow and stalls.

The technique you are describing, in effect, consists of pointing the nose at the target (the runway) with rudder because it gets results faster. This results in uncoordinated flight, and lots of people have died from spins originating from this habit.

Even in a glider with a ponderous roll rate, all maneuvering turns must be coordinated banked turns to make the necessary heading changes. You have to think of it in terms of headings on final, and adjustments to headings when required.

Like gliders, large airliners have similarly ponderous roll rates and they make coordinated turns on final like all other airplanes. It takes a bit of patience. If you are drifting off track, you make a heading change, the normal way, with a coordinated turn to a new heading, then another coordinated turn to adjust as required. None of this pointing the nose with rudder stuff.

The thing is, in gliders, you often need large aggressive rudder inputs just ahead of the aileron input to get a coordinated turn in the first place, and that's fine, it's the way gliders have to be flown. But the objective is still to keep the yaw string straight as possible while maneuvering.

Answer 3

In a cross wind landing one can stay coordinated and crab their way down to the round/flare phase, but over the runway rudder and ailerons have specific purposes.

The rudder points the nose of the plane straight down the center line. The ailerons are used for lateral control, banking into any crosswind component.

For example, a 5 degree roll in a Cessna 172 at 2000 lbs lift would produce sin 5 × 2000 = 174 lbs of side force! So, even "ponderous" rolling is effective to hold track.

Interestingly, short final can also be flown this way, at a safe airspeed.

For example, approaching "cross controlled" in a 172 would be suicidal at 50 knots, but much safer at 65 knots. Importantly, while on short final, flying in a straight line, slipping is not nearly as dangerous as in a turn, where differences in wing tip speed can become critical.

Over the runway, basically one does what is needed to control the aircraft until it is down and stopped. Effect of rudder will vary with different aircraft. In a flaring 172, it does little more than yaw the nose back and forth without appreciably changing the fight path.

All uncoordinated manuevers should be thoroughly tested at altitude before use in a pattern.

Regarding "the world going by too fast for the slow rate to yield precision", this is definitely true for very light wing loads such as paper airplanes, models, and birds. Here, the "tricks" of controlling side wind gusts can be seen in the common seagull: high wing design with anhedralled wingtips and no vertical stabilzer. This design automatically banks into the wind without excessive yawing.

Full scale aircraft are seen in all shapes and sizes because their much higher wing loading (or mass to surface area) creates a much slower acceleration from a side force, that is more easily controlled.