Say a pilot "bobs" his aircraft's nose up and down. The rapidly changing AOA should generate more drag than if the aircraft were to fly straight at a constant pitch where the AOA shouldn't be significantly large, right? Is the extra drag significant enough for this technique to be used in a practical landing approach?
No, that kind of landing technique is too risky and dangerous.
You’re most likely to just exchange kinetic energy for potential energy, gain altitude, then slam into the ground with the aircraft exhausted of airspeed.
Also it’s superfluous, considering that a properly managed approach on glide path and airspeed will accomplish pretty much what you intended to do with your proposed landing technique. A stabilized approach and entry into the roundout will result in the aircraft exhausted of energy and settling onto the runway once the power is pulled to idle, with little float.
Landing is pretty tricky:
- it can only be done in certain areas with a suitable landing strip;
- contact point with the ground should be at the start of the runway;
- vertical speed should be kept to a minimum;
- the speed vector must be aligned with the runway;
- lower horizontal speed is convenient for correcting wind gusts etc.
So the aircraft speed must be low, while the speed vector must remain carefully pointed at a specific point on earth. Both these requirements are equally important for a successful landing, and voluntarily varying one for improving the other is not a good idea. Which is what you would do if you would be pumping the stick forward & aft.
In order to reduce airspeed, the drag of the aeroplane must be increased - this can be done significantly by increasing Angle of Attack or sideslip, where AoA variations also change direction of the speed vector. The change in pitch attitude (a rotational velocity) contributes very little to drag: rotation axis is the wing centreline, so only an extra speed around the horizontal tail surface is gained. Nothing compared to what happens with the wing and fuselage surfaces.
Repeatedly stalling and recovering the aircraft could be a way to quickly lose energy. Still at the cost of the vertical speed which will not be constant.
As @Ryan Mortensen points out in a comment, a much more controlled way of losing airspeed is by slipping: the sideslip angle increases drag while leaving the lift vector alone. Much easier to hit that suitable spot on the landing strip.
In R/C model aviation beginners sometimes do inadvertently try this technique. While they are on the final they keep working the elevator up and down up and down because they can't really judge a good glide slope at that stage of their learning cycle.
So when they do this on approach, it also happens after touch down. You have a bumpy ride going down to the landing strip you are going to have a bumpy landing and more often than not a broken prop and a nose wheel.
Why does it happen? Because the very basic premise is incorrect. Final approach is a very stable glide slope. If you try anything fancy there you can’t expect a great landing.
Also, when you are coming in like that and touch down you will have a natural tendency to overcorrect the pitch on the first bump and it then becomes a catch 22
No. This would be the direct opposite of a stabilized approach, moreso it would be opening the possibility to cause an accelerated stall.
As has been mentioned, it would be an affront to the aircraft and to the humans aboard this aircraft.
Trying this is a sure sign of a bad approach, and would be better replaced with a go around and a better controlled second attempt.
The issue you face here is that pitching will affect your rate of descent rather strongly, which is exactly what you don't want while you're trying to land. Keeping on your glideslope is already tricky enough at times; trying to follow the glideslope while deliberately wavering up and down would be very difficult indeed - and when you start getting close to the ground, obviously the pitching risks bumping into the ground earlier than you planned, and at a flatter angle, which risks hitting the ground with something other than your landing gear (depending on your aircraft's configuration and how sharply you pitch down).
On top of that, landing brings you close to stall speed, and rapid changes to your AoA will therefore risk stalling the aircraft, and given that you're getting quite close to the ground during landing, you may not have the altitude to spare when recovering from that stall, which would result in a hard landing or crash somewhere short of the runway.
The principle, however, is sound. Repetitive use of maneuvers that increase drag in order to reduce the aircraft's speed is the idea behind "fishtailing", which is basically the horizontal equivalent of what you describe. Basically, a fishtail maneuver involves pushing the rudder over to one side or other while keeping the wings level, resulting in a slip. Ordinarily you'd push it to one side for a moment, then let it come back and do the same thing the other way, resulting in a motion not unlike a fish swimming, hence the name. This reduces your speed whilst having very little impact on your overall rate of descent (aside from the effects of your reduced speed), making it easier to achieve safely.
All the same, I probably wouldn't advise fishtailing as a deliberate intention unless you absolutely must land first try, for whatever reason. If you feel you're coming in too fast and you're not sure you can slow it down in time, you're probably better off going around and having another try at getting it right.