Can flaps be used to perform rotation and flare?

Question

TL; DR using flaps to directly control sink/climb rate near ground = simpler and faster response = easy to fly airplane?

Rotating is the pitch up during takeoff that causes the aircraft to leave the runway.

Flare is the pitch up just before touchdown that causes the sink rate to drop.

In both maneuvers, a downforce is applied to the tail, to re orient the aircraft to increase the angle of attack, so as to increase lift. The objective is this lift, rather than a change in orientation.

Hence, would it not make sense to use a method to directly alter lift, if available? Specifically, would it not make sense to increase the flap settings just for the moment these maneuvers are done? Takeoff is done at a non-maximum flap setting. When the airspeed nears the required value for liftoff, flaps could be lowered to full. Final approach can be done with takeoff flaps, and just above the runway, instead of pulling back, the flaps could be lowered.

I know the flaps are slow to deploy, but the moment of the fuselage makes pitching slow as well, doesn't it? Even if the slow movement of the flaps erodes any advantage in terms of response time, the decreased angle of attack at the leading edge would be favorable in terms of stall characteristics, wouldn't it? And the requirements to touch the main gear first on landing could be satisfied with even a relatively small pitch angle. And tail strikes could be avoided.

Answer 1

While this is an interesting idea, there are some key issues with it. As a GA pilot, here are my issues with that concept.

1. Changing flap settings causes signifigant differences in aircraft handling. If I was, say, on short final, and I changed my aircraft's configuration - it would be difficult to maintain a stable approach profile to the runway. If I were to deploy my flaps just above the runway, the aircraft would lose its "stable approach" and I would not have as much control authority over where, when, and how my aircraft touches down. There is a reason GA and Commercial pilots alike are taught to maintain a stable approach and have the aircraft fully configured well before landing.

2. I would never adjust flaps on take off or climb out (until at a reasonable altitude). The reason is simple. First of all, aircraft have operating limitations. You would not take off with full flaps because they generate too much drag, and hence limit aircraft performance. They are great for landing because of that same drag. However, If I were to take off with (theoretically) full flaps in a Cherokee or Cessna, it would not be safe to retract them until sufficient airspeed and altitude has been attained. Raising flaps at low airspeeds causes a noteworthy forward pitching motion and a temporary arrest of the climb rate. This is dangerous when in a low-airspeed, low-alitude situation and is warned against by CFIs.

Answer 2

Yes, but.

Flaps will change the pitch equilibrium to nose-up when moved up, so when you want to increase your lift coefficient, you need to reduce the maximum lift coefficient to make the airplane pitch up. This makes flaps much less suitable for take-off and landing than the elevator, but you can control airplanes with flaps only under some conditions. You can lower flaps momentarily to increase lift, but this will make the airplane pitch down. Doing this shortly before touchdown will in effect result in a hard landing: The airplane will initially climb with the flap change and then gradually lower its nose and consequently drop to the runway like a brick. This is the same effect that makes landing a flying wing tricky.

There were several fatal crashes of the ASW-20 glider when the elevator link had been forgotten to be connected. This can happen after the airplane has been disassembled for road transport or maintenance and, upon reassembly, the pushrod was not re-connected to the elevator control horn.

Gerhard Waibel became so upset that he personally drove to a glider club where that happened and demonstrated how to fly an ASW-20 with a disconnected elevator.

The flaps have less control authority than the elevator, so you need to think ahead a bit. Also, they will reduce the maximum lift coefficient when commanding a nose-up pitch, so the landing will be at a higher speed. But since the flaps of gliders are controlled manually, they will be sufficient for pitch control, including landing.

This might be different on airplanes with hydraulic or electric flaps where the speed of flap angle change is much slower than the short period mode, so that pitch changes will result in PIOs or even a runaway pitch change which cannot be counteracted fast enough before control is lost.

The shortest take-off technique for the Me-262 used flaps up during the takeoff run to minimize drag until the airplane approached lift off speed. Then the flaps were lowered in order to lift off at the lowest possible speed. This is generally not recommended - when the flaps malfunction, the take-off run will become a lot longer.

Answer 3

1. Can you use control surfaces to control an aircraft? Yes.
2. Are flaps controls a way to control the control surfaces? Yes.
3. Can you use them to control an aircraft? Yes.
4. Could you land this way? Possibly.
5. Is this something you should try? No.

Flaps are a slow, clumsy and coarse method to modify your control surfaces and there are all sorts of attendant problems with this method, especially in exactly the conditions likely to dominate while landing. Last resort. Better than no control at all, I suppose. As a thought experiment it's interesting.

If you want to try it out, use a simulator.

Answer 4

The question is more complex than one might think, at first blush. Can lift be lessened, in order to land, or assure a landing? Of course. It's the reason that spoilers deploy after landing, either manually or with auto ground spoilers, after main gear touchdown; to kill lift, put weight on the wheels, improve braking effectiveness, and to increase ground control by virtue of the gear, as aerodynamic speed decreases.

There are practical issues associated with retracting flaps during landing. If one increases flap setting, one increases both lift and drag. If one reduces flap setting, one reduces lift and drag, and also increases stall speed. Because approach speed is a percentage value above stall speed, this also changes the relevant approach speed. Because the percentage margin above stall speed, approach speed, also directly relates to stall margin, it impacts descent path, angle of attack and other factors, including power required.

When flaps are lowered, the slipstream angle behind the flaps is also altered. This alters the downwash on the horizontal stabilizer. A change in downwash on the horizontal stabilizer affects pitch, pitch trim, and controlability for a given power setting, weight, blance, and speed. In certain extreme cases (carrying a load of tailplane icing, for example), a change in flap setting can cause a control departure or significant pitching moment (eg, nosedive). Outside of such extreme conditions, a change in flap setting will necessitate a speed change, which will also require a pitch change and in most cases, a power change. Simply retracting or extending flaps during takeoff or landing is not an isolated event: it invokes every other element of the approach control as well, including requiring a pitch change.

Another aspect of changing flap setting on the runway or while approaching the runway is the possibility of moving the wrong control. I know a crew that retracted gear upon landing, thinking it was flap, and subsequently had a gear retraction on the go...the aircraft settled back, ended up overturning, and burned. Crews generally do not make aircraft configuration changes on landing until clear of the runway, and on takeoff, no configuration changes after the aircraft has been set up for takeoff.

That said, under very limited circumstances, mostly in light single and multi engine aircraft on rough dirt or gravel airstrips, I have applied flaps late in the takeoff; it's a bush technique to reduce flap damage during the takeoff roll, from rocks and gravel. Applying flaps late in the takeoff has limited application. Likewise, under similar circumstances landing, retracting flaps does put weight back on the gear and settle the airplane in on short, rough runways, especially where braking may not be effective due to the surface. These aren't standard, and should be considered very carefully before trying, and then only by those who understand the practice and have experience with it. There are a lot of things one can do wrong that can get one hurt, which is why the nearly universal standard practice is to not touch the flaps or configure until clear of the runway, on landing, and to be fully configured before every entering the runway to takeoff.

Answer 5

Using flaps as a means to shorten the run both on take-off and final is quite common among bush pilots. They can more or less 'kite' down or 'jump' off the runway with the help of more than marginal use of flaps. In commercial aviation it could be automated, but that demands certification, which just doesn't add up financially.

The other way around, pulling up the flaps is the right thing to do when encountering a tail stall, which mostly occurs during final at low speed and altitude as a consequence of the combination of mixed icing and flap induced turbulence. It being a counter intuitive move does not change that.

Answer 6

Even if we disregard the problems current flap systems have if they were to be used as flare controls (speed of operation, lag, discrete positions), and assumed they could be controlled in the same manner as flaps, there are still (at least) two major downsides:

Flaring with pitch control (the elevator) is superior in simplicity. Just pull on the stick or yoke, and that's it. If flare was initiated with flaps, you'd have one extra contorl to operate, and being a creature with two hands, you run out of interfaces with the plane. Remember, during flare you should have hands on throttle and stick.

If that problem was somehow mitigated, there's the problem of control uniformity across different types of planes. Different planes react pretty much in the same way to pitch inputs, but flaps, not necessarily.

Both of the aforemenrioned could be overcome by help of automation that would "level" out the differences and handle some of the control, but better not let Boeing design that system.

All in all KISS is a good principle. We have a perfectly functional and redundant way of managing flare.