How can an airliner have low altitude stall recovery?

Question

I was going through the air crash record and found out that a lot of them happened when the aircraft stalled at a low enough altitude where the pilots couldn't get enough speed to pull back out. I was curious to see why this was the case when I learned about flow separation hysteresis where to reattach the flow the wing needs to pitch down further than the static stall angle. Reading that and the crash record brought up a question in my mind:

How could an aircraft be engineered to have safe low altitude stall recovery? (aside from having a VERY low wing loading, ballistic parachutes, or a computer to prevent pilots from entering low altitude stalls)

From a cursory analysis, this situation seems ideal for some form of emergency engine, either electrically powered fans or a few solid rocket motors to right the aircraft and provide enough power to at least make the touchdown not a crash. Or the wing and stabilizers have some form boundary layer acceleration to reduce the amount of time needed to recover from a stall.

I'm curious to see what approaches have been tried to try and solve this problem or if the newer aircraft have some other technology incorporated.

Answer 1

The most important reason for aircraft stalls is due to high angle of of attack, which causes flow separation.

Usual stall recovery procedures consist of reducing the aircraft angle of attack (pitch angle) and increasing power so that the stalled wing starts producing lift and the aircraft forward speed is increased to the required amount. Then the controls are applied in such a way to return to level flight as soon as possible.

The problem with low altitude stall is that stall recovery usually involves some loss of altitude and if the altitude is low enough, recovery from stall may not be possible before contact with ground.

However,the recovery altitude may be decreased to a certain extent by incorporating the following technologies in civil aircraft.

• Use of high lift devices- Use of high lift devices like slats and flaps increases the lift coefficient and also delays the onset of stall. In case the slats have not been deployed before stall, they can be deployed to get the airfoil (wing) produce lift again.

Source: zenithair.com

• Increasing the power- In general, greater the power applied, the less the loss of altitude. In this case, the engines usually have excess emergency power that can be applied for reducing the loss of altitude.

In general, it is better not to enter into a stall and most of the technologies are geared up towards prevention of stall than recovery due to a number of reasons

• In case of civil aircraft, below a certain altitude, stall recovery is usually not possible.

• Addition of systems to ensure a soft landing in case of stall increases the cost and complexity and in any case may be impracticable in case of large aircraft. As such, it is better to train the pilots to prevent entering into a stall in first place.

Answer 2

The main solution to speed up stall recovery has been to install more powerful engines.

Stall is a total energy problem. If the aircraft is low, it has little potential energy left to top up its kinetic energy, and a stall is exactly when the kinetic energy drops below the value needed for staying in the air.

As you correctly note, the boundary layer needs some time to recover, and the further back on the wing you look, the more the boundary layer is shaped by what went on before. This helps to delay stall when the pitch rate is sufficiently high, but makes recovery take longer. Boundary layer suction would certainly help, but all cases of suction which I know of were either installed to keep the flow laminar for longer or to delay separation at high angle of attack. The suction volume needed to speed up stall recovery is a magnitude above that needed for delaying stall, so even when suction was installed, it would have been insufficient for shortening the stall recovery.

The idea of quickly adding kinetic energy is much more promising, and your proposal of a rocket engine will certainly help. If it points slightly upwards, it can compensate for the lift loss during stall and speed up the airplane so it returns quickly to the linear flight regime. The same can be achieved with sufficiently powerful engines, and since they are much more fuel efficient than rockets, they have been preferred over rockets to make fighter and aerobatic aircraft essentially unstallable if their thrust-to-weight ratio approaches one.

Next, a benign lift curve slope at stall can be engineered into the wing by using a big nose radius, a linear pressure recovery gradient and washout, so the outer wing still has attached flow when the inner wing stalls. The first two factors help to keep lift fairly constant well into the stall, so vertical acceleration is low. The third factor ensures proper aileron response and low roll moments due to flow separation, so the aircraft will not divert and stays controllable. The drag increase due to separation, however, will decelerate the aircraft and lead to a loss of lift if the stall is not ended quickly. The benign lift curve slope of the wing made the stall in case of AF447 so uneventful that the copilot never realized that he had pulled the aircraft into a stall.

However, most aircraft are flown such that they never enter into a stall. Instrumentation and warning devices will prevent even incompetent pilots from stalling the aircraft unless they intend to do so, eliminating the need for emergency rockets or oversized engines.