Video #9: Aircraft Stalls

Script (with weird spellings and lots of comma’s to make the helpful American gentleman narrator speak properly!)

Hello viewers! This episode will be looking at stalls.

A stall in an aircraft is the point where the wing is no longer generating enough lift to keep the aeroplane up in the sky. This, is somewhat undesirable.

It is caused when the angle of attack of the wing exceeds the critical angle, and to remind yourself of what those terms mean, I suggest checking out the exceptional video number four in this series. When the relative airflow is meeting the aerofoil at a small angle of attack, it curves around the wing nicely, and doesn’t have too much problem sticking to the surface of the wing, like sticky the stick insect, stuck on a sticky bun, on a stick.

However, when the aerofoil is meeting the relative airflow at a higher angle of attack, it is much harder for the air to curve around the surface of the aerofoil, and it will separate. The air that separates then becomes turbulent and eddies, and it does not generate as much lift for the wing compared to the air still attached.

At a certain point, known as the critical angle of attack, there will be so much of this separated air that the wing no longer generates sufficient lift. This, is a stall.

You’re not going to want to be in a stall for too long, as other wise the houses get big. Getting out of a stall is very easy, you just need to reduce the angle of attack. Usually this is done by moving the stick forward. This pitches the nose of the aircraft down and gets the wing back below the critical angle. This then allows the airflow to resume it’s sticky duty on the wing.

You can stall an aircraft in any attitude and at any airspeed. You could be in this attitude, moving really fast in this direction for example, although if you are, it is safe to say you are having an exciting flight. If your flight path is in this direction, the relative air flow is in this direction. If we put in the wing chord, you can see that we are stalled, even though the nose is pointing way, way, way down towards the ground and you are probably already weeping, as you stare at your impending doom.

Your instincts, and my brother, creepy american gentleman, will be telling you to do the opposite, and to However, you must actually push forward on the stick, as only reducing the angle of attack will get you out of a stall. This will move your nose even further downwards, possibly causing some brown seepage in your pants, but also moving your wing back within the critical angle. This will restore control, and you can ease out of the dive, all be it very slowly.

Let’s now look at a stall in a slightly more realistic scenario. Say you are on final approach for landing. Your airspeed decays and you are too busy trying to hide the illegal immigrants to notice. Lift formula shows us that velocity is quite important to stay up in the sky. Coming in for landing our attitude is like this, but our flight path is like this. The relative airflow, being the opposite of our flight path is like this, and you can see that there already exists a high angle of attack. If our airspeed reduces, and attitude maintained, our flight path will now alter to become steeper, which alters the relative airflow, which increases the angle of attack further.

You can see that this now means you will have stalled the aircraft, and this is actually a stall you practice in the PPL syllabus, called a power off stall. Now, imagine that your right wing begins to drop. Your normal instinct will be to use aileron to raise it again, but you are in a stall! Look at what will happen if you use aileron to raise the right wing. Yes, you will stall that wing even more, and it will drop suddenly. You will now have one wing pretty stalled and one wing very stalled. This will result in a spin.

This is why you are taught to control the aircraft with rudder when close to or in a stall.

How do you know when your aircraft is close to a stall? Aircraft often have horns or other warnings to announce impending excitement. Physically, the aircraft will begin to buffet. This is caused by the turbulent airflow created by the separated air moving over the airframe.

Hope this helps! Happy falling out of the sky!

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