Video #10: Aircraft Stability

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

Hello internet! Welcome to episode 10 of the profpilot dot co dot ewe kay flight training video series.
This episode will be traversing, the perilous waters of aircraft stability, paddling, across the reason why it is important to understand as a pilot, and streaking, across what design features are used to change stability levels in the design of your aircraft.
As the angered aviator baby is now demonstrating, A stable aircraft will resist external forces from changing its flight path, in this case by an enraged airborne toddler. A less stable aircraft will be more easily influenced. Once it has been disturbed, a stable aircraft will return to its original flightpath quickly, whereas an unstable aircraft will not. There are two types of stability, static stability, and dynamic stability.
It’s a typical, normal, run of the mill day, and here is an aircraft happily flying in equilibrium, straight and level flight. But suddenly, the sperm whale from the Hitch hikers guide to the galaxy makes an unwelcome appearance, and clips our aircraft into a nose down situation.
Static stability is the initial action that the aircraft will do as a response to this upset, remembering that there is no input from the pilot. Not that you will be controlling the aircraft at this point, as you will be too busy watching your new friends terrifying descent into obvlivion.
At least the Japanese will be happy. So. No pilot input. If the aircraft has positive static stability, the initial action of the aircraft, having been knocked into the nose down position, will be to put its nose up again. Neutral static stability will mean the aircraft doesn’t do anything, it will carry on in its new, whale induced attitude. Negative static stability will mean that the aircraft will actually nose down even further.
So that is static stability. Dynamic stability is the overall tendency for the aircraft to get back to its initial flight path. A dynamically stable aircraft will eventually return to the flight path after a few oscillations. A dynamically neutral aircraft will carry on oscillating like some horrendous rollercoaster. A dynamically unstable aircraft will carry on oscillating but with increasingly large oscillations, which will probably end in tears. And lots of vomit.
So far, we have only been looking at stability in pitch, but as we shall see in a moment, there is also stability in roll, and in yaw too. For training aircraft, you generally do not want to be flying an unstable aircraft, like if it was made of amy winehouse. But what is it that determines how much stability the aircraft has? Let’s look at directional stability first, which is the stability in yaw. Directional stability mainly involves this wonderful piece of apparatus, the vertical stable iser. The vertical stable iser aligns the aircraft with the wind in flight, so the wind moves equally over both sides. As we learned in episode 5, if the aircraft yaws one way, then an angle of attack will be created at the vertical stable iser, which in turn creates lift on the vertical stable iser. This lift will then drag the aircraft back towards its original direction. However, momentum will keep the aircraft going through this point, another angle of attack will be created, and the process will repeat through a series of diminishing oscillations. This shows the aircraft is both statically and dynamically directionally stable.
Longitudinal stability, the stability of the aircraft in pitch, involves the horizontal stable iser. In an aircraft, the centre of gravity, an imaginary point where all weight acts, is in front of the centre of pressure, an imaginary point where all lift acts. Since an aircraft always moves around its centre of gravity, without anything to stop it, the lift from the wing will want to tumble the aircraft through the air like this, which provides a challenging environment in which to serve an inflight beverage. To prevent this, the aircraft has a horizontal stable iser that provides a constant down force at the tail. This is because the downwash from the wings creates a negative angle of attack at the horizontal stable iser, which in turn causes lift to act downwards at the tail. This should hopefully be enough to counteract the nose down force during normal flight. If the nose of the aircraft is pitched downwards by, for example, a glancing blow from the remaining bowl of petunias falling from the sky in the hitchhikers guide, the airspeed of the aircraft will increase. This will make the lift from the wings increase, which means a larger downforce at the tail is needed to pull the tail down again. Thankfully, more lift from the wings increases downwash, which increases the negative angle of attack at the tail, which increases the downforce there. The nose raises, lowering airspeed and lift, and like before, the aircraft will oscillate back onto its old flightpath. Obviously, if the aircraft is a t-tail, then all that is slightly different, but I won’t go into that.
Lateral stability, the stability of the aircraft in roll, is affected by dihedral. Dihedral is where the wings are in a very shallow v shape, like this. If you fly over a source of very localised hot air, this blast of tedium may raise your wing, banking the aircraft. This will be an uncoordinated turn, as you have not kept it balanced with the rudder, and adverse yaw will rear its ugly head. So, banking to the left, will yaw the aircraft to the right, sticking the left wing out a bit. You can see that the dihedral of the wing presents a higher angle of attack on the lower wing for the air to get its teeth into. Higher angle of attack means more lift, raising the lower wing, and the aircraft goes back to level again. If the aircraft is too stable in roll, and will barely even let you bank the aircraft, like in the ilushin barn from episode seven, then you can give the aircraft anhedral, for the opposite effect.
Stability is a big topic, and this video has just merely caressed the surface of the thrilling subject. It there is demand, then I will plunge into the murky depths still further.
See you next time!

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