by adding more wheels to a bike you make it more stable but it makes it a bit more difficult to maneuver in an aircraft we need to be highly maneuverable but we also need a high level of stability so that we don't get knocked around by the turbulence and the wind so how do we achieve this tricky balance between the two let's find out [Music] hi i'm grant and welcome to class 16 in the principles of flight series today we're going to be looking at stability as we fly through the air we experience a lot of bumps and shakes in the form of turbulence changes in wind and thermals coming up off the ground for example if we don't correct these it would throw us off course and lead us somewhere we don't want to be we therefore have to generate some sort of force that opposes these small shakes and we do that in the form of stability i'll be breaking stability down again into two classes with the first part looking at the overall concept of stability and in the second part we'll take a deeper dive into how we achieve stability around the three axis of the aircraft so there are two main types of stability we have static and dynamic stability and they both can be either positive negative or neutral starting with static stability this is what we call the initial response of an object once the force that is displacing it out of equilibrium has been removed the easiest way to think about positive static stability is a ball on a curved surface like a bowl so if we move the ball away from the equilibrium state equilibrium stage is represented by this dotted line here the initial tendency of this ball will be back towards the equilibrium state this has positive static stability if we flatten out the surface into just a plate then we displace the ball over away from our equilibrium point there is no initial tendency to either get closer or to get further away this is neutral static stability the final is negative static stability or static instability if we place the ball over here on the top of this curved surface this hill for instance then the initial tendency is to continue rolling further away from our equilibrium point this is negative static stability so dynamic stability is a response over time for the object to return to the equilibrium state once that initial force that displaces it is removed so we'll start again with the ball in the curved bowl-like surface so if we let go of the ball over here the initial tendency is to return towards equilibrium that's our positive static stability and what you'll see is it rolls down it goes through the equilibrium point it rolls up the other side up to about this sort of point here and then it will roll back and it will roll back and it will slowly tend towards back to the equilibrium point if you represent that graphically you would have the displacement up here it comes back it passes through the zero point it comes back it comes back slowly reducing slowly reducing until it gets back to the equilibrium point over time for an object to have positive dynamic stability it must first have positive static stability so this here is an example of oscillating periodic dynamic stability so the displacement oscillates back and forth until settling down on that equilibrium point you can also have a periodic uh dynamic stability where there is no oscillation essentially and the object will just stop at the point of equilibrium so for instance if we put a little plate in here the ball would then have the initial response towards the equilibrium state i would roll roll roll and just stop here this is still positively dynamic stability but instead of oscillating back and forth it just tends back to the zero point if we were to remove all friction and drag forces from our curveball we'd we would see that same static stability initially but we wouldn't have the friction slowing us down and bringing us closer and closer back to this neutral point when it crosses the neutral point there would be no friction and it would go up to the same point and then it would bounce back and go to the exact same point again so you have this oscillating back and forth motion so whilst we are statically stable we are dynamically neutrally stable we don't tend closer over time so if you think about it graphically our displacement starts off up here it goes all the way down to the opposite side and it comes all the way back and you get this sine wave of displacement positive static but neutral dynamic if we take the flat plate example we know that this is neutrally statically stable because if we remove the force it doesn't tend closer to the equilibrium point or further away and over time that doesn't change the displacement remains exactly the same so dynamically neutral instability is just a straight line over time it doesn't get any closer or any further away from that initial equilibrium point negative dynamic stability is a tendency to diverge over time or dynamic instability you could call it so if we think about the ball in the bowl again you could have positive static stability but if for some reason it gains speed over here i don't know why it would but just imagine it gains speed when it goes through this zero point that means that it would deflect all the way up here and then come down with more force and then as it passes through the zero point it gains even more speed it would end all the way back up here and it just keeps getting further and further away as it rolls up and down this bowl so if you think about it in a graph you have the initial displacement it oscillates back and forth getting slowly and slowly further away as the time increases if we think about the ball on top of the curve again we saw that the initial tendency was instability negative static stability because it tends to diverge away from that equilibrium point and over time it just continues to diverge further it doesn't get any closer back to this equilibrium point so in terms of its dynamic stability when you graphically represent it it starts off by diverging and it just continues to diverge over time this is an example of static and dynamic instability whereas this is positive static stability but negative dynamic stability generally speaking the more positive static stability an aircraft has the more force is needed to displace it from that equilibrium point if it's really stable basically it likes returning to that equilibrium point so to get away from it we need to apply more force an easy way to think of the strength of stability is think about the steepness of the curve in the bowl so over on the left here we have sort of a low static stability we don't need to apply that much force to get it over here and it will pretend to rollback whereas on the right here we've got a very high statically stable object we'll need to just apply a lot of force to get it up the steepness of the curve of the bowl when you apply that to an aircraft it means that a stable aircraft such as on the right here will require a large amount of force and therefore a large amount of control input to actually make it diverge and maneuver it think of a very stable tricycle is harder to move than a normal sort of bike once we've managed to displace the aircraft if it has a high dynamic stability it will want to remain in this new displaced state so we've broken out of this previous equilibrium and we're into this new one and any disturbances to this new equilibrium state will tend back towards this new equilibrium state this means that dynamically stable aircraft are easier to control and we don't need to constantly edit the inputs as we stop any further divergence so if we apply those concepts of stability to an aircraft we can see that we have the three dimensions to work in essentially we've got the normal axis the lateral axis and the longitudinal axis rotation around normal is yaw rotation around longitudinal as pitch and around lateral is roll but in terms of stability we don't talk about roll stability hitch stability and your stability because we already have names for that sort of motion so we get a bit confusing so instead we call the stability in yaw or directional stability the ability to keep heading in the right direction our stability and role we call lateral stability that's our stability not to rotate the aircraft and in terms of pitch our stability is known as longitudinal stability it's the stability of the longitudinal axis not to go up and down and we'll see in the next class that you get essentially stability moments that you deal with and it's important to note that it's positive to the right for directional stability and that is from the perspective of the pilots always so if you're heading along and you get a corrective moment to the right clockwise that would be a positive corrective moment in terms of lateral it's the same in terms of the pilot's view if you roll to the right you roll clockwise that's a positive corrective moment and in terms of pitch you view it as positive nose up so a nice quick class there this is just the sort of outline of the concepts of stability and we're going to break down how the aircraft actually implements stability concepts in the next class so you get three types of static stability positive neutral and negative and then you get dynamic stabilities within them as well so let's first talk about positive positive and positive you can either have an aperiodic or oscillating motion a periodic is just a tendency to return to the equilibrium point and the oscillating passes through the equilibrium point and bounces back and forth until eventually settling down on that neutral positive static and negative sorry positive static and neutral dynamic is again an oscillating motion and you have it passing through the equilibrium point but never getting any closer it just passes back and forth back and forth like a pendulum it keeps basically swinging back and forth positive static stability and negative dynamic again is an oscillating motion but each oscillation brings us further and further away from the equilibrium point important to note is that if an object is positively dynamic in stability it must also be positive in static stability but not necessarily for everything else in terms of neutral static if you have neutral dynamic as well that's an aperiodic motion it doesn't repeat and you essentially display something and it stays displaced it doesn't get closer over time in terms of negative static stability if we've got negative dynamic again that's an aperiodic motion and that's just a tendency to initially diverge and then keep diverging over time you can think of the levels of stability in terms of the steepness of these bowls something that is not very stable will have a low curvature to the bowl and it's easy to get out of this and in something with a high level of stability the sides of the bowl are a lot steeper and it requires a lot more force to diverge from that equilibrium and then when we apply stability to our aircraft axes we've got stability in pitch known as longitudinal stability stability in row is known as lateral stability and stability in yaw is known as directional stability