<< Page 18 Page 19: Page 20 >>   Stability In a few words, stability is the tendency for angles of attack and sideslip to eliminate themselves. Imagine you are pulling a wagon, one of those little-kid “Radio Flyer” red wagons with the handle in front that connects to movable front wheels. So you’re walking in front of it, pulling it by the handle. If the wagon for some reason turns a bit to one side – say one of its wheels is deflected by a small rock, or the wagon’s contents move or shift, or you change the direction you’re pulling – then the wagon will automatically turn itself to again face the direction its going (because of the way you’re pulling it). This is stability. The opposite, instability, would be the tendency for an angle between the direction the vehicle is facing and the direction it’s going, to increase itself. Say you’re pushing the wagon by the handle. If the wagon starts facing and going a bit to the left, your pushing it will cause it to veer to the left. The faster you’re going, the more quickly you’d have to respond (by moving the handlebar to the left so as to make the wagon go back to the right) in order to keep the wagon in front of you. If you do nothing, the wagon will turn 180 degrees and you’ll end up pulling it. Here’s another analogy: Say you’re at a pub and you throw a dart at a target. But say that you throw the dart partly sideways, so the dart is not pointing quite the way it’s going. (Maybe your motor skills are worsening due to the consumptions of beverages at the aforementioned pub). Will the dart just keep flying sideways? Above: A dart is thrown at an angle: It is flying to the right but pointing slightly away from its target. How will it react to the air that comes from the right? The greatest deceleration-due-to-drag will be experienced by the tail, due to its greater surface area and lower weight. So the tail gets pushed back. Not only that, the tail’s angle deflects air away from the center of the dart, creating a lift force that pushes the tail towards the centerline, also helping the dart fly straight. Eventually, this lift and drag push the back end of the dart so that it is flying directly behind the front end. No, the dart will not keep flying sideways. The air hitting the dart will create drag. Since the tail of the dart is lighter and has a bigger area, it will get decelerated by the air more quickly than the rest of the dart – in other words, it gets blown back by the air. Not only that, but the flat surfaces at the back of the dart deflect air like a wing: they push air away from the centerline of the dart, and thus they get pushed by the air towards the centerline (equal and opposite reaction). Due to this lift force created by the tail fins when they are at an angle, and by the extra drag the tail fins experience when they are at an angle, the dart orients itself so that the tail flies directly behind the front, and so that the pointy end points the way the dart is going. This is why darts and arrows have light tails in the back: The wind pushes the tail back more strongly than it pushes back the rest of the dart/arrow (since the rest of the dart/arrow is heavier and not as flat), and the tail fins create lift that push them towards being behind the front, causing the dart/arrow to point the way it flies (which leads to less drag, and to a higher chance of it sticking its pointy end into the target). Airplanes have tails for much the same reason: If the plane starts pointing in a direction other than where it’s flying, the air blowing past the airplane pushes the tail backwards, and it is deflected outwards by the tail which pushes the tail towards the centerline of the airplane, and this keeps the airplane pointing in the direction it’s flying. Stability in aircraft works very similarly to a dart or, say, a wind vane. i.e. one of those things many people have on their roofs, consisting of a flat plate (usually shaped like an arrow or a chicken or other bird) which swivels around an axle, and of the 4 cardinal directions (which do not swivel, but always point to the corresponding directions). To understand the balance of moments that makes airplane stability possible, it’s useful to think about windvanes. The arrow always points to the direction the wind is coming from. If the wind is blowing from a certain direction, and for some reason the arrow starts pointing off to some other direction, the wind will blow it back so it points into the wind. Why? Because the arrow’s tail has a larger surface area than the arrow’s head, so the wind hitting the tail will blow the tail back with greater force, and the tail will end up behind the axle, which is as far back as it can be. Most meteorological wind vanes don’t even have an arrow head, they’re just a tail. Obviously, the wind will blow the tail so it is behind the axis of the vane, so the pointy tip will always point into the wind. When aircraft are in flight, any force that does not act through the center of gravity will cause (or try to cause) the aircraft to swivel about its center of gravity (CG). A turning force like this is called a moment, and is stronger the farther from the CG it is applied. Imagine a windvane which is a flat vertical plate shaped like an airplane seen from the side. The swiveling axle goes vertically through where the plane’s CG would be. Here’s that windvane, a plane simplified so it is a flat plate shaped like a plane’s profile view: The fuselage is roughly symmetrical with respect to the CG/axle: there’s as much behind as there is in front. The big asymmetry comes from the tail. The wind blows on it harder, making it go to the back, so the plane points into the wind. (However, if the CG – our axle – is far enough back, there will be a lot more fuselage ahead of the axle than there is behind, and this might be a larger area than the tail, so the wind will blow on it harder and make the plane face the opposite way. This plane, with the CG way back, is unstable). Now, if the windvane were an actual model airplane, not just a plate – so it has a cylindrical fuselage, wing, horizontal tailplanes, etc – it will still work as a windvane: The wind’s force pushing the thin wings and horizontal tailplanes is very small, and the area of the cylindrical fuselage works like a flat plate when hit by the wind from the side. So our flat-plate approximation was actually quite accurate. This is Yaw stability. Everything we’ve been talking about has been about the Yaw axis (the swivel axis on the windvane, like steering a car right or left). So, yaw stability is easy: Stick a big flat vertical thing near the back with a lot of surface area, and no big flat things (or as few as possible) near the front. And the further forward the Center of Gravity is, the more the plane is blown to face into the wind (the more stable it is, since the more of it is behind the center of gravity and thus “blown back” by the wind). A plane that is unstable in Yaw would have its nose blown to the side quickly, and would tend to want to face backwards, at the slightest increase in the sideslip angle. It could be impossible to fly. So the bigger your vertical tail is, and the further back it is, the more your airplane will be stable in Yaw – the more it will turn into the direction of sideslip, so the less it will “slide” when you bank it. Remember, the turning moment gets larger the farther from the CG (or swivel axis) the force is applied. This is why a tail far back is better for stability than a tail that’s close to the CG. It’s like why kids of different weights can play in a seesaw, if the heavier kid sits closer to the middle than the lighter kid: they produce about the same moment. So a small tail that is further back could provide the same stabilizing moment as a larger tail nearer the middle. But, of course, then you would need a bigger tail boom, which is heavier and more draggy, although the larger tail that is not as far back will itself be pretty draggy. So you have to compromise, maybe experiment with a few configurations (or look at the ones commonly used) when deciding how big you want your plane’s tail to be, and how far back. Now let’s talk about Pitch stability, which has even more consequences to the way an airplane is designed (because the wings also affect it). It works the same way, but about the Pitch axis. Imagine our flat-plate airplane is now mounted by the pitch axis (the axis that goes from wingtip to wingtip). There it is below: There is about as much wing behind the pitch axis as there is in front, so the turning forces of the wind blowing on the back half of the wing and the front half of the wing cancel themselves out (and they’re close to the CG anyways, so they’re small turning forces). The horizontal tails, however, get blown back just like our previous examples, so it still works. So, with the CG in the middle of the wing, and a horizontal tail behind the CG, we’re done, right? Possibly. But say that, to reduce weight and drag, you wanted a smaller tail closer to the front. How can you do that and still keep your plane stable? By using the wing as a stabilizing surface, i.e. by putting the CG ahead of the wing! Although this would make a fine windvane, there is obviously a problem if you try to fly a plane like this. All the lift is being generated behind the CG, and this causes a nosedown moment! How do you compensate for this? Two ways: One, the tail in the back, is turned slightly downwards, so as to deflect air upwards and push the tail downwards. (This generates some induced drag, but less than what was caused by the weight and viscous drag you saved by having a smaller tail and shorter tail boom). Again, because the tail’s downwards lift is further out than the wings’ upwards lift, the downwards lift can be smaller than the upwards lift (so most of the lift is left over to keep the plane in the air) and still cancel out the moment. Again, this is just like the lighter kid who sits further out on the seesaw: The plane is the see-saw, its support is the wing, and it is balanced on the wing by a big downwards force close to it on one side and a small downwards force further from it and on the other side. The other way to have the wing behind the CG and to counter the nose-down moment this causes is to have the engines as low as you can. Drag hits an airplane around the centerline, and if thrust is applied under the centerline, then the thrust is producing a nose-up moment – you know, like when the wheels of a car cause it to accelerate really fast and the car pitches up a bit. So airplanes try to have the CG as far forward as they can, the tail as far backwards as is practical, and the wings just behind the CG, with lift moments being balanced by the tail. (How are tail-less airplanes, like flying wings, stable? They use wing-twist. See page122. And canard airplanes take an entirely different approach to pitch stability. See page 145). Now, let me say that the engine is by far the heaviest part of an airplane. Remember how I said aviation is a battle against weight? How airplanes are made out of light materials? Well, not the engine. An engine deals with extremely high-pressure high-temperature gases, and it cannot melt or explode. So it is made with tougher materials than the rest of the plane. So the plane’s center of gravity is roughly where the engines are. Below: The F-104 (right) has its engine near the back. So it has its little stubby wings at the very back, for stability. So the nose wants to tip forwards. Having the stubby tail deflect downwards enough to compensate would cause a lot of induced drag, so the engine nozzle points upwards (pushing the exhaust gases upwards), and this pushes the tail of the plane downwards instead of the tail pushing air upwards. This is kinda like thrust vectoring, but is not moveable. The other way to make up for the wings being far back is to have the engines really low. Below (left) you see how newer 737s have engines set so low to the ground, the bottom is “flattened” to allow for enough clearance. (These engines are also wider than older 737 engines, as they are more efficient, higher-bypass-ratio turbofans). So stability is why most airplanes have engines at the very front (so that the wing is behind the CG). This is why airliners have the engines sticking out the front of the wings, reaching as far forward as is practical. Also, stability considerations explain why the engines are really low to the ground (to create a nose-up moment to “balance” the weight being at the very front). Airplanes with engines set high up and far back (like business jets and MD-80s, ERJ-145s, etc) must have larger, draggier tails in order to still be stable, especially since they must have a lot of fuselage ahead of the wing for balance (i.e. so that the center of gravity is near the wing). So Pitch and Yaw stability works just like a windvane. If most of the flat surfaces (like the tail, and ideally also the wing) are behind the center of gravity, they will get blown backwards and the plane will point forwards. There is also Roll stability, but it is very weak and makes little difference to the flying of an airplane, and works a little differently from pitch and yaw stability in that it does not involve the tail. Wings canted up – dihedral – will have a tendency to un-roll a banked airplane back to a level orientation. Dihedral wings on a KC-135, Anhedral wings on a Harrier This is because in a banked airplane, the higher dihedral wing is at a slightly lower angle of attack than the lower one, so the lower one makes more lift and tends to rise until both wings are at the same orientation, bringing the airplane back to wings-level. The extreme case of this is easy to imagine and understand: Imagine an airplane with dihedral (canted-upwards) wings. It rolls to the right almost 90 degrees, such that the left wing is pointed straight up, and the right wing is NOT pointed straight down (they are dihedral, so there is an angle between them that is not 180 degrees). The left wing, because it is pointed straight up, it at a zero angle of attack and not producing lift at all. But this is not true for the right wing, so the right wing’s lift tries to roll the plane back to level. Even when the left wing is at some angle of attack and producing lift, the right wing will be at a slightly higher angle of attack, producing slightly more lift. Why is stability such a big deal? Well, because it is almost impossible to fly an airplane that is unstable. Remember yaw stability. Imagine that every time an airplane sideslipped, there were no tendency to correct the situation. An airplane could develop a large angle of sideslip, and instead of facing into the wind, it would keep flying sideways. This is Neutral Stability (angles of attack and sideslip not automatically increasing or decreasing themselves). The wake of the fuselage would disrupt lift on the trailing wing, the engine would not work so well because air would be coming in sideways, and the control surfaces would also not work well because air would not be coming from the front. This would be pretty bad. Airplane engines, propellers, wings and control surfaces are designed to work with air coming from the front. (However, the Sukhoi Su27 fighter is almost neutrally stable, which allows for continuous and easier hi-alpha flight. For example, an Su27 pilot desiring to lose airspeed fast (say, during aerial combat) might pull up his Su27 into a 90 degrees angle of attack, i.e. flying belly-first into the air, using the entire airplane as one giant speed break. This is called the Cobra maneuver and can be imitated by few other airplanes, like the MiG29. All western fighters that do the Cobra “cheat” by using thrust vectoring to fight the stability of their airplanes). An Su27 flying to the right at about 100 degrees alpha What would be worse than Neutral Stability is true Instability. Neutral stability means an angle of attack or sideslip does nothing to try and fix itself, as was just described. Instability means an angle of attack or sideslip makes itself worse automatically. Like the barely-controllable Radio Flyer wagon you’re pushing instead of pulling. You have to keep turning it in order for it to go the way you want it to go, otherwise it turns itself sideways and backwards. Instability in an airplane would be similar. If the angle of attack increases – because, say, you pulled up – it would keep increasing until the plane was facing backwards or until you push it back down. Again, the plane always works like a windvane about its center of gravity. If most of the flat surfaces are ahead of the center of gravity, they will try to be blown back. This would mean if the nose started going a certain way, it would rush and go that way faster and faster, unless you stopped it. If you used too much force, however, it would go the other way, and then you’d have to bring it back to center again… you would spend all your time wrestling with the instability (like trying to balance a broomstick on one palm) and never fly in a straight line. However, you should see that, if an unstable airplane will have an angle of attack increase progressively faster, then a slightly unstable airplane would be very agile and maneuverable. Fighter planes and aerobatic planes are much less stable than airliners or general aviation planes. Military fighters have what is called fly-by-wire, where the pilot’s inputs do not directly move the control surfaces. Instead, inputs are read by a computer that in turn decides how to move the control surfaces to best achieve what the pilot wants. One of the advantages of fly-by-wire is that it can be programmed to recognize unwanted increases in the angle of attack and sideslip, and to quickly fix them automatically, while they’re small, by deflecting the control surfaces in the opposite way. This means a fly-by-wire airplane can actually be unstable, and the computers keep the angles of attack low enough for the plane not to go out of control. Such a plane could be extremely agile. (It could also have very small or inexistent tails, and thus be less draggy). That’s what the makers of the F-16 decided around 1970. When the F-16 flew in 1973, it was the first jet without hydraulic or mechanical control surfaces – they were all electric, operated by a computer. The computer checks the plane’s angle of attack and sideslip thousands of times per second and makes minute adjustments to the elevators and rudder to keep the plane flying in a straight line. The growing forces of a tail blown by the wind of a high angle of attack or sideslip are simulated by the control surfaces, which deflect to push the tail back if the angles of attack or sideslip increase. And if the pilot wants to turn (by telling the computer to increasing those angles), boy can the F-16 turn. Engineers say the F-16 computers face the same quickly-changing instability as a bicycle being pushed backwards by the handlebars (like the unstable kids’ wagon) at 80 mph. Artificial-stability systems are present in many jets and rockets, from the Space Shuttle to the Blackbird to the 777. The Space Shuttle is naturally unstable. That is probably what caused it to come apart in the air. The wings are near the back, which means the center of gravity is near the back. This means the nose is pretty light, so if it points off to one side relative to the direction the air is coming from, it would get pushed to go farther to the side, and the shuttle would end up flying sideways, then backwards, like the bike or wagon in the thought experiment, if the tail’s computer-activated rudder does not come in to push the nose back. Of course, there is only so much the rudder can do – if the nose gets to be at too high an angle relative to the air, it gets blown to the side with a force too big for the rudder to counteract. This is probably why the Columbia crashed. As thermal tiles fell from the left wing, drag increased on the left side (a hole in a flat surface can be very draggy), and the rudder had to be trimmed for yaw to the right. As more and more tiles fell off, the rudder was trimmed more and more, until it was stuck all the way to the right. At this point, if more tiles fell off, the Shuttle would yaw left faster and faster, and be flying onto its right side. The sides are made of relatively flimsy and low-melting-point aluminum that is not meant to be exposed to the high temperatures and pressures of compressed leading-edge air, so at this point the sides either melted or came apart. If not, the tail would have been blown back, rolling the shuttle to the right, exposing the belly again but sideways… and at this point the Shuttle is just tumbling through the air, being heated in places it was not meant to be heated, facing aerodynamic loads and pressures it was not made to face. This is why stability is very important: planes are not made to fly sideways, especially not at 20 times the speed of sound. And on an almost unrelated note: Now that you know that most airplanes are naturally stable, like a windvane, because of their tails, then there is no reason why a model airplane would not make a good windvane. I fact, searching the internet, one can find these windvanes for sale:   - - - - - - - - - - - - - - - - - -   Summary: Stability -Stability keeps an airplane pointing the direction it’s going (and thus not changing direction unless actively turned) in the same way that a dart, or a windvane, or a kid’s wagon, points the way it’s going: An angle of attack will generate forces on the tail and often on the wings (in any case, on something behind the center of gravity, or pivot line) which will turn/”blow” the back of the plane backwards, reducing this angle of attack. This is Pitch and Yaw stability. In short, angles of attack (pitch) or sideslip (yaw) automatically try to decrease. Same thing for angles of bank if the wings are canted upwards (roll stability). -To be more stable, more of the surfaces need to be further back. This means a bigger tail set way in the back, and a center of mass that is far forwards (so usually engines are near the front). -If the center of mass is ahead of the wings (so the wings act as stabilizing surfaces – a big plus), then the lift behind the CG will make the plane nose-down. To counter that, the tail is set downwards (negative alpha), generating a small amount of negative lift. Because it is further away from the CG than the wings, its moment arm matches the wings’ (smaller force, bigger distance) and it cancels the nose-down moment. Another way to create a nose-up moment is to set your engine really low. -A plane that is less stable is more agile, but a plane that is truly unstable is almost impossible to fly without the aid of computers, and will go out of control at high angles of attack or sideslip.