Last time around we looked at how the wings work to generate lift. Once we have lift we can go flying, but we're not quite there yet. We need to make sure it flies in a straight line and not tumbling end of ever. The ability to make it go up or down would be useful as well. The ability to turn it would also be really nice - we might be heading for Newark and I really don't want to go there.
I'm writing this for those who are not pilots or aviation buffs. This is mostly old hat for them. Still, a little review never hurt anyone. I find that by writing these it forces me to think about how this stuff actually works.
In the last chapter, we determined that in order to fly we need lift equal or greater than the weight of the airplane.
Wings and Stuff
To fly straight, we need the lift and the weight to be in certain places.
Pick up a pen and balance it on your finger. The point where it balances is called the Center of Gravity or CG. An aircraft also has a Center of Gravity and its location is very important. It actually changes in flight as we burn fuel, but as long as it stays within an acceptable range we're OK. In fact, when you get up and walk to the bathroom in flight you actually change the CG slightly.
The CG of a large jet is normally somewhere just forward of the wing. I won't go into how we calculate it because it's pretty dry stuff. We actually used to use a type of slide rule but now it's done by computer.
When I have a choice of seats on an airliner I always pick one just forward of the wing. In turbulence the plane pivots around its Center of Gravity. The closer you're sitting towards the CG the smoother the ride. The tail moves a lot more so if you like that sort of thing sit back there.
Here's a picture. The orange circle represents the Center of Gravity.
Center of Gravity (Orange Circle) and Center of Lift (Blue Circle)
Hey Mom! Look at cool picture I made!
The blue circle is called the Center of Lift or Center of Pressure CP. It represents the point at which the forces of lift act on the airplane. It's a bit more complicated than that, but this works for purpose of explanation.
The Center of Pressure changes a bit with airspeed and angle of attack, but not enough to be really significant here.
The most important thing is that Center of Gravity must always be forward of Center of Lift or the plane will not be stable. Move the CG too far aft and the plane would want to flip over on its back (bad). Too far forwards and we'd be so stable that we wouldn't be able to control the plane (also bad).
A quick side note here. Some military aircraft are indeed unstable (F-16, F-117, B-2) and would not be able to fly without computers. This was done to enhance maneuverability in the F-16. The F-117 and B-2 were designed to be stealthy and aerodynamics was very much secondary. They need a computer just to be able to fly.
We still need one more thing. With the Center of Lift behind the Center of Gravity, the plane would actually want to flip over forwards. That's why we have that little wing back in the tail. That's called a "horizontal stabilizer". It works just like a wing except it's upside down. It produces just enough downwards force to keep the plane level.
Of course we don't always want to stay level. Me might want to go up or down. By moving a small surface on the back of the stabilizer, called an "elevator", we can change the amount of downwards lift and thus change the pitch of the airplane.
Tail surfaces
The elevator is connected to the pilot's yoke either by cables and pulleys (707, DC-8), hydraulics (most airliners) or electric actuators (787). I'll go into those in greater detail in another chapter. For now we'll just say "Pull back on the yoke and the houses get smaller. Push forward, the houses get bigger."
So far so good. If we want to go up, we pull back on the yoke and raise the nose. Since we're now going "uphill" we need to add power. You don't get something for nothing.
To go down, push forwards and lower the nose. We now need to reduce thrust because we're going "downhill" and will start to speed up otherwise.
Of course it's not quite as simple as it sounds. This aerodynamic balancing act that keeps the plane level has to be tweaked every time we speed up or slow down. That's called "trimming" the airplane. If the plane is trimmed for 250 knots, speeding up to 300 will cause the nose to want to come up on its own. If we slow to 200 the nose will want to drop. You could fight this with the elevators, but you'd get tired after a bit.
What we do is trim the horizontal stabilizer. That whole surface back there can pivot up or down, usually by hydraulics. We have a switch on our yoke that controls the stabilizer. On the 727 there was a manual trim wheel in the cockpit for backup. When we ran the trim that wheel would spin around and make a loud "ka-chunk ka-chunk ka-chunk" noise.
So now we know how to stay level, how to go up or down and how to trim for different airspeeds. We still need to be able to turn.
That vertical fin on the tail is called the "vertical stabilizer". It keeps the plane going straight. Pretty simple. The moveable surface on the back is called the "rudder". Unlike the rudder on a boat, it doesn't turn the airplane. If you tried to turn the jet using just the rudder you would get an ugly, sloppy turn and you'd probably spill your coffee.
The wing is actually what turns the airplane. I like to equate turning a plane with turning a bicycle or motorcycle. Lean it over and turns in that direction. Bank the wings left or right and the plane will turn that direction.
Lift vector in a turn
All we're doing by banking the plane is using some of the wing's lift turn the plane. Fighter pilots talk a lot about the "lift vector". Change the direction of the lift vector and the plane will go in that direction.
Since we don't get something for nothing we need to increase the total amount of lift when we do this. We're using some of our total to turn the jet but we still need the same amount of lift in the up direction to fight gravity.
Another way to visualize this is we're making the wing do extra work by holding the plane up and turning it at the same time. So when we turn we have to pull back on the yoke a little bit to increase the angle of attack and make the wing take a bigger "bite" of the air. More lift means more drag because, you guessed it, nothing is free. So we need to add a little thrust or we'll lose airspeed.
Turning the plane is simple enough in concept, but to do it precisely takes a little bit of effort. Bank, add a little back pressure, add a little power, level the wings, take out the back pressure, put the power back where it was.
So how do we get the plane to bank? We have control surfaces on each wing called ailerons (it means "little wing" in French). Turn the yoke and one comes up and the other goes down. The one wing now makes more lift than the other and we bank. Large jets also use the spoilers on top of the wing to help bank but I'll talk about those more in the next chapter.
Aileron
Turn your car's steering wheel and the car will turn until you center the wheel, at which point it will stop turning.
Doesn't quite work that way in a plane. Turn the yoke and the plane will bank (roll) and will keep rolling for as long as you have the yoke turned. Since we don't want to go upside down in airliner we need to stop rolling at some point. Yeah, I know, Tex Johnson rolled the 707 prototype but I'm no Tex Johnson plus it would scare the chickens.
So, move the yoke, the plane will start to bank. The bank will keep increasing until we center the yoke. Once the yoke is centered, the plane will keep that amount of bank and will keep turning until we do something else with the yoke.
Want to stop turning? Move the yoke in the other direction until the wings are level again and then center it or you'll end up turning the other way.
So if the rudder doesn't turn the plane what's it there for? It has several uses. It keeps us going straight down the runway on takeoffs and landings. It also keeps us going straight if we lose an engine on one wing.
The big thing it does is "coordinate" the turn. When we move the ailerons to bank the plane, the wing that's moving up is now creating more lift than the one moving down. This extra lift creates extra drag (nothing is free) on that wing. This makes the nose of the plane want to yaw in the opposite direction that we want to turn. The rudder counters this so we make a nice coordinated turn.
People that fly light aircraft become very adept at using the rudder. On a heavy jet we really don't need to use the rudder when we turn because the jet has some other tricks built into it.
First off, much of our bank comes from the spoilers instead of the ailerons. More about how those work in the next chapter. The important thing here is, the spoiler only comes up on the wing that's going down. This counters most of the drag from the wing that's going up. Very convenient.
Spoilers Fully Deployed
Then there's a little gizmo called a "yaw damper" that takes care of the rest for us. Swept wing jets tend to be a little unstable from side to side. The nose wants to wander around in a little figure 8 called a "dutch roll". The yaw damper constantly moves the rudder a tiny bit to keep the nose pointed where it's supposed to. Any little bit of adverse yaw left over will get taken care of by mister yaw damper.
So all of this means that us lazy jet pilots don't have to use the rudder except for takeoffs and landings.
That's it. You now know the basics of powered flight. Next time I'll talk a bit more about the flight controls.