In the last diary I talked about how the engines on an airliner work. Unless we plan on driving it down the interstate we need wings to actually make the thing fly.
The wing on any large aircraft is an incredible piece of engineering.
It has to be able to support several hundred thousand pounds of weight while still being light enough to fly. It has to be able to withstand the stresses of maneuvering, high speed flight, turbulence and some of my landings. It has to be able to fly close to the speed of sound yet still let you fly slow enough to take off and land in a reasonable amount of real estate. It also has to hold fuel tanks, hydraulic lines, electrical wiring, pneumatic ducting plus maybe the plane's engines and landing gear.
In the last chapter I talked about what makes the plane go:
Today, without getting too technical, I'll try to explain a bit about what actually makes the plane fly.
Let's start with how a wing actually works.
Some of us may remember from science class way back when that it was all about Bernoulli's Principle. Bernoulli says that the air going over the curved top part of the wing goes faster than the air traveling under the wing. All things being equal, higher speed means lower pressure. This creates higher pressure under the wing which creates lift.
Turns out it's more about Isaac Newton (remember him?) than Bernoulli. For every action there is an equal and opposite reaction yada yada yada.
A good demonstration is worth 1000 words in this case. Remember when you were a kid and your Mom yelled at you for sticking your hand out the car window? Now I would never recommend that you stick your hand out the window of a moving car, but suppose you did anyway.
With your palm parallel to the ground all you'd feel is the air pushing your hand towards the back of the car. That's called "drag". The faster the car is going, the more drag you're going to feel. Specifically this is called "parasite drag" because you're not getting any lift yet. Parasite drag is the drag from trying to push something through the air. It's a function of the shape of what we're trying to push through the air times the speed squared. That's incidentally why most cars today are shaped the way they are - to reduce drag and get better gas mileage.
Because drag is a factor of the speed squared (speed x speed), to get something to go through the air twice as fast we'd need four times as much power. That's why my V12 Jaguar only went mumble mumble cough.....statute of limitations...miles per hour instead of twice as fast as your Camry.
Drag and lift are different forces but they are related. Suppose you tilted your hand (the one sticking out the car window) up about 20-30 degrees now. You'd feel your hand being lifted. You're now generating lift! How much? However much your hand weighs plus a little more. Angle it the other way and you can make lift in the downwards direction. And your hand isn't even shaped like an airfoil. Take that Bernoulli!
As you feel your hand lifting, you might also feel more drag on it. Intuitively it makes sense. Instead of our hand slicing through the air like a knife it's moving at an angle. The air is pushing back harder but some of that push is in the up direction.
By changing how far you tilt your hand you can feel the change in lift. The fancy term for this is "angle of attack". More angle of attack equals more lift - to a point. Tilt your hand too far and you'll feel it stop making lift. That's called a "stall". Go past a certain "critical" angle of attack and the air just can't keep up anymore and goes all turbulent.
Okay all you aeronautical engineers out there, I know about "laminar flow" and "boundary layer separation" but I'm grossly simplifying things here. Try to make the wing work too hard and it gives up. That's my story and I'm sticking with it.
Increase thrust and we can speed up, but at some point drag will increase to the point where we can't go any faster.
In cruise flight all four of these things: thrust, drag, lift and weight are in perfect balance. We're not going up or down and our speed is not changing.
That's the basics. Of course the devil is in the details. How large does the wing need to be? How thick? Short and stubby or long and tapered? Straight, swept or delta (triangle shaped)? The shape of the wing determines if it will be a fast sprinter, a nimble acrobat, a super efficient long-distance runner or a powerful weightlifter.
There's more. Do we stick the wing on top of the fuselage (high wing) or in the middle (mid wing) or on the bottom (low wing)?
If you want to see this in more detail here's a Wiki article on all the various wing designs. Note that many of these never got past the prototype stage and some didn't even get that far:
Personally I'm a sucker for anything with a delta wing. Give me an F-106 or a Mirage IV any day. I don't care how they fly they just look so damn cool!
It turns out that the optimum wing for a jet airliner is going to be swept back around 30 degrees and have an "aspect ratio" of around 8. That just means it's about 8 times as long as it is from front to back. Gliders have super high aspect ratios (long thin wings), jet fighters have very low ones (short stubby wings). Airliners are in between.
The wing will probably have a "supercritical" cross section, which is tough to explain except that it's fatter in the front and curved in the back and is supposed to be more efficient at higher speeds.
Now you can impress your fellow passengers "Say, did you know this aircraft has a supercritical wing"?
The wing will most likely be mounted low on the fuselage. There are probably other reasons, but mainly this lets us get away with shorter landing gear and thus less weight to carry around.
The wing will also sit a little higher up at the tip than at the root. This is called "dihedral" and is used to make the plane a bit more stable. If the plane gets tilted a little (maybe by turbulence) the lower wing now makes a bit more lift than the other one and will straighten the plane back out. Stability is a good thing in an airliner - I don't like to spill my coffee.
Now we don't want to stall the wing on airliner - ever. However we want the wing designed so that it gives us some warning before it stalls by buffeting. We'd also don't want anything really hairy to happen if it stalls like the plane spinning or worse tumbling. Some jet fighters have incredibly violent stall behavior - talk to anyone who's flown the A-7.
So what's all that really mean to us? It means we've got something that will fly efficiently at mach .80 up around 35,000 feet and might be able to go .84 mach or so. Because fuel is so expensive we care more about efficiency than speed these days. The wing on a 727 liked to cruise at .84 and could go up to .90 mach - but it needed a lot of gas to do it. You reach a point of diminishing returns due to drag where you burn a lot of extra gas to go a little bit faster. That's why we stay around our cruise speed. It's the most bang for the buck.
We have some other tricks to increase efficiency. Those things that stick up at the end of some wings? Those are called "winglets". They help reduce drag by limiting the wingtip vortex. What's a "vortex"? Imagine a little horizontal tornado trailing back from each wingtip. Run into one and it can ruin your day, but that's another story. They're a byproduct of producing lift and reducing them can cut down on drag.
Now what's really amazing is, through the use of flaps and other high lift devices, we can get this wing to fly as slow as 115 knots or so. That makes takeoffs and landings a lot easier. I'll talk more about those in the next chapter.
Why does the wing flex so much in flight? Just be thankful you're not in a B-52. Those wings flexed 18 feet at the tip (I stopped looking after a while). The reason it flexes is that if it didn't have some "give" it would break. Don't worry, it's very strong. Strong enough to withstand normal flight plus turbulence plus a little extra just for good measure. It's also inspected regularly to detect any metal fatigue long before it becomes a problem.
Will it glide? Pretty well actually. An airliner will glide maybe 3 miles for every 1000 feet of altitude. From cruise altitude you might go 100 miles or so. Best glide speed is somewhere around 220-240 knots, so they may glide a long distance but they won't glide for a long time.
So now we know how to make it fly. We haven't talked about what keeps it flying in straight line or what makes it turn. That's a whole chapter unto itself and we'll talk about that next time.