It is true! The popular theory of flight is just a theory. From the material I've read in the classroom, my textbooks, and my flight instructors, my writeup is considered correct. However, it may be that vuo and his new-age science is right. Then again, this is all theory, and all of these writeups could turn out to be wrong in the long run! Who knows? If there was a single definitive answer, the writeup would be titled "how things fly" not "the theory on how things fly." Keep an open mind and decide for yourself!
The Theory of Flight is something that is difficult (but not impossible) to explain in text-only format, so if you really badly want to learn and have it make sense, it is best to find a visual resource. Microsoft Encarta
has some good visuals, and there are many websites out there that can show you nice pretty moving diagram
s. If you decide to barrel on ahead anyway, put on your imagination caps and be prepared to visualize situations by waving your hands around in the air like miniature airplane
s. It might be best to lock your door.
I'll do my best.
Before I get too technical, I'll discuss a very simple way of looking at how a wing works. You've all stuck your hand out the window of a fast moving car
and "played" with it in the wind, right? That is essentially an inefficient wing. As your hand is pointed straight into the wind, the air pressure above and below your hand is equal - so your hand stays pointed straight into the wind. If you angle your hand upwards a little bit, the pressure below your hand gets very high (depending on the angle of your hand) and the pressure above your hand becomes quite low. As air pressure
likes to equal itself out, your hand is going to be "sucked" upwards by the low pressure
area above your hand, and pushed upwards by the high pressure
area under your hand; resulting in your hand
flying upwards and backwards, and smacking your arm into the window frame
in your car
As I said, your hand is a very ineffecient wing
; you can only get lift
if you angle your hand upwards in the wind. Since your hand presents a bigger frontal profile in doing this, there is a lot of air friction
pulling your hand backwards and slowing you down. This would be a bad thing in an aircraft
. What they found, however, is if they made a roughly-teardrop
-shaped wing, you could get lift even when the wing is facing straight into the wind. This reduces that frontal profile
, there is less friction
, less drag, and therefore the airplane can go faster. Just like you notice in your car - the faster you go, the more lift you create! The high pressure/low pressure thoery is the basic fundamental behind lift, which is known as Bernoulli's Principle
Now for the technical stuff. This will take a few paragraphs before it all seems to come together for you. It may benefit to read this twice.
A wing of an aircraft is roughly teardrop shaped, or, as I like to think of fit, garden-slug
shaped. I'll do my best at an ASCII art
___...___ (--- a
....,,,----''''''' \ (--- b
------------------------------' (--- c
Does that make sense? Sure it does. Ignore the "a b c" for now, I'll get to that later. It is a bit hard to draw out in ASCII
format, but the top of the wing is supposed to bubble upwards a little bit and taper backwards; no part of it is really flat. The bottom side is, for the most part, flat. The front of the wing will be the right hand side, the rear end the left. The front edge is rounded and is called the leading edge
, and the back edge is tapered to a point and is called a trailing edge
. A line drawn directly from the frontmost point on the wing to the rearmost point is called the chord
of the wing, sometimes referred to a as a chord line
with fixed wing
s have the wings set at a slight angle away from the aircraft's body. As I mentioned a few paragraphs back, you have to angle your hand into the wind quite a bit to get any noticable lift. The same goes for a wing, though the angle doesn't have to be quite as exagerated. The Angle of Incidence
is known as the amount the wing is designed to be angled away from the axis
of the aircraft... To call upon the hand-in-the-wind example again, your car would have an axis from back to front and your hand would be sitting in the wind, not producing any lift. If you added a few bags of brick into the rear seat, the back end of your car would sink but the front would remain at the same height - this would not change the angle that you have your hand at in reference to the car, but it would change the angle of your hand in reference to the wind. With your hand - or a wing's chord line
- headed straight down the vehicle's body, it would be said that the angle of incidence
is zero degrees. Most wings are in the vicinity of 5 degrees or less.
states that the pressure of a fluid
decreases as the speed of the fluid increases. This also applies to gas, and therefore, to the atmosphere
... That is why when the weatherman
says a Low Pressure system
is moving in, it's usually more windy than when he says a High Pressure system
is on it's way to your house. If you look at the crudely drawn diagram above, I have labelled three imaginary "channels" of air that are about to pass over the wing, channel A, B, and C. For the purposes of this example, pretend that the wing is at a very slight angle - say, 5 degrees of an upward bank. (So small of an angle that even if I wanted to draw it, ASCII art doesn't have a high enough resolution to show it) Channel C is going at, say, 100 knots and is going to pass straight under the wing and deflect off of it a tiny bit. This will press on the underside of the wing (generating some lift) and the airflow
is going to slow down - to about 95 knots. No big deal. Air channel A is going to pass straight over the wing at 100 knots, and isn't going to be touching the wing at all. The interesting channel is Channel B. It hits the wing right on the nose and travels up and over the wing, and down the backside. This wind, due to friction and being deflected all over the place, is going to initially
slow to about, say 60 knots.
Now it really gets messy. If the air on top is moving slower (60 knots), and the air underneath the wing is moving faster (95 knots), what is filling that gap on top of the wing? Look at it this way: In front of the wing, you have, let's say, 10 units of air. Behind the wing you also have 10 units of air. 5 units pass on top of the wing, and 5 units pass under the wing. Yet the air on top is moving slower! This, in an extreme circumstance, would create a vacuum on top of the wing. In normal aircraft, however, this merely results in a very low pressure zone. What does low pressure do? Suck! It sucks the air back over the wing, speeding it up to match the speed of the air below the wing.
Great, so now you're thinking "so the air on top of the wing is travelling at the same speed as the air below the wing." Wrong. The top of the wing, if you remember, is bubbled upwards - the air travelling over the wing has to go a farther distance to reach the back of the wing as compared to the wind under the wing that just flies straight to the back. If you were to take a string and pull it tight over the top of the wing, and then take a string and pull it over the bottom of the wing, the top string would be longer would it not? For air to travel over the wing and meet up with the air under the wing, it would have to travel FASTER to arrive at the same time. I'll try to demonstrate this with another example.
b b b
b b b b ___...___ b
b b b .-----------------' \ b b b b (--- b
c c ------------------------------' c c c c (--- c
c c c c c c c c c
(I changed the wing design to make it a bit easier to see, but imagine it is a normally shaped wing.) As you can see, Airflow B and C both have the same volume of air - 15 units each. You can also see that airflow B and C are also always in line with each other - from front to back, B never lags behind C or vice versa. Air before the wing and behind the wing is nearly undisturbed. Airflow B, however, travels a much farther distance, and the spacing between each B is quite a bit farther apart than the spacing between the Cs. So now that the air has "caught up with itself," you can visually see how the upper portion of the wing has less air per square inch; therefore a lower pressure. Lower pressure, again, equals suck - since it cannot suck the wind any faster (it's already caught up with itself!), it is the wing that gets "sucked upwards" into the low pressure zone, trying to fill the void. This is the main source of lift, and is the basis of Bernoulli's Principle
That should just about explain it. I strongly recommend you understand this concept before you learn how to fly
; it makes ground school
all that much easier.
Some common questions are being asked! Here I will attempt to explain them.
How do aircraft fly inverted?
Well, turn your hand upside down the next time you have it hanging out of the car window. Tilt it upwards until you feel your hand "lifting." Ta-da! Remember: The upward bubble in the wing only allows the aircraft to face relatively straight forward yet still produce some lift, thereby decreasing the frontal surface area and therefore reducing drag. This reduction in drag results in more speed, and more speed produces more lift! It simply makes the wing more effecient. You are correct in assuming that the upward-bubbled wing will "fight" the lift and drag you down a bit while inverted, but this is overcome by angling the wing upwards a little bit more. Arcing the wing into an upward bubble or slug-shape alone is not enough to make an aircraft fly - you still need that fast flowing wind "striking" the bottom of the wing to get Bernoulli's Principle working!
What about fighter jets and other fast jets? I don't see big thick wings on those aircraft!
Thinner wing designs, also known as laminar wings, work on the theory that if you fling the aircraft forward fast enough, you don't really have to have that extra lift. Imagine you are driving down the highway at 600 miles per hour. You put your hand (with some effort) straight into the wind, and it doesn't move. But at the slightest rotation upwards, the air pressure is so great that it rips your hand off. Ta da! Even better lift. The problem with these wings is that they don't produce a lot of lift at low speeds, so their minimum speeds are higher - they need longer runways for takeoffs and landings, and they also need bigger and more powerful engines. If you put one of those Bernoulli teardrop-shaped-wings on an F-16, it would actually add weight and drag to the aircraft and do more harm than good. The teardrop-shaped-wings are best placed on slower, less expensive aircraft.
How does the air on top of the wing know it has a "date" with the air under the wing?
It doesn't "know," but keep in mind that there is very fast moving wind behind it, urging it to keep moving! If the air on top of the wing moved slowly, there would be a huge buildup of air in front of the wing. If you put a rock in the bottom of a river, the river still flows at the same speed! But the water immediately above the rock has to speed up, lest it be "crushed" by the water behind it. This is why a great big wide river moves rather slowly, but as it passes through a narrow canyon it speeds up (rapids!).
What about the Coanda effect?
First, the Coanda effect is a theory just as Bernoulli's Principle is. But, if you think long and hard about it, the Coanda effect results in a Bernoulli pressure-differential effect, and thereby produces lift. By saying that the Coanda effect is the true source of lift, it's like your saying that a bicycle doesn't run because the pedals are moving, it runs because your legs are moving the pedals. Yes, that's why the system works, but it isn't how the system works. In any case, time will eventually show that all of these crazy theories were all wrong and some grand unified theory will explain everything. (Similarly; Did you know that E does not equal MC^2? It's very close for all currently practical applications but when you start doing math with really big numbers it makes quite a difference.)
Sources: Most of this info is straight from my head, but the class that I got the knowledge from uses From the Ground Up as a reference. Very good book; has quite a few diagrams and better drawings of wings than I could ever do. ISBN 0-9680390-5-7 published by Aviation Publishers Co.
There is another book named Theory of Flight that happens to deal exclusively with this material in great, great detail; unfortunately I do not own it and cannot reference it. You can find it on amazon.com.