Why does it take off? (Or, *HOW* does an airplane fly?)

[Ray Garrison]

Gosh, we had so much fun with the simple concept of a plane taxing on a moving surface to take off I thought I'd ask a rather more interesting question.

How, really, does an airplane fly?

Until recently most textbooks said it's because of the Bernoulli pressure equation - you know, the shape of the airfoil (wing) causes a reduced pressure on the upper surface, resulting in relatively higher pressure on the lower surface, thus pushing the wing (and plane) up, up and away.

Do you agree with that? Any reasons why or why not?

I'm not gonna volunteer any opinions just yet, kind of curious what people say.

Then again, maybe nobody else cares... I'm just easily amused.

[bodcaw boy]

by placing it on a movable conveyor!!

boy!!

[PhilMays]

Duh, with Pilots....I thought you were smarter than that!

[space_cowboy]

My avatar doesn't mean I know squat about the aerodynamics of plane flight, but I generally agree with what Ray described. The higher pressure on the bottom of the wing provides lift. Thats why at lower speeds (takeoff and landing) the flaps and whatever the thing is called on the leading edge of the wing are *generally* adjusted down to create a more exagerated wing shape. This helps generate more lift at lower speeds. Watch the next time you take off and land.

That's also why planes are de-iced before take off, if the airflow over the wing is disrupted, enough lift can't be generated. That's also why if wings ice up while in flight it usually results in bad news.

Ok, that's about all I know on this subject.

[oldtimer]

See "Chicken Run".

[Ray Garrison]

See "Chicken Run".

[customsteve01]

Ray,

Students of physics and aerodynamics are taught that airplanes fly as a result of Bernoullis principle, which says that if air speeds up the pressure is lowered. Thus a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift.

Here is some reading material if you have some time on your hands.

The link is on the description of lift, but if you click around in the site you can find a lot more.

http://www.allstar.fiu.edu/AERO/airflylvl3.htm

Steve

[Ray Garrison]

Cool link. Much better than several I was going to post a little later.

I still like the simple summary in Stick and Rudder - "An airplane flies because the wing pushes the air down."

Here's a link (ta da) to a really neat JAVA program called FoilSim on NASA's website than animates many of the variables involved, and let's you try different parameters to see what happens. Doesn't seem to understand the concept of stall, however.

[space_cowboy]

Guess maybe I had things backwards in my reply........LOL. The main thing I care about is that when I'm at 35,000 feet however it works, it works.

[JetJockey]

Those "things" on the front of the wing that extend for takeoff and landing are generally refered to as lowspeed lift devices of which there are two widely used types being "slats" and "leading edge flaps". They work by increasing the camber (bend) in the wing which causes the airflow over the top of the wing to travel further to reach the trailing edge of the wing(increased low pressure) at the same time as the flow over the bottom of the wing which has a much shorter distance to travel(high pressure) which creates increased lift at low airspeeds.

[Ray Garrison]

Those "things" on the front of the wing that extend for takeoff and landing are generally refered to as lowspeed lift devices of which there are two widely used types being "slats" and "leading edge flaps". They work by increasing the camber (bend) in the wing which causes the airflow over the top of the wing to travel further to reach the trailing edge of the wing(increased low pressure) at the same time as the flow over the bottom of the wing which has a much shorter distance to travel(high pressure) which creates increased lift at low airspeeds.

Well, close. Again, the increased lift when the leading edge slats are extended is *NOT* due to an increase in pressure differential through some sort of Bernoulli effect, but rather from the slats affecting the boundry layer so the wing can be flown at a higher angle of attack before the Reynolds number threshold inducing a stall is exceeded.

See this page from the government's centennial of flight website. Quoting them:

"The maximum coefficient of lift CL,max may be increased through the use of a slot formed by a leading-edge auxiliary airfoil called a slat. When the slot is open, the air flows through the slot and over the airfoil. The slot is a boundary-layer control device and the air thus channeled energizes the boundary layer about the wing and retards the separation. The airfoil can then be flown at a higher angle of attack before stall occurs and thus get a higher CL,max value. For angles of attack less than the stall angle, however, the airfoil lift curve is relatively unaffected whether the slot is opened or closed."

The old Bernoulli explanation has pretty much been replaced by simple Newtonian force analysis.

[JetJockey]

And leading edge flaps do what? They are not the same thing.

[Ray Garrison]

They increase the cord angle (with respect to forward motion) thus increasing angle of attack and lift.

[jacksonbart]

How many monkeys and 747s would it take before a single monkey was to successfully pilot a 747 from LaGuardia to Charles de Gaulle?

[bodcaw boy]

just one.....only it needs to learn to type first....[8-|]

boy!!

[pauln]

It may help to understand the underlying principles rather than stating the names of the principles.

The shape of the wing's cross section is basically curved on the top and flat on the bottom. When the air molecules pass under the wing they go from the front to the back in a fairly straight line. When they pass over the top of the wing, they have to take a longer path because it is curved. The wing "cuts" the air as it passes through and sort of leaves the air the way it was before. Since the molecules at the front of the wing before passing over or under all meet up at the same time at the back of the wing after passing over or under, it is clear that the ones going over the top had to go a longer distance in the same amount of time as the ones under the wing and therefore did it faster. So the air going over the top went faster than the air going under the wing. That's the main thing. But what difference does that make?

Air pressure is just the molecules of air banging against some surface. The ones going under the wing are going a little slower than the ones above the wing, so the ones under the wing have a few more opportunities to bang the underside of the wing than those above. If you have a ridgid two sided surface where more molecules are banging one side than the other, you have a net resultant force. If that surface is a wing, that force is called lift. That is, more molucules of air are under the wing banging in the up direction than there are molecules above the wing banging in the down direction (because the ones going over the top of the wing go faster and don't have as much opportunity to hit the wing). The faster the air moves over and under the wing, the greater the difference in the proportional banging in favor of the ones banging under the wing, so the resultant net force - lift - increases.

With engineering and lots of experimentation, it is possible to get the lift force under the wing to exceed the static down force of the planes' mass due to gravity, which does not change. This is done by running the plane down the runway faster and faster until the lift force is enough to take off.

The modifications to the plane's wing shape during take off and landing is meant to increase the curvature and front to back distance so that the lift effect may be preserved at slower speeds used for take off and landing. Once the plane is up to fast crusing speed the wing shape only needs a little curve to stay up and overcome the gravitational down force of the plane.

There is much more involved to getting a plane to fly than just this... the other two forces at bay are thrust generated by the prop or the jet engine, and drag, which is the plane's resistance to being shoved through the air. As long as thrust beats drag and lift beats gravity, well you are on your way to flying... the Wright Bros put the control surfaces that we now have in the tail on the front of their aircraft (but it still flew). Stability, control surfaces (the elevators and rudder in the tail) and many other things have to worked out to get a flying plane... for example, many planes' wings are not level but make a slight v- shaped angle as veiwed from the front or the back. This angle is called dihedral and increases the stability of the plane. It does this by having each angled wing direct it's lift force direction (vector) slightly outward down. If the plane were to twist or rotate a little to the right or left (one wing too high), that higher wing's lift vector would be angled out from the vertical, and the vertical concoment of that wing's lift would be decreased (and the other wing's would be increased). This acts to self correct the planes orientation back to flat flight (it's a negative feedback loop!).

But I digress... just think of the molecules banging on the wing - more bangs underneath means airplanes fly...

[pauln]

.Lift is up

Gravity is down

Thrust is forwaed

Drag is backwards

[oldtimer]

Why does it take off? "It was stapled to the chicken."

[Capin-Newbie]

But.............. What if you are flying at the speed of sound (slightly faster than the speed of smell) would you still be able to hear the subs in the trunk?

Brian

[Ray Garrison]

Trick question. Airplanes don't have trunks.