I have a stupid question, please excuse my ignorance, I basically understand the lift principle involved in lift (I think ) where air goes over the top wing, I believe due to the wings shape creating lift. How does it work when the plane is inverted? Or when the wings are symmetrical?

I need to review my phyics.

Thanks

Jon

Heres an explanation from a beginner:

Air is a mass. All airplanes have the ability to create thrust. With thrust, the plane pushes against the air (a mass) and the air (mass) pushes back against the plane. Wings are shaped to create lift, like you had described. With thrust, the orientation of the plane does not matter (whether it is upside down or not), as long as it is going in a forward manner. Am i right?

It is all about the angle of attack. Notice that when you fly inverted, you need to add a bit of down elevator to maintain altitude? This isn't because you plane suddenly changed its balance, its because in order to produce lift inverted, you have to raise the nose a bit to increase the angle of attack. I guess in a sense, I might be wrong, that by doing this you are "modifying" the shape or profile (not literally bending) of the wing that is being "presented" to the air? I might not have worded that statement the best in the world, but there is a good thought behind it anyways! Shoot me down if necassary, I'm not an aeronautical engineer.

Think of a fully symmetrical wing. How does it create lift?

It creates lift by using a positive angle of attack. When inverted, it does the same.

With a flat-bottom wing, a much larger AOA is needed when inverted

You are all hinting around it but not quite hitting it.
Aerodynamics 101.
An airplane flys because of the wing. It is designed so that air takes longer to go over the top of the wing than the bottom and that creates negative air pressure or lift.
OK that is the very basics of it. Now comes symetrical wings. Angle of attack comes in to play more. As you angle the wing in relation to airflow you present a larger surface area on top as compared to bottom resulting in the same negative air pressure or lift.
Inverted flight? same thing and that is why flat bottom wings have more lift and require a higher angle of attack when inverted.
It all boils down to air traveling farther on top (or bottom when inverted) to create lift.

Thanks that is the type of answer I was looking for. I never thought about AOA and the question just buged me.

Thanks againg and hope i did not waste your time

Jon

Sheer force of will and by holding your tongue right.

And enough up elevator (while inverted) to give the wing a proper angle of attack and forward motion enabling it to create more lift on the sky side than the earth side.

There is no such thing as a bad question and it is never a waste of time answering one

Hold on here.. The only thing you need to know about flying inverted is... Up is Down and Down is Up. If you forget this and are in a low pass over the runway you will give your spectators one heck-of-a show

At our clubs 50th anniversary when doing limbo, one pilot was flying under the limbo inverted and went a little too low and little too slow and landed right under the limbo line inverted, not much if any damage to his plane. http://www.tcrconline.com/photos/MAD2007/58.html

Jon

Same reason it flies right side up.

To fly right side up, the wing finds an angle into the airflow that creates lift. We can't see that angle from the ground.

Same thing upside down.

We usually can't see that angle from the ground either on most models. Some you can. In almost every case, to hold altitude when inverted, we have to hold down elevator. Think about that.

Thats what im saying!! I does not matter theoretically if the plane flys inverted or right side up. The same forces apply. Think about it:
Plane right side up, needs up elevator to increase altitude (+,+). Plane inverted, needs down elevator to gain altitude (-,-). It does not matter, the plane gains lift the same way (with more elevator in either direction depending on orientation). So to say its different is wrong.

So, if im right, lift really depends on AOA to current created by trust, NOT NECESSARILY the wing shape. So, wing shape merely helps out with this effect in slowing the airflow above the wing, it does not solely determine the effect. Lift can be changed to the reverse side of the wing with more manipulation from elevators/thrust thus creating the same exact effect.

Be careful here as the "sky side earth side" thing becomes exponentially more confusing as they converge.

So this is the same reason spad planes work well.

But the flat bottom wing does help increase the "lift" without A O A

Cool

The biggest point is to not let them converge.....

Its tough to say if a flat bottom wing flies with 0 angle of attack.. This may depend on where the balance point is.. But on a flat bottom wing, you are getting the majority of the lift from the shape of the wing, or it's camber.. this is why a flat bottom aircraft flies like c@@@ when it is upside down.. You need a tremendous amount of AoA in order to overcome the fact that the wing is now producing lift in the direction of terra firma..

The AOA for a symmetrical wing section can be very small. If you ever get a chance to see a CL stunt model flying then try to see what angle it flies at both upright or inverted. Better still, ask if you can come out to the centre to watch because then you'll be only about 60 to 70 feet away from it all the time and have the ground as a reference. You won't be able to see any angle because it's so small, something like 1 degree or less, but this tiny angle is enough to give 3 pounds or so of lift (the weight of the model). Admittedly an RC model weighs a lot more for the same size wing but even so the angle needed to give maybe 6 pounds of lift is still extremely small.

Not quite right again.
Wing shape can generate lift depending upon the shape. Flat bottoms with curved upper surfaces in level flight create lift with NO (or little) angle of attack. Adding AOA increases the amount of lift until you reach a point where you actually stall due to seperation of the laminar flow boundary layer over the top of the wing. Symetrical wings generate lift ONLY via AOA and eventually stall for the same reasons.
Lift is created in the flat bottom version NOT because the air over the upper surface moves slower but because it moves faster. The seperated (by the leading edge) air mass wants to rejoin at the trailing edge. Since it is a longer distance over the curved surface than over the flat surface the air much move faster. This increase in speed causes a slight reduction in air pressure upon the upper surface relative to the lower surface. So you can say the upper surface is being sucked up or the lower surface is being pushed up, That point is irrelevant.
It doesn't take thrust per se, not much thrust in most gliders. Gliders glide DOWN and while doing so generate lift by moving forward. The lift must exceed the a/c weight for the plane to fly. To move forward you need to exceed the amount of DRAG the a/c presents to the air.
A flat bottom wing flying inverted MUST have a higher AOA to maintain altitude.

Even the experts don't agree on this. Some say Bernoulli's principle some say AOA. I think its either depending on the shape of the airfoil.

Fischer,
Then they are all correct -
It is partly Bernouli's and partly AOA.
A flat bottom, and even a semi-semetrical airfoil can produce lift at a neutral AOA. If travelling fast enough, the lift can overcome the weight of the vehicle, and the plane will fly straight and level with a slightly negative AOA.

My Texan with gear up and full throttle crossing the runway after a steep dive looked to be pointing slightly toward the earth as it went by at 80+mph.
I had to hold what I felt to be considerable down elevator to keep it from balloonig towards the clouds.

In closing,
The shape of the wing provides the lift with or without efficiency. AOA and speed changes the amount of lift.
I can get a brick to produce lift if it travels fast enough and has the right angle of attack.

Bob

Bobmac- excellent synopsis!

awesome job bruce,,,

Jon, there is plenty info up above to provide a model flier enough info to fly toy airplanes. However there are many factors that may be nice to know, but definitely not required. There are significant items that may be difficult to explain, such as 3D, however if one analyses the pressure patterns plus the source of the airflow, one can explain 3D just as simply as regular lift. Now in subsonic airspeed where we RCers operate it all is relatively easy, but trans-sonic and supersonic each have their own properties which is not yet our RC problem

Lift is = to the Coefficient of lift (Angle-of-attack) X 1/2(roe{air-mass density} x Velocity squared X wing area)

We also know that at any given level in this atmosphere, the total pressure -- weight of the air -- remains the same and undisturbed it is STATIC pressure. Therefore when the air is moved, DYNAMIC pressure is created which lessens STATIC pressure as TOTAL pressure is a constant.

Now as a pilot, what can you control? You can control Angle-of-Attack (AOA) and you can control Speed (Velocity).
Notice that Velocity functions by the square which means that velocity is one big key-player in the production of lift. Air-mass density is controlled by forces higher than we can reach, and wing area is pretty well set, however items like Fowler-Flaps provide some options for a pilot. Not a factor here!

No matter the airfoil, at some AOA lift is produced as the speed is increased flowing over one side reducing the static pressure vice the higher static pressure on the opposite side.

Every item you move on the control surfaces redirects airflow via a different AOA, which changes the lift equation around that surface, which in turn places a load factor on another area. Move the elevator on the horizontal stabilizer UP and you make lift on the bottom of the stab which places a load on the wing, thus increasing the wing's AOA which produces more lift, thus producing movement in the realm of wherever the top of the wing happens to be pointing.

If your airplane has adequate thrust, or power, however derived whether jet, propeller, or dive, that wing/surface will produce a lift-force when loaded by another surface. Without that power to override DRAG (same equation-Induced from the AOA and/or Parasitic resulting from the airflow) then speed will decrease resulting in rapid loss of that lift-force.

Notice that most high wing trainers have significant down-thrust (all should have). Without Down thrust, increase the speed on the high lift flat bottom wing, the machine climbs, reduce speed it tends to dive. Using adequate downthrust, higher engine speeds cancel out (or reduce) the extra lift. Get the machine well trimmed out, and the lift along with the thrust-vector-down at higher speeds, keeps the attitude fairly constant throughout the use of throttle. reducing the throttle, the thrust vector relaxes and the machine is trimmed for lower airspeed. Add throttle and the increase speed(lift) is offset by the down thrust vector.

Bernouli's theorem is very effective, however Bernouli was concerned with liquids that cannot be compressed. Air can be compressed which does provide some differences.

Last point, when you are inverted, and you pull "up" elevator, the machine does as you tell it. It doesn't know where mother-earth is but it will go there. BTDT enough. []

No.

It is theoretically possible to design a wing for a plane that will NOT let it fly upside down but fly OK upright.

You could throw all the thrust you want at the plane, but upside down you'd still have almost no lift.

( assuming of course you are not producing SO much thrust that you've basically turned the plane into a ballistic missile ).

Most airfoils DO provide lift when inverted, but some are MUCH better than others.

Symmetrical airfoils tend to do the best job, while a flat bottomed wing will not produce as much lift when inverted... hence the extra required speed ( which generates the missing lift ).

As stated AOA also needs to be changed when the wing is inverted.