What makes an airplane fly

Why does an airplane fly?

An aircraft flies when its propulsion is greater than its drag. The lift must be greater than the weight of the aircraft. The lift occurs when it moves with its wings in the air. We're talking about dynamic lift.

There are four forces in competition when it comes to flying. In reality, these forces are not as clearly separated as they appear in the drawing. They are more connected than the drawing can show.

Dynamic lift occurs when the aircraft moves through the air. It turns on the wings. This means that the wings are tilted upwards at the front.

An airplane without a drive is on the ground.

You have already felt the dynamic buoyancy yourself when you stretch your hand out of the railway window and tip it slightly upwards at the front.

By flying we do not mean take-off or landing, but cruising flight. Evenly. In the air.

Do you know that?

An airplane doesn't fly ...

○ without wings
○ without air
○ without speed
○ without the wing angle
○ without suitable control

A plane flies when

◉ it has wings
◉ there is air
◉ it is fast enough
◉ the wings are turned on
◉ it is appropriately controlled

 

 

How exactly does dynamic lift come about?

The more understandable the explanation, the more wrong it is. Unfortunately, this is the case, but it should not prevent us from taking a closer look at the various explanations. What is certainly true is the following:

The air in front of the plane is just coming. After the plane it flows away downwards. Because something cannot simply arise that goes down, a force must arise that cancels it. The lift.

The aircraft manages to deflect air coming from the front downwards. This is where the buoyancy arises.

The exact and correct explanation of why the lift occurs is a very mathematical explanation. She works with “circulation” and “rotation”, with “gradient” and eddies, with vector fields and conservation laws. With visualizations through streamlines. With start-up vortices that separate. - You can learn that when you go to a university or a technical college - but not here.
  • Conservation of momentum: The mass and speed of the air particles determine the momentum. When the direction of the air particles after the airplane has changed, their momentum has changed. That must not be the case without further ado. Only when the aircraft is pushed up does the impulse remain.
  • Conservation of energy: The air particles move faster on the upper edge of the wings. That would be a higher energy that arises there. That must not be. Fast currents have lower pressure. So the energy is retained.
  • Vertebral sets: A vortex forms behind the wings and remains behind. However, vortices can only arise in pairs. Another vortex is created around the wing, which is responsible for the lift.
  • Air pressure: There is less air pressure above the wing than below the wing. That means a resulting force upwards - the lift.

A positive angle of attack increases the dynamic lift.

 


Wikipedia keywords:Airplane, dynamic lift, streamlines, Bernoulli effect, Magnus effect, Coanda effect, circulation, vortex, conservation laws, conservation of momentum, Jutta-Joukovski theorem, wing


 

Four possible understandable explanations

1. The "reflected stone theory"

That is Newton's explanation.

What is right:

◉ the angle of attack is necessary

What is not right:

○ the lift is only generated by the air on the underside of the wing
○ different tops do not matter - always the same buoyancy

What cannot be explained:

◉ an ascending plane creates a downdraft that cannot be explained
◉ different tops result in different buoyancy
◉ Spoilers (raised panels) along the top of the wings can change the lift

Correct would be:

◉ we also have to include the top
◉ Buoyancy is created by anything that deflects the airflow downwards
◉ the top also deflects the air flow downwards - even more

What we can say for sure:

Buoyancy is created by anything that can deflect the air downwards. The aircraft receives an upward force that allows it to lift its weight. The whole thing takes place dynamically - in motion.

Note: Newton's explanation is not entirely wrong. At high speeds and low air density, few particles can come into contact with the top of the wing, most of it happens under the wing and the predictions are correct - a space shuttle at 50 miles altitude and 10,000 mph on reentry.

2. Venturi Declaration

A second explanation is also not entirely correct. It's called the Venturi theory. When gas or liquid flows through a pipe, the speed increases as the pipe becomes narrower (Venturi Nozzle). Paradoxically, the pressure drops in faster currents. The "Bernoulli sentence" describes the phenomenon: "In faster currents there is negative pressure." On the surface of the wing, the air flow condenses due to the curvature. “The air pressure drops, the aircraft is sucked upwards.” - this is the wrong theory.

What is right:

◉ Negative pressure on the top
◉ The curved surface hinders the air flow (flow lines are compressed)

What is not right:

○ The negative pressure on the surface is large enough to make the aircraft fly
○ The top is not a Venturi nozzle, where is the top?
○ The air does not flow right past the top and bottom, as is often said to explain the higher speed at the top (longer path, curved), and then to try Bernoulli with the negative pressure.

What cannot be explained:

◉ Aerobatic planes can also fly on their backs - they would crash
◉ The prediction is generally wrong for a flat plate as a wing
◉ different undersides result in different buoyancy

Correct would be:

◉ The conservation of mass also plays a role, not just conservation of momentum (Newton) and conservation of energy (Bernoulli)

What we can say for sure:

Bernoulli's theorem correctly describes that there is negative pressure in fast-flowing fluids.

Note: It is wrong to assume that the curved surface causes the speed change. From then on, Bernoulli's explanation would be correct. Overall: The prediction is also incorrect in the second incorrect explanation.

3. Vortex

That's a pretty correct explanation, if not very understandable. There is a circulation - a vortex - to the right around the wing, when a vortex to the left is detached from the wing edge and remains. Helmholtz vortex theorems. The whole thing can be calculated with a vector field. All the information for buoyancy lies in the streamlines. There is a conservation law for eddies. Left and right must balance each other out.

Problematic: it's all right two-dimensionally (wing cross-section), three-dimensionally not completely (the wing is also quite long).

4. Differences in air pressure

A fourth explanation is based on the pressure differences in the flowing air on the top and bottom of a curved wing. We still have to discuss the direction and length of the arrows in the following drawing - but the difference is essential. There is less air pressure at the top than at the bottom. Down wins. The difference is the lift - which makes the plane lighter.

In total

Everything that turns the air flow causes the dynamic lift. We need it because it takes off the weight of the plane.

There is an analysis of the various explanatory models by Rita Wodzinski in Plus Lucis magazine. The article is interesting for teachers, we will probably not fully understand it here.

Flying is probably one of the phenomena that we observe in nature and simply have to accept. - Rudolf Voit-Nitschmann, Time

 

Three axes are very important for stable flight. Three types of rudders can redirect the airflow so that it rotates around the appropriate axis.

Roll, nod, yaw. Try to imagine these movements.

The aircraft's rudders change the air currents.

Show with the mouse

Explanation by streamlines

Explanation by differences in air pressure

 

"Flying" even without a drive

It all depends on the relative wind speed.

  1. How does a helicopter fly? Listen to the radio show from SWR2 Knowing about it.
  2. Paper planes fold with lots of instructions. Link: https://www.foldnfly.com/
  3. A script with explanations on flying and instructions for paper planes that fly well can be found on Werner Gruber's website. Link (PDF)
  4. App: wind tunnel, enables flow simulations

Text, photos, illustrations: Lothar Bodingbauer
Source / background: https://www.grc.nasa.gov/www/k-12/airplane/

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