Why can a helicopter fly at all? Part 2

This part is the second part - it might be useful to read the first part first. You can find it here:  Part 1: Introduction

Part 2: Bernoulli, pressures, buoyancy in detail

This chapter deals with the physical basics. If you are interested in physics in general, you should of course read it through. If you were at war with physics classes at school, you should read it even more.

It doesn't work without physics

The pressure of the surrounding fluid, for example gas, air or water, always acts on an object. When the object is at rest, it is only the static pressure. When the object moves, there is additional dynamic pressure against the direction of movement due to the movement. The static pressure decreases perpendicular to this direction. All in all, static and dynamic pressure always act together on an object - the two together add up to the total pressure.

According to Bernoulli, the total pressure continues a fluid such as gas, air or a liquid composed of static and dynamic pressure. p is derived from the English word "pressure" for the German word "Druck". This total pressure can be regarded as constant under certain assumptions:

pgesamt =pdynamic +pstatic = constant

The dynamic pressure is the pressure generated by the movement of a fluid (gas, air, water) and acts in the direction of flow. For example, if you hold an umbrella in the wind, you will feel the dynamic pressure of the moving air.

helicopter blog dynamic printing wind umbrella

Figure 4: The wind on the screen and the person creates dynamic pressure

The static pressure is the pressure that acts on the side surfaces at right angles to the direction of flow (Fig. 5). It decreases with increasing dynamic pressure. It therefore pulls objects in its direction due to the lower pressure. Like a grand piano.

helicopter blog static pressure moench

Figure 5: Since the monk and its surrounding air are at rest,
only static pressure affects him

The so-called dynamic buoyancy finally arises from the flow against a wing, on which the air on the top of the profile has to travel a further distance than on the underside. To make the longer way along the top of the profile at the same time as the colleagues at the bottom, the air moves much faster there. The static pressure on the underside of the profile is therefore greater than on the top. Since objects want to move away from high pressure and towards low pressure, the wing wants to move upwards with a certain force. This buoyancy is generated by the pressure difference and is of course influenced by many other factors such as the wing shape and wing area.

lift wing movement bernoulli why can a helicopter fly

Figure 6: The wing wants to move up 

What you have just learned in practice: Take off step by step

  1. The aircraft is at the beginning of the runway and is not moving. There is no wind. Only static pressure acts on the wings from all sides - as in the previous example on the monk. The weight of the plane pushes down.
  2. The plane accelerates forward. There is dynamic pressure from the front on the wings. The air that flows past the arched top of the wing must be faster and there reduce the static pressure. However, the lifting force that arises on the wing due to the still low speed of the aircraft is not sufficient to lift the aircraft. The pilot knows that and goes full throttle.
  3. The plane races along the runway. A lot more air now flows past the top of the wing much faster. The dynamic pressure there is now very high, which means that the static pressure is very low. A very low static pressure creates a strong buoyancy. It is now strong enough to counteract the weight of the aircraft - it lifts off the ground with the landing gear.
Helicopter lift air flow mechanics wing wing

Figure 7: The wing pitch increases the lift effect

We could also tie the plane to the ground and instead place a lot of fans in front of it to generate the necessary flow. This is how wing profiles are tested in wind tunnels. The lift could be increased if the path of the air around the top is artificially extended, for example by changing the profile or tilting the entire wing (Fig. 7). That would have brought us a little closer to the helicopter. In the next part, greetings from Isaac Newton.

In the next part:

  • Principle of reaction and conservation of momentum by Isaac Newton
  • From car windows, lunch boxes, office chairs and Bud Spencer
  • What distinguishes the helicopter from the paper plane

Part 3 can be found here: Why can a helicopter fly part 3

April 6th, 2018|

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