Part 1: Introduction and Buoyancy

This part is the first of a few to answer the question of why a helicopter can now fly. The series is based on the e-book of the same name written by Florian Gronau and myself. However, the entire e-book is too long for a single blog post, which is why we have made small, easily digestible snacks here and made a blog series out of it. But first, the short foreword from the e-book, which illustrates our motivation:

Foreword

As passionate helicopter pilots and flight instructors, we naturally have a lot to do with helicopters. And on sightseeing flights, passengers very often ask similar questions. The challenge is to answer these questions in spite of complex technical matter in such a way that they can be understood without a technical background. The do-it-yourself pilots are always a special challenge, as they should at least partially control the helicopter themselves after a brief briefing. And without any technical background or other previous knowledge, only after a briefing on the floor of about 30-45 minutes. Fortunately, there is also the double tax with which the flight instructor can help permanently. There is also a good dose of humor and fun, so that we do not forget why we are actually on this planet. The aim is for the pilot plane to get out of the helicopter after the flight hour with a truly unforgettable sense of achievement. The focus here is on the essentials and the simplest possible explanations. The first attempt before you is to answer the question as exactly as possible why a helicopter can fly and how it is controlled. Have fun!

Introduction

While an aircraft can only take off from the ground and remain in the air at high speeds, the helicopter simply lifts itself into the air from a standing start. It is not only able to float forwards, sideways and backwards, but also to rotate around its own axis. But how does it work and how does the pilot control these precise flight movements? These are questions that many people ask themselves. For example, when a helicopter flies over your own house again - be it the rescue helicopter on the way to an operation or one from the police who is in the air searching an area. Or anyone who has ever needed a rescue helicopter but couldn't really enjoy the view from above. These questions are explained in a clear and understandable way in this series of blog articles. We make sure that it is understandable for people without prior knowledge. Among other things, we learn what distinguishes a helicopter from a paper airplane, what it has in common with a lid in the dishwater or with office chairs, and what role Bud Spencer plays in this. Because the helicopter uses the same physical laws as an aircraft, but these relationships are easier to understand for many people using the aircraft as an example, we will first explain them on the aircraft before we carry out the transfer to the helicopter. Here we go.

Completely detached: How heavy things learn to fly

A powerful jump in the air from your knees is not enough to be able to say with a clear conscience that you have flown. Rather, you have to deal with whether you like it or not, how the gravitational pull of the earth can be permanently overcome or balanced. This gravitational pull, which pulls all objects to the ground, is called weight or, more scientifically, “gravity”. It's very useful to us - without it we'd be floating around a little awkwardly. The image of objects floating around in space is known to us thanks to numerous television transmissions from the space shuttle or the ISS space station. Of course, also from various sci-fi films such as Star Wars, Star Trek, Guardians of the Galaxy, Der Martianer ... ahem - you understand what I mean. On the way to the bus stop or through our apartment, it means that we are always standing on solid ground instead of floating away like a soap bubble. So if you jumped in the air with the intention of flying, gravity pulls you straight back to earth. The little soaring comes to an end quickly and you are back at the same place from which you started. Unless you are a bit clumsy and lose your balance on landing. If you are a bird, you could flap your wings hard enough to counter this force and your buttocks will not make a more or less painful encounter with the ground.

why can a helicopter fly weight force bird

Figure 1: The weight dreams of flying like a bird

So if we really want to fly properly, we need a force that permanently counteracts the weight and therefore stays in the air for more than a few seconds. We owe it to the Swiss scholar Daniel Bernoulli (1700-1782) that we can consciously create such a force today. Put simply, he found that in a flowing fluid (gas or liquid) a speed increase is also accompanied by a pressure drop. So if something flows faster, there is less pressure at the same time. Imagine that an object that happens to be shaped like the wing of an airplane is moved through the air (Fig. 2). The air must split up when it hits the wing. A part of it flows above and has to travel a longer distance due to the curvature of the wing. The other part flows past the bottom and has a shorter path.

why can a helicopter fly weight force bird

Figure 2: If you have the longer way, you have to be faster

So that the air particles can meet again at the back of the wing, the "upper ones" have to flow a little faster with the longer way. According to Bernoulli, the so-called static pressure decreases at the top. A buoyancy force arises when air moves quickly over the specially shaped profile top and slowly over the bottom. The faster flowing air has a lower static pressure than the slower flowing air, so the high pressure at the bottom raises the profile. The following very simple experiments help to understand this.

Experiment 1: How are fast-flowing air and buoyancy related?

Hold a sheet of A4 paper horizontally on one of the short sides so that the other side hangs limply due to gravity. Now blow air over the curved top of the paper with a blow dryer. If you don't feel like getting the hair dryer out of the bathroom now, just blow on it. The paper rises! The whole thing still works if small stones are used to stick the tape to the underside of the leaf to increase weight.

Experiment 2: How can you visualize pressures and buoyancy?

If you don't have an airplane wing at hand, just place a playing card on your flat outstretched fingers instead. The map is now the wing. A second person presses the card from above. If both people press equally hard, the card does not move. The fingers correspond to the air pressure on the resting profile. Now let's mentally let air flow over the profile (Fig. 2), the air pressure on the top decreases and with it the force. In the example, this means that the second person with the upper hand significantly reduces the pressure on the card, while the first person with the lower hand maintains the original pressure. The card moves up.

why can a helicopter fly weight force bird

Figure 3: The early predecessor of the Airbus A380 here during secret tests

Summary

The low pressure of the air molecules on the upper side of the profile is comparable to an innumerable number of rather weakly pressing fingers, while the higher pressure on the underside of the profile is comparable to an equally large number of strongly pressing fingers. The force resulting from this pressure difference is called buoyancy. If this buoyancy is greater than the weight of an object, that object leaves the ground. Our little hopper will finally become a flight! Aviation pioneers of the late 19th century in particular could sing a song about this if you were still alive. However, despite numerous failed attempts, they did not give up and made the difficult step from countless small failed hops to serious flights of at least several meters. The principle described above - applying a lifting force that is greater than the weight - is still the basis of all aircraft today. It almost doesn't matter how heavy an aircraft is as long as a force can be generated that is greater than the weight that holds the aircraft on the ground. How? That's all? Yes. Sounds strange, but it is true. For this reason, the world's largest passenger aircraft, the Airbus A380, can fly at all - even with its maximum takeoff weight of 569.000 kilograms. That corresponds to about three adult blue whales. Or 400 VW Passat. The A380 can even travel up to 15.200 kilometers! Passat and blue whale, on the other hand, still cannot fly.

In the next part:

  • The physical basics of buoyancy - don't worry, it's easy to explain.
  • We accompany an aircraft from the stand to take off - practice up close. Please put on your helmet and glasses before reading.

Straight to the next part: Why can a helicopter fly part 2