# Are fighter pilot cockpits airtight

## airliners.de

The legend of the bumblebee has been circulating in motivational guides for many years: The bumblebee doesn't know that it can't fly. She makes it easy and it works, although it cannot be explained physically. In fact, it took from the legend's origins in the 1930s to 1996 for the aerodynamic facts to be proven beyond doubt.

This development shows two things: On the one hand, aerodynamics is a relatively young discipline in physics. Even if the beginnings go back a long way, it wasn't until the late 19th century that people began to understand the processes better and to record them scientifically.

On the other hand, many processes are extremely complex. While a basic understanding of a circuit is easy to convey and calculate, it is far more complicated in aerodynamics.

### Why doesn't a balloon fly? Why is he driving?

There are basically two types of vehicles for aircraft: The first group are aircraft that are lighter than air. This group includes all balloons which, with the light gas or hot air in the envelope, are overall lighter (i.e. gas + envelope + basket with load) than the displaced ambient air. They do not generate their lift dynamically and therefore do not fly. They sail like a ship on the water, which gains its buoyancy according to the same principle.

The second group are aircraft that are heavier than air: these aircraft must actively generate the lift that lifts them into the air - they fly. This group includes airplanes, helicopters, gyroscopes and others.

In both cases, the buoyancy is a force that acts against the weight. There is a simple lift equation for the lift of an aircraft. There are three elements in this equation that significantly determine lift: the wing area, information on the flow and the lift coefficient.

### The buoyancy - actually quite simple

The first quantity in the equation is the wing area. Of course, depending on how big or small the wing is, the lift is also. The second involved is the current that is flowing around the wing. In our case it is the ambient air. Two data are important: how fast is the air and what is the density. While the importance of speed is also evident, density is not as common.

Fortunately, for us humans, differences in air density are barely noticeable in everyday life. After all, we know that the air becomes "thin" on high mountains and that we have to breathe faster. This effect has a major influence for the aerodynamicist: the density of the air - i.e. how many mass particles of air are in a certain volume - changes with the air temperature and the air pressure. If we have a high air density, such as at an airfield like Amsterdam at sea level on a cold winter day, a profile provides a lot more lift than on a warm day in Denver at 1,600 meters above sea level.

Last but not least, the lift coefficient remains. This value is a dimensionless quantity that describes the properties of the wing: the shape of the profile and the angle of attack in the flow. If you have this information together, you can calculate the lift.

On the other hand, it's also easy to see how to play with the values. A mind game: An airplane flies straight ahead at an altitude. It burns kerosene and becomes lighter, which reduces weight. But because the lift remains constant, the aircraft would climb slowly and continuously. What can the pilot do? He cannot change the wing area while cruising. On the other hand, he can change the flow by flying more slowly. It could also reduce the angle of attack and thus the lift coefficient, but would then have to maintain its speed.

How exactly does the lift force develop on the wing profile?

Lennart Z.

If you miss a wing, you can determine pressure differences on the wing: On the upper side of the wing there is a strong negative pressure ("suction"), on the underside there is a slight overpressure ("pressure"). These pressure differences - and above all the dominant negative pressure on the upper side of the wing - ultimately generate the lift force.

At this point in the explanation you will find many explanations for the pressure differences in the relevant literature, which on the one hand are as popular as they are easy to understand, but on the other hand almost all of them are wrong across the board! NASA has explained the errors of the common theories here and on the following pages.

The disappointing truth is that all further correct explanations can only be explained to the specialist audience. If you are looking for a direction for further orientation: There is a circulation around the profile, you need the "Navier Stokes equation", an extension of the Euler equations, and the Kutta-Joukowski theorem.

NASA has explained many facts around the topic of aerodynamics very well in the "Beginners Guide to Aerodynamics" - at least as far as it is possible without integral functions. Aerodynamics students deal with these integral functions in detail, like those students of the famous Prof. Ludwig Prandtl who thought up the story of the bumblebee in a pub in the 1930s.

### About the author

In the airliners.de series "Answers from the Cockpit", airliner pilot Nikolaus Braun regularly answers questions on pilot topics relating to aviation technology and flight operations. If you also have a question, write to [email protected]

Nikolaus Braun is a pilot with a large German airline and currently flies on the Airbus A330 / A340. The studied Dipl-Ing. (FH) for aviation system technology and management also advises part-time with his company Nikolaus Braun Aviation Consulting (NBAC) on projects in research, development, legislation and teaching.