Applications of aerodynamics

Applications 

After a very long time, I am trying to come back to write my blog about scientific topic, especially aeronautics. It has been quite intense time, where I was needed to think what direction I would like to go. 

Currently I am teaching a course covering many topics of physics at the same time of future power engineers. It is not easy to compress everything in one course. I think it is too little time and also I am throwing so many stuff on the students that I am not even sure they are able to catch that. 

But it is interesting to get to the basics again and talk about the basic mechanics, power and energy, energy generation and transmission, fluid mechanics, a lot of applications, thermodynamics and heat transfer, including again million of the applications. And it wouldn't be me, not using rockets and aircrafts as an example. 

Aerodynamics is the study of motion of air, which is part of the study of fluid dynamics. I would call aerodynamics as a study the way objects move through the air. The rules of aerodynamics explain how an airplane is able to fly. Anything that moves through air is affected by aerodynamics, from a rocket blasting off, to a kite flying. Since they are surrounded by air, even cars are affected by aerodynamics, but as well as the airfoils of the wind powerplants. 

First, I needed to talk about the lift, and drag, because many things related to each other. For example the airfoil of the wind turbine has very similar approach. And it is really interesting to follow the stuff. Airfoil can be described in the sense of the propeller, in case of propeller plane, or as the wing creating the lift, or the airfoil in case of the wind turbine. All of them are using the same physics, but you need to realize what is needed as an outcome. The primary application of wind turbines is to generate energy using the wind. Hence, the aerodynamics is a very important aspect of wind turbines. A modern wind turbine blade is designed in a shape that is similar to the wings of an airplane.

Blades of wind turbines, airplanes and helicopters are essentially the same. They are all wings, roughly to do the same job. However, each one is designed or enhanced to do their special job. The differences can be: number of revolutions, wing span, rate of flow, tip speed, density of medium, material, and load. Turbine blades are designed to move in large volumes of rather slow moving air in order of 15 to 50 km/h and not create turbulence that fouls the next blade. Aircraft propellers are designed to move high velocity air and avoid the turbulence by moving out of the way. Wind turbines use blade pitch to adjust the rotation speed and the generated power. Wind turbine's control system adjusts the blade pitch to keep the rotor speed within operating limits as the wind speed changes. In aircraft, blade pitch, often shortened to pitch, refers to the angle between the propeller blade chord line and the plane of rotation of the propeller. Blade pitch is most often described in terms of units of distance that the propeller would move forward in one rotation assuming that there was no slippage.  "Fine" pitch refers to a fine or low pitch angle which yields good low speed acceleration (takeoff and climb) whereas "coarse" refers to a coarser or higher pitch angle which yields optimum high speed performance and fuel economy (cruise).

And the aircraft wing itself creates it responsible for flying. You know that for example gliders do not have any engine to create thrust, so how they can fly or remain in the air without direct falling down? Airplanes fly as a result of Bernoulli's principle, which says that if air speeds up the pressure is lowered. Thus a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift. That is very usual and popular explanation used almost everywhere. But that is not the only one. 

Lift occurs when a moving flow of fluid is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

And not to forget, lift is a mechanical force. It is generated by the interaction of a solid body with a fluid (liquid or gas). It is not generated by a force field, such as gravitational field, or an electromagnetic field, where one object can affect another object without being in direct contact. For lift to be generated, the solid body must be in contact with the fluid, which concludes that if there is no fluid, there cannot be lift. The Space Shuttle did not stay in space because of lift from its wings but because of orbital mechanics related to its speed. Space is almost a vacuum. Without air, there is no lift generated by the wings.

One would be able to ask now, why Space shuttle actually had the wings at first point at all. The first reason is just simple, aerodynamics during the lift off. Aerodynamic shape is needed in any rocket for lift off. And the second reason is, while space shuttle was returning after the mission, it was gliding down. I called myself space shuttle as a locomotive with wings. 

 

And I really like that I can use one example to students and say, hey there is only one physics behind it. Wings keep an airplane up in the air, but the four forces are what make this happen. They push a plane up, down, forward, or slow it down. The way the four forces act on the airplane make the plane do different things. Each force has an opposite force that works against it. For example lift force works opposite of weight, or thrust force works opposite of drag force. When all the forces are balanced, a plane flies in a level direction. The plane goes up if the forces of lift and thrust are more than gravity and drag. If gravity and drag are bigger than lift and thrust, the plane goes down. 

What I like to ask what is the difference between the weight and gravity. Gravity is a force that pulls everything down to Earth. On the other hand, weight is the amount of gravity multiplied by the mass of an object. Weight is also the downward force that an aircraft must overcome to fly. A kite has less mass and therefore less weight to overcome than a jumbo jet, but they both need the same thing in order to fly, and that is lift.

But what is the difference then between the weight and mass? Weight is a measure of the gravitational force exerted on an object. Weight changes as gravity changes.  Mass is the amount of matter in an object, which concludes that mass never changes. How it affects for example astronaut?

Astronaut can have mass about 120 kg including the suit. If he is still on Earth the weight is then 120 times 10, being 1200 N. If he is on the Moon, the mass doesn't change and remains still 120 kg. However what changes is the weight, being now 120 times 1.6 which equals to about 200 N. The acceleration due to gravity on the surface of the Moon is about 1.6 m/s^2.

And at this point it is so easy to come to the topic of rockets. And there is plenty of connection later as well. Rockets are engines, which generate thrust force. Rockets and airplanes both work on Newton’s third Law of motion to produce thrust. For every action, there is an equal and opposite re-action. A gas or working fluid is accelerated to the rear and the engine and aircraft are accelerated in the opposite direction.  

For example, if a 500 kN action force is generated downwards by expelling the gas out downwards of rocket, a 500 kN re-action force is also generated upwards and it is known as lift or thrust force as a result of the force generating downwards. 

How the jet engine or engine of the commercial aircraft differ from the rocket flying in space? The work on same principle. They produce thrust based on the Newton's third law of motion. The main difference between and jets and rockets is that jets and aircrafts burn their fuel using the oxygen that is present in the atmosphere, however the rockets carry their own oxygen to space, which leads to another difference. Jet engines have two openings, an intake and an exhaust nozzle. On the other hand rocket engines have one opening, an exhaust nozzle. 

You can see the schematic picture of the jet engine below. 

Most space rockets carry their own fuel and oxidizer with them. Furthermore, you don’t necessarily need fire to provide thrust to a rocket. You can obtain thrust by simply ejecting mass out of the rocket. 

Rockets can be classified as suborbital, which is kind of brief visit to space, orbital and escaping Earth’s gravity. I think that important note here is, where actually begins the space. The official beginning of space is 100 km above the Earth’s surface. 

This is a table showing the height above the Earth's surface and weight force of 10 kg object. 

From the table you can see the classical value of g you learn at school 9.81 m/s^2. When you are on the International Space Station, wish I am there, the g drops down to 8.8 m/s^2. Where the drop comes from?

g at a certain altitude = ((𝑔 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒) 𝑥 6371^2)/ (𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑐𝑒𝑛𝑡𝑒𝑟)^2 . Altitude distance from center for 100 km above surface is 6371 + 100km = 6471 km. Altitude distance from center for 360 km above surface, approximate position of ISS,  is 6371 + 360 km = 6731km. 

There are plenty of reasons why to use rockets, if we try to forget any was examples. For example scientific rockets, such as sounding rockets that are designed to take measurements of the atmosphere or to perform microgravity experiments. They are typically suborbital type of rockets, but they get much higher than weather balloons. You can use rockets in emergency case, or just as a hobby in case of fireworks. 

I have one more point to ask and you can send me your opinion. I will write next article about that topic:  When real rocket lifts off, the rocket’s velocity increases with the altitude.