Who Says You Cannot Make your Own Airplane? Series #3: Understanding How Jet Engines Work And Effect Of Atmosphere In Flight

The cost of an airplanes may be on the high side but have you ever considered building one by yourself? Like seriously. Just that you cannot do that unless you understand the mechanism behind flight and how aircraft is built. So Stay with this page - @teekingtv

So I welcome you guys back to this priceless page. So first I am going to give you a brief recap of the previous series. That is for the sake of those who are just joining us, and like I did on the previous post, I am going to break the concepts discussed on this post down into the absolute basics. Okay here we go.

Having talked about the forces of flight, weight, lift and how the aircraft wings cause lift, here we are looking at thrust, the basic take-off speeds and that object that is responsible for thrust - engine. The three basic take-off speeds are V1, Vr/"Rotate" and V2. V1 is the speed beyond which the take-off should no longer be aborted. Vr is the speed at which the pilot begins to apply control inputs to cause the aircraft nose to pitch up, after which it will leave the ground, while V2 is the speed at which the aircraft may safely climb with one engine inoperative. How these speeds are used are carefully explained so as to avoid confusions. Engine is responsible for the forward movement of the aircraft. As the engine pushes a massive air backward, the aircraft is pushed forwards in an opposite direction, as proposed by Isaac Newton. Some aircraft have their engines mounted at the back while most have theirs mounted below the wings.

We had lots more than I can brief you here in the previous series and in case you are just joining this page for the first time, you may try to check it out here.

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Okay, we are just going to start exactly where we stopped in the previous series. Still on aircraft engines and here on this post we re going to discuss how the jet engines work.

How the jet engines work.

Jet engines have been the successful drivers of aircraft for nearly a century now. In this section, I will explain the technology behind the jet engine in a logical step by step manner. A jet engine keeps the aircraft moving forward using a very simple principle. The same that makes the air-filled balloon move. Yes, again, Newton's third law of motion. Just like the reaction force produced by the air moves the balloon, the reaction force produced by the high speed jet at the tail of the jet engine makes the aircraft move forward.

So, the working of jet engine is all about producing a high speed jet at the exit. The higher the speed of a jet, the greater the thrust force. The thrust force then makes the aircraft move forward. Such high speed exhaust is achieved by a combination of techniques. If you can heat the incoming air to a high temperature, it will expend tremendously and will create a high velocity to the jet. For this purpose, a combustion chamber is used. Effective combustion requires air to be at moderately high temperature and pressure. To bring the air to this condition, a set of compressor stages are used.

The rotating blades of compressor add energy to the air and its temperature and pressure rise to a level suitable to sustain combustion. The compressor receives the energy from the rotation from a turbine which is placed right after the combustion chamber. The compressor and turbine are attached to the same shaft. The high energy fluid that leaves the chamber makes the turban blades turn. The turbine blades have a special airfoil shape which creates lift force and make them turn. As the turbine absorbs energy from the fluid, its pressure drops. From these steps, we have achieved our objective - a really hard and high speed air emitting from the exit of the engine. The engine case becomes air or toxic outlet which results in even greater jet velocity. In short, the synchronized operation of the compressor, combustion chamber and turbine makes the aircraft moves forward.

Modern aircraft use a slightly improved compressor-turbine arrangement called a Two Spool. Here, two independent turbine compressor stages are used. The shaft of the other compressor turbine passes for the inner one. The outer turbine is subjected to a low energy fluid and will run at a lower speed than the inner turbine. Low pressure blades are longer, this low speed helps to reduce centrifugal stress thus improves the blades life. Some modern aircraft even use a Three Spool engine. The engine we have discussed so far is more specifically called as Turbo Jet Engine. Turbo Jet Engine tends to produce a high level of noise. A revolutionary improvement was made to this engine by fielding a large fan with a low pressure spool. Such engines are called Turbo Fan Engines and almost every commercial aircraft run on them.

A Turbo Fan Engine bypasses a huge amount of air. The ever nearly bypassed duct provides a good jet velocity to the bypassed air. In a Turbo Fan Engine, the majority of the thrust force comes from the fan reaction force. Further, the fan greatly improves airflow in the system by sucking in more air, thus it helps to improve the thrust. This means high thrust creation with an expense of slightly more fuel. This is the reason why Turbo Fan Engines are highly fuel economical. The noise produced by a jet engine is highly dependent on the exit jet velocity. Since in a turbo fan, the bypassed cold air gets mixed with the hotter, it is possible to keep the high velocity within the limit. Thus, it overcomes the noise problem. With a quiet exhaust and better fuel economy, Turbo Fan Engines continue to dominate aircraft proportion systems.

I hope this provides a nice introduction to the working of jet engines. We have more ground to cover on that later but that should be okay for now in order to not get you guys confused. I guess I can take a sip from my coffee cup now (you should get used to that already).

Now I am back. We discussed about thrust in the previous series. I need to let you understand that thrust is not only used to move the aircraft forward. In fact, it is also used during landing but this time, it is a reverse thrust. But then what exactly is reverse thrust, when and how is it used on aircraft?

Reverse thrust

Reverse thrust is used to slow down the aircraft at the runway as it touches the ground. There are three main components which slow down the airplane on the runway. Primary is break, secondary is reverse thrust which is what we are going to talk about here and third is the dynamical breaking with the ground spoilers. So we have two words in reverse thrust. It is reverse because the turbine's output is being guided into the reverse direction and as you might have known, we apply thrust in order to increase the breaking action.

As the aircraft touches the ground, the flaps beside the engine open up. These are called reversal doors. They act as guiding wings in the mid-section of the engine and force the accelerating air from the fan into the opposite direction. Now, these reversal doors do not open automatically at touchdown. No, you are going to have to lift the levers which activates the hydraulic system to apply pressure to open the reversal doors. Keeping the levers in this position will only give you reverse. But as soon as you remove the force levers to the off position, the engine will spool up, creating more force, increasing reverse output. The engines will increase thrust up to 70% within full reverse. Again, it sounds absolute odd that we are applying thrust again just as we touch down. But you can definitely hear the difference between idle and full reverse thrust, trust me. Using the reversal significantly decreases the landing distance. Bearing between aircraft weight and environmental factors. Landing on a snow covered runway, we apply full reverse thrust and you could see the snow blowing in front of the aircraft. Wow! Beautiful.

So when do we use reverse thrust?

Using idle reverse at touchdown is mandatory by many aircraft manufacturers and airlines. First and foremost, to immediately decrease the aircraft speed and stabilize the aircraft plus to reduce the break usage. Landing on wet or snow contaminated runway using reverse thrust is absolutely viable to decelerate the aircraft and keeping it in a straight line. Full reverse thrust is not permitted at some airports. Nevertheless, pilots may use full reverse thrust when necessary but might have to state that in their decision in report. Airplane with wing mounted engines may only use full reverse thrust until slowing down to specified speed because using reversals below that speed could blow up particles through the runway and those particles may enter the engines and thus cause damages. For example, in some aircraft, you will have to reduce the full reverse to idle reverse at 70 knots and retract reversals at thee speed below 40 knots. There are many different types out there but all work after the basic principle of forcing the air into the opposite direction.

Very often, you see executive jets using reverse thrust during taxi to reduce break usage while strolling down the taxi way. Because of their high mounted engines (at the back), pilots do not need to worry about damaging the turbines.

Okay that is that about the four forces of flight. You could see there are pretty much more to them than what you have read in most books. Now, like I said in the first series, there are other principles that make flight possible (the four forces aside). And now we are going to take a look at the effect of atmosphere or better still climatic factor as regards to flight.

How the atmosphere effect flight.

Understanding how the atmosphere effects the flight is very important not just to pilots but to everyone who intends to understand the mechanism behind flight and build one by himself from scratch, since it is the environment the aircraft is flying in. There are many processes that take place within the atmosphere but we will take a look at them in the future. For now, let us talk about what the atmosphere is and what its components and properties are.

An atmosphere is defined as the layer of gases surrounding a planet or other material body to its healthy place surrounding that properties.

There are more than twenty gases in the atmosphere but we will focus on the four main gases. The most common gas is Nitrogen, covering 78.08% of the atmosphere. Oxygen is the next, covering 20.95% and supports life and also for the combustion of fuel of the aircraft engines. Less than 1% of the atmosphere is covered by Argon (0.95% to be exact) while Carbon Dioxide covers 0.05%. There are many other gases covering the remaining percent but they are in such a low percentage that we will not be focusing on them. They include Carbon Monoxide, Helium, Methane, Ozone, Hydrogen and are called Trace Gases.

There is however one gas found in the atmosphere that is very important for the performance of the aircraft. We are talking about water vapor. It is invisible as a gas but it can be seen when in solid or liquid state. Clouds and rains are examples of its liquid state and while ice and snow are example of its solid state.

The ratio of water vapor present in the air expressed as a percentage of the amount needed for saturation at the same temperature is known as relative humidity.

100% humidity is visible as a cloud while the air cannot hold the vapor in the vapor state and it condenses into water. The amount of gases in the atmosphere remain constant up to sixty kilometers. From there on, gravitational separation changes the composition of the atmosphere. For the purpose of the principle of flight, we will just consider the lowest part of the atmosphere and we will always assume that the composition remains constant. The atmosphere is divided into four main layers. Starting from the nearest to the earth, we have the Troposphere which extends to approximately 11km above the earth. Then we have the Stratosphere which extends up to fifty kilometers above the earth. The next layer is the Mesosphere which extends up to eighty kilometers above the earth. And finally, the Thermosphere.

Between the Troposphere and the Stratosphere is a boundary called the Tropopause. The top altitude of the tropopause changes depending on the latitude but the average is 36090ft. Conventional jet aircraft are limited to fly up to the Stratosphere while the light propeller aircraft fly inside the Troposphere. Some special aircraft are designed to fly within the Mesosphere while only the space rockets are able to fly in the Thermosphere. The temperature changes with increase in altitude as the earth is heated by the sun. The temperature decreases as altitude increases within the Troposphere as we get further away from the earth surface. We will only focus on how the temperature changes within the Troposphere and Stratosphere as it is in these layers where most aircraft fly.

The temperature remains constant from the top part to the lower part of the Stratosphere just before rising again. General aviation pilots are restricted to fly not higher than 10000ft in most of occasions by several factors one of which is the weather, general aviation aircraft are not designed to go higher than normal and more importantly, the oxygen we need to breath. We have already learned that the composition of the atmosphere remain constant at lower altitude but there is one more factor that takes place in this situation. That is the Atmospheric Pressure. The air is a substance which contains molecules and these molecules have their own mass.

Pressure is defined as the force applied perpendicular to the surface of an object

Force is defined as a push or pull and will be produced when a mass is accelerated.

The unit of measurement for mass and acceleration is thus in standard international system. The unit for mass is kg and M/s2 for acceleration. The product of these two is Force, measured as Newtons. The mass of the air is accelerated towards the earth by gravity which will give us a force. We will consider that force acting on an area of one square meter extending upwards. This is pressure, acting on earth surface and is called atmospheric pressure. The unit of measurement for pressure is N/m2 called Pascals. The Pascal results in a very small unit. In aviation, we use the HectorPascal. Hector means 100, so 1 HectorPascal means 100Pascals. Another unit that is used in aviation is the Millibar.1 Millibar is equal to 100 Pascals. The air Pressure act in all direction, so the air pressure will act on the aircraft that is flying in it. This atmospheric pressure acting on anything different from the surface of the earth is also known as Static Pressure.

It was explained before that the pressure of the atmosphere was given because of the gravity pulling the massive air downwards onto the earth surface. This will lead to a higher molecules as it gets closer to the earth surface and will directly affect the mass. We conclude that the pressure is not constant as we go further away from the surface of the earth. Static pressure reduces with an increase in altitude. This is why breathing is difficult at high altitude since there is less pressure to force oxygen into our lungs. It is a common mistake to think that the amount of oxygen reduces with the altitude and we learned it is not that way that it is the pressure which reduces. Same thing happens with the combustion of engine reducing the flow of oxygen through. This fact of number of molecules in the air altitude leads us to another important term called Air Density.

Density is defined as the mass of a substance per unit per unit Boolean of that substance.

The density of the air is measured as kg/m3. Similar to pressure, density reduces with the increase in altitude but this is not the only factor as the temperature also affect density. Higher temperature reduces density same as the cooler or lower temperature will increase the density of a substance. We have learned above that the pressure and temperature decrease with an increase in altitude, but it is the pressure which takes the greater effect on density. The amount of water vapor will also affect density as it is the amount of air we consider in m3, not the amount of water. If we take a m3 and we introduce molecules of water, there must be a reduction of air molecules. We conclude that the specified volume of water is less dense than the volume of air molecules. The more water vapor in the air, the lower the air density is.

The atmosphere changes everyday, so a Standard Atmosphere was set in 1964 also known as ISA or International Standard Atmosphere. ISA assumes that at Mean Sea Level or MSL:

• The temperature is 15°C
• Pressure is 1013.25 hPa (HectorPascal) pr Mb (Milibars)
• And the air density is 1.225 kg/m3

From Mean Sea Level (MSL) to 11km or 36090ft;

• The temperature decreases by 1.98°C per 1000ft or 0.65°C per 100m

From11km to 20km;

• Temperature remains constant at -56.5°C

Conclusion

Notice that at 11km and above, the temperature no longer decreases with altitude. That tells us that the temperature decreases as as we altitude increases from the earth surface to about 11km which happens to be within the Troposphere. Hence, the air at the surface of the earth is warmer than the one found up to 11km altitude. This is something we were not expecting as many of us believed that as we leave the earth surface and get closer to the sun, the air would be hotter. It is not so because what makes the air hot is not the sun itself but the deflection of the sun on the earth surface. This weather conditions have a very great effect in flight. **I understand that what we discussed here is kind of confusing and hence, I strongly request you read this effect of weather once again. We are going to take a break now but not to worry, we are meeting again pretty soon.

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References

• Jet Engine
• How jet engine work
• Thrust reversal
• Aircraft performance due to weather
• Weather
• ISA

Previous Lessons In The Series

• Series #2: Understanding The Thrust Mechanism And How The Engine Works
• Series #1: Understanding The Mechanism Behind Airplanes And The Misconceptions

Till we meet again on my next post, well I am not changing my name. I am @teekingtv and I write STEM! THANK YOU FOR READING.

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