Who Says You Can't Build Your Own Airplane? Series 13: Can We Store The Fuel Elsewhere Aside The Wings

I welcome you back as we resume on our journey to build an aircraft. Like I usually say, we cannot build something we do not fully understand how it works. Understand the chore aspects might be hard but it is important if we really want to do this. A brief recap f our previous series below:

The subfields under Aerospace and Aeronautics Engineering include Aerodynamics, Propulsion, Controls and stability and Structures. There are different fields to specialize on under the above listed sub-fields. Among the most common ones are analysis, designing and testing.

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Just joining us for the first time? Now I tell you what, I am going to fine you for that but first check out the previous series: Series 1 | Series 2 | Series 3 | Series 4 | Series 5 | Series 6 | Series 7 | Series 8 | Series 9 | Series 10 | Series 11 | Series 12.

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Pixabay image (Public Domain - CCO Licensed)

Most machines either run on fuel or batteries. Even when some of them make use of batteries to kick start their operation, they eventually live on fuel. Among these machines that operate on fuel are these fastest objects conveying us through long distance in a matter of hours, thereby closing to or even breaking the speed of sound. In another word, the aircraft. While we have the modern day light aircraft that fully run on batteries, commercial aircraft that convey higher payload and travel through longer distances still run on fuel. This makes fuel one the most important entities that drive the cost of flight.

If you have been following these series from the very beginning, you would understand that aircraft store their fuel right on their wings. This is one of the reasons why aircraft get blown up in smoke whenever there is a serious crash or have their wings slightly hit an obstacle. With respect to this, many of us have been wondering if there are better and safer places we can choose to store our aircraft fuel as we work on building our very own aircraft from scratch.

Reasons why fuel is stored in the wings

Just as you might have thought, the manufactures are considering if there are safer places to store aircraft fuel so that when accident happens, quickly blowing into smoke would not be the first thing that comes to aircraft minds. But it turns out there are good reasons why the fuel is being on the wings. Let us look into that.

Storage room

Just like other transportation machines, the aircraft is built to be able to convey as much payload as possible. When I say payload, it does not end on you as a passenger and your baggage alone, other cargos are included. While the loads (cargo) are stored in the fuselage below the cabin and the nose houses the pilot cockpit, there is no such room for us to our fuel.

However, the almost completely hollow wings turn out to be a perfect place to store the fuel. The wing box that form the wings can take great amount of load while something we call ribs inside of the wing make up the shape of the wing and contribute to the structure of the wing.

The aircraft is not like the cars we drive around the cities. They sometimes spend half a day flying in the sky. This long time of operation means they require huge amount of fuel especially when you are flying cross-country. Do you know how long it takes to fly from Florida to Middles East? So the wing is structured in a way that inside is like a box so big and wide enough to store enough fuel to fly almost anywhere. In a nutshell, we can utilize the big part of the wing as tank to store the fuel.

Weight and balance

Another reason the wings are chosen in storing the fuel is for the aircraft weight and balance. Like I have said in one of our previous posts, weight and balance is very crucial to the overall performance of an aircraft not just in flight but starting from the moment you bring it out of the hanger and start taxiing. And you cannot get your aircraft in a balanced position without considering where its center of gravity lies. Not just that, stability and good control of the aircraft are achieved by weight and balance. Most aircraft (top tailed aircraft excluded) have their wings almost at the center of the fuselage. The fuel is going to weigh many tons and for this reason, it important to be careful of where to store it.

Pixabay image (Public Domain - CCO Licensed)

The aircraft's center of gravity changes with respect to several factors one of which is where the cabin seats are situated. When the aircraft is loaded in the right manner, the center of gravity is situated more or less over the center of the wings. The wings give the aircraft all of the needed lift in order to fly.

Now let us look at this in a way we can all understand quickly. Let us say the aircraft is carrying two thousand gallons of fuel before embarking on the flight journey. As it takes off and starts flying in the sky, the fuel reduces, right? Of course the fuel is being used by the engines and as these engines continue to use the fuel, the fuel cannot remain i a constant level. At this point, if the fuel had been stored at the nose or towards the till of the aircraft, what happens as the fuel reduces?

Just as you have thought, the center of gravity changes, and seriously? This changes a lot of things. Stability will continue to reduce as the center of gravity moves around and this affects the weight and balance. At the same time, the
In a nutshell, weight and balance is the second reason why we want to have our full stored in the wings.

To counteract wings flexing

Flight itself is possible because the wings create the needed lift to take the aircraft off ground. Most f this lift is required during the take off because that is when the aircraft i heaviest. The engines are fully operational and more power is needed to get the aircraft in a straight and level flight. Also, with passengers, crew members and cargos in intact, this is the moment when we get he maximum weight of an aircraft.

All of these; the passengers and payload are situated at the center of the wings. Now that we have all of the weight at the center and all of the parts of the wings creating lift, we end up getting the wings flexing and by flexing, oh no I do not mean taking rounds of alcohol and barbeque in a joint. The wings are constructed to flex and when we have an extremely aircraft, the wings are going to be flexing a lot. However, there is a limit to the amount of this needed by the aircraft so as to not impact negative effect on air dynamics.

To counter this and ensure that the wings' flexing is not off limit, we need to do something in order to distribute th weight equally on the wings. To do this, the thought of having the fuel tank situated around the ings came to the minds of the engineers. The fuel will act as a counter to ensure that the aircraft does not have all of the weight right at the center but evenly distributed or at least, the weight of the fuel across the wings will create some balance and this will reduce flexing.

With this, when an aircraft takes off and flies in the sky, the wings will still flex but not as much as it would without some weight balance across them. This is a really important reason for having the fuel stored in the wings.

To ensure that weight distribution stays even like we have discussed above, the aircraft makes use of the fuel in a place called Center Tank first before the ones in the wings. As the aircraft operates, the engines heat up the fuel as it goes into them. The AC Controlled Fuel Pump is responsible for ensuring that the fuel in the Center Tank get depleted before using the one in the wings.

These are the reasons why the fuel is situated in the wings and unless we can think of how to solve the problems that this fuel position solves in different ways, we cannot possibly think of storing our fuel elsewhere.

Other things about the fuel system are:

Fueling station

This is situated on the right side of the aircraft. This system is responsible for passing the fuel through the three tanks situated on the aircraft - Tank 1 (right wing), Tank 2 (left wing) and the Center Tank. This system fills up the wings first after which the Center Tank is filled. Each tank has two pumps. So when one fails, the other can be used.

Nitrogen generating system

The fuel pump uses the fuel as lubricants themselves. So let us take a look at an instance where the fuel gets depleted in one of the tanks. Surely, you do not want the tank to heat up, in fact, you do t want anything to heat up where the fuel talk less of the fuel pump itself but that is what happens when the fuel is depleted. At that point, the fuel pump runs dry and starts heating up.

To counteract the problems that may generate from this heat, there are two systems that are set in place. As the pump i felt to have lowered down and getting depleted in a tank, the first system passes a warning to the cockpit thereby alerting the pilot to turn off the system. Well, even when the pilot dos not turn this fuel pump ff, it will off by itself automatically before it gets hot.

The second system , Nitrogen generating system enriches the air inside the tank with Nitrogen and makes the air almost impossible to ignite. These systems are to ensure that the aircraft is safe.

Jettison Procedure on aircraft

One of the things many of us find mysterious is why aircraft dump fuel. I mean how could a pilot just decide to dump this enormously expensive liquid that powers the very aircraft? To understand why this happens, there are some things we need to know first and I am going to show you here.

Pixabay image (Public Domain - CCO Licensed)

There something we call DOW, that is Dry Operating Weight. This is the actual weight of an aircraft before being onboard with payload, passengers and cargo. When the payload are situated inside the aircraft, we have what we call Zero Fuel Weight. As soon as the fuel systems are connected to fill up the three tanks we mentioned above in the aircraft, we have hell long names for this and these are Trip, Contingency, Alternate, Final Reserve and Taxi Fuel. All these added together give us what we refer to as Take-off Weight. There is also what we call a Landing Weight.

Let us use the Boeing 747 as a case study here. Its maximum landing weight is 312000kg, a structural weight limit. All other limiting weights are not subject to impact forces as compared to the landing weight and therefore, it is much lower. Engineers have done their math and decided that if the pilots land the plane with the descent rate of not more than roughly 300 to 400 feet [per minute, at a maximum landing weight of 312000kg, the aircraft will not suffer any structure damages primarily in the landing gear and airframe structure.

However, any landing weight above the maximum will have the consequence of suffering internal damages. This can inflect damages that may cost huge amount of money and several moths to fix. When this happens, we have what we call Overweight Landing.

We really want to avoid this and this is where the Jettison procedure comes to play. The pilot needs to quickly reduce the airplane’s weight by opening a spring-loaded valves via fuel dumping panel which leaves specific hose on each wing (usually farther away from the engines)and with this, the unnecessary literal amount of fuel get dumped overboard. As regards our case study, the Boing 747, the rate is approximately 2 tons per minute alongside the current procedure fuel burning.

Final Thoughts

Fuel is what drives the aircraft. The batteries might be responsible for the electrical works going on in the cabin and I the cockpit but the engines use fuel as their driving force. It is important for us as engineers to understand everything regarding this in order to know how to plan our design. We may have to take a break now but we are meeting again very soon.

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References

• Aircraft fuel system
• why fuel is stored in the wings
• Fuel dumping
• Structure of the wing

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Previous Lessons In The Series

• Series #12: Understanding Aerospace & Aeronautical Engineering And The Careers
• Series #11: Yo Nigga! You Don't Mess With Flight Instruments
• Series #10: Pilot Navigation - Flight Planning, Flight Logs, Map Readings, Correcting Heading Errors & Actions To Take When Lost
• Series #9: Understand The Concept Of Flight Approach And Landing

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. GOD BLESS.

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