大気観測装置開発から搭載までの経過

Additional Material (Aircraft)

１．The air conditioning system of the Boeing 747

Modern airliners cruise at 9,000 m ～ 13,000 m, where the outside air temperature is as low as -45～ -55 degrees C and the air pressure is 1/4～ 1/5 of its ground level. So, it is necessary to pressurize the cabin and also regulate the cabin temperature, as we cannot retain our lives under these conditions. For these purpose, modern airliners are equipped with efficient air conditioning systems.

To pressurize the cabin, high pressure air is required, and this air is usually supplied from the jet engines of the airliner. Jet engines consist of compressor, combustor and turbine. The high pressure air is extracted from the compressor section, which usually has approx. compression ratio of 30. Since the pressure and temperature from the compressor, however, are extremely high, distributing the air to the cabin directly is not practical. Then, the air conditioning system controls and regulates both pressure and temperature of the air, and deliver the conditioned air to the cabin.

In our atmospheric observation program, the sample air is extracted from just downstream of the air conditioning system and supplied to the ASE. Namely, it is a conditioned air rather than fresh outside air.

２．Basic of Jet Engine

Figure shows the details of the inside of jet engine.

This figure, however, not appropriate to explain the function of the jet engine, then, another simplified figure is provided.

In the region shown as①, several stages of compressor blades (just like as fans) are stacked. When these compressor blades are driven to rotate, the air from the intake of the engine is forced to send rearward direction. As the passage way of the air is narrower and narrower in the rear part, the air is compressed.

As the air reaches to the portion shown as②, the pressure and the temperature of the air get higher and higher, at which jet fuel is injected. By burning the fuel the air temperature rises and expands its volume.

As the expanded air rushes in the turbine section shown as③, the turbines (just like a windmills) are driven to rotator. This rotation drives the compressor section through the engine spool shown as ④.

So, compression process, burning process and expansion process are progressing simultaneously and continuously in the jet engine, and excess fuel energy causes the exhaust gas to spout to rearward the engine, which in turn generate the propulsion force or thrust.

Air conditioning air in the airplane is extracted(bleed air) from the compressor section of the engine. However, this is not an appreciated mechanization for the engine, since extracting the compressed air will lower the thrust, or will increase the fuel flow to recover the loss.

３．CO₂ Emission from Airliners

In Japan, almost 21 % of total CO₂ emission is from the transportation system, and almost 4 % of the CO₂ emission from the transportation system is from the airline industry.

So, the CO₂ emission from the airline industry is about 0.85 % of the total CO₂ emission in Japan.

This unexpectedly small contribution of the airline industry is due to two reasons. One is a relatively small contribution of air transport in the term of km-person comparing to the total transportation system including train etc.

Another reason is unexpectedly better fuel performance of the aircraft. For example, when Boeing 777-300 is operated between Sapporo and Tokyo which has 900 km distance, about 10,450 litters of fuel will be consumed. This implies that the fuel performance of the 777-300 is limited to only 0.086 km/litter. However, considering 470 passengers onboard, the total transport efficiency would be 40 km-person/litter (0.086 X 470).

This fuel performance were achieved by aircraft manufacturer's tremendous effort, answering to the airline requirements to reduce the fuel burn and then reduce the operating cost, and also enhancing the long range operation connecting the large cities in the world directly.

Same as cars or trains, aircraft fuel performance is affected by ; aerodynamic performance, engine's fuel performance, and airframe weight. Although recent improvement of the aircraft fuel performance seems to have achieved mainly by improvement of the engine performance, the other factors still have very important meanings.

Japan is contributing to the aircraft manufacturer's industry through supplying the major part of carbon composite which can reduce the aircraft weight.
Airlines are also making effort to reduce fuel burn by applying various methods. One example is to extend high lift device(flaps) and landing gears as later as possible, because these devices have large drags.
Another examples are ; to fly the shortcut route, to fly the fuel optimum route and/or fuel optimum altitude considering the wind , under the cooperation of the air traffic controller. Also, airlines are making effort to reduce the aircraft weight by adopting light weight passenger's seats and tableware etc.

４．Factors affecting the aircraft's fuel consumption

Same as cars or trains, aircraft fuel performance is affected by ; aerodynamic performance, engine's fuel performance, and airframe weight. In formula this can be expressed as follows.

Where: M is mach number, L is lift, D is drag, TSFC is fuel flow per thrust, W is weight

The L/D in the first term is usually called as “lift drag ratio”
If the aircraft weighing 400 ton is supported by thrust generated by four engines as shown in right hand figure, the required thrust per one engine is to be 100 ton. This is quite unrealistic, because the current biggest engine used for Boeing 777 can generate only 50 ton.

On the other hand, if the aircraft is supported by wing instead of thrust as shown in left hand figure, the required engine thrust is to balance the airplane drag. By the way, current usual aircraft have the lift drag ratio in range between 15 and 20, but let assume that the ratio is to be 20 for easiness of calculation.
In this case, the drag would be 20 ton (= 400/20), and required thrust per engine is reduced to 5 ton. And this is the reason why the airplane can fly.

Substituting the relation of L = W, and D = T to the L/D yield that L/D is equal to W/T, then, physical meaning of L/D (= W/T) can be understood as an index to show how many thrust will be required to carry an unit weight.
From this point of view, the L/D for trains, cars or ships will be very large because these weights are supported by ground or sea water, and the aircraft will give a bad impression in terms of the L/D.

However, each transportation system has each optimum speed range. Then it will be more fair to compare the efficiency of each system by using the index which is the product of the speed and the L/D. This is M X L/D shown in the first term in the formula. This can be understood as an index to show how many thrust will be required to carry an unit weight at a specific speed (then, for a specific distance).
Figure shows the L/D and the M X L/D for Boeing 747-400, which is one of the best masterpiece airliners in today's era.

747-400　の L/D と M×L/D

From the figure showing her M X L/D, it is obvious that cruise should be at mach 0.85, and actually she usually flies in that speed range. Further, as shown in the figure showing her L/D, the lift drag ratio will sharply reduced at between mach 0.85 and 0.9, it is due to the drag increase associated with shock wave formation.

５．Factors affecting Aircraft's Fuel consumption

Next term is the TSFC (Thrust Specific Fuel Consumption), and this is an index showing the engine fuel consumption per hour to generate a unit thrust, then it shows the efficiency of the engine. By the way, the engine efficiency is determined by two factors ; a cycle efficiency and a propulsion efficiency.

To improve the cycle efficiency, it is important to raise the compression ratio and burning temperature in the combustor. Jet engines equipped on the early era of jet airliner had a compression ratio around 16, and burning temperature around 1100 degrees C, but current engines have a compression ratio around 33, and burning temperature around 1300 degrees C to improve the cycle efficiency.
Generally, when the burning temperature rises, NOx tend to increase. Because of this, the technological development to reduce NOx is getting more common at the same time of maintaining the high burning temperature.

To improve the propulsion efficiency, it is important to reduce the exhaust speed. This is very similar to the rowing a boat; When rowing a boat with very slender oar the small amount of water will run very fast but the boat does not go forward, and if rowing a boat with very wide oar the large amount of water will run very slowly and the boat does go forward very easily. Then it is important to reduce the exhaust speed and increase the volume of the exhaust gas.
To achieve this, current jet engines have a very large fan just in front of the compressor. In this type of engine, only a minor part of air will pass inside the engine core, and majority will exhausted without passing the engine core, i.e., this part of air is merely bypassed. The ratio of the bypassed air to the air through the engine core is called as a bypass ratio, and current big engines have bypass ratios of 5 approx.

This was a breakthrough to improve fuel efficiency of the engines, and thanks for this engines, current large airliners are able to connect far distant cities directly.

６．Factors affecting Aircraft's Fuel consumption (Body weight)

Last one is the aircraft weight reduction. The strongest reason requiring the weight reduction is that the aircraft cannot fly if the weight is too heavy, but there is another reason requiring the weight reduction. That is, there is no gas stand in the air, and therefore each aircraft must carry the fuel by herself.

For example, The Boeing 747-400 flying to New York from Tokyo weighs about 375 ton at takeoff including fuel weight of 145 ton, and the fuel to burn is about 125 ton. (Remain fuel of 20 ton is reserve fuel)
Then, the takeoff weight is 375 ton and the landing weight is to be 250 ton (375 - 125), so the landing weight will be about 2/3 of the takeoff weight. This is quite different from cars; the weight of a car is about 1 ton and the weight of the fuel would be about 50 kg at maximum.

Considering the fuel flow of the Boeing 747-400 flying to New York from Tokyo, the fuel flow at the very beginning of the cruise would be about 12.6 ton/hr (15,700 litters/hr), and the fuel flow at the end of cruise (just before descending to New York) would be 8.4 ton/hr (10,400 litters/hr).
As these values indicate, the fuel flow varies very largely as the aircraft weight changes. This is due to the fact that aircraft must carry the fuel to be burned in a later period; 5 hours or 10 hours later.

And this means that the fuel flow will increase if the aircraft weight increases, as the fuel to be carried by herself will also increase, then, the aircraft weight will again increase and the fuel flow will again increase, i.e. the weight of the aircraft will continue to increase in a vicious circle.
This is a reason why the aircraft manufacturers are making tremendous efforts to reduce the aircraft's empty weight.