Question

A3: Explain how graphs shown below assist you to make decision about the type of propulsion system suitable for an aircraft with maximum speed of M<0.7. (Note: First describe what information can be obtained from the graphs, then, explain your answer by reference to the graphs.)

Answer 1

The two graphs (Figures 1 & 2) are typical plots between

Figure 1: Flight Mach Number Vs Thrust Specific fuel consumption (1/hr)

Figure 2: Flight Mach Number Vs Engine Maximum Thrust to Weight Ratio

The Thrust Specific fuel consumption (TSFC) [1/hr] and Engine Maximum Thrust to Weight Ratio (T/W) [Dimension-less] is typically ranged based on the flight mach number for the following propulsion systems:

1. Piston Engine/ Propeller
2. Turbojet
3. Turboprop
4. High By-Pass ratio turbofan
5. Low By-Pass ratio turbofan
6. After-Burning turbojet
7. After-Burning turbofan
8. Ramjet
9. Rocket

Thrust Specific Fuel Consumption: The Thrust Specific Fuel Consumption also abbreviated as "TSFC" signifies the amount of fuel the engine burns per hour divided by the net thrust. For jet engines the TSFC is generally measured in pounds of fuel consumed per hour (lb/hr) per pound of thrust (lb)

TSFC = $\frac{\frac{lb}{hr}}{lb}$ =

hr

The Foot Pound System (FPS) is followed in the above equation.

The main performance factor to note is that,

• Low Thrust Specific Fuel Consumption (TSFC) = High Efficiency
• High Thrust Specific Fuel Consumption (TSFC) = Low Efficiency

Now arranging the propulsion systems based on their TSFC in ascending order (Low to high): (Based on the information provided in the graph or Figure 1)

Piston Engine/ Propeller < Turboprop < High By-Pass ratio turbofan < Low By-Pass ratio turbofan < Turbojet < After-Burning turbojet < After-Burning turbofan < Ramjet < Rocket.

Now getting to the figure 2 i.e. Engine Maximum Thrust to Weight Ratio. As the altitude is not provided it can be considered at sea level which signifies the maximum thrust an engine will be capable of producing. Thus it is mentioned as Maximum Thrust to Weight Ratio (T/W). The Maximum Thrust to Weight Ratio (T/W) is a dimensionless quantity

$\frac{T}{W} = \frac{N}{N}$

Here N is the Newton, as both the Newton's gets canceled out this becomes a dimensionless quantity.

The main performance factor to note is that,

• The Thrust to Weight Ratio (T/W) is directly proportional to acceleration i.e. If the (T/W) is high, the aircraft will accelerate more quickly, capable of climbing more rapidly, has higher turn rates, will be able to reach higher speeds.
• But higher the Thrust to Weight Ratio (T/W), more fuel is consumed by the aircraft. This increases the overall weight of the aircraft.

Now arranging the propulsion systems based on their Thrust to Weight Ratio (T/W) in ascending order (Low to high): (Based on the information provided in the graph or Figure 2)

Piston Engine/ Propeller < Turbo-prop < High By-Pass ratio turbofan < Turbojet < Low By-Pass ratio turbofan < After-Burning turbojet < After-Burning turbofan < Ramjet < Rocket.

As the information from the graphs (Figure 1 & 2) are well established, now we can move on to make decision about the type of propulsion system suitable for an aircraft with maximum speed of M<0.7 (M-Mach Number).

As per the aircraft's requirement, the type of an aircraft is be identified as "Subsonic aircraft" as M<0.7.

First we can eliminate the propulsion systems which do not belong to this operational condition. The following engines can be eliminated as their operation condition is irrelevant to the required operational condition:

1. Ramjet (Engine cannot be operated efficiently below M<3)
2. Rocket (High TSFC & (T/W)) means lower efficiency.
3. After-Burning turbofan (Generally afterburners are not used in case of subsonic aircraft due to its higher fuel consumption)
4. After-Burning turbojet (Generally afterburners are not used in case of subsonic aircraft due to its higher fuel consumption)
5. Turbojet (Turbojets can also be neglected because of its lower efficiency and higher maintenance cost)

With this the following options of propulsion systems remain

1. Piston Engine/ Propeller
2. Turboprop
3. High By-Pass ratio turbofan
4. Low By-Pass ratio turbofan

Deciding Factor:

• Low Thrust Specific Fuel Consumption (TSFC) = High Efficiency

In general it is best to choose the propulsion system with the lowest TSFC which can provide optimum Thrust to weight (T/W) ratio. This practice is followed for adhering better efficiency, as efficiency is the major deciding factor in subsonic plane. The aircraft may not perform maneuvers like a fighter jet, but can efficiently travel from one location to other with minimum fuel-consumption.

Typically the thrust produced should be greater than the weight of the aircraft, so that the aircraft can lift. But in general practice the lower (Thrust to weight (T/W) ratio) can be compensated by adding a lifting surface such as wing. With (T/W)>1 the aircraft will be capable of taking-off vertically. This (VTOL) Vertical Take-off capability is not generally required in general aviation subsonic aircraft. Thus (T/W) is a secondary factor, this makes the Thrust Specific Fuel Consumption (TSFC) the primary factor deciding the type of engine. It is best to opt for a propulsion system which holds the lowest position in the graph (Figure 1):

Based on this following two propulsion systems can be shortlisted with the assistance of graph:

1. Piston Engine/ Propeller
2. Turboprop

Let us consider the advantages and disadvantages of both the propulsion system:

Piston Engine/ Propeller

Advantages of Piston Engine/ Propeller

• Substantially cheaper
• Lower Thrust Specific Fuel Consumption (TSFC) than Turbo-prop Engine.
• Higher efficiency
• Lesser Fuel Used

Dis-advantages of Piston Engine/ Propeller

• Heavier
• Higher noise level
• Less Reliable

Turbo-prop

Advantages of Turboprop

• Lighter
• Reliable
• Quieter
• Can produce additional thrust in-terms of engine exhaust additional to the propeller thrust

Dis-advantages of Turboprop

• The turbo-prop engine generally uses more fuel for generating the same horse-power or thrust of a piston engine.

Final Decision of power-plant:

Based on the comparative analysis in terms of graphs, advantages and disadvantages, a "Turboprop propulsion system" is suitable for an aircraft with maximum speed of M<0.7. Though the piston engine has the advantage of lower TSFC, its increased weight, high noise level and lesser reliability makes us choose the "Turboprop propulsion system" over the piston engine.

The disadvantage of higher fuel consumption in "Turboprop propulsion system" is substantially compensated with the lesser engine weight, which means more fuel can be carried on board. Additional advantages of Reliability,
quieter operation and ability to produce additional thrust in-terms of engine exhaust in addition with the propeller thrust, makes "Turboprop propulsion system" an ideal choice for an aircraft with maximum speed of M<0.7.

With advancements in "Turboprop propulsion system" technology, it is gradually replacing the traditional piston engines used in business jets, helicopters and shorter range commuter airplanes.