Roskam Airplane Design Part Iv Pdf 14
The simulation results show that the electric motors can have a maximum power output of 68kW in the case of a maximum passenger weight of up to 132kg. If the electric motors are assumed to have a maximum power output of 50kW (worst case scenario), the maximum electric motor power can be increased by 4kW in this case. In the majority of the aircraft designs, electric motor power can be increased by 20% in order to keep the electric motor power output in line with the 50kW maximum power output of the electric motors. Note that this power increase is only needed for passenger comfort. The electric motor performance does not change when more electric motor power is available in order to reduce the number of passengers and therefore maximize comfort. The maximum passenger weight of 132kg is fairly realistic because this value is only slightly over the maximum weight. Based on this, it is estimated that the electric motors can provide a total power output of 3180kW (50kW + 68kW) in this worst case scenario. It should be noted that the turboprop design is not intended for short flights. The X-Plane will therefore be limited in the number of aircraft registered to 3 in this simulation study. This also implies that the passengers should be equipped with parachute systems in case a high altitude emergency occurs.
The optimal power split for the electric motors is determined through the power optimization process. This is done with respect to passenger comfort. This can be seen in the results of the CVT study of the X-Plane designs in Fig. 11. It can be seen that the optimum power split lies between 35% and 55%. The corresponding MTOM values can be found in Table 7. If electric motor power is too low, the air resistance is significantly increased, which is also evident in Fig. 11. If electric motor power is too high, the drag and therefore the passenger comfort level are significantly lowered. The power peak of the electric motors is obtained when the electric motor power is as high as possible. The air resistance is increased and passenger comfort is improved when the electric motor power is increased above the optimum. The next step is therefore to find the optimum passenger comfort level. The probability density functions for lift and drag (for the passenger seat) are normalized based on the areas under these curves. The mean values are calculated as a function of the electric motor power. The results are shown in Fig. 12. It can be seen that comfort can be improved by a factor of about 1.5 when electric motor power increases from 35% to 55%.
If the same engine is used for both the propeller and turboshaft engine, a gear box is necessary to share the propeller shaft between the turboshaft engine and the electric motor. The overall propulsive thrust is determined by the gas turbine engine and the electric motor, whereas the mass specific power (W/kg) of the turboshaft and electric motor determines the maximum propulsive thrust. As an example, in Fig. 3, the results of the propulsive performance for the basic ATR-72 model are displayed. Three different propulsive modes for a hybrid electric turboprop aircraft can be distinguished: Plug and play, pure electric and combined electric/gas. The pure electric mode corresponds to the situation, where the electric motor drives the propellers directly. The combined electric/gas mode corresponds to a situation where the gas turbine engine is connected to the propeller shaft to power the propellers. The pure electric mode is expected to be the optimal solution for regional aircraft. The combined electric/gas mode is expected to be the optimal solution for commercial airplanes. The plug and play mode should be used in case there is no need for the basic configuration, such as maintenance considerations or if a further enhancement of the hybrid system is required during the life time of the aircraft (e.g. variation of propeller geometry or blade pitch).
As aircraft operating conditions increase, the turbine load decreases which leads to a significant loss of thrust. This is particularly important for turboprop engines because the hot gas temperature in the turbine is significantly lower than in a turbojet (hot compressor-cold turbine). Furthermore, at a given turbine inlet temperature and hence thrust, the nozzle exit area is significantly smaller in the turboprop engine than in a turbojet. These two factors can be seen as an enlarged pressure ratio in the turbine cycle. Turboprop engines are known to be quite sensitive to pressure ratio, and hence it is important to work closely with the turboprop engine manufacturers and consult them on the design. This is also the case for the turboshaft engine used for the hybrid electric configuration. For a basic configuration, the various configurations of the turboshaft engine will be evaluated, and subsequently the most suitable configuration will be selected for additional parameters are taken into account (e.g. the components available on the market, manufacturing cost). For optimal performance, the gas turbine needs to operate at its best efficiency (minimized fuel consumption) and the electric motor should be able to deliver full power for the purpose of propulsive power. In this case, the gas turbine is used to maximize the electrical efficiency at the electric motor output. The electric power splits of the basic hybrid configuration are evaluated. A minimum of a 80% turbine load requirement is expected to be the primary target for the power splits. This corresponds to an electric motor power of about 12 kW. A power split of 60%/40%/20% was chosen for the basic configuration.
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