Individual Propulsors

The previous section presented the overall performance of the baseline propulsion system design at a number of key operating points. It is also useful to break down the performance

0.0 2.0 4.0 6.0 8.0 50% 60% 70% 80% 90% 100% N PF (k N ) Fan RPM Centreline Outer Edge

(a) Net propulsive force.

250 275 300 325 50% 60% 70% 80% 90% 100% SPC (W /N ) Fan RPM Centreline Outer Edge

(b) Specific power consumption.

Figure 3.29: ADP performance of propulsors at the centreline and end of the propulsor array as a

function of rotational speed.

of individual propulsors in the array to identify the differences in performance as a function of the propulsor’s location. Whilst each propulsor is sized to produce the same net propulsive force at design point, they will perform differently at off-design conditions due to differences in propulsor size and location. To demonstrate this difference, the performance of two propulsors in the array, the centreline propulsor and the propulsor at the far end of the array, was simulated at the ADP flight conditions (Figure 3.29) and sea level static (Figure 3.30) for a range of fan rotational speeds.

Both propulsors are sized for the same NPF at design point, therefore, the reduction in net propulsive force as the fan RPM reduces is very similar (Figure 3.29a). However, the centreline fan produces slightly more net propulsive force than the end fan as the rotational speed reduces. Both fans operate on the same fan map (scaled to their respective non-dimensional mass flows). Their running line is therefore similar, with the same relationship between rotational speed, fan pressure ratio, and efficiency. The difference therefore arises due to the different flow characteristics of the two propulsors. Net thrust produced by the propulsors reduces for both propulsors as the rotational speed is reduced. However, as the ratio of h/δ for the the centreline propulsor is lower, its momentum drag is lower (lower average velocity at station i). The centreline propulsor is therefore able to produce more net thrust than the outermost propulsor. As the fan at the centreline has a lower ratio of h/δ and a higher propulsive efficiency than a propulsor at the end of the array, the SPC of the centreline propulsor is consistently better than propulsors further along the span (Figure 3.29b). Reducing the rotational speed of the propulsor reduces its power consumption and increases its propulsive efficiency (due to a lower exhaust velocity), hence SPC improves as the rotational speed is reduced. In addition, h/δ decreases as the fan rotational speed reduces, due to a lower mass flow demand and hence a lower mass flow ratio. A minimum SPC point becomes apparent for the propulsors at the edge of the array. This is similar to the trend which would be observed for a typical turbofan engine, where a minimum SFC point can be found as engine power is reduced.

At sea level static, the differences between the two propulsors reduce (Figure 3.30). The aircraft is static and the airframe boundary layer is negligible, meaning location-specific differ- ences do not play a part in performance. In addition, the high mass flow ratio of sea level static operation means that the ratio of h/δ is high enough to make any ingested boundary layer negligible in comparison to the ingested free-stream flow. As both propulsors are effectively operating with free-stream air at a Mach number less than 0.35, the inlet total pressure recov- ery predicted from Seddon’s method is 100%. Therefore, the only difference in performance

3. Propulsion System Modelling 0.0 10.0 20.0 30.0 40.0 50.0 60.0 50% 60% 70% 80% 90% 100% N PF (k N ) Fan RPM Centreline Outer Edge

(a) Net propulsive force.

60 70 80 90 100 110 120 50% 60% 70% 80% 90% 100% SPC (W /N ) Fan RPM Centreline Outer Edge

(b) Specific power consumption.

Figure 3.30: Sea level static performance of propulsors at the centreline and end of the propulsor array

as a function of rotational speed.

between propulsors is the difference resulting from their size. As the performance benefits of the boundary layer do not contribute at static conditions, both propulsors have the same trend linking the decrease in rotational speed to a decrease in specific power consumption (Figure 3.30b). However, the difference between the two propulsors is apparent in their net propulsive force. The propulsor at the array end is larger than the centreline propulsor, as it is sized for a larger non-dimensional mass flow in order to produce the required net propulsive force at ADP. The propulsor at the array end is therefore able to produce more thrust than the centreline propulsor at sea level static conditions.

As the free-stream flow speed increases, the boundary layer begins to play a larger part in performance and the efficiency of the propulsors in the array begins to diverge (Figure 3.31b). As with the change in rotational speed, the centreline propulsor is more efficient, due to the ben- efits of ingesting a thicker boundary layer. As the flight velocity reduces, the capture area ratio of the propulsor increases. This is matched by a similar increase in the ratio h/δ , which tends the inlet flow characteristics, and hence performance, towards that of a free-stream propulsor (Figure 3.32). Flow characteristics as modelled are asymptotic to their free-stream values, how- ever, for the simulated flight altitude of 30,000 ft, average total pressure and velocity reach 99% of their free-stream value at a mass flow ratio of approximately 4, and 99.9% of their free-stream value at a mass flow ratio of approximately 50.

As the rotational speed of the fan is reduced, its mass flow demand also reduces. A low mass flow demand (below 1.0) implies an adverse pressure gradient, which leads to the risk of separation of the boundary before the inlet. The mass flow ratio is also linked to operating conditions (altitude and Mach number), with an increase in speed leading to a decrease in capture area ratio. For the simulated configuration of the propulsor array, the propulsors at the outer edges operate at a lower capture area ratio. However, the outer propulsors also operate with a thinner boundary layer, which may counterbalance the separation risk. The risk of boundary layer separation at low capture area ratio must be addressed for BLI propulsion systems operating at high velocity and low power settings, such as flight idle during descent.