CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from British Patent Application No. GB 1818684.1, filed on 16 Nov. 2018, the entire contents of which are herein incorporated by reference.

BACKGROUND

Technical Field

This disclosure relates to boundary layer fan systems for aircraft.

Description of the Related Art

Boundary layer ingestion (often abbreviated to BLI) is a technique that may be used to improve aircraft propulsive efficiency by reducing downstream wake and jet mixing losses. In practice a designer must try and exploit this benefit without adding excessive weight, drag or inefficiency into the aircraft or propulsion system.

Whist in theory the maximum benefit arises if all the aircraft's boundary layers are ingested, in practice it is only practicable, without distributed propulsion, to ingest the boundary layers which are most concentrated. On a conventional tube and wing aircraft this means ingesting the fuselage boundary layer only. Fuselage BLI is therefore considered the most likely embodiment of BLI for the next generation of aircraft. Typically, in straight and level flight at optimum lift-drag ratio the fuselage drag is responsible for around 20 to 25 percent of the total aircraft drag.

The propulsive efficiency of an aircraft propulsion system relates the power input into the flow, Pflowrelative to the propulsive power which is the product of the thrust, T and the flight velocity, v∞:

ηp=PpropulsivePflow=(Tv∞Pflow)[Equation⁢⁢1]

If all streams are evaluated at ambient pressure then the power into the flow is the difference between the kinetic energy of the jet and inlet streams:
Pflow=½{dot over (m)}(vj2−vi2)  [Equation 2]
whilst the thrust is the difference between the jet and inlet momentum:
T={dot over (m)}(vj−vi)  [Equation 3]

Combining Equations 2 and 3 with Equation 1 gives:

ηp=2viv∞+vjv∞[Equation⁢⁢4]

In a conventional engine where vi=v∞, i.e. the inlet velocity is equal to the flight velocity, then the propulsive efficiency must be less than or equal to one (ηp≤1). The limiting case of ηp=1 occurs when the jet velocity matches the inlet velocity; however this is unachievable with a finite propulsor size that produces thrust.

In the case of BLI, vi<v∞. With a depressed inlet velocity, the left-hand term in the denominator of Equation 4 reduces. The propulsive efficiency increases and can even exceed one. For a fixed specific thrust (vj−vi=constant), both terms in the denominator reduce. For a given inlet velocity, the propulsive efficiency increases further.

For the propulsive efficiency to exceed unity a dissipation that would otherwise have occurred must have been prevented. The origin of this dissipation can be seen by rearranging the above equations to obtain the lost efficiency, (1−ηp):

1-ηp=⁢PdissipatedPflow=⁢(12⁢(vj-v∞)2-12⁢(vi-v∞)2Pflow)[Equation⁢⁢5]

Equation 5 may be interpreted with and without boundary layer ingestion.

In the case of a conventional engine where vi=v∞, only the left-hand term is non-zero. The lost work is therefore the kinetic energy excess that is dissipated in the jet downstream.

In the case of BLI where vi<v∞, the right-hand term is now non-zero due to a depressed inlet velocity. This represents prevention of dissipation that would otherwise have occurred in the wake. Again, for a fixed specific thrust (vj−vi=constant) the left-hand term also reduces whilst vj>v∞.

Whilst significant simulation and prediction has been performed in multiple research programmes to quantify the theoretical benefit of BLI, little published work to date has investigated the required aerodynamic design of a turbomachine that may be used in a fuselage BLI installation on a conventional tube-and-wing aircraft.

SUMMARY

In an aspect, there is provided a boundary layer ingestion fan system for location aft of the fuselage of an aircraft, comprising:

a nacelle defining a duct;

a fan located within the duct, the fan comprising a hub arranged to rotate around a rotational axis and a plurality of blades attached to the hub, each of which has a span from a root at the hub defining a 0 percent span position (rhub) to a tip defining a 100 percent span position (rtip) and a plurality of span positions therebetween (r∈[rhub, rtip]); and

a hub-tip ratio of the fan, defined as the ratio of the diameter of the hub to the diameter of the fan measured at the leading edge of the blades, is from 0.45 to 0.55.

This increases the blade speed in the hub for a fixed tip Mach number, which reduces hub diffusion to tolerable levels in this application.

