Wingtip propellers

The generation of lift with finite-span wing inevitably produces lift induced drag. Due to the three dimensional nature of the flow around the finite-span wing, the difference in pressure between the upper and lower surfaces of the wing forces the air flow to curl around the wing tip resulting in wingtip vortices. The circulation around the wing tips increases the downwash velocity just behind the wing trailing edge – in particular near wing tips – without producing additional increase in lift. On the contrary, this increased downwash velocity lowers the effective angle of attack of the wing and thus

requires a physical increase in wing angle of attack to maintain the required total lift of the wing. This change in angle of attack will result in induced drag also referred to as vortex drag.

Winglets are commonly used on current aircraft to decrease the lift induced drag.

This additional device at the wing tip extends the wing vertically without increasing its span. The vertical extension provides an increase of the effective aspect ratio of the wing [80] and therefore reduces the lift induced drag by further approaching the two-dimensional flow. Depending of the type of winglets, a 3-6% reduction of the total drag in cruise is usually achieved [17].

In the past, many wingtip devices have been studied: end plates [105], wing grids [65], spiroid wing tips [50], wing tip turbines [90] [48], wing tip blowing devices [68] and others. In particular, a wingtip propeller can be used as a wind turbine in order to harvest some energy from the vortex and attenuate its strength at the same time. This results in additional available power and a decrease of drag due to lift [95] [2].

But rather than adding some extra systems, can it be winglets or wingtip turbines for energy harvesting, several studies [96] [124] suggest to direct the high energy mass wake of a main aircraft powerplant into the tip vortex in order to interrupt the vortex core axial flow. This dissipates the vortex and results in a decrease in induced drag. In particular Snyder et al. [124] conducted a series of exploratory wind tunnel tests on a wingtip propeller in a tractor configuration. The results of the study confirmed the theoretical prediction that the use of a rotor turning in the direction opposite to that of the tip vortex produces a simultaneous lift increase and drag decrease. Rotating the wing tip propeller in the same direction to that of the tip vortex leads to opposite results. The rotational component of the propeller slipstream is then available for amplifying or attenuating the wing vortex system. In other words, wingtip propellers could be used as an active control of the lift over drag of the aircraft through the variation of the induced drag. This seems very attractive, as low values of induced drag are generally targeted for take-off and climb while approach and landing requires high values of induced drag.

Patterson Jr and Bartlett [93] [94] also conducted experimental studies on wingtip mounted propellers in a pusher configuration. Placing the propeller just behind the wingtip increases the propulsive efficiency of the propeller as a result of the influence of the wingtip vortex flow, and simultaneously attenuates the wingtip vortex by injection of the propeller wake into the vortex to reduce induced drag.

However, mounting all the propulsive power of the aircraft at the wing tips also raises possible disadvantages. While the wing of an aircraft with conventionally installed

powerplant near the center wing box is generally sized by positive load factor cases, a wing with tip-mounted powerplant would probably be sized by negative load factor cases (such as hard landing). Indeed, placing the engines and propellers at the wing tips will relieve the wing shear and bending moments under positive load factors but will increase them under negative accelerations. In addition, the wingtip powerplant installation highly changes the torsional moment of inertia of the wing which could result in aeroelastic problems due to unfavorable coupling of bending and torsional modes of flutter or vibration. For these two previous reasons, the structural design of the wing must be carefully considered in the evaluation of such aircraft configuration.

Finally, having all the propulsive power installed at the wingtips can lead to impossible trimming of the aircraft in case of engine failure. In order to avoid this situation the propulsive power must be redistributed. Electrical power transmission provides a great flexibility for this purpose. Wingtip propellers are therefore considered in different hybrid aircraft concepts by NASA [19](Figure 1.23) [9](Figure 1.28). Borer et al. [19]

calculated a 5-10% improvement of lift over drag in cruise coming from the wingtip propellers of the X-57. They also pointed out that this first estimation most probably under predict the effect due to the method used. Antcliff and Capristan [9] expect an increase in effective propulsive efficiency (which includes the induced drag reduction) of 18% for the wingtip propulsors on the PEGASUS concept.

Fig. 1.28 NASA PEGASUS concept [9]

At that time, no simple aerodynamic model can reflect the potential benefits of the wingtip propellers listed before with a relatively good level of accuracy. For this reason, placing the main propellers at the wing tips has not been studied in this thesis and will be considered in the prospects for future works.