Design considerations

One design problem is to find a compact layout, as in Fig. 7.2, where the passenger cabin can he fitted properly into a wing of given planform. A parameter which may be used to characterise this aspect is N/S, where N is the number of passengers to be carried and S the wing plan area. We also make the assumption that the plan area of the cabin is related to the number of passengers, e. g. by Sc = 0.7N[m^] . This passenger density can be determined from (7.5) for given values of the wing loading. It turns out that, since Wp/W reaches maximum values at certain values of W/S, it also reaches maximum values at certain values of N/S, i. e. it is not worthwhile to make N/S as large as possible. The optimum values of N/S lie between about 0.6/m^ and 0.8/m^, depending on the value of the specific structure weight factor ui^ , Lighter structures allow a lower passenger density. Again, the maximum of Wp/W is very flat, and, if the optimum values of N/S should be too large for practical purposes, some departure from them need not result in a large payload penalty. Practicable values of N/S may lie below the optimum values, between 0.5/m^ and 0.6/m^. The wing loading must then be adjusted accordingly to slightly lower values than the optimum values in Fig. 7.4. It should thus be possible to find a satisfactory solution to the layout problem. The design of the cabin itself is then mainly a structural problem.

The balancing problem is closely related to the layout problem. It should be somewhat easier than that for a slender supersonic aircraft. Again, the fore – and-aft position of the engines may be used to balance the aircraft about the position of the low-speed aerodynamic centre.

Equation (7.5) can also be used to work out how the payload fraction depends on the value of the effective aspect ratio A/K , for different passenger

densities and structure weights. All the results show the same trend: a slight improvement as A/К is increased, and a levelling out and no further improvement beyond about A/K = 2 . We draw from this the important conclu­sion that the values of A/К of interest lie between about 0.5 and 2, i. e. the values of s/l lie roughly between 0.25 and 0.5. This is a belated justification of the assumption made at the beginning: that we are dealing with slender wings. Thus the various design limitations of the allwing aero – bus lead again to a region of no conflict, like that for supersonic slender aircraft in Fig. 6.72, but this region is now larger since the supersonic cruise restriction has been removed.

It is still beneficial to achieve values towards the higher end of the A/K – range: this will allow lower wing loadings and lower CL-values on the airfield, and also lower take-off thrusts. But, again, it is not worthwhile to strive to go beyond about A/K = 2 . In the aerodynamic design, therefore, the methods discussed in Chapter 6 can be applied again. In particular, the information in sections 6.5, 6.6, and 6.9 can be used, with the main data on lift, drag, and stability in Figs. 6.48, 6.49, and 6.51. The warped wing in Fig. 6.58 was designed with an application to an allwing aerobus in mind.

As yet, no such aircraft has been fully designed and built; but there is no doubt that the early studies of S В Gates and G H Lee were on the right lines. Much more work needs to be done on all aspects of the design to establish the usefulness of the concept more firmly. But the available information indicates that the prospects are promising and that a slender allwing aerobus could use­fully fill a gap in the range of air transports.