Summary:

Three features combine interdependently to give very low drag and low weight:

1 an axisymmetric retractable tail-cone, which eliminates base drag,
2 convergence of the cabin sidewalls rearwards, over the full cabin height; and
3 the entrances to the front row of seats have been moved rearwards, to the second row of seats, to give continuous structural sidewalls uninterrupted by doorways. This leads to low weight. The roof is domed, both for excellent aerodynamics and for ease of access from the second to the front row of seats.

It is considered that the fuel consumption and the required engine power could each be reduced to about a half of conventional, because of the combined very low drag and low weight. The manufacturing costs should be low, because of the low stresses in the continuous structural sidewalls and the small size of the engineering components. The performance should be high.



(Please excuse the slightly light hearted Introduction.)


Well, well ….. what “problems” there are with these changes:-

Imagine a car with an axisymmetric retractable tail-cone, which tapers to a point at its trailing edge, and which fairs-in cleanly at its join to the rear of the cabin. “Not possible” you might say, “the shape does not fit at the join.” That’s right, or at least it is for existing cars.

Imagine a car with the cabin width decreasing rearwards, so that the cabin, externally, is considerably narrower at the rear than it is at the driver’s seat, over its full height. “Yes”, you might say, “and that might be made to fair-in quite nicely with the axisymmetric tail-cone, but what about the rear wheels? Either they will need to be reduced on pitch, such that the car loses stability, or they and their suspension and fairings will cause very bad blockage just as the flow approaches the tail-cone, such that you will lose much of your aerodynamic advantage. Effectively, you will have converged only the upper part of the cabin sidewalls, with the lower part still being the large fairing around the rear wheels.” Yes, of course.

Imagine a car with the entrances for the driver and front passenger placed immediately behind the front seats, instead of right at the front seats, with no routine entrance right at the front seats, but instead continuous structural sidewalls in place of the conventional doorways. Imagine the improvement in stress distribution during a front impact on a car of such a design when the load can be taken directly through fixed light structure built into the position of each present front doorway. Imagine, also, the greater stiffness of such structure making it much more compatible with an extremely light, “near-shell”, type design for the rest of the car. “Well, that whole arrangement is even more absurd,” you can be heard to say, “for how would the driver and his favoured passenger get into their seats?”

In a sense of fairness you may add that, overall, the above proposals include a light and stiff tail-cone, because the tail-cone has been specified as axisymmetric and reduces to a point at its trailing edge. Such a design of cone, you accept, could be a good protection against aeroelastic excitation of the cone, the cone would be of low cost, and it would also allow the car to turn rather sharply without getting its tail-cone into the way of other traffic - but, really, nothing else seems to fit!


Until all the changes are combined!

When all the changes are combined there is a circle of interdependence between them. The circle will be broken for the convenience of the discussion, by assuming at the start that the rear wheels and suspension will be very much slimmer than conventional. The circle of interdependence will now be discussed starting from that situation.

The interdependence can be put broadly as 4 effects:

(i) the slim rear wheels and suspension make practicable the convergence of the cabin sidewalls rearwards, in a manner that is both over the full cabin height and has all of the cabin sidewalls exposed to the flow;

(ii) the slim rear wheels and suspension, together with the above cabin convergence, makes the axisymmetric tail-cone practicable;

(iii) the above cabin convergence, together with the axisymmetric tail-cone, makes the dome practicable, and therefore also the rearward movement of the front doorways;

(iv) the rearward movement of the front doorways, together with the presence of the axisymmetric tail-cone, makes the slim rear wheels and suspension practicable.

And so back to item (i).

These relationships will now be considered in turn.


(i) The slim rear wheels and suspension make practicable the convergence of the cabin sidewalls rearwards, in a manner that is both over the full cabin height and has all of the cabin sidewalls exposed to the flow.

The car proper, ie without its tail-cone, is taken to have about the same overall dimensions as a medium to large “estate” in the current European market. That range of size and proportions has been chosen in order to provide a fairly generous length for streamlining, while remaining within the accepted sizing of the market.

With the above size and proportions the lower part of the body shape as shown in the diagrams is generated by first converging rearwards the cabin sidewalls, starting from immediately aft of the front seats. If, now, the rear wheels are ignored, an edge radius, between the sidewalls and the undersurface, starts, from about zero immediately behind the front wheels, and increases until the whole cabin section has become about circular at the junction with the axisymmetric tail-cone. The geometry would be straightforward for no rear wheels. On the other hand, for fully pitched rear wheels with each wheel wide, ie thick, in its own right , the wheels and their suspension and fairings would probably be so wide that they would still merge into the converged cabin sidewall. Effectively therefore, over the lower part of the car, no convergence of the flow would have been achieved. However, if, on the other hand, the rear wheels, suspension and fairings were all very slim, but of full pitch for stability, the whole “module” on each side of the car would fit into the space available and yet leave width for an airflow passage between the wheel fairings and the cabin walls. It may be noted that the geometry is eased by the large radius at the lower edge at that position, where the nominal section of the cabin will have become close to circular as described above. Also, some indentation in the region local to the wheels is put into the cabin sidewalls and lower surface in order further to ease and optimise the flow.

It therefore seems reasonable to confirm the statement (i), that the slim rear wheels and suspension make practicable the convergence of the cabin sidewalls rearwards, in a manner that is both over the full cabin height and has all of the cabin sidewalls exposed to the flow.

(ii) The slim rear wheels and suspension, together with the above cabin convergence, makes the axisymmetric tail-cone practicable.

From the above discussion the slim rear wheels allow the cabin side walls to match the tail-cone at their junction. In addition, with the lengths that are available for the size of car as indicated above, one would expect to be able to streamline the fairings over the rear wheels in a reasonable manner, given the starting point that the wheels are slim.

The situation is therefore that the car proper matches the geometry of an axisymmetric retractable tail-cone very well, for an initial diameter at the junction between the tail-cone and car as chosen and indicated in the diagrams. The chosen diameter seems to the writer’s judgement to be in the sort of region that would be ideal. The diameter has been made about as small as would seem reasonable relative to the width of the car at the driver’s seat, given the need for good streamlining and some allowance for imperfections and sidewind. The resulting length attainable for the cone would then be about a minimum. The advantages of being axisymmetric have been mentioned above. The tail-cone is thought of as having a tough plastic sheet construction, lightly pressurised for extension and with a network of “bungee” chords for retraction, with the whole probably in about 4 sections, and with small supplementary additions where the shape needs to depart slightly from strictly axisymmetric. The tail-cone would be extended and retracted probably in the region of 40 to 50 mph.

It therefore seems reasonable to confirm the statement (ii), that the slim rear wheels and suspension, together with the above cabin convergence, makes the axisymmetric tail-cone practicable.

(iii) The above cabin convergence, together with the axisymmetric tail-cone, makes the dome practicable, and therefore also the rearward movement of the front doorways.

The rearward convergence of the side walls gives a reduction in the car base area, while the axisymmetric tail-cone continues the reduction of the base area to zero. Consequently there is no longer a necessity, from the point of view of aerodynamic base drag reduction, either to keep the height of the roof to a minimum in its forward area, or to reduce it as much as possible in the rearward area. It is more important to use a roof shape that promotes efficiency in the overall aerodynamics and, in particular, in the diffusion flows of the side walls and tail-cone. Now, it can be argued that both the overall aerodynamics and the diffusion flows will be improved if the streamwise curvatures of the roof are kept small – basically because the roof forms an “end wall” when the toal aerodynamic field of the car plus its reflection in the ground is taken into account. The central meridional plane of the roof is therefore allowed to continue to rise rearwards from the top of the windscreen, and, then, to curve only very gradually to a downward slope sufficient to fit the tail-cone, as in the diagram. The central meridional region of the roof at the fore and aft position a little behind the driver’s seat is therefore considerably higher than in a conventional car. The region forms a dome.

The effect on the centre of gravity would be small as the roof weight is usually small. The effect on side wind loads would need attention, but would probably be kept reasonably low by the rounded transverse shaping of the roof and windscreen.

As a result of the dome, movement for the driver and passengers can be made to be very easy between the front and rear seats. The more rearwards entry to the front seats is therefore practicable. Consequently, the required conclusion again seems reasonable, in this instance that (iii), the above cabin convergence, together with the axisymmetric tail-cone, makes the dome practicable, and therefore also the rearward movement of the front doorways.

(iv) The rearward movement of the front doorways, together with the presence of the axisymmetric tail-cone, makes the slim rear wheels and suspension practicable.

The discussion for item (iii) has established that the rearward movement of the front doorways is practicable. The very light but stiff structure that is then built into the position of the conventional front doorways, giving continuous structural sidewalls, would also therefore be practicable. The structural sidewalls, for the purposes of resisting front impact, would effectively have become continuous for the full length of the car.

The total structure in that region can be both lighter and stiffer than for a conventional car. Conventionally, loads, such as during front impact, have to be transmitted around the doorway using heavy structures, probably in steel, and probably with the load in that region acting partly in bending. The new structure would be in direct compressive loading during front impact. It would therefore have only relatively small deflections compared with the conventional structure. As a result, it would be much more compatible with the small deflections of an ideal and extremely light shell, if such could be used for the total main structure of the car.

Now it will be seen from the sketches that the lines of the structure can run nearly straight, in the position that conventionally would have the sharply bending upper frame of the front doorway. The whole structure of the car can therefore be made with very good load lines, further supporting the use of a light and rigid type of shell for the main structure.

Furthermore, the edge radius, or curvature, laterally, between the roof and the sidewalls, is made very gradual, continuing a correspondingly curved windscreen. This would be mainly for the same sort of aerodynamic “end wall” reasoning as has been applied to the streamwise roof curvatures, above. It is also because of the eventual need to join with the circular sectioned cone. Passengers’ heads are still cleared, despite the large edge radius, because of the additional height at the dome. Such gradual curvatures at the “edges”, giving curvatures everywhere that are greater than conventional away from the edges, would further favour a light shell type structure.

With all of the above advantages it seems reasonable to expect that the overall design for the car would move much closer to the ideal and extremely light shell. The shell would perhaps be made even more closed by manufacturing any non-opening windows in a semi-structural material and bonding them to the shell.

From the above discussion the rearwards movement of the front doorways seems likely to lead to a very considerable lightening of the total car structure. As a result, there would be some slimming of the rear wheels and suspension.

There is, however, a further effect on the rear wheels and suspension.

Now the presence of the tail-cone can cause the turbulent wake that conventionally comes from the bluff base of the car to fill entirely and so eliminate entirely the very large base drag. Then, if other more detailed matters are dealt with, such as cleaning-up the flow on the under surface of the car, the aerodynamic drag could be reduced to a fraction of the conventional value. From very rough calculations the drag could be reduced to about a half of the conventional value, when all the "cleaning-up" effects, as well as the effects of the reduction in size of the components, are taken into account. Some discussion is given below, under the heading ~ "The total effect on the aerodynamic drag".

Once there exists the combination of a drag that has been substantially reduced because of the tail-cone, together with a structure weight that has been substantially reduced because of the front doorway changes and the other changes discussed above, then consideration of the design situation immediately reaches the well known virtuous circle. The power required is reduced, both for acceleration, because of the substantially reduced weight, and for the maintenance of steady speed, because of the substantially reduced drag. Consequently the total power required is substantially reduced. As a result, the engine can be of substantially lower power than conventional, and therefore smaller and lighter, the filled fuel tank can be smaller and lighter, while the brakes, wheels and suspension can also be smaller and lighter. Consequently, the structure, which has to hold all of thes