Indifference
Extreme efficiency can lead to extreme problems. In general the cleaner a shape and the closer the airflow over it, the more sensitive it will be to disturbances in that airflow.
For FFs these problems chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.
Poor indifference is caused when attached (lamminar) airflow over a surface changes direction or pressure. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.
This is avoided, on cars as well as FFs, by enforcing separation of the airflow from the surface at chosen points, rather than allowing the separation line to wander about the surface generating unpredictable lift effects.
At the front this is usually done almost immediately, certainly within 500mm of the leading edge. 001 used immediate separation devices on all the leading edges and some of these sharp direction changes, can be seen in the picture.
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on the track | FF Web). 002 had less separation on flat surfaces, as nose lift was not seen as a problem but has clearly visible channels running up each side of the nose to enforce separation of any airflow across the nose surface
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002 | FF Web). The ledge in front of the lights is intended to enforce separation of the flow over the lights and features on all the Voyagers in various forms.
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FJ original nose, plus Production nose, for comparison. | FF Web)
002 however had a minor indifference problem. Strong turbulent side winds could be detected as tail buffeting. The probable reason for this became apparent when wool tufting tests were studied. 002 has no separators on the upper tail for cross flowing air. It's tail ridge is a gentle curve. .
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002 | FF Web)
The production Voyagers are a simple step on from 002. There is more tail area, chiefly due to the use of slides on the head fairing and a complete head fairing. The separation feature on the nose is somewhat more 'styled' and the nose is shorter due to packaging improvements in the basic design. The result was very encouraging with excellent stability and indifference, while efficiency seems as good as any other of these shapes.
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03 | FF Web)
Enforced separation of flow across the tail is just as important as across the nose. 001 and FJ both use separation enforcing edges on the upper edges of the tail. Subsequent study revealed that other vehicles have suffered from cross-flow generated tail lift and buffet and this device is a fairly common solution. The sharp upper edge of FJ's tail may be extreme but some device like this is important.
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Banana and Fat Jogger | FF Web)
FJ's nose shape paid only lip service to enforced separation. The shut line gap of the nose opening panel was supposed to bleed air into the airflow over the nose to provide separation. It's pressurised by the air entering the headlight opening. There is no evidence that this works and indifference was initially so bad that strip separators were retro-fitted, very crudely, to the nose.
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Royce | FF Web)
Indifference was still only moderate in some conditions and a new nose was made with explicit separation features and other changes.
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New Nose | FF Web). This shape improved indifference and flow into the radiators.
Finally the plan view of the vehicle should be considered. If the shape tapers too sharply after the point of maximum width then there may be too much separation towards the tail leading to poor indifference in some conditions. FJ has a slight problem in this respect, whereas the fatter-tailed production shape is markedly more indifferent. Remember the airflow will separate below 2 degrees positive angle of attack, so the tail should be almost parrallel untill the cut-off (Kamm-style) or whatever final separation point is chosen.