Evija vs Elite - aerodynamic designs

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richardw
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Evija vs Elite - aerodynamic designs

Post by richardw »

I've just come across this interview with Richard Hill, Lotus aerodynamicist, on the aerodymanic design of the Evija: https://www.pistonheads.com/news/ph-fea ... car-/42507

Love or loathe the Evija, he makes some interesting comments about its design which reminded me a bit of the Elite. An important part of his philosophy is 'porosity' or increasing the ability of air to pass through the car's structure rather than just be displaced around it. He says "With no internal combustion engine to work around, battery profile and distribution allows us to try new things with the shape. The main one here being the directing of air flow through the vehicle." Of course the absence of a conventional ICE makes some difference, but I would have though the main restriction is the unfortunate need to carry people! Even with an ICE, the Elite managed to do some of this rather well - the ducted cooling airflow, for example, helps reduce air pressure under the vehicle by preventing it's exit under the rear of the engine compartment. It also contributes to the airflow over the screen which is generally aligned with the direction of the central wiper at any point in its arc, thus improving wiper performance at high speeds. Hill says "Airflow is also channelled through the front openings and diverted around the cockpit, with the intention of affecting pressure and also to keep powertrain components at operating temperature." The Elite further exploited the 'porosity' concept 45 years ago by channeling yet more air from the engine compartment through the longitudinal chassis box section, having a cooling as well as an aerodynamic effect.

Further back on the car, the interview recounts: '...the airflow’s effects towards the rear of the car “reduce wake and exit air at a higher Z plane than at the front”. This, Hill explains, “introduces a negative camber”, with the “actual overall profile on both the top and bottom surfaces working together to induce this angle." Well, the Elite does this as well, with just sufficient downforce at the rear (aided by the roof lip above the rear window) to avoid lift at speed.

Hill goes on to say that "...with the Evija design, we wind off the workload at low speed to create the efficiency needed for range, then at higher speed our design induces the downforce so the performance is there." I think this is a bit misleading; aerodynamic effects increase exponentially with speed - it is not the achievement of the designer to have them do so. What he is really saying is that the car is, quite slippery at low speed where aerodynamic factors don't play a huge part, but that the downforce built into the design will emerge above these speeds! Downforce is the enemy of aerodynamic efficiency, which leads me to conclude that the 45 year old Elite (and Europa before it) will turn out to have a lower drag coefficient than the Evija; that figure - tellingly - isn't being released until the production shape of the Evija is finalised.

I'll put a fiver on the Elite's Cd and CdA being lower than those of the Evija (but will only take one bet!)

I wonder what Oliver would have to say?

Cheers, Richard
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Lotus-e-Clan
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Re: Evija vs Elite - aerodynamic designs

Post by Lotus-e-Clan »

The aero on the Elite wiper was horrendous IME. I've seen it lift off the screen at over 100 mph - Excel too. You must have seen that for yourself?

Pretty sure the Elite broke ground on aero, but I think there was far less scope to apply as much contemporary aero knowledge to production cars during the 1970's as there is today. On that point alone, one is comparing apples with pears in the context of cutting-edge aero concepts applied to Elite and Evija. Besides, the basic Elite aero design was flawed - it produced lift that had to be dialed-out with a spoiler - and Chapman was not happy about the compromise IIRC. Using modern flat floor aero concepts proper downforce is possible without relatively crude spoilers. Pretty sure Chapman would have understood underfloor aero (he pioneered the concept) in the context of F1- but I'd be surprised if it crossed his mind to apply that to a four-seater road car in the seventies...whereas the Evija has plenty of current F1 aero cues.

Being an F1 fan and a bit of an experimenter, I panelled the floor of the Clan to enclose the suspension voids to see what's what...
BELOW: Looking toward the front, the large voids which house the front suspension are covered with black ABS plastic panels.
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BELOW: The std Clan rear suspension is housed within a canvenous untidy void too ...
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BELOW: subsequently panelled with ABS ...
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These underfloor mods resulted in scary downforce toward 100mph! Scary because around 100 mph the car, now sucked onto the track, followed every small surface undulation because the front dampers were compressed on to the bump stops. I had to increase the front spring rate and damping by 30% to compensate. Simple underfloor mods do work ...the Elite/Excel would benefit too.
Peter K

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Re: Evija vs Elite - aerodynamic designs

Post by DavidOliver »

While enjoying Richards enthusiasm for the Elite Aerodynamics, I believe certain comments are not strictly in line with airflow
characteristics.
-Preventing air exit under the rear of the engine compartment- The front air scoop is what directs the air through the radiator,
and exiting air over the bonnet is hardly laminar to induce loading.
The porosity of the longitudinal chassis box section is severely restricted by the gearbox which sits directly in front.
Regarding drag coefficients, at the speeds to be considered, they will change dramatically as speed increases and downforce is induced,
as Hill notes for distance efficiency the drag (and downforce) is designed to dramatically reduce at lower speeds. The Elite does not
get to the same speeds where important downforce is developed and huge electric traction can be delivered.
At an early age Colin Chapman had a good feel for Aerodynamics after his RAF experience and time designing roooves
for the British Aluminium Company. No doubt the Elite was developed to include his aerodynamic knowledge to the full.

Dave O.

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Re: Evija vs Elite - aerodynamic designs

Post by richardw »

Oliver Winterbottom was responsible for the aerodynamic design & testing of the Elite. His book - "A Life in Car Design" - contains a lot of interesting information about the project.

Richard
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