Aerodynamics
From Apterawiki
Aerodynamics are a key contributor to the Typ-1's outstanding fuel efficiency. The body design is the most visible component of this, but design elements like shrouded wheels, rear-view cameras and recessed windshield wipers also reduce drag.
Contents |
[edit] History and Inspiration
[edit] Body design
The Typ-1's body design is probably its most immediately noticeable feature. Whether you think it looks funny or whether you think it's sleek and sci-fi, the aerodynamics of the body is one of the key reasons for the Typ-1's outstanding fuel efficiency.
The body shape has been optimized in a virtual wind-tunnel with CFD to be as aerodynamic as possible. All the composite body panels are flush-fit so as not to create turbulence.[1]
In the earliest concept renderings of the Aptera, the overall shape is strikingly similar. The swooping front-end, shrouded outrigger wheels, and horizontal tail line are all easily recognizable in this early stage of design.[2][3] The body design is similar to the human-powered 'Pegasus' and 'Fusion' vehicles and the MIT Aztec[4]. Steve Fambro has said the direct inspiration was the Volkswagen 1-litre concept car.[5]
To an extent, the ideal aerodynamic shape is a teardrop. In an ideal-shaped car, the front widens faster than the rear narrows because the longer the airstream has to adhere to the vehicle, the more drag there is. Despite that, the rear must take its time when narrowing in order to help the airflow stick to the car so that the vehicle doesn't drag a partial vacuum behind it. All curves are as smooth and continuous as possible to similarly reduce flow separation.[6] Practicality and legal constraints dictate the use of tail lights and a license plate, so the Aptera terminates in a flat back.[7] To fill in the small wake, air from the cabin is vented on either side of the license plate.[8] To reduce ground turbulence, the boat tail is higher off the ground than the nose.[9]
Like the body, the wheels, too, are shrouded to reduce drag. Drag would be less if the wheels were mounted directly beneath the body, but they are extended on outriggers to enhance stability in cornering and at high speeds. Altogether, the Aptera has a drag coefficient of 0.11[10] -- worse than the best eco-racers, which get as low as 0.07, but lower than the best cars on the road[11] (i.e. the Prius's 0.26 and the EV1's .19). Aerodynamics is only compromised for seating position, since Steve believed “the two passengers the vehicle will hold had to sit side-by-side or the car wouldn't be acceptable.”[12]
Aptera claims an overall aerodynamic drag 1/5th that of a Prius and 1/25th that of a Hummer H2[13], which would be a CdA of just over 1 ft2 = .093 m2. This would imply a frontal cross section of 9.1 ft2 = .84 m2
[edit] Boat-tail
Some people claim that a Kammback like on the Prius or Insight would be more aerodynamic, but this is incorrect. Wunibald Kamm created the cut-off design as a concession to practicality, trading off efficiency for a shorter body. The Typ-1's body, however, is around 13′1″ long[14], compared to the Prius's 14′7⅓″. Boat-tail designs with a vertical cutoff can become unstable in strong cross-winds[15], but the Aptera's horizontal tail avoids these problems.
