solar car aerodynamics & body design - american...
TRANSCRIPT
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Solar CarAerodynamics &
Body Design
21 March 2015ASC Conference, Ann Arbor MI
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Why is the aerobody important?
Arguably the single most important part● Touches every other part of the car: Array, chassis,
suspension, etc● Large majority of power is expended pushing the car
through the air● Determines shape of the array/amount of power
available● May be a major structural component
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Basic Performance
Critical equations:
PAero = (½)ρVa2VCdA
PRR = CRR1mgV + CRR2NV2
Other important factors: Power system efficiency (motor, battery, etc), constant quiescent
draw of electrical system, etc.
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Basic Performance
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Basic Performance
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Basic Solar Car Aerodynamics
Low speed aerodynamics!● Skin friction dominates pressure drag● Minimize pressure gradients● Smooth blends between main body and protrusions
(wheel fairings, driver canopy, etc) to minimize junction drag
● Interactions between wheel fairings are critical
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System Design
Aerodynamics do not exist in a vacuum - the design of the aerobody touches all other parts of the car.● Array concerns● Fitting mechanical systems● Perfect physics world vs. real world
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System Design - Array
● Aerodynamics vs. Solar Array: Maximize system performance.
● Careful analysis and simulation required● Balance between the two may change race-
to-race
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System Design - Array
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System Design - Array
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System Design - Array
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System design - Mechanical
“Theoretically ‘better than the competition’ is only relevant if your car is not in pieces in the ditch.” - Laurie Miller, former Minnesota team lead, on "over optimized" designs.
No matter how good on paper, the design is no good if you can’t race it.
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System design - Mechanical
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System Design - Theory vs Reality
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Wheel Fairings
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Wheel Fairings
● Aero drag force of a spinning wheel in an airstream goes up with V3 rather than V2
● Many un-aerodynamic things attached to wheels - suspension member, brakes, brake lines, etc
● Complex flow interactions between different fairings under the car
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Wheel Fairings
● Must be designed in concert with the aeroshell, not as an afterthought
● “Little baby solar cars” - wheel fairings will take almost as much design and manufacturing effort as the rest of the aeroshell combined.
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Dynamic Wheel Fairings
Actuate in some way to allow the wheels to steer● Pros: Narrower and not as long; less frontal and
whetted area● Cons: Mechanically complex. Panel gaps. Fit&finish
issues may negate theoretical aero advantages
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Dynamic Wheel Fairings
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Wheel Fairings - other features
● Sealed wheel wells and tight fenders to minimize air circulation inside the car
● Sacrificial fairing bottoms to get as close to the ground as possible
● Spoke covers on wheels - claimed up to 100W PTD savings at highway speed by some teams
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Crosswind Sailing
Generate thrust via crosswinds to reduce drag
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Crosswind Sailing
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Crosswind Sailing
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Body Structure DesignMany different levels of structure required in aerobody● Aerodynamic shell around welded tube spaceframe?● Composite chassis bonded to aeroshell?● Monocoque without removable topshell?
Regardless, care and precision required at all steps to maintain good aerodynamics
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Molds
As many features in the mold as possible. The more detail is in the mold now, the less work there is to do later. PLAN AHEAD.● Scribe lines for trimming egress hatch, windshield,
fairing cutouts, chassis alignment, etc● Joggles for access panels, fairing attachment● Array inset and wiring channels
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Surface Finish● Array edges● Driver egress hatch● Fairing attachment● Wheel access doors● Top/bottom shell junction
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Molds
Build or Buy?
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Molds - Build
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Molds - Build
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Molds - Build
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Molds - Build
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Molds - Buy
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Layups
Plan, plan, plan. ● Know your materials, resins, cure times and
temperatures● Write up a complete procedure. ● Cut all material ahead of time and store in labeled
containers● Do a practice layup on the full mold with some cheapo
fiberglass● PPE: For your sake AND the car’s
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Layups
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Surface Finish
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Body Structure
● Add extra support to keep array surface from flexing while driving
● Provide features to register location of the top shell and attach it to the chassis
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Body Structure