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Post Info TOPIC: Why no vertically symetrical airfoil bodies?


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Why no vertically symetrical airfoil bodies?
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Hello I recently discovered electrathon. 

At first I was surprised that horizontally and vertically symmetrical airfoil shaped bodies where not being used.  Add on a pair of airfoil profile a arms and airfoil profile wheel fairings and you'd have the lowest possible drag coefficient.  But this is not the case with electrathon cars.

I have read that vertially symmetrical airofil shape is not used because a half airfoil shape has lower drag at low 1-2" ground clearance.

Can anyone tell me at what height the half airfoil shape is no longer more efficient than vertically symmetrical airfoil shape?

The reason I ask is because I am required to have 10cm (3.93701") ground clearance for on road vehicle and I want to know if I should go with vertical and horizontal airfoil body or use a half airfoil instead.  The half airfoil will obviously have better stability due to ground effect but low drag coefficient is my main concern.



-- Edited by ElectricStreamliner on Sunday 10th of January 2016 12:51:24 AM

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This turns out to be fairly complicated and changed with speed, surface roughness, width, thickness, phases of the moon. Ok, I made up the last one.

According to Leading Edge, which is mainly focused on what Solar Racers have learned, a symmetrical airfoil NACA 0015 should run a ratio of minimum height divided by body thickness in side view of around .5. In their example, this works out to .2-.35 meters.

For a car model, flat bottom airfoil shape in side view, should run a ratio of minimum height divided by body Length of around .02. In their example, this works out to .1 of a meter.

Note that for the first, you are looking at body thickness and the second, you are interested in body length.

This information is from page 119 of The Leading Edge by Giro Tamai. Aerodynamic design of Ultra-streamlined Land Vehicles. There is a great deal more detail there.

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If I understand this correctly.

A 50cm thick vertically symmetrical airfoil body such as those used by solar cars should have a minimum ground clearance of 25cm.

It seems to be that solar cars direct their airflow to the top and bottom of the vehicle body with very little to the side.  Is it possible that a sea glider shape would have a different minimum ground clearance considering it directs air to the left, right, top and bottom equally?



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Although a late reply, still might be worthwhile if people are new to fluids.
Four main reasons, three of which are critical for Electrathon (Items 1, 2 and 4)

- Center of gravity height
Profiling the underside to be anything other than flat utilises a very valuable space claim for components to achieve a low CoG, which reduces the cornering capacity. This is probably a differentiator between hyper-mileage vehicles and Electrathon, as peak performance skews enough towards drag mitigation that they will chase it wherever possible. And they don't really need to turn much, while our handling is a bit more important.
Flat 1 - 0 Symmetrical

- Ground effect for drag
Recommend having a look at the below image. The x-axis represents the ratio of the length against the distance from the ground; if you're in a long vehicle that's low to the ground, your data point is on the far left.
www.researchgate.net/profile/Joseph_Katz6/publication/228616843/figure/fig8/AS:301998801997836@1449013533967/Figure-9-Effect-of-ground-proximity-on-the-lift-and-drag-of-two-generic-ellipsoids.png
The dotted blue line on top represents the drag force on a symmetric airfoil, while the lower, fixed blue line is the same conditions on a flat undertray.
You'll note that the coefficient between the two designs isn't very different; you might gain an extra 5% reduction of drag in total. This is a bit of a loss, but it's up to the designer if it offsets the other items...
Flat 1 - 1 Symmetrical

- Ground effect for lift
In the same image, the green line represents the downforce generated by the shape.
A flat undertray allows smooth air flow on the underside, making a low-pressure zone.
The symmetrical profile has to allow the air that goes underneath to compress as it gets to the biggest cross-section, and expand as the profile goes away from it. This compression causes a high-pressure region.
The difference in pressures above and below is what gives you lift (like an airplane) or downforce (like a racecar).
The symmetrical profile, particularly for aggressive length-height ratios like ours, causes lift. This means your car loses adherence to the ground as it travels faster.
The flat profile gives you downforce, pushing the tyres into the ground and increasing the available grip.
Flat 2 - 1 Symmetrical

- Ease of manufacture
A flat undertray is simple!
Final score: Flat 3 - 1 Symmetrical

My overly-elaborate method of explaining a simple situation!

Happy designing and kind regards,
Brendan

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Except that downforce increases the effective weight of your vehicle at speed and therefore increases rolling resistance. Electrathon vehicles likely never travel fast enough to require increased grip (i.e. downforce) and would be best served by having neither lift or downforce. Flat 1- symmetrical 1

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Well, you definitely don't want lift. Downforce is always mitigable by increased tyre pressure if it were to exist in any great quantity. With the light weights, neutral or slight downforce would help to give a positive feel of turn in when correcting the vehicle's steering, while overall lift will contribute to a skittish or "loose" feeling when initiating a turn.

I'm not talking about extra shaping specifically for downforce; merely selecting the body shape that would give best handling. My current iteration suggests 14N drag for 60N downforce at 80km/h, which is simply enough to ensure the body isn't floating between gravity and lift.

But sure, you can have that back if you like. Flat 2 - 1 Symmetrical.
On the balance sheet, do you want to sacrifice center of gravity height and manufacturing ease (flat undertray) for absolute minimum drag (symmetrical)? As your parts and operator positioning hinges on this decision, it's pretty much the first choice on a build that should be made, alongside wheel arrangement and chassis type.



-- Edited by Brendan_Smith on Friday 23rd of June 2017 01:05:35 AM

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(That should read "You definitely don't want lift". I put hat brackets on it forgetting it'll switch that text to HTML. Rookie mistake.)

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Archer321 wrote:

Electrathon vehicles likely never travel fast enough to require increased grip (i.e. downforce) and would be best served by having neither lift or downforce. Flat 1- symmetrical 1


On an oval (like Five Flags) this would be true but on most tight tracks we regularly exceed the traction limit and would be able to carry more speed if we had more grip.

The theoretical maximum range would be achieved by running at the exact unchanged speed achieved by discharging the battery at the steady 1C current with the motor set up to be it's most efficient at this exact combination of current and RPM. Any variation such as slowing for a corner or speeding up for the straightaway will lessen the range.

The challenge is finding the sweet spot between the efficiency cost of varying from the theoretical ideal by slowing for a corner versus the efficiency cost of what ever method used to increase grip to allow the vehicle not to have to slow down such as downforce, lower tire pressure, etc.



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ProEV wrote:
Archer321 wrote:

Electrathon vehicles likely never travel fast enough to require increased grip (i.e. downforce) and would be best served by having neither lift or downforce. Flat 1- symmetrical 1


On an oval (like Five Flags) this would be true but on most tight tracks we regularly exceed the traction limit and would be able to carry more speed if we had more grip.


 Perhaps, but the speeds you would have to be traveling to generate enough downforce to matter via aerodynamics would far exceed the speeds Electrathon vehicles achieve.

The spoiler on most high end performance cars don't achieve any results until above 60mph

It seems to me that if your grip when cornering is limiting your speed you might want to look at your tire width, size, pressure, and/or profile (rounded shoulder bike type or square shoulder car type). Contact patch size could probably be improved.  Weight distribution might be a factor too.



-- Edited by Archer321 on Friday 23rd of June 2017 05:54:17 PM



-- Edited by Archer321 on Friday 23rd of June 2017 08:20:00 PM

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I agree with Archer--if your car is set up right with great steering you seldom if every do any more than a slight side drift over the whole corner...but the car remains going straight and there is not any swerving, etc. So if I am taking a 50' diameter 180 hairpin I plan on a few feet of sideways motion over the corner and aim that far + inside the barrier on the outside of the corner. One thing also that I see with my and other kids is the way they approach a corner. Many go into the corner and then as they come the apex they try to turn quickly. If you can start the turn well before the apex and are at least 1/2 way turned when you get to the apex you can take the corner much faster without tire scrub, loss of power, and the back end, etc swerving.

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Mike, I agree with your point about picking the right line through a turn and allowing for drift but short, crowded tracks sometimes make that virtually impossible. You wouldn't believe the crazy tight, short tracks they sometimes run in South Florida. 10 turns in a 1500' track (at the last race) and 19 cars wadding up in the corners, all while you are trying to actually use up your pack capacity means trying to maintain fairly high speeds through some pretty tight, crowded corners. Spin outs do occur.

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Despite deviating slightly away from the original topic, everything that's being discussed certainly should be on a designer's mind early in a build, and all entirely valid and correct points!

Again from an outsider's/newcomer's view, I'd imagine that competitive teams would change out tyres depending on events? Thin road tyres for when you need to remove drag and rolling resistance and thicker slicks running less pressure for carparks and other obstacle-laden courses?

Back on topic though, if you go with a symmetrical profile, you will definitely generate lift when it's that large and so close to the ground. Lift on a traction vehicle is pretty well never a good thing. If you go with a flat underside, you swap to downforce and minimal increase of drag, while having more/better space to put things. Again, all up to the designer to choose; go for absolute minimum drag, or improve your packaging and improve stability.

Spoilers and the like would be totally ridiculous to place on an Electrathon car; the drag vs. grip cost is hugely in drag reduction's favour. As a comparison, Formula Ford allow just a flat undertray for aero packaging, no wings, and get a decent amount of chassis press from nothing more than shaping the floor flat. I'm not talking about driving a car upside down on the roof; merely enough so that a side-wind won't upset the body of the car.

Control arms and frame elements exposed to wind, though, are both smaller and further from the ground, so would likely benefit from a teardrop shape rather than a cylinder or square section. But whether that's worth the weight and complexity penalty...

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