Low Drag Prototype Car

Discussion in 'Hangar Flying' started by Mighty Mira, Jul 17, 2006.

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  1. Jul 17, 2006 #1

    Mighty Mira

    Mighty Mira

    Mighty Mira

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    Hi,

    I'm in the throes of designing an low drag car, and I figure that I will get more help here than on a kit car forum, due to the preoccupation with aerodynamics. Here is some background:
    http://www.gassavers.org/showthread.php?p=19764#post19764

    Ultimately, the plan is to design an electric vehicle (EV). The key (for me) is to minimize drag while making it more practical than a motorbike.

    To minimize drag fully, the vehicle should be shaped like an airfoil, particularly in the vertical plane.

    I realize that a design will have to cope with crosswinds. Therefore, I figure that if my vehicle resembles an airfoil at every point for every possible angle of attack at 60mph and in any sub-galeforce wind, that will minimize drag while enabling a range that is consistent within a wide variety of crosswinds. (I figure that orientating the vehicle via 4 wheel steering to be impractical, dangerous and expensive.)
    Failure to take into account crosswinds is a failing of the Bede designed Pulse motorcycle - in a crosswind it won't be much better than a car as far as drag is concerned. At least as far as an electric vehicle is concerned, where consistent range is vital. I.e. one must have a reliable "point of no return" in an EV. No good going half the journey and then being stranded due to high winds.

    [​IMG]

    In order to be practical, I have decided on the following constraints:
    1. must have maximum width under that of a Suburban. (preferably less)
    2. must have maximum height, will still either having airfoil cross section or very close to it. Hence need an airfoil that has a high thickness. (Note that this is NOT a solar car and hence I don't need to maximize flat area. I want to maximize usable cabin space provided drag is minimal.)
    3. the airfoil should be low drag up until about 100mph.
    4. the vehicle should be stable at high speed. I figure that so long as the rear is pointed slightly up, this should solve it. Remember that even if the rear is lifted up slightly, and if this moment causes the front of the vehicle to lift up, we can weight the front with batteries to counteract this.
    5. I will not require that the vehicle must be waxed at all times. It should be expected to have dust, bugs, etc splattered on it.

    Here is a rough model:

    [​IMG]
    [​IMG]

    I decided to sharpen the edges to totally eliminate the rear low pressure region.
    [​IMG]

    So, my plan is to get a car with minimal height (esp. at rear) and width. This narrows down my choice to some sort of convertible. I am going with a Ford (Mercury) Capri, 1989. (Similar to 1991 Mercury Capri).

    [​IMG]

    I plan to build a wood frame around it and then use clear plastic sheet on the frame. Then do a coast down test to see what the drag coefficient is. Once I know the drag coefficient, I can decide whether the chassis will carry enough lead acid batteries for the range I desire, or if I will need to upgrade to a small truck chassis.

    I am not sure how to deal with the ground/air interaction. Hence, I'm not sure if a symmetrical airfoil is going to cut it. (i.e. I am assuming that air pushed out of the way below the car does not have an easy path out of the way at the front of the car, and an easy way to get back under the car at the rear. This may be an incorrect assumption, after all, assumption is the mother of all **** ups.) Obviously, the section at the very rear of the car where it is angled up slightly to induce negative lift will be different from most airfoils.

    If the air under the car is a problem, another way I've thought of going about it is with a splitter and then a gentle curve up to the front of the car, while keeping the underneath section of the car flat or nearly flat.

    If there is no drag penalty, I would like to keep at least some taper at least under the back, as that results in the shortest (and hence narrowest) possible car for the given height, and hence the most practical.

    Please let me know any thoughts/tips/suggestions you might have about this, including contructing a proper prototype (assume composite would be quickest and easiest?), and what would be the optimum shape/airfoil to choose for minimizing drag in this circumstance. I think the plan view is pretty much correct, it's just the vertical plane that might need some work.

    I realize that building proper elevated struts may reduce drag even further, but it brings additional problems such as stability (due to high centre of gravity) and prototyping it quickly by modifying an existing car.

    MM

    p.s. sorry if this is the wrong forum, originally the intent was to get feedback on the construction, but I'm more interested in feedback on airfoils and how they relate to the ground.
     
    Last edited: Jul 17, 2006
  2. Jul 17, 2006 #2

    wsimpso1

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    I would actually think that the Bonneville crowd would be a better group to get low drag and stable aerdynamics for ground vehicles.

    The ground vehicle interactions are strong, so much so, that race cars gave up on airfoil shapes. Most use flat bottoms and shape at the front to minumize air running underneath the vehicle. After that, you have to get the air around the people and stuff with a minimum of total disturbance, which does not necessarily mean an airfoil shape, nor does it imply a flat car. I would actually look at the Le Man's and Benneville vehicles.

    Also, there are books from the EAA book service on Laminar flow shapes, both foils and other bodies, and this may prove quite interesting, being as Laminar flow can reap large benefits, particularly at the low vehicle speeds of automobiles. The drawback is that the boundary layer can trip on small interuptions, so the front 35-40% of the body should be one piece. Perhaps the cabin would open by pivoting the entire front half of the body body forward.

    Some classes of racecars use ground effects to produce negative lift whereby there are ducts carrying air through the car (Indy and CART come to mind). It seems to me that minimizing drag of essential ducting is the best plan for internal flow.

    Rememeber this about drag, the biggest variables that you can manipulate is frontal area. So, minumize frontal area, then skin area and shape. I shall look up the authors on the laminar flow books.

    Billski
     
  3. Jul 17, 2006 #3

    orion

    orion

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    One set of books that discuss car bodies and include references are "Fluid Dynamic Lift" and "Fluid Dynamic Drag" by Hoerner. Otherwise, as Billski alluded to above, car design and aircraft design have only a few parallels (unless of course you're looking at doing a flying car). For specifics, you'll have to reserch text references that deal specifically with the issues of land-borne aerodynamics and of course, those dealing with wheel-borne stability and control.
     
  4. Jul 18, 2006 #4

    Mighty Mira

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    Partially. However, they do race at higher Reynolds numbers than regular cars. They also can orient themselves so that they are only ever inline with the wind. Hence, minimizing frontal area is as simple as being long and thin, which enables them to maximize the amount of energy and power output/over weight while minimizing frontal area and Cd.

    A practical road going vehicle does not have this option. A long thin car will be good where conditions are expected to be still. However, where crosswinds are more frequent, the car should be streamlined over the range of angles of attack it has towards the air if range is to be predictable. (Of course, the faster you go the less you have to worry about crosswinds.)
    Race cars have much different requirements to electric vehicles. Lateral Gs at speed is a major requirement, not so with economical vehicles. For example, on the interstate highway even an economy car with little thought given to aerodynamics can easily travel at 100mph safely, as it can on the autobahn. Lateral Gs aren't a factor.

    The other thing is that lead acid vehicles are typically heavy if range is to be decent. Those batteries are great for making downforce, and stability especially as their density allows a low centre of gravity.

    And so a long range but more practical EV (i.e. better than motorbike, on par with sports car or econobox) needs an ultra low coefficient of drag (as low as possible, hopefully 0.1 or lower). A race car needs high lateral Gs and so has a Cd of 1+.

    High fuel economy cars are very much approaching an airfoil in shape. For example, the EV-1, the UFE-III, the Probe V. All are rather similar looking, although their crosswind performance would not be great IMO. Solar cars are also along that path, but I suspect that reducing frontal area and maximizing flat surface area for solar panels takes precedence to minimizing Cd for a variety of wind conditions at highway speeds.
    Now that is an interesting idea. Perhaps it could be made practical. Another thought is to make the wheel fairings start after the majority of the body starts tapering backward, i.e. after max cross sectional area is reached. This idea comes from the ar-5.
    That is certainly true.
    Thanks. Minimizing frontal area means minimizing height and width. Hence going with something that will be not much higher than a capri, if at all, and start out not much wider.

    Another thing is to realize that frontal area is normal to the angle that the air hits the car. Since this varies, it pays to think that what is a smooth shape on a still day (a bonneville racer) is kind of like a broom going sideways on a day with a good crosswind.

    Anyway, thanks for the reply and refs.
     
  5. Jul 18, 2006 #5

    Nilsen

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  6. Jul 18, 2006 #6

    wsimpso1

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    First things first – If you are concerned about cross wind behavior and nominal drag, you are best served by looking at racecars, including the Bonneville crowd. I do not care if their q is as much as 16x your q, the same goals exist and same rules apply. The primary difference is that the right amount of net down force for you and the Bonneville crowd is much closer to zero than it is for Formula 1. And the lower Re makes use of laminar flow more practical. Next, the Bonneville crowd spends a bunch of effort on aerodynamic cleanup and keeping aero-losses for cooling to a minimum. They also need no more down force than you. I continue to think that you will do well to look at race cars of various types, including Bonneville vehicles, for minimizing aero drag and stability in cross winds.

    Stability in cross winds is an interesting topic. Auto makers and racers have struggled with the topic.

    Crosswind Stability (fully developed) is satisfied when:

    Sum of Lateral Forces is zero;
    Sum of Yaw Moments is zero;
    Sum of First Derivatives of Yaw Moments with respect to Yaw Angle is Negative.

    Elaborating, this means that the vehicle's side loads and yaw rotation angles are all in balance, and that the nose will tend to return towards straight ahead when disturbed. Details:

    1 Sum of Aerodynamic Side Force - Sum of Tire Lateral Force = 0;
    2 Sum of Aerodynamic Yaw Moment - Sum of Tire Yaw Moment = 0;
    3 dMy/dY – dF1*X1/dY – dF2*X2/dY – dF3*X3/dY – dF4*X4/dY < 0

    Since tire forces are a function of the roll stiffness, roll angle of the vehicle, camber change angles, camber thrust functions of the tires, and rolling moments, well, we need another equation:

    4 Sum of Roll Moments is Zero

    This equation will balance the roll stiffness and tire normal forces, and is normally where suspension geometry is used to tune transient response.

    Then comes the dynamics of a transition from changes in cross wind, how much control feel you want when in a cross wind, and how much self correcting you want the vehicle to have. You can go on to add terms and equations as you find need for them.

    Race cars may run at higher velocities and Re, but they are VERY concerned with cross wind behaviour, and would be an excellent place to start. Looking at most race cars where aero design is allowed, (and even in stock or sedan racing), the vehicles are deliberately set up to have more lateral area and more vertical area aft of both the CG and the center of lateral traction. Note that even in Europeon Saloon racing, which is similar to the way NASCAR used to be, they run low profile air dams forward and large sideplates on the rear spoilers, all of which adds greatly to this effect.

    This gives some automatic yaw angle into the cross wind (helping to satisfy Rules 1-3) and minimizing steering correction needed. Nominal caster angles will provide tactile feedback into the steering system to notify the driver that the cross wind is present and being corrected for. Rule 4 gets complicated because we want a basically balanced to slightly understeering yaw response in most cornering, and for/aft roll stiffness bias is the best tool for adjusting this, and yet it also feeds back into the normal forces and thus the lateral forces in each of the tires, showing up in yaw resistence…

    I will leave the determination of the terms and derivatives to the engineer on the project.

    You did mention adding batteries to correct for aero lift. Hmmm. Forward that might be a good idea, but if you add weight aft to correct for lift at the rear end, you will enter a dance with aero instability in yaw and perhaps in pitch too. If the CG is aft of the effective center of lateral area, the beast will have a positive first derivative, which means it will try to ground loop like a tail wheel airplane.

    Billski
     
  7. Jul 20, 2006 #7

    h_zwakenberg

    h_zwakenberg

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    Bump that up to 130 mph. Been there, done that. As a matter of fact, I routinely cruise at those speeds when commuting weekly back and forth to my current contracting client, which is a 390 miles' drive.
    Mind you, the Autobahn often is so crowded, that attaining let alone maintaining those speeds is difficult at best or even completely impossible...
    The point is: contemporay cars can easily cope with it.

    As to the project at hand: since you're contemplating an EV, I wondered why weight obviously is less of an issue for you. Aren't those base vehicles you are considering way too heavy for the job?

    bye
    Hans
     
  8. Jul 20, 2006 #8

    orion

    orion

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    I know this one - having many years ago unknowingly driven my 1966 Corvair Corsa convertible onto a downhill asphalt road that was treated with an oil/sand mixture. One minute I was pointed forward and a split second later, not. Quite a wild ride.
     
  9. Jul 20, 2006 #9

    wsimpso1

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    Oooo, the backwards automobile event... In the Corvair, it was atleast as much cg aft of the center of lateral resistance from tires as from aero effects, but produces the same result - Ground loop or Spin out - It is exciting.

    I feel a need on the aero shape issues to pop a few issues. First, Abbot and von Doehoff's "Theory of Wing Sections" is a starting point for airfoil theory. Please note, the airfoil folks assume unrestricted flows around both sides of their foils, while cars have the air attached to the ground plane. Second, they discuss 3D wings and talk about how much effect the aspect ratio has in messing up the flow situation, and they were talking about numbers of 3 on up towards infinity. A car with an airfoil shape has an aspect ratio less than unity, and thus is all wing tip...

    Next, Carmicheal's "Personal Aircraft Drag Reduction" gives you a starting point for how to shape things for minimum drag. He covers a number of airplanes, and also covers Laminar flow methods including some gravity powered devices. Interesting stuff. He also covers methods of computing overturning moments (destabilizing).

    Seriously, low drag without lift or downforce does not require an airfoil so much as it requires looking at the shape and saying to yourself, does it look like a continuous flow of air here will want to flow around this thing. If you try to trick it into going over the top, it will likely spill over the sides. Then go back and look at how Le Mans racers are shaped. Find the down force gadgets and back them out, and you will have a pretty low drag, stable system. Remember where their CG is relative to their bodt shape, and I think you can gain all sorts of insights.

    Good Luck,

    Billski
     
  10. Jul 21, 2006 #10

    Mighty Mira

    Mighty Mira

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    Yes, I would certainly not consider putting batteries in the back, that sounds like a recipe for disaster.

    My thoughts are to have the air leaving the back end of the car going either horizontally or slightly up as to add some downforce. My thoughts on batteries are to put them either close to the front or under the floor, between the wheels.
     
  11. Jul 21, 2006 #11

    Mighty Mira

    Mighty Mira

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    Hans, I am attempting to design (or at least, going through the thought processes) a practical, cheap, EV. The main limitation with EVs powered by lead acid batteries is range. So I am attempting to maximize range, while having a usable interior, reasonable acceleration, stability and indifference to crosswinds.

    Given the same battery, range is proportional to battery weight/ CdA. This is counterintuitive, but makes sense when you think about it. (An object will continue along its path of motion unless acted upon by an external unbalanced force. Minimize the external unbalanced force (drag) and you can go forever. If you can get a large battery weight to highway speed, you have the same losses (except rolling resistance, the automotive equivalent of induced drag, and it's minor) as you do with a light car.

    In miles, lbs and square feet, at 70 miles an hour,
    Range = .314 * Battery Weight / CdA
    (roughly).

    If I can get drag to 0.11, frontal area similar to a Capri, I can get 300 miles range with 900kg of batteries. Hopefully you can see my preoccupation with achieving extremely low drag figure WHILE not being an impractical solar vehicle type thing.

    I want to design it to get AT LEAST 300 miles of range at a cruising speed of 70mph. That's 5 hours of driving at 60mph.

    I think it's better to first optimize CdA and usable interior first. That way, weight is minimized. Minimize weight (especially, non-battery weight) and maximize acceleration, minimize city driving (and likely some country) energy usage, and hence cost.

    Hence, that is why I would start with a capri. Due to its small size, I can craft a rough prototype shell with cheap plastic and wood in order to test CdA with a coast down test. Unfortunately I can't vary ride height as well, but them's the breaks.

    After I have that spec, I can select an appropriate chassis to build a proof of concept vehicle, likely a small truck chassis.

    This thread goes over some of the issues:

    http://www.gassavers.org/showthread.php?p=19948#post19948
     
    Last edited: Jul 21, 2006
  12. Jul 21, 2006 #12

    Mighty Mira

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    I suppose that seeing as the air is attached to the ground plane, there may be larger gains than a person might otherwise think by elevating the body from the ground. This of course has adverse effects on stability, however, an electric vehicle has an ace in the hole with battery placement. It is easily possible for an EV to have a CG that is lower than a regular car while having an undertray significantly higher than regular cars. If we consider that the worst case scenario is a van or minivan, we could potentially craft a vehicle with several feet of clearance before we reached the worst case. This might well result in further drag reduction.

    I'm not suggesting making the car into an airfoil btw. You should be able to see that with my prototype. It's not as if I am expecting the car to act like a wing. However, it remains true that for ultimate low drag, the front should be rounded while after the point of max frontal area there should be at most a gradual taper with any one dimension. (Note, I am not including the Kamm back which is an engineering compromise and not actually superior to a taper to a point.)

    And likely, that taper happens to be zero for the bottom. It stands to reason that at one extreme you have cars inches off the ground and flat, while in the air you have an airplane fuselage which looks like an airfoil rotated 360 degrees... so in between, something that minimizes drag will look like something in between. Since an airplane fuselage is going to be better than a car (assuming no wind), likely it is going to be better to elevate the car and have a more fuselage-like shape.

    And again, while bonneville/le mans craft are interesting to look at (i.e. general streamlined shape), note that with typical crosswinds, at high speeds, a racecar will not be getting much drag from a crosswind because the crosswind vector is small in comparison to the car velocity vector. Hence, the angle will not be severe enough to cause flow separation at the side of the car and hence increased drag. All they have to worry about is stability in crosswinds, and not the drag that occurs at lower speeds.

    I am most concerned about the drag. With 900+kg of dense batteries to be placed in the car, I don't think stability will be an issue. Drag will be, however.
    Thanks. You have certainly challenged my thinking, which I appreciate. I am starting to think that I should be thinking seriously about elevating the body of the car somehow. However, the crosswind drag on these larger tyres might kill off any efficiency gains.
     
  13. Jul 25, 2006 #13

    CNCRouterman

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    EVs

    Hello Mighty Mira

    I am intriged by your project. I have a 60 mile round trip commute myself, and the cost of either gas or diesel is sorta like a shop vac reaching into my wallet, it sucks.

    My research into EVs is focusing on storage (batteries) and economical traction motors. The DC drives seem to be the least expensive to get into, but are a little harder to get regen braking out of, and have a few other issues. The three phase inverter drive packages appear more efficient, more flexable, more suited to tying into the home power grid (like a big ups) but carry a hefty cost as well. I spoke with one DC motor supplier to resell their product as well, they want me to show them that I have been directly involved in building an EV to demonstrate a level of compitence before they will talk about a reseller authorization.

    Your investigation into the aerodynamic aspects has enlightened me quite a bit. Thank you.

    Where are you building your POC project?

    I will check out your links to other discussions as well.

    Eric
     
  14. Jul 21, 2010 #14

    Tom Nalevanko

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    There was a lot of effort in this domain going back even to the 30's. Here is an interesting French design, the Mathis 333, from 1946. Under the logo reads, "Weight is the Enemy"; very true!

    And here are a few more notable low drag cars with their drag co-efficients; Cx"; I am sure that there is more detail readily available on these.

    Mathis 333 0.22
    Citroen Eco 2000 1984 0.21
    Renault Vesta 1987 0.186
    Loremo 2005 0.218
     

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  15. Jul 22, 2010 #15

    roverjohn

    roverjohn

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    There are lots of 100mpg one offs running around. Here's one:

    DIY, 100 MPG Car is Back on the Road After 20 Year Hiatus | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com

    Find the Loremo. It is pretty much optimized now so you could just copy it and then save your energy for doing something useful like building a prototype. My guess is that you are making a big mistake by deciding to use LA batteries. Much too heavy and you then need a truck chassis to carry them. The car above needs about 7hp to maintain hwy speed and is very light. Until magic batteries are developed you are probably wasting your time unless you live somewhere where there is no fuel. Gas is still very cheap.
     
  16. Aug 1, 2010 #16

    autoreply

    autoreply

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    The Nuna-5 racecar is specifically designed to produced thrust from lateral winds. The bottom/side-design is very interesting:
    [​IMG]

    You might also take a look at the eco-challenge cars. Both designs have a major issue with side wind-drag, while in "real" racing cars that's mainly limited to maintain sufficient stability with sidewind, while drag isn't too interesting.
     

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