Mighty Mira
Member
- Joined
- Jul 15, 2006
- Messages
- 6
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.
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:
I decided to sharpen the edges to totally eliminate the rear low pressure region.
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).
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.
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.
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:
I decided to sharpen the edges to totally eliminate the rear low pressure region.
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).
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.
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