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Aerodynamic design for cars

  1. Jan 2, 2004 #1
    What are all the advantages of good aerodynamic design and how can these be proved mathematically?
    For example what is the difference between a truck with a wind deflector and the same truck without the wind deflector?
  2. jcsd
  3. Jan 2, 2004 #2


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    I'm sure the designers do a computational fluid dynamic analysis, but the easiest way to prove it is with a wind tunnel. Aerodynamics plays a significant role in top speed and fuel economy.
  4. Jan 11, 2004 #3
    Well I have been interested in aerodynamics a long time now, and I can say some things that are basically wrong with some vehicles:
    *trucks and buses have flat fronts wich propably make 100kgs drag at 100kmph
    *cars have spoilers because the designers are DUMB and make the shape of a car like an airfoil (most common design) and then struggle to keep the wheels on the ground with spoilers
    *cars having a flat rear that automatically generates a low pressure. Though it collapses to create a vortex, that is still a waste!

    The list can go on if you just look carefully...
  5. Jan 12, 2004 #4


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    You should distinguish between spoilers and wings. Spoilers act to break lift, and can make cars more aerodynamic by breaking down the vortex/vacuum at the tail.
    Wings generate lift, on cars this is typically negative lift, and are used essentially only on racecars.

    drag is typically measured in Newtons, or pounds. Kilograms are a unit of mass.

    Less drag means higher fuel efficiency, and a potentially higer top speed.

    Using wings for downforce means that cars can accelerate more because there is more friction between the tires and the road. At speed, Formula 1 cars could drive upside down because the downforce is larger than the weight of the car.

    In practice, serious interest in aerodyamics on cars is mostly in the context of racing.

    A second environment where fuel economy is important is trucking, but in that environment the need is for inexpensive aerodynamic improvements.
  6. Jan 21, 2004 #5
    Good aerodynamic design reduces drag which improves fuel economy. The mathematics of aerodynamics is well known and the best overall shape (from a low-drag point of view) is called a Rankine Oval. However, in the real world, you have to compromise good aerodynamics with the need to fit in the engine, passengers, loadspace, etc and the cost of manufacture. An excellent example of a good compromise is the Citroen Zsara Picasso - almost a perfect Rankine Oval.

    One of the aims in streamlining is to reduce the vortices that dissipate energy. These tend to occur when you get sharp re-entrant corners and flat surfaces. However, the frontal shape of the vehicle is not too much of a problem because it pushes a wedge of air before it, effectively streamlining it. And trucks tend to be slim relative to their length - which also helps. More important is the back - a flat back drags a lot of turbulent air behind it and should really be streamlined. However this isn't practical with large trucks because it would compromise the shape of the loadspace. Streamlining the turbulent re-entrant corner between the cab and the box body is a cheap and useful addition.

    Of course, none of this is to do with roadholding. To improve roadholding using aerodynamics is possible (e.g ground effect in F1 cars) but you may have to compromise on drag. All engineering is a compromise.


  7. Dec 15, 2006 #6
    Hi, Many excellent responses. Take a look at Buckminster Fuller's
    DYMAXION CAR. This as an excellent design , ahead of it's time.
    A three wheeler with a tear drop body, and low sprung weight suspension, which needed a front wheel drive system like we see everywhere today.
    Remarkable for 1935. And it did all this with a 90 HP flathead V-8.
    I can only imagine what Bucky would have done with a 2.5Lsubaru and carbonfibre.
    Best regards, Dan
  8. Dec 16, 2006 #7


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    I hate to discourage you, danscope, but there is no 'Resurrecting Dead Threads Guru' badge. :tongue:
    I'm glad that you're doing it, though, 'cause they're good ones. :biggrin:
  9. Dec 16, 2006 #8
    I know this is off thread but I'm trying to get a hold of Danger. Can you PM me please?
  10. Dec 16, 2006 #9


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    Done, and nice to meet you. :smile:
  11. Dec 18, 2006 #10
    Hi, Thanks for the reply. Nice to meet you.
    Enjoy your Holidays, and Best regards, Dan
  12. Dec 18, 2006 #11


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    Likewise. :smile:
  13. Dec 19, 2006 #12
    Hi, You should see my design for a three wheeled flying motorcycle.
    A section of glider fuselage with one wheel in back and two wheels forward,
    tandem seating, and a modest 2 cycle engine for power. Rotax water cooled would be ideal. 65mph ground speed.
    Flying package in the form of a Burt Rutan "Long-EZ". 150HP swept wing with tip rudders and an canard wing forward(detatchable). Would like to see this as a home-built with factory parts( wings, fuselage skins, tube chassis ..)
    Under $40,000, 175Kts cruise, land 60kts,1200 nm range.
    Just on paper for now.
    Best regards, Dan
  14. Dec 19, 2006 #13
    Sorry, couldn't help but picture the 2CV as an anti-example of aerodynamic design. :rofl:
  15. Dec 19, 2006 #14


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    That actually sounds pretty reasonable. I assume that you'd have to stow the wings along the sides while ground-bound. Detachable, or folding?
  16. Dec 19, 2006 #15
    Hi Nice of you to reply, Here is an extended description of the design.
    Bear in mind:all of the expensive components "Stay" at the airport. We don't want some fool running into our expensive wings, engins, propellers etc.
    They stay safely where they should must needs be. Here is the picture:
    I have had an interest in refining a design for a
    roadable aircraft for some years. Here is an essay on that possibility....as a three wheeled flying motorcycle.

    May I say that I have been following this particular subject for some time.
    After many years of design work, I shall say that the only things required for
    the advance of roadable aircraft are a "reasonable" aproach by government,both state and federal, in regard to experimental aircraft.
    One of the first approaches to an affordable roadable aircraft will be a
    flying motorcycle. The reason for this is that once you call something a "Car",
    you open a can of worms with regard to regulation. My design offers a reasonable and practical work around. The three wheeled motorcycle.
    It will be a two passenger, tandem seating (one front, one back) section of
    fuselage similar to a sail plane, with a bubble top canopy, and having
    "TWO WHEELS IN FRONT" and a single rear drive wheel. This is a stable road configuration, as opposed to the single front wheel which can easily roll over in a turn, especially a braking turn. This fact considered, shall we look at the flying configuration. This part is a well known and applauded design plan
    by Burt Rutan called a "Long - EZ" . This has been flying for well over 20 years and is a superb flying platform, stable, efficient, stall resistant and having an
    excellent glide ratio, being one of the more slippery designs out there. Just go to Oshkosh for the fly-in , and you will see these aircraft in surprising numbers, and they are all hand built. One of the interesting things about the design is that the main wing spar which carries 90 % of the load is located
    behind the rear seat. This most interesting feature allows for the convenient
    sectioning of the forward fuselage from the main wing-landing gear-engine-
    propeller-fuel tanks section which I shall call the "flying package".
    This also includes the forward wing, known as the "Cannard" wing, which is removed and stays with the flying package. Don't be afraid. Sailplanes take the wing off each day with a couple of secured bolts. Part of a good design.
    The landing gear remains fixed, for economy and simplicity,although there are many examples of canard aircraft with the luxury of retractable gear.
    You can gain another 20 knots or so. But....you're going to PAY!!!
    The forward landing gear is a single retractable nose wheel, light and simple
    which retracts with a hand lever. The "Road Wheels" do "Road Work". This is important in design considerations. Aircraft wheels are light, thin, and have no tread, and work marvelously as aircraft wheels. They are not welcomed on the street or highway and do not last in that purpose. They are thin. Simple.
    Do not use aircraft tires on the road. This design has two motorcycle wheels
    with disc brakes which may be coupled or released from an internal torsion bar suspenion, saving weight, drag, and locating some suspension weight further aft, a good thing. rs come off; the wheels come down and get pre-loaded to bear the vehicle weight. Then the wheel covers are re-secured in reverse to become fenders. The nose wheel is now retracted,and gear door locked closed.
    When on the parking tarmac, we secure the wing in it's tie-down configuration
    and then deploy a rear support strut, just under the engine support. This will bear much of the weight ( nearly as much as the road vehicle ) and keep the flying package from tilting over backwards , and pranging an expensive propeller. I should perhaps mention that we don't ever want to go down the street with expensive and delicate aircraft parts and compete with trucks,
    suv's and $50 cars with bad drivers. My design leaves all of this at the airport, and it is through this rational approach that we will agin have need of and praise our local airport. You will never have a situation where you land on highways and streets. This is non-sense and shall never be accepted by the majority. I am not opposed to emergency utilization of highways for landing and....there are ways to do this safely. Ask me later.
    Now, as to the road vehicle. As the aircraft sits at tie-down, with the support strut in place, we can deploy the front suspension. This is a Sikorsky design (circa 1930 ) and well proven as anyone who has seen a catalina flying boat or an F8 Crusader . The fuselage covers for the wheels are removed and the wheels come out, and with the aid of a small speed wrench (a crank )
    the weight of the road assembly is recieved.... a "Pre-loading" of the suspension. The covers that come off are then secured back onto the body and serve as fenders (it pleases the DOT to have fenders and sort of protects the canopy.) Clever design.
    Next, the rear drive wheel is deployed by the speed wrench, allowing it to touch ground and pre-load the weight of the road vehicle.It utilizes a shaft drive. Now, we can release the electical (multi-pin amphenol connector) and flex tubing for the Pitot tube ( air speed indicator..you see.) The mechanical connections for flaps, elevons rudders,brakes,throttle,mixture control etc are touch-based mechanical designs, thought out long ago by such visionaries as Molt Taylor. It comprises a pad, like a hockey puck which lines up with another like it. They push against each other. Right aileron against left aileron,
    rudder against rudder, throttle belcrank, mixture control etc.
    It's not rocket science,folks. A high school kid can do it. This would be all possible in about ten minutes or less. And perhaps taking more time for re-assembly for pre-flight. Still with me? Now, we can remove the four bolts and securing pins , and separate road vehicle . The road vehicle is powered by a "separate engine". After all, aircraft engines are somewhat expensive,and you only get 2000 hrs before major overhaul; although there are some extraordinary designs for automotive engines with forged crank shafts , different cams, and of course a cog-belt reduction gear to reduce propeller RPM from that of the V-8...usually from
    4100 rpm down to 2150 where the propeller is most efficient. This has been done, and is done every day. They even have a 3 litre Subaru flying.
    The point is that it is fool hardy to waste the flying hours that you get out of a 25,000 dollar aircraft engine on road travel, when we can get an inexpensive 2 cycle motor cycle engine to do the job much better, with popular parts anywhere, and other advantages. And less emissions.
    The canard wing (as mentioned before) is removed with two bolts and securing pins, and is secured to the wing. All of the expensive stuff stays at the flying field. Simple. Don't expose your aircraft to the street.
    The road vehicle is simple, light enough and ,if necessary, replacable.
    The fuselage,in fact has been proven to enjoy a better than 30 MPH crash rating. Substantially better than a bare motocycle in which YOU are
    "The Roll Cage". In performance, I should be satisfied with a road speed of 60 MPH, and reasonable braking performance.
    The aircraft is flown with a side-arm stick, and doesn't interfere at all with the steering wheel. Brake pedal is above the rudder pedal on the right, and the clutch is above the left. And if you can't drive stick, you have no business flying. Now...... is this for "Everyone" ???? Well, certainly not for those with a reckless driving record. The "Privilige" of flying any aircraft remains with those who prove themselves competent to earn an aviation licence, and justly so. What has kept the general public from embracing general aviation untill now has been the extraordinary costs (defrayed by home building),
    The difficulties of navigation ( now demystified by the advantages and miniaturization of GPS navigation, and glass cockpit displays enabling you to land in the fog anywhere...it's already here.)
    The required talent in sculpting an laminar air-flow wing from polyfoam ,s-glass and epoxy rein and graphite fibre...(this can be satisfied by a manufacturer in an autoclave ( a heated mold which produces a perfect wing
    in hours ,not months...Perfect! )
    And the will and confidence to fly. This has been satidfied under your nose by a generation of kids who can fly an helicoptor on their computer screen at the age of twelve. Yes...things have changed....a little bit. In fact, you can learn to fly on your computer . It's done all the time. You still need to go to school
    with a certified flight instructor. But it will be worth while. Having your transportation with you at all times is an idea whose time has come.
    What do we need??????
    Federal and state co-operation.
    And perhaps, a sort of race. Really.. a competition.
    All forms of racing have reched the envelope. Formula One, Nascar, NHRA
    Drags, USSC, unlimited air racing..you name them. They are all "Aberations of transportation" and presently do little for the advancement of our own transportation . But let's consider:
    A race between point A and B on the road with two people on board and a modest payload....say a bag of golf clubs and an overnight case for each person. Extra grand prix points for payload. Upon reaching point B, the vehicle
    prepares for flight. No speed required here. Half hour should be reasonable.
    No pit crew allowed or required. Then , a tech inspection....half hour.
    Then, pre-flight and to point C, for instance, Barnestable,Cape Cod to
    Nantucket; land, re-set as a road vehicle, go to town and get a tee shirt and a hamburger, and return, reconfigure for flight, fly to Provincetown,fuel up,stretch your legs and return to Barnestable. Now, THERE is a competition I should like to see on ESPN!!! After 5 years of competition, we shall have isolated a design worthy of production "As A Kit ", so as to make the idea affordable and possible. A kit could be made for under 15,000 dollars, and then powered how so ever you can afford. As of today, our GA fleet is getting so long in the tooth that we are on the point of replacing half of them anyway, and we haven't even mentioned the corrosion problems associated with alluminum skinned aircraft. By the way; fibre glass and epoxy aircraft don't rust. More importantly, you are the manufacturer, and responsible party.
    This has been the hallmark of the EAA for many years. They have a good record for quality, high preformance aircraft, and as a community, serve themselves well in all respects.
    As to the performance specifications we may see with this design :
    Typical performance charactaristic for such aircraft are :
    Take-off velocity----60 knts
    landing vel -- 55
    cruise vel --- 160kts and better
    top vel --- 190 and better
    range --- 800 NM and better ( just how long do you want to sit in one place anyway).
    Glide ratio ---10 to 1 and better, which means that, unlike some of the hair brained ideas lately, this aircraft, in an engine-out situation can linger in the air a damned sight longer than most anything else out there, and for me, a keen consideration in anything I should fly. I dislike copters...too many disposable parts. ANY AIRCRAFT WHICH LOSES POWER SHOULD STILL FLY AND CONTINUE TO FLY. tHIS IS VERY IMPORTANT. iF i AM AT 5000 FEET, I
    Also, with the advantages of an onboard nav computer with an excellent database, you can hit a function key, and the computer will tell you where to steer and make a best decision on where to put down in an emergency.
    Some comfort there. There also exists an balistic recovery parachute which will safely bring you down at the push of a button. REALITY, not SF.

    Well, thank you for your patience and persistence in reading my essay.
    If I find time, I shall certainly read your replies, and if possible answer your
    questions as best as I can.
    May you find this at least....interesting, and perhaps, one day inspiring.
    Yours, in the interest of aviation , Dan Bessett
    PS: I shall be happy to enertain questions.
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