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Thread: Xfsaer wants to build a fan car type FSAE car for autocross type demonstrations

  1. #31
    @Z I did your water bottle experiment with a shop vac cleaner (a big one) - could pick up up to 0.8 lt of bottle without any gap .Using tooth picks i found (apx 2mm thk - not 1mm like in your experiment i couldnt lift it and went for 0.5 lt which i could not lift either -settled for apx 0.35lt which i lifted

    Upon close inspection there is something important that happens to the bottle as the vac hose approaches - they deform making the leakage smaller so the suction stronger (try to approach the hose slowly to the bottle and see the bottle deforming (at least my 0,4-0,5 thk bottle did that

    So the gap between the road and the skirt seems to be a super important factor

    Curiosity led me to ask a friend to look at the back of his vaccum former/thermoforming machine where he found a radial flow snail type fan - as Z proposes but in this application we have no gaps. Try leaving a thermoforming polycarbonate sheet to stay a bit more over the oven of a vac former, during the upcoming forming process it is possible that due the extra sag from the extra unwanted heating that we introduced , the sheet will become so thin that upon vacuum over the mold a small hole will open up somewhere and suddenly all pressure is lost and we possibly have to bin the part...

    So why leave ANY gap if it drops the vacuum performance so much , why did Chapparal 2J as well as the Cheapparal Vette leave apx 12mm distance from skirt to road? (the 2J went to great length to keep the movable lexan skirts at constant gap with the road with a specially designed mechanism) , why not just leave 2-3mm and form the front part of the skirt V shape as a ship (or put a V shaped rubber sweeper in front of the skirt ) as Z commented so that the debris is not stuck in the minimal gap ?

    The answer (in my opinion) is to let the actual engine of the vehicle propel the thing forward and not stay stationary!! Remember the road & track competition mentioned on the "Build my own sucker car " thread ? Someone mentioned there the same thing happening to a modified vacuum kart - had difficulty moving! I guess since he had no suspension or adjustable height skirt he actually had to play with the vac engine throttle or openings in the vacuum floor to get the thing moving ....

    Try to put the vacuum hose of your vac cleaner on the ground and moving the vertical hose across the ground and then putting some distance and moving it - it was much easier for me

    I guess when introducing a gap the benefits from the radial fan evaporate as you obviously have to draw much volume of air to create the needed pressure difference - i dont know obviously the characteristics of a compounded radial fan set up but the gut feeling i am getting is that there is significant airflow taking place in the case of the 2J and BT46 so that their axial fans are not stalled
    What do you people think?
    Last edited by Xfsae; 11-03-2013 at 07:30 PM.

  2. #32
    Senior Member
    Join Date
    Mar 2005

    Your posting is still "way too scattered". You are unlikely to make progress by randomly plucking ideas and numbers out of thin air. You have to think a bit harder about these things. Here are some examples:

    1. The "Bernoulli and continuity equations" are all but meaningless here. I know the Cheaparral crew used them, but copying stuff off the web is not a well-reasoned engineering approach.

    2. A 50 kg, 10 hp industrial diesel is good enough for static testing, but rather expensive. On a directly linked webpage I saw a cheaper, ~30 kg, 16 hp petrol engine. For use in the actual racing car I am sure you could find an engine that is a lot lighter again, and possibly with even more horsepower.

    3. A fan (any type) can have atmospheric pressure at its inlet, and positive (above atmospheric) pressure at its outlet. Or it can have a "vacuum" (below atmospheric pressure) at its inlet, and atmospheric pressure at its outlet. Or a bit of both. You should be looking at the second type of curves.

    4. The "power lines" in my earlier fan link indicate the outer envelope (upper-right boundary) of Pressures and Flow-Rates that such a sized motor can deliver. Engineers are expected to be able to read such diagrams (even when such diagrams are drawn up by the tea-lady...)

    5. As I have been suggesting since the 2005 thread, a VERY IMPORTANT part of this problem is SKIRT DEVELOPMENT. Specifically, it is beneficial to minimise skirt leakage, thus lowering power requirement, and increasing downforce. So, once again, (hopefully more clearly):

    5a. If you MAKE THE DECISION that you want SKIRTS-OFF-THE-GROUND (for whatever reason, say, personal preference, or "no-scraping" Rules), then you will have a HIGH LEAKAGE FLOW-RATE, and consequently you will (most probably) need a LARGE DIAMETER AXIAL-FLOW FAN powered by a HIGH HORSEPOWER MOTOR.

    5b. However, if you AIM TO MINIMISE SKIRT LEAKAGE, then you should be able to get MORE DOWNFORCE from a SMALLER RADIAL-FLOW FAN with a LOWER HORSEPOWER MOTOR.

    6. Note that if you take option 5b, but then PUSH THE SKIRTS ONTO THE GROUND with the FULL VACUUM DOWNFORCE, then you are being REALLY STUPID (!!!) because, of course, you will find it very hard to move the car!

    7. So look closely at the Bowland skirts for inspiration... Or, more plainly, the SKIRTS CAN SEAL QUITE WELL even if they ONLY TOUCH THE GROUND LIGHTLY! Or, even more plainly, DO NOT HAVE A RIGID CONNECTION BETWEEN THE SKIRTS AND THE ROOF OF THE VACUUM-BOX!!!

    8. I can't do all of this for you...

    Last edited by Z; 11-04-2013 at 08:53 PM.

  3. #33
    Z- you writing in capitals like shouting is not appreciated ,your thoughts come across as scattered too in many threads but i still do appreciate your contributions ,as a matter of fact i am looking closely at a Z....... suspension for a future project I thing it can be good for aero vehicles and i hope to see some implementations of it in the future

    The "Bernoulli and continuity equations" are not meaningless if there is significant airflow (and at least in the case of the BT46B it is)- they are used in the initial design stage of all types of axial flows - been there done that before (aircraft props - non ducted) .Maybe i am coming across as a bit uncertain and wishing for participation of others because i am not a know all type of guy ... I like to hear what others want to say .I have never built a fan car before. I am honest

    In any case if you don't use Bernoulli and continuity equations to find the CFM , how are you going to find the ballpark CFM of your system? Which other method there is? (You need static pressure and CFM to predict BHP required properly according to typical methodology as seen on the document attached below)

    Regarding your point (4) - i am an engineer

    (though i dont appreciate your comment - engineering is very broad and i could have been an engineer with no knowledge in fans - that doesnt mean someone cannot point me in the right direction without my status as an engineer being shaken - knowledge can be acquired , the willingness for knowledge is the hard thing to find)

    and i think the horsepower curve being a straight line did not make sense and also its inclination and point of origin seem strange - i cannot make anything useful out of it . Also the vertical scale showing BHP ratings (which we need) is missing and instead of it we get another vertical scale of static pressure (we now have 3 when we need only one) - so we cant use the curves even if they are correct.

    Another thing that seems to be missing totally also are system lines of which we need several according to the methodology i attach in the document below:

    Just for clarity (and please correct me if i am wrong) i follow these steps

    1) I start from the static pressure and CFM calculations (which i asked a second opinion-double check here because those data determine all the rest)
    2) I need at least one static pressure curve (we get many for different rpms in Zs attached document: http://www.aircontrolindustries.com/.../01._ep10a.pdf) with its corresponding horsepower curve (none provided)
    3) I need several system lines that show how the blower performs inside systems of differing impendance which is also necessary (according to the methodology below-last pages) to predict the hp ratings for our system (none system line provided)

    Methodology described Here: ftp://www.nyb.com/Letters/EL-03.pdf

    Also i posted all my calculations and results for MY project for anyone - not only you - to check
    The fact that i followed the guidelines of Cheapparal ( in any case if you read about them they are all engineers and they did contact the manufacturer of the fan so the whole process they followed is bound to have some ground) does not subtract anything from my effort to learn - we all have to start from somewhere

    Your comment :" if you take option 5b, but then PUSH THE SKIRTS ONTO THE GROUND with the FULL VACUUM DOWNFORCE, then you are being REALLY STUPID (!!!) because, of course, you will find it very hard to move the car!" might refer to me trying to move the vacuum hose across the ground - remember the 1979-1980 ground effect cars ? they were using the harder springs they had to push the skirts to the ground -same thing - they just used Bernoulli effect to create the suction instead of the fan

    My gut feeling is that there many ways to implement suction/active aero and looking at the pictures the BT46B team switched from one to another one in a matter of 1-2 days during the qualifying and race days of the Swedish GP 1978 ( they must have been living a nirvana of active aero explorations that weekend) .By the way the Swedish GP 78 is all in the youtube (German commentator) - funny to see all other team principals like Patrick Head running to take a look at the fan in any way they could after Lauda stopped the car

    And one more engineering question (i think i posted several others good ones and well thought out that were unanswered): why are the final version of the BT46B side skirts bent inwards? ( they are the wrong way around if you are just looking to seal the floor)

    There is a lot to be learned on the subject from anyone- it is not just redneck black and white stuff- i hope more people with access to specialized Uni /industry facilities like wind tunnels get involved - i will be happy to participate even as a tiny part of a movement of resurgence of active aero on ground vehicles... (how cool would be to take internal pressure and drag values for an active aero car model on a rolling road )

    As long as we live we are all students
    (By the way what we are discussing here is also useful to people designing FSAE car cooling systems -air cooled single for example or choosing a bigger fan for the radiators of water cooled engines with minimal impact on aerodynamic drag and max efficiency)
    Last edited by Xfsae; 11-05-2013 at 02:10 AM.

  4. #34
    Senior Member
    Join Date
    Mar 2005

    1. My capitalised shouting is an attempt to get the message across, so that I do not have to keep repeating myself.

    2. The "continuity equation" is applicable here in a limited, though very trivial, sense. The conventional "Bernoulli equation" (= static pressure inversely proportional to velocity squared) is most definitely NOT applicable. If you think it is, then please give a detailed example of such a calculation. More appropriate equations can be found somewhere on this Forum (or on the web, or in a library...), relating to "the restrictor" (which behaves like a small skirt gap).

    3. Regarding fan horsepower, you are comparing documents from two different manufacturers and wondering why they look different. It is quite common for different companies to present their information in different formats! Anyway, I have already explained how to estimate the required horsepower (either from the curves, or from first principles), so I won't repeat myself here. Yet again.

    4. The idea of "SLIDING" skirts is that they move up and down INDEPENDENTLY of the car body. This is so that they can be designed to push down against the ground quite lightly (say, with only a few kgs of force), even while the body itself is being pushed down with many tons of force. This is a VERY IMPORTANT CONCEPT to understand (hope I don't have to repeat it...).


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