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Thread: WINGS

  1. #161
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    Originally posted by Dunk Mckay:
    I'm currently tussling with the interference in airflow that arises from turning wheels...
    Dunk,

    "Turning" = steered?

    Briefly (it is after midnight), I think the bigger issue here is that the wind approaches the front of the car at a large angle, maybe, ~45 degrees from straight ahead (ie. from the direction the front wheels are pointed). That is, the airflow is effectively curved. More correctly, the air is stationary, but the car is travelling on a curved path.

    I reckon most CFD models ignore this effect. Wind tunnels are incapable of modelling it (they only do straight flow).

    But not really a problem, IMO, for the right design of undertray...

    Z

    PS. Consider Monash's huge front end-plates, as the car goes around a hairpin!

  2. #162
    Err, yeah sorry, steered. I knew there was a proper word for it. I think the heatwave here is frying my brain.

    I guess I can be a bit too CFD minded sometimes.

    So let me think... (insert grinding gear noises here).

    If the under tray was divided down the length of the car into separate ducts by various fins/internal skirts, whatever you want to call them, with the outermost ducts further out than the internal distance of your front wheels.
    They remain relatively untouched when in a straight line as they are behind the front wheels, except for any air you can pull through with a good diffuser section that is (diffuser section that narrows between the rear wheels of course, but as it is expanding upwards this can all be compensated for.
    But when cornering, the outside one recieves air "scooped up" by the upright mounted body work (big turning end plate), and the inside one recieves air directly from the outside of the car, maybe also some flow redirected slightly by the wheel as well. I'm thinking cleverly designed brake ducts and carbon hub caps that could direct air from above the underwing out around to the lesser used side in cornering.

    Ok, so that last part might be a bit excessive. That hard part will be getting any useful test data. The only real way is to build it, stick it on a car and run some pressure sensors and tassles.

    Has no one made a CFD program that can simulate stationary air and moving objects? I wonder if you could make a really simple "wind cylinder" that spins car models on the end of big arms and looks at the air flow around the model? Easily variable corner radius, just slide up and down the stick. Surely if it were practical someone would have done this before. Perhaps the airflow is too disturbed after the first pass, also any significant sideways disturbances are going to have some effect when you get to the other side, unles you have a big piller, but then you're too close to a wall...

    Hmmm... interesting.
    Dunk
    --------------------------------------------------------
    Brunel Racing
    2010-11 - Drivetrain Development Engineer
    2011-12 - Consultant and Long Distance Dogsbody
    2012-13 - Chassis, Bodywork & Aerodynamics manager

    2014-present - Engineer at Jaguar Land Rover

  3. #163
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    Originally posted by Dunk Mckay:
    I wonder if you could make a really simple "wind cylinder" that spins car models on the end of big arms and looks at the air flow around the model?
    ...
    Surely if it were practical someone would have done this before.
    Dunk,

    And indeed they did! Right at the beginning of this great big aero adventure the Lillienthal brothers, Otto and Gustav, used the above sort of mechanism to discover that thin curved plates made much better wings than flat plates. These "arcuate", or "sail-like", aerofoils are very efficient.

    Otto wrote a book in 1889(?), "Birdflight as the Basis of Aviation", that is a fascinating story about two farm-boys who decide that they are going to fly. Any students wanting to push the envelope on FSAE aero could learn a lot from this book. It is basically about taking a "big picture" view, doing the fundamental calculations, learning from nature, perserverance, etc., etc.

    Unfortunately, in 1896(?) after ~2,000 glider flights Otto crashed and died. Nevertheless, his and Gustav's work inspired many similarly minded young men, including the Wright brothers, and the rest is history.
    ~~~o0o~~~
    ... any air you can pull through with a good diffuser section ...
    VERY IMPORTANT! You do NOT have to have a high mass flow of air under the car for good downforce.

    Joe Katz, on page 50 of his book "Race Car Aerodynamics", says,
    "There are those stylists who make every effort to block the flow under the vehicle
    and those brave few who try to push as much air as possible under the car.
    ... if high downforce is sought then the latter approach is inevitable."


    NOT TRUE! And, surprisingly, on page 184, Figure 6.7 D, Di, & Dii, Katz actually explains how to get high downforce with minimal airflow under the car.

    All this is easier to explain with a few pictures. When I've finished draining the swamp (literally!, and in the middle of this cold, wet, and miserable winter!), I'll see what I can do.

    Z

  4. #164
    Hi all, long time silent, I designed a (spool) diff for UQR in 2007, had a car accident, now I'm designing a wing kit for Melbourne. No one I can find in the real world knows anything (and it's more than a little late in the year!) so can you guys help? I've been reading lots but I can't find anything about this.

    https://www.dropbox.com/s/l9rshmsceb12dv9/wing.png

    That is a 3 element wing with 2 elements at 50° to the ground. It isn't my actual proposition, but I wanted to try a high angle of incidence to confirm my intuition that (*) wings with high incidence can only provide "downforce" normal to the wing surface - ie drag, and not much downforce.

    Yes, I was right. But, the best I can design (with a good angle of incidence) gives 8 kg of downforce, nowhere near the required 100kg to ensure the inside wheel is loaded through a corner. Or, I suppose it doesn't need to be. But still, we need ~150kg to generate 0.5G of downforce. Are my targets too high, or how can I reach them?

    Thanks, and sorry if I'm missing a post that I should have found.

  5. #165
    What wing profiles do you use?

    First i would chose a wingprofile for low speed and then go with that and add. Some more wings.

  6. #166
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    Originally posted by Kent Slaughter:
    That is a 3 element wing with 2 elements at 50° to the ground.
    At a brief look, I think the main (first) profile has a too big AoA.

    I think you can find useful looking for aeronautical flaps (I found useful to have a look at Abbott).
    Don't forget that aeronautical airfoils you can find easily are designed for low drag.
    Lorenzo Pessa

    D-Team UniPisa (alumni of E-Team - Università di Pisa)
    FSG & FSAE-I 2009-2010

  7. #167
    Senior Member
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    Oct 2002
    Location
    Melbourne, Australia
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    Kent, I would be careful when making big assumptions based on little data and intuitions.

    There are many reason why that design might not have made much downforce, and the one you mention is just one of them.

    How big was the wing that you tested anyway?

    Diving straight into a complex multi-element design is a bit dangerous. Better to run some CFD on simpler and well documented single element profiles first and build from there. Compare with the published data as you go, and refine your model and techniques, and check your sensitivities to turbulence models, domain size and mesh refinement etc.

    Regarding the high angle of attack of the flaps, you are correct that the flaps themselves will not generate much downforce in and of themselves. But what they do do is drive the circulation, and pump heaps more air under the main plane, and that is where the downforce is actually generated.

    So think of it as a system with each part affecting the other parts. This philosophy (perhaps sadly!) extends beyond just the wings. They interact with the rest of the car quite strongly.

    So once you get your wing "perfectly optimised" be sure to add it to the car and see how much performance is lost, and then start the process all over again
    Regards,

    Scott Wordley


    Scoring in every event for the last 12 comps running!
    http://www.monashmotorsport.com/

  8. #168
    I think if you do it right as in keep it light and not ruin the CG they can be effective. I can't imagine under trays do a lot for us at the speeds we see though. Our last one made 20lbs of down force at 40mph and weighed 12 pounds... But what do I know I'm the engine guy.

  9. #169
    i have a question about 2D Airfoil design. There are a few methods - Airfoil optimisation/Inverse Design ETC. Does every FSAE team out there write their own code for airfoil design or are there publicly available codes? (even PROFOIL is not available). Writing a code for inverse airfoil design will take at least a month or two. I presently have 3 airfoils which I want to use for a optimised airfoil and am stuck with only one option- write my own code? Considering the fact that I have to design the complete package by January, is there any way out?

  10. #170
    In the Human Powered Submarine we used JavaFoil (available online) to make preliminary design. It worked quite well for that purpose. There is also X-foil that is available for download under a GNU license.
    :::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::
    2007-2012 - Suspension, chassis, and stuff (mostly stuff)
    Université de Sherbrooke

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