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Thread: WINGS

  1. #171
    I haven't read this whole thread, but from what I have read, some teams have used adjustable wings. I was wondering if anyone has tried using an airbrake like on the McLaren MP4-12C, or if it is against the rules.

  2. #172
    Senior Member
    Join Date
    Feb 2004
    Costa Mesa, CA
    Nope, never.

    Not sure if this is "Active DRS" or an "Air Brake". http://www.youtube.com/watch?v=7wozSqFXitY

  3. #173
    Read the aero rules - they're very short. Seems like air brakes are 100% allowed.

    You're basically only limited in plan area of where you can have wings/aero devices, and no fans sucking air from under the car. ANything else goes.
    Rex Chan
    MUR Motorsports (The University of Melbourne)
    2009 - 2012: Engine team and MoTeC Data acquisition+wiring+sensors
    2013 - 2014: Engine team alumni and FSAE-A/FStotal fb page admin/contributer


  4. #174
    That's basically what I was thinking of, thanks.

  5. #175
    This year we are going to introduce front and rear wings and I would like to know some method ( appart from CFD's) to uplift this part in the design part.
    We have thinks tubes pitots but we can't afford this solution and the result of using gauges in the carbon fibre wishbone suspension will be insignificant.

    Thank you,

  6. #176
    If you have (a) consistent driver(s) simple lap times would be a good place to start. Check you fuel consumption as well though.

    Tuft tests are also very useful, some flo-viz paint maybe. But you don't me to tell you this, you'll have read this in all the relevant literature already wont you?
    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

  7. #177
    Interesting discussion so far - although a little heavy on the 'gut feelings'.

    Z is correct when he states that if you're looking to win economy/efficiency you should look to underbody aero heavily.

    However, if you're interested in winning competitions you need to look at the point sensitivity (not just lap time) of these vehicles to changes in parameters such as CL.A, CD.A, Power, Weight, CG height, Track width, etc. You're looking for the best COMPROMISE between them all. You also must consider the resources available to your team and what resources you can acquire within your area/country through sponsorship. Basically you want to focus your resources in the area where you will get the highest points return on investment. Bang for buck.

    Back to aero... Yes underbodies alone are very efficient at making a decent amount of downforce, if they are designed well. That is the problem. It is enormously computationally expensive to capture all the important flow structures, mainly from the large velocity gradients between the undertray surface (no slip) and the ground (moving no slip - symmetry/free streamline assumption is wrong if you think about it) - you need a lot of elements to capture the boundary layer, depending on your turbulence model try to get a y+ (wall/surface distance made dimensionless with friction velocity and kin. viscosity) between 30-100 and use a wall function. The other major resource sink is capturing the details about your longitudinal vortices and accurately predicting their stability and how they interact with your boundary layer growth which in turn controls where parts of the diffusing section will separate. Typically for a given circulation, the higher the vorticity the tighter and more stable the vortex. Don't even bother with 2D CFD or trying to design the undertray without rotating wheels and the rest of the car in there (level of model complexity will be driven by your computing resources, mainly meshing so time and ram), you'll get an educational benefit but it won't represent reality at all. Also, beware a lot of things claimed about the early ground effect cars and what is 'good' for an undertray. Due to the short wheelbase of FSAE cars seals are not particularly helpful, full ones anyway, as a lot of the undertray flow needs to come from the sides.
    ^^ all of this said, it's still a good idea to take your results with a large grain of salt. Nothing will beat physical testing in the real world - either via simply driving different radius circles and logging G's or through pressure tapping surfaces.
    The main advantage I see in CFD is using it as a visualization tool which helps you better understand what's happening (or maybe not happening!)..

    ^^On-track pressure contour vs. CFD pressure contour for our 2011 undertray.

    One thing Z has mentioned previously, is how it is advantageous to use ‘slotted’ flaps (like the flaps on most of our current wings) as part of the diffusing section of the undertray. What this actually does (this is very simplified..) is ‘restart’ the boundary layer growth by injecting high velocity air into the underside of the diffuser tangent to the surface, (hence why slot gaps are converging – to accelerate the air) – this also ‘pumps’ the diffuser and draws more air through the throat further increasing downforce. One major problem with ‘conventional’ FSAE cars is getting usable air to these slots, typically behind the driver the flow is quite stagnant.

    Start rant..
    You’d be surprised at how few engineers actually understand how wings work – the actual mechanism for lift.. and if you start saying anything to do with Bernoulli you get an F – how can a flat plate make lift? Go study Helmholtz’s laws of vorticity (if you’re really interested check out Hans J. Lugt).
    End rant..

    As for not using/using tunnels... Tunnels in my opinion are a consequence of packaging and vortex management – they’re useful for building and controlling vortices which you can use to entrain more flow/cleaner flow into the undertray as well as keeping your boundary layers under control. One concept I’d love to explore is taking the full frontal packaging area (762mm in front of front tyres) for the undertray inlet, you’d likely end up with the throat occurring around/before the front tyres (not an adverse pressure gradient so you can converge quite quickly), and then diffusing for the length of the vehicle (305mm after rear tyre)... A major problem here is packaging and CG height (which dictates how narrow track you may go – which for FSAE legal tracks is more important than aero –and any load sensitivity) – you don’t want to go sticking a 30-70kg engine 500mm above the ground to clear the diffuser. You’d have to come up with a pretty funky vehicle layout to make all this work... I’m thinking engine/batteries between the left side front and rear wheels, which is less ‘aerodynamically usable’ than the centre of the car, and the driver between the right pair of wheels. This would give you ~1.85+ sq. m of effective undertray, at 60 km/h with an average Cp. of 3-4, you could get 1000+N of downforce for very little drag. As the throat is so far forward, the centre of pressure would be around the front wheels so you’d need a rear wing to get a decent aero balance, which will also drive the diffuser harder (you generally want your centre of pressure at ground level to be on or just rearward of your CG).. so you could end up with 1500-2000N downforce at 60km/h for relatively modest drag.

    So, this brings me to another important point – risk management. The above (batshit crazy) idea would be a large risk – you’re basically stepping into unchartered territory with any number of likely problems. Teams already have enough issues designing the same car they did the previous year with a few minor modifications. I’m not trying to say this approach is wrong, but it’s something you as a team need to decide on – what is important to you guys? An example of this is probably Marylands 2012 car, the car had an awesome aero design but they seem to have compromised too much in terms of serviceability and reliability (hope you guys get this concept going!).
    You can design and build the above supercar but chances are you won’t have it running well before competition – if you turn up to competition with only one week of testing and driver training you will still be slower than the traditional 200 kg non aero 600 cc that has been testing for 3 months. An important part of engineering is acknowledging your capabilities and limits. You need to balance your risk vs. reward.

    As for CFD of the car actually ‘cornering’ with curved flow, we’ve just developed a model that does just that. So we can set any chassis slip angle, front wheel slip angles, and corner radius to see the effects (especially aero yaw moments). Essentially it’s just a curved domain with a rotating reference frame and an added inertial component to the fluid body forces. This is a lot more computationally expensive, so it’s only really used to double check designs and help make the compromise on front and rear endplate sizing. (As just yawing in a conventional wind tunnel is incorrect)

    ^ Shows the asymmetry of the turbulent kinetic energy of the wake while cornering in the 2011 car.

    The first ‘wind tunnel’ was actually a whirling arm (as Z said) made in the 1700’s by Benjamin Robins, and slightly later used by Cayley who built the first glider from what he learned. This approach was later abandoned due to the model running in its own wake (from the previous revolution).

    ^ Whirling Arm

    To put a number on how much downforce I think a well developed undertray on a traditional FSAE car could make.. I'd say between 350-450N at 60 km/h, with negligible increase in bare vehicle drag. All the underbodies of the top Euro/US/Australian teams I've seen (most of them) sans wings, I'd say they're between 100-250N at 60 km/h.
    For a team looking to try out aero for the first time, I'd recommend front and rear multi-element wings. Basically because it's easy and low risk. This all clearly depends on your team, resources, and what you actually want to get out of the competition.

    Wow, that was some good procrastination! Sorry about the mass of text!

    Monash Motorsport
    Aerodynamics/Vehicle Dynamics/Management

  8. #178
    Senior Member
    Join Date
    Nov 2010
    NSW, Australia

    thanks so much, that was epic!

    UoW FSAE '07-'09

  9. #179
    His name is Luke Pherrson.

    *slow clap*

    Good post. Glad to see this contribution surface from one of the top aero research groups.

    Luke, are you familiar with the front mounted diffusers from formula 1?

    This one being the Willianms W15:

    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  10. #180
    What makes you say that the ground being a streamline is wrong? In 2D, neglecting the fact that it's rough, the air cannot penetrate the ground, so the velocity perpendicular to the ground is zero. Assuming no wind, the ground has the same speed as the on coming air. This means that the velocity vector of the air at the ground is tangent to the ground, so the ground is a streamline.

    My fluids prof has tried to beat it into us that wings make lift because they curve streamlines which creates a pressure gradient due to centripetal acceleration. Although I know there's another reasoning based on circulation and what not.

    Thanks for your advice on underbodies, especially regarding analysis. My team is looking at adding aero next year and given that we don't have much in the way of computational resources(or people very knowledgeable about what CFD is actually doing), I think we'll stick to wings. I was thinking of looking at something in 2D to get a rough guess of how much downforce we'd be making to see if it was worth our time, but given what you've said I don't think I'll bother.

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