Aerodynamics discussion

Discussion in 'Car Modding' started by jtbo, Jul 26, 2012.

  1. jtbo

    jtbo Registered

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    Mystery and black magic surrounds topic of aerodynamics, some know stuff, some guess stuff, maybe we could try to discuss subject to level of common car guy.

    First mystery is this one, comment is based from what I have gathered during the years, but it still is in testing phase:
    Code:
    [BODYAERO]
    BodyDragBase=(0.477945)           // base drag Volvo 360 cd 0.38, rFactor needs adjustment for frontal area. Cdrag = 0.5 * 0.38 (cd) * 1.95 (frontal area) * 1.29 (air density) = 0.477945 must test if works, alternatively leaving out * 1.29 may give better results
    Result is 38.4N (Lazza's motec plugin) of drag at 100kph, top speed of about 180kph.

    220 000km driven engine as I measured it, (which probably is not 100% accurate as drivetrain loss etc. might be bit inaccurate)
    Powerdyn software was used and ignition coil pickup for rpm data. Specs of new motor are 101hp and 156Nm of torque.
    Code:
    RPMTorque=( 0.0, -19.5, -19.5)
    RPMTorque=( 500.0, -18.2, 79.8)
    RPMTorque=( 1000.0, -18.3, 119.6)
    RPMTorque=( 1500.0, -21.3, 132.0)
    RPMTorque=( 2000.0, -27.0, 136.0)
    RPMTorque=( 2500.0, -33.8, 142.9)
    RPMTorque=( 3000.0, -39.9, 147.5)
    RPMTorque=( 3500.0, -43.5, 148.7)
    RPMTorque=( 4000.0, -49.0, 145.6)
    RPMTorque=( 4500.0, -53.0, 142.3)
    RPMTorque=( 5000.0, -57.7, 139.5)
    RPMTorque=( 5500.0, -61.3, 128.8)
    RPMTorque=( 6000.0, -63.7, 117.5)
    RPMTorque=( 6200.0, -67.4, 95.0)
    RPMTorque=( 6300.0, -69.8, 71.1)
    RPMTorque=( 6400.0, -73.2, 45.3)
    RPMTorque=( 6500.0, -77.5, 5.2)
    I think that top speed in specs were around 180kph, but can't remember exact value now, I think it was 188kph, but need to check that.

    Here are some formulas and table showing how much BMW 8 series has drag at different speeds (looks to be nice source anyone making 8 series BMW for rF2):
    http://www.e31.net/resistance.html

    Copied from URL above:
    Code:
    FAir = A/2 × Cd × D × v2
    
    with
    
    A being the frontal area of the car in m2,
    Cd being the drag coefficient,
    D being the density of air (1.29 kg/m3) and
    v being the velocity in m/sec.
    I get 1.95/2 x 0.38 x 1.29 x 27.78^2 = 368.8437301N, which is roughly 10 times (9.6) higher of what I see logged when looking logs with Motec.

    Obviously something in somewhere is at fault, maybe formula I found? Suspiciously close to 9.81 times smaller number, maybe somewhere is kg/N conversion error?

    It usually is me however that has made error, so I suspect there is more to learn from this.

    In rF1 using this formula for drag my car was bit slower, not much, some did say that their car became too slow. But then again I did use lot lower rolling resistance values which I did calculate from real tire data that I happened to find and I did get really close to results what I excepted from top speed.

    What do you think, why logs show only 38.4N? What do you think about car being able to run 180kph, is that too much for worn motor? Anyone has other ways to calculate this specific parameter?

    I guess these two are next most important values to get right for street car without wings:
    BodyCenter=(
    BodyFore=(

    But, there is big but, how to find aerodynamic center of vehicle without wind tunnel, same for lift (rarely downforce with street cars).
    I can see front and rear downforce being logged too, but problem persists of finding what real values are, internet is useless for those, also even somewhat close estimate would be better than nothing at all, still not a clue how to get even that.
     
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  2. erale

    erale Registered

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    Maybe the plugin does output kg instead of newton?
     
  3. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    There is no black magic... just maths :)
    I can't tell you, whether the telemetry plugin is ok or not but rF 2 itself does the job as expected in that matter.
     
  4. lordpantsington

    lordpantsington Registered

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    FWIW I thought the suggested D in rf was 1.2029 kg/m3 ?
     
  5. erale

    erale Registered

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    Still doesn't explain the huge difference.
     
  6. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    Just checked one of my cars and yes, it looks like the telemetry plugin shows drag values in kgs. So, that's the reason for your issue.
     
  7. jtbo

    jtbo Registered

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    That is interesting, it differs just slightly from +20C sealevel value of 1.2041kg/m3, maybe it is 100kpa and +20C?


    Has others got sensible readings using plugin? It can always be issues with my undertray or other aero settings too.

    Mathematics is black art for many of us, as long as I can find formula to put into excel everything is fine, I don't need to bother with mathematics too much, but so far haven't had luck finding formula which would give center of aero forces.

    Oh yes at 100kph I got -71 downforce at rear, which I take is lift, but it is not much more that I know from that part, it probably is not impossible, but most of stuff seem to be more of college student or professor level instead of common car guy level, that complicates things a bit with this aspect.

    edit: Thanks LesiU, that is good to know, maybe Lazza will release update as some point for this.
     
  8. lordpantsington

    lordpantsington Registered

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    I hope you don't mind the tack on posting. Not sure if this was a "lets talk about..." thread or not. Relevant to discussion at least.
    I'm trying to calculate DF @ 100mph (44.704 m/s). From my understanding the force is equal to C*V^2. To find C I need to use
    RWSetting=6 &
    RWLiftParams=(-0.0697,-0.0130, 0.0003)

    -.0697+(-.013*6)+(.0003*6*6)=-.1369
    -.1369*44.704*44.704=-273.5875N This is not a lot of DF for a rear wing (61.5#).
    Am I missing something? Car has no diffuser.
     
  9. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    It depends on what wing, from which car. In the end, you use base value to balance the whole f/r downforce. Set it correctly, just to match downforce at setting 1 (or 0) and then make sure, df increase per setting is OK.
     
  10. lordpantsington

    lordpantsington Registered

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    I see I missed posting the obvious question.
    Is that downforce example correctly calculated?
     
  11. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    Yes, it's correct :)
     
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  12. newtonpg

    newtonpg Registered

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    Thank you guys, I've learned a lot with you.

    I have a question and I suppose it is simple but I really dont know.

    Now that we can estimate the vertical forces and were they act, how they are distributed to each wheel?
     
  13. jtbo

    jtbo Registered

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    Any topic I open are open discussions and information sharing free for anyone, I learned again something, I think so at least, so it was very good that you did post here :D

    I hope to have these discussion threads from every area of car modding so that at the end we have covered everything, but it takes time of course.
     
  14. lordpantsington

    lordpantsington Registered

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    Found out what I was missing/why the wing is not producing expected DF. I need to also consider the "BodyFore=" entry which also adds drag and downforce into the system. Values are N/m/s for the 3 entries in the usual rf coordinate system: (Left+/right-,Up+/down-,Fore-/Aft+). Multiply each by speed to get a moment of force.
     
  15. jtbo

    jtbo Registered

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    There was comment about that, lift was important, drag was overwritten if I recall it correctly.

    Body itself has lift or downforce and wing has it's own too, difficult if one does not have data from without wing and with wing or wing tested separately, anyway one should always try to have them separate for adjustments to have correct effects, or that is what I think at least.
    There can be odd things if all downforce is put to body or the wing, changing adjustments then would make things bit odd as in reality body has what it has and only wing part is changing.
     
  16. lordpantsington

    lordpantsington Registered

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    Ah yes,

    From Formula ISI '06

    "BodyFore=( 0.0,-0.170, 0.370) // Aero forces from moving forwards (lift value important, but drag overwritten)"

    Has anyone zeroed drag out then and noticed a difference?
     
  17. jtbo

    jtbo Registered

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    I think that I have some cars without drag value, some with completely different values that what it should be and some with proper value and if there is effect it is not big as I get very good results either way. Of course some proper testing would be required to verify, just simple steady 100kph and checking drag value from motec with value being zero and with some proper value should do the trick and tell if there is any reason to have value there at all.
     
  18. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    N/m/s are units for dampers (how much damping they provide). Here, for the BodyFore parameter, it's just force in Newtons

    The last value is not used anymore, as you have that covered by BodyDragBase. So, yes, you can zero it and no one will get hurt ;-)
     
  19. lordpantsington

    lordpantsington Registered

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    Multiply the coefficients [unknown units] by velocity [m/s] and get a force [N], the unknown units must be N/m/s.
     
  20. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    F = 1/2*C*p*v[SUP]2[/SUP]*A

    F - Force [N]
    C - coefficent... of drag, in our case (no units, anyway)
    p - density of...air, in that case [kg/m^3]
    v - velocity [m/s]
    A - planform aera [m^2]

    rF takes:
    1/2*C*p*A as a single coefficent, so its unit is [kg/m] (becase you have (kg/m^3) * m^2 )- let's call that coeff as "Cf"

    So, for rF, instead of...:
    F = 1/2*C*p*v[SUP]2[/SUP]*A

    ...you use:
    F = Cf*v[SUP]2[/SUP]
    ...and Cf is what you put into HDV.


    Now, the units from the above:
    Cf in [kg/m]
    v in [m/s]

    Cf*v[SUP]2[/SUP] => (kg/m) * [(m/s)^2] = (kg/m) * (m^2/s^2) = kg*m^2/m*s^2 = kg*m/s^2 = N


    So, to sum up, those particular coefficents are expressed in [kg/m].
     
    Last edited by a moderator: Oct 20, 2012
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