+ Reply to Thread
Page 6 of 10 FirstFirst ... 4 5 6 7 8 ... LastLast
Results 51 to 60 of 94

Thread: Plotting Cn-Cy and Cn-Ay Graphs.

  1. #51
    Senior Member
    Join Date
    Mar 2008
    Location
    Brighton, MI
    Posts
    677

    Reading Material

    Claude:
    I have enjoyed reading these articles very much. Most of my career was targetted on the 'pleasability' and 'safety' aspects of vehicle dynamics. Your talking points show that the racing focus demands attention almost entirely to the balance and ultimate/max limit control regimes.
    Great stuff ! (Check your e-mail).

    I remember way back in time (1969) as a Summer Co-Op at GMPG, we had a car equipped with front and rear rocket engines mounted on the front and rear axle locations on the body (powered by hydrogen peroxide) that could 'augment' yaw moments by a rear seat operator that could surprise a driver or help them through a course.

    I still have a few pieces of that system in the barn and the a pyrex bottle that held the peroxide that I smuggled out many years later.

  2. #52
    Senior Member
    Join Date
    Mar 2008
    Location
    Buffalo, NY USA
    Posts
    321
    Bill,

    Looks like that rocket motor setup goes back pretty far, https://www.sae.org/publications/tec...ontent/640001/ "Vehicle Handling Response to Aerodynamic Inputs", 1964. Did you ever talk to Tom Bundorf about this bit of history? CAL/Calspan had something similar used for crosswind simulation around the same time, but I'm not sure where it came from.

    RCVD Chapter 8 has quite a bit of detail on racing applications of MMM, for anyone thinking of rolling their own.

    -- Doug

  3. #53
    Senior Member
    Join Date
    Mar 2008
    Location
    Brighton, MI
    Posts
    677
    Doug, the best aerodynamic wind gusts came from 2 full blown Continental piston airplane engines, props and all with wire guards. Mounted on steel frames painted yellow as I recall. They would be placed along side a road course and you would drive by them. By today's OSHA standards, it would never 'fly', so to speak.

  4. #54
    Senior Member
    Join Date
    Mar 2008
    Location
    Buffalo, NY USA
    Posts
    321
    Quote Originally Posted by BillCobb View Post
    the best aerodynamic wind gusts came from 2 full blown Continental piston airplane engines, props and all...
    We made a survey of these types of facilities, c.1980. From memory, there were nearly a dozen around the world, some had several big prop fans as you described, up to six(?) in a row. Many had been used for a few years until test work dried up and were no longer operating. Judging by the crosswind handling of some current cars, it might be a good idea to resurrect this testing in some form.

    We did some low-buck projects to look at crosswind effects on single track vehicles (2-wheelers). One used a solid propellant model rocket engine (not much thrust) and another used a Cessna taildragger. Before the USA went nuts about airport security, it was relatively easy to set up in a corner of the Buffalo airport. The little plane was tied down so the propwash blew across an unused taxiway and we rode through the sharp-edged "gust".

    The most elegant (and lowest cost) setup used a string--which could be attached at different places to simulate different locations for the center of the aerodynamic sideforce (simulating different types of streamlined bodywork)...a second vehicle rode/drove alongside the test bike and the experimenter tugged on the string.

  5. #55
    Bill, Thank you for your remarks. From you, I take them as a big compliment.

    At OptimumG we always focus on balance more than grip for one simple reason: Drivers can feel the balance not the grip.

    I defy any driver to feel the difference between 3.1 and 3.2 G of lateral acceleration. Some will tell you that the car has more grip simply because they look at the lap time. 0.1 G difference in each corner will make a huge lap time difference

    If you remove the lap time from the dashboard some rare drivers will tell you that the car has more grip "because I shift from 2nd to 3rd 20 meters earlier at the corner exit" But that is it.

    But give them a very small difference of front and rear cornering compliance and they will fell it.

    Put a big guy of 100 kg 1 meter away from the center of a swing and another small guy of 50 KG 2 meters away of the swing center. The swing is horizontal, balanced. Put a ballast of 1 Kg under the seat of the guys. The swing angle will change. Both guys will feel and see it.

    Give 1 Kg under the seat of the big guy and 2 Kg under the seat of the light guy. What do they feel?

    OK that is not the perfect comparison because mass and force is not the same thing: we should do the same experience on the earth and on the moon.

    It is true that the driver will feel in his shoulder, hips, ribs, head/ helmet a different acceleration. But can he feel the difference between 3.1 and 3.2. G? At 100 or 200 or 300 Km/h with the noise and vibration of the track and the car? I doubt it and my experience has shown they don't.

    But if they miss the apex by 1 meter they will complain about understeer.
    Last edited by Claude Rouelle; 02-10-2018 at 04:53 PM.
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  6. #56
    I have always found it interesting that most dual-axis acceleration diagrams (e.g. Cn-Ay etl al) ignore body slip angle rates (see below). While not a huge concern when the diagrams are being used to look at max planar accels, it is especially of concern when using the diagrams to 'control' and 'stability' derivatives.

    (N.b X-Y is the ground frame fixed to the CoG of the vehicle, x is the chassis heading, y is orthogonal to x, t is the heading of the velocity of the CoG, n is orthogonal to t, the box is a rendition of a vehicle and the shoe is on the other foot just FYI)

    So it's obvious there is a coupling between normal acceleration, body slip velocity and chassis yaw velocity (and from chassis yaw velocity, there is an influence on the effective slip angles of each tyre). Side note: I think there is a great benefit to thinking in terms of normal acceleration {n-direction} rather than lateral acceleration {y-direction}, insofar as the math is actually correct when using nt-coords. Similar point with using yaw acceleration (Az) rather than net yaw moment (Cn); why compare moments with accelerations when you can compare accels with accels (perhaps I am missing something)?

    Well it will obviously depend on the magnitude of the body slip angle velocity, and body slip angle velocity (betaDot) depends on a bunch of stuff, but crucially, depends on slip velocity. For a typical FSAE vehicle, Slow steer speed, betaDot <10 deg/s; steer velocity greater than full lock in less than 0.2 sec, betaDot > 50 deg/s. This is obviously dependent on a huge array of factors, including yaw inertia. Anywho, Az-An diagrams below for betaDot = [0; 30; -30] deg/s.



    (N.b. Body Slip Velocity is betaDot, just FYI)

    So, as I said, the magnitude of accels not much altered, but the control and stability derivates are reasonably different at the limiting accels. Side note mk2: I see a lot of CnAy diagrams developed for arbitrary chosen steers and body slip angles, this will give you arbitrary limits of your diagrams. The above diagrams determine the limits by looking for saturation of the tyres on each axle, limiting load transfers, steering and slip bandwidths etc.

    Lastly, even though I know the bounds of the above diagrams are reasonable, I still don't know if the steers and body slip angles provided to generate the vehicle responses are actually achievable for the stated velocity and betaDot (as ya'll know, the driver only has input actuation for steer angle, not so much over body slip). I think it would be reasonable to say that some sort of transient, Lagrangian assessment (whether through modelling or experimentation) would be useful to examine the actual space of steer, slip, slip rate and speed acheivable by a particular vehicle before placing full, unconditional trust into these Eularian models.
    Last edited by rory.gover; 02-17-2018 at 05:54 AM.

  7. #57
    Senior Member
    Join Date
    Mar 2008
    Location
    Brighton, MI
    Posts
    677

    You have it both ways.

    Hence the conundrum: "Racing is all transient", but racing analysis is apparently all steady state.

  8. #58

    Yes we take r into account

    At OptimumG we always take r into account
    Attached Images
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  9. #59
    Racing is nothing else than transient but racing analysis could be QUASI steady state
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  10. #60
    Senior Member
    Join Date
    Mar 2008
    Location
    Brighton, MI
    Posts
    677

    Rcvd

    Quote Originally Posted by Claude Rouelle View Post
    Racing is nothing else than transient but racing analysis could be QUASI steady state
    That would be pages 308 and 309. Beat ya to it, Doug !

    Besides, smooth (smoove) is fastest, with minimal r-dot. Tires like to be in a momentary steady state. The fastest drivers look slow. When you see a driver sawing at the wheel, its not gunna be a good day.

+ Reply to Thread
Page 6 of 10 FirstFirst ... 4 5 6 7 8 ... LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts