From v03.n762 Fri Oct 3 14:11:10 1997 From: "Christopher M. Dicely" Date: Wed, 05 Mar 1997 19:39:47 -0800 Subject: GURPS Vehicles: Ground Maneuvering Take II (long) Okay, revised to address race cars, etc., using aerodynamic downforce to enhance maneuvering, here I go again... In the Real World(tm) ground vehicles are limited to maneuvering at about 1G (wheeled vehicles can push this a little due to tire deformation, or a lot at higher speeds by using negative lift, but more on that later), but GURPS Vehicles (even with the max maneuver = 2xMR "reality" rule) allows some ground vehicles to maneuver at up to 6.5G! So, I thought maybe... GENERAL RULES FOR GROUND VEHICLE MANEUVERING Ground vehicles calculate MR normally, then reduce it to a max of 0.5G. Those that had an original (before limiting) MR above 0.5G get a bonus to handling rolls: Original gMR Handling Bonus 0.75 +1 1.00 +2 1.5 +3 2.0 +4 2.5 +5 3.0+ +6 If the original gMR falls between two numbers on the table, use the lesser bonus. All ground vehicles are limited to attempting maneuvers of 1G or less *except* that wheeled vehicles may attempt maneuvers of up to 1.25G. This rule replaces the 2xMR restriction for ground vehicles only. Wheeled vehicles attempting maneuvers above 1.0G are at an additional -2 in addition to usual penalties. ADVANCED RULES FOR AERODYNAMIC LIFT Remember the comment up above about "negative lift". Well, in fact, aerodynamic lift is a real concern for all ground vehicles. Currently, in the GURPS Vehicles rules, neither aerodynamic lift nor aerodynamic drag are major factors in ground vehicle performance, which is simplifying but not particularly accurate. These rules add consideration for aerodynamic lift for ground vehicles, but rules for drag (alas) will have to come later. Lift Area for Ground Vehicles Ground vehicles, like aircraft, calculate lift area, based on body area and type. Aw = Surface Area of (lifting) wings, Ab = Surface Area of body Lift Area (Al) = Aw + Bf x Ab Bf = 0.3 for lifting body, 0.067 for lift neutralizing body, 0.033 for ground effect body (see BODY OPTIONS, below), 0.1 otherwise. Ground vehicles may also have a *negative* lift area, surface area providing negative lift. Negative lift is produced by negative lift wings (see NEGATIVE LIFT WINGS, below). An = Negative lift area = Area of negative lift wings. So, what do these lift areas do? Well, if the vehicle has net negative lift, it the airflow will hold the vehicle to the ground, increasing maneuverability and traction, while if it has positive lift, the reverse will occur, reducing maneuverability and traction. Use the following formulae to calculate speed effects on maneuverability: If An > Al calculate the Speed Interval (Si) at which the vehicle gains 0.25G to maximum maneuver and 0.125G to MR with the following formula -- note that this gain occurs at each interval of Si, so if a car had an Si of 50mph, it would gain 0.125G to MR at 50, 100, and 150, and 200mph for a total +0.5G at 200mph: Si = [ 0.25 x W / ( An - Al ) ] x Rs x Sl Where Rs and Sl have the same meaning as when calculating Stall Speed, and W = loaded weight. If An < Al, use the following formula to find the Speed Interval (Si) at which the vehicle *loses* 0.125G from MR and 0.25G from maximum maneuver: Si = [ 0.25 x W / ( Al - An ) ] x Rs x Sl With all symbols used as in the previous formula, except that Rs is 2.67mph for vehicles with responsive structure. BODY OPTIONS Ground vehicles can have the following body options: Lift Neutralizing Body (TL7): The vehicle is equipped with airdams and spoilers for negating some of its body lift. This option may be taken at no cost for ground vehicles with Good Streamlining, or at (Structural Cost/Structural Area) x Body Area for vehicles with Fair or No Streamlining. Vehicles intended as air vehicles generally will not take this option, as it degrades air performance. This option cannot (obviously) be combined with the Lifting Body option, or the Ground Effect body option below. Ground Effect Body (TL7): This is a skirted body designed to trap a *reduce* air pressure area under the vehicle. It is in many ways the opposite of a GEV skirt on a hovercraft The vehicle must have Good streamlining, but is treated for body volume and structural cost purposes as having Very Good streamlining. Most vehicles with Ground Effect bodies will also have Negative Lift wings, but this is not a requirement. This option cannot be combined with Lift Neutralizing body design, or with Lifting Body design (obviously). NEGATIVE LIFT WINGS (TL7) A vehicle may be fitted with a negative lift wing to enhance road holding. A negative lift wing is treated as a single wing with 0.05 x body volume, it can have the STOL, biplane, or triplane options. It *does* incur the 10x cost multiple on structural cost. In practice, this single wing will be high over the rear wheels of the vehicle, with a (generally) smaller wing or set of side-wings in the front of the vehicle. Vehicles with negative lift wings will generally have Good streamlining, and once they are available Ground Effect Bodies as well. - -------------------------------- Most of the serious work on negative lift in the Real World has been in the contest of Formula One racing, so the data I found was rather skimpy on actual performance, and heavy on anecdotal (i.e., some vehicles have performance around 3.5g with negative lift wings) and qualitative description, so I just tried to generalize from the rules for wings in flying vehicles. I make no guarantee that the stats of these vehicles will reflect real world stats, on the other hand they do reflect real world concerns and design issues and trade-offs. If there's interest, I can try to write up a new version of the ground performance rules to keep the current relative speed stats but incorporate drag and lift into top speed and acceleration stats. - -------------------------------------------------------- Meddle not in the affairs of Wizards, For thou art Crunchy, And Good with Catsup. - -------------------------------------------------------- THIS SPACE | Christopher M. Dicely | THIS SPACE INTENTIONALLY | cmdicely@ccnet.com | ACCIDENTALLY LEFT BLANK | cmdicely@ucdavis.edu | LEFT BLANK