Don't Flip Out: Prototype Design Rules
A new year and once again we face changes to the LMP chassis regulations.
For 2012 the most significant change worth noting is the addition of mandatory openings over the front and rear fenders. These openings are in addition to the mandatory louvers (at the front) but are different in that they must be open (to a specified area) and allow the tire to be visible. And while it seems the regulations are constantly being tinkered with haphazardly, understand that the ACO is striving to ensure that cars are safe to both driver and spectator. The thought of a 900-kilogram car taking flight surely keeps the ACO up at night and is the motivation to constantly address the yaw-flip issue. The ACO has its task cut out for itself; when a car of approximately 10 square meters floor plan area is suddenly pitched into an unintended yaw at high speed, the tendency will be for that vehicle to take flight. Because at angles relative to the direction of travel exceeding 30 degrees, downforce drops substantially; this is a fact of life. But since 2004 we've seen a number of additions to the regulations aimed at reducing this possibility and helping a car recover from disaster.But let's talk some basic terminology first. Understand that while a race car is essentially grounded to a two-dimensional plane, the chassis reacts dynamically, in three dimensions, just like an aircraft. For instance, the car will squat under hard acceleration (settle on the rear springs) and dive under hard braking (settle on the front springs). Consider the car in side elevation view, the rear goes down (squats), the nose will come up and vice versa. This is pitch. Looking at the car from the frontal elevation and as it corners, note that the side to which the car is cornering will move closer to the ground while the opposing side does the opposite. This is roll. And now peering down on top of the car in plan view, deviations to either side of the intended direction of travel about a central pivot point is called yaw.It's also important to understand the mechanics behind the yaw-flip incidents. When sudden yaw occurs, the chassis rolls away from the spin and the leading front fender generates lift, pitching the nose up and – most significantly – exposes the underfloor area to the oncoming airflow. This results in (1) air packing into the underside of the floor which negates underfloor low pressure and (2) net lift overcoming static weight. Thus the car takes flight when that net lift exceeds 900 kilograms, give or take. The other determining factor is the speed in which that lift exceeds static weight; that's called the critical take-off speed. Rewinding back to 2004, the new-rules LMP aero package was designed and developed with reductions in yaw sensitivity in mind. For starters, the entire underfloor architecture was redesigned and the old-style LMP900 flat bottoms with the added on rear diffusers were done away with. Instead the ACO instituted a regulated tunnel floor, though the shape of the tunnels was heavily controlled in lateral and transverse cross section as well as in plan, with minimal tolerances for deviation. This was in an attempt to keep absolute downforce in check. But the primary device to reduce yaw blow-over was the length-wise chamfer on either side of the floor. The concern was that as the car yawed and rolled inboard, away from the spin, airflow through the underbody was effectively choked as that inboard ride height reduced, compounding the problem of airflow packing into the underside of the now exposed (large) floor plan area. The chamfer helped maintain a consistent ride height, needed as a pressure relief valve for the airflow wanting to pack into the underfloor in adverse roll situations. And the quantified result was a substantial increase in the critical take off speed compared to the old-rules LMP900s.Along with the heavily regulated tunnels and chamfered underfloor, a 20mm underfloor skid was also introduced. The skid was a 20 mm thick step made out of Jabroc that attached to the bottom of the car. The idea behind the skid was simply to increase minimum ride heights, reduce downforce while adding drag (an additional factor of increased ride height), and in and of itself wasn't designed to address the yaw issues. But in 2009 a “domed” skid was introduced. The curve of the dome, 20mm at apex, simply helped to maintain underflow velocity while the car was in yaw by smoothing the transition across the skid.2009 also saw the introduction of narrow span rear wings and reduced chord rear wings. The ACO loped 400 mm of span off the wings in addition to 50 mm of chord. The immediate effect was a roughly 35 percent reduction in rear wing downforce though preseason development saw the downforce reduction ultimately negated. The end result was a slight increase in drag resulting in slightly reduced overall efficiency.In 2011 the ACO introduced the rather infamous center-line shark fin. This vertical fin was originally mandated to be between 920 and 1030 mm above the reference plane (car bottom). In a yaw event the shark fin generates an overturning force in it's own right. The key is the height of the fin and its surface area. As yawing airflow tumbles over the top of the fin it generates a high-up force acting across the rather ample surface area of the fin. This actually leverages the lifting moment generated by the leading fender, helping to force the lifting front back down.For 2012 the ACO pushed the minimum height of the fin upwards and now mandates it to be between 1020 and 1030 mm tall. This was in response to concerns after Peugeot's Paul Ricard preseason testing accident last year where the fin-shod 2011 908 actually became airborne when a mechanical failure induced a yaw incident. It was understood the 908's fin was designed using the 2011 minimums (920 mm) and data following that accident showed the solution was much less effective when compared to solutions using the 2011 maximums.So in the ACO's constant pursuit to address the yaw issue they have adopted large openings over the front and rear fenders for this season. The concept behind the fender holes is to relieve the pressure build up once the car is suddenly yaws. The holes also act as a defacto method by the ACO of “dirtying” up the car; the fender holes increase drag and reduce downforce. Though as we've seen before, development will negate these effects giving us a net gain of zero in season-to-season improvements.Is there a final solution to the yaw flip issue? It is hard to overlook the lack of tendency toward yaw-flip in cars of the IMSA GTP/Group C era. They had totally free underfloors and aerodynamics. They also had much larger total downforce numbers. But all of the solutions introduced by the ACO since 2004 have a very band-aid feeling. Will the ACO in 2014 have a “grand solution”? Clearly more study is needed. Here's to hoping.
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