I hope this provokes a lively discussion, this is a subject that I believe everyone in LSR can understand and benefit from with the end result of faster records and safer vehicles.
First, I mean no disrespect to Costella or any builder or contender in LSR. My concern and comments on the previous thread come from my knowledge of vehicle mechanical-aerodynamic stability and control. My concern is that speeds have now exceeded the area of mechanical stability and entered the area where aerodynamic stability dominates and the knowledge base of LSR is not yet wide or deep enough for safety.
I spoke to a senior SCTA board member last year about the need for the higher speed vehicles to prove their stability with analysis before going too fast, and we had a good discussion about the costs and where the break point could be for that analysis. I am still searching for a reasonable CFD cost solution for everyone and have not found it yet. However, my experience in S&C (stability and control) concerns me about some of the designs I see vs. the speeds people are seeking.
Most LSR designers use the idea of a "Cp" as a plot of the lateral area of their vehicles and presume that if the CG is forward of the 50% point of this plot, then the vehicle is stable in yaw. This is simply not true.
First, most symetrical aerodynamic surfaces rotate around the "quarter chord point" or only 25% of the length. This is called the "yaw neutral point". Very few LSR vehicles have their CG forward of this. Even so, at relatively low airspeeds (below 200 mph) the dynamic pressure is low enough that mechanical stability can override the aerodynamics. Above 300 mph, the opposite is true and any vehicle that is not solely stable aerodynamically will not be recoverable if it loses traction. Downforce can increase the mechanical advantage, but it is a bad trade since downforce usually leads to pitch instability.
Tails or other large vertical surfaces mounted far aft are used in some designs and can radically improve the overall vehicle's yaw neutral point. However, blunt tails (like chute tubes) can reduce their effect. Some of the vehicles currently seeking 400 mph are nearly neutral in stability due to their aft CG and high degree of aft separation. There are solutions and a few in the 400 mph club have done a very good job of addressing this issue. Some haven't, and that scares me.
At least a first-order, algebra-based stability calculation should be required of any motorcycle going over 200 and any car going over 300. As speeds increase, the mechanical stability is going down exponentially with speed (dependent on surface condition, traction, and tire dynamics) and up linearly with downforce. Countering this, aerodynamic instability increases with the square of speed. At some speed the two lines cross and things can go bad very quickly. Since most motorcycles do not have downforce, this equation leads to the need for positive yaw stability at the starting line. Worse, downforce-based stability is at the mercy of driver skill; and I like to be kinder to my drivers.
The REAL danger is that this "negative stability" speed may have already been achieved without external upset and then the vehicle makes another similar run and encounters an upset due to surface or wind conditions and suffers an uncontrolled departure; i.e. SPIN. Think about all of those guys who have gone fast in roadsters or stock body cars and then spun at less speed. Their driving skill may have saved them in the past, this does not mean it will forever. At any combination of speed, surface, and wind condition it is the LSR vehicle's job to go straight, not to demand an ever-increasing level of dynamic driver input.
In aviation, we call the ability to handle instability the "velvet glove": a VERY complimentary term for the pilot. And a not-so-complimentary one for the engineer who made it necessary. As an engineer I don't like being the butt of jokes, so I make the things that I design stable and controllable. My pilots appreciate this and bitch about other engineers instead.
All of this relates to yaw stability and spins. Pitch and roll stability is another subject entirely and much more complex.
