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.
Here is my take on CFD for what it is worth:
Swift Engineering has just finished a detailed CFD study of the ACK Attack to try to determine why the bike becomes unstable at speed with the rear doors attached. This was done on their brand new Cray Supercomputer and CFD software which was installed in July. http://www.swiftengineering.com/
The bike originally was designed with the rear of the body cut off and open with an open area of about 8” X 24”. After setting the record in 06 and running as fast as 349 mph the bike handled very well and showed no signs of instability.
In 07 after adding doors to the rear we crashed at the Bub event when the bike became unstable and began to oscillate side to side at about 300 mph. In 07 we blamed the crash on track conditions.
In 08 at the Shootout we spent the first three days trying to figure out why the bike began to experience a side to side phugoid oscillation at about 330 mph which increased in frequency as the vehicle accelerated. After many tweaks on the bike I finally told Rocky to open the doors when the oscillation began. When he did the bike became absolutely stable. We set the record with the doors off in 08.
We commissioned Swift to do the study and try to determine why the bike became unstable with the doors on. The first phase of the study looked at the bike at 350 mph and 6500 ft density altitude at 0, 2, 4 and 8 degree of yaw. The study showed that the bike was unstable at these speeds and tended to wind cock away from a cross wind. They said the rider could correct for this up to a point at which the bike would be uncontrollable.
The second phase studied the bike with the tail off my friend Ken Mort who helped with the design and has many years experience with the 130 ft wind tunnel at NASA Ames bet me a beer that they would find it is unstable with the doors off and that is exactly what they found. There was a slight increase in stability but nothing that would correlate with what the empirical data told us.
The other interesting thing they ask was what was keeping us from going faster with the doors off. They calculated we needed 180 HP with the doors off and 163 HP with the doors on to go 350 mph. The reality is we used all of at lease 800 HP with the doors off to go 360 and the bike was just not going to go any more than a few miles per faster. While these calculations might hold true for a moving body in free air they don’t work for ground vehicles.
The folks at Swift are very smart with practical knowledge and I appreciate the work they did on this project. While the CFD is well developed and understood for the Champ and Indy cars which they work with at 200 MPH and aircraft at higher speeds as my friend Ken Mort says there is really no good understanding of the dynamics of ground vehicles running at half the speed of sound especially a motorcycle. We did receive a lot of interesting data from the study such as boundary layer thickness, pressure gradients, drag in different configurations and other data but the basic question about the change in vehicle stability with the door on was not resolved.
I would not trust CFD data to reliably predict the stability of a ground vehicle at 300 MPH + and knowing the cost of CFD; to require such studies at this point I belive would be counter productive and detrimental to the sport.
One last comment as the bike has been displayed at a number of venues including the SEMA shows and a number of people who claim aerodynamic knowledgeable and expertise have come by looked at the vehicle and they know exactly what’s wrong with it and explain how I have designed it wrong. I always answer “how fast has the motorcycle you designed gone”?