Aerodynamic vs. vehicle stabilty

[racergeo]

  On a front wheel drive streamliner do I want an open diff or a spool?

[John Burk]

"I'm a little unsure of motive here so maybe you can help me out here Blue.... Why in the he11 would you have a talk with a Senior SCTA official and drop a bug in his ear to require aero stability testing on our vehicles?.... what in the he11 are you thinking? Do you have some ego problem that you feel the need to feel important to SCTA officials and others.... I for one would appreciate it if you would shut your mouth, especially to officials.... You seem to be a sharp guy why don't you spend your time actually building a race vehicle..... I know you are a aero guy but please don't suggest we all install propellers...."

I'm sure some people said the same thing when they had to add roll cages .
John

[jl222]

 
Blue...you talk of your pilots and drivers, who are they and what planes-motorcycles-or cars have you designed?


                        JL222

[desotoman]

what if you rode it backwards? We all know the front of the bike has the forks right?

JonAmo

Cool, a two wheel forklift. I love it.  

Tom G.

[jl222]

 
 Double post


                          

[desotoman]

I'm sure some people said the same thing when they had to add roll cages .
John

Yep, I could hear them from years ago saying, don't tell me what I need. I know what I am doing, it will just cost me more money, and is not needed.  Sometimes it is hard to save people from themselves.

Tom G.

[jl222]

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.

 For the record


           JL222
  

[jl222]

I'm sure some people said the same thing when they had to add roll cages .
John

Yep, I could hear them from years ago saying, don't tell me what I need. I know what I am doing, it will just cost me more money, and is not needed.  Sometimes it is hard to save people from themselves.

Tom G.

  Tom...if SCTA listens to Blue you better plan on putting a real small engine in that beautiful roadster of yours.


                                    JL222

[panic]

Sometimes it is hard to save people from themselves.

Why is it assumed that this is anyone else's business?

[Blue]

Wow, spend a few days in the shop and look at the response!  I'll try to get back to all of the questions and criticisms tonight.  For a few quick notes:

I think I started out with a compliment to Jack.
Aero stability is related to the bodywork, not the wheelbase.
I've seen plenty of blunt cars that were aero stable, at least in yaw.
I never cross-talk between teams.  That's why they talk to me.
If it ain't cheap and easy to do, I wouldn't recommend it.

Most important:

NO PROPS!!!!!

[saltfever]

Looking forward to it, Eric. Please check post #19.

Speaking of bluff bodies, I think NASA Tech Briefs published a white paper 3 or 4 years ago. The research was primarily focused on big semi truck body shapes and/or trailers, IIRC. The results were not intuitive in that a blunt shape was more efficient than a semi-aerodynamic shape in some unique cases.

[jl222]

Sometimes it is hard to save people from themselves.

Why is it assumed that this is anyone else's business?

  You ''save the people from themselves bunch'' would surely ban hare and hound desert racing.

            JL222

[desotoman]

Sometimes it is hard to save people from themselves.

Why is it assumed that this is anyone else's business?

IMO,

Not anyone's business if no other person is affected directly.

It is someone's business if other people are affected directly.

Tom G.

[A2WindTunnel]

Blue,
I am not an aerodynamic expert, but I have built experimental airplanes and lots of motorcycles. I was interested in your comment that motorcycles have "no downforce". I have to disagree. If you look at most bikes with factory bodywork and windsheilds, you can see that there is a lot more surface area providing downforce than upforce. Because the frontal area of bikes is such that the bottom half is much more dirty and upright with relation to the relative wind than the top side, there is ample downforce.
Maybe the A2 wind tunnel member here can vouch for that (or not).

From what I have seen it is not a matter of lift or downforce but the major imbalance between front to rear that seems to be typical of most motorcycles.  I typically have seen lots of front lift associated with negative rear lift (or rear downforce).  I’m curious if you could post a side shot of what you think is adding downforce to your bike?  If it is more or less the same "type of shape" as below it is similar to an airfoil shape and this is not a characteristic of creating downforce.  (Cambered top with flat bottom)

[saltfever]

A2 thanks for posting that great pic. I never thought of it that way but the rider could be the top shape or surface of the airfoil and actually be adding lift. However, from what I can see, the smoke is detached, or turbulent, from back of the rider. This would be typical of a wing stalling and a wing in stall does not add lift. So regarding your post have your wind tunnel scales actually shown lift is being produced? Or are you saying the bike's CG is in the turbulent area and that is the negative lift (downforce). Not trying to be argumentative . . . just trying interpet the results of the pic.