High Speed Stability

[Chubbs]

Evening all,

I am currently working a project looking into the influence of vehicle aerodynamics on a vehicle's high speed stability.

After having a quick search of the forum I couldn't find any posts which discuss the matter and was wondering if anyone knew of any articles/reports/publications that discuss any studies on how different aerodynamic aspects affect the high speed stability of a car (mainly in a straight line)?

I am focusing on both automotive and motor sport applications so anything within these topics will suffice.

Any knowledge, that you could post in here would be very helpful too.

Many Thanks

Chubbs

[Jersey Tom]

There is public domain information out there regarding vehicle stability, and what influences it (primarily a handful of tire parameters, speed, and inertia).

Generally speaking, more aerodynamic downforce helps stability and the ultimate understeer gradient or stability index of the car.. unless you do something like have all the downforce on the nose of the car.

Big question is.. downforce helping stability, and velocity hurting stability through decrease in yaw damping.. which wins?

[Chubbs]

Thanks for the quick reply.

So far my research has discovered that early studies in this area generally say that low lift coefficients will improve vehicle stability as you stated and that studying the lift coefficient of a vehicle was generally the conventional evaluation method for assessing a vehicle's stability. This coupled with a negative pitching moment was seen to provide a stable car at high speeds.

However, looking into more recent studies, it has been found that while this is a general indicator, the lift coefficient by itself is not sufficient to evaluate a vehicle for real life situations where a vehicle will experience unsteady flows, buffeting, cross winds and so on.

Some of the areas where the industry is researching in modern times includes wake from the rotating wheels, underfloor aerodynamics (highly exploited in motor racing for high down force as you all know) and much more. It's these areas of study that I am seeking more detailed publications.

I hope I am making sense.

Thanks

[DaveW]

You might receive a constructive reply if you could define "stability". Mercedes & Audi encountered & solved one type of instability with their LMP vehicles. Ron Ayres addressed successfully several types of instability for the Thrust SSC project (although Andy Green made significant contributions). He is currently busy with the Bloodhound. Any IRL team could also contribute, I'm sure.

[Tim.Wright]

Generally stability is related to the restoring force/moment generated by the tyres when a lateral or yaw disturbance is imposed on the vehicle. Basically, the more stable the vehicle, the more it resists changes to direction. So, obviously too much stability is not a good thing.

As JT mentioned it has a lot to do with speed, but also cornering stiffness of the tyres and location of the aerodynamic CP (plus a load of other things)

Basically, stability is a well known charactieristic and can be described with dynamics equations. If you find somewhere (Milliken etc) a derivation of the bicycle model, your stability constants are in there. You will see what parameters influence stability and from there you can make the link back to the aerodynamics.

Tim

[DaveW]

Here is an example of the LMP high speed instability I referred to above. Mainly "straight line", I think you might agree. That particular type of instability is not confined to road vehicles, see (for example) here. A search of publications by J.L. Stollery might be productive, if you are interested.

[alelanza]

Wasn't center of pressure important as well? i can't quite recall, but I think having the center of pressure behind the cog also made a more stable car, hopefully someone can confirm or correct this.

[DaveW]

Wasn't center of pressure important as well? i can't quite recall, but I think having the center of pressure behind the cog also made a more stable car, hopefully someone can confirm or correct this.

I think the problem is a little more complex than you suggest. My guess would be (& it is only a guess) that Mercedes' problem was a result of moving the rear wing aft to improve airflow, & restoring the centre of pressure with increased front (ground effect) down force. The result was stable unless/until the front down force was reduced (by the wake of a leading vehicle &/or a large road input). Then the c.p. would have moved aft, pitching the vehicle nose up which ultimately caused the observed "critical point" departure. The thrust & drag vectors would have contributed to the pitching moment, of course.

The reasons for the Bluebird departure were even more complex, I believe, involving thrust, lift & drag vectors, but it was still a "critical point" problem (became unstable only when both the airspeed & incidence exceeded critical values).

[DaveW]

p.s. Extreme triple hulled power boats are very interesting, actually. Drivers control stability by adjusting the direction of thrust. If they become too ambitious, however, they can become "marooned on a island of stability" - the only way to reduce speed is to cut power, but that will cause the boat to flip. An analogous problem can occur laterally in some rear-drive road vehicles....

[alelanza]

Wasn't center of pressure important as well? i can't quite recall, but I think having the center of pressure behind the cog also made a more stable car, hopefully someone can confirm or correct this.

I think the problem is a little more complex than you suggest. My guess would be (& it is only a guess) that Mercedes' problem was a result of moving the rear wing aft to improve airflow, & restoring the centre of pressure with increased front (ground effect) down force. The result was stable unless/until the front down force was reduced (by the wake of a leading vehicle &/or a large road input). Then the c.p. would have moved aft, pitching the vehicle nose up which ultimately caused the observed "critical point" departure. The thrust & drag vectors would have contributed to the pitching moment, of course.

The reasons for the Bluebird departure were even more complex, I believe, involving thrust, lift & drag vectors, but it was still a "critical point" problem (became unstable only when both the airspeed & incidence exceeded critical values).

I'm sorry if my post seemed like a direct reply to yours, when i wrote it i did not have youtube access so i didn't know you posted that. I was just making a comment in response to hi speed longitudinal stability in general terms, i recall that the positioning of center of pressure vs center of gravity was important, in a similar way to a fwd car being more stable in general terms vs a rwd one.