abnormality in my lift coefficient graph.

[cotty32]

today i was plotting the variation between laptime and cl when i came across this interesting point on my graph. I find that my laptime increase after the critical angle of attack which is 4.7 but there is a deviation from the normal just before the critical angle of attack at around cl=2.7 . can anyone explain what this means?[URL=http://s1294.photobucket.com/user/ ... .png[/img][/url]
note that x axis has Cl and Y has my laptimes

[Tim.Wright]

At a guess I'd say its something due to the track characteristics rather than the car. Though, not knowing what software you are using, it could also be something wrong in the model. It will be pretty hard for anyone to explain anything unless you at least explain what the model is, what track and what car.

Just wondering, are you also changing drag when you change lift? If not, I find it a bit strange that a pure increase in downforce with no drag penalty can give you a worse laptime.

Its always good to do a sanity check of the speed traces for a few CL values to see where the gains/losses are happening. Typically a change to the car will improve some parts of the track, and worsen other parts. So with a track of say 10-15 corners, its quite possible that the overal laptime response to CL could be quite a strange shape as some corners improve and some worsen at different rates.

The sharp transitions seem a bit strange to me though. Is this a steady state type sim?

Tim

[cotty32]

should have explained it clearly my bad
the graph is from matlab
i looped my problem from a cl of 0.1 to 8 through steps of 0.1 and the drag changes cd = cd0 + 0.04cl^2.
I was given a track with the values of straight lengths,turn radius and turn angles and was asked to find out the optimal lift coefficient for a car in that circuit. i caulculated the speed into the straight and the speed at the beginning of the curve and found the overall lap time.
[/url][/img]
I chose my Cl range as an assumption and my optimal value of 4.7 is correct

[Tim.Wright]

If you plot the speed trace for 4 laps (CL=1,3,4 and 6) it will probably answer your question.

If you are interested in what happens at the continuities, plot the speed trace for CL=4.5 and 5.0 since they give give near identical laptimes either side of the discontinuity.

[Blanchimont]

I see two main problems. The matlab program itself (how does it handle the change from a straight to a turn?) and the assumed model of cd, as it is a function of cl^2.

Maybe you should limit cd to a maximum of let's say 1,5, rerun the code and look how the graph looks with this limitation added.