Body panel's vs Airflow

[Clive Austin]

Originally Posted by SteveSlowboy

The RCE article on Wallace's DJ, reckoned that they were seeing aero effects from a low as 45mph.

Even if you're not producing significant down-force at low speeds, reducing drag is still beneficial.

Steve

I think this was in reference to the monster triple plane rear wing that was fitted to the DJ Steve. From memory even SMB was surprised that it was showing some effect at this speed but you are right in that it appeared to be generating some downforce - alebit probably with the drag of an average parachute. On the hills with that much power then the downforce is more important than drag I guess.

It is probably worth making the explicit distinction between aero which is principally designed to reduce drag (streamlining in old speak) and that which is designed to generate downforce. Drag reduction I believe kicks in from the off whilst meaningful downforce kicks in later - I suspect this is to do with how the boundary layer builds up with speed and to what extent it is attached to the aero profiles. As I understand it, not present or no attachment = no downforce. The speed at which this occurs will depend on the viscocity of the medium (air) and the aero profile. The more extreme the profile (ie the triple plane wing) then the lower the speed.

I think in respect of side pods and engine covers their main aim is packaging to avoid drag and to channel the maximum volume of "smooth" air flow to the rear wing where it will then help deliver downforce. I can't help but feel that the air flow from cars with no engine cover must be very disturbed by the time the roll bar and engine gubbins have all had an impact. It would be interesting to see one in the MIIRA tunnel. Perhaps RCE could do an aero bytes feature on it? (Are you out there somewhere SMB?).

C

[The DOA]

I think the key is finding a balance.

A lot of formula student cars run massive aero packages in America/Australia and get meaningful downforce from very low speeds. I remember one Australian team at FSUK2010 saying that at 30MPH their car was definately benefiting from the aero effects and watching videos of them in action proves just that, the cars ability to change direction at low speed is phenomenal. This is fine because they have relatively little power and run at relatively low speeds so drag isnt much of a problem as long as they can keep the weight low (the Australian car was circa 200KG which is relatively competitive).

I think as soon as you start raising the fast corner speeds and straight line speeds then you need to start worrying about aero efficiency rather than weight as aero drag starts to have a relatively large effect on acceleration in proportion to the weight a vehicle carries in the form of efficiency increasing bodywork such as sidepods and engine covers.

Simon McBeath covered this pretty well in a recent copy of Racecar Engineering, his conclusion seemed to be that even if the extra bodywoprk made the car ultimately slower in theory, it was worth the extra bits and weight to gain confidence through fast corners due to the extra downforce achievable.

[Clive Austin]

I think the confidence point is a good one. I run a "barn door" on the back of my 1100 OMS - I know I don't need to but knowing it is there has more of an impact on my times than the physical downforce!

I'd still like to see a "bare" vs "engine cover" comparison in the tunnel though.

C

[image]

I'm with Asklepios on this --- a microlight or glider generates enough 'Australian Downforce' even at the sort of speed you can run at, to lift all that gubbins and a couple of blokes off the ground --- it's a matter of the right combination for the speeds you're expecting to run at to keep the drag manageable.

He's refering to one event at Forrestburn when I ran out of front wings (I'll gloss over the multiple miscalculations leading to that!! ) so took off the rear on on the basis of 'balancing the car' --- even on that quite slow track the loss of aero grip was so marked that I didn't quite believe it ---- to the point where I fell off the track on the next three runs trying to redial my driving!! :->

I'm convinced even our home-brewed aero works ---- however I reckon in the absence of a wind tunnel it's still far from optimum --- I run with no engine cover and my thinking on that is to run a rear wing as wide as the rules allow and (like Clives) a serious Barn Door's worth deep on the basis that whatever you do the air behind the driver's head and shoulders is going to be dirty and you want as much wing area out beyond that central strip as possible.

K