2 replies to this topic

[mariner]

Again not strictly car related but this paper, while way too heavy on maths and Matlab for me is a study in reducing aircraft drag and fuel burn by shifting the centre of gravity backwards.

 

https://www.scienced...590123023006096

 

There is I think a relevance to car aero in that the optimum  CG is so far back it is only just inside the defined control static margin for aircraft ( B747-400). Presumably the flight control systems can cope!

 

Somewhat similar to drivers finding it hard to manage a fast set uo but with a low stability index.

 

There is a  small practical problem, I think,  in that the fuel optimum CG is behind the rear undercarriage point so the plane would fall backwards if sat on tarmac  - hence dynamic fuel or cargo shifting in flight is needed 

[Greg Locock]

It doesn't work for cars because for aircraft the gain in efficiency is from using the tailplane for lift, whereas for the usual cg location the tailplane is providing about 1% (ish) of downforce, still creating drag. (Now I'll look at the paper)

[GreenMachine]

Intuitively I believe the gain comes from not using the controls (trim tabs, adjustable tailplane), as they increase drag as they deflect - shifting the cg allows the airframe to adopt the optimum AoA (best L/D ratio?) with all controls in a neutral position, and drag is minimised.  Be interesting to see how that relates to the centre of pressure. [EDIT: On reflection, I think this is all about keeping the centre of pressure (lift) exactly aligned to the cg.]

 

Mariner, by definition, it requires load shifting, as fuel is burnt it needs to be actively and progressively moved to ensure the cg stays put - nbd, it happens now, just not as precisely as this might require.  And yes, the flight control system can handle it, the pilots only think they are flying the big bird because the computers humour them - FBW FTW   

Edited by GreenMachine, Today, 10:52.