5 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, 27 April 2025 - 10:52.

[10kDA]

This is why many airplanes with retractable landing gear retract them toward the rear. When landing, slow speeds with less effective controls, more mass toward the front increases stability. At cruise speeds, stability is decreased with mass shifted aft but the tradeoff for increased efficiency is worth it. Cessna 182 RG (aka Cessna 182 "Aaarrgh!")

 

This has been known for quite some time. See also: Grumman F6F, Curtiss P-40, Bellanca 14-13 series, Beech 18, many others.

[GreenMachine]

I kinda doubt that,  On the puddlejumpers  I think it is about structural loadpaths, packaging, and ensuring the mainwheels are sufficiently far back to avoid wheelbarrowing and/or provide sufficient control authority for the nosewheel (see Tripacer).  Enhanced stability may be a result.

 

On combat aircraft, that consideration would be so far down the list that I doubt it would ever be raised in its own right - it may be an outcome, but those stability decisions are not going to be made on where the wheels rest in the 'up' position.  INMHO 

[10kDA]

Though not first and foremost consideration in the design of combat aircraft, the effect was noted to enhance maneuverability, especially in the cases of the F6F and F4U. Cessna exploited the phenomenon in the design of their retractables with solid gear legs, resulting in reduced trim drag which = better fuel burn numbers. When extendde the wheels are in the same location relative to CG as fixed gear models of the 182 and 172. The load path is from the tire contact point to the airframe during operations in contact withthe ground, again, the same load paths as fixed gear models. Secondarily air loads which determine max gear extension speed which is usually intended to keep the gear doors from departing when opened. Some twins such as the Beech 18 retracted gear toward the rear of the engine nacelles. All this was known pre-WWII. Flight testing confirmed the phenomenon, and some mfrs put it to use.