New Aircraft CofG question.

[noisy]

When a new aircraft is constructed, how is the centre of gravity determined? The Cof G obviously has critical implications for aircraft stability, but I can't see how it can be determined without flying the a/c.

I could visualise hanging the aircraft from the ceiling, but this obviously wouldn't work for your new A380!

How is it done? Is it blindingly obvious or is it a real head scratcher?

Thanks
:O

[Shawn Coyle]

Just about the same weigh (pun very deliberate) that it's done for routine maintenance. Weigh scales on the wheels or jack points, etc.
The only thing that might be different for a first time prototype aircraft is to determine the vertical CG position, which might entail using some (calibrated) lateral tilting of the aircraft.

[Genghis the Engineer]

There are formulae based upon aerofoil characteristics and aircraft geometry to estimate the CG range. Theoreticians will tell you that they are exact - which is certainly not true, but they give a reasonable estimate of the range. You then structurally prove the structure to a slightly further wider range than planned, to be on the safe side. If you want to see the formulae, Darrol Stinton's "Design of the Aeroplane" or Dan Raymer's "Aircraft Design - a conceptual approach" are the most popular standard textbooks.

When it comes to a first flight, you aim to fly as near as possible to the predicted mid CG position - this is on the basis that this should be somewhere within a safe CG envelope. The aircraft is not initially flown - a series of fast taxis (getting the nose/tail off) or very low hops are flown whilst all the controls are explored somewhat briefly. This bit is clearly easier for a light aircraft, but then again Boeing and Airbus usually have better analysts than everybody else and their CG limit predictions are generally much closer to the truth.

Once the aircraft is flown, a series of handling checks are flown - if you want those in detail try DS's"Flying Qualities and Flight Testing of the Aeroplane" but in outline the static and dynamic stabilities in all three axes, the stalling characteristics and control in critical modes of flight (especially take-off rotation and landing flare) are investigated. This is firstly done at progressively forward CG positions in very small steps until what is considered the fwd limit is hit. Then you start repeating all the same tests towards the aft limit.

Ultimately after a lot of flying hours and a lot of repetition of the same tests at different conditions, you have established a CG range - which hopefully will be reasonably close to the original predictions but rarely identical, at least for any aeroplane with reversible controls.

Brief disclaimer, this is all fixed wing - I have no idea how it's done for a helicopter you'll have to ask somebody cleverer than me who understands those things (Shawn?).

G

[noisy]

Thanks for the responses so far...
I hadn't thought of scales beneath the wheels even though I have seen it done with racing cars.

I am aware that the cg wanders around in flight due to fuel use, passengers moving about and occasionally lunatics with parachutes departing the aircraft(!) and that this is sorted out using the trim wheel.

Can the theroretical equations really even give ball park figures for the cg range? How does this work with large aircraft? I have had the opportunity to wander around beneath a TriStar & 747 and been impressed with the sheer size of such aircraft.
Surely the equations are going to begin to break down outside certain aircraft size limits?

John Cunningham may have 'hopped' the first comet - I wouldn't fancy being the poor so-&-so who has to flight test the first A380!

[Flytest]

With regard to helicopters, generally speaking, the method of using pressure pads on the jacks, and jacking the aircraft into its S & L attitude is adopted. If you contact me direct, I can put you in touch with a colleague, who specialises in this field.

Further to my last, I worked at Boscombe Down in the early nineties, they had an aircraft weighing / C of G facility in one of the hangers at the ETPS. Don't know if it still exists, but worth a shot??

[ICT_SLB]

"John Cunningham may have 'hopped' the first comet - I wouldn't fancy being the poor so-&-so who has to flight test the first A380!"

Noisy,
Nowadays most aircraft have been "flown" in the simulator for many hours before the first flight of the hardware. For FBW aircraft like the A380, the characteristics will have been deliberately tuned to be as close to that of the rest of the Airbus range as possible.
There are also aircraft that can be set up to simulate the handling of much larger types - examples of which are the Calspan Learjet and Bombardier's ACT Challenger (A/C 3991). Crews also will fly similar aircraft if they're available.

Even without these facilities, the usual work-up is low speed taxi, high speed taxi (both with full systems checkout including Electro-Magnetic Compatibility) and then, finally, first flight.

P.S. Weighing to verify the C.G. is one of the most common flight test procedures plus most flight test aircraft have a water ballast system (two tanks at either end of the cabin). This allows the FTE to set the CG to the precise test requirement

[Mad (Flt) Scientist]

The theroetical estimates for cg range are accurate for any size of aircraft, to similar extents. Everything just gets scaled by wing area and chord anyway.

Most large manufacturers will wind tunnel test a configuration, giving further refinement to a prediction, and/or use CFD, and/or a body of corporate knowledge of similar aircraft configurations.

The end result is it would be something of a surprise to have to make significant changes to either cg range or trim authority on a modern programme.

[noisy]

Thanks to everyone who cotributed to this thread