View Full Version : CG/CP Position

Dick Whittingham
8th Apr 2002, 21:32
Can anyone state with absolute certainty whether his big jet cruises with the CG behind, at or ahead of the CP? Or the AC if you prefer. Got the theory, but would like to know what happens in specific aircraft.

Thanx in advance

Dick W

Stan Evil
9th Apr 2002, 20:10
I don't know the answer but why not give the boffins up the road at Filton a call - after all they build the Airbus wings so I should hope they know.;)

9th Apr 2002, 20:46
Strong implication in Boeing article on increasing Take Off performance by limiting the fwd CofG position that the CofG is forward of the CofP on the 767-300.

Article entitled: Alternate CG Takeoff Limits; Airliner Oct-Dec 1994; Author Martin M Wiuthington, Boeing Aerodynamic Performance Engineer. Obviously, even if the CofG is forward of the CofP then the further aft it is the less downwards force will be required from the tail - hence a performance improvement.

Dick Whittingham
12th Apr 2002, 16:55
Thanks guys. Any more out there?

Let's try a different tack. Do you fly in the cruise with the longitudinal trim nose up or nose down from neutral - asssuming there is a neutral setting marked?

Dick W

John Farley
12th Apr 2002, 19:07
Hi Dick

As you are an instructor I would like to ask you a question if I may.

When you drive up a staight motorway do you have the wheel to the left or the right - assuming you can tell when it is straight ahead?

12th Apr 2002, 23:10
The take-off/neutral setting is an arbitrary reference point. Arbitrary in that the flight crew don't necessarily know whether it causes the tailplane to provide a downforce or an upforce.

An a/c can be stable with an upforce type tailplane as with a 'conventional' downforce type tailplane. Think of 'tandem wing' designs.

Without data from the manufacturer, how is a pilot supposed to know? All we can know is that for a given CG position then the trim should be set to 'x' [b]units[/i] with 'x' units providing adequate control authority/control pressure for that condition of flight.

The obverse of this is that if the control pressure is relieved then the trim is providing an appropriate amount of 'alternate' pressure for the condition.

This still doesn't inform about whether a tailplane is an 'upload' or 'download' design, only that there isn't a control force required from the pilot.

The designer decides where 'neutral' is, and what the mainplane/CG/tailplane relationship is.

Dick Whittingham
15th Apr 2002, 09:35
Hi John, Hi Tinstaafl

I suppose it depends on whether you are in the northern or southern hemisphere!

I'd better come clean on this. Alex and I have been arguing about stability. He holds that most transport aircraft have the CP at or just aft of the basic CG, giving a wing couple that is stable and just needing the tail to add more stability. I hold that the CP will be forward of the CG, giving an unstable wing couple that is then adjusted by the tail design to give the stability required. This may sound like "how many angels can dance on the head of a pin?" but we need to keep all our P of F sections, Performance, M & B, etc. in line and would like to have a real, definitive, answer. Trim, trim forces and the up and down load on the tail are secondary problems, and not in dispute.

There are lots of experts here on Prune, so I tried to get an answer without starting a wide-ranging discussion about stabilty. I guess we will have to ask the design authority.

regards to all, Dick W

15th Apr 2002, 16:23
I believe the Boeing article addressed this, perhaps reading between the lines, that the B777 and MD11 have the CG behind the CP of the main wing, by virtue of their augmentation systems.


“The trend in the design of modern airplanes is to have less static longitudinal stability--frequently referred to as relaxed static stability (RSS)--to capture the benefit of improved fuel efficiency. Simply stated, some airplanes are now designed to be aerodynamically efficient, and stability is augmented electronically so that stick force gradients will meet certification requirements. Many methods exist for augmenting stability. For example, the Boeing 777 and MD-11 use flight control computers that adjust the elevator actuator positions to give the appearance of more longitudinal stability than the airplane actually has. In other words, computers absorb the extra workload caused by flying with RSS.

Augmented stability provides better cruise performance with no increase in workload and no adverse effects from flying at an aft CG. This technology also allows for a smaller tail size, which further reduces drag and weight. However, FAR Part 25 requires that handling qualities remain adequate for continued safe flight and landing following an augmentation system failure. Therefore, a practical limit exists for how far aft the CG can go.

The Boeing 777 uses redundant digital flight control computers to provide positive (static longitudinal) stability and enhances that stability with airspeed feedback. The MD-11 uses computers to provide neutral speed stability. In other words, the CG of the MD-11 appears to be at the neutral point. The MD-11 uses elevator deflection to hold attitude at any speed within the normal flight envelope, then trims the stabilizer. This is known as an "attitude hold" system” (Quote from Vol. 2 Boeing Aero Magazine).

The center of pressure is at the trailing edge when no lift is being produced, as CsubL increases the CP creeps forward toward the LE with a divergent slope. At around 25%MAC :) (couldn't resist) the alpha to create the CsubL required will be at stall. After stall the CP starts moving toward the trailing edge and does some non-linear dynamic stuff. As CG is measured relative to MAC, the CP may sometimes be in front or behind the CG. The BE-02 has a CG range of 4% to 40% MAC, a result of the stabilons. The CP range must approach and pass the CG during normal flight operations with a rear CG. The relative position of the CP to CG in the ought two is based on how the aircraft is loaded.

Hope this sheds some light on your question.

Dick Whittingham
17th Apr 2002, 17:21
Excellent info in the Boeing paper highlighted by %MAC, and a very good analysis of a 747/400 run by Dan Winterland all agree that modern swept wing transports fly with aft CG positions and a tail upload for fuel economy reasons and handle the reduced longitudinal stability by various fixes within the autopilot and control systems.

My thanks to all who contributed. I have the info I needed now.

Dick W