Tuesday, 31 March 2015

Aerodynamic Centre, Centre of Gravity, and pitch stability in aeroplanes

A 'lively' discussion between me and Malc on pitch stability of aeroplanes led to a most interesting morning and early afternoon today at Salford University with my mate Thurai (Professor of Aeronautics).

Thurai freely gave his time to explain to Malc, Pete, and myself the lift force on the wing due to camber (always static in that its position doesn't vary) and lift force due to Angle of Attack or alpha (which moves fore and aft with varying alpha), and how these can be combined into a non-moving but varying total force called the Aerodynamic Centre (AC).

As I had tried to get Malc to accept, in order for the wing (therefore the aeroplane) to be stable in pitch (i.e. returns to trimmed alpha after a disturbance)  the AC has to be behind the Centre of Gravity (CG). The further behind the more stable is the aircraft in pitch, but also less maneuverable. If the CG ever got behind the AC, the aeroplane would be divergent in pitch rather than stable and therefore unflyable by a human pilot. And because the CG is ahead of the AC, a nose-down moment is induced which is countered by a downforce from the tailplane in the form of downward lift (the Americans call the tailplane the horizontal stabiliser which I think is a much better name - 'cause that's what it does!).

Generating lift generates drag (it's called Induced Drag) and drag uses more fuel, so the download on the tail is something it would nice to get rid of. What I didn't know until Thurai told us today is that some clever aeroplanes, such as the A380, can do just that.

With the CG carefully positioned at 35% of the mean aerodynamic wing chord the tailplane negative lift (downforce) can be reduced to zero. This requires that the aircraft has an intelligent 'CG shift' system, moving fuel fore or aft to achieve the minimum load on the tail, and hence the highest flight efficiency. This 'zero tail lift' is only possible to achieve while the aeroplane is in steady cruise. When any maneuvering is required, the CG has to be moved to the conventional position, and the tail download returns.

Thurai also went on to explain that of course it isn't only the wing that has an AC. Every part of the aeroplane has an AC; the fuselage, the tail, the engine pods, even the winglets on the wing tips. The average AC of the entire aeroplane is called the Vehicle Aerodynamic Centre (VAC) or Neutral Point.

And just as I pointed out to Malc that for longitudinal static stability the CG must be ahead of the AC, so it must also be ahead of the VAC, or Neutral Point.

After that intensive lesson in aerodynamic theory Thurai took us for a tour of the laboratories. He showed us research projects being carried out by his students, model aircraft used for research, wind tunnels, both subsonic and supersonic, and various aero engines from a 1940s Rolls Royce Nene centrifugal compressor jet engine developed in Barnoldswick (the 'B' in RB 211) to a giant General Electric CF6 turbofan engine from a DC10.

By now it was mid afternoon and time for a pint. Peter left us in Manchester to head off home while Malc and I searched out a cosy Holts pub for a couple of jars of excellent bitter. By late afternoon we were getting peckish, but a handy and remarkable value Chinese buffet restaurant across the road from the pub cured that.

Thank you Thurai for an excellent and most informative day!



  1. I wondered whether to be pedantic, but an aircraft can have the CG forward of the main wing AC, so long as it puts its horizontal stabilisers at the front in the form of canards.

  2. Hi Dan. CG forward of AC is the norm for aeroplanes with a tailplane. Did you mean CG behind AC?

  3. I think I did, but more thought reveals I was incorrect. CG forward of AC as normal, only "corrected" by an upward lift form the canard as opposed to a downward lift from a tailplane. I also happen to thing canard bearing aircraft often look great, too!