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!



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Bye bye to a faithful friend

The C90, no longer mine, in the hired van of its new owner this morning

Those who have followed these ramblings for the past three years will be familiar with K101 XMO, my lovely little C90. It has taken me on many 'little bike' adventures but a couple of months ago I bought a Honda Innova to replace it. The Innova is Honda's ultimate development of the Cub (which began life as the Honda 50) with the engine enlarged to 125cc and fuel injected, and four gears instead of the C90's three, so it is a bit more usuable. However, I doubt the Innova will ever be the classic that the C90 is. Honda's latest small bikes use a completely different engine with continuously variable belt drive instead of the Cub's conventional gearbox and chain drive, so are no longer 'Cubs'.

Some videos of the bike taken last Saturday. Malc is the pilot:

Start and ride away: http://youtu.be/Ss8a_fFHLOc

Ride past: http://youtu.be/zCCHgwPSdKc

Ride up and park: http://youtu.be/olMK69R-Vl0

Walk around with engine running: http://youtu.be/PU401CxO_zg

For a while I contemplated keeping the C90, but with the Suzuki Freewind 650 and Moto Guzzi Griso 1200, space in the garage is tight. Also, the Innova does everything the C90 was formerly used for so chances are the C90 would see very little use. So it had to go.

A mature chap from Longton, Stoke on Trent, came today to collect it in a hired van. He seems a sensible type, who I hope will give XMO the same level of care and attention it has received while resident in my garage.

My Honda Innova, which has replaced the C90

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Winston Churchill's funeral train at the NRM

The great man died 50 years ago - January 1965. I can remember watching his St Paul's Cathedral funeral on black and white TV, and afterwards his remains were carried by special train from London Waterloo station to his place of burial in Oxfordshire. The train was hauled by Southern Region pacific locomotive bearing his name, and that locomotive together with the luggage waggon that conveyed his coffin have been cosmetically restored and are currently on display at the National Railway Museum's Great Hall in York. Today, Malc, Pete S, Pete A, John and me went to see them.

John, Malc, Pete A, and Pete S by the locomotive in the museum today

'Battle of Britain' class locomotive 'Winston Churchill'. Many of these engines were re-built without the air-smooth casing and with the removal of such unsuccessful features as chain-driven valve gear in an oil bath casing. When rebuilt they were considered to be much better locomotives fully realising their basic good design of boiler and running gear which had been hampered by Bulleid's (the designer) more eccentric features.

The locomotive's somewhat untidy cab

The restored luggage waggon which conveyed Churchill's coffin

This view of the locomotive shows its white 'Southern Railway' headcode indicators and the gap in the streamlined casing designed (none too successfully) to lift the smoke from the chimney clear of the boiler to aid the train crew's view ahead

Furness Railway 'Coppernob'  in the Great Hall at York 

Probably the first locomotive to travel at more than 100mph, the Great Western's 4-4-0 'City of Truro' 

Majesty in steam; LMS Stanier Pacific 'Duchess of Hamilton' displayed in its original streamlined condition. Many Duchess Pacifics were originally built in this condition, but all were later 'de streamlined' as the casing offered little aerodynamic advantage and added more than 3 tons of weight. 'Duchess of Hamilton' was re-streamlined some years ago, but is unfortunately not in working condition at the moment.  

Me 'driving' a top steam loco!

The view from Hamilton's driving seat 

The massive Duchess firebox. These locomotives were the most powerful steam engines to run in Britain. Their ultimate power output has probably never been realised as a fireman cannot feed this massive fire box fast enough to fully satisfy a Duchess in full cry. Nevertheless, Duchesses have put up efforts of power output, in service and in preservation, unmatched by any other British steam Locomotive. 

Duchess of Hamilton's cab 

Nearby in the Great Hall is the Great Western 'King', 'King George V' complete with the bell (not the original) it acquired when it visited the USA before WW2. These were designed by the GWR's then Chief Mechanical Engineer Charles Collett in the 1920s and were the most advanced steam locomotives of their time. The GWR never developed any main line steam locomotives to follow the 'Kings'. It was Collett's assistant at the GWR, William Stanier, who, when head-hunted by the LMS (London, Midland, and Scottish) Railway to develop a 'Super King' embodying much of Swindon's best practice and enhancing it. These were the 'Princess Royal' pacifics built at Crewe in the early 1930s. Stanier's development of the Princess Royal, the Princess Coronation or 'Duchess' class of the late 1930s and 1940s really was 'the ultimate in steam'.    

Another look at Princess Coronation 'Duchess of Hamilton' and its iconic art deco train 

In the NRM workshop is the boiler of the NRM's biggest embarrassment. 'Flying Scotsman' (4472) purchased many years ago without a thorough enough survey it has proved to be a money-pit, soaking up millions of pounds that could have gone to restoring other locomotives. And years later, it's still a long way from seeing service. 

Flying Scotsman's tender in the workshop 

'Evening Star', the last steam locomotive to built (in 1960) for British Railways. Perhaps if 4472 hadn't absorbed so much of the NRM's funds this locomotive could have been restored to working condition.

'Evening Star' is a BR standard class 9F, a freight locomotive. However, as she was the last Swindon gave her a name, a lined green paint finish instead of black, and a GWR-style copper-capped chimney  (the only 9F to receive these embellishments). Evening Star and many other 9Fs actually proved very capable engines on passenger trains, especially on hilly routes such as the Somerset & Dorset. But only in summer, as they were never fitted with steam heating apparatus to provide heat for the coaches.  

Back to the 1800s. A Midland Railway 'Single' in the Station Hall, our last look at the museum before returning home. By train of course!

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