Jetliners – Why do they look alike?

Tuesday 18th September
Talk, “Jetliners – Why do they look alike?”
Mr Ken Wheeler RCEA
Do jetliners look alike? At least to me the current crop do and there are good reasons for this. At this time the Boeing 787 Dreamliner is being portrayed as representing the state of the art. Externally it looks conventional except for the raked wingtips but compared to older planes from the same stable it is dramatically different as for one thing it is almost entirely made from plastics in an effort to substantially reduce structural weight with enhanced aerodynamic performance. For this aircraft, 50% is made from load bearing plastic composites. Steel takes 10%, titanium a further 15% and aluminium just 20%. This great act of faith in plastics is closely followed by its marginally larger and possibly superior competitor, the Airbus A350.
There have already been problems with the first in service models of the 787 in that the wing roots have needed to be stiffened by the onsite introduction of tailored titanium plates on both the top and bottom surfaces. The solution of all problems involved with the introduction into service of both these aircraft is of prime concern to the UK as substantial manufacturing outsourcing for both Boeing and Airbus is placed here in the UK sometimes surprisingly with SME’s having CadCam facilities making quantities of complicated very expensive components to Boeing or Airbus “drawings”. This is additional to major proprietary items such as wings, undercarriage systems, electronics etc.
The prime elements of the jetliner, the wings and the engines, have been driven by history.The hull ,or fuselage as it is properly called is a circular tube which is best suited to cutting holes for windows, doors and hatches for a structure subject to internal pressurisation.
Wing design has been influenced by the utterances of the great George Schairer, Chief Engineer of Boeing in 1964 who said “ it is by the wing design you will fail”. A modern transonic wing is far removed from what Schairer could have imagined in 1964; he had been influenced by swept wing data captured from Germany at the end of WW2 which has historically proved to be a palliative at best surpassed by what is known as a “supercritical aerofoil section” of which more later.
Sir Frank Whittle in 1947 had already forseen that for his turbojet engine “ the addition of a fan section to the front with the main flow bypassing the core engine will considerably improve the fuel economy”
Historically, this has resulted in the High Bypass Engine now common to all jetliners.
The almost universal use of the wing mounted podded engine suspended under and forward of the wing leading edge giving at the time unforeseen benefits for engine interchange ability in spite of horrendous design difficulties resulted from a US Airforce directive cira 1954 that “ the Air Force will not accept future bombers having jet engines mounted within or directly on the wings” thereby setting in motion the engine configuration for what was to become the Boeing 707.
The constant driver over the years since around 1970 has been fuel economy with modern engines using 70% less fuel. There are other influences such as pollution and noise which have also been addressed.
In searching for other percentage changes which have a major influence on fuel saving is the reduction in tare weight and the reduction in aerodynamic drag, specifically the wings ,for aircraft which are required to fly much faster.
A jetliner does not operate in isolation but is contained in a series of subsets with three primary constraints . One subset is the atmosphere which means operating at the correct speed and altitude. Another is the aircraft subset which involves its aerodynamics,its structure and its engines. The remaining subset is the business model for the airline.
The aerodynamics of the aircraft subset are controlled by the “Flight Management System” for the structure, the “Flight Envelope Management System” and the engines the “Engine Condition Monitoring System”.
You could say by way of example that a motor vehicle is designed by experts to be driven by idiots where the kinetic energy of impact is absorbed by the strain energy of a progressively collapsing structure.
For a jetliner ,these are designed by experts to be driven by experts which these days includes “ expert” electronic systems. Fundamentally, jetliners are designed not to crash because such an incident invariably leads to substantial loss of life. The flight management system is basically programmed prior to a flight to perform all the flight manoeuvres through the autopilot including automatic take off and landing monitored by the pilots.The flight envelope management system ensues that under all conditions of normal operation and emergency the actions of the pilots are over ridden by the system to ensure that the pilot cannot apply a control command which would overstress the structure. Airbus have fully implemented this system whereas Boeing at this time still allow a measure of manual over ride and they have” bent” a few planes in the process which has meant in some cases taking the aircraft out of service.
Engine condition monitoring is typified by all Rolls Royce Trent engines flying are able to transmit 24/7 the condition of the engine determined by transducers to a central analysis Centre at Derby where engineers on duty can discuss with the pilot any apparent concern. The transducers also produce a continuous time log of an individual engine’s parametric data.
Typically, Rolls Royce don’t sell you an engine these days but instead a complete package including monitoring and servicing for the life of the engine. Revenues from service contracts exceed those for basic engines.
Regarding economics, the business model subset, this relates to the airline operating jetliners and involves computer generated algorithms predicting outcomes from normal day to day and forecast activities.( algorithms you will recall are just recipes for solving problems and of particular value for unitised quantities with airlines as you cannot sell half a passenger seat). The business model also includes outsourcing of maintenance and the leasing rather than outright purchase of aircraft, with a list price of some $200 million each beyond the resources of even the biggest airline. The business model is a whole story in itself.
Our initial subset entitled atmosphere I have left until last as it is involved with the aerodynamics of the wings in which crucial conceptual changes have been made. The atmosphere made up of molecules starts to behave in a compressive manner ,rather like a log jam, at velocities near the speed of sound resulting in density compression. As the speed of sound depends solely on the ambient temperature, comparative aircraft speeds are best expressed as a ratio to the speed of sound known as the aircraft’s Mach number and in order that we all sing to the same hymn sheet an International Standard Atmosphere( ISA ) has been agreed.
Between 36000 and 65000 feet altitude the temperature remains constant at 214K ( -56C) and the speed of sound is 295 m/s ( 660 mph) which means that aircraft travelling at the same speed within this altitude range have the same Mach number. For example, an aircraft travelling at 574 mph at between 36000 and 65000 ft has a Mach number of 574/660 = 0.87. Although temperature is constant , air density and pressure do change. Therefore, we always talk of speeds in terms of Mach number and the structural integrity of the aircraft is vouched in these terms and a Machmeter forms part of the flight management systems.
You may well ask at this stage where this is leading us .Because aircraft manufacturers in the initial stages offer guaranteed performance and fuel consumption for models frequently tailored to an airline specific requirements on an agreed delivery schedule at a fixed price, prescriptive design is used. This means that improving what you have entails less risk so the aircraft tend to have similar configurations. Circa 1997 defines the jetliner as “ wide bodied aircraft with high aspect ratio super critical wings and winglets powered by two high bypass ratio turbo fan engines mounted in pods under the front of the wing giving exceptional range and economy manned by two crew on the flight deck. Cruising speed Mach 0.86 at 36000feet”. Little has changed since then apart from this love affair with plastics.
The efficacy of the high bypass engine where it is more efficient to pass a large volume of air through an engine at low speed than a much lower volume at high speed emphasises the difference between the older turbojet and the modern high bypass turbo fan is illustrated by the Rolls Royce Avon of the 1950’s and the current Rolls Royce Trent 1000.
The use by Airbus of the supercritical wing on their A320 series aircraft came as a nasty shock to Boeing with their competing 737 which had a conventional sub critical wing section. The word supercritical is unfortunate viewed in a layman’s context but it is a technical expression related to specific values of the negative pressure coefficient on the forward top surface of an aerofoil.( Don’t forget that an aerofoil is a cross section through the wing). It is this negative pressure coefficient that is related to the lift generated by the wing. Broadly speaking, the rate of change of curvature over the top leading edge of an aerofoil produces an increase in local velocity of the air passing over and this produces an increasingly negative value of pressure coefficient such that at the point on the aerofoil section where this local velocity is just sonic the value of pressure coefficient at this point is called “ Cp crit”. Imagine a Cp crit line running spanwise along the top of the front of the wing where the airflow passing over is just sonic. Thus at high speeds on conventional aerofoils the local airspeed over the wing can be just sonic along a particular span wise line in spite of the fact that the free stream velocity of the air approaching the wing will always be less . For example, a wing having a local airspeed which is just sonic at Mach number of unity could have a free stream Mach number of 0.61.
As you have already surmised, this is where the trouble starts. The wing is a prime energy conversion system in providing lifting forces and in doing so produces consequent drag forces. These drag forces have serious implications for fuel economy It is the aerodynamic equivalent of there being no such thing as a free lunch. Increase in free stream Mach number means density compression of the airflow over the wing and consequent increase in the value of the drag force and the wing’s nowhere near flying at Mach 1. At Mach 0.61 the freestream compression is already up by around 18% over the static value.
For example, increasing the free stream Mach number with a conventional sub critical wing can mean that at values less than M 0.61 there is no sonic flow present over the wing .Increasing it to M0.61 produces the Cp crit situation a small supersonic bubble starting to appear behind the Cp crit line until say M 0.74 is reached when a strong shock wave occurs known as M div (drag divergence) on the surface of the aerofoil just aft of the Cp crit point resulting in substantial increase in drag due to separated flow.
So it looks as though a Mach number of 0.74 could be a limit but as the Germans did, you can cheat by sweeping back the wing such that aerofoil section sees only the resolved angle of the flow normal to the wing’s leading edge ,the other component sliding spanwise down to the wing tip. In this way the Boeing 707 managed M 0.84 but not very economically. The Comet managed a thirsty M 0.74 with little effective sweepback.
I expect you are wondering about what happened to the supercritical wing and now the moment has arrived. The objective is to reduce the magnitude of the shockwave and to delay the onset of drag divergence to a more acceptable free stream value of around M 0.86 with the substantial benefits of reduced fuel consumption this gives and at an enhanced speed.
You will recall from before that an aerofoil is a cross section through a wing where due to the curvature of the top area just aft of the leading edge the local velocity of the airstream always exceeds the forward speed of the wing (the wing’s freestream Mach number) and where in this area there is a point of maximum negative pressure where the local velocity is at its greatest ,this point being denoted as Cp and if the local air velocity is just sonic the point is called Cp crit. Further modest increases in the freestream Mach number results in a severe shockwave just aft of the Cp crit point ,known as the point of drag divergence ( Mdiv ). The flow is supersonic in front of the shockwave and always subsonic behind accompanied by severe air turbulence causing a rapid increase in drag.
A distinctive feature of a super critical wing is a more bulbous leading edge followed by a flattening of the top such that as little change in curvature exists, the local air flow velocity remains constant resulting in a much reduced shockwave with the Mdiv point much further aft and occurring at a higher local air velocity and a reduced level of turbulence. In essence you can fly the super critical wing up to M 0.86 before the Mdiv value occurs compared to M0.68 for the conventional sub critical aerofoil. The super critical wing also has higher lifting capability which leads to fuel savings of around 15%. The profiles of the new supercritical wings are surrounded by commercial secrecy as they represent a proprietary advantage to the manufacturer but in general maintain the above concept. The modern super critical wing also incorporates extensions called winglets or raked tips which reduce induced drag (discussed later) with consequent further fuel savings of up to 6%. Winglets are beyond the scope of this presentation.
Finally, we need to consider the major influence of the high aspect ratio wing. Aspect ratio which is the slenderness of the wing is expressed as the span squared divided by the wing area. This is the same as saying the aspect ratio is the span divided by the average chord (width). The wing has higher pressure on the lower surface and lower pressure on the top surface with an airflow trying to equalise this leading to trailing edge vortices particularly from the wing tips. This gives rise to another coefficient, that for induced drag (Cdi) which is a function of the aircraft’s weight divided by the aspect ratio ,all squared so that increasing the aspect ratio has a considerable influence on Cdi.
Summarising, we see that the jetliner has three prime elements which effect its efficacy and these are the wings ,the engines and the need to save weight to match the principal driver which is fuel economy and as a result of prescriptive design has produced similar configurations.
The historical technological progress to the super critical wing, the high bypass turbo fan engine and the massive use of plastic composites is typified by two current new builds, the Boeing 787 and the Airbus 350. When you consider that around 40% of your airline ticket is to pay for the fuel this gives an element of perspective to this presentation.
Ken Wheeler CEng