The Lifting Fuselage Body

The Lifting Body or Lifting Fuselage concept dates back to the earliest days of aeronautics.

The concept is not to be confused with the Blended Wing Body that McDonnell Douglas were considering launching in 1997. In the BWB, the fuselage is not distinct but blended into the wings.

The Lifting Fuselage is also distinct from the Flying Wing designs of Jack Northrop and the Horten Brothers. In these designs, the centre section of the wing is thickened to accommodate the payload but there is also no distinct fuselage as such.

The Lifting Body or Lifting Fuselage design incorporates lifting wings and a distinct fuselage which however, unlike the conventional cylindrical fuselage that is the almost universal norm in civil aircraft today, is designed to provide a major part of the lift of the aircraft.

Induced Drag of the Lifting Fuselage Design

However this criticism is based on a misconception. If you consider the following two planforms, the conventional wing planform on the right has a standard high aspect ratio. The wings by themselves of the lifting fuselage design can be relatively short; but the fuselage body itself is now also part of the wing and its width is in effect added to the entire span of the wing we are considering.

The high aspect ratio conventional design is supposed to have lower induced drag because the effect of the wing tip vortices is proportionally less:  as air escapes from the higher pressure underside of the wing to the upper surface, it pushes down on the upper surface with consequent loss of lift.  With a high span wing, the wing tip vortices affect a smaller proportion of the total surface area of the wing and therefore induced drag is supposed to be less.

It is in fact well known that aspect ratio does not necessarily predict drag.  As Daniel Raymer shows (Aircraft Design:  A Conceptual Approach, P20), aspect ratio cannot be used alone to predict the entire subsonic L/D ratio because the friction drag is a function of wetted area, not just wing area as expressed by aspect ratio.

The B47 bomber had an aspect ratio of 9.4, while the Avro Vulcan delta winged bomber designed for the same mission has an Aspect Ratio of only 3.0; however both aircraft have a L/D of 17.  Both aircraft have about the same wing span, but the B47 has a narrow chord leading to lower wing area and higher AR.  But the Wetted Area for the B47 is actually higher (due to the tail and fuselage) so L/D comes out about the same as the Vulcan. 

In fact, the B47 has an S_wet to S_ref ratio of 7.9 compared to 3.0 for the Vulcan:  the Vulcan is clearly a far more efficient design from that perspective.

The Northrop Flying Wing had an S_wet to S_ref ratio even lower than the Vulcan of about 2.2.

Therefore as Raymer says, L/D depends primarily on Wetted Aspect Ratio, defined as wing span squared divided by total wetted area - not just simply the ordinary Aspect Ratio.  This takes into account both the induced drag due to lift and the parasite/ skin friction drag.

The article "The Burnelli Aerofoil Body" by Dr M Watter, published in "Flight Magazine and the Aircraft Engineer", December 1935 (available here) gave a detailed evaluation of the advantages of the Burnelli lifting fuselage.

The wing span of a lifting fuselage aircraft could be extended if required to reduce induced drag:  this was not found to be necessary with the Burnelli lifting fuselage concept, leading to weight saving and stability advantages.

Straight Sided Fuselage

In looking at a Burnelli Lifting Fuselage design in comparison with say the B2 bomber, the Blended Wing Body or the Irkut 111 below,  the Burnelli lifting fuselage has straight angular sides. The other designs use various curved blends to merge the top and sides of the fuselage into the wings.

This is an important design difference which contributes to the stability, performance and aerodynamic efficiency of the lifting fuselage.

One effect of the sharp angle between the upper or lower surfaces and the side panels is to prevent cross flow of air from the higher pressure underside to the lower pressure upper surface.  In other words, it fulfills the same function as winglets or end plates on the tips of conventional wings.

This geometry is conceptually similar to the well known C Wing of Professor Ilan Kroo at Stanford University.  One C Wing design concept created by Kroo is shown below - a hybrid blended wing body B747 with non planar wing tips. Next to it is the McDonnell Douglas BWB with C Wings added by Kroo.

Advantages of the Lifting Fuselage Design

The advantages of this concept over conventional cylindrical fuselage designs are:

• Improved Payload - Range
• Superior Field Performance
• Low Take Off and Landing Speeds
• Lower Manufacturing Cost
• Simpler Structural Analysis
  
• Vastly Superior Crashworthiness and Survivability
  
• Improved Fuel Efficiency
  

Performance Advantage

In the 1970s, Boeing proposed to launch an aircraft that would have been called the Boeing 754.  This was a classic lifting fuselage design.  It was cancelled in favour of the B767.  The table below (adapted from the Burnelli website) compares the two designs.

The Annual Report of the cargo airline Cargolux for 1975 showed on its front cover an artist's concept of the 754 front loading cargo through cavernous cargo doors.  Cargolux intended to be a launch customer for the aircraft.

Like many other aerospace technology innovations, the Boeing 754 was abandoned in favour of the inferior conventional approach as we are still seeing with the A380 and 787 today.

Crashworthiness

Notwithstanding all the other advantages, the case for the Burnelli lifting fuselage design is overwhelming based on safety grounds alone.

In 1935 a new Burnelli UB14 executive aircraft was being delivered to a customer. On arrival at Newark it tipped to one side and the wing tip caught the ground at 135mph. The aircraft cartwheeled down the runway before coming to rest with the main fuselage and cabin structure intact. Everyone on board survived uninjured. There was no post crash fire.  The cause of the crash was incorrect rigging of an aileron.

This post-crash scenario is unimaginable with a conventional cylindrical fuselage aircraft. The structure of a cylinder is such that it will always crumple or buckle when subjected to a compressive or shear load. In aircraft crashes we frequently see the fuselage break apart at the joints between the different sections - where different fuselage sections were manufactured as individual units before joining.

Even in the straightforward case of a commercial aircraft running off the end of the runway at relatively low speed, the fuselage frequently fails to maintain structural integrity and breaks apart.

There is simply no comparison with the structural rigidity and design philosophy of the modern automobile. In the 30 years since the late 1960s in which cars have become rigorously designed for crashworthiness and passenger survivability, civil aircraft structural design has not changed at all. If anything, modern aircraft may be less crashworthy due to their reduction in weight, use of composites and possible composite associated health hazards.

Irkut 111

The Russian aircraft manufacturer Irkut presented a "wide body fuselage" 100 seater aircraft at Le Bourget in 2003.

Unlike the Burnelli designs, the fuselage is not designed as a lifting surface.

This would be an interesting candidate for CWD analysis.

Links and Further Information

More information on the Burnelli Lifting Fuselage concept can be found here.

Download AIAA 2003-0292 "The Contributions of Vincent Justus Burnelli" by Richard M. Wood