What were the disadvantages of early prop driven blended-wing aircraft?

Question

I've been researching the history of pre-jet aircraft and I've come across several famous, and not so famous, blended-wing aircraft. The idea seemed to show promise but never really caught on, even for a limited span of time or confluence of technological factors e.g. weight vs engine power etc.

Just to be clear, I'm not interest in flying wing and/or tailless aircraft whose early problems are well known, but rather those designs that have tails but have a fuselage shaped to produced a significant component of lift.

Just a few of the designs I've stumbled across:

Burnelli CBY-3

Cunliffe-Owen OA-1 (Burnelli license)

French de Monge 7.4

Soviet LK-1 a.k.a. Fanera-2

There are quite a few more I don't have handy web links for. Junkers did a couple that would probably qualify.

The large number of designs from all over the world show that the basic idea offered promise but all the designs eventually went no where despite being tried in many countries, economies and even political systems. That in turn seems strange because in the 1920s-early-1940s, lift seemed more important than speed for larger multiengine aircraft i.e. just getting a load airborne at all was worth quite a few other tradeoffs. As engines grew more powerful that tradeoff lessened but for at least 15-20 years a high lift design should have been more desirable than a high speed one for larger aircraft.

The failure of many different designers over the course of three decades to make the blended wing work suggest to me that the basic idea has some inherent disadvantage/s that outweighed any gains. Unfortunately, none of the resources I have consulted offer any clues to why the designs did not "take off."

I suspect the designs showed some rapid onset of stall of instabilty but that's just an uninformed hunch on my part.

Any clues?

Answer 1

Generally, to create lift means to deflect the oncoming airstream downwards. How and where this is done in detail on an airplane is less important than doing it with a smooth spanwise lift distribution such that the wake behind the aircraft is trough-shaped.

A fuselage is disturbing this lift distribution, but not severely. The length of the fuselage means that it creates considerable lift at small incidences, even with the low lift slope of slender bodies. In case of the N-250 it was found that an incidence of 2° produces the lowest drag. However, this also means that the lift is only just right at one angle of attack, and when the aircraft is flown in different conditions, the fuselage can mess up the lift distribution quite easily, especially if it is wider than necessary.

During the 1930s and 40s, when this was understood in detail not only in academia, but also in the design offices, progress in engine technology pushed operating altitudes up until planes needed pressurized cabins to transport passengers comfortably. Pressurisation demands a round or nearly-round fuselage cross section to keep structural mass down, so any attempt at widening and shaping the fuselage would result in an unacceptable mass penalty. This ended the experimentation with lift-creating fuselages.

As for the N-250, the idea of fuselage inclination in cruise is correct in theory, but resulted in massive protests from the cabin crew which now had to push their carts uphill, ending this kind of optimization quickly. This is why it never caught on.

Answer 2

I remember having read that in these airplanes, at high speeds and low angle of attack, it was the center section, the lifting fuselage, that provided most lift, but at an steeper attitude, it was the regular wing that lifted. The shape of the 'lifting fuselage' sides makes suspect an increased 'marginal vortex' drag, this reducing the efficiency of idea. For a new procedure to be added to series, at least in the Automotive industry, they say it must provide a 10% increase in efficiency, along with a 10% reduction in costs, this is why changes that a private owner can introduce in their engines or vehicles may be worth for them but not for a company.

The mail boxes open to suggestions by people in the production lines, that once received concepts such as the way to use a single screw or bolt instead of the several previously used, saving the cost of thousands of bolts a year, are closed now, changes in the production line for this minimalist improvements were too expensive to implement in recent times.

Besides this, a patented idea must offer an overwhelming advantage for a builder to pay for it and incorporate to their production, if improvement is not big enough as to offer noticeable gains from the patent, the time will be left flowing, when the patents have expired, in most cases advances in technology offer better things.

However, some XXI century airplane designs incorporate concepts analog to the Louis de Monge and Burnelli's lifting fuselages. The time will say if it are good enough. Salut +

Answer 3

Looking through your first link about the CBY-3, it seems there may be political reasons for the failure of this design. To this day, many organizations still refuse to acknowledge the merits of the design. Part of the issue is probably the interests of much larger companies which were already deeply entrenched by this time.

Your second link is related to the Burnelli designs, and the page also offers this explanation:

The Cunliffe-Owen OA-1 Flying Wing was almost certainly too late to be a success in the civil marketplace. By the time it was ready to fly it was already 4 or 5 years behind the times in terms of performance and economy.