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NASA TECHNICAL Ivf_¢IORANDI_ NASA TM-77952

AIRCRAFT OF THE FUTURE

NASA-TM-77952 19860003850

S. Yeger

Translation of "Flugzeuge der Zukunft," Technisch-oekonomische

Information der zivflen Luftfahrt, Vol. 13, No. 2 1977, p. 88-90

(Grazhdanskaia Aviatsiia, No. 7, 1976).

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

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15.supp1....,o,yNo,.,Translationof "Flugzeugeder Zukunft,"Technisch-oekono- b

mische Informationder zivilenLuftfahrt,Vol. 13, No. 2, 1977, pp. 88-90

(GrazhdanskaiaAviatsiia,No. 7, 1976) (A77-39452).

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Some Basic problemsconnectedwith attempts-toincr-ea_sethe Size and _padit_ )

of transportaircraftare discussed. Accordingto the square-cubiclaw, the

increasein structuralweight is proportionalto the third power of theiin- -

crease in the linear dimensionsof the_.aircra_twhen geometricsimilarityis "

maintained,while the surfacearea of the aircraft increasesaccordingto thct

•secondDower.A Consequenceis thatthefract_on-0fusefulweight--wiil decrea.'

asaircraftincreaseinsize. HoWever,in flying-wingdesignsinwhichthe

whole load on the wing is.proportionalto.the distributionof lifting forces,"

the total bending moment on the wing will be sharply reduced,enabling

lighter-construction.- Flyingwings may have an Ultimate.capacity of 300Q

passengers. -.

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AIRCRAFT OF THE FUTURE

S. Yeger

The steady growth of air traffic in a number of /88

countries has lead to a substantially increased number of

commercial aircraft in air traffic operations. The result has

been that the carrying capacity of many, even very large

airports, is approaching its upper limit. As a result, air

traffic specialists are confronted with the problem of managing

continued air traffic growth without further increasing the

number of aircraft.

The problem has been solved in various ways, among others by

$

designing and constructing jumbo aircraft with 300 to 500 seats

to replace several conventional aircraft. One representative of

this genre is the jumbo aircraft IL-86, developed in the USSR.

A number of specialists involved in designing, constructing,

and operating aircraft are of the opinion that the period

between 1980 and 2000 will witness further increases in

dimensions and capacities of passenger aircraft up to 1000 or

2000 seats. Opinions on this matter vary greatly, however.

This may be explained by certain technical problems involved in

producing gigantic aircraft, mainly with difficulties in

obtaining necessary mass relations.

Aircraft exhibiting the largest payload at a given range

possess the highest efficiency. As dimensions change, however,

the relative masses of such elements as the power plant change

while those of fuel and equipment basically do not. For this

reason, the increase or decrease of the load mass caused

*Numbers in the margin indicate pagination in foreign text.
by dimension variations is dependent above all on the aircraft's

relative structural weight which has direct influence on

economic efficiency.

The battle over reduction of the mass of such structural

elements as wings, fuselage, empennage, and undercarriage goes

clear back to the origins of air travel. Research on the laws

governing changes in dimensions and mass of aircraft parts began

at the same time. They came to be expressed in the

"Square-Cubic Law."

According to the square-cubic law, the increase in structure

weight is proportional to the cube of the increase in the linear

dimensions of the aircraft when geometric similarity is

maintained (while the surface area of the aircraft, including

€

_ the wing surface, increases proportionally to the sqare of the

linear dimensions). Since the surface load must be kept

constant to maintain take-off and landing characteristics, as

well as for flight behavior, the take-off mass has to change

proportionally to the square of the linear measurements.

Therefore, the relative structural mass increases proportionally

to the aircraft's dimensions and the payload portion will

decrease.

One consequence of the square-cubic law is an uninterrupted

increase in construction mass greater than that of other mass

portions as when linear dimensions increase. The necessary

relative portion is not preserved for the other elements, making

it impossible to produce an aircraft with the desired

characteristics. This means that the extent to which an

aircraft can be enlarged is limited. Practical aircraft /89

construction has, however, already extended beyond the

limitations on aircraft growth formally dictated by the

square-cube law.

4
The principle lies in the fact that only a portion of the

structural mass is determined by exterior forces, and it

represents mass relevant for guaranteeing strength.

The remaining portion is determined by additional

installations and auxiliary elements, with their portion

decreasing as the aircraft becomes larger. This increases the

structure's mass efficiency. At the same time, excess strength

decreases, a factor which is determined by various technological

factors and the structure's local strength margins. Stresses

increase in the elements working on pressure. Furthermore,

practical enlargement of linear measurements allows the

similarity of the aggregate and the inner structural elements to

remain in tact. This, too, helps to slow down the growth of

relative structural mass. i

The most essential point, however, is exact consideration of

the basic forces involved in the airfoil's progression through

the square-cube law. In fact, the bending moment's resultant

which puts stress on the airfoil's elements of longitudnal bond

is determined by a force which is smaller than the thrust. The

amount of this force is smaller than the airfoil structure's

mass force, the entire load stored in the airfoil (fuel,

equipment), and the load attached tq it (engine with cockpit,

landing gear).

This type of surface discharge caused by mass force lightens

the structure substantially. An aircraft consisting solely of

an airfoil would be the ideal situation. If the entire load in

the wing can be adjusted proportionally to engine distribution,

as well, bending moment will approache zero, and the structural

mass will reache a minimum.

This idea was the basis of the flying wing aircraft which

was very popular between 1930 and 1950. In spite of its

5
popularity, however, this construction was no more efficient

than conventional aircraft forms.

The flying wing aircraft requires large dimensions and

surface areas to accommodate passengers and freight. The

specific surface load is small with low flight mass and

relatively large surface area. The thrust coefficients attained

during flight are small, as well. Therefore, the attainable

aerodynamic efficiency in air travel by no means reaches its

maximum value.

Speed increases intensify this disadvantage: As thrust

increases, lift/drag ratios deteriorate even more. It was more

advantageous to use a smaller wing which could be flown at

maximum efficiency in air travel. This allowed for J

accommodation of the load in a fuselage elongated in flight

direction. The fuselage possessed a small cross section.

However, this made it possible to reduce the total drag only by

retaining the conventional set-up.

Calculations reveal, however, that the flying wing aircraft

is advantageous for a lO00-seat aircraft. The airfoil

dimensions required to attain maximum aerodynamic efficiency are

so large that the flying wing can accomodate passengers,

baggage, fuel, and equipment.

At present, model procedures are being prepared, and methods

have been developed to calculate the mass of aircraft parts, so

that it is possible to control structural mass changes while

making large alterations on the aircraft (even without

maintaining geometric similarity).
0 I000 _00 3000Poss.

Figure 1: Change of payload portion on the total mass of

a commercial aircraft as a function of the

number of seats.

Figure 1 shows the payload portion of an enlarged,

conventional subsonic aircraft with an assumed range of 3000

kilometers.

First, the mass of an imaginary aircraft is calculated

without each payload portion (GKoM = 00. Then payload is

increased slightly, so that the total mass Go becomes greater

for a payload portion Gxos = G<_ > 0. Go

Continuing this procedure by adding to the seating capacity,

we _eka_mine that the payload portion increases rapidly. After

reaching a maximum, Gx0s decreases again, since the relative

' structural mass becomes larger.

The figure shows that there is an optimum size for aircraft

with a smaller number of seats at GKoM = max. The maximum

payload portion corresponds to aircraft dimensions for which

stress has reached its highest point and strength margin its

lowest point. A two-story fuselage is best suited for more than

500 seats. Such a fuselage is smaller than a one-story

fuselage, can be more densely loaded, and has a smaller mass. A

three-story fuselage is recommended for aircraft with more than

1500 seats.
O_M

bo

f

o 1ooo 2oo8 sooo_.

Figure 2: Change of payload portion of the total mass as

a function of number of seats for various types

of commercial aircraft. A - Conventional

set-up (I - single deck fuselage, 2 - two-story

fuselage, 3 - three-story fuselage); B - Flying

wing aircraft with cockpit, accomodations for

€

passengers in cockpit and in airfoil, and C -

"Pure" flying wing aircraft.

Different aerodynamic aircraft designs under given

circumstances are compared in Figure 2. A conventional aircraft

with two-story fuselage and a seating capacity over 1000 begins

to be disadvantageous with regard to GKOM when compared to a

flying wing aircraft with a small cockpit in place of the

fuselage (and accommodating passengers in the cockpit as well as

in the wing). Partial use of the airfoil for loading payload

permits a moderate enlargement of the surface and high

aerodynamic efficiency values in flight.

Use of a pure flying wing aircraft is most advantageous for

further enlargements of dimensions and increase in seating

capacity over 2000, as well as for corresponding surface load

enlargement.

8
Thus, the use of new designs and aerodynamic solutions and

the introduction of multiple-story fuselages, aircraft without

empannage, and flying wing aircraft lead towards shifts of the

maximum attainable payload portion in larger aircraft. This

increases the cost-effectiveness of aircraft with lO00 or more

seats. More refined construction methods, materials, and

equipment, as well as more efficient engines encourage this

scientific-technological development for the future.

(Taken from Grazdanskaja aviacija 7/1976; Translator: Dr.

Peter Bork)
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