The Bases of Modern Aircraft Design | Design Analysis and Design Configuration | Aspect Ratio Review

An aircraft design requires a media to build, as well as tools and other instruments to design the aircraft. The media can be a piece of paper or a computer application. To continue creating aircraft to build, the media need to prepare first to help designers to configure basic geometric description of the aircraft. Every media needs different preparations and treatments before used depend on the thing to be built.

To find proper methodology in designing process of an aircraft, begin with basic geometric descriptions of what a thing to build which I call “The Bases of Modern Aircraft Design”. That is vertical line and horizontal line where both lines intersect at one point, called zero-point, many say root. The vertical and horizontal line are similar to perpendicular and longitudinal axis of the aircraft design. Then from the zero-point, root of the aircraft design, begin to draw line of what aircraft need to design. Basically, in aircraft design the first element to design is wing. So, from the root then draw a line to design the wing while it sweeps back or sweeps forward. Due to aircraft design is symmetrically aligned, vertical line will help it aligned. Means aircraft element on the right hand of vertical line will be a mirror design of element on the left hand, and in turn. While horizontal line helps to measure square and size of the elements, as such length of the element measured from the horizontal line to its chord. Similar measurement is also going to vertical line.

As aircraft design often classified by its wing configuration, the first thing to do is how to determine span of wing and what parameter to measure it. New approach has been analyzed to determine span of the wing. That is by placing angle 60 degrees to the bases of modern aircraft design at about 1.7 cm right from zero point along the horizontal line. Configurations of the angle will perform wingspan by 14.1 meters, while angle at 1.8 cm right from zero point will perform 14.7 meters of wingspan. Note that there is range 0.60 meter from 14.1 to 14.7 meters. The same thing occurs when the angle placed at 1.9 cm right from zero-point, it will perform wingspan by 15.3 meters.

How do the configurations of angle 60 degrees result such measurement? To determine the configurations, firstly we need to define every single element, position and even deflection of running lines in square of aircraft design. Familiarization and talent about definition of the elements help designer(s) to build advanced aircraft design. The angle 60 degrees, for example, need to define correctly. Hypotenuse line, the longest side of angle 60 degrees, that intersects with vertical line will be called as root of the angle, while hypotenuse line that intersects with horizontal line called as chord. For useful simplification, here the angle 60 degrees placed at 1.8 cm right from zero point at horizontal line. From the root, make straight line along the horizontal line. And then make perpendicular line from hypotenuse at its chord. The perpendicular line is equal to 90 degrees if measured from the hypotenuse side, or equal to 60 degrees if measured from imaginary vertical line of the chord. And note that the straight horizontal line will intersect the perpendicular line at a certain point, called tip chord. If measured from root to tip, it will form 7.35 cm length. The value of 7.35 cm will be a span of wing at the right hand. Since span of left wing is the same as span of right wing, then wingspan is number of both span of left and right wing, which is equal to 14.7 meters of wingspan. This wingspan configuration exactly matches span of wings Sukhoi Su-27 Flanker.

The configuration of angle 60 degrees provides compact design. Means that if main configuration of the aircraft, span of wing for example, is changed, other elements will (be) change(d) and need to re-design. In other words, the configurations will affect overall elements and measurements, and even parameters, and the design automatically compacted by itself. So that one element to the others are unified and inseparable.

As every line running and flow through the aircraft design square, many running forward or backward, even horizontally, and many deflected to other side, the angle line will do so. Many of the lines might intersect and cross-meet, with each other at a certain point, as if it wants to point something idea. Talented designer(s) shall see the phenomenon as guide to design proper placement, shape, length, and width of elements in aircraft design. Following and playing with running lines in square of aircraft design is quite challenge.

Square of aircraft design as mentioned above, is imaginary square surround the aircraft design formed by imaginary horizonal and vertical line. The imaginary square is measured from length times width. Length is the length of fuselage, while width is the wingspan. Configurations and functions of the square will be described any further.

Chord, as one of the definitions used in this design analysis and configurations, is the imaginary straight line joining the leading-edge and trailing-edge of an airfoil. The chord length is the distance between the trailing edge and the point where the chord intersects the leading-edge. The point on the leading-edge used to define the chord may be either the surface point of minimum radius or the surface point that maximizes chord length. The chord of a wing, stabilizer and propeller determined by measuring the distance between leading and trailing-edges in the direction of the airflow.

Designing an advanced aircraft requires many parameters to build with. Designers need the parameters to measure aerodynamic characteristic of an aircraft to be built. As aircraft is built as a fighter, it needs to fly very fast and to move smoothly while flying in the air at credible speed. The aircraft also need stability at high angle-of-attack. Parameter required to build such an aircraft is aspect ratio. In aeronautics, aspect ratio of a wing is the ratio of its span to its mean chord. It is equal to the square of the wingspan divided by the wings area. Furthermore, the aspect ratio applied not only in wing configurations but also applied in many elements of aircraft design, such as tail fins, fuselage, square of aircraft design, etc. Aspect ratio and other features of the planform are often used to predict the aerodynamic efficiency of a wing because the lift-to-drag ratio increases with aspect ratio.

Wings design and configuration:

Modern high-performance fighter aircraft has one of three wing types: trapezoidal, delta and sweptback wings with low aspect ratio, where the three wing types can fly at subsonic or supersonic speed. Such wings have low aspect ratio and are thin in order to minimize wave drag (drag due to shock waves). An aircraft with high aspect ratio of sweep back wing cannot fly supersonic, except under special circumstances e.g. power dives. Such wings are thicker in order to accommodate a thick main spar.

Delta wings have a long root chord and therefore can have a thick main spar while retaining a low thickness-to-chord ratio. Delta wings also have larger wing area than trapezoidal wings with the same aspect ratio. This means low wing loading even during maneuvers. At low speed conditions they can produce a lot of additional lift when placed at high angle-of-attack, thanks to the leading edge vortices. However, delta wing aircraft do not require a horizontal tail.

As disadvantages of delta wing: higher viscous drag due to the large wing area, high induced drag at subsonic conditions due to low aspect ratio, bad deep stall, pitch control is achieved by deflecting upwards the trailing edge control surfaces in order to produce a nose up moment. This reduces the total amount of lift generated by the wing. This problem can be overcome by incorporating a horizontal tail or lifting canards.

The pitch control and bad deep stall disadvantages of delta wings led to several variations:

Compound delta or ogive delta: the inboard sweep is generally higher so as to create even stronger leading-edge vortices and delay stall to even higher angles.

Horizontal tail: Provides additional stability in pitch and therefore the wing can produce more lift. Tail elevator provides pitch control.

Canards: They are usually all-moveable and provide additional pitch control. The aircraft is usually statically unstable. Note also that flaps are difficult to use with a tailless delta configuration. Delta wings are generally cropped. The pointy wingtip is difficult to manufacture and structurally weak. Furthermore, cropped delta wings delay vortex bursting. The center of lift lies aft on a delta wing. This means that the horizontal tail can only be effective if it lies even further aft. Usually the tail is highly swept and can be placed on a highly swept fin.

 

Trapezoidal wings: in trapezoidal wings, the leading-edge sweeps back but the trailing-edge sweeps forward. But some delta wings also feature a slight sweep forward angle at the trailing-edge. Trapezoidal have better performance than delta wings at transonic speeds and during transition to and from supersonic conditions. However, do not require upwards deflection of the trailing edge control surfaces for pitch control and therefore do not lose lift. The center of lift lies further forward and therefore the tail must not lie too far back on the fuselage. And preferred for stealth applications. Flaps can be easily used.

As the disadvantages of trapezoidal wings: high wings loading and stall at much lower angles of attack than delta wings. This problem is overcome using leading-edge (root) extensions (LEX or LERX) and/or canards. Modern trapezoidal wings have very highly swept leading-edges, approaching the sweep of delta wings.

Sweptback wings: both leading and trailing-edge are sweep back. Sweptback wings have very similar characteristics to trapezoidal wings but trapezoidal has larger wing area.

The advantages of sweptback wings are: lower wing area compared with trapezoidal, and hence low wing loading. The center of lift lies between that of trapezoidal and delta wings. Sweptback wings have the trapezoidal wing advantages compared to delta wings.

And as the disadvantages: require LEX, just like trapezoidal wings, and even create higher viscous drag than trapezoidal wings.

The basis of modern aircraft design could help to configure the designing process of geometric descriptions for all three type of the wings. That is by placing the angle of 60 degrees at 1.7 cm right from zero point at the horizontal line. Configuration of this angle will perform 14.1 meters of wingspan. Then begin to draw line from root to tip sweeps back by 42 degrees to perform leading-edge of the wings. Make line from tip to tip, the span of wings, will form delta of the wings. Again, at the root of span draw line from root to tip sweeps back by 12 degrees. The configuration will perform sweptback wings. And continue to make line from wingtip to wingtip, the wingspan, to perform cropped delta. And finally forming tapered trailing-edge sweeps forward by 15 degrees. The configuration results a trapezoidal wing which is like an inverted shape of a kite.

From the wing configuration above, we can see that one wing to the other is such an evolution of the wings. Transforming one wing to the other is by adding straight line from wing tip to wing tip or even by adding sweep line that will result different aerodynamic characteristics of the wings. Main difference among the wing configuration is sweep of the leading-edge. Cropped delta for example, must have higher angle of leading-edge than sweptback wing, due to higher wing area of cropped delta. As well as trapezoidal has highest angle of the leading-edge. Another difference is wingspan, the highest wing area the narrowest wingspan. That is because aspect ratio of wingspan square to wings area, where the aspect ratio must be low, as requirement of low aspect ratio for fighter aircraft.