Morphing Aircraft Concepts, Classifications, and Challenges
Akhilesh K. Jha and Jayanth N. Kudva
NextGen Aeronautics, Inc.
2780 Skypark Dr., Suite 400, Torrance, CA 90505
ABSTRACT
A morphing aircraft can be defined as an aircraft that changes configuration to maximize its performance at radically different flight conditions. These configuration changes can take place in any part of the aircraft, e.g. fuselage, wing, engine, and tail. Wing morphing is naturally the most important aspect of aircraft morphing as it dictates the aircraft performance in a given flight condition, and has been of interest to the aircraft designers since the beginning of the flight, progressing from the design of control surfaces to the variable-sweep wing. Recent research efforts (mainly under DARPA and NASA sponsorships) however, are focusing on even more dramatic configuration changes such as 200% change in aspect ratio, 50% change in wing area, 5o change in wing twist, and 20o change in wing sweep to lay the ground work for truly multi-mission aircraft. Such wing geo
metry and configuration changes, while extremely challenging, can be conceptually achieved in a variety of ways – folding, hiding, telescoping, expanding, and contracting a wing, coupling and decoupling multiple wing segments, etc. These concepts can be classified under a few ‘independent’ categories and sub-categories so as to permit a systematic evaluation of benefits and challenges. This paper presents: 1) a review of prior work leading to current R&D efforts, 2) classification of morphing designs, and 3) a summary of technical challenges encountered in designing a morphing aircraft.
INTRODUCTION
If a morphing aircraft is defined merely as an aircraft that changes its configuration in-flight, it can be seen that morphing aircraft is not a new concept. Even the first successful controlled and powered flight by the Wright brothers involved differential twisting of the wings for generating the control forces. Since then, control surfaces in the form of ailerons and elevators have been extensively used for providing stability and control to an aircraft. Furthermore, in order to reduce the drag, almost all aircraft are now fitted with retractable landing gear systems. One can also see configuration changes such as flaps deployment for landing or taking-off. While these small changes can be technically termed as morphing, they are either necessary enablers for a controlled flight or contributors to the i
mproved aerodynamic performances of an aircraft.  As a result, these technologies do not necessarily allow an aircraft to perform different types of mission tasks. For example, while a long-endurance aircraft (e.g. Global Hawk, Fig. 1-a) aided with the necessary control surfaces can loiter over a target for a long time, it cannot fly at high-speed like X-45 (Fig. 1-b). Table 1 summarizes how increasing or decreasing a wing parameter affects the performance of an aircraft. As an example, it can be seen that for efficient low-speed flight, the aircraft should have high aspect ratio and low sweep angle. Contrary to this, high-speed flight requires low aspect ratio and high sweep angle wing. Therefore, in order to have the same aircraft fly diversified missions, it should be capable of making large configuration changes in an efficient and reversible manner. This is what aircraft designers aim to achieve by developing morphing aircraft technology.
retractable
Wings are the most influential parts of an aircraft as they create most of the lift required for flight. Also, their shape and size determine the suitability of the aircraft to a particular mission. Therefore, wing morphing has been of a major interest to the aircraft designers for a long time. One can find several previous designs, which bring small and large shape and size changes in the wings in order to make the aircraft suitable for multiple missions. Designers have also attempted changing the fuselage and engine characteristics of an aircraft for the same reason. In the next section, we briefly
describe a few of these notable attempts. The survey is limited to those designs which have been either tested in a real flight or in a wind-tunnel. Thereafter, we classify these designs in several categories based upon the basic mechanism on which the shape and size changes have been achieved. Finally, we present the challenges faced in the design of morphing aircraft, which are in addition to those found in designing a conventional aircraft.
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Table 1:  Effects of wing geometric parameters on aircraft performance
(a) (b)
Figure 1: Widely different aircraft configurations are involved in high and low-speed flights:
a) Global Hawk and b) X-45.
SURVEY OF MORPHING AIRCRAFT
In this section, we present brief discussions on some unique morphing aircraft designs of the past. It should be noted
here that this list is not all-inclusive as it is difficult to survey all previous designs. The survey has been divided into
large and small wing morphing designs, fuselage morphing, and engine morphing. The following subsection presents
examples in each of these categories:
1.Large wing planform change
Here, we include those designs which enabled the aircraft fly in at least two speed regimes. Apart from the conventional
wing flap designs for landing and takeoff, one of the first efforts in this direction was the variable camber wing design
by H. F. Parker of NASA in 1920. He designed a variable camber wing for bi-planes and tri-planes, and performed
structural and wind tunnel test to validate the concept (Fig. 2, Parker, 1920). The main idea here was to use flexible
wing (one for bi-plane and two for tri-plane) in order to supplement the lift generated by the rigid wing while the plane
is landing or taking-off. In this design, one spar was placed at the leading edge and another about two-thirds of the
chord back from it. The portion between the spars was made flexible, and that behind the aft spar rigid. The rib was
allowed to slide over the rear spar. The camber in the wing was produced automatically by the load it shared. Therefore,
at high speed, the variable camber wing was placed in such a way that it aligned with the air stream with no camber, and
hence carried no load and camber. At a low speed, when the angle-of-attack of the plane was increased, the flexible
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wing experienced aerodynamic forces causing its flexible section to deform upward. This resulted in a cambered wing that shared the weight of the aircraft. The internal truss structure prevented the wing from cambering after a certain limit.  He also discussed merits and demerits of changing wing surface area and the angle-of-incidence for reducing the landing and take-off distance and increasing the flight speed. As we will see in the next few paragraphs, several designers later considered these morphing ideas.
(a) (b)
Figure 2: Smooth camber change design proposed by Parker in 1920: a) variable camber rib design and b) vector
diagram of the forces for bi-plane in negative stagger (Parker, 1920).
A variable camber wing was designed by V. J. Burnelli of Uppercu Burnelli Aircraft Corp. In addition to change in the
camber, the wing also changed its area. The design had a chord expansion mechanism for a one-seater monoplane, and
was granted a patent in 1933 (Fig. 3-a; Burnelli, 1933). This wing was used in the Burnelli GX-3 aircraft, which first
flew in 1929 (Fig. 3-b). The main portion of this aircraft wing was mounted and braced rigidly between the spars. The
leading and trailing edge portions moved outward and downward in order to change the area and the camber of the
wing. The shaft parallel to the forward spar was controlled by a hand wheel while that running parallel to the rear spar
was driven by a chain from the forward one.  In order to minimize the movement of the center of pressure, the
mechanism was designed in such a way that it provided higher movement in the leading edge than in the trailing edge.
(a) (b) Figure 3: Variable area and camber design by Burnelli: a) mechanism as shown in the US Patent 1,917,428 (1933) and
b) aircraft Burnelli GX-3 fitted with the variable area and camber wing (1937)
In 1937, G. I. Bakashaev of USSR designed a telescopic wing aircraft, named RK (Fig. 4-a; Shavrov, 1994). The
telescopic mechanism consisted of six chord-wise overlapping wing sections that used to extend from each side of the
fuselage till 2/3rd of the wing span. The telescopic part was retracted and extended using steel wire, driven manually
from the cockpit during take-off and landing. In 1941, Bakashaev modified the design by fitting telescopic wings (Fig.
4-b). The aircraft was designated RK-I. In this design, a telescopic glove use to extend from the fuselage to cover the
whole span of the tandem wings of the aircraft. The control surfaces were located in the rear wing, and they were not
covered with the glove. The telescopic glove was operated using an electric motor. Compared to the area change of 44%
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in the RK design, the RK-I design was able to change its wing area by as much as 135%. Though these aircraft did not
go into the production phase, the designs were considered successful.
(a) (b)
Figure 4: F ighter prototypes developed by G. I. Bakashev: a) RK (1937) and b) RK-I (1941)
The first variable incidence wing was designed for the aircraft XF-91, also known as “Thunderceptor”, by Republic
Aircraft Corporation in 1949 (Fig. 5-a). The incidence of wings of this rocket-powered supersonic aircraft could be
varied in flight. While the high angle-of-attack was used for takeoff and landing, the low angle-of-attack configuration
was used for high-speed flight. The angular range of the incidence was from -2o to 5.65o. Although the maximum speed
of this aircraft was 984 mph, its endurance was quite low (25 min). This was one of the reasons why XF-91 was not put
into production despite making several successful test flights.
Another variable-incidence wing was designed in 1955 by Chance-Vought for F-8 Crusader (Fig. 5-b). The wing was
able to rotate about its rear spar by about 7o. Apart from providing low speed take-off and landing capability, it also
enabled the pilot to maintain the fuselage parallel to carrier deck or runway for better visibility in the elevated
incidence. In addition, the entire leading edge and the ailerons could be lowered to increase the effective camber of the
wing and consequently reduce approach and landing speed.
(a) (b)
Figure 5: The variable incidence wing aircraft: a) Republic XF-91 "Thunderceptor" (1949) and b) Vought F8U
Crusader (1955).
Variable sweep wing (swing wing) design has been the most successful and popular morphing design so far. By
sweeping the wing, an aircraft can fly at both supersonic and subsonic speeds. It also helps the aircraft in take-off and
landing. The first attempt in this direction was made by German Messerschmitt, who developed P-1101 (Fig. 6-a) in
1944. The sweep angle could be altered only from 35o to 45o, and that too while the aircraft was on the ground.
In-flight sweep angle change was first achieved in X-5, built by Bell Aircraft Company of USA in 1952 (Fig. 6-b). The
sweep angle could be changed from 20o to 60o in 20 s. It successfully demonstrated the swing wing concept by showing
reduced drag and improved performance resulting from wing sweep as the aircraft approached the speed of sound. One
of the major challenges faced by the designer of X-5 was to compensate for the change in the center of gravity location
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of the airplane as the wing is rotated about its pivot. To this end, they developed a mechanism to push the entire wing
assembly by as much as 27’.
(a) (b)
Figure 6: The first swing wing aircraft: a) Messerschmitt P-1101 (1944) and b) Bell X-5 (1952).
The swing wing design became very popular from its inception. In 1952, Grumman built a fighter, named XF10F-1,
which could change its sweep from 13.5o to 42.5o (Fig. 7-a). It demonstrated the handling problems of swing-wing
aircraft, later solved by the development of fixed wing root “gloves”. It also used full-span slats and Flower flaps
extending over 80% of the trailing edge for shorter landing and take-off distances. While the aircraft suffered from
several airframe, engine, stability, and control problems, the swing wing technology was proven to be effective and
reliable. Only one of the two XF10F-1s was built and the program was discontinued due to several technological
difficulties.
The first production aircraft with swing wing capability was General Dynamics built F-111 Aardvark (Fig. 7-b). It was
developed under the Tactical Fighter Experimental (TFX) program aimed to produce a single aircraft to fulfill a Navy
fleet defense interceptor requirement and an Air Force supersonic strike aircraft requirement. The first F-111 flew in
December 1964. With wings fully extended, the F-111 could take off and land in as little as 2,000 feet. With wings fully
swept back, it could reach a speed in excess of Mach 2.  F-111 exhibited a very high trim drag at supersonic speed due
to inboard mounting of wing-pivot. This was later corrected in the design of F-14, which used a more outboard pivot
location. The wing of F-111 could sweep from 16o to 72.5o. Though F-111 went into production line for the Air Force, it
did encounter several problems due to structural failures, loss of directional stability, engine surge, and stall (later
solved by a major inlet redesign). The swing wing design did not stop here, but continued to be the part of several high
performance aircraft such as Mig-23 (USSR, 1967), Grumman F-14 Tomcat (1970), and Rockwell B-1B Lancer (1983).
(a) (b)
Figure 7: The swing wing aircraft: a) XF10F-1 Jaguar (1952) and b) F-111 Aardvark (1964).
The application morphing technology has not been limited to powered aircraft. It was successfully implemented in a
single-seat experimental sailplane built by Fritz Johl of South Africa in 1970 (Fig. 8-a; Stinton, 2001). It achieved a
chord change of 100% using an internal “lazy-tongs” mechanism. It retracted the chord to increase the speed of the
sailplane in order to reach a thermal in a short time. In this configuration, the wing had high aspect ratio and low area,
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