Larger aircraft: Metallic wings
How much fuel could you store in the Rogallo wings if you need some?
The wings provide lift, but not only. In large aircraft they usually receive thrust from the engines, in order to transfer it to the fuselage, they are also fitted with hyper-sustentation devices and aerobrakes. And finally they can transmit a good part of the aircraft weight to the outer landing gears when the aircraft is on the ground.
The latter functions are not compatible with a flexible structure. Only metallic or composite wings are used. They can be adjusted for an accurate envelope with optimized performances.
Smaller aircraft: Wood and canvas or metallic wings
The structure can be stiffened, this is commonly done, and leads to the concept of wood and canvas, as the DR-400 and many other GA training aircraft (all first aircraft were built this way):
However these airframes don't scale well, and are not compatible with fast GA aircraft, leaving alone airliners speed range. Larger and faster aircraft, which anyway must protect their wing tanks, use metallic wings.
Ultralights and paragliders: Only categories with Rogallo wings
The advantages provided by flexible wings (easy storage, light weight, low cost) is a significant choice criteria for use in ultralights and paragliders, and of course for parachutes. These advantages overcome their limited aerodynamic performances, which is of lesser consideration for vehicles that are not specially aerodynamically designed.
To summarize: The flexible wings are more appropriate for smaller and slower aircraft, in particular those not requiring wing tanks, wing control surfaces, or wing landing gears. Namely ultralights.
The wings in the aircraft fulfill a number of functions, which would be quite difficult and inefficient with the Rogallo wings (or parafoils, for that matter), like:
- L/D ratio- The main purpose of wings is for efficient generation of lift i.e. with minimum drag. The L/D ratio of modern airliners are anywhere between 15-20, while in case of the parafoil/Rogallo wing the values are in the range of 5. One thing to note is that an important way to improve the L/D ratio in normal wings is to increase the aspect ratio, which is severely restricted in case of these flexible wings- even for aircraft design, flutter is a serious problem.
L/D ratio of flexible wings, table from Gliding Parachutes for Land Recovery of Space Vehicle by W. H. Eilertson
Fuel carrying capacity- Apart from creating lift, an important function of aircraft wings is to carry fuel, which isn't practical in case of flexible wings- this alone would be enough to restrict these devices to smaller aircraft.
Controls- The wings house the controls, which would not be possible in case of flexible wings (future combat aircraft are expected to have wing morphing, but is still a bit far away). The control of flexible wings are by using cables, which are suitable only for small aircraft. Another issue is redundancy, which would be quite difficult with these exposed cables.
Control of rogallo wing using cables; from NASA's Gemini program; image from vintagespace.files.wordpress.com
Engine mounting- Wings serve as an important place for mounting engines, which would not be possible in this case- the aircraft are restricted to single or inline engines.
Landing gear mounting/retraction- in some cases, the landing gear is mounted/retracted into the wing, which again would not be possible in case of flexible wings.
Safety- The flexible wings do not offer the same level of safety as the rigid (elastic) ones. What if they fail to deploy properly or start to retract in flight? NASA carried out research on the usage of flexible wings for the spacecraft landing programs which brought out a number of issues related to structural safety and deployment:
The wing’s structural problems persisted. Further drop tests saw more sails fall apart ...
Wing deployment tests were also proving problematic. The sail wasn’t deploying consistently, at times not opening fully and failing to inflate with enough time...
Even though most of these problems were fixed, it would have been difficult to convince regulatory authorities and paying public for their use.
It should be noted that most of the advantages of the flexible wings are precisely because of their limitations- it is collapsible for reduced storage and weigh less because it doesn't carry fuel or have control surfaces and doesn't have mountings for various things (like navigation lights etc).
It should noted that the people involved were quite conscious of the restrictions of this type of wing- For example, Francis M. Rogallo, the inventor of rogallo wing was quite cautious when he noted,
... if we could discover how to make flexible wings that could be packaged and deployed somewhat like a parachute, such wings would have many new applications as well as replacing some parachutes and rigid wings.