Designers are continually trying to create new and better machines that will outperform and outsell what is currently available. Periodically, a new “magic configuration” or concept will attract a large amount of attention. There will be claims that this configuration is overwhelmingly superior to all others and that the majority of future airplanes will inevitably use this new design. Some of these concepts survive and go forward. Some are used briefly and then disappear, and some never make it into service.
Some examples from history include:
- Triplane fighters (WW-I)
- Canard business aircraft (Avtek 400, OMAC-1, AASI Jetcruzr 250 and 400 and the Beech Starship)
- Tail-pusher propeller airplanes (Lear Fan, Cirrus VK-30 and Prescott Pusher)
Last month, we took a look at the role of marketing in the choice of design features of an airplane and how the need to attract financing can drive the choice of an “innovative” configuration. Some recent examples include blended wing-body airplanes, electric propulsion and E-VTOL. All are getting plenty of attention and attracting significant investment.
We have also seen that a designer must weigh two propositions when making the decisions about adopting innovative design features.
First: Conventional practice is conventional because it works.
Second: Innovation can be the path to something better than what can be achieved with conventional practice.
Both of these speak to a common story that applying the two basic principles above can help avoid.
The Hype of the Magic Configuration
The magic configuration will often show up in marketing material put out by emerging companies. Usually these combine computer-generated graphic images with a glowing description of the projected capabilities of the airplane and the advantages of its design. Often these presentations are almost entirely devoid of technical realism. One of the downsides of modern CGI technology is that it is possible to produce very realistic images that give the illusion of reality to something that is little more than an early-stage idea.
The new magic configuration will likely get a lot of publicity, and many people will be persuaded that it is the way of the future. However, not all discussions of magic configurations are this hyperbolic. The concept may, in fact, have some actual virtue that attracts the attention of serious design engineers and appears to offer an attractive solution to a real-world design problem. The technical advantage being claimed may be real, at least to the first order. The real challenge is to evaluate it rigorously and not get caught up in the surrounding excitement.
The Designer’s Dilemma: Is It Really the Way To Go?
The designer should try to avoid being caught up by all of the superficial enthusiasm and make sound technical decisions. The first thing to consider is whether the concept actually works. The second is to determine if it works better for the mission the airplane being designed must perform.
Does It Work?
The new configuration or technology is usually presented to have some specific advantage over conventional practice. The questions to answer are:
- Is the advantage real?
- If the advantage is real, is it useful?
All airplanes are governed by the same laws of physics, so claims of magic quickly evaporate in the face of properly comprehensive physical analysis.
First Principles
It is usually possible to get a small number of physical parameters that tell the story. For example: The maximum lift-to-drag ratio of an airplane (L/D) is directly proportional to the ratio of the wingspan and the square root of the wetted area of the airplane [span/sqrt(Swet)].
Similarly, the useful load fraction (useful load/gross weight) gives a good idea of the structural efficiency of the configuration.
For the emerging electric propulsion systems, the energy density of the batteries (kilowatt-hours/kilogram mass) dominates the problem.
Comparing any new configuration or technology to current airplanes at this fundamental level can give a lot of insight into whether or not the new configuration has enough value to be worth further effort.
Vehicle Performance
It is vital to evaluate the potential effect of a concept on overall system performance. A claimed advantage might in fact be real, but the concept may have other disadvantages that offset or overpower the initial gain. A very common mistake is to look only at the positive and not consider the negative.
One example of this is canard configurations. When the modern canard concept first appeared, two advantages were claimed:
- Trimming the airplane with an up-load on the foreplane rather than a down-load on a tail was more efficient.
- The canard configuration packaged better than an aft-tail configuration, making the fuselage lighter and reducing drag.
In practice, the up-load on the short-span forward surface was higher than the down-load of the horizontal tail it replaced. Also, carrying so much lift on a short-span surface produced enough extra induced drag to offset the advantage of substituting an up-load for a down-load to trim the airplane. While some canard airplanes (notably the Long-EZ and its derivatives) were successful, they were not overwhelmingly superior to conventional aft-tail airplanes, and in some cases (see Starship versus King Air) compared poorly to conventional airplanes.
Sometimes, the advantage can be successfully exploited and lead to a successful airplane. The Piaggio Avanti, for example, used a three-surface configuration. The small “forward wing” has a flap that is used to trim out the pitching moment from powerful flaps on the main wing, while the aft tail provides enough stability that the forward wing is not loaded heavily in cruise. This combination enabled the airplane to have very effective high-lift systems and a highly efficient cruise-optimized wing as a result.
Does It fit the Mission?
It’s important to keep in mind that there is no single magic configuration for all types of airplanes. What may be a good configuration for one mission might be completely inappropriate for another. This is even true for conventional or well-accepted concepts: A high-wing strut-braced monoplane might be a good concept for a backcountry bush plane but totally inappropriate for high-speed, long-haul operations.
Early in the life cycle of a new concept, it is common to see it everywhere and being applied to all kinds of missions. But any new concept or technology, even if it works as well as its advocates believe, will be more applicable to some missions than others. The more successful concepts will find a niche where they are effective, but they will not take over everywhere. Even if the configuration has real advantages, they will almost always give a more meaningful improvement for some missions and be either less advantageous or actually unworkable for others.
The Cycle
The history of any new configuration or technology follows a predictable trajectory known as the “Gartner Hype Cycle.” The stages of the cycle are:
- Innovation Trigger: A new technology appears and generates a lot of interest before any meaningful demonstrations have happened.
- Peak of Inflated Expectations: Expectations of great success develop, often accompanied by claims of great early success.
- Trough of Disillusionment: Interest fades as most applications of the technology fail to deliver on the early, inflated expectations. A few determined entities soldier on. Many concepts die off at this point if successful applications prove elusive. This is the point at which physical reality cannot be ignored and fundamental flaws in the technology must be solved if it is to survive.
- Slope of Enlightenment: Successful applications of the technology appear. They may be less dramatic than the initial hype but they are useful enough to continue. At the same time, practitioners develop a useful practical understanding of the technology and how to implement it successfully.
- Plateau of Productivity: If the technology has survived this far, it has found a place where it is useful and it is adopted as part of standard practice in that arena.
Airplane design is not immune from this cycle. Some magic configurations survive and find their place, and some, having had their moment of initial enthusiasm, die in the trough of disillusionment.
