Definition: Aircraft that is still in the experimental testing phase of development, or craft built by amateurs from kits or plans.
Significance: Experimental aircraft are important for the progress of the aviation industry. Flight test of an experimental aircraft validates expected performance criteria, tests structures, evaluates handling characteristics, and more. Scientific research is dependent on flight test results from experimental aircraft. A new airplane as such is experimental, but a proven and certified aircraft is experimental when mounted with new, untried applications requiring testing.
Types of Experimental Aircraft
A distinction must be made between true experimental aircraft and amateur-built airplanes constructed from kits or plans. The former category contains aircraft involved in research and design at costs reaching into the millions, while the latter group contains aircraft constructed by individuals for fun, at a substantially smaller cost. An experimental military project or airline endeavor may reach hundreds of millions of dollars before the production run gains approval. The homebuilt or kit plane project, on the other hand, is notably less expensive. Single airplanes built for personal use have flown at a cost well below $10,000. Another difference between the two groups of experimental aircraft includes the time spent in flight test. The large company projects may spend more than a year in flight test while a homebuilt or kit plane undergoes a basic forty-hour testing period.
The Design Process
Regardless of the group to which the airplane belongs, every aircraft flying today began in the much the same manner. Industry discovers a need for a particular design or a mission requirement. After the creation of a new concept, pilots, engineers, and mechanics discuss and research ideas for the production of the new craft.
The next step is sketching the design idea. More than a few new ideas begin as pencil drawings on napkins in restaurants over lunchtime discussions. From the first idea sketches, aeronautical engineers further refine the drawing by use of computer-aided design software. During the initial design period, engineers exchange ideas and make concessions and compromises that are eventually lofted into the drawing process. Modelers next construct a model of the new aircraft.
Models of the new airplane are necessary for many reasons. Wind-tunnel testing requires models of different sizes. Models enable designers to visualize proportional sizing. Problems not visible on a drawing board may vividly stand out in three dimensions.
After studying the models in depth, technicians and engineers develop mock-ups of various sections and components of the new aircraft. These mock-ups allow others to test the airplane and offer their opinions to the designers regarding positive and negative aspects of the new craft. Mock-ups also allow pilots, engineers, and mechanics to spot problems before production of the aircraft. The earlier design changes can be made, the more economically they can be incorporated into the production schedule.
The Prototype and First Flight
At the completion of the design process, construction of the prototype begins. Construction of one or two copies of the new craft is required for testing. This is an expensive proposition; until the craft goes into mass production, per unit cost of the craft can be phenomenal. The purpose of the prototype is further research and development. Changes follow in quick succession as shortcomings become evident and better construction methods become available. As the first prototypes are readied for flight, flight profiles are developed for the initial flight and the flight test program to follow. The most exciting event at an aircraft plant is the first flight of a new design, an event anticipated by everyone in the company.
Typically, first flights do not last more than about three-quarters of an hour. The only concern the company and the pilots have with the aircraft is whether it will fly. Initial flight testing does not try to test the edge of the flight envelope. Pilots, engineers, and managers are not interested in how fast or high the airplane is capable of flying on the first flight; they only want to see it fly and see it handle the way it is expected. Test crews will address other questions regarding the performance envelope later.
As test pilots put the aircraft through its paces, they keep meticulous data on every aspect of each flight. The company uses the data to refine the flying qualities of the prototype and suggest changes in the production run.
After completion of the flight test program, the new airplane will finally reach acceptance. If it is an airliner, company officials from the airline will either accept or reject the new craft. Military acceptance is somewhat different, in that the airplane flies through a more intense flight test program. Additionally, flight tests of military aircraft involve weapons systems compatibility.
Homebuilt Experimental Aircraft
The other type of experimental airplane is the burgeoning kit plane and homebuilt industry. In 2001, approximately twenty-two thousand homebuilt aircraft constructed by amateur builder-pilots were flying in the United States.
One organization dedicated to homebuilt enthusiasts is the Experimental Aircraft Association (EAA). The national headquarters of the EAA is located in Oshkosh, Wisconsin. There are local chapters of the EAA throughout the United States that allow homebuilding proponents to exchange technical information and discuss problems or other building concerns. Members also enjoy the camaraderie of other members, along with the encouragement offered at monthly or biweekly meetings.
Advantages and Disadvantages of the Homebuilt
There are many reasons pilots choose to build their own aircraft. One reason is performance. Since the 1950′s, aircraft manufacturers have done relatively little in the way of increasing or improving aircraft performance. For example, a popular older model production aircraft flies at 100 knots, burns 7.5 gallons of fuel per hour, and can travel up to about 500 nautical miles before refueling. The latest model of the same aircraft today can fly about 106 knots, burns approximately 8 gallons of fuel per hour, and has a range of about 500 nautical miles. There has been virtually no change over the years in its performance.
However, ingenious amateur aeronautical engineers and pilots have produced airplanes using the same engines in newly designed airframes. These home builts are capable of 175 knots, with ranges beyond 1,000 nautical miles, using the same power plant, producing the same power at the same fuel flow. The backyard engineers have managed to attain much greater speeds over longer distances for the same amount of fuel.
While an increase in performance is advantageous, there are limitations to the use of a homebuilt airplane. For instance, home builts or kit planes are restricted from use in commercial operations. They also require a large amount of time for construction. Some pilots complain about the handling qualities of the smaller airframes. However, in many instances the advantages far outweigh the disadvantages.
One very important advantage is that of knowing the aircraft. As the owner-builder of the aircraft, the pilot is intimately familiar with all the systems of the airplane. Another benefit to constructing an airplane is that as the manufacturer of the craft, the builder is qualified to perform all maintenance and inspections on the airplane according to the Federal Aviation Regulations (FARs). In other words, each year, when it is time to inspect the airplane, the builder of the craft can save hundreds if not thousands of dollars in shop fees.
Another advantage homebuilt owners have over production aircraft owners is that they can build the airplane at their own pace, according to their own budget. A home-builder can spend $600 one month, and if funds are lacking the next month, the building process can slow down while the builder spends only $50, or nothing at all. For the pilot who has purchased an aircraft with a loan, however, the bank will want payment each month.
The time required to build a kit plane varies. With a fast-build option, a builder can have a plane airborne in less than a year. On the other hand, some builder-pilots have dragged out a project for twenty-five years. Average build times, depending on the make and model of the airplane, is about 2,000 to 3,000 hours of work. Working part time, this equates to two to four years.
After the airplane is finished, it is time for the first test flight. For this important first flight, many homebuilders opt to hire a professional test pilot familiar with their design. This is a smart choice for pilots who have allowed their flying skills to degrade during the construction process. Many would like to fly their homebuilt on the first flight, but this is a case where vanity must defer to common sense.
Following the initial flight, the homebuilder is free to fly the airplane through a test program. During this time, the airplane will be restricted to one geographical area for forty flight hours. After the airplane is proven through this test period, the restrictions are lifted. Now the owner-builder-pilot is free to use the airplane for personal use as any other airplane.
Some experimental aircraft begin as homebuilt inventions to test new designs, such as Fred Weick’s homebuilt 1934 W-1A, which experimented with the use of tricycle landing gear.
Types of Homebuilts
While the production market is limited, homebuilts offer a wide selection of airplanes to the potential builder, running the gamut from very simple single-seat ultralights to highly sophisticated, six-place family airplanes. A pilot desiring a production four-place family airplane that can cruise faster than 145 knots will spend more than $400,000, plus ongoing maintenance and upkeep. On the homebuilt market, however, one can choose from among many relatively inexpensive four-seat, high-speed, long-range airplanes. Similar savings can be made in home-building seaplanes and amphibious aircraft.
After choosing the group of airplanes from which to select, a pilot might research the safety aspects of particular designs. The next decision is the airplane’s appearance, which is a matter of personal preference. Not only are there a wide variety of kits available for homebuilt plane types, but builders who want more than what is available on the market also can design a new craft incorporating all the desired attributes.
Most homebuilts and kit planes are smaller in size and weigh less than manufactured planes. Coupled with proportionally larger engines, this tends to increase the performance of the design. Many builders opt for two-place designs that provide opportunities for a great deal of com-promise. Most pilots find themselves flying alone or with only one other person. When they have a need to carry more, they rent larger airplanes.
Construction Techniques and Materials
An exciting aspect of homebuilding is the selection of construction materials. Construction techniques vary with different airplanes. A popular airplane that uses construction techniques of the 1930′s and 1940′s is the Pitts Special. The fuselage of the Pitts is constructed of steel tube and wood formers covered with doped fabric. The wings are constructed of wood spars and ribs and covered in fabric. Other homebuilts constructed completely of wood are beautiful examples of artistic creation. Some use woodworking techniques that date back to World War I.
Conventional construction of modern light airplanes is sheet aluminum riveted on formed aluminum structures. In the early 1970′s, homebuilders began experimenting with foam, epoxy, and other composite materials. Many of the new materials, such as Kevlar fabric and carbon graphite, are lighter than steel and provide greater tensile strength. In addition to being stronger and lighter, some of the new composite materials are easier to work with and enable the builder to form the compound curves of aeronautical structures more easily than when working with conventional materials.
With the use of Stirling engines or engines that use other alternative fuels, pilots may be able to fly farther and faster than ever imagined. Modern engines are one reason homebuilts are capable of such great speeds. The fact that the aircraft is experimental allows owner-pilot-builders to select the engine of their choice. Selection of the engine can be just as varied as selection of the aircraft itself.
The modern certified aircraft engine is a costly item. A new Lycoming or Continental aircraft engine can easily exceed costs of $20,000. Propellers and other accessories on the engine can drive that cost up another $5,000 or $6,000. Although such high prices may be discouraging, the homebuilder has many engine options.
The engine on a homebuilt does not have to be a certified aircraft engine. Because the airplane does not adhere to the specifications defined by the Federal Aviation Administration, builders can use any power plant they find suitable for their design. Indeed, many homebuilts are flying using engines from Volkswagen cars, chainsaws, snowmobiles, or outboard motorboats.
An automotive V-8 engine powers one of the most popular homebuilts, the Lancair IVP. The IVP can carry four passengers in pressurized comfort at altitudes above 25,000 feet, at speeds greater than 330 knots. Another example for those who doubt the use of automotive engines is that of the Volkswagen engine. Properly adapted for aerial use, this little engine has powered airplanes as fast as 230 miles per hour while getting more than 80 miles to the gallon.
While some doubt the validity of homebuilt and kit-plane flying, this class of airplane is here to stay. For individuals with some technical background and the ability to work with their hands, homebuilding is a way to acquire an airplane inexpensively. The rewards they reap flying their own creations are many; chief among them the cost savings the homebuilder will realize over the years of flying the airplane. Saving money, however, is only a part of the compensation. The greatest reward is watching people admire the airplane. Most homebuilders are very pleased to hear the comments others make regarding the craftsmanship and work invested in the airplane.