Flight Testing of Homebuilt Aircraft

For Presentation At The 35TH SETP Symposium
Los Angeles California

Chuck Berthe (AF), Calspan Corporation and Dick VanGrunsven (M), Vans Aircraft

ABSTRACT

When one stands at the main entrance of the Smithsonian Air and Space Museum there are just three aircraft hanging in the central hall. They are singularly true to the basic theme of their selection. As one would expect they represent important steps in aviation development; the Wright Flyer, the first successful flight of a controllable airplane; the Spirit of St. Louis, the first nonstop flight from the United States to Europe; and the Voyager; the first nonstop flight around the world. However a more important theme is that they all represent extraordinary efforts by small groups of people dedicated to the accomplishment of a seemingly impossible task. In addition all these aircraft were privately funded. Controlled by the same people who did the work, and unencumbered by bureaucratic and corporate "help", these programs were efficient and successful (this used to be the "American way", Voyager proved that it still just might be).

It is also important to note that two of these aircraft were homebuilts and the other was about as close to a being a homebuilt as a factory built aircraft can be. The Wright Flyer was the ultimate homebuilt, the Spirit of St. Louis was a labor of love by a very small factory, and over fifty years later the first aircraft to fly around the earth nonstop, the Voyager, was again a homebuilt. Consequently it should not be surprising, but probably only fitting, that light aircraft aviation in this country should be saved by homebuilders. It is, in reality, a continuation of a well established tradition.

This home grown movement to preserve light plane aviation in this country could use professional help. Members of this Society have provided assistance and will probably continue in increasing numbers to do so. However we should take the time to understand the unique problems associated with the kind of flight testing required. It isn't as simple as it might appear. Some comments on the subject from a test pilot that designs homebuilt aircraft, and from another who builds them, might shed more light on the subject.

INTRODUCTION

The character of General Aviation in the United States has changed drastically during the last fifteen years. Beginning in the early 1950's the average private aircraft was a product of the vast production facilities of Beech, Cessna, Piper, etc. Today there are virtually no affordable General Aviation aircraft available to the private aircraft owner from these or other manufacturers. The industry has turned instead, for the most part, to the production of a few but very expensive units per year for use as corporate aircraft. The reasons for this transition are many and include: shame, blame, lawyers, poor management, economics, lawyers, over conservative engineering and design, lawyers, and etcetera.

Fortunately, and due to the foresight of people, such as the founders of the Experimental Aircraft Association in the 1950's, the light aircraft part of General Aviation is again flourishing (and will continue to flourish if the FAA can continue to be persuaded from "helping" too much). Somehow these persistent founders convinced the bureaucrats that it was all right for individuals to build and fly their own airplanes (in other words they made the Wright Brothers legal). As a result the design of light aircraft has been allowed to leap ahead at a rate not imagined, and considered impossible, when left at the hands of the major manufacturers (a current, but incomplete, listing shows over two hundred eighty designs of homebuilt experimental aircraft flying). As a consequence today's homebuilt experimental aircraft not only provide the Sunday pilot with an affordable 60 mph puddle jumper, but also for the more experienced, designs are available that include 300 mph, high altitude, all weather aircraft that incorporate some of the leading technologies. In many cases the designers of these aircraft are developing the new technologies ahead of the traditional industry.

As a consequence of the magnitude of this "homegrown" airplane movement much of the flight testing being done today is on homebuilt experimental aircraft. The down side is that much of this testing is being done by pilots who are not only untrained in flight test procedures and techniques, but are in many cases not even aware that they are conducting flight tests. The professional test pilot community can provide a valuable service to this movement by providing some of our expertise to these homebuilders. Our donations could range from an informal source of information to planning and flying their test programs. There are ample opportunities for first flights, no money, but much satisfaction to be gained by helping to save light plane aviation in this country. But before you rush off in a mad search for a homebuilder who needs a test pilot there are some things you should know about testing these seemingly simple and straight forward little machines.

UNIQUE ASPECTS OF FLIGHT TESTING HOMEBUILT EXPERIMENTAL AIRCRAFT (A BUILDER/TEST PILOT VIEW)

Flight testing a homebuilt aircraft requires the same techniques and technical scope that we are used to in our professional testing. The flight envelop must be demonstrated, performance measured and documented, and the systems tested. There are some major differences from what we are normally accustomed to, and they can make it a whole new ball game.

FLIGHT TEST TEAM

You will not have a flight test team with its backup of specialist in flight controls, flying qualities, propulsion, avionics, and structures. The test airplane was most likely manufactured by a single person (it might even have been yourself). He was the taxpayer, the customer, the contractor, the program manager, the production manager, the production staff, and what ever else it takes to make an airplane. You will be the flight test commander, flight test director, safety review board, flight test engineer, range clearance officer, and what ever else it takes to conduct a flight test program.

Probably for the first time, you will be totally responsible for the entire flight test evolution. Sounds like the perfect scenario doesn't it? "Me, finally in total command of the operation and I get to do all the flying!" You also get to do all the work, and in most cases you will be all alone. You will have to analyze problems, in addition to recognizing them, and come up with a pretty good idea of how to fix them. Imagine flying a test program where you have to personally fix, and pay for, every problem you find. If you're also the builder that's the way it is. That makes it a little more difficult to maintain professional objectivity.

Most of us, no matter how sensitive we think we might be, are not totally aware of how much we depend on the "support group" available to us as test pilots in our professional flight test activities. In addition to all the indispensable technical assistance we receive there is also a strong sense of moral support that we depend on. When we experience a very harrowing situation that could result in loss of the aircraft (and us), and we save the day solely due to our exceptional skill and technical knowledge, we know that the event was observed by many members of our "support group". We expect, at the very least, to have an attentive audience to hear the details at the debriefing. If we are lucky, and the support group is perceptive, we will receive some degree of praise which we will handle very professionally by pretending to minimize the importance of our actions. When one returns from a similar experience in a homebuilt there will normally be only you and the aircraft at the debriefing, and the airplane isn't very good at listening or heaping praise. If the builder is there he may not understand the skill it took to save the airplane, but he will understand quite well that their is something more to fix, and that is liable to limit his praise somewhat. Not only is there no money to be made in homebuilt testing, there are also fewer ego perks.

TEST FACILITIES AND EQUIPMENT

The material assets of a homebuilder are generally quite limited when compared to industry norms. The only real similarity to industry are the tradeoffs that must be made between requirements and available funds. The similarity ends when one counts the zeros behind the numbers in the available fund column. Flight test equipment is most often limited to cockpit instruments, timers, knee pads, and note taking items. The more sophisticated might utilize a voice recorder, and the ultra sophisticates might obtain, or make, a force gage, but the norm will be cockpit instruments and hand held data. We all know how to take hand held data, however it does increase the cockpit workload and requires a more detailed design of the flight cards. Personal experience indicates that handwriting clarity is directly proportional to the square of cockpit volume. Homebuilts are not noted for their cockpit volume.

Test facilities for homebuilt aircraft vary from well appointed, heated hangers on large airports to a lean-to on a grass strip (Dick did the development testing for the RV-3 from a six hundred foot strip in his back yard). In general it will be from an uncontrolled airfield with between two and three thousand feet of grass or blacktop, with runway widths varying from twenty five to one hundred feet. Where it gets a little more interesting is when one considers the approaches to some of these facilities. A two thousand foot strip can shrink to five hundred feet usable in a hurry when there are tall trees at either end.

Most of the homebuilts are somewhat portable due to their size, consequently the test pilot should be able to choose a suitable test facility within driving range of the permanent home of the aircraft. In fact he should insist on a reasonable facility. One should consider the size of the runway (as well as the approaches) as it relates to the performance of the aircraft. In addition consideration should be given to the availability of suitable landing or ditching areas in proximity to the runway (some of these engines are experimental too, and all of the engine installations are, if not experimental, at least custom).

The flight test area will be specified by the FAA preliminary airworthiness certificate, however they will generally go along with any reasonably located area of twenty five miles radius requested by the builder. Consequently the test pilot will have an input as to the location (naturally, selection of the test area and that of the test facility are related). Ideally, one would prefer an area of low traffic density that was also unpopulated. These are becoming more and more difficult to find, however, an area of five miles on a side can generally be located within the overall test area that meets these criteria and can be used for the more eventful flight test procedures. The common sense rules that we adhere to in our professional work apply, i.e. don't place innocents at risk.

For some of the test evolutions a chase, or at least someone on the ground with a radio, would be beneficial. Good experienced chase pilots, while plentiful in the professional arena, are rarely to be found around your local small uncontrolled airport. They become even more rare when cost of the chase plane is discussed. An additional factor unique to these operations is the time availability of those few you have found to have the required capabilities. Homebuilt testing is done during spare time. When you add the factor of Joe's wife letting him come to the airport on one of his few days off, to the other test factors of weather, aircraft availability, and your availability, your test team starts to dwindle. The best bet is to plan the flights as solo efforts and be grateful for help when it is available.

UNIQUE TECHNICAL REQUIREMENTS

A basic requirement is that the test pilot be qualified to fly the aircraft in question. In our professional work this is rarely a problem. In the case of homebuilts it more than likely will be. The aircraft are much more simple than what we are used to, but that fact does not necessarily relate to how they are to fly. Most of them will require piloting techniques that are different than those required in our professional work. Some of the techniques will be easier and some will be more difficult. The point is that they will be different and we must prepare for that aspect and obtain some experience in these techniques prior to test flying these machines, or prior to even providing guidance to someone else that may be flying them for that matter. The word experience was used instead of training in an effort to soften the blow. If, for example, you expect to flight test a tail wheel configured aircraft, you should have "tail dragger" experience (these configurations are directionally unstable on the ground). It's not a good idea to fulfill this requirement, during a solo flight test, in an airplane that you or someone else has spent five years building.

Test pilots must have detailed knowledge of the aircraft they test and most are justifiably proud of their ability to seek out and learn these details. This trait is equally appropriate to homebuilts. The difference is that this learning must be applied to basics of the aircraft that we normally take for granted. For instance:

Virtually all of homebuilt aircraft are piston powered. Some use certified aircraft engines, some are modifications of certified aircraft engines, some are special purpose aircraft engines not certified, some are modified automobile engines, some are modified snowmobile engines, and some are engines designed for other purposes that were bolted in an airframe in the faint hope that they would work (they seldom do). The thing to remember is that all but the certified engines (installed in a certified manner) are experimental in nature. An experimental aircraft with a certified engine can be an interesting challenge, as can a certified aircraft with an experimental engine. An experimental aircraft with an experimental engine will definitely have a long and interesting flight test period. The bottom line is that the test pilot must know the power plant, not only technically, but also operationally. This is no different than what we are used to. The difference is that in the case of homebuilts the importance of this knowledge can be masked by the simple appearance of these little power plants. These little engines have a great bit in common with those F100 series engines that normally push you around, they both keep you in the air, that makes them equally important.

Most of the engine and propeller installations will be custom, designed by the builder, as will most of the electrical systems and avionics installations. For instance, how many of us know what to inspect on an installation of a Lycoming 0360-F1A6 with a modified carburetor, a light weight starter, a Honda alternator, and an experimental crossover exhaust system all installed on a Glasair? If you don't you had better find someone who can, if you expect to test the aircraft. This is another example of our subconscious reliance on our normal support group, this time the highly skilled people who design and manufacture our airplanes.

Almost all homebuilt aircraft are propeller driven. There are few current generation test pilots who have significant experience in propeller driven aircraft, and even fewer who are familiar with fixed pitch propeller performance (most of which are made of wood laminations). A fixed pitch propeller has no engine overspeed protection and is optimized for a single flight condition. One with too high a pitch may not get you off the ground in the space allotted. One with too low a pitch will overspeed at the slightest provocation. Flight through even light rain can ruin a wood propeller worth around seven hundred dollars or so. If wood propellers are not retorqued on a regular basis they can fail the attach bolts and come off in pieces of various sizes. This kind of stuff is nice to know if you are about to test one of these simple little airplanes. An experience of Dick's might help us to learn more about simple wood fixed pitch propellers:

The purpose of the flight was to conduct flutter testing of the prototype RV-4 to ten percent beyond Vne. It was necessary to overspeed the fixed pitch wood propeller somewhat to achieve the high air speed required for the test. This overspeed stressed the propeller to the extent that it disintegrated (the propeller should have been able to accommodate the overspeed, however it was found to have substandard glue lines between the laminations). Due to my concentration on the flutter tests, I assumed the shaking to be due to a failure in one of the control surfaces. Only after visually checking the control surfaces and control reactions did I notice oil coming from the engine cowling. An immediate shut down of the engine revealed only short stubs of the propeller blades remaining on the hub. Since I was conducting flight tests, I had plenty of altitude and was wearing a parachute. I used the altitude, but didn't have to use the chute. There then incurred an interesting example of homebuilt flight test psychology. I had two airports available within gliding distance, my 3000 foot grass strip at home, and a 7000 foot tower controlled airport with emergency and support equipment which included an FAA GATO office. I chose the grass strip, partially because that was where my tool box was, but primarily due to fear of the bureaucratic help that I felt would certainly be thrust upon me at the larger, but safer, airport. It may well have been a poor decision on my part, unnecessary paranoia, but it does underscore the regard many of us have for the help available from the FAA."

Another area that requires more than normal attention is the basic structure and construction of the aircraft. When an aircraft is tested in our professional environment the test pilot can accept as a given that the wing spar was designed and manufactured properly. In the case of homebuilts it was probably designed properly however the quality of manufacture varies. There is no such thing as "production flight test" in the case of homebuilts. Each one is different, even when two aircraft of the same design are constructed by the same person the two will be different. The test pilot must be able to inspect the structure to assure that the manufacture was according to the plans. This of course requires the test pilot to study the drawings.

The unique technical requirements for flight testing homebuilt aircraft are well within the capability of the professional test pilot, however they can be in areas that we normally don't have to worry about. This requires some study on our part. Don't expect to simply drive out to the airport Sunday afternoon and test a friend’s homebuilt. Give it the same attention as the big iron that you will test Monday at work.

UNIQUE ASPECTS OF FLIGHT TESTING HOMEBUILT EXPERIMENTAL AIRCRAFT (A DESIGNER / TEST PILOT VIEW)

DESIGN CONSIDERATIONS

Historically, many of the most popular light planes have been the result of evolution rather than revolution. With each new design more was borrowed from the past than was innovated. While this is not the metal of which legendary designers are made, it is a good basis for developing sound, predictable airplanes. If the designer has done his job properly, his design is rather conservative, and if he also has some luck, the test pilot will have a rather straight forward, and hopefully uneventful, job.

In the case of the "designer/test pilot" co-author, the genesis of the RV series of aircraft was the Stits Playboy dating back to the early 1950's. The first of the evolutionary steps were customized modifications to the basic design. This was followed by a major redesign that included an all metal wing. The first completely new design of the series was the RV-3, an all metal, single placed, low wing monoplane, with a bubble canopy. Recognition of designer limitations, and the subsequent conservative design approach, was a large factor in the success of this airplane. There was no intent to invent a new wheel, and the use of lots of wing area and large tail moments provided the intended performance and flying qualities. No changes were necessary prior to marketing the design. The RV-4, while similar in planform to the RV-3, was actually a new design incorporating a tandem dual cockpit. This design did require a one half degree reduction in tail incidence prior to marketing. The RV-6 was again a new design, a side by side version of the RV-4, that maintained the basic aerodynamic features of the previous designs. The RV-6A is a tri-gear version of the RV-6.

It has been reported that the flying qualities of the RV-4 were tested, at one point, by the legendary Bob Harper who gave it a Cooper-Harper rating of "Two" for the up and away flight tasks (The owner/builder, a co-author, wouldn't let him execute the landing task. Perhaps good thinking, but not a good career move, Bob was his boss). In typical fashion however, Bob noted that it wasn't a "One" because the rear control stick was a little too short.

While this series of designs was quite conservative, not all homebuilt designs are. Perhaps the most successful "unconventional" series of designs are those of Burt Rutan. Today designs of this type are not considered unconventional at all. However in 1976 the Vari-EZ came as a real shock to the industry. While it certainly posed some flight test problems, it was tested by professional engineers and pilots, and in its marketed form possessed admirable flight characteristics and outstanding cruise performance. Burt's expertise in stability and control tamed the canard configuration which served as a basis for numerous other canard and tandem wing homebuilt designs which followed. Other unconventional designs have not been so successful and provided quite a challenge to the test pilots involved.

It is sufficient to say that a test pilot of homebuilt aircraft must know enough of aerodynamics to be able to identify potential problems. There are some aircraft that "look good" (aerodynamically) that don't necessarily fly that way. On the other hand most that don't "look good" tend to be somewhat lacking in their flying qualities. The design quality of homebuilts can range from the thorough professional approach that Burt uses, to something scratched on the back of an envelope. Most of Burt's started that way, as do many designs. Just be sure that the design you are testing didn't stop, technically, at that point.

SOME SPECIFIC EXAMPLES OF DESIGN IMPACT ON FLIGHT TESTING

As previously discussed, there are a large number of design types currently flying as homebuilts. Some examples of flight test experiences and flight test considerations concerning some of these design variants might be in order at this point.

CANARD CONFIGURATIONS

Canard configurations offer some notable advantages in the area of stability and cruise efficiency. They also offer some interesting flight test experiences. Many of the canard designs utilize relatively new and highly efficient airfoil sections that maintain laminar flow over much of the surface. Unfortunately if for some reason laminar flow is not maintained, their lift characteristics can change significantly. It turns out that bugs, raindrops, and other imperfect things that happen to perfect airfoils in this imperfect world, are not at all helpful in maintaining laminar flow. A good friend of the designer co-author was the first builder to complete a homebuilt version of the Q2 aircraft. His test flying was progressing well until he decided to improve the cosmetic appearance by painting leading edge stripes on the lifting surfaces, a rather common paint scheme. On the next flight, and after a rather interesting take off, he discovered that the forward wing was unable to generate enough lift to provide the pitch attitude required for a safe landing speed. The paint line, not understanding the importance of laminar flow to this airfoil, had rather dramatically changed its lift characteristics. After a rather prolonged test flight of over three hours duration, the fuel burn had moved the center of gravity aft to the point that a marginal landing could be made. Sandpaper and polish solved the problem (until the next swarm of gnats appear).

Another example of homebuilt flight test pitfalls is the four seat design called the Velocity. A number of these design had flown successfully for many hours when the flight testing of a gap seal on the canard elevator included an aft center of gravity condition. Under these conditions the aircraft entered a deep stall which was unrecoverable. The pilot remained with the aircraft as it made a vertical descent, in a flat attitude, and crashed into the water. The pilot was not injured and the aircraft was only slightly damaged. The aircraft has since been undergoing some very innovative high alpha, on the ground, testing to uncover the problem.

There have been other examples of follow on testing of canard designs, after years of uneventful operation, that uncovered deep stall and flat spin modes. In some cases the designer had specifically tested for these modes and not experienced them. Canard and tandem wing designs seem to be susceptible to uncertain high alpha related aircraft responses. Slight differences in wing construction, a different method of coupling a spin entry, etc., can create new and exciting modes of falling.

"UNCONVENTIONAL" CONVENTIONAL DESIGNS

In the early 1970's there was a homebuilt design that provided the initial excitement that gradually became the boom in homebuilding activity whose results we enjoy today. That was Jim Bede's BD-5, an all metal low wing single placed pusher with retractable gear and a side stick controller. It was an unsuccessful design for many reasons, however it did demonstrate the fact that there was a large market base for high performance homebuilts. It also proved that it wasn't lack of technology that was keeping production general aviation aircraft in the 1930's in design innovation and performance. From a test pilot's standpoint the BD-5 was an interesting airplane. It comes close to holding the record for first flight crashes by homebuilder test pilots. It had a high thrust line, the tail was not immersed in slipstream, and the pitch control was sensitive. You can develop your own takeoff scenario.

The Questaire Venture is an excellent example of unconventional looking conventional aircraft. The immediate impression is "not enough tail length", however, if one looks more closely the tail length is not that short, the fuselage is just relatively wide. If one looks at the planform, the configuration is even more conventional. The result is an aircraft that has set a number of speed and climb records and has excellent flying qualities, the latter due in large part to a cleverly designed mechanical feel system. The test pilot of this airplane has been seen to smile a lot.

Replica and scale replica historic aircraft are popular among homebuilders. Homebuilt replicas of WW-I aircraft will probably have predictable flying qualities i.e., marginal or neutral stability about an axis or two, and could be almost as demanding to fly as the originals. Replicas of WW-II fighters are almost never full-size and could suffer from scale effects and relatively high wing loadings. Some of the originals of these didn't handle all that well either by today's standards.

The lack of affordable jet engines has not totally deterred homebuilt designers from jet like designs. A recent example was the RANS S-11. This was not a replica of a specific aircraft but was more of a generic space age jet in appearance (after the F-117 anything appears possible), but propeller-driven. The prototype was destroyed in a crash following an engine failure, however it is reported that flying qualities (difficulty in achieving a power-off flare) contributed to the severity of the damage. Perhaps fly-by-wire shapes should have fly-by-wire systems.

UNCONVENTIONAL CONFIGURATIONS

Non-conventional stability configurations can cause problems for any test pilot, but they can be particularly hard on inexperienced homebuilder/test pilots. An early example which was successfully flown was the Baker Delta Kitten, a one-of-a-kind homebuilt of the 1960's. It was a true delta wing airplane with a delta-like fuselage as well. Its flying qualities and performance turned out to be satisfactory, however the trim drag and the induced drag associated with low aspect ratio subsonic tail-less designs limited the practicality of the design.

Another tail-less design of that era was the Dyke Delta. Plans for this design were marketed and several dozen were built. Not really a delta, it could best be described as a low aspect ratio flying wing. The dramatic reflex of the wing trailing edge, needed for stability, resulted in rather high landing speeds.

Prior to flight testing these sorts of designs a historic data search of similar designs would probably be in order. Almost everything has been tried at least once. Lessons learned 40 or 50 years ago are still valid and can be quite useful for we younger test pilots to put in our bag of tricks.

HIGH PERFORMANCE DESIGNS

Probably one of the greatest challenges facing the homebuilder/test pilot today are flight characteristics of some of the higher speed kitplanes now popular. Several designs like the Glasair-III, Swearingen SX-300, and the Questair Venture are capable of speeds of 300 mph in level flight and their landing speeds and subsequent runway requirements are far above those of typical light planes. This class of airplane places the homebuilder in the arena of serious flutter consideration. Poor building techniques can have less obvious, but more serious consequences than most homebuilders are able to identify or cope with. A simple thing like not checking balance after painting can result in catastrophic flutter.

These aircraft are more similar to those we test professionally. Consequently our professional experiences can be directly applicable. This is also a performance level where lack of technical backup can have the most serious consequences. Flight testing these aircraft requires the same level of technical approach that we are used to.

DESIGNER "CONSTERNATIONS"

A homebuilt designer has little control over his product once the plans and materials leave his shop. He can only hope that the builder follows the plans and utilizes satisfactory construction technique. A stroll down the line at Oshkosh will show that this is most likely the case. Most of the current homebuilts show better workmanship than factory airplanes. There are, however, instances of less than clever workmanship the thought of which can cause the designer many sleepless nights. There are other instances where the workmanship is fine, but the piloting skills leave something to be desired. The results are the same, the designer loses sleep. Some selected examples might serve to demonstrate the point.

HOT SHOT "TEST" PILOT MEETS FLAWED HOMEBUILT

An RV-3 builder, recognizing his piloting limitations, had an experienced pilot (a flight instructor) test fly his plane. The initial part of the first flight went without incident. After a few passes over the field the pilot departed the area and flew to a friends country home some twenty miles away. He then proceeded to buzz the home several times. On the last pass he made a sharp pull up and the wings departed the aircraft. The investigation of this fatal accident disclosed that the builder had used only one fifth of the wing spar rivets specified, due to misreading the drawings.

As in many accidents there is plenty of blame to go around on this one, however, any one of three key people could have prevented the accident had they done their jobs properly; the builder, the FAA inspector of the aircraft, and the pilot. On how many test flights have you personally inspected the wing spar? With homebuilts it's not a bad idea.

GETTING THE "BUGS" OUT

An RV-4 builder assembled his airplane at the airport. After several weeks of finishing touches and waiting for the FAA inspection, he undertook the first test flight. Upon becoming airborne he noticed that there was no airspeed indication, and elected to land immediately. He attempted to lower the flaps at an excessive airspeed and bent the flap handle in the process. What followed was a series of high speed, no flap, landing approaches. Finally a successful landing was achieved. The airspeed failure was caused by bugs nesting in the pitot tube (nothing new about that).

This was a situation where, given the limitations of the pilot involved, someone on the ground with a radio (perhaps one of us) could have placed the lack of urgency of the situation in proper perspective and made for a more uneventful first flight.

"PREVIOUSLY OWNED" HOMEBUILTS

In most instances homebuilts made from kits are of good quality and are structurally sound. However, the builder/test pilots don't always do complete limit testing. If and when the aircraft is sold, the new owner will probably assume that it has been tested and proven as one would expect from a use factory aircraft. However, even if it was built from a high quality kit, there is always a possibility of a hidden flaw. A well intentioned modification or mistake made by the builder might have escaped detection during a incomplete test program and low stress usage history of the aircraft.

Dick once had an experience that illustrates the point. A friend invited him to fly a homebuilt Stits Playboy. It had been highly modified from the plans but had flown successfully for over ten years and several owners. During the flight, while executing a three "G" pull, the elevator control arm separated from the control column. To quote Dick:

"Fortunately the airplane had an independent elevator trim system. However, due to a forward center of gravity the trim wasn't capable of holding the airspeed below about 1.6 V-stall. With the trim set to full nose up, the control stick held between my knees (for roll control), and using both hands to manually pull on the elevator control cable, a safe landing was theoretically possible. When I had to momentarily use one hand to close the throttle, and then double up again on the control cable, I gave up my last option. This incident taught me something about making assumptions."

It was later determined that a seemingly innocent material substitution in the control column had reduced it's strength to one third of the design specification. Apparently the airplane had never been tested to high "G", nor had it been exposed to high stick forces in it's ten year history.

CONCLUSIONS

The authors had two specific goals in mind for presentation to this elite professional body. The first was to generate interest in providing flight test assistance to the homebuilt movement. The second was to demonstrate that this sort of "toy airplane" testing requires all the professionalism that we take to work with us, and in fact may require a more basic knowledge than we normally are required to demonstrate (inspecting wing spars for heavens sake!). That having been said we don't want to minimize the sheer joy of flying some of these little airplanes. Imagine going out to airport of an evening, and for the cost of five gallons of auto fuel, jump into an airplane that will do almost any maneuver that your government iron is capable of, with the exception of dropping ordinance, but including being able to look over your shoulder on climb out and see a plan view of the airport. All this with no hard-hat, mask, operations duty officer, flight schedule add-on, tower chatter, radar control chatter, boundary calls, etc. Think about it.

The members of this society can provide a valuable service to a part of our aviation heritage that could well become extinct. We would encourage members, who have not yet done so, to join the Experimental Aircraft Association and become acquainted with this unique movement. Take some time to learn about these little airplanes and participate. It is the only way we know of for a professional test pilot to be able to afford to own his own high performance, or just plain fun, airplane. In addition we can contribute to something important to the future of aviation in this country while having a lot of fun doing it.

There's a lot of interesting, demanding, and fun testing to be done with homebuilt aircraft. Have fun, but......."be careful out there".

Last Update: 5/4/97
Web Author: Juan Jimenez