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Build your own airplaneBuild your own airplane

DN Staff

September 11, 1995

16 Min Read
Build your own airplane

How can you justify a hundred dollar hamburger? For Charles Kohler, it's easy. All the justification he needs sits on the Tarmac, just beyond the condiment trays: One brand new Lancair IV-P, homebuilt and completed last December--the twelfth such airplane to fly.

Refueling and lunch at Florida's Flagler County Airport is a weekly ritual for Kohler and his neighbors from the Daytona Beach fly-in community where he lives. The 40-mile hop provides a good excuse not only to fly, but to talk shop. Lately, the talk has focused on Kohler's new plane. Its stats are impressive:

As the only four-seat, high-performance airplane on the market, the Lancair IV-P features a structural composite airframe, primarily of carbon fiber, and NASA-designed airfoils. Its six-cylinder, 550 cu. in. engine, built by Teledyne Continental, generates 350 hp at 2,700 rpm. Cruising speeds exceed 340 mph--well beyond the airspeeds of comparable production planes.

"I read that the plane had set a record flying from San Francisco to Denver," recalls Kohler, a career pilot for Pan American World Airways. "The average speed was 362 mph, which I thought was spectacular."

For comparison, Kohler cites a more recent record set in a four-place production aircraft. Flying from Daytona Beach to Atlanta, the pilot averaged 173 mph. "That's about as good as you can buy, factory built," he says. "But at the same low altitude, the Lancair IV-P cruises at 250 mph."

To enjoy such performance, however, one must build the plane, which takes time, perseverance, and money. Kohler ordered his kit in the spring of 1991 and started building in December that same year. Three years later, the project was complete; first flight took place December 9, 1994. Even when subtracting the interruptions--Hurricane Andrew and a new house, for instance--build time required more than 7,000 man hours. Total cost? $185,000.

Begin at the end. Kohler began construction at the airplane's tail, assembling one of the more simple parts of the kit: the horizontal stabilizer. The procedure is straight-forward and explained--with the aid of simple drawings, parts inventories, and check-off list--in the corresponding instruction manual.

Using templates supplied by Lancair, Kohler first builds a wooden cradle. The stabilizer's lower shell, which is laid up and cured at Lancair, sits in the cradle as the builder works on it. Made of carbon fiber in an epoxy resin matrix, the shell is two-and-one-half times stiffer than similar fiberglass composite structures. It also posseses 10% to 30% more strength. To achieve equal strength, a wet lay-up shell would require more material, adding more weight.

The layup itself comprises five plies of carbon fiber cloth over a 3/8 inch DuPont Nomex honeycomb core. Lightweight and thermally stable, the Nomex core will not sustain a flame. Additionally, it reduces toxic fume outgassing common with the foam cores of wet layup systems such as polyester or vinylester.

After placing the stabilizer's lower shell in the cradle and weighing it down, the builder installs the spars and ribs. Pre-molded at the factory, these parts are also made of prepreg sheets around a honeycomb core. One simply cuts them to size with paper templates and a jigsaw, and glues them in. Capstrips, glued to the top of each spar and rib, form an I-beam structure for added strength and rigidity.

"The most complex step in building the horizontal stabilizer," Kohler reports, "is installation of the elevator hinges, which mount to the aft spar." To ensure that the brackets are perfectly level and straight, it is necessary to build up the mounting surfaces with epoxy and microballoons. A string line guarantees proper alignment to prevent binding later on during operation.

With the hinges in place, a builder completes the stabilizer assembly by gluing on the top shell. The resulting airfoil is a high-laminar flow design with non-critical characteristics. This means that the airfoil is capable of maintaining laminar flow over 50% to 60% of its chord, generating greatly reduced drag.

"The composite structure," Kohler explains, "lets me reproduce the Lancair IV-P's computer-generated airfoil without drag-producing rivets, lap joints, or corners." The material's stiffness, he adds, maintains optimum contour under most flight conditions.

Builder's responsibility. Federal aviation regulations define an amateur-built aircraft as "an aircraft, the major portion of which has been fabricated and assembled by person(s) who undertook the construction project solely for their own education or recreation." They also define "major portion" as 51% of the fabrication and assembly.

But while the FAA requires a daily log and photographs of all the work done, as well as bills of sale, FAA inspections are limited to ensuring the use of acceptable workmanship and construction practices.

"The FAA makes a cursory inspection to see that certain principles have been held up," says Kohler, "but they don't go around and check every nut. Once the skin is put on, the responsibility is yours."

Kohler and many other kit plane builders, therefore, belong to the Experimental Aircraft Association in Oshkosh, WI. Technical Counselors from the EAA, usually associated with the more than 850 local chapters across the country and around the world, are available to help kit builders inspect workmanship and structural integrity.

In addition, Kohler claims the technical manuals available to EAA members prove helpful. This is especially true, he says, in those areas where an average kit builder may not have much experience, such as calculating voltage loss or determining wire size for instrument installation. Whenever in doubt, Kohler suggests contacting the factory directly. "If there was a problem, I would just call them."

After cutting his teeth on the horizontal stabilizer, Kohler moves on to the fuselage and wings. Like the stabilizer, shells for the wings and fuselage are formed in molds, cured in an autoclave under precise controls, and FAA certified. Construction methods mirror those used to build the horizontal stabilizer.

For example, the Lancair IV-P calls for full slotted Fowler flaps. This is because the airplane's wing is relatively small, designed that way to enhance top speed. This higher wing loading gives a smoother, more stable ride, but higher stall speeds as well. Incorporating Fowler flaps increases the maximum lift coefficient and reduces the stall speed.

Installing the Fowler flaps involves a lot of trial and error in the fitting to get the flap to move correctly. Two different flap tracks with two different travels describe the flap's motion. Locating the straight brackets to achieve the desired arc, Kohler recalls, requires a fair share of patience.

Two-way street. Because Kohler's Lancair IV-P was only the 39th kit sold and the 12th built, he was sometimes able to improve upon the hardware design. For example, Kohler found that the cables for securing the door latches tended to slip off their drum. "I saw that there might be problems with this system," he says, "so I got some of my friends and we engineered a 0.25 pitch chain system."

Kohler also reconfigured the landing gear doors. On the aft edge of each door, he reports, there wasn't enough room to accommodate both wheels retracting into the wells at the same time. So he put a little flapper door with a mousetrap spring on the back, allowing the doors to touch one another and open an extra inch or so.

While hardware and other components are not required to be certified in a home built airplane--a builder can install an electrical switch from an automotive parts store, for instance--Kohler believes it is best to maintain the highest level of certification. That means using certified components supplied by the manufacturer.

"The engine in my plane," he explains, "is certified as long as I maintain it in the condition I received it. But if I want to modify the ignition system by replacing the magneto with an electronic automotive type trigger ignition, I would lose the certification of that engine."

In short, the engine builder--Teledyne Continental--would claim that the engine is modified outside the parameters of FAA certification, and demand that the dataplate be removed. And while Kohler notes this hurts resale status, it can also harm the "peace of mind" certification provides.

A record run? Peace of mind is good, but another kit-builder named Jim Rahm will tell you he's after speed. Specifically, a world record for a four-seat single: Honolulu to New York City, non-stop, in 12 hours.

President of Engineair Inc., Daytona Beach, FL, and member of Charlie Kohler's fly-in community, Rahm is building a production V8 which he intends to install in a Lancair IV-P presently under construction. The engine will be offered as an option to builders of the Lancair IV-P.

The all-aluminum engine, derived from a Chevy small-block V8, offers 420 hp at takeoff, 350 hp for climb, and 315 hp continuous cruise. Displacement is 375 cu. in. A reduction gear set, designed and built by NSI, Arlington, WA, slows propeller rpm to 1,950 at cruise. A patented linear cam device, also from NSI, eliminates the dogfight between the propeller that thinks it's a flywheel and the engine which is firing with power pulses.

"Basically, we've taken a successful engine, enhanced it, and adapted it for aircraft," says Rahm, one of the founders of Hurst Shifters. "We have the lowest specific fuel consumption of any engine in the entire aviation engine inventory; we have the highest power-to-weight ratio of any engine in the aircraft engine inventory. By turning the propeller slowly, and using an additional blade or two," Rahm predicts, "we will go like stink when we get up high."

Kohler, conversely, bought his stock engine directly from Lancair, which is the OEM. Because the engine arrives by crate, fully assembled, the builder's toughest task is achieving the proper tilt to the firewall. This is because the engine mount bolts directly to the firewall which glues into the fuselage. Proper orientation of the firewall ensures a straight propline.

"The aft tilt and the twelve o'clock arrangement have to be exactly right," says Kohler. This, he points out, depends on the fuselage being exactly level in its cradle. "All these angles have to be worked down to less than a tenth of a degree." As with the firewall, a builder doesn't glue the shear web--which holds and aligns the wings--in place until all measurements are exactly correct.

Worth the effort. Prior to flying the finished airplane, a builder must adhere to FAA rules by: registering the aircraft; applying for an airworthiness certificate; passing FAA inspection; and obtaining a radio license.

Of course, the person flying must have a valid and current pilot's license. Additionally, amateur-built airplanes are initially limited to operation within an assigned flight test area for at least 25 hours when a type certificated (FAA approved) engine/-propeller combination is installed, or 40 hours for a noncertificated engine/propeller.

With these procedures realized, Kohler says the 25 months of labor, the occasional frustrations, and the expense were well worth it. "I enjoyed building the aircraft. The drawings and instructions were well-written and clear, and the building procedures relatively simple. Probably the most sophisticated piece of equipment I used was a Smart Level(TM)" Bodywork, he remembers, was the worst part of construction.

Had it been available when Kohler started building, the company's fast-build option of preassembled wing and tail assemblies "would have saved a year's work." But Kohler has no regrets as he finishes off his ice tea at Flagler County Airport's Runway Restaurant.

"Let me give you a couple of numbers," he says with a grin. "The Lancair IV-P has 2.1 sq. ft. of flat plate area. That Cessna over there, which also carries four people, shoves 6 sq. ft. of flat plate through the air. That's one reason my home-built is 120 mph faster!" It is also one reason the rest of his lunch buddies probably wish they had Charlie Kohler's patience, perseverance--and ultimately--his airplane.

This story does not cover the many steps of building a kit plane, and is not intended to be used for instructional purposes.

An alternative in aluminum

"You fought wide open, full throttle...You felt that engine in your bones...Maximum power, lift, and maneuverability were achieved mostly by instinctive flying: You knew your horse...The excitement of those dogfights never diminished."--General Chuck Yeager

They say most aviators are would-be fighter pilots. If so, there's a big market for Jim Stewart's kit plane: A scaled P-51 Mustang, the single-prop airplane in which World War II flying ace Chuck Yeager first became famous.

Introduced at the 1994 International Experimental Aircraft Association's annual Fly-In Convention, Oshkosh, WI, the all-aluminum "S-51" is engineered to replicate the aesthetics, performance, and flight characteristics of the full size original. Its airfoil, for instance, is the same as the big Mustang. Achieving those numbers, Stewart says, demanded recalculating control surface sizes to get good aerodynamic characteristics back into the airplane after scale down.

Because there is little off-the-shelf hardware that matches the plane's specifications, Stewart manufactures most of the components himself. "Even with the landing gear, there's nothing we can buy other than the brakes," he points out. "We have to make our own wheels, struts, and shocks."

Such attention to detail, however, pays off with load paths virtually identical to those of the big plane. Consequently, the S-51's structural configuration is very similar to that of the P-51, with ribs located where ribs were, and heavy pieces where heavy pieces were. The only significant departure from the original is the engine.

The full-size P-51 employs a 60-deg, V12 Rolls Royce Merlin engine. It barely fits within the big plane's narrow cowlings, let alone those of a 70% version. The scaled P-51, therefore, uses a big-block Chevy V8 modified to aircraft requirements. Stewart does not supply the engine with his kit, but he does advise builders about the additional parts and modifications needed.

Weighing 740 lbs, the engine produces 400 hp at 4,700 rpm. Stewart claims these figures make the S-51 the most powerful production two-seat sports plane in the world--ever.

As with the original P-51, the propeller runs at about half the engine speed. Stewart explains: "We run our tip speeds up to a Mach number of about 0.8, where the propeller is very efficient. But if the engine turns at that speed--around 2,200 rpm--there isn't enough power."

A custom gearbox solves this problem. Designed to manage more than 600 hp, it uses 6-pitch gears and a reduction ratio of 2.13:1, almost identical to the P-51. There is only one size difference between the original spur gear and that of the S-51. Both gear sets, moreover, display virtually the same face. To highlight the overkill built into his scale version, Stewart is quick to point out that the Merlin engine is rated at a whopping 1,400 hp.

For extra safety, the S-51 carries two electrical systems. The first is a main bus that services the whole airplane and one ignition system. A battery, which is on-line all the time, powers a second ignition system. Because the battery is diode protected, a downed first system cannot short through it. "Our main concern," Stewart says, "is dependability."

His other concern is build time. Unlike composite kits where a builder glues the top half to the bottom half, and left side to the right side, the aluminum S-51 has more than 3,500 parts; riveting alone takes 1,000 hours.

Much of the kit, therefore, is pre-fabricated. Pop rivets hold most of the primary pieces in place for delivery. When the crates are opened, a builder sees something that already looks like an airplane. Final assembly, however, requires replacing the temporary pop rivets with solid aircraft rivets--30,000 of them.

"We provide the buyer with about 50 engineering assembly drawings," reports Stewart. "A builder with good mechanical aptitude can finish the project and be in the air within two to three years max."

How much does all this cost? Right now, the basic kit sells for $53,000; the fast build kit, delivered with the wings and much of the fuselage already riveted, costs $78,000. Factoring in the engine, propeller, and instruments drives the price to $100,000, and $120,000 respectively. Finally, a builder must be aware of hidden expenses. Labor charges, special instrumentation, upholstery, and painting can easily add up to $50,000.

Stewart says some customers buy two kits to avoid building expense; selling one pays for the other. Still, the kit price is far less than owning an original, while the adrenalin rush is nearly the same. Those who have flown the S-51 attest that the control forces are heavier than most amateur built aircraft--they actually meet military speci- fications for training and fighter aircraft. The result, they say, is a kit plane that flys and feels like a "heavy" aircraft--like a classic WWII P-51 fighter.

Writer's impressions

It's one thing to admire a "homebuilt" on the ground--quite another to stake your life on the builder's competence by accepting a ride. As I climb into the cockpit, the first thing I confront is the following mandatory plaque:

"Passenger Warning--This aircraft is amateur-built and does not comply with federal safety regulations for standard aircraft."

Comfortable leather seats, Bose headsets, and the pilot's high confidence level slows my heartbeat some, but take-off drives it right back up. "This thing is powerful," I think.

I've flown in 4-passenger aircraft before, but can't recall the same exhilaration blasting down the airstrip. What I do remember is a lot of noise--nothing close to the quiet, smooth performance of the composite structure I'm flying in now.

After cruising above the clouds, trading turns at the stick, and landing for a "hundred dollar hamburger," we head for home again--a confident pilot and passenger.

Kit planes are booming! Kit sales increased 58% in 1993, and 30% in 1994. U.S. production aircraft, conversely, are at an all-time low. The average fleet age for a single engine aircraft is now over 25 years.

It's no surprise, therefore, that the ratio of amateur-built to factory-built piston aircraft, newly registered with the FAA, exceeds 3:1. But as this article can only highlight, building a kit plane remains a time-consuming project. The forth-coming Design News CD-ROM Suppliers Directory gives the full story.

There, you'll find considerably more background and graphics than we could publish in the magazine, as well as a glossary of kit plane sources. In several sections of the story here, we've included a symbol that indicates additional information contained on the CD-ROM version. When you see the symbol go to the CD-ROM version. It will put you on the shop floor with the builder himself.

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