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William Stinson's Thatcher CX4

Story and photos by Patrick Panzera, EAA 555743, ppanzera@eaa.org
Construction photos by Wilson Leonard


Bill Stinson and David Thatcher
We caught up with Bill Stinson and David Thatcher at EAA AirVenture Oshkosh 2008. They were parked in a place of honor, just outside the Homebuilders Headquarters.

William Stinson’s first attempt at an amateur-built experimental aircraft is certainly a winner. The single-seat, all-aluminum, semi-monocoque construction beauty is powered by a 1915-cc Great Plains Aircraft Supply (GPAS) Volkswagen conversion, reported to make 69 hp max, 65 hp continuous.

The first time Bill laid eyes on a CX4, the plane was spotted on the ramp during EAA Chapter 485’s pancake breakfast at Ferguson Field in Pensacola, Florida, the very airport where Bill took his first flight. David Thatcher (the designer, builder, and pilot of this plane) was standing beside his prototype and was approached by Bill who began to converse with him, not realizing he was the designer. David proceeded to tell Bill that he had plans to develop a builder’s manual and a set of aircraft construction documents for a scratch-built version. Bill didn’t think much more about it until he went to Sun ’n Fun the next year and saw the airplane there again. He sat in it, fell in love with it, and made the decision to try his hand at manufacturing an experimental aircraft.

Bill Stinson’s flying adventures started back when he was nine years old. A coworker of his father’s took him for a ride in a Cessna 150. That was of course during the days before Young Eagles (YE), but in a sense, it was a YE flight, having the same impact on young Bill that we YE pilots hope to impart to each child we introduce to flight – one that will last a lifetime.

At Auburn University. Bill studied aviation management and ended up administering air medical helicopter programs for the next 15 years. Bill earned his wings and has owned aircraft in the past, but he always wanted to build and fly his own.

Like many of us, he built his share of model aircraft and worked some with his hands, completing honey-do’s, but had no formal training in metalworking. David encouraged him with his creation since his design goal is to allow the builder to complete his project primarily solo in a two-car garage and in less than 1,000 hours. With that, and with the construction estimate in the $15,000 range ($13,500 advertised), Bill decided to take it on as a personal project. It was also very comforting for him to know that he lived in the same city as the designer and that if he had any technical questions, answers were a local phone call away. As it happened, David would stop by occasionally to check on Bill’s progress.

As things normally go, Bill exceeded his $15,000 budget by a little. He opted to have his GPAS engine kit professionally built and test-run by GPAS, and he also selected some instrumentation that was a little bit more expensive than he originally planned. Bill purchased the UMA micro gauges, which are substantially smaller than traditional steam gauges, thus allowing for more instruments in the same size panel while still retaining that classic steam gauge look. That look is part of Bill’s personal preference for a “little personal make-believe fighter.”

Along with going over budget a bit, Bill also missed the mark by exceeding the advertised 850 hours build time by about 100. Not too bad, all in all.

Getting Started
Beginning with the construction of the center section spar, virtually everything else goes from there. Once the center spar is built and jigged on the bench, the wing panel spars are set for the proper dihedral. Once the assembly is completed, the builder can either set the wing spars aside and attach ribs and skin later to concentrate on the center section, fuselage, and empennage, or he can work on the wings first.

Thatcher
Spar construction can be intimidating, but the plans and construction manual are detailed enough to give confidence to even the novice builder.

Bill chose to complete his wings first and get them out of the way before turning his attention to the center section, working toward getting it on its gear. “Once the airplane’s on the landing gear it goes fairly quickly after that,” Bill said.

“It’s just an enjoyable project, and of course I started it because I wanted my own personal airplane, one that I built myself,” he said. “But I tell ya, the thing that is probably the most rewarding for me is not the bright shiny airplane sitting here in front of us; it was the knowledge that was imparted to me by David Thatcher and the friendship that developed over time.

Thatcher
With three of the primary performance instruments at 3-1/8 inches, there was room for eight more 1-1/4-inch UMA gauges. On the left side: tachometer, manifold pressure, cylinder head temperature, and volts. On the right: oil pressure, oil temperature, fuel quantity, and fuel pressure. No vertical speed indicator.

“Dave is a 79-year-old gentleman who was an airplane and powerplant mechanic [A&P] and IA [inspection authority] all of his life and always wanted to design and build his airplane because he could never afford to buy one. He’s just a genius when it comes to a simple design that was easy for me to put together, and certainly his help was invaluable.”

Not everyone will have the access to David that Bill has enjoyed, due simply to location. However, I’ve been monitoring the CX4 Internet builders group for quite some time now, and from seeing David’s active role in the group, I’ve come to know and respect him through the process. Although I don’t have any immediate plans to build a CX4 myself (although I’d love to), I’m confident that I would develop a lasting relationship with David through the process, just as Bill.

It’s Not All Scratch-Built
There are some components available that Thatcher produces and offers to those who purchase a plans set. Among these parts are the upper and lower fiberglass cowl halves, the acrylic canopy, and the fiberglass wing and elevator tips. Available from approved vendors are preformed ribs and bulkheads; complete wing and center section spars; aileron sub-assembly; quick-build fuselage (includes spars); quick-build wings with ailerons; and the engine mount. But it’s not critical that any of these parts be purchased. The builder is free to build them; the plans show how it’s done. Details are also provided on fabricating the cowl, but for some it may not be as cost-effective as purchasing one pre-made.

Wings
The spar as shown in the photo above is constructed using squeeze rivets (conventional), but that’s about the only place where Avex pull rivets aren’t used. The plans call for either counter-sunk or dimpled construction (in conjunction with T-88 structural adhesive), but the builder can choose to follow the “Chris Heintz” method of concaving the nose of the rivet puller’s mandrel, folding over the head of the rivet to form a low-profile dome pull rivet.

Landing Gear
The landing gear “borrowed” from a Sonerai II is available off-the-shelf from GPAS, with the complimentary wheelpants from Aircraft Spruce (ACS). The wheels and brakes were also supplied by GPAS and are comprised of standard Azusa aluminum wheels and go-cart style hydraulic disk calipers rated at 1,200 pounds gross. With the CX4’s 850 gross, they should work fine. Additionally, with master cylinders affixed to each of the rudder pedals, and a full-castering tail wheel (attached to a leaf-spring – both of which are offered by ACS), ground operating the little taildragger is rather “conventional.”

Thatcher

Although the design originally called for the rudder pedals to be suspended from an overhead torque tube fitted between the instruments and the header tank, the most recent iteration has them floor-mounted. This adds almost infinite flexibility with the location and can allow for more leg room as there’s substantial distance between the specified pedal location and the back of the firewall.

The issues with the old design, as shown above, were a little more involved than just leg room. In the original rudder pedal design, the weld joints were under an eccentric load. Some of the people who looked at this setup worried that at some point the pedals might break off at the weld. David came to agree with that and redesigned the system as shown below.

Thatcher

Additionally, the attachment of the brake master cylinder is also eccentrically loaded. This tends to cause the brake extension rod to bend and buckle if applied under heavy pressure. Another point: the new design also gets the rudder pedal controls out of the way so a larger (10.5 gallon) fuel tank can be installed. And as one last good aspect, the new design is being looked at by the Light Aircraft Association (LAA) in the United Kingdom, and the organization likes the new design much better, although it wants 1/8-inch cables instead of the push-pull cables now installed.

Hydraulic master cylinders operated by depressing the upper portion of the rudder pedals are incorporated into either iteration of rudder control.

Thatcher
Keeping it simple, and for the most part, affordable, dictates using off-the-shelf parts wherever possible. The tail wheel and leaf-spring are both available from Aircraft Spruce and Wicks Aircraft Supply.

Canopy
The aft section of the sliding canopy operates smoothly through the use of off-the-shelf, cabinet-grade, full-extension drawer glides, sliding aft to clear the seatback bulkhead to allow for easy ingress and egress. The windscreen is robust enough to act as a handhold.

Thatcher
The curvaceous lines of the canopy are reminiscent of the P-38 Lightning or the Supermarine Spitfire.

Although the airplane can’t be flown with the canopy slid back, it can be removed for open cockpit flight.

Thatcher
Click for larger image

David’s personal aircraft, the prototype and serial number one, has lived a rough life, having survived both Hurricanes Ivan and Rita and also all of the load testing done to date. Refurbished more than once, it’s become a staple at Sun ’n Fun for the past few years, and hopefully beginning with 2008, a staple at Oshkosh as well.

Aesthetics
David is an aviation artist, putting brush to canvas on a regular basis. When he was developing his conceptual design, which I’m told had begun life on a napkin in a McDonald’s restaurant one morning during breakfast, he incorporated certain independent elements into his creation that he admired in other aircraft. The paint scheme is classic. I can’t help but believe that the graceful lines of the airframe and complementing paint job probably contribute to a good portion of its success. The affordability and ease of construction are just the icing.

Recognizing the inherent synergistic beauty of the CX4 package, Bill opted to copy it verbatim, save a marginally different instrument panel and a slight color difference in the accent striping. Where David chose black, Bill went with dark blue.

Thatcher

The Engine
As previously noted, Bill opted to deviate a little from the plans by going with the 65-hp, 1915-cc engine rather than the 55-hp, 1700-cc specified by David. Although the most common displacement for a 60- to 65-hp VW is the tried and true 1835 cc, Bill found that it costs the same to bring it up to 1915 cc, making it the best bang for his buck. The next logical jump in displacement is 2100 cc, but the price increases as well by almost $1,300 (for a 5-hp increase) as the crank stroke is increased and modifications to the case are required including a heavy-duty front bearing.

At the time of this writing, for an additional $950, GPAS will build and test-run its kit engine for the customer, a cost that Bill was willing to incur as cheap insurance. Overall, he’s very satisfied with his decision and with the personal service received from Steve Bennett, the owner of GPAS, telling us that he has been very helpful through this process and very supportive as well. “I would not hesitate to do business with Great Plains again,” Bill said.

Thatcher

The ignition system is typical of most commercially available VW engines to see the market in the past few decades. The setup includes dual ignition with the top set of plugs fired by a Slick 4316 aircraft magneto driven by the crankshaft and hung from the accessory case that houses the internal alternator. The lower plugs are fired by a solid-state, waste-spark (lawnmower style) electronic ignition, with the ignition module driven by the camshaft installed where the original distributor once resided. It isn’t elegant, but it gets the job done.

A personal watercraft battery (from Walmart) offers plenty of cranking amperage for the geared (Subaru-style) starter, with enough left over to power the 12-volt electrical system.

Thatcher
The wheelpants and the spinner are off the shelf from Aircraft Spruce. The 54-inch diameter by 44-inch pitch wooden prop is provided by Tennessee Propellers.

The float bowl carb is a very simple and proven updraft unit made by Zenith and features a cable-controlled choke, low-speed idle bleed screw, mid to high range fully cockpit adjustable mixture control, idle adjustment screw, and a float bowl vent to help prevent vapor lock.

GPAS recommends the use of either an electric or mechanical fuel pump, or no pump at all if you have at least 0.5 pound of fuel pressure by gravity feed and can flow a minimum of 10 to 12 gallons/hour at your highest angle of attack. If you’re going to run auto fuel, a fuel pump is a must according to GPAS.

Performance
The reported stall-speed on Bill’s plane meets the design target of 40 mph. There are no flaps, just full-span ailerons. The landing speed that works for Bill is 60 mph on approach. A comfortable cruise is 120 mph with 130 mph possible at 3,100 rpm at about 22 inches of mercury on the manifold pressure (MP) gauge. Top speed is 135 mph. Bill’s preference is to operate the engine at 2,900 rpm, or about 19 inches of MP which usually nets 120 mph. Fuel burn at this “cruise” setting is approximately 3 gallons/hour, running automobile fuel – 40 miles/gallon in no-wind conditions.

The CX4 is reported to be a very stable yet fun-to-fly aircraft. With ample dihedral in the wing, additional dihedral in the tips, full-span ailerons, and sufficient elevator control for balance in all three axes, the plane is responsive but by no means a squirrelly aircraft. Bill reports that it’s very rewarding to fly in smooth air, with his feet off the rudder pedals and his hands off the control stick. When properly trimmed, it will fly straight and level for a long time. But when it’s time to have fun, very small control inputs are all it takes. According to Bill, “...basically look, think, and you’re turning, but I expect if you were really to yank the control stick you could roll this thing on a fairly tight axis. I have not tried any aerobatics and I don’t plan to.”

The dihedral in the tips isn’t as great as it appears. There’s an illusion that the wingtips are upswept even though the top of the wing is a straight line from root to tip. Sighting down the wing, it becomes evident that the wingtip dihedral is all in the bottom surface.

Thatcher
Click for larger image

This photo of David Thatcher’s prototype parked during Sun ’n Fun 2009 clearly shows the straight line of the wing.

David says that the only reason he designed the wingtips as he did was for aesthetic purposes – no particular aerodynamic reason. But I can’t help but think that there’s some benefit gained over the traditional alternative of bluntly terminating the wing at the tip.

Thatcher

During takeoff, the tail lifts almost immediately. Once ready for departure, Bill applies full power and immediately puts the control stick in a position slightly forward of neutral, the tail lifting within a few seconds. By simply holding it in that position until the airspeed indicator reads 60 mph, a slight release of pressure gets him airborne within about a 500-foot roll.

Thatcher
This screen shot of a video of Bill’s first flight shows exactly what he stated; the tail came off the ground in under six seconds with gentle acceleration.

For slow flight, with stall at 40 mph, Bill is very comfortable flying 60 all day long. “I’ve had no sluggishness in the controls in slow flight,” he said. “Out over the Gulf of Mexico is where I did most of my 40 hours of Phase 1 testing, taking advantage of the nice smooth air over the water. It was very easy to hold altitude during slow flight, and the controls continued to have complete authority. In the stalls, it’s very similar to how a Cessna product stalls in the sense that it’s more of a mush and a settle; you don’t have a surprise break, and it doesn’t tend to drop a wing one way or the other.”

In rough air, it’s comforting to know that the elevators and the rudder are mass-balanced.

Night Flight?
Bill set up his CX4 for night flying and has it certificated that way as well. Although he’s not anticipating flying at night, he does fly around an area in Pensacola where there’s a high volume of military training aircraft, so his desire was visibility. Selecting a combination navigation, position, and strobe light system, Bill opted to use light-emitting diode (LED) technology over incandescent. As an emerging technology it seems clear that LED for aircraft lighting is making incandescent a thing of the past.

The Paperwork
Certificating it with the FAA as the first customer-built version was actually straightforward. As soon as Bill received his plans, serial number 74, he reserved his N-number, , hence N74CX. Admittedly “kind of cheesy,” Bill said, but he also liked saying the abbreviated call sign "Four Charley X-ray." With completed paperwork and a deal struck with a designated airworthiness representative (DAR), the inspection took place once the payment was made. The airworthiness certificate was issued that same day with no snag in the registration process at all.

An exhaustive builders log with lots of detailed photos is credited for the ease with which the inspection was completed. If there was anything that the DAR wanted to see but couldn’t because it was covered or closed, Bill provided an acceptable photo.

Plans
$360 includes shipping for a comprehensive set of hand-drawn plans complete with a color builders manual. David was kind enough to provide a complimentary set of plans to us for evaluation. I was a little surprised to see so many pages dedicated to such a small and simple plane, but since I’m a big fan of detail, they sure work for me. Each of the 15 photocopied 36-inch wide by 18-inch tall pages is packed with information, painstakingly hand-drawn by David but noted mechanically. In almost every case where text (notes and dimensions, etc.) is required, it appears that David pasted computer-generated strips of paper to the original drawings. This makes for some very comprehensive reading.

Thatcher

In addition to the well-written color building manual, our set included two amended pages and one sheet of full-scale drawings of the turtle deck, windscreen, and windscreen skirt.

Support is best handled by way of the Internet-based Yahoo group. Although I’m sure that David will answer any and all questions by phone, posting your issue or concern on the Yahoo group will usually net an almost real-time answer from someone who has already been there. Additionally, other members have posted photos and links to assist other builders.


Length

18 feet, 3 inches

Wingspan

24 feet

Width

1 foot, 11 inches

Height

4 feet, 8 inches

Wing area

84.4 square feet

Empty weight

520 pounds

Gross weight

850 pounds

Useful load

330 pounds

Wing loading

10.7 pounds / square foot

Fuel

9 gallons

Fuel type

92 octane auto

Engine

1700 cc to 2180 cc VW

Electrical system

22 amp 12 volt – alternator

Brakes

hydraulic disc / toe brakes

Construction

All-metal 6061-T6***

Design load

4.5g

Miscellaneous

heater and vent

Performance**

Cruise

125 mph @ 3,000 rpm

Stall (Vso)

40 mph

Rate of Climb (Vy)

825 feet/minute @ 75 mph

Best angle of climb speed (Vx)

63 mph

Never exceed speed (Vne)

165 mph

Takeoff roll

700 feet

Best descent speed

65 mph

Plans and manual printed in color

$360 includes shipping

Construction time

Estimated 850 hours

Cost of materials

$9,200

Miscellaneous:
• aircraft may be flown with canopy off
• wings removable for transport in 20 minutes.

Specifications are based on David Thatcher’s standard prototype.
**Performance is based on 750 pounds with a 1700-cc VW engine.
***Cowling and wingtips are made from fiberglass.


Products available directly from Thatcher

Canopy

$175

Windscreen

$80

Windscreen retainer

$80

Canopy and windscreen bows with skirts

$75

Canopy latch

$45

Control stick with boot

$125

Rudder pedals and assembly

$200

Elevator horn

$85

Exhaust system

$200

Nylon bearing set

$21

Thatcher
David Thatcher, designer of the Thatcher CX4, pilots serial number one over Lake Hancock, FL during Sun 'n Fun 2008 while EAA Chief Photographer Jim Koepnick squeezes the shutter. Photo plane pilot: Bruce Moore. Thanks go to the EAA for allowing us to use this photo.

A Little About David Thatcher Sr.
The thing that has always attracted David Thatcher to airplanes is their beauty and graceful lines. From the time he was a boy, he wanted to design and build an airplane incorporating all the elements he liked best. It also needed to be simple and economical to build and operate. A closed cockpit was important for year-round flying enjoyment but needed to be removable for open-cockpit flight. And the cockpit needed to be big enough to handle a 6-foot-plus person comfortably, so a wooden mockup of the cockpit was constructed first.

David’s credentials include being a graduate of the Embry-Riddle School of Aeronautics for Aircraft Mechanics and continually participating in general aviation for over 50 years, with countless hours of hands-on experience in sheet metal, welding, engine build-up, and all the other skills necessary to be an aircraft mechanic. Since he retired, he found he had the time, space, and most importantly, permission from his wife to build an airplane, one which might be well received by other homebuilders that don’t have unlimited funding but share David’s dream.

A special thanks to David Thatcher, Bill Stinson, and Wilson (Will) Leonard for their help with this article. Will and his brother Phil built twin CX4s which will be featured in an upcoming issue.

Thatcher
Twin Wilson brothers Thatchers.

For More Info
Thatcher CX4
1020 E. Jordan St., Unit A
Pensacola, FL 32503

Phone: 850-712-4539
Email: ThatcherCX4@cox.net
Website: www.ThatcherCX4.com/index.html

 

CX4 Evolution and Growth
Bill Stinson’s CX4 was the first customer-built CX4 to fly in 2007. Since that time, the worldwide body of CX4 builders has grown dramatically; kits and major assemblies are now available, and the visibility of the airplane around the country is taking off. We now take a deeper look at some of the milestones in bringing this design to the homebuilding community. Read more

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