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Tri-Gear Thatcher CX4 Flies

Another landing gear option

Story and photos by Peter Beck, EAA 19566, for Experimenter

New gear gives the CX4 an aggressive yet artistic stance.

As if divinely preserved to lead a new life, Dave Thatcher’s CX4 escaped major storm damage at the 2011 Sun ’n Fun International Fly-In & Expo. Back home in Pensacola, Florida, Dave now has fitted it with a tricycle gear, making good on promises to CX4 builders and fans. He has test-flown and taxied the new configuration himself and finds it to have the same responsive but stable handling characteristics as the original conventionally geared CX4. Plans and a kit of tri-gear components for new aircraft construction, as well as a retrofit to completed CX4s and projects under construction, are now available. An appearance at EAA AirVenture Oshkosh next month is planned.

The original CX4 with its conventional landing gear is highly controllable and poses no serious ground-handling challenges, even for pilots with limited tailwheel training. Indeed, it has been flown by a freshly minted 17-year-old pilot with only 4 hours of tailwheel experience. Since the CX4 is a single-seat aircraft, an expensive tailwheel endorsement isn’t required.

Nevertheless, according to CX4 designer and builder Dave Thatcher, many modern pilots and prospective builders have requested a tri-gear version. Handling of a conventionally geared plane is different from that of a tricycle-geared plane, and access to post WWII-era planes like Cubs and Champs, to get any level of experience, is simply becoming more and more difficult to come by. Not to mention qualified instructors and often substantial costs. Dave’s hope is that the tri-geared version of the CX4 will give a higher level of confidence and assurance to modern pilots.

Flying and Handling
The new gear configuration was first taxied and flown by Dave during the month of May 2011. Unlike the traditional gear, the castering nose wheel requires the use of differential braking at low ground speeds. Early taxi maneuvering and flight testing indicate ample ground control and minimal reductions in in-flight performance, due to increased drag that comes with most tri-gear configurations.

A critical moment in handling any airplane comes as it transitions from a foot-controlled go-kart to a full flying machine. According to Dave, the CX4’s generous rudder area makes this transition smooth, both from taxi to flight and from flight back to taxi. At relatively low taxi speeds, the rudder provides directional control, while tight turns can be accomplished by full rudder and an engine burst without use of brakes at all.

Installation of the new gear in the prototype that was already fitted with conventional gear required less than a week to complete once all the parts were fabricated. And in addition to the gear being easily retrofitted to completed aircraft as well as projects still under construction, the new nose gear strut mounts on the cockpit side of the firewall and bolts to the main engine support structure. Because the nose wheel casters, no steering control linkages are needed. Nothing in the engine compartment needs to be disconnected. The fuel tank and its cover must be removed, however, to gain access to the rear of the firewall, but they don’t need to be modified to accommodate the new gear strut.

The main gear uses the same aluminum leaf called out on the conventional gear plane. The leaf strut is simply reinstalled just aft of the main spar, and the brake lines are rerouted. Easily installed reinforcement of the aft fuselage floor is required and uses off-the-shelf hardware and aluminum angle stock.

The yoke, tire, and wheel, as well as the S-strut, are very reminiscent of those found on many other successful experimental aircraft.

New Major Components
The new nose gear uses an S-strut bent from 4130 steel tube, which is specific to the CX4. Ground steering is achieved by differential braking – the nose wheel casters on a wheel yoke that is cut and milled from 5 x 5 x 1/4 inch 6061-T6 square tube. A standard aircraft quality 400 x 4 inch, 4-ply tire (and tube) with an aluminum rim makes up the front wheel. It carries about 120 pounds of weight at takeoff and landing speeds, so no grocery cart wheels on this one. All other pieces are cut or milled from the standard 6061-T6 angle that is used elsewhere on the airplane. The wheelpants are a narrowed-down version of the 500 x 5 inch main gear pants.

The uniqueness of this retrofit is that it doesn’t interfere with anything under the cowl. All attachments are from the passenger compartment of the firewall.

Naturally, a CX4 built with the nose gear from the outset requires no special recertification. The same would apply to a reconfiguration midway through the project that hasn’t been through the certification process. A CX4 that is built with the nose wheel from the start, or reconfigured to tri-gear during construction, requires nothing more than its airworthiness certificate when issued.

Retrofitting once the plane has flown is considered a “minor change” to the type design, and the paperwork involved for the legal aspect of the retrofit requires only an entry in the aircraft log, as long as the airworthiness certificate allows this. A minor change to the type design is a change that “…has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are “major changes.” For more info on the rule, read more.

Plans, Kits, and Availability
Plans for the new gear and an installation manual are now available from Dave Thatcher, as are parts that are unique or CX4-specific. These include the S-shaped steel tube gear strut and the castering nose wheel yoke especially.

In addition, the same CX4-specific components will be available as no-additional-cost options on the CX4 fuselage kit from Thatcher CX4 Works (www.CX4works.com).

Look for the CX4 prototype at AirVenture on the grass in front of the Homebuilders Headquarters building.

AirVenture Appearance
The tri-geared CX4 is scheduled to be flying in review at AirVenture Oshkosh on two separate days during EAA’s Homebuilts in Review.
For more information, contact Dave Thatcher of Thatcher Aircraft via e-mail ThatcherCX4@cox.net. You can also contact Peter Beck of Thatcher CX4 Works at www.CX4works.com.


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