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Gear Retraction Details
By Tony Bingelis (originally published in EAA Sport Aviation, March 1980)
ONCE YOU DECIDE to equip your airplane with a retractable gear, no blank piece of paper, no tablecloth, or napkin will ever again be safe from your sketches and doodles of retractable gear details. I dare say that by the time the gear construction is underway you will have become thoroughly familiar with the theory of folding landing gear mechanisms. After that, all that remains is the job of changing theory to substance by constructing it and making it work smoothly and consistently (ha!). Simple paper sketches often fail to convey the complexity of construction, alignment and adjustment. This is uniquely true when you undertake to develop a retractable gear installation in an airplane originally designed for a fixed gear.
Modifying an airplane that you had no part in designing must never be taken lightly. Sure, a few builders have successfully done so, but even most of them express doubts that they would ever again attempt such a conversion. They say it is not worth the effort; that you would often find yourself rebuilding as much as 50 percent of the structure, and with each part made and installed, some other part would have to be moved, removed or redesigned . . . all of which in turn forced still other changes. Weeks would often stretch into months and what had originally seemed to be a simple undertaking drags out for a year or more . . . sometimes with disappointing results. Sounds discouraging, doesn't it? But, let's take an objective look and see what is really involved in building a retractable gear installation; and then, you decide for yourself.
The Gear Legs
Retractable gear legs (struts) are no different than those used in fixed gear installations. I've seen oleo struts, slab spring gear legs, Wittman whip-rod legs, and welded steel tube truss gear legs all successfully modified for retraction. So, the type of gear leg used should not, of itself, present an insurmountable conversion problem to a determined builder.
Far better than building the entire gear would be the acquisition and adaptation of stock gear legs (complete with the retraction mechanism) removed from some demised type-certificated aircraft. Since a lot of machine shop work would ordinarily go into a retractable gear, this acquisition would eliminate most of that expensive work.
Builders, being an innovative lot, have cannibalized and used assemblies from aircraft like the Culver, Navion, the Piper Arrow and others. This approach, however, is frought with difficulties and can sometimes result in a botched-up structure much less a botched-up landing gear weighing more, and costing much more, than if the builder had stayed with the designer's original plans.
The Jack Points
Sometime during construction you have to decide where to install three external jack points. Yes sir! Two jack points are not enough, not for a retractable gear airplane. The three jack points are as necessary as are three special (tall) jacks which will be used henceforth to provide the means for raising the aircraft high enough to clear the landing gear for retraction testing, not only during construction but later for the annual inspections.
You may even have to modify three regular jacks so that the aircraft can be raised high enough.
If the jack points are not properly located on the aircraft, you might have to attach weights to the tail-end or tie it down to keep the airplane from tilting . . . it all depends on the location of the main jack points. Remember too, retraction tests will have to be performed at least annually.
In order to make a gear leg retractable, its upper end must be attached to some sort of a trunnion. Unlike the attachment bracket which immobilizes the fixed gear leg to the structure, the trunnion provides the support and pivot axis necessary in a retractable installation. A trunnion suspends the gear, usually between the main spar and an auxiliary spar.
You might get lucky and locate a couple of stock trunnion castings which could be adapted to the gear legs you will be using, otherwise you will have to fabricate your own. Anyway, not all aircraft utilize cast-aluminum trunnions . . . some, like the Beechcraft Bonanza, stay with the sturdy built-up welded steel tube trusses. Constructing this type structure is well within the capabilities of the average builder.
Some designs incorporate a large.tube welded to the upper gear leg. It serves as a trunnion and pivot axis for the gear leg. This suspension method is often used in lightweight retractables, but because only one end of the installation is supported, it can result in unusual loads being imposed on the spar. Not only is a large hole through the spar necessary to accommodate it . . . the spar and the adjoining structure must be beefy enough to take the landing and taxiing stresses. If your spar design cannot accommodate such a large hole, or if the hole must be drilled too close to the top or bottom edges of the spar, a failed spar could be the future consequence . . . and what if that should happen in flight? Certainly, a careful calculation should first be made to determine if the spar can handle this type of installation.
Let me interject a note of caution by saying, no builder should attempt to install a retractable gear that requires modification of essential structure without first working a stress analysis for the change. Of course, each builder must evaluate his own qualifications in this regard if he intends to decide on his own whether or not his changes will weaken the structure.
If welding is necessary to modify the attachment area of the gear legs, there will always be the risk that the oleo mechanism could become damaged or distorted. On the other hand, shock struts utilizing coil springs instead of oleos are less sensitive to the effects of distortion from welding. Nevertheless, the welds must be good and the distortion must be controlled within acceptable limits. The alignment of each attachment point is always critical because some gear installations cannot be adjusted for toe-in or toe-out once completed and installed.
Shock Struts and Scissors
Since any good retractable gear will have some shock absorbing capability, this provision can sometimes result in conflicting measurements. For example, the gear legs will ordinarily measure longer when retracted than when under load on the ground. The wheel-well dimensions and even their shape and location should take this into account. Don't let your first retraction test surprise you with a gear that won't fit into the wheel wells.
The primary function of a scissors assembly is to hold the wheel in correct alignment and to keep it from swiveling like a bar stool.
The scissors may be positioned as necessary during construction to keep them from protruding when retracted, or from interfering with any of the aircraft structure. Scissors do not have to be located in front or back of the strut. A number of aircraft have them situated on one side or other. This determination, however, will have to be made prior to the jigging and welding.
Before construction proceeds too far, a method of retraction must be selected. It will undoubtedly be a manual, electric, or a hydraulic one . . . and probably in that order of preference initially.
If you elect to build an electric or hydraulic system, you will have to provide an alternate or standby emergency extension system. This is usually a manual system.
Manual Retraction is the most reliable and the least expensive of the three to install. It entails only a minimum of maintenance and best of all, an alternate or back-up system is not needed. Manual systems work through direct linkage from the cockpit control handle, or lever to the individual gear legs using gears, universal joints and torque tubes. To aid in retracting the main landing gear, large springs (usually garage door springs) are hooked into the linkage.
Manual systems require either a cockpit located control lever (as in the Mooney), a conveniently located hand crank or a ratchet. A manual lever-operated system permits you to raise the gear quickly with one continuous sweep of the lever, while the hand crank arrangement is much slower and requires considerable cockpit activity on your part during its operation.
Electrical Retraction is more sophisticated and seems to appeal to more builders. The heart of this system is the electric motor which is geared and slowed down to provide the brute force necessary to do the job. In addition, the installation requires additional gear comprising a jack shaft arrangement to convert the motor's rotary motion into the push-pull action require to retract the gear.
The gear retraction switch is installed so that when it is in the UP position, the geared motor hums into action, causing the gear to rise to its fully retracted an UP-locked position. When the gear switch is flipped DOWN, the UP-lock is released, the motor reverses an the gear is extended to a fully down and locked position.
However, this is too simple an arrangement and may cause you unbelievable embarrassment should "anyone" accidentally flip the switch to the UP position while the aircraft is still on the ground. To save yourself from future embarrassment, you can install an electrically operated safety switch which will not allow the retraction system to work when the gear switch is flipped to the UP position until the weight of the aircraft is off the gear.
Hydraulic Retraction systems in homebuilts are comparatively rare. They are often more complex systems made up of surplus actuating cylinders, selector and sequence valves and other hydraulic components all interconnected with tubing. In hydraulic retraction gear systems, the downlocks and uplocks are ordinarily operated hydraulically.
After you consider the advantages and limitations of each of the three systems, it is difficult not to conclude that the manual system has to be the most practical of the retraction systems for light homebuilts. The electric retract system would, of course, run a close second choice. I suppose the deciding factor would be the availability or non-availability of the major components.
. . . And What About The Retractable Nose Gear?
A retractable nose wheel introduces more complexity because some means must be built in which will permit it to be steered while on the ground and not while airborne. Some way must also be worked out to guarantee that the wheel does not get cocked crosswise during retraction, and finally you may have to add a shimmy damper to make the wheel behave during take-off.
An airplane with a non-retractable, but steerable nose wheel provides pleasurable ground handling as it responds immediately and in direct proportion to rudder pedal input. But providing the same capability in a retractable nose wheel takes a bit of mechanical ingenuity because it must be built to automatically disengage the rudder pedal steering when the nose gear is retracted. One way you can achieve this capability is to build in an automatic disconnect feature similar to that used for the ailerons in folding wing installations.
A bell crank operated directly by the rudder pedals is positioned to make positive contact against a couple of rollers fitted to the ends of a horn (bell crank) mounted on top of the nose gear strut. As long as the nose gear is extended, the bell crank (connected to the rudder pedals with push-pull tubes) works against the nose gear rollers transmitting the rudder pedal action directly to the nose gear causing it to respond. Then, anytime the gear is retracted, the rollers move away from the rudder pedal operated bell crank (horn), effectively moving it away from all rudder pedal input. The rudder pedals are now free to operate the rudder only and it is not until the nose gear is once again extended that the pedals take on the additional duty of providing steering input to the nose wheel. On the ground the steerable nose wheel imposes a noticeable increase in the rudder pedal load.
And finally, a nose wheel centering device must be devised to insure that the nose wheel is not cocked during retraction. If the proper geometry can be built into the retracting mechanism, the nose wheel will automatically center itself in the neutral position as the gear retracts.
Landing Gear Warning Systems
Faulty gear? That raises the question . . . How do you know if the gear is faulty? How do you know it is down and locked . . . or retracted?
To guard against an inadvertent gear-up landing, all retractable gear aircraft must have warning systems which are automatically activated when the throttle is pulled back below normal approach power settings and the gear is not down and locked. Most factory-built retractables have a manual cut-out feature to inactivate the infernal racket of the gear warning buzzer (horn). After doing so, should you fail to reset the system you will have eliminated your safety warning feature.
Your retractable must, therefore, be equipped with the traditional micro and limit switches which cause lights, buzzers and even mechanical indicators to activate, showing you . . . nay, proving that the landing gear is down and locked, or that it is doing something else you may not want it to do.
What Others Have Done . . .
Among the retractable homebuilts now flying it is difficult to single out any particular retractable landing gear design as being more popular, more successful or more prevalent than any other.
One frequently used retractable gear design is the one engineered especially for the CA-65 by Anton Cvjetkovic. It made the homebuilt scene sometime around 1966. This gear was among the first designs to receive the attention of homebuilders who wanted to convert their fixed gear installation to a more sophisticated retractable mode. Undoubtedly, one good reason for its popularity may be attributed to the fact that plans were immediately available. And another because of its simple and very light manual gear retraction system . . . one that requires very little machine shop work. Because of these attributes, variations of this gear appear in more homebuilt retractables than any other design.
The Cvjetkovic Retractable Gear
The Cvjetkovic gear was originally intended for use in low-wing aircraft grossing up to 1500 pounds. It is a surprisingly light landing gear for a retractable and when equipped with 500 x 5 wheels, weighs a mere 73 pounds. In comparison, the Emeraude's fixed cantilever strut gear weighs a hefty 75 to 78 pounds. However, in all fairness, I should point out that the Emeraude gear utilizes the larger, heavier 600 x 6 wheels and is better suited to rough field operations.
Other interesting features of the Cvjetkovic design are also attractive to the homebuilder. Besides the minimum machine work required, the gears, universals and bronze bushings are standard commercial parts. The Cvjetkovic gear, too, employs the ordinary easy-to-obtain coiled springs used in garage door mechanisms. These springs are pre-stretched inside the wing to help ease the manual cranking load during the retraction effort. In its original configuration, the gear was cranked up with a built-in standard 3/8 inch drive ratchet positioned between the seats. To operate the system, all you had to do was to pull the manual down lock release and ratchet away. The gear retracts slowly because it takes about 14 flips of the ratchet handle to get it up all the way. But for simplicity, it is hard to match.
Although there are numerous adaptations of the Cvjetkovic among the homebuilt retractables, there are perhaps as many flying around with retractable gear systems salvaged from certificated aircraft.
In addition to the retractable plans available from Cvjetkovic, I understand that the Sidewinder and the Hawker Hurricane designs' retractable gear plans may likewise be purchased separately. Look through the classified advertisements in the last few issues of SPORT AVIATION for the addresses and costs. You may even find others listed by the time you read this.
No matter how you look at it, constructing and installing a retractable landing gear is a long, expensive ego trip and one should ponder it a lifetime before embarking on it. For my part, I would never build a retractable tricycle gear job like the Falco if it didn't look so good with the wheels tucked up.