Retractable undercarriage?

Retractable undercarriage?

A good reliable retract system will transform a model like this

Having wrung out a sport aircraft or two, many modellers decide that they want to build and fly something more challenging. This often leads to fantasies of flying a sleek scale job, a design that will more than likely date from W.W.II or later, and whose full-size counterpart will have been fitted with a retractable undercarriage. Nothing looks worse than a Spitfire flying around with a fixed undercarriage, so to do such a model justice we must fit retracts. But which type to install, and how do they operate? Hopefully this article will help clarify some of the misconceptions about retractable u/c systems and encourage those who, as a result, have steered clear of building a favourite aircraft.

Although electrically operated retracts are occasionally seen, the choice is really between three basic types: mechanical, air and hydraulic, each having 'pros and cons' when being considered for a particular application.

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MECHANICAL

Mechanical retracts are normally the cheapest solution and are now being included in the kits of many higher quality ARTF models (particularly warbirds). The vast majority of mechanical retracts are operated via servos and pushrods, which limits the size and weight of the undercarriage legs and wheels that can be lifted by the available servo power. As a result the mechanical method is only suitable for smaller models up to a maximum weight of around 5kg (11 lb).

In general terms mechanical retracts are fairly simple units, comprising of a main body with a rotating pivot block thats actuated by a pushrod from a built-for-purpose retract servo. This servo differs from a standard unit in that it provides 180° rotational angle and operates at a slower speed (to give a more realistic retraction / extension speed), whilst offering greater torque. The u/c leg / oleo is firmly clamped into the pivot block and as this block rotates the leg will retract / extend. Whilst being low cost and fairly easy to install, there are some significant disadvantages to mechanical systems. Firstly they can be quite difficult to set up for reliable operation, requiring a great deal of time to be spent accurately adjusting the overall movement of the pushrods to ensure full extension / retraction of the wheels without stalling the servo. This is more difficult than it sounds as retract servos are non-proportional, so the 180° of non-adjustable travel often requires the use of a round output disc with very carefully positioned holes for ball joints / clevises, to get the exact pushrod movement required.

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A retract servo in the wing of a Spitfire

Secondly, if either the nose leg or tail wheel has to be retracted as well as the mains then a second retract servo will be required, as its almost impossible to install a reliable system with both wing- and fuselage-mounted retracts being actuated by one servo. Finally, in the event that a rough landing bends one of the u/c legs such that it wont fully retract, the associated servo could well become stalled. This will cause an excessively high current drain and possible damage to the servo. Even if the servo doesnt suffer any damage, the greatly increased current consumption will flatten a battery very quickly, as a stalled servo can draw several amps. If the servo is being powered by the Rx pack then this could quickly become depleted, with the potential to lose control of the model completely due to the flat battery.

To eliminate this problem, many models utilising mechanical retracts employ a secondary, lower capacity battery pack that powers only the retract servo. Fitting a second pack in this way does require an additional switch and a 'Y' lead, the latter modified to allow the second battery to provide power exclusively to the retract servo. To this end the red (positive) lead is either cut through or removed from the receiver end of the plug, thus ensuring that only the secondary battery pack provides power to the servo, eliminating any possibility of a stalled retract servo flattening the Rx pack. The 'Y' lead is then connected to the retract channel of the Rx, with the second battery being plugged into one of the sockets of the 'Y' lead via a switch. The retract servo is plugged into the leads remaining socket.

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A simple mechanical retract unit here in a Hangar 9 T-34 Mentor, you can see the pushrod that activates the unit

AIR

There are two basic air systems in popular use: 'air up / spring down' and 'air up / air down'. Both types utilise an on-board air tank filled with compressed air via a one-way fill valve, using either a hand or electric pump. The air stored in this tank is distributed to the retract units either by a servo operated valve or an electronic valve actuated by a spare channel of the radio system.

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Air retract units are normally comprise main side frames or a moulding, within which a pivot block is moved by the pushrod from an air ram. The undercarriage legs / oleos are mounted in the pivot block, using either grub screws or a split block with clamping bolt for security and to ensure the leg cannot rotate out of position. The air up / spring down system (probably the more popular of the two types) has a spring installed inside the air ram to extend the undercarriage, the air being used purely on the reverse side of the piston to overcome the spring resistance and retract the legs. Simplicity and safety are key features here; there's only a single air line to each retract, and in the event of an air leak the spring pressure on the piston will cause the u/c to extend automatically as the air pressure reduces, reducing the likelihood of a 'wheels up' landing and the possible damage this can cause.

A complete air up/air down set including the air cylinder, switch unit, t-junction, joiners, air tubes and input valve.

The downside is that it can be difficult to regulate retraction /extension speed, as the spring works against the air pressure during retraction. This, together with the air pressure under extension, can lead to wildly different retraction / extension speeds. On a practical level, as the undercarriage extends upon loss of air pressure, it's often necessary to be inventive to keep the undercarriage tucked away into wings and fuselages during transport. In this respect, then, tape or elastic bands are popularly used as temporary straps.

Air up / air down systems operate in a similar manner, but each air ram has two air connections, one on each side of the piston. When the system is pressurised and the control valve operated the compressed air on one side of the piston is vented to atmosphere whilst the on-board air tank supplies compressed air to the other side of the piston, thus moving the ram and, in turn, retracting / extending the legs. This is a more complicated system as there are two air lines to each retract unit and the operating valve has more air connections, together leading to an increased likelihood of air leaks. In the event of a serious leak the u/c will not automatically extend as there's no internal spring to force the legs down, and the first the pilot may know of an problem is when the u/c refuses to extend, or even worse, extends partially but without locking down. Mind you, there is one rather neat way of overcoming this potentially damaging situation, and this requires the use of a Tamjets Gear Fail-safe unit. Developed and marketed very recently, this is an extremely clever little gizmo that I'm sure will eventually become a must have for all models using air up / air down retract systems. The electronic unit plugs between the retract servo / valve and the Rx, and has an airline connection to the main retract air system. The function of the unit is to command the retract servo / valve to extend the u/c should air pressure in the system fall below a preset value (the recommended pressure for most systems is 50psi). Once connected and programmed, which is a very simple procedure, the unit is tested by retracting the undercarriage and then reducing the air pressure by allowing a small air leak to occur. As soon as the pressure falls to the 50psi level, the retract servo should be operated by the gear fail-safe unit and the u/c extended normally.

The Tamjets unit is a major safety benefit to all models with air up / air down retracts and has particular relevance to jets, as a pressure loss leading to either a belly landing or a landing on less than three legs will at best lead to a damaged model and at worst a write-off. Had this unit been available when I test flew my Mick Reeves Hunter earlier this year, I wouldnt have had the embarrassment of making a two-wheeled landing on the first flight and the consequent repair to the dragging wing tip!

One further benefit of using air systems is the relative ease with which operating u/c doors can be included, using miniature air rams and a sequenced secondary valve. Mechanical retract systems usually require the doors to be either servo operated or operated by the u/c legs / wheels themselves, using an over-centre wire pull / pushrod system. Fitting and operating u/c doors is a subject in itself and beyond the editorial space allowed here, so well ignore it and move swiftly on.

HYDRAULIC

Commercially available hydraulic retract systems have only been available for a fairly short period of time and are still a rarity at flying fields. However, they're likely to become increasingly popular due to the ever-growing number of larger models (particularly jets) with high wing loadings and consequent increased landing speeds / loads. Eurokit is currently the only manufacturer offering a wide range of hydraulic retracts to suit a variety of models, whilst some kit manufacturers offer tailor-made retracts to suit specific models.

A typical hydraulic system

In general, hydraulic retract units and operating valves appear very similar to those supplied with air up / air down units, the only real difference being the incorporation of seals that are compatible with the oil / fluid being used in the system. The air tank and fill valve are done away with, and in their place are an oil tank, electronic control unit (ECU), hydraulic pump and dedicated battery pack. Linking of the various items is carried out much like an air system, with two hydraulic lines running to each retract unit. Oil is introduced to the system by filling the tank, then running the pump to distribute it, bleeding any air from the lines. Once most of the air has been removed, the bit remaining tends to be forced back into the oil tank as the system is cycled.

The operating method is fairly simple in that the control valve and ECU are simultaneously activated via the retract channel, at which the pump forces fluid down the lines into the retract unit rams, operating the gear. Models with wing-mounted main undercarriage units are accommodated by the use of self-sealing, two-piece valves in the lines where the wing panels separate; when these valves are parted theres no oil loss, and air is not introduced into the system.

There are a number of advantages with hydraulic systems, including the ability to control very accurately the speed by either changing the input voltage to the pump or by adjustment of the electronic control unit. Another positive is the extremely secure up /down lock (due to the incompressible nature of oil), and reduced leakage, given the much greater viscosity of oil over air. My relatively limited experience of hydraulic retracts suggests that they're generally more reliable than air systems, as long as they're correctly installed. Of course, you don't get all this accuracy and reliability for nothing, and hydraulic retracts tend to be more expensive, given the requirement for a high precision pump, ECU etc. They're also heavier due to the weight of oil carried and additional components.

It's a fact of life – big jets need good retracts but don't forget to set aside a budget for the system too!

HYBRID

There have been a number of very large models built where its been necessary to adopt a mix of technologies in order to simulate the retract operation of the full-size aircraft. In the case of my large C-17 Globemaster, the three-wheeled main u/c bogies had to rotate through 90° before the u/c could retract and then, once extended, they had to rotate back into position. This required the use of air rams to rotate the bogies, and electric screwjacks to retract /extend the u/c. The retracts on this model were amongst the most complicated I know, with a total of 17 air rams (including some for the doors), four geared electric motors, three servos, three air control valves, 16 microswitches, a fuse box, a 10-cell battery pack and great lengths of wiring and air tubing. The model even has an on-board air compressor to maintain a constant 90psi air pressure within the system!

CHOICE

As you can see, there's a wide range of retract possibilities available, ranging from the very simple through to the most complicated. Within this vast array there's a system to suit every model and every pocket. The choice is yours!

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