Design your own Class 1 Carrier model
A scratch built Class 1 Carrier model is undoubtedly one of the most complex types of C/L models possible to make, particularly for those designers who want to incorporate a 'full house' of low speed/ high lift devices as well as a powerful engine.
Choice of engine
It bears repetition that Class 1 is primarily a horsepower contest. You should therefore always use the largest engine size permitted by Rule 3.1., ie: .40 cu.in. Ideally, the most powerful .40 you have, or are prepared to buy, should be used to give your model the maximum speed possible. If this engine happens to be fitted with a conventional R/C carburettor and throttles well, then this will also address the needs of the slow run and landing.
Unfortunately, many of the racing/speed .40s, whilst undoubtedly the most powerful .40s of all, neither throttle well (despite what the manufacturers say) nor can be fitted with adequate conventional silencers (Rule 3.4) since they have usually been designed to produce their high power output in conjunction with a tuned exhaust system.It is therefore most sensible, at least for your first model, to equip it with your most powerful R/C .40 that you know will throttle properly and will run happily on a normal silencer. This will serve as a (relatively) gentle introduction to Class 1 and will enable you to learn how to operate the model without the additional worry of coping with 25,000rpm's worth of extra adrenalin! Subsequent engines can then be selected on the basis that you will thereafter have more attention to spare for the effects of raw horsepower and the greater fussiness such engines often exhibit.
The most powerful US engines used for Class 1 do not usually have conventional carburettors, since the R/C venturi sizes available are far too small to realise the power levels possible . Instead, a plain venturi is fitted, but this is of the largest diameter that the engine will run on using a pressurised fuel system. Throttling is achieved by the combination of a special fuel metering valve and blanking flaps (or their equivalent) over the venturi intake and/or exhaust stack. Most of this mechanical kit is very specialised and not available except by special order.
Although tuned exhaust systems are specifically banned and therefore engines that are designed for such operation cannot be used, the large venturi/fuel metering valve/exhaust flap set up still enables some really hot (but non-pipe-timed) racing engines to be seen in Carrier. In conjunction with high nitro fuels and the appropriate props and plugs, these engines really are fire breathing monsters!Choice of prototype
The most important stage in Class 1 model design is selection of the actual full size aircraft to be modelled. Care in prototype choice will usually go a long way to ensuring good flight behaviour, potential low speed performance and ease of construction.
If you have already chosen your motor, this may influence your selection - a heavy motor will require a prototype with a short nose, whereas a light motor needs a prototype with a long nose. Prototypes with long tail moment arms (the horizontal distance between the wing and tailplane) are less likely to be twitchy during the fast run than those with short moment arms and will also provide the elevators with greater leverage to enable the model to 'sit up' more easily whilst it mushes along during the slow run.
Prototype elevator size may be a further consideration. Ultimate slow flight performance often depends on the model being capable of flying at up to 600 angle of attack! No matter how non-scale you may consider this to be (...and it is!), it is permitted by Class 1's rules (Rule 11.2.2.2). Top competitors will always take full advantage of this beyond-the-stall prop-hanging mushing-along style simply because there is no slower type of flight, even though it requires considerable practice. The model's ability to 'fly' at this angle not only depends on its longitudinal balance, but also on the power of the elevators. Small elevators will clearly make this attitude more difficult to attain and maintain.
The use of various methods to assist slow flight should be seriously considered at this early stage. Whilst usually fascinating in their own right, they should really only be used if they have a good chance of increasing slow run performance. Use for their own sake produces unnecessary complexity, unreliability and unwanted extra weight. Whatever systems you elect to use - if, indeed, you use any at all - you are not allowed to model anything that was not used on the full size prototype, nor are you permitted to change the size of any control surface, except by the +5% linear variation allowance.
For slow speeds under 'aerodynamic' flight conditions (see the slow run in the Set up and fly section for an explanation of the two different methods of achieving slow flight), flaps are usually the best way to enhance wing lift. If you are intending to use flaps during the slow run, you should therefore choose a prototype with large flap surfaces for maximum extra lift, and flap design of the most effective type. The tables attached show a comparison between the lift effects of the most common full size flap systems and the spectacular variation between the simple plain flap and the treble-slotted area-increasing Fowler flap is obvious. Table1 Table2 (78k each file.)
Nevertheless, be careful! These comparisons are technically valid only for full size aircraft at full size speeds. There exists little or no documentation for the effect of flaps working under model conditions, so it is not possible to say with certainty whether the comparisons will show the same variation for Class 1 purposes, or more, or less.It is also worth remembering that the more effective the flap type, the more complex the actuating mechanism required usually is. Just think what the slotted Fowler flap will need! Anyway, whatever you choose to incorporate in your model, the rules allow you to simplify complex flap systems if you don't want to replicate the full detail, provided you don't increase the effectiveness of the flaps.
By convention, plain flaps must always operate as plain flaps and split flaps always as split flaps. However, Zap flaps, slotted flaps and Fowler flaps can all operate according to their individual designs, or they can be made to work less effectively: Zap flaps become split flaps and slotted and Fowler flaps become plain flaps. Got that?The tables also show the effect of slats and slots. These are boundary layer control systems and work by delaying the onset of airflow breakaway, ie: the stall. The (fixed) slot is a permanent wing feature and simply directs air across the upper surface of the aerofoil and effectively 'sticks' down the main airflow boundary layer so that breakaway takes place at slower speeds and higher angles of attack. At high speeds, the fixed slot effect is negligable, although there is a small increase in drag due to its presence. The slat is merely a moveable aerofoil that creates a slot between itself and the main wing when the extra lift is needed. The slat can be shut at high speeds when not needed, but the price for this greater in-flight flexibility is the extra mechanism needed to operate it. Both of these devices are potentially quite involved to construct, and although they can sometimes make a noticeable difference to low speed wing performance, care should be taken with their design to ensure that their contribution to extra lift is more than the extra weight they will inevitably involve!
Ailerons may be used in a variety of ways. The Class 1 rules require that they move in opposite directions (Rule 8.1.2.1.), but there is some latitude permitted here by convention. It is acceptable to operate only one aileron; this is usually on the outboard wing where the up-deflected aileron not only rolls the model out of the flight circle but also acts as extra drag to also yaw the model towards the outside of the circle.
Some prototypes used drooping ailerons to provide extra low speed lift to supplement the effect of the flaps. Models of such prototypes can feature this effect provided that aileron droop does not match the extreme deflection angle typical of flaps.Note that flight in stalled attitudes, however, is obviously outside normal aerodynamic considerations and lift-increasing devices such as flaps, slats and slots, and roll-inducing surfaces such as ailerons, can be considered to be inoperative. Nevertheless, the drag increasing function of flaps and ailerons, and the wing-area-increasing function of flaps where relevant, can still contribute to slow flight performance.
An outward-deflecting rudder is often used, since the yaw effect continues to be present irrespective of flight attitude and for the practical reason that it is relatively easy to operate in comparison with some of the wing-mounted systems.
If the prototype used dive brakes, these may also be used on the model. Although such drag-inducing surfaces can significantly affect model speed, there needs to be a commensurate increase in lift from some other source, otherwise the model may simply fall out of the air. Care should also be taken that large fuselage-mounted dive brakes do not blank off the elevators, or cause sufficient turbulence to render the elevators useless. Some means of varying the deflection would be prudent in order to mitigate or avoid these situations.
Line sweep devices can be extremely effective indeed, so much so that some models rely on a line sweep as the only slow run device. Rule 4.3 effectively limits the full range of movement to fore and aft limits defined by the wing root chord. Use of a line sweep must be planned at an early stage. If the prototype has sharply tapering wings, the amount of sweep available at the tip will be minimal if the leadouts are planned to run inside the wing. In this situation, a choice has to be made between the drag reduction of enclosing the leadouts and the greater sweep movement available for external leadouts, since the sweep guide can extend back past the trailing edge.
Most prototypes have some wing dihedral (or anhedral) and this must be reproduced by the model (Rule 8.1.2.3.). However, complex dihedral (eg: polyhedral, cranked and gull wings) can not only be more time-consuming and difficult to construct, it can also make internal leadouts, control runs and control surfaces quite complicated. They tend not to be as strong as straight wings of equivalent weight.
It may be convenient to choose a prototype with a fairly slabsided fuselage so that model construction is straightforward. A round or oval section fuselage can be very attractive, but it can be much more complex and require more effort to build. Greatest use is probably made of the +5% linear scale tolerance here: fuselages with sectional curvature can be redrawn as slabsided in part or in whole, depending on the degree of curvature, to make them substantially easier and quicker to construct.
Rule 8.1.2.1. requires that the undercarriage exits the model in the scale position (though it does not have to be scale thereafter) and this may be another consideration in the choice of the prototype. Few prototypes possess fuselage-mounted undercarriage, though this is probably the most potentially robust arrangement for coping with arrested landings. The next best variation is narrow track wing mounted gear. The full size prototype might have had some stability problems with narrow track gear, but this instability will be largely absent in the model by the nature of C/L. Narrow track gear means that the wings are least stressed during heavy landings. Wide track gear may look impressive (as it undoubtedly did on the full size naval aircraft), but this imparts the maximum landing bending load into the wing structure.
The prototype's layout of fuselage, wing and tail can have a material influence on how easy it is to design the internal structure and to provide hatches for maintenance and repair. Before a prototype is ever chosen, the approximate arrangement of engine, fuel tank, speed control system, slow run systems, undercarriage, arrester hook, main structural components (engine bearers, wing spars, bellcrank mount, u/c mount, fuselage spine) and access areas must all be considered together so that, if necessary, the prototype can still be rejected without either excessive design time or perhaps even construction work being wasted.
Prototypes with large transparent 'greenhouse' cockpit canopies can be very impressive, but such shapes can sometimes be very difficult to mould and can also reduce fuselage strength. Since canopies do not have to be modelled in clear material (Rule 8.1.4.), the designer has the option to reproduce this part of the model in solid if desired. This section, whether solid or clear, is also often made removable to provide internal access.
Size of model
This aspect should really be considered at the same time as the choice of prototype, since the two are actually inter-related. A prototype may appear to have every feature desired, but once it is scaled down to the appropriate size, the actual model may simply not have enough internal space to accomodate all the systems you need. Since Class 1 means high speed, the main criterion that determines model size is simply drag. The smaller the model, the less the drag, of course. Unfortunately, if you make the model too small, not only may competitive slow speeds become difficult to achieve, but the handling at such speeds can be particularly poor. This will then have an adverse effect on your ability to make successful landings.
The parameter usually taken to determine model size is wing area. Although Class 1 rules encourage models to be built primarily for high speed, it has now been found in the UK that using the smallest possible airframe leads to exactly the problems outlined above. In fact, current US practice appears to favour models of very substantial wing area, and some are even built to the maximum span (Rule 3: 44ä). This means that wing areas are not dissimilar to those of BCD; Class 1 speeds are much higher simply due to the extra horsepower involved. The drag of a large wing may approach that of a small wing if constructed from thin sheet rather than being built up in a thicker section, with the slow speed lift qualities served by the choice of a prototype that used large flap areas. In any event, UK Class 1 experience is at an early stage and there are still many questions to be answered. Nevertheless, it is prudent to avoid wing areas much below 200 sq.in. as models smaller than this, almost without exception, have proved to be very difficult to handle at low speed A good starting point would probably be in the region of 250-300 sq.in., with the wing thickness reduced to the minimum commensurate with adequate strength.
Model structure
A Class 1 Carrier model structure is probably more of a compromise than most other C/L model types. Like all aircraft, large or small, the old adage 'simplify and add lightness' always applies. Nevertheless, Carrier models are usually constructed to withstand landing loads which are heavier than in any other aeromodelling activity, and this has an inevitable weight penalty. It is impossible to assess what a reasonable 'design' limit would be, since landings can vary from the gentlest right through to an arrival which cannot be classed as anything but a crash!
A Carrier model cannot function without at least the engine, fuel tank, speed control system, undercarriage and hook. These items will frequently determine much of the structure sizes and locations.
Side mounted engines may produce a prettier model from the pilot's viewpoint, but the required silencer may project so far below the nose that it is likely to become the first item damaged in a hard landing. Upright engines are much better, and frequently easier to start. If conventional bearers are used for upright engines, a fuselage-mounted undercarriage can also be attached to them. Bearers for upright engines can also be extended backwards and used as an attachment for the throttle control system and perhaps for any sub-spar system that forms part of a wing-mounted undercarriage.
Inverted engines? Guess the first thing that'll get whacked in that hard landing?Fuel tank position can be very important, and it is often worth juggling the local structure so that tank position is exactly right. Remember that Class 1 is a horsepower race: what price your model design if the engine won't run properly because your tank has been poorly positioned to miss a structural element?
Undercarriages are perhaps the most problematic structural item of all in Carrier. Ideally, the landing forces should be absorbed by something seriously beefy. For wing-mounted gear, loads really ought be transferred to the fuselage. This either requires a completely separate structure inside the wing, additional to the wing structure itself and with an obvious weight penalty, or the wing structure itself has to be strengthened throughout, with local reinforcing where the U/C is attached.
Many Class 1 designs seem to terminate the undercarriage on a plate merely attached between the wing ribs, with little concern given for transferring loads through to somewhere stronger. Heavy landings can easily punch such arrangements right through the wing!Whilst undercarriages are damaged far more frequently than arrester hooks, never underestimate the forces that can act on a hook. Not only do hooks catch on arrester wires (...you hope!), but they can also snag on the joint edges of the carrier's deck panels and on the end of the deck at the top of the ramp. Just as your hook can be up to 1/3 of the fuselage in length (Rule 3.), and hence legally non-scale, there is no requirement to locate the hook in the scale position. The further to the rear a hook is attached, the further below the model the business end will dangle and the easier it will be to hook up. Unfortunately, the further to the rear the attachment point is, the more extended needs to be the structure which transfers the arresting forces back into an area of strength. Wherever the hook is located, do not skimp on this matter!
Airframe structure must also take into account all mechanisms and linkages, leadouts where internal (and particularly if subject to a line sweep) and access for fuelling, adjustment, maintenance and repair.
Many of the mechanical systems associated with slow flight require modifications to conventional wing structures, and cognisance of this must take place at the design stage. Some arrangements are either so complicated or difficult to envisage (or both!), that modellers will go to the extent of building localised mockups to work out the installation problems before the actual structure itself is built.
Internal leadouts can severely compromise wing structure if a line sweep is desired, the leadouts often sweeping through main structural components such as spars or undercarriage mounts. Even if this is minimised, ribs are always weakened by the front-to-back slot required. If line sweep is to be incorporated into the model, it is usually far better to mount the leadouts externally and accept the slight amount of extra drag in return for the resulting enormous simplification - and frequently extra sweeping space as well.
All the model's enclosed systems should have a means of access. Entirely built-in mechanisms with no access at all either mean that the model's useful life is governed by its shortest-lived component, or, more likely, that the model has to be butchered to get at the offending item when it needs attention. Nothing works perfectly forever, and sooner or later something will need to be got at. Although access hatches or removable sections undoubtedly involve slightly extra weight, this can be minimised by proper consideration of the problem and is invariably worth the thought and effort spent.
Class 1 rules
Introduction to Class 1
Set up and fly Class 1
Existing Class 1 designs
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