In the last article we covered the construction of the head area of the helicopter. In this article, we turn our attention onto the main frames and engine installation with a brief look at the two types of control that is associated with model helicopters, namely CCPM and mechanical mixing.
Main Frame assemblyI have mentioned before that helicopters have three types of main frame materials:-
- GRP Plastic
- Carbon fibre
Each material has both advantages and disadvantages.
GRP Plastic FramesThese frames, known as ‘plastic fantastic’, are generally associated with cheaper model kits, but although they are often cheaper, they offer amazing resilience to the riggers associated to model helicopters. Vibration is a common part of helicopter flying, and if we can reduce this vibration then the helicopter will give a long service life.
GRP frames offer excellent resilience to vibration, damping its effects on other parts of the helicopter. So what does GRP stand for? It stands for Glass Reinforced Plastic.Advantages:
- Relatively cheap.
- Good vibration suppression qualities.
- Mouldable into any shape.
- Threads can strip easily.
- Frames can crack and they may be difficult to spot.
- Impurities in the plastic can weaken the casting.
- Expensive moulds are required for manufacturing thus large production runs are required by the manufacturer.
Top Tip 1:If a thread strips in the GRP casting, a small amount of Cynoacrolite will repair the thread until the screw is removed. This process can be repeated but not indefinitely. I have successfully inserted a helicoil into the GRP frames as an upgrade and these have lasted many years, and by replacing the screw with am M3 cap head screw, I feel is a better solution anyway.
Aluminium Frames.Aluminium frames are easy to manufacture as they are normally supplied as flat plates which build into a stacked frame design, they are accurately drilled and are cut into some elaborate shapes. Originally, these aluminium side frames only came in their natural colour, but recently, they can come in many different colours achieved by anodising the aluminium. Advantages:-
- Easy to spot if the frames are damages by lying them on a flat piece of glass.
- Aluminium is a relatively cheep material.
- Fixings are metal to metal, don’t forget to use threadlock!
- Less work is required by manufacturers as the left and right sides are the same.
- Can easily be upgraded to carbon fibre at a later date.
- They are very poor in vibration suppression.
- Frames cannot be straightened as this weakens the material.
- Aluminium has a tendency to shrink in cold weather.
- Extra care is required to ‘square up’ the main frames.
Talking about aluminium shrinking in cold weather, have you noticed that the tail belt of a helicopter appears very loose on a cold day, but when you check the belt tension at home in a warm room, the belt tension is ok! This is due to the aluminium boom shrinking in the cold weather. This does not happen with carbon booms.
Carbon Fibre Frames
These are not only strong but they look fantastic. Again, carbon fibre side frames are manufactured in flat plates which lend themselves to the stacked frame construction.Advantages:-
- They look fantastic!
- The frames are easy to tell if they are broken.
- They are tolerant to vibration.
- The frames are very strong.
- Carbon fibre is very light.
- Less work required by manufacturers as the left and right sides are the same.
- Carbon fibre is expensive.
- The edges of the carbon fibre frames are sharp and have to be smoothed off.
- Extra care is required to ‘square up’ the main frames.
Now, I have mentioned stacked frames. What does that mean I hear you ask? No, I don’t always have little voices in my head, it is a fair question. Stacked frames mean that there is a wide lower set of side frames that house the engine and fuel tank, and fitted above this (stacked on top) is a narrower set of frames that support the tail boom, main mast bearing holders and the top bearing support for the engine start shaft. This construction is required as the side frames are flat and the bottom set need to be wide enough to hold the engine between them. The two sets of frames are connected together with short spacers, some of which are hard to fix in place.
There is always an exception to the rule, and this time it is from Miniature Aircraft in the form of the Fury where the top and bottom frames are made from one piece of aluminium, or carbon fibre in the Extreme, which are very close together and a metal engine block extends out both sides of the frames to support the engine in place.
In the eighty’s and early ninety’s, models were made where the aluminium frames were bent to cater for the wider lower frames and narrower upper frames, but in the event of a bump with mother earth, it was difficult to determine if the frames were bent or the bend in the aluminium weakened the integrity of the material.
Now, GRP frames do not suffer from this problem as complex moulds are made and this offset between the top and bottom is easily catered for. A lot of the modern ‘plastic fantastic’ helicopters are so well designed that cooling shrouds for the heads of the engine are moulded into the side frames as one piece. The bearing housings are also moulded into the side frames, the ones on the Thunder Tiger Raptor are so well moulded, that the bearing fit is amazing, just put the bearing in one side and it stays there. If the same is done to a Hirobo Scaedu, the bearing falls back out until the two half’s of the frame are assembled together, this could result in the outer part of the bearing rotating which is not a desirable effect. In this case the bearing outer must be ‘glued’ in position with a bearing retaining compound.
When assembling the GRP frames together, the two halves are pretty much self aligning with each other. With the stacked frames, not only do you have to make sure that the side frames are parallel with each other but that the top and bottom sets of frames are parallel with each other! This can only be achieved by constructing the lower and upper frames on a smooth piece of glass using a set square. This is why stacked frames are usually associated with the intermediate to advanced fliers, and the ‘plastic fantastic’ GRP framed helicopters are aimed at the beginner.
Fitting the engineOnce the frames are assembled with all the associated bearings fitted, and the main mast is fitted to the main frames, the engine is now required to be fitted. In the “plastic fantastic’ helicopters, this is fairly straight forward as the gear mesh between the clutch bell and main gear is non adjustable, the only adjustment is how high the engine is inserted, too high and the clutch can be pushed up too tight in the clutch bell stopping the clutch disengaging. This is a common problem with the Thunder Tiger Raptor, but it affects most models in this class. The clutch bell must have a little vertical movement, known as float, in order for the clutch to work efficiently.
With the stacked frames, the engine mounting holes are usually slotted, as well as the top bearing block fixings for the engine. This is done so that any anomalies with the build can be adjusted out and the gear mesh can be adjusted between the main gear and the clutch bell pinion. This may sound straight forward, but think about it. There is movement on how high the engine can be mounted, the engine can lean forward or even be twisted side to side. Both the latter parts are very detrimental to the overall build of the helicopter causing parts to wear out quicker and thus cause unwanted vibration.
So be warned and take your time in lining up the helicopter frames and the engine installation.
Top Tip 2:Use a thin strip of paper, I normally use a thin strip of a post-it-note, with the glue bit removed in case you were wondering, and run it in between the two gears. If the paper is cut by the gears, then the gear mesh is too tight. If there is little definition in the paper, then the mesh is too far apart. Consequently, if the paper is nicely pan folded, then the mesh is good, but don’t stop there, make sure that this definition is even across the paper pointing to a miss aligned engine!
Helicopter Controls SimplifiedI mentioned at the beginning of this article that I would cover the two types of control systems associated to model helicopters. These are:-
- Mechanical Mixing.
- CCPM mixing.
These above systems control the movement of the swashplate. The swashplate is a clever design in which the bottom section remains stationery whilst the top section rotates on a large bearing turning a static motion into a rotating motion.
Mechanical MixingThis basically means that there is one servo for each action of the helicopter movements namely the pitch, elevator and aileron. Mechanical leavers then mix these servos together stopping any interaction between each control. There are many different ways of achieving this, some methods are better than others. There are systems that allow the servos to rock in ball raced cradles, other systems use servos that are rigidly fixed relying on the ball joints on the control rods to take up the movements. The latter type tends to stress the servo output bearings, or bushings on cheaper servos, and I tend to stay away from this type of rigid control preferring the rocking cradle design.
CCPMCCPM stands for cutting corners promotes misery. Only joking! It stands for Cyclic/Collective Pitch Mixing where all servos are directly connected to the swashplate and all servos move in the same direction for pitch, for aileron, one servo moves up, one moves down and the front servo stays still!
The transmitter works out the amount of movement each servo is required to make in order to achieve the desired result. The most common CCPM swashplate setups are 3 servo 120deg CCPM and 3 servo 140deg CCPM.
In 3 servo 120deg CCPM, as the name suggests, three servos that are directly connected to the swashplate where the three connections to the swashplate are 120deg apart. This type of control was not favored by many pilots as some interaction between the servos was very noticeable in flight. This was mainly due to the difference in the servos themselves. If for instance pitch is required and all servos are required to move up, any hesitation or difference in servo speed will introduce unwanted elevator and/or aileron movement as well. That is why it is imperative that the three servos are exactly the save type. With the advent of digital servos, this interaction is negligible, and the benefits far out strip the hassle of setting up the system mechanically.The benefits are:-
- The pitch is far more powerful as the loads are spread across three servos.
- The pitch is faster as less strain is required thus allowing ‘Pitch Pumping’.
- Fast and crisp elevator and aileron is achieved as less overall movement is required for each servo e.g. one servo moves up and two move down.
- Good quality servos are required to be of exactly the same type.
- The initial mechanical setup must be accurate.
That just about covers it for this article, as I will cover actual setup in another article. I hope this answers some of the questions you may have had with regard to why there are so many different types of helicopter main frames and control systems, until next time when I will be covering the tail section and tail rotor control, have fun and keep looking at the models used at your local field for inspiration.