An Introduction to Model Helicopters
Click on the images to enlarge
During the last couple of years there has been an explosion of interest in model helicopters (if the requests for information to my club are anything to go by). By far the majority of this interest has been fuelled by the micro and sub-micro electric variety. Modern electronics and new technologies have made the once impossible, possible. Most of, but not all of these small models are for indoor flight only and there are several regular events held in large sports halls locally to enable these to be flown safely and well away from the rigours of ‘normal’ British weather! The PicooZ was initially regarded as nothing more than a toy when it first appeared, however the technology used to create such a marvel has enabled many would be flyers to get a taste of rotary winged flight. Be warned! It can be very addictive.
Whilst the PicooZ uses a conventional ‘main and tail rotor’ setup, there are many slightly larger and easier to fly model helicopters that use the co-axial arrangement of two, main, two-bladed rotors set one above the other and rotating in opposite directions means that the small tail rotor (used as an anti-torque device on conventional helicopters) is not needed. Whilst these co-axial machines are relatively easy to fly and very controllable, they are not capable of any form of aerobatics (well not intentionally anyway!) but do provide an insight into the flying capabilities of model helicopters. There are probably now hundreds of such models on the market and most have some sort of fuselage depicting at least a representation of a full size helicopter, there are even several ‘Chinook’ style models with twin co-axial contra rotating rotors! Some of the larger ones can even be flown outdoors, but only in near calm conditions.
Co-Axial Twin Rotor
There have been several attempts over the last twenty years or so to market a ‘proper’ small electric helicopter with all the capabilities of their bigger (I.C. powered) brothers. Some were more successful than others. However, it wasn’t until quite recently with the advent of lithium polymer (‘Lipo’) batteries, brushless motors and truly miniature servos and gyros that the dream of a ‘full function’ model electric helicopter became a reality. Over the last 4 to 5 years the floodgates have opened and the number of such ‘proper’ small electric helicopters has been both astonishing and slightly bewildering.
Align T-Rex 450S
A Short History lesson............
Model helicopters generally didn’t appear until the late 1960’s with the introduction of ‘proportional’, multi-function radio control systems. Full control of a model helicopter requires very precise movements of the control sticks on the transmitter and like the history of full size fixed wing aeroplanes it was some years after reliable control of them was achieved that helicopters started to appear on the scene. Virtually all early model helicopters used 2 stroke internal combustion engines using glow plugs for ignition and with a capacity of around 10cc (‘60’ size or .60 of a cubic inch). They were large machines with main rotor diameters of about 1 ½ meters plus (around 60 inches) and they were all modelled on full size machines (mainly those manufactured by Bell Helicopters).
Bell 206b Jet Ranger. This one was originally built in 1975 from a German kit made by Kavan. Refurbished a couple of time since, but is still flying!
It would be another 15 years before smaller more agile ‘sports’ machines came onto the market - from Japan mostly. These were initially of half the size (5cc or ‘30’ size engines) and main rotors of around 1 metre. But the main difference was that they were almost ready to fly and made almost entirely of moulded plastic. These were known as the ‘plastic fantastics’! This size of helicopter is still available and most helicopter clubs regard them as possibly the ideal training model. Relatively inexpensive to buy, spares are easily obtainable (you need plenty of these when learning!) and will handle our typical Sunday morning weather.
The original ‘60’ size machine with a full scale fuselage has almost disappeared. ‘50’ size (8.5cc) machines have taken over to a large extent. The engines are more powerful than the ‘60’ size of 30 years ago and the fuselages are only slightly smaller and easier to transport. The overall flying performance is also much better. As model helicopters improved and control of the main rotor became understood more, a large number of flyers wanted to fly aerobatics like their fixed wing cousins. This lead to the development of the large ‘sports’ helicopter. Built very light and with the most powerful engine you could get hold of! Aerobatics in turn lead onto what’s become known as 3D flying.
This is a style of flying that has overtaken almost all other types of model rotary winged flight. This is now dominated by large electric model helicopters using very large ‘brushless’ electric motors and even bigger lithium polymer batteries to create a power system producing, in some cases over 6 horsepower! Machines are generally referred to by the size of a single rotor blade, so a 700 size electric machine would have a couple of 700mm blades, that, with the rotor head produces a rotor span of around 1.6 metres. The sizes now available from several companies start around 150 through 250, 450, 500, 550, 600, 700 and 800. These are all predominately electric, with only the 550, 600 and 700 available in glow or ‘nitro’ versions.
3D flying consists of flying the helicopter in all 3 dimensions. Up and down, left and right and forwards and backwards. All done a very high speed and including pirouettes and rolls in any direction. It involves immense concentration and incredible skill. There are competitions at all levels from club to even a World Championships.
This 3D style of flying also lead to a huge leap forward in electronic design of control systems.
Typical of the sort of high powered 3D model.A T-Rex 600E. Electric powered and extremely capable.
From the end of the 1970’s control of the tail rotor anti-torque had been assisted by a mechanical gyro that would dampen down any unwanted movement by the tail. Changing pitch on the main rotor or opening the throttle would cause the amount of anti-torque required to change hugely. Trying to fly a model helicopter in the early days was dominated by the challenge of keeping the tail from swinging from side to side because of changes of torque. With the advent of the early gyros this helped enormously. By the middle of the 1990’s the mechanical gyro had given way to the electronic ‘solid state’ gyro, which in turn led to the ‘heading hold gyro’ which now dominates. Now that the tail had been ‘sorted’, attention turned to the main rotor head. With one of two exceptions, the vast majority of model helicopters used a ‘flybar’ to stabilize the rotor head. This consists of a shorter steel rod with a ‘paddle’ at either end. These paddles have an interaction with the rotor head to provide some degree of stability. They do not contribute anything to lift, indeed they absorb a not insignificant amount of power and it was this that led to various experiments with using a further two gyros to control the stability of the main rotor head. This in turn led to the 3 axis gyro that most 3D helicopters use today. Gone is the flybar and in its place is a micro miniature 3 axis gyro where two of the axis refer to fore and aft pitch (elevator) and to left and right cyclic (aileron) and the third axis is yaw (rudder). This has transformed the ability of model helicopters to perform all sorts of incredible manoeuvres.
It has also enabled the scale model helicopter pilot to be able to use multi-blade heads without the fear of many of the adverse side effects that multi-bladed heads were renowned for…..
The death of the scale helicopter has of course been greatly exaggerated over the last few years and there is still a dedicated band of eccentrics (I have to admit to being one) who insist on flying ‘toy’ helicopters pretending that they’re flying a real one! Virtually any size of model helicopter can be turned into a scale model.
Agusta A109a. This model built from scratch with commercial mechanics and retracting undercarriage.
10cc engine and 1.5M main rotor.
Bell 47G III
¼ scale with 26cc petrol engine (onboard electric start) and 2.3 metre main rotor diameter.
Built from a Vario kit from Germany.
There is no denying however that the bigger they are the more realistic they tend to be. This then leads me onto the group of model helicopters where there are almost no ‘sports’ machines and the sky really is the limit. Engines tend to be big, very big and almost exclusively internal combustion engines (I have however recently seen a couple of big scale models powered by equally big electric motors). Glow plug (nitro) engines tend to be ‘90’ size (15cc) plus. Modified industrial spark ignition (petrol) engines from 20cc to 30cc are frequently used in models with main rotor spans in the region of 2 – 2.5 metres. These engines produce perhaps the same as the ‘90’ size glow plug engines, but at much lower RPM and produce much greater torque. They are also much cheaper to run as they use ordinary 2 star petrol with a small percentage of 2 stroke oil added.
With a few notable exceptions, all full size commercial and military helicopters use gas turbine engines for power. In fact it wasn’t until the invention and introduction of the gas turbine engine that the development of the helicopter took off - literally. The helicopter isn’t a particularly efficient method of flying and requires a lot of power just to stay airborne. It can however do things that ‘normal’ aircraft can’t do (Harriers excepted!) and that is to hover, fly sideways and backwards and land and take-off vertically and it is this facility that makes the helicopter such an asset for Search and Rescue, Point to Point and as a stable military weapons platform. Early helicopters like the Bell 47G (the first commercially available helicopter to get an official certificate of airworthiness in 1947) used a very large and heavy 6 cylinder aircraft engine to lift just two people and no payload when the same engine in a high wing, cabin aeroplane could seat four and a small amount of luggage. The introduction of the small gas turbine engine gave helicopter designers a small power plant that was light and reliable. In the last ten/fifteen years miniature versions of these engines have started to be produced, first for fixed wing ‘planes and then for helicopter use. With the increased power available from these engines models between 1/5 and 1/3 scale are now possible. They have the ability to turn main rotor blades of 2.5 metres plus diameter. They do consume an extraordinary amount of fuel (paraffin or kerosene), but they do produce copious quantities of power (8.5 horsepower plus) and the sound, and even the smell is ‘just like the real thing’.
And at the end of the day that’s what it’s all about........................
BMFA Area Chief Examiner (Helicopters)