Math's Helicopter Website

Construction starts at last

This is where the fun begins. As you may have noticed by now, I have a great passion for model helicopters, but I would like to point out at this early stage in construction about safety.

Cringe you may and boring I know but let’s put a few things into prospective first. Can you remember when you were little and you went to your local park and there was a roundabout, you always started out by trying to spin it as fast as you could, the closer you were to the outside, the more the roundabout wanted to throw you off, the closer to the centre, the less the effect. Now let’s apply the same principle to the helicopter’s rotating parts, namely the main rotor hub and tail rotor.

The main blades of a helicopter turn between 1400 rpm for a beginner to over 2000 rpm for 3D flying. Our old roundabout was only rotating about 30 to 60 rpm, depending how sick you felt! The tips of the main rotors can exceed 200 mph, and now is not the time to think, did I use thread lock on those bolts? when it is in the air hovering at head height! These are all impressive statistics, but don’t forget that the tail rotor turns about 5 times faster. The main point I am trying to get over here is that a model helicopter is NOT a toy and must be treated with respect; this is especially true during construction.

Most instruction manuals, both verbose and pictorial ones, will specify when and where to use thread lock on the screw threads.

Top Tip 1. Use thread lock on all metal to metal fixings i.e. when fixing metal screws into metal components.

When applying thread lock, you will only need a very small amount applied to the screw thread which has been de-greased, too much and the screw may need a heat source to remove the screw at a later date, and this on a plastic helicopter is not always practical.

I will cover construction of the main rotor head first as this is the most critical part of the helicopter, and as experience has highlighted, has caused many new to the hobby, the greatest of problems. This area can be further split into two sections:-

The main rotor grips and rotor hub.

The main grips are linked together by a metal shaft known as a feathering spindle. This spindle passes through the rotor hub and is supported in the rotor hub by rubber dampers that will give the spinning rotor disk a degree of flexibility depending on the hardness of the rubber. When learning, these rubber dampers supplied are fairly soft to give the model a more stable flying characteristic, 3D pilots will use much harder durometer rubber giving a much crisper and faster feel to the helicopter. I have just inserted nylon dampers with a very thin rubber ‘O’ ring into my Raptor 50 to give the head a very crisp feel and this will eliminate the rotor blades from flexing towards the boom under extreme 3D conditions.

Top Tip 2. Lightly grease the feathering spindle as this will reduce vibration as the shaft centralises itself when the head is spinning and this will also reduce head noise.

The grease I use is a Teflon Fortified Grease obtainable from bicycle shops and is guaranteed not to thicken and can handle extremes of heat without changing its characteristics. The tub I bought should last me for many years to come and is a good investment.

The blade grips are supported on the feathering spindle by two ball bearings, and on the better kits, a thrust race is used as well to take care of the end loads (i.e. the blade grips can still rotate when the rotor blades are trying to pull the blade grips apart as in the roundabout scenario earlier). This thrust race causes a lot of problems for beginners.

Firstly, the thrust race is made up of three parts, not including the spacers that some models have, there is a back ring washer with a slightly larger internal hole that must be placed onto the feathering shaft first, closest to the rotor hub, then the thin cage that holds the balls which must be packed with grease, the instructions very rarely state this fact, followed by smaller internal diameter ring washer that is placed closest to the blade grips. I also install the ball and cage part, t hat must be packed with grease, so that the side which exposes the balls most is facing the centre hub.

Top Tip 3. How can I find out which hole is smaller? Trial fit the washers over the feathering spindle.

Once all the bearings, thrust races and blade grips are in place, don’t forget to use thread lock on the securing screws. This is one of the most important places to use thread lock and don’t forget to de-grease the screw thread first, you don’t want to throw a blade on your first flight!

The Flybar Assembly.

The next step is to assemble the flybar. This is a straight forward part of the building process as long as you take into consideration the following points:- Insert the flybar and before fitting the paddles, take time to centralise the flybar by measuring the amount exposed each side is exactly the same, when happy, only then fully tighten the grub screws using threadlock and then add the flybar paddles, again making sure the distance of exposed flybar is the same on both sides.

Top Tip 4. Weigh each paddle, if one paddle is heavier than the other, carefully remove the excess weight by sanding the entire surface of the heavier paddle. This will not change the dynamic balance of the flybar.

If the two paddle weights are very different, say over 5 grams, then send them back. I have only had this happen to me once, and it was obvious that there was a manufacturing problem.

Flybar Paddle alignment, although not as critical as main blade tracking, will reduce vibration and leads to a very smooth running helicopter. There are many aids on the market, and some helicopter head designs lend themselves for easier alignment than on others. So what is it you are lining up? Well, for a start, both paddles must be lined up with each other. Is this enough? No. In most cases the paddles should be lined up with the flybar control levers that are affixed to the flybar. In some cases, these control arms are angled down slightly, but by the same amount making the flybar paddles level.

The Flybar paddles are essential in the handling characteristics of a helicopter, the heavier the paddles, the more stable the helicopter is in the hover which is ideal for the beginner. The lighter the paddle, the faster the response rate is, making the model livelier. This quicker response will help 3D fliers as the helicopter will roll faster and stationery flips and rolls are a breeze!

Main Blades.

I will cover main blades in this section as this completes the head area.

The main blades of a helicopter support the total weight of the helicopter in flight just as a wing on a plane, but unlike a plane, the lifting section of the ‘wing’ can be changed just by bolting on a different set of blades.

Main blades are sold in sets of two ‘matched’ blades. So what does matched sets mean? It means that both blades not only weigh the same but each balance in the same place, therefore a helicopter rotor blade needs to be balanced statically and dynamically.

Statically just means that both blades must weigh exactly the same (e.g. 100 grams each). Now, if one blade is 100 mm long and weighs 100g and the other blade is 50mm long and also weighs100g, then if this set of blades is spun up on a rotor head, the model will shake itself to bits, why, well the dynamic balance of each blade is in a different place.

To find the dynamic balance point, balance one blade on a round pencil and at the point of balance, you have found the dynamic point of balance for that blade. This point is important when balancing a set of blades because it is at this point the lighter blade is balanced to find out where the weight is added to make both blades weigh the same.

So the procedure is to find out the lighter blade, say for example it is 3 grams lighter, now find the balance point of the heavier blade, say 50mm from the root. Place a mark on the lighter blade 50mm from the root and use this as a point of balance and add the 3 grams of PVC tape and add it at the point where the blade will balance. This will make both blades the same weight, static balance, and both will balance in the same place, dynamic balance, now the blades are a matched set. Please note, this procedure only relates to wooden blades as the modern techniques involved in producing carbon fibre blades is far advanced and the user just requires to bolt them on without checking their balance points. The blades will require to be tracked, but I will be covering this point in ‘Setting Up a Helicopter’ in a later issue.

Now, there are different types of aerofoil when selecting blades. The different types are:- The list goes on so I will only explain three of the above types.

Semi Symmetrical Blades.
These blades have an aerofoil shape to the top side of the blade but have a fairly flat bottom section. This section gives the helicopter an efficient lift factor for a slow rotor speed which is suitable for the beginner and scale model. The faster the head turns, the more drag is produced causing the engine to work harder causing the engine to overheat. The slow head speed these blades operate in will lead to a slower reacting model giving the helicopter a more stable feeling. The disadvantage is that the helicopter can’t fly inverted as the blade is not designed to fly in that mode. This type of blade is often supplied made of wood in new model kits for beginners.

Symmetrical Blades.
These are also known as 3D blades as these blades have the same curved aerofoil section on both sides. Now you may be thinking that this will create more drag, well they don’t because the aerofoil section is not as thick, this makes the blades less efficient at creating lift but allows the blades to spun faster with less loading of the engine, this faster RPM artificially creates more lift and with a faster head speed, the helicopter becomes more lively lending itself to the 3D pilots and the blades will create lift which ever way up they are. Being a thinner section blade, not by much, but enough to make a difference in flight, blades need to be stronger, that is why most symmetrical blades are made from carbon fibre.

Mild ‘S’ Blades.
These are a half way house between the semi symmetrical and the symmetrical blades where the bottom of the blade had a small amount of lift compared to the top. This type of blade will perform loops, rolls and mild amounts of inverted flight but has the advantage of good upright lift ideal for learning auto rotations with, more about those later in the series.

That about takes care of assembling the head of your new model, in the next article, I will be covering the mainframes and engine installation together with the difference between mechanical mixing system and CCPM systems. I hope this has wetted your appetited, until then, have a look at your blades and have a go at checking their balance.


©2005 Matthew Ullman