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Tips and Tricks

Aircraft CG. What where why how?

I often get asked. What is CG and why is it so important.
Well without going into the heavy aeronautical terms the CG is the aircrafts Centre of Gravity. 
Basically it’s the point on the wing at which it balances and does not tip nose down or tail down
Usually (NOT ALWAYS) a aircrafts CG is about 30-35% of the way down the chord (width of wing) of the wing,
For most aircraft this happens to be more or less where the main spar is located
Please note this only applies to wings with constant chord (ie straight wings without taper)

So to find out if your trainer, or Piper Cub, Or Cessna is balanced. Pick it up (WITHOUT fuel) and put your fingers under the wing (if it’s a High wing) on its CG Point, 
If it’s not marked or known then at about 33% of its chord (width of the wing)
If your aircraft is a low wing, turn it upside down to balance it.

So basically put your index finger point at a position a 3rd of the way down the wing (looking from the front)
Now check to see how the aircraft balances

If it balances level or is slightly nose down (at most 5 degrees) then it’s 100% balanced
However if the nose or tail drops violently then it’s said to be Nose Heavy or Tail Heavy

The effects of each condition are quite different.

a mildly Nose heavy aircraft will tend to be more stable in flight and less prone to Tip Stalling(dropping a wing)
However if it’s too nose heavy you will find that on landing you will battle to flare and it will basically fly
Nose first into the ground no matter how much up you give. Extreme Nose heaviness also makes some types of aerobatics almost impossible. Ie Spins, Snap rolls to name a few

But nose heaviness is definitely the lesser of the 2 evils

Tail heaviness can result in nasty crashes
If an aircraft is tail heavy it’s often said to be Twitchy, Prone to ballooning or Porpoising, 
Stalls easily. Drops a wing at the slightest provocation and generally is a little cow to fly.
Without referring to specialist aircraft such as funflies which are intentionally tail heavy, in general tail heaviness is bad
And it’s even worse with scale aircraft. Nothing as nasty as a tail heavy spitfire.

After doing the balance check and you are unsure if its nose or tail heavy as it’s basically in the middle, then test it out in the air
It’s fairly easy to check if your aircraft is nose or tail heavy. (Please let an instructor do this if you are still learning to fly or are unsure of your aircraft)

Take it up quite high, about 150ft. fly straight and level, slowly pull back the power until it’s at idle
While feeding in up elevator slowly and evenly. The aircraft will slow down, might start to pitch nose up and at this point when it’s at or near stall point it will give away its CG tendencies
For instance, if your trainer is balanced or slightly nose heavy. it will slow down, basically stop in mid-air for a second, then drop its nose gently pick up some speed and fly away normally.
If your trainer is very tail heavy, it will slow down quickly, stop in mid-air and will stall suddenly and potentially viciously
I.e It might drop a wing, which is falling to one side and might even go into a spin (unlikely in a trainer)
To recover from this apply power, LET GO OF THE ELEVATOR, let it gain speed and pull out gently (this is why we gained 150ft of altitude in the first place

If you continue to hold in up elevator while trying to recover you will most likely keep the aircraft in a stalled condition and cause it flick its wing from side to side eventually ending in a pile of parts on the ground.

The reason i say check balance without fuel is simple. with fuel in the plane will be nose heavy, but as the fuel is burned it might start to become tail heavy which is NOT what
you want when you're coming in to land.

So check your your plane before flying, don’t just assume it’s balanced. If in doubt get someone in the know to assist you.

(Info supplied by Willie)



Blade grip tightness

What is it? Quite simply, how tight the main rotor blades and tail rotor blades are secured in the blade grips. This is adjusted by the securing bolt and sometimes (depending on the size of the heli) by a nylock nut on the other side.  

So what is too tight and what is too loose? Simply put, a blade that is too loose can end up hitting your boom when there are changes in pitch or hard landings. A blade that is too loose also has vertical movement where it meets the holder, which can degrade the blade at that point and send your blade flying.
 A blade that is too tight, will give you head wobble in the lower RPM and if the centrifugal force cannot straighten the blade, result in an exploded heli...

Here is a simple way of determining the correct blade tightness for your heli

Extend the blades fully & turn the heli 90 degrees to the horizon. The blades should remain in the extended position and not collapse as a result of gravity alone. Give the heli a little shake, and the blades should drop slightly, but not flop completely. This applies to the main as well as tail rotor.

As the heli’s get larger, you have to tighten the blade bolts more as the blades have much more mass. Once over 700mm blade size (not rotor diameter), blade tightness is so important and on big blades like these, you generally have to tighten the bolts so tight, the blades won't move when the heli is held sideways and shaken. If large blades like this are too loose, they will lag during spool up or down. Depending on both mass of the blade and the amount of lag, this lag could shake your bird apart due to the effects of ground resonance and an out of balance head caused by blade lead/lag.

(Info supplied by Willie)



What is CCPM?

Other than just a fancy term used by the RC heli fraternity, it is really the heart of Heli operation and needs careful setup and understanding to make heli flight possible.

Cyclic/Collective Pitch Mixing or CCPM is a system which incorporates the collective movement of servos in order to manipulate the swash plate of a heli. This can be achieved by a number of different systems, either 2 input, 3 input or 4 input. We will be discussing the 3 input system as that is the most commonly used.

3 input CCPM is quite simply a swashplate that is controlled by 3 servos. Each servo has a specific function, i.e. aileron, elevator & pitch, however all 3 servos work together in order to achieve aileron, pitch or elevator movement. This intricate movement is controlled by the computer in your TX; so quite simply, CCPM is the software that makes this mixing movement possible. In a 3 point system, the ball links are spaced 120 degrees apart around the swashplate and each point connected to its corresponding servo.

Basic CCPM setup on 120 degree

So now that you have a basic understanding of what CCPM is all about, here is a basic guide to setting up CCPM on your heli.

For this guide, we will be using the common 120 degree swash setup, as it’s the most commonly used. This is also taking into account that you have a compatible TX system, designed for heli use. It is possible to configure CCPM, using an elaborate combination of Y-leads and mechanical arms, but as this has to 100% precise, we advise against going this route (Most modern TX's have the ability to switch between Heli/Acro/Glider modes)

First disconnect the motor, by separating 2 of the 3 wires from the ESC. This way you will not damage the Heli or yourself, whilst toying with all the settings and features of 'Collective Pitch'. Trying to negotiate with a rotor, spinning at 1500RPM whilst you are trying figure out what each button does is going to definitely leave a mark!

Next step is to get into the transmitter and start configuring it for Heli use. Let’s assume you have a compatible transmitter and it is equipped with Swashplate electronic mixing. It is a good idea to have your manual handy as you will be looking for CCPM ( Cyclic / Collective Pitch Mixing). You will need to select the 120 degree swash setup. This may either present as a diagram of the swash, or simply just a degrees option. So with the 120 degree swash, your Aileron and pitch servos tilt the swash left and right. With elevator inputs, all three sevos tilt the swash fore and aft. With Pitch inputs, all three servos raise or lower the swashplate.

Most modern Transmitters allow you to reverse the direction of either of these servos in order to achieve the correct movement as per your inputs.

When plugging servos into your receiver, make sure that they are plugged into the following channels. This setup is specifically for Futaba (JR and other systems may vary - check your user manual for your TX to verify the correct location)

channel 1 Aileron (roll)
channel 2 Elevator (fwd/aft)
channel 3 ESC
channel 4 Rudder ( connected to Gyro and then to Servo )
channel 5 (Gyro Gain remote )
channel 6 (Pitch)
 

So now that we have all the servos plugged in and the TX setup is complete, we need to make sure the swash is moving in the correct direction. Make sure not to have your linkages from your servo connected to the swash yet as wen testing, you could bind and burn out your servos. What you are looking for is when you give left or right aileron that the left and right servos move in that direction.. I.e. push left aileron, then the left servo (standing behind the heli) will drop, and the right servo will move up. Vica versa for right aileron. Pushing forward elevator, rear servo will lift, left and right servos will lower. Raising collective, all the servos must lift in unison.

You have now completed the basic CCPM setup and can move onto the next step.

Now what you need to do is reset all trims and sub trims to zero.

Turn on your Transmitter, and all servos will centre themselves.

Now that servos are centred, you can now attach your servo horns at 90 degrees or parallel to the ground. Now you can mechanically start adjusting the links (by turning in or out the ball links) until the swashplate is level or until the 3 links are the same length. Make sure the swash is level about halfway up the main shaft so that full movement positive & negative can be achieved.

You can either use a swashplate levelling tool to get the swashplate level, or by using a button spirit level to get as close to level as possible.

(Info supplied by Willie)

 

Wheels - Setting up your plane for perfect take-offs

If you have ever had a tail dragger I’m sure you’ve experienced the frustration of trying to keep it straight on the runway while taking off.

For instance on your take-off run it may seem to run straight then all of a sudden veer off to one side ending up in the rough or doing something called a ground loop. Where it basically stops and spins around in a circle on its main wheels.

This problem is especially prevalent on Piper cubs,Decathalons,Tiger moths etc.

In reality it’s quite easy to remedy this problem. Firstly the problem is most likely caused by the alignment of your main wheels. It may sound trivial but the toe (alignment) of your main wheels on a tail dragger has a marked influence on its take-offs

Much like your car. If your planes wheels are badly out of alignment it’s difficult to control. The problem with a badly ground looping tail dragger is usually toe out. What is this?

If you stand over your plane just behind the wings facing forward and hold the plane level to the ground(you may have to lift the tail a bit to simulate a take-off) and look straight down the leading edge at the ground you should see 50% of your wheel. I.e. half of your wheel from the axle forward. If you see more or less you may experience problems with your aircraft nosing over on the ground if the wheels are too far back or ground looping if the wheels are too far forward.

Note that some aircraft have difficult wheel positions. Such as Spitfires and Harvard’s, there is not a lot you can do about this apart from hold a little up elevator as you’re accelerating to keep it from nosing over. Once it has picked up speed ease off on the elevator to stop a premature take-off and a potentially disastrous stall.

Ok back to “normal” tail draggers. Once you have ascertained that you can see 50% of the wheels and the wheels are not too far forward or back, look at the alignment. In other words check each wheel and see if it’s pointing straight ahead, outwards or inwards. Chances are that one if not both wheels are pointing outwards. This is called toe out and makes life difficult for the pilot and amusing for your club mates. Ideally both wheels should have a little toe in (pointing inwards) about 2 degrees or so. They should at least be pointing straight forward. But you’re handling will be better if they are slightly nose in. Obviously if they are pointing inwards too much they will drag on the ground and shred themselves so a bit of common sense should be used here(I.e. ask a fellow club mate for his opinion or use the tried and trusted theory that if it looks right then it probably is)

With both your main wheels pointing slightly inwards (toe in) you should find your aircraft a totally different animal on the ground. It will run straight and will not have as much tendency to veer to one side or ground loop

Hope it helps

 Happy Flying
(Info supplied by Willie)