KNOW YOUR CENTER OF GRAVITY:

Things certainly change.  Gone are the old days when all we had to do was strap an engine and the radio gear into the model an off we go. 

With our new rules it seem apparent to me that if you do not have a very fast turning model and one that accelerates quickly you will find it hard put to win races. 

To do this you must have a smooth flying model with a rearward COG:

 Things are now so technical.  Flying a new model can be somewhat of a frightening affair. If you have not got it right - you have a beast on your hands – gyrating around the sky  - sometimes out of control.  These are the instances when you can prove to your mates how great a pilot you are.

 Anyhow who wants to do this – isn’t it better to just have you model perfect prior to launching it.  Then all you have to do it set back and enjoy the flying of such an exhilarating and smooth flying aircraft.

 First you must make sure that your model is  spot on in relation to warps and decklage etc.  This means line up the wings, tail surfaces and engine thrust so that they are all Zero.   Also balance the wings so that one side is not heavier than the other. Well that it all that is normally required for a Pylon Racer.

 There is one thing that I have missed out and that is {“Where is the COG?”). We need to know this so that we can control the turning ability of our racer.

The COG of course is the Center of Gravity. This is the pivot point of you aircraft – this is where your model balances in flight.  If you have the COG well forward – the model becomes very stable and it requires rather large movements of the control surfaces to made the model do anything.

Most people balance their models by placing their fingers quite near the root cord of their model. (The Root Cord is the Center Cord of the model) (The Cord is the distance across the wing at any position of the wing). This method can only work if you have either a non tapered wing or a wing that has both tapers the same . SO DO NOT USE THAT METHOD.

If you have a model with a reward COG - it become very responsive on the controls and a small movement of the control surfaces causes a severe reaction.  Sometime the model becomes so touchy that it is almost impossible to fly.  I had one such occasion lately.  And this caused me to pull up my socks.  I will never fall for that again!

We spend a long time building models.  There is a lot of skill and learning to get the right techniques.  It is really stupid to blow all that away with carelessness prior to test flying the model.

I think that this is the main problem with new comers into our sport.  It is real easy to say “She’s Okay Mate”.  You say that when you are willing to take a risk  - you are hoping that everything will come out right.

Actually the chances are that they will not.

  In the past I have just placed the radio gear in the model – usually to the back of the model in the service bay – where the wing goes.  The Battery is placed just behind the Fuel tank followed by the Receiver, and then the servos.  This is a standard configuration.  I would assemble the model and find the balance point of the model by assembling it and balancing it by placing my fingers under the wings.  If the COG is forward enough – I thought the model was okay. Of course this is totally incorrect for a non standard wing plan form - such as a sweep back or forward - straight leading - or trailing edge tapered wing.

How do you get a model to turn quickly?  Well you do that by positioning your COG well back in the model.  However do not go too far as the model can become uncontrollable.

 The COG is expressed for our purposes as a percentage of our wing cord.  Normally a 33% from the leading edge of the wing is about what people want for a sports model. Between 25 and 33% also seems best for Pylon Racing.  Depends how you like it!

In the old days to make a pylon model to make it fly smoothly – we have opted  to run far less sometimes 10% COG.

One other point that should be mentioned is that a forward COG model is often somewhat difficult to land.  It is hard to flare out the aircraft on landing and it  has a propensity to nose over.  Also the model is more difficult to take off  as it wants to run a considerable distance along the ground prior to take off.

 So now we are all moving our COG’s Back.  Now how do you measure you COG position accurately. The problem here is all out wings are tapered – so where do you balance the model along the wing to get an accurate measurement?

 I decided to go into this and find out a little bit more.  The problem is  - all fast pylon models have tapered wings with rather high aspect ratios. At what point along the wingspan should you balance the model?

 If you are lucky enough to have a copy of the Winfoil Program (Advertised in Airborne) that was written by Malcolm Hardy from Sydney – all you have to do is plot the wing plan form in the program and it will tell you where the “Mean Aerodynamic Cord”  (MAC) of the wing is positioned. From this information I was able to draw a line across the bottom of the wing with a non-permanent marker - to indicate this position.

 I then made up a balancing device which consisted of two 3/16” wooden dowel pieces – about six inches long.  I just clamped then in two drill press vices – pointing upwards.  (When I have time one day I will make up a nice stand) This I will probably make out of a Flat piece of 5 mm Ply 25x25mm Square – with two dowels stuck in the center.

 Anyhow I positioned these two balancing points – one at each MAC of the wing.  Then moved the assembled model along the MAC until the model balanced.  I then measured the balancing point from the leading edge of the wing, and from that I was able to calculate the COG%. 

For example: -

 Say the MAC was 19 cm and the balancing point was 5.5 cm. 

The % COG is  (5.5x100)/19 = 28.9 %.

This original analysis of that model that was out of control, disclosed a COG of 41%.  No wonder I had problems!

 What I believe that you should aim for is about 25% to start with and then – slowly bring the COG slowly back.  If it gets too twitchy you may have to use the Exponential feature on your transmitter to get the model less sensitive around zero stick position.  I personally am flying a F400 model at 28% with 20% exponential (To smooth out landing) – and it seems to handle okay.

 Andrew Davies has been working very hard on this aspect of modeling and has had considerable success. He maintains that a model that has a very forward COG becomes draggy as it flys at a nose down angle of attack.  I fast model with a rearward COG  - around 33% flys at a more level angle of attack and thereby corners, accelerates better – and flys faster. That sounds logical to me – but I do not profess to be an expert in aerodynamics.

 Now many of you do not have the Winfoil program to find the MAC.  So I poked around on the Internet and found this – and it seemed to work out okay. So just check it out.

 http://www.wcflyers.com/reference/calculators.htm (See example above)

 I now have this program permanently on my computer and if I want to calculate where the MAC is I just feed in all the variables and the computer automatically calculates it for me.

It asks where the COG is on my wing – I just feed in anything – It does not really matter – what I want is the measurement out from the Root Cord to determine the MAC position and it does this.  After I have determined the MAC position – I balance my model on that MAC position and measure where it balances with a ruler. I then calculate the COG% position using a hand held calculator against the Cord length of the MAC.

On the above example I have selected a sweep back of ZERO that means that the wing has no sweep back.  All those measurements are in centimeters and you will note that the MAC is 25.68 cm outboard of the root cord.  (That position will not change and that if where you take your COG positions from on that wing. (Note that this program does not plot the wing that I have specified)

 I played around with a few methods of determining the MAC that people told me. (For those who do not have a computer Adrian Wall has a manual method that he detailed in Airborne 184)

 Some people just measure half way along the wing – and say that is the MAC.  This is only an indicator of where your COG is in relation to the true place where your model should balance – and that is along the MAC.