After reading this , the pitch curve setting were crystal clear to me. So just wanted beginners like me to understand it the simpler way
The normal Pitch settings for a helicopter consist of 3 things.
A. Negative Pitch for inverted flying and getting the heli down in high winds.
B. Zero Pitch - the point of transition from negative to positive, and positive to negative Pitch
C. Positive Pitch for upright flying.
As we know, servos have limited travel. Picture a servo lying down horizontally with its servo arm pointing horizontally. In this position it would be at position B as listed above....Zero Pitch . From position B as the servo arm moves downward, it would be at position A....Negative Pitch From position B as the servo arm moves upward, it would be at position C....Positive Pitch
Most modern radios allow us to set Pitch curves based upon a 5-7 point "Pitch curve". Her we are going to base everything on a 5 point Pitch curve. Today’s radio's think of a total servos travel as 0,25,50,75,100.
Think back to the horizontal servo, where the radio sees 0 as having the servo arm all the way down, 50 as dead level, and 100 as all the way up. So if we wanted a servo to move from position B to position C. We would have to tell the radio that we want the servo to move from 50 to 100...not 0 to 100. Get the idea? Remember the radio sees 50 as centered...not 0 and a properly set up head sees zero pitch.
Ok so where does this lead us.
Well on a collective Pitch helicopter. Servo movement directly translates into blade Pitch . So when we set up a collective Pitch helicopter we want to use what the radio sees to program and set it up. Now there are two basic different ways to fly a collective Pitch helicopter.
1. Normal mode. - For upright flying, consisting of positive Pitch only
2. Idle up (3D mode) - For inverted flying, consisting of positive and negative Pitch
Lets look at Normal mode first. BUT...you want to keep thinking in terms of full collective...that is Negative to Positive Pitch .
Lets assume that Pitch is controlled by a single servo. Since servo travel is directly translated into blade Pitch , we want the radio to understand that we only want the servo to move the blades into positive Pitch , and only a small amount of negative to fight high winds 4 degrees. This means that we want the servo to be limited to moving from lets say 40 to 100 not 0-100. So with the servo centered, and the servo arm centered at 50/ then 40 would give us that little bit of negative we need. Get it??
Point 1 would be 40.
The next point...Point 2 would be 45
The next point...Point 3 would be 50
The next point...Point 4 would be 75
The next point...Point 5 would be 100
The throttle curve would be set as
0,25,50,75,100
This means that when the throttle stick as at dead bottom. There is 0 throttle, and negative roughly-4% Pitch .
As you move the stick up, it increases throttle to 25%, and increases Pitch to 45 (roughly negative 2+1 degrees Pitch based on a max of +12)
When you get to mid-stick, throttle will be at 50%, and the Pitch will be at 50 (0 degrees pitch)
...and so forth.
In this manner, when you have the heli sitting on the ground, the blades aren’t spinning, and there is a very small amount of negative Pitch .
As you increase the throttle stick, the blades start spinning, and the heli lifts off around mid-throttle.
Now...on to Idle up or 3D mode.
Now we want to take things to the next level. In order for us to do this we have to tell the radio that we want to make use of the servos travel below 40. So how do we do that? Well via a switch on the radio. The Idle Up switch. When we flip this switch we are telling the radio that we now want to discontinue use of the normal mode, and that we want the throttle stick to now control the entire range of the servos travel
Remember the 5 point Pitch curve. This is what really confuses beginners.
Now the 5 point Pitch curve contains points for the entire servo travel instead of only roughly half of it and therefore, the 5 points have to represent the entire servos travel.
So the first point would be 0...full down on the servo, full negative Pitch -12
Point 2 would be 25....mid-way down on the servo, half negative Pitch -6
Point 3 would be 50...centered servo...zero Pitch
Point 4 would be 75...mid-way up on the servo, half positive Pitch +6
Point 5 would be 100...full up on the servo, full positive Pitch +12
Notice also that the throttle curve changes. In idle up mode we never want the throttle below 50% or else the blade RPM will get too slow and the heli could fall from the sky. This is where the collective Pitch independent of throttle comes into play. In idle up, the blade Pitch is no longer dependant on throttle. So throttle settings in Idle up often look something like this, 100,75,50,75,100.
Courtesy :
http://www.rchelicoptertown.com/forum/showthread.php?t=3480