Overview of Throttle and Pitch Curves

Curves are used to match the pitch of the blade to the RPM of the motor.  If you were to turn the blades so they were pointing up at 45 degree angle, it would slow the motor down due to the increased resistance by air against the blades.  So, whenever you’re increasing the angle of the blades, you need to also increase the engine speed.

This is similar to how the cruise control works  in your car.  If you’re driving on a flat road at 100km/h, the engine’s RPM is constant.  As you start to go up a hill, the engine RPM will increase in order to keep the car going at 100km/hr.

To accomplish this, the output of the left thumb-stick is used to drive BOTH the motor speed and the swash plate servos that set the blade pitch.  The image below shows this relationship.

Click on the diagram to go full screen.

Throttle and Pitch Curves

The relationship between the thumbstick, the blade pitch and motor speed is defined using two curves or graphs:  the pitch curve and the throttle curve.

In the Futaba T7C transmitter, the factory default NORMAL curve is a linear curve.  That is, when the thumbstick is pulled all the way down, the engine is at 0% and the swash plate is at the bottom of the shaft.  When the thumbstick is pushed forward, the swash plate goes to the top and the engine goes to full power.  That is, there is a one-for-one relationship between both.

The linear curve is fine for building the helicopter and doing initial servo adjustments.  However, it is not that great for flying since it will be hard to hover and land.   Also, you cannot do acrobatics (e.g. fly upside down) with a linear curve.

Most transmitters provide throttle-pitch curve sets:

  • NORMAL – used for take-off and landings and precision hovering.
  • IDLE UP1 – regular flight.
  • IDLE UP2 – 3D acrobatics for advanced pilots.

Step 46 – Determine The Range of Blade Pitch

Using the default linear pitch curve (0/25/50/75/100), we need to determine our maximum and minimum pitches.  Here is where we use a pitch gauge.

  • Set your radio to have linear throttle and pitch curves.
  • Put the pitch gauge on one blade.
  • Put the thumb-stick to 0, 25%, 50%, 75% and 100% positions.
  • Using the pitch gauge, measure the angle at each position.
  • Repeat for the other blade.

Shown below are my measurements at P1 (0%), P2(25%), P3 (50%), P4 (75%) and P5 (100%).  The black squares below are the “Actual Pitch” line.  I took the 5 measurements and put them into a spreadsheet then interpolated the data points to see if it was a linear relationship, which it was linear.  The is the “Calculated Pitch” is the solid line below.

As you can see, my TREX 500 has incredible pitch range, from -20 to +17 over the range of the thumb stick.

UPDATE: I found out afterwards that I can limit the travel of the swash plate using the radio.  I’m not too happy with the fact that my pitch range is “off-the-charts”.  Apparently using the ENDPOINTs settings to do this is not wise since all the servos move together.  To simply this process, my Futaba 7C radio provides a SWASH-AFR feature where you can limit the range.   Here are my SWASH-AFR settings:

  • AIL: -50%, no change required.
  • ELE: +50%, no change required.
  • PIT: from +50% to +40%.  This setting has an impact on the COLLECTIVE PITCH.  By reducing PIT from +50 to +40, when the thumb-stick was @ 100%, the pitch gauge measured +12 degrees, instead of ~ +20 degrees.  It also reduced the lower end as well, when thumb-stick was @ 0%, I measured -19%.

Step 47 – Configure the curves

I have a 13T pinion gear and a 6S 22.2v LiPo that puts out 2650 mAh, so according to the Align manual, my curves should be:

For My TREX 500
Normal Idle 1 Idle 2
General Flight Sport Flight 3D Flight
P5 100% 100% 100%
P4 85% 75% 93%
P3 70% 70% 85%
P2 40% 75% 93%
P1 0% 80% 100%
P5 +12.1° = 87% +12.1° = 87% +12.1° = 87%
P4 78%
P3 +5.4° = 69% +5.4° = 69% 0° = 55%
P2 59.5%
P1 -2.0° = 50% -5.5° = 40% -12.1° = 22%

A really really cool feature of the Futaba T7CH is it will automatically compute P4 if you specify P3 and P5!  Do this:

  1. Enter P5 = 87%
  2. Enter P3 = 69%
  3. Turn the dial to P4.
  4. Hold down the silver dial wheel for 1 second and voila, it will compute 78%.

The graphs are shown below:

Blade pitch vs. Thumbstick Position

Now enter these values into your Futaba transmitter.  Remember, these curves are a feature of the transmitter.  The helicopter is not aware of these curves, it is simply told to apply more or less blade pitch and throttle.