Sunday, 2 October 2011

Throttle and Pitch Curve Setup

RC Helicopter Pitch and Throttle Curves a Simple Guide

Introduction to Throttle and Pitch Curve Setup


Pitch and Throttle curves. People get to easily confused by this subject. It is mostly because they look for other peoples settings to use as there own, and don't truly understand the mechanics behind them. So when they don't work, they are unsure of what to adjust to rectify the problem, for instance if you are suffering from a low headspeed and "wagging" RC helicopter.
This subject is in fact very easy, and it just requires a little careful planning to get the perfect pitch and throttle curves for your RC helicopter. I will also briefly look at idle up pitch and throttle curves for the budding 3D pilot. My blog will list the latest RC helicopter guides and tutorials.
RC Helicopter Servo End Points
A little digression first, if you followed my previous guides, and in particular the mechanical setup guide. You will have seen we set the end points for the servo travel on the helicopter. It is useful to just spend a few minutes understanding how curves fit within those end points. When we have the servo travel set at 100%, we can increase or decrease that value, which then restricts or increases the servo travel. The pitch and throttle curves work within these pre-set end-points. So 100% at point 5 on a throttle curve corresponds to 100% servo movement if it is set to 100% as the end point, or 125% if set to this in the end point menu.

Graphical Helicopter Throttle Curve Menu 

So remember, pitch and throttle curves work within your end points. When we initially set up the radio system, we gave the pitch a linear curve. Normally most radios use a 5 point pitch and throttle curve, but some more advanced radios use a much higher granularity allowing for a higher resolution curve. So in a linear curve we have the 5 points corresponding to the 5 stick positions in a linear fashion. P1=0%, P2=25%, P3=50%, P4=75% and P5=100%.
How this information is displayed will depend on your particular hand-set. Some will display it as a visual graph, and also as you move the throttle stick, will also update the relative position on the graphs axis. Others will just list them as text, so P1, P2 and so on.

Pitch Curves
  After the mechanical setup, we arrived at a ±11° pitch range, with a linear curve this then means at P1 we have -11° pitch, and +11 degrees at P5, with a linear change between the two points. Crossing P3 at zero degrees pitch. It is possible to change these points to tailor how the pitch changes as we move the throttle stick. In normal mode we most certainly don't want -11° of pitch at zero throttle. But we do want this kind of pitch curve in idle up for 3D flying and upside down antics, but more on that later.

Graphical Helicopter Pitch Curve Menu
Getting back to end-points again briefly, we can use the throttle and pitch curves to set "fake" end points. So for instance if P1 is changed to 30% rather than 0%, then low stick now starts at a 30% value of the linear curve. So we will not get the full travel distance. This can be very useful. So with the pitch curve for normal mode flying you would make a higher starting point to make less pitch range between bottom and centre stick.
Changing P1 and P2 of the linear pitch curve, will allow us to reduce the negative pitch range of the helicopter. We are after a linear curve between P1 and P3, the same for the curve between P3 and P5, but at a different rate of increase. Ideally -3°, or -2° for P1, this means we wont slam the helicopter into the ground when trying to land, but also we need negative pitch to be able to bring the heli down in high winds.
The best way to set P1 and P2 is to use a pitch gauge. Attach it to the blade, and set P1 to when you read approx -3°/-2° of negative pitch. P3 will still be at zero degrees pitch, so set P2 halfway between the two. This is now your normal pitch curve Leave P3,P4 and P5 the same to match the idle up settings for these points. As the idle up switch is flipped whilst in the hover, so they need to match, usually at around 5°/6°. This normally corresponds to around ¾ stick for the hover.

Throttle Position Example Normal Pitch Values
P1 40
P2 45
P3 50
P4 75
P5 100
Throttle Curves
  In a RC helicopter, the motor doesn't always provide power in a linear fashion, especially electrics, the power band is normally quite high. If it was linear curve, it would match the pitch change exactly, which wouldn't work. If the rpm is low on the headspeed for the given pitch, we need a higher motor rpm to increase the head speed. As our pitch values are now set. (curve rising steeply then plateau). A symptom of low head speed is oscillating in the hover, we now adjust the throttle curve to give the correct headspeed at the appropriate stick position.
The idea now is to match the power band to the correct pitch, so at 11°, we need the maximum motor rpm, again we are in normal mode, so we need a curve with a high power band. Rising fast, and then levelling out, to provide the maximum power as loads are applied to the blades, and drag becomes a factor.
Ill give a "text book " example here, what we are looking for is for the throttle to ramp up sharply then flatten out. So this would be P1=0%, P2=50%, P3=80%, P4=90% and P5=100%. Remember as the throttle stick moves we are going through two curves, pitch and throttle. Both have an effect on lift, but our pitch values are fixed, so the throttle values are the ones that need "tweaking" in order to maintain the correct headspeed of the helicopter.

Throttle Position Example Normal Throttle Values
P1 0
P2 50
P3 80
P4 90
P5 100


Idle Up Throttle and Pitch Curves


So, now for the scary bit (although, it really isn't that scary!) idle up! So, eventually you will want to fly upside down. If you try that in normal mode you will have an expensive repair bill, and dented pride. So we need to adjust our pitch and throttle curves to take this into account, remember, we set the helicopter up mechanically to be able to fly upside down, but we did not allow for this in our normal flight mode. Luckily, modern computer radios allow you to have a second, and even sometimes a third set of curves, that you can swap to at the flick of a switch.

Idle Up: Pitch Curve
  So what we are looking for is the same amount of pitch but negative in the bottom half of the throttles travel as we have in the upper half for positive pitch, so lets say ±11°. This is simply our original mechanical linear pitch curve, this will allow for an equal amount of positive and negative pitch at the extremes of the stick movement. Set the idle up pitch curve to these numbers.

Throttle Position Example Idle Up Pitch Values
P1 0
P2 25
P3 50
P4 75
P5 100
Idle Up: Throttle Curves
Idle Up Pitch Curve  So as I said, we
need/want to fly upside down, and as pulling back on the throttle is now negative pitch, a linear curve on throttle would give zero throttle. We most definitely don't want this to be happening, in fact we want an increase in the motor to pull out from our manoeuvres as we press on through increasing negative pitch values.
This equates to a throttle curve looking like a "V" as we need the correct power from the engine at the appropriate pitch value. At mid stick, P3, we do not want 100% throttle, as there will be no load on the blades at zero degrees pitch, so we reduce the throttle here to stop the headspeed increasing like a banshee, we apply a linear increase either side of this, with P2 and P4 to match a linear curve. Similar to the normal mode, these values may need "tweaking" in order to get the correct head speed. We could also use a "U" shape, but the same theory applies and for this example we will concentrate on the linear increase either side of centre stick.
With these settings we will get an increase in rpm when the idle up switch is engaged, this is normal, and also the way it is normally kept. If this is unnerving, the throttle curves can be adjusted so at the ¾ stick position of the hover, when we engage idle up, they match to stop the slight and sudden jump in the helicopter.

Throttle Position Example Idle Up Pitch Values
P1 100
P2 95
P3 90
P4 95
P5 100
  So if you have followed the mechanical setup, radio setup, blade balancing, and blade tracking guides this should be the final step in the setup of the helicopters rotor head. The above examples are suitable for a wide range of RC helicopters, some may differ slightly, if in doubt, always refer to the manual, or a reputable on-line forum. You will find the members of this community more than willing to help you. Thanks for reading.


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