07-02-2011, 07:40 PM
Given there were lots of questions on heli set-ups and that our club is electric only, I though I would post some excellent information for the heli guys on setting up throttle and pitch curves and the theory behind it.
The author is "Martin" from SSSFA who is widely acknolwedged as one of the most experienced electric heli pilots around.The original post came from www.rcrotorz.com.au which is a broad based heli forum. Martin has many technical posts on this forum and I would encourage heli pilots to take a look at his contributions.
http://www.rcrotorz.com.au/showthread.ph...ric-Heli-s
Throttle Curves for Electric Heli's
Electric motors work differently to nitro motors,
as such we use very different throttle curves
and slightly different operating procedures
What is different?
Nitro engines will over rev at high throttle settings and under no load
Electric motors don't do this - their speed is always exactly relative to the voltage they see
(Your TX throttle curve and the ESC control the voltage delivered to the motor)
Nitro engines will decrease in RPM when a load is applied
Electric motors don't do this, they just draw more current
(When a lot of current is drawn the battery voltage will drop a little, so will the RPM)
These two nitro characteristics are why we need a throttle curve (and ideally a governor),
to try and keep the headspeed constant
These two electric characteristics explain why we can use a flat throttle curve to get a constant headspeed
and even run an electric motor on a 100% flat throttle curve with no risk of over speeding
(A flat throttle curve means same throttle position across the entire curve, e.g. 100%, 100%, 100%, 100%, 100%)
Why is a constant headspeed an important goal?
Because the collective (lift), aileron and elevator all respond differently with respect to RPM
At low RPM they are weak, slow and mushy
At high RPM they are strong, quick and crisp
When learning to fly you notice this most in the collective
Your collective reflexes are not yet developed
You take off and find that the heli rises quickly at high collective stick positions
But when you drop the collective stick, the heli comes down very quickly indeed,
yet the heli is slow to arrest this decent if you let the headspeed drop
This is even worse on a windy day
Wind gives lift, a gust of wind and the heli climbs suddenly [uncommanded]
You need to use much more negative collective that you would normally to descend,
and if you're using a throttle curve, this will slow the headspeed badly
The combination of a changing headspeed and changing blade pitch
creates a lot of possible reaction speeds that you need to contend with - it's hard work
Even expert pilots are seeking a constant headspeed
They way they quickly trash around the sky,
they are expecting the heli to respond in a certain manner and speed to each stick input
It's a lot harder to time these moves correctly if the headspeed and reactions are changing a lot
(and when you're doing this on the deck, timing and response are vital)
Modern ESC's
These days, most ESC's have a soft start facility, it's vital
Electric motors can change speeds very quickly indeed
Without a soft start we risk stripping the main gear during spool up
(This may be another reason to not use a throttle curve!)
Make sure your ESC is set to heli mode (mainly turns the brake off) and enable the soft start
If you are using a CastleCreations ESC there is a lot of information for you here (PDF's and Videos):
http://www.rcrotorz.com.au/showthrea...ghlight=castle
You need to run the ESC at 75% throttle or higher (Kontronic ESC's can run lower)
At lower throttle settings, the switching components in the ESC get too hot and can burn out
This is a factor we take into account when selecting the number of cells, motor KV and gearing
Electric Throttle Curves
Due to the way electric motors work (constant speed at a steady voltage)
we tend to run flat throttle curves, same value right across the curve.
This delivers a steady voltage to the electric motor and gives us a steady headspeed
So what do we want to get out of our various throttle curves?
The normal mode throttle curve is normally targeted at getting the heli through the spool up procedure
and gives us a usable but calmly behaved heli
When we encounter problems with the heli in the air,
we will either want to take the heat out of the heli and drop back to a calm normal mode
or in more serious situations we will go into hold mode (crashing with power off creates much less damage!)
So for example, normal mode: 0% 75% 75% 75% 75%
The stunt1 or idle1 throttle curve for beginners would typically be the same as normal mode
without the 0% setting that we use when spooling up and down
This enables us to use full negative pitch and still be fully powered
For example, Stunt1: 75% 75% 75% 75% 75%
The stunt2 or idle2 throttle curve may be used to give a quicker headspeed for hot dogging around
For example, Stunt2: 90% 90% 90% 90% 90% (but you can use 100% flat if you want)
The specific setting values will depend on your setup and the headspeeds you get - adjust as desired
Spool up & down procedures
The ESC soft start enables us to use a different spool up procedure,
and supports the use of our flat normal mode throttle curve.
We simply smoothly raise the collective stick to zero degrees (normally middle stick position) in one single motion
The heli will start to spool up slowly thanks to the ESC soft start
Once the headspeed is fully spooled up (~5 seconds) we then raise the collective further and take off
After landing in normal mode you smoothly drop the collective stick to the bottom,
the power to the motor will be cut, the heli will spool down nicely
Also note normal mode pitch suggestion in next section
Pitch Curves
If you read older RC heli guides, they say beginners should learn to hover without any negative pitch
That was fine back in those days, but these days heli's are both lighter and have more lift
I've seen numerous beginners struggling to bring a heli down on a windy day
I tend to start beginners off with these 3D curves (hovering at 75% stick)
(the values are pitch degrees - not TX point values)
Normal: -5 -2.5 0 +5 +10
Stunt1: -5 -2.5 0 +5 +10 (When learning to hover)
Stunt1: -10 -5 0 +5 +10 (When learning basic 3D)
Stunt2: -10 -5 0 +5 +10 (Full linear 3D setup)
Hold: -5 -2.5 0 +5 +10
This enables you to bring the heli down on a windy day
This means when you spool down in normal mode,
the heli does not push hard into the ground
(-10 degrees is a lot - it can squash skids on small helis, you'll snag the tail)
The hold curve is designed to avoid you grabbing too much negative
Just aim for 1/4 stick and guide it in
Normal, Stunt1 and hold all feel the same (especially when you loose power!)
When you're much more advanced you will change these - but that's another story/article
Caution
When running a flat 100% throttle curve,
check your headspeed to ensure it's safe!
Generally smaller heli's can use faster headspeeds
due to shorter blades and associated forces
Here are some rough guidelines for beginners:
325mm blades ~ 2750 rpm
425mm blades ~ 2600 rpm
500mm blades ~ ???? rpm
550mm blades ~ 2100 rpm
600mm blades ~ 2000 rpm
700mm blades ~ 1900 rpm
The author is "Martin" from SSSFA who is widely acknolwedged as one of the most experienced electric heli pilots around.The original post came from www.rcrotorz.com.au which is a broad based heli forum. Martin has many technical posts on this forum and I would encourage heli pilots to take a look at his contributions.
http://www.rcrotorz.com.au/showthread.ph...ric-Heli-s
Throttle Curves for Electric Heli's
Electric motors work differently to nitro motors,
as such we use very different throttle curves
and slightly different operating procedures
What is different?
Nitro engines will over rev at high throttle settings and under no load
Electric motors don't do this - their speed is always exactly relative to the voltage they see
(Your TX throttle curve and the ESC control the voltage delivered to the motor)
Nitro engines will decrease in RPM when a load is applied
Electric motors don't do this, they just draw more current
(When a lot of current is drawn the battery voltage will drop a little, so will the RPM)
These two nitro characteristics are why we need a throttle curve (and ideally a governor),
to try and keep the headspeed constant
These two electric characteristics explain why we can use a flat throttle curve to get a constant headspeed
and even run an electric motor on a 100% flat throttle curve with no risk of over speeding
(A flat throttle curve means same throttle position across the entire curve, e.g. 100%, 100%, 100%, 100%, 100%)
Why is a constant headspeed an important goal?
Because the collective (lift), aileron and elevator all respond differently with respect to RPM
At low RPM they are weak, slow and mushy
At high RPM they are strong, quick and crisp
When learning to fly you notice this most in the collective
Your collective reflexes are not yet developed
You take off and find that the heli rises quickly at high collective stick positions
But when you drop the collective stick, the heli comes down very quickly indeed,
yet the heli is slow to arrest this decent if you let the headspeed drop
This is even worse on a windy day
Wind gives lift, a gust of wind and the heli climbs suddenly [uncommanded]
You need to use much more negative collective that you would normally to descend,
and if you're using a throttle curve, this will slow the headspeed badly
The combination of a changing headspeed and changing blade pitch
creates a lot of possible reaction speeds that you need to contend with - it's hard work
Even expert pilots are seeking a constant headspeed
They way they quickly trash around the sky,
they are expecting the heli to respond in a certain manner and speed to each stick input
It's a lot harder to time these moves correctly if the headspeed and reactions are changing a lot
(and when you're doing this on the deck, timing and response are vital)
Modern ESC's
These days, most ESC's have a soft start facility, it's vital
Electric motors can change speeds very quickly indeed
Without a soft start we risk stripping the main gear during spool up
(This may be another reason to not use a throttle curve!)
Make sure your ESC is set to heli mode (mainly turns the brake off) and enable the soft start
If you are using a CastleCreations ESC there is a lot of information for you here (PDF's and Videos):
http://www.rcrotorz.com.au/showthrea...ghlight=castle
You need to run the ESC at 75% throttle or higher (Kontronic ESC's can run lower)
At lower throttle settings, the switching components in the ESC get too hot and can burn out
This is a factor we take into account when selecting the number of cells, motor KV and gearing
Electric Throttle Curves
Due to the way electric motors work (constant speed at a steady voltage)
we tend to run flat throttle curves, same value right across the curve.
This delivers a steady voltage to the electric motor and gives us a steady headspeed
So what do we want to get out of our various throttle curves?
The normal mode throttle curve is normally targeted at getting the heli through the spool up procedure
and gives us a usable but calmly behaved heli
When we encounter problems with the heli in the air,
we will either want to take the heat out of the heli and drop back to a calm normal mode
or in more serious situations we will go into hold mode (crashing with power off creates much less damage!)
So for example, normal mode: 0% 75% 75% 75% 75%
The stunt1 or idle1 throttle curve for beginners would typically be the same as normal mode
without the 0% setting that we use when spooling up and down
This enables us to use full negative pitch and still be fully powered
For example, Stunt1: 75% 75% 75% 75% 75%
The stunt2 or idle2 throttle curve may be used to give a quicker headspeed for hot dogging around
For example, Stunt2: 90% 90% 90% 90% 90% (but you can use 100% flat if you want)
The specific setting values will depend on your setup and the headspeeds you get - adjust as desired
Spool up & down procedures
The ESC soft start enables us to use a different spool up procedure,
and supports the use of our flat normal mode throttle curve.
We simply smoothly raise the collective stick to zero degrees (normally middle stick position) in one single motion
The heli will start to spool up slowly thanks to the ESC soft start
Once the headspeed is fully spooled up (~5 seconds) we then raise the collective further and take off
After landing in normal mode you smoothly drop the collective stick to the bottom,
the power to the motor will be cut, the heli will spool down nicely
Also note normal mode pitch suggestion in next section
Pitch Curves
If you read older RC heli guides, they say beginners should learn to hover without any negative pitch
That was fine back in those days, but these days heli's are both lighter and have more lift
I've seen numerous beginners struggling to bring a heli down on a windy day
I tend to start beginners off with these 3D curves (hovering at 75% stick)
(the values are pitch degrees - not TX point values)
Normal: -5 -2.5 0 +5 +10
Stunt1: -5 -2.5 0 +5 +10 (When learning to hover)
Stunt1: -10 -5 0 +5 +10 (When learning basic 3D)
Stunt2: -10 -5 0 +5 +10 (Full linear 3D setup)
Hold: -5 -2.5 0 +5 +10
This enables you to bring the heli down on a windy day
This means when you spool down in normal mode,
the heli does not push hard into the ground
(-10 degrees is a lot - it can squash skids on small helis, you'll snag the tail)
The hold curve is designed to avoid you grabbing too much negative
Just aim for 1/4 stick and guide it in
Normal, Stunt1 and hold all feel the same (especially when you loose power!)
When you're much more advanced you will change these - but that's another story/article
Caution
When running a flat 100% throttle curve,
check your headspeed to ensure it's safe!
Generally smaller heli's can use faster headspeeds
due to shorter blades and associated forces
Here are some rough guidelines for beginners:
325mm blades ~ 2750 rpm
425mm blades ~ 2600 rpm
500mm blades ~ ???? rpm
550mm blades ~ 2100 rpm
600mm blades ~ 2000 rpm
700mm blades ~ 1900 rpm
