Cyclic to throttle mixing-
When a pilot performs
a roll or a flip, higher loads are placed on the helicopter’s engine.
During the maneuver the engine will bog down under load and give sluggish
response. To help maintain engine power during demanding maneuvers,
cyclic to throttle mixing is used. This means that when a cyclic
input is made, throttle is also automatically added by the radio.
Depending on your radio, there are certain ways to set up this mix.
If you own a Futaba 9z, the radio already has a built in mixer designed
especially for this purpose. I do not know if the JR 10x also has
it, but I presume it does. For the rest of us mere mortals with 8103’s,
Super8’s, 9C’s, and other lesser radios, manufacturers have elected to
not supply this mix (probably in a feeble attempt to get us to upgrade
to their radios that do). However, we do have two “programmable mixers”
in the radio that we can use as cyclic to throttle mixers.
Aileron to throttle
and elevator to throttle each need their own programmable mix. They
will enable you to specify a percentage mix to the throttle, and usually
25-30% is an average number. This simply means that during a full
stick aileron roll, 25% additional throttle is added to supply additional
power. If both aileron and elevator are input together (under normal
flight conditions this should not happen!), up to 50% extra throttle may
be added. Now, we run into a problem if our throttle is already at
80% and then we attempt to do a full stick roll, adding an additional 25%
throttle. 80% + 25% = 105%. In other words, the radio will
be telling the throttle servo to open the carb to 105%, which is impossible.
If this happens, the carb will reach its limit and stop while the throttle
servo will attempt to push it farther, resulting in servo stall, throttle
arm bending/breaking, and/or other bad things. Imagine if you were
at full throttle and then also tried to do a roll, this would be 125%!
Not good.
The problem here is that
our travel limits, known as ATV or EPA, do not specify the maximum allowable
servo throw. They work as advertised for normal servo operation,
but as soon as a mixer tells the servo to go farther, ATV’s go out the
window and the radio freely exceeds them. Many articles have been
written about this- MHT comes to mind here, so I won’t get into the details.
But because of this problem, we need away to deal with the extra servo
travel. A few methods are used today, one is to electronically max
out the servo so it can go no farther, and the other is to use some type
of device that absorbs the extra travel without anything being damaged.
Method one is done
by first subtrimming the throttle servo in the full throttle direction
as far as the transmitter will allow (120% for Futaba). Make sure
your linkages are disconnected! This uses up most of the extra
servo travel, but not all. Now increase the high throttle ATV just
until the point where the servo stops turning. This is the point
where at 100% throttle, the servo is right at the end of its maximum travel.
Any commands to go farther by the transmitter are disregarded by the servo.
This method works well, but does have some problems. First is that
the high throttle ATV is a large number, probably 115%+. If we wanted
to have a linear throttle response, we would also need to set the bottom
ATV also to 115%. Now the servo is traveling very far, and in order
to get the correct amount of throw at the carb we have to use a very short
arm on the throttle servo. With this arrangement, high ATV’s and
a short arm, the throttle is going to act very sluggish, since the servo
has to turn a long way. Even if we set the bottom ATV to a smaller
number such as 60%, this results in nonlinear servo response and throttle
curves that don’t appear to make any sense. It will work, but I don’t
feel that this is the perfect answer to the problem.
Method 2- Most radios
allow you to turn on and off your programmable mixes at a specific throttle
stick position. You can tell the radio to turn off your p-mixes just
at the throttle point where the mixes would just begin to overdrive your
carb. HOWEVER, only one throttle stick position can be used. If you
set this up to be high throttle stick position, this works well for upright
flight, but what happens if you are doing an inverted climbout at low stick
(high throttle) and then command a roll. The mixers will not turn
off and your servo will still overdrive the carb. Until we can set
two stick positions to turn on and off the cyclic to throttle mix, this
method is not effective.
Method 3 is to use a device
to absorb the extra servo travel. This can be anything such as a
very flexible servo arm, a spring type “Servo saver” as seen on r/c car
steering servos, a spring steel pushrod with a U bent in the center, or
a solder clevis sliding on a pushrod up against a piece of fuel tubing
that will compress when the carb reaches its limit. Not all of these
are pretty, but the last one gets my vote for the most effective and “trick”
looking setup. Using a device like this, the absorbing part of the
pushrod is stiff enough that the carb responds as usual within its operating
range. When the full throttle limit is hit, the device will compress
until a lower throttle setting is commanded. The advantage to using
this setup is that the throttle servo can be set up normally, using linear
ATV’s of the users choice. Preferred throttle setup is another whole
can of worms that I won’t get into, but hopefully you understand why this
method gives the greatest number of options.
I hope this clears up a
thing or two that you may have wondered about cyclic to throttle mixing.
Good luck with whatever method you choose. Please write me an email
if you have some comments or experiences you would like to share.
email me: jbond007 at lycos.com