My third platform type also implements heave function; so
now becomes a 3 degree of freedom platform with pitch, roll and heave.
A complete overview of the motion cockpit with all features is show
All the small additions help to make the simulation as real as possible.
Above setup was shown at FS Weekend 2008 in Lelystad Airport in the
Netherlands, Nov 1&2 2008.
http://www.fsweekend.com/ . The
FSweekend page describes the demo's I gave.
The basic idea shown below was already described in
Motion Systems page.
The system needs three actuators, and together they will need to carry
the weight of the platform.
To reduce the power in the servo motors, the actuator was build in
such a way that most of the static platform lifting force is accomplished
via bungee cords. Weight balance is absolutely key in achieving heave with
relatively low power requirements. The drawing of one actuator is shown below.
The bungee cords need to be long to keep the pulling force equal over the
actuator travel. For details see
Mechanical components III
Platform basic setup:
The dimensions are such that there is sufficient space to tilt the Simpit in
all directions. Heave range is about 40cm.
A later addition was the
stabilizing rod in the center. Without that, the platform would make
swinging motion after sudden pitch or roll motion, which was annoying and
Note that the 25x25mm square metal used in some older pictures is not up
to the job: The extreme forces during heave will bend the rear bar (see
below). A new supporting frame was made from rack-shelve metal.
Since the 3 actuators are not in line anymore, they need to be build with
more side-support. I have also increased the total height of the actuator,
in order to increase the amount of travel. The V-belt is 68 inch outer
diameter. The metal bars are connected to the V-belt with cable clamps.
The 3 actuators are placed in triangle. The orientation is such that there
is space for the bungee cord of each actuator. In the middle, the
stabilizing rod can be seen, that protrudes through the bottom-plate of the
The position pickup of each
actuator is via bottom guiding wheel, that is gear coupled to the 10 turn potmeter.
Update: The original position pickup via gear on the bottom pulley has a
drawback: It has some slip; After a couple of flights, the mid position will
shifted. An improved position pick-up makes use of rope coupled to
spring-loaded gear drive to potmeter. See:
Improved position pick-up.
Because the platform is hanging above ground via 3 V-belts, there is some
room for movement in horizontal directions. During tests, it was found that
the platform will swing back and forth after sudden pitch and roll
excursions, which can be annoying. I therefore added a mechanical
stabilizing structure, that will dampen this rocking motion.
The stabilizing structure consists of a 30mm diameter metal pipe, that is
welded onto a 5mm metal plate.
This metal pipe then protrudes through the floor of the simpit.
Since the simpit floor moves up and down and also tilts with roll and pitch,
I build a ball joint that can slide over the metal pipe. The ball joint is
not completely fixed to the simpit floor, but attached via a wooden
plate that can slide in all horizontal directions. Without this freedom of
horizontal movement, the simpit pitch, roll and heave movements feel too
stiff and unnatural.
The ball-joint consists of a wooden sphere, that is mounted in a tight
fitting enclosure. To make the enclosure match the sphere, I applied epoxy
putty between wooden enclosure and sphere.
I used petroleum jelly between the plate and the simpit floor to create
friction. In this way, there is sufficient horizontal movement, but
the swinging motion is completely damped out.
The metal pipe will protrude through the simpit, but it will be in between
your legs, and will just not touch the switch console.
Mounted the simpit on the structure: There is some bending and squaking, but
The pitch-up and pitch-down angles are much bigger than platform I & II. The
ceiling will limit total heave to 2/3 of max travel. Need to be limited via
Also measured the weight pressure on each actuator: The rear actuators need
45kg of balance weight, (~ 8 strings of bungee cord), the front needs
40kg (~7 strings of bungee cord, later changed to 8 strings as well)
The bungee cord mounting is shown above: 8 strings for the rear actuators,
and 8 strings for front (I found out by testing). With actuators in
top position, the bungee is slightly stretched. This can be adjusted by
choosing another hole in shelf-metal.
Stretching the bungee cord can be difficult, as you need to pull around
45kg. I made a stretch-tool that makes use of a lever and cable tightener to
hook and unhook the bungee cord holder.
The other side of the cable is fastened to the V-belt. The cable loop passes
underneath alongside the V-belt. Grooves are added to the pulley to keep the
cable away form the V-belt.
Getting the simpit on the platform is easy: move the metal support beams
downward and block the actuator drive with a screwdriver or something. Then
the simpit can be slid onto the support beams.
Without pilot, the front actuator already gets depressed due to the simpit
With pilot, all actuators get depressed. Although on the picture they seem
to go down all the way, but overall there is relatively good balance. This
needs to be tuned to the pilot's weight. I found that without good balance,
the servo system consumes a lot of steady state power, but after
re-balancing, average power is around 60W, with heave peaks to 1500W.
First flight tests have shown that heave is a great motion addition,
although it is very dynamic and requires lots of tweaking to get it right.
See Motion Software III and
Fooling your brain only works when you are sitting in a completely enclosed
cockpit, so I installed the cockpit covering that I also used in the #2
platform. The big fresnel lens will result in a very wide and deep scenery
view, which will help a lot in making the brain 'believe'.
Motion is a whole different experience now! You actually can reduce the
pitch and roll excursions; Small motion cues will be more easily accepted by
the brain when it goes together with a wide visual scenery view without
other external clues.
One problem: With the cover installed, the rig will easily hit the low
ceiling. Ouch! I have now lowered the pilot seat and cockpit by about
New enclosure construction is shown below.