I would like to discuss this issue of Torque Twist on paramotors that I hear so much talk about.
In an ideal world, there should NEVER be any Torque Twist seen on any paramotor, yet everyone talks about it... I have a feeling that most pilots completely misunderstand it, so let́s try to clear the matter up.
Before you read further, you might wish to first brush up on your understanding of the various forces on paramotors, specifically Gyroscopic Precession, Assymetric Blade Thrust and the Torque Effect, by reading this article:
http://xplorer.co.za/articles/prop fx.htm
OK, let us proceed...
What we expect from our paramotors when we squeeze the accelerator, is Thrust!
However, paramotors create a number of other forces at the same time, of which
some of these are rather undesirable and must be managed, either in the design
of the paramotor, and/or by the pilot in flight.
The most common and definitely the MOST PERSISTENT undesirable force is the
Torque Effect, which everyone knows about.
The MOST UNDESIRABLE of all these forces, is Gyroscopic Precession, which
causes many incidents and accidents (and has killed a number of PPG pilots).
Gyroscopic Precession is mostly likely to cause the paramotor to Yaw to one
side (twisting the risers), pushing the pilot to that side, causing the wing to
dive off to the other side. The wing looks like it is experiencing "lock out"
on a winch launch when the pilot does not keep the wing facing the direction of
the tow ropés pull.
Probably 99% of times when a paramotor "twists" under his wing in the Yaw axis,
this is caused by Gyroscopic Precession.
So why do pilots keep talking about Torque Twist, when it accounts for perhaps
1% of such "twisting" under the wing?
Torque Twist will NEVER happen on a properly designed paramotor.
I repeat, NEVER!
Read the previous paragraph again... Make sure you understand, that Torque Twist
will absolutely NEVER happen on a properly designed paramotor!
So when you see a pilot and paramotor twisting to one side (in the Yaw axis),
usually immediately after takeoff, it is highly unlikely to be caused by Torque
Twist, especially as this is the moment most likely to deliver maximum
Gyroscopic Precession, caused by three things that often happen almost at this
moment, namely:
1) The pilot has just gone from idle, to full power in order to launch and,
2) the pilot tends to apply brakes to increase his climb angle, plus...
3) the pilot is likely to be transitting from a standing/running position to a
seated position in his harness.
All three of these actions tend to attempt to tilt the motor backwards, which
will cause Gyroscopic Precession which could cause problems, especially on a
belt drive (and very seldom, perhaps never on a gearbox model).
On a belt drive, the pilot and motor will yaw to the Left, while the wing dives
off to the right. If the motor twists more than 90 degrees, then the pilot is
in serious trouble. At 180 degrees, the thrust is no longer pushing the wing
forward, but acting as a brake, resulting in drastic reduction in airspeed,
causing sudden and dramatic altitude loss, usually leading to impact with the
ground. Often this is a fall from 20 feet or less, so does not usually lead to
serious injury (but is likely to destroy the propeller, frame and other engine
components if the pilot crashes at full power which is often the case).
On a gearbox model, the gyroscopic forces of the propeller and large gear act
in the exact opposite direction as those from the crankshaft, flywheel and
small gear. The net result will be the resultant of those two forces, so the
motor might yaw either to the Left or to the Right, depending on which of the
two groups has the more powerful Gyroscopic forces. A well designed paramotor
might have Zero resultant Gyroscopic forces (or so little that it becomes
insignificant.)
Though the propeller is turning at much lower RPM than the crankshaft, it has a
far larger diameter. Seeing as the rotational inertia is mass multiplied by
radius, multiplied by RPM, the greater radius of the propeller makes up for its
lower RPM. The mass may change between a lightweight Carbon prop and a heavier
wood prop, so be aware that changing from one prop type to another, or changing
propeller diameter will inevitably change the resultant force.
OK, so where does Torque Twist come in?
There are two causes of torque twist:
1) The Torque Effect causes the paramotor to roll over slightly to one side on
a pilot́s back. Clockwise on a belt drive, and anti clockwise on a gearbox
drive.
On a poorly designed paramotor, the paramotor might not roll around the same
point as the center of the propeller, which shifts the center of thrust to the
side of the pilot́s spine, pushing more on one side, causing the pilot to
"twist" beneath his wing. This can happen just after takeoff, or any time the
pilot opens the throttle wide.
Let me re iterate... this should NEVER happen on a properly designed paramotor.
The center of thrust should remain as close to the pilot́s spine as possible.
Now for another problem... what if the manufacturer has deliberately moved the
entire engine slightly to one side of the frame, to shift its weight to
counteract Torque effect? I am sure you can see what other problems this will
cause...
2) The second potential cause of Torque Twist, happens only when the paramotor
is tilted back with respect to the vertical risers in flight. As the Torque
Effect causes the motor to roll over to one side, if the paramotor is not
perfectly vertical, then the torque effect will have a vertical and a
horizontal component. The horizontal component will cause the paramotor (and
pilot) to yaw or "twist" around the risers.
HOWEVER, a small amount may actually be desirable and often done deliberately...
Here is why. Let́s take a practical example, of a belt drive with the prop
spinning anti clockwise (as viewed from behind).
The Torque Effect causes the motor to roll over to the right, causing the wing
to steer to the right. By tilting the motor back slightly, we induce another of
the usually undesirable effects, in this case, Assymetric Blade Thrust (this is
not the place to describe what it is or how it is formed, so go read the above
linked article if you need to).
On this belt drive paramotor, the Assymetric Blade Thrust causes the paramotor
to Yaw to the Right, inducing a left hand bank on the wing, causing the wing to
steer to the Left.
By adjusting the tilt angle of the paramotor, one can balance these two effects
until the resultant is close to zero bank on the wing and therefore no tendency
to steer to Left or Right.
However, it is a bit more comfortable for the pilot in flight to tilt the motor
further back than that perfect setting, as well as improve the climb rate by
thrust vectoring (thrust deflected slightly downwards).
Fortunately the motor yaw action of the Assymetric Blade Thrust, and that of
the Torque Twist caused by tilting the motor backwards, act in opposite
directions and are not linear to the same scale, so on many paramotor designs
(most notably gearbox drive models), once can have the motor tilted backwards
as much as 15 to 20 degrees in great safety. On most belt drives, that would be
considered rather excessive and downright dangerous.
This is what often sets the better designed paramotors apart from the lesser
models. Safety and Comfort in a well balanced design where these undesirable
forces are balanced and managed properly in the geometric design of the
paramotor.
Like everything else in life, there are better versions and worse versions.
Caveat Emptor... Let the buyer beware.
Make sure you understand these issues and make an educated, intelligent
purchase decision and that you set your paramotor up correctly according to the
manufactureŕs guidelines.
If you require further clarity on this topic, feel free to respond via email.
all the best,
================================
Keith Pickersgill. Cape Town, South Africa
