|
Chapter 7 — Takeoffs and Departure Climbs
Runway Surface and Gradient
Runway conditions affect takeoff performance. Typically,
powered parachutes take off from level grassy
surfaces. However, runway surfaces vary widely from
one airport to another. The runway surface for a specific
airport is noted in the Airport/Facility Directory
(A/FD). Any surface that is not hard and smooth will
increase the ground roll during takeoff. This is due
to the inability of the tires to smoothly roll along the
surface. Tires can sink into soft, grassy, or muddy
runways. Holes or other ruts in the surface can be
the cause of poor tire movement along the surface.
Obstructions such as mud, snow, or standing water
reduce the powered parachute’s acceleration down
the runway. Many of these same hindrances are multiplied
in effect by the use of soft or wide tires that
increase resistance themselves.
The gradient or slope of the runway is the amount of
change in runway height over the length of the runway.
The gradient is expressed as a percentage such
as a 3 percent gradient. This means that for every 100
feet of runway length, the runway height changes by
3 feet. A positive gradient indicates that the runway
height increases, and a negative gradient indicates that
the runway decreases in height. An upsloping runway
impedes acceleration and results in a longer ground
run during takeoff. A downsloping runway aids in acceleration
on takeoff resulting in shorter takeoff distances.
Runway slope information is contained in the
Airport/Facility Directory.
Takeoff Performance
Takeoff performance is partly a condition of accelerated
motion. For instance, during takeoff, the powered
parachute starts at zero speed and accelerates to
inflate the wing, then to takeoff speed and becomes
airborne. The important factors of takeoff performance
are as follows:
• The takeoff speed.
• The rate of acceleration during the takeoff roll.
• The takeoff roll distance is a function of both
acceleration and speed.
The minimum takeoff distance is of primary interest
in the operation of any powered parachute because
it defines the runway requirements. The minimum
takeoff distance is obtained by taking off on a length
of runway that allows sufficient margin to inflate the
wing, perform the LOC procedure, and then satisfactory
room to initiate a lift-off and climb.
The powerplant thrust is the principal force providing
the acceleration and — for minimum takeoff distance
— the output thrust should be at the maximum after
the wing is inflated and successful LOC procedure preformed.
Use smooth, gradual throttle settings to avoid
porpoising. Drag is produced as soon as the powered
parachute moves forward. The drag of the wing decreases
as it rotates into position over the cart.
In addition to the important factors of proper procedures,
many other variables affect the takeoff performance
of a powered parachute. Any item that alters
the takeoff speed or acceleration rate during the takeoff
roll will affect the takeoff distance.
The most important variable to affect the takeoff
performance is how fast the pilot can get the wing
overhead, centered, and ready to take the load of the
cart. Often, most of the runway used will be for the
inflation and wing LOC procedure. Unlike almost any
other type of flight, a powered parachute pilot has to
create the airfoil and clear it on the ground before liftoff.
It is always best to practice this skill at a longer
field where mistakes can be made and corrected in
plenty of time before taking off.
Even a slight headwind will have a dramatic effect
on takeoff distances for powered parachutes because
a wind helps inflate a wing much faster than can be
done on a calm day. Even light winds can be a large
percentage of the flying speed of a powered parachute.
A powered parachute that flies at 35 mph taking off
into a headwind of only 3.5 mph is working with a 10
percent headwind. A headwind that is 10 percent of
the takeoff airspeed will reduce the takeoff distance
approximately 19 percent. In the case where the headwind
is 50 percent of the takeoff speed (a brisk 17.5
mph), the takeoff distance would be approximately 25
percent of the zero wind takeoff distance (75 percent
reduction).
Gross weight also has an effect on takeoff distance.
Proper consideration of this item must be made in
predicting the powered parachute’s takeoff distance.
Increased gross weight can be considered to produce
a threefold effect on takeoff performance:
1. Higher lift-off speed,
2. Greater mass to accelerate, and
3. Increased retarding force (drag and ground
friction).
If the gross weight increases, a greater speed is required
to produce the greater lift necessary to get the
powered parachute airborne at the takeoff lift coefficient.
As an example of the effect of a change in
gross weight for a typical PPC, a 21 percent increase
in takeoff weight will require a 10 percent increase in
lift-off speed to support the greater weight.
A change in gross weight will change the net accelerating
force and the mass that is being accelerated.
The takeoff distance will vary at least as the square
of the gross weight. Adding a 200-pound passenger
to a machine that already weighs 400 pounds, with
a pilot weighing 200 pounds, will increase the gross
weight by 33 percent. That increase of one passenger
will degrade the performance of the powered parachute
dramatically. The 33 percent increase in takeoff
gross weight would cause:
• At least a 25 percent decrease in rate of
acceleration, and
• At least a 76 percent increase in takeoff
distance.
For the powered parachute with a high thrust-toweight
ratio, the increase in takeoff distance might be
approximately 76 percent, but for the powered parachute
with a relatively low thrust-to-weight ratio, the
increase in takeoff distance would be more. Such a
powerful effect requires proper consideration of gross
weight in predicting takeoff distance.
The effect of pressure altitude and ambient temperature
is to define primarily the density altitude and its
effect on takeoff performance. While subsequent corrections
are appropriate for the effect of temperature
on certain items of powerplant performance, density
altitude defines specific effects on takeoff performance.
An increase in density altitude can produce a
fourfold effect on takeoff performance:
1. Greater takeoff speed.
2. Decreased thrust and reduced net accelerating
force.
3. Reduced rate of climb.
4. Increased runway required.
If a powered parachute of given weight and configuration
is operated at greater heights above standard sea
level, it will still require the same dynamic pressure
to become airborne. Thus, the powered parachute at altitude will take off at the same indicated airspeed as
at sea level, but because of the reduced air density, the
true airspeed will be greater.
Proper accounting of pressure altitude (field elevation
is a poor substitute) and temperature is mandatory for
accurate calculation of takeoff roll distance.
The most critical conditions of takeoff performance
are the result of some combination of high gross
weight, altitude, temperature, and unfavorable wind.
In all cases, the pilot must make an accurate calculation
of takeoff distance from the performance data
of the AFM/POH, regardless of the runway available,
and strive for a polished, professional takeoff
procedure. In the calculation of takeoff distance from
the AFM/POH data, the following primary considerations
must be given:
• Pressure altitude and temperature — to define
the effect of density altitude on distance.
• Gross weight — a large effect on distance.
• Wind — a large effect on wing inflation and
overall distance.
• Runway slope and condition — the effect of an
incline and the retarding effect of factors such
as snow, ice, or uncut grass.
|
