Flight Principles of Powered Parachutes Light Sport Aircraft

Here are the Basic Flight Principles of a Powered Parachute Light Sport Aircraft.

The ram air chute wing retains its rigid shape during flight due to air pressurization, just as a conventional aircraft's wing is rigid due to internal structure. Both crafts wings have a top skin, a bottom skin, a leading edge and a trailing edge. Both have curved upper surfaces and relatively flat lower surfaces. The only difference is the fabric construction and the cell openings in the leading edge. The chute is made of zero or low porosity material which prevents air escaping. Once air flows in, it has no means of escape except back through the leading edge. In flight, outside air cannot enter the pressurized wing and is forced to flow around the leading edge. This results in the prevention of air escaping from the wing, and in the formation of an aerodynamically correct wing.

 

 

Lift, Weight, Thrust, Drag

In straight and level, un-accelerated flight...Lift equals Weight and Thrust equals Drag. When the wing, with its curved upper surface and flat lower surface, moves through the air, it forces the air to flow around it. Air flowing over the curved upper surface has to travel further, and therefore faster, than air going under the wing. Air traveling at a higher velocity creates a lower pressure than air traveling at lower velocity. This higher pressure air traveling beneath the wing produces an upward force called lift. Weight is produced by gravity which is equal to the weight of pilot and aircraft. When lift and thrust exceeds weight and drag, the craft will climb. When weight and drag exceeds lift and thrust, the craft will descend. Thrust is produced by the propeller driven by the engine. Drag is produced by air resistance as the craft moves through the air. Resistance is produced by the chute, the lines, the craft and pilot.

 

Throttle Control

The throttle controls your climbs and descents in powered parachutes rather than your speed. It does this by changing the angle of attack of the wing or that is, the angle at which the wing meets the air. When the throttle is pushed forward to increase power, the thrust of the prop moves the craft forward. This changes the angle of attack of the wing, resulting in a climb. Maintaining increased thrust will maintain the climb. When the throttle is pulled back to decrease power, the craft moves back under the wing. This changes the angle of attack of the wing, resulting in a descent. During normal climbs, cruise and descents, the wing automatically adjusts to the proper angle of attack. The rate of climb is controlled by the throttle setting. Each pilot must experiment with the throttle setting required for climb and descent given the pilot weight and the atmospheric conditions.

 

Pendulum Stability

The key to the powered parachutes safety and ease of operation lies in its pendulum stability. This principle allows powered parachutes to fly straight and level all by itself. Consider a pendulum which consists of a suspension point with a weight attached to the other end. With a powered parachute, the wing acts as the suspension point, the craft and pilot are the weight. When the weight of a pendulum is moved out from under its suspension point and released, it will immediately swing to get back to its original stable position. An object is considered stable when it has a natural tendency to return to its original position after being disturbed by an outside force. A stable aircraft will return to level flight whenever the controls are at cruise. Likewise, whenever a force such as a wind gust moves the powered parachute, the suspended weight of the pilot and craft will always return to its natural stable position. With such excellent stability, the powered parachute almost flies itself. Due to this feature, pilots should be aware that there is almost constant gentle motion in a powered parachute. While most noticeable during the first flight, the gentle movement soon becomes quite natural to the more experienced pilot.

 

Effects of Wind Gusts on Pendulum Stability

A wind gust will cause the larger and lighter wing of the powered parachute to move first, displacing the suspension point of the powered parachute. Its pendulum stability will cause the smaller and heavier craft to swing back into its normal position directly below the wing. A side gust will move the wing to the side first. Then the craft will swing back under the wing, resulting in a flight path slightly to the side of the original. A gust from the front will move the wing backward, increasing the angle, of attack. The powered parachute begins a climb. Then the craft will swing back under the wing, reducing the angle of attack. The powered parachute begins to descend. When the craft centers itself under the wing again, level flight will continue. A gust from the rear will move the wing forward, decreasing the angle of attack. The powered parachute begins to descend. Then the craft will swing back under the wing, increasing the angle of attack. The powered parachute will climb. When the craft centers itself again, level flight will continue.

 

Axis of Movement. Yaw, Pitch and Roll

The aircraft pivots about three axis' of movement. The axis' are: yaw (movement around the vertical axis), pitch (movement around the lateral axis) and roll (movement around the longitudinal axis). Conventional aircraft generally control yaw through use of rudders, pitch through elevators and or canards, and roll through ailerons. Learning control of these systems increases the complexity of flying conventional aircraft. The unique pendulum effect of the powered parachute, controls movement through the lateral and longitudinal or pitch and roll axis'. The pilot need only control the vertical or yaw axis. To turn the craft the pilot pushes either rudder pedal. This introduces the yaw, or turn. The craft's pendulum stability automatically controls the roll, or banking effect. Pitch movements introduced by wind gusts are controlled by the crafts self-stabilizing pendulum design. Pitch movements for climbs or descents are controlled by the throttle control. The very simple control of movement about these three axis' make the powered parachute extremely easy to fly.

 

Steering Controls

Ground steering is accomplished by pushing the control stick right to go right or left to go left. If you properly align the craft prior to takeoff or landings (into the wind) the control stick is used very little. The pilot controls in flight steering with the two rudder pedal tubes. The steering control lines from the parachute run through a pulley on the outrigger, down to a tang and quick link on the rudder tubes. When the pilot pushes either rudder for a turn, the steering control lines pull down that side of the parachute to which it is attached. Push left to turn left, right to turn right. The drag created by the lowered trailing edge slows that side of the chute, causing the other side to fly faster creating a turn. The more the trailing edge is deflected, the faster the turn, the shorter the radius and the more altitude lost. Whenever drag is increased there is a loss of lift. This altitude loss, which can be 30 feet or more in a very tight turn, can be minimized or eliminated by simply adding throttle. Lost altitude in a steep turn, coupled with gusting winds and no additional throttle can result in a potentially unsafe situation if done too close to the ground. The rudder pedals have another use not related to turning the powered parachute. This use is to assist in opening closed end cells on takeoff. On initial roll-out, it is not uncommon for one or more end cells to be closed due to outside pressure on the canopy surface. To the pilot, it will appear the canopy is folded down over the cell openings. To open these cells, once the chute is overhead, the pilot pumps the rudder pedals forward and releases them quickly. Do not attempt a takeoff with closed end cells. Abort the takeoff and try again. End cell closures will eventually correct themselves as air being forced into the open leading edge will push through the cross ports within the chute to fully inflate the chute. Pumping the rudder tubes forces air forward in the outer cells, opening them and allowing air to come in for full inflation.

 

The Effects of Wind Direction

When the craft is on the ground the parachute will seek to face directly into the wind. This tendency is called weather vaning and the parachute will try to pull the craft into the same direction. A difference between the wind direction and the takeoff direction will not allow the parachute to inflate properly and could conceivably cause the chute to pull the craft over. You must know and understand the wind conditions and characteristics of your flying site and always line the craft into the wind. The rudder pedals have another use. As you start your forward ground roll the chute may fall to one side or the other. Push the rudder pedal on the opposite side. This will create drag on the opposite side of the chute that is down, thus pulling it back to center. Once centered, push both rudder pedals and release them quickly. The chute should "pop" overhead and you can add power for immediate takeoff.

 

Air Speed and Ground Speed

Non pilots are often confused by the terms "air speed and ground speed". Think of air speed as simply a performance statistic of the craft. Powered parachutes fly through the air at 26 mph, regardless of whether the throttle is at full setting or cruise. Powered parachutes do not increase in air speed with an increase in power because as power is increased the angle of attack is also increased, which produces more lift and drag coinciding with the increase in thrust. Therefore, the powered parachute climbs but does not fly at a greater air speed. The air speed must be used in conjunction with the wind speed in order to compute the ground speed. Ground speed is the speed which the craft is actually moving over the ground. It varies with the wind speed and the direction the powered parachute is facing in respect to the wind. Going into a 10 mph head wind produces a ground speed of 26 mph minus 10 mph or 16 mph ground speed. A 10 mph tail wind produces ground speed of 26 mph plus 10 mph or 36 mph ground speed. This must be considered when flying cross country. For instance, you cannot fly to a given point with a tail wind using half your gas supply and expect to get back on the other half facing into a head wind.

 

Engine Out

©Copyright © 2001 Freedom Flight Inc. All Rights Reserved. Portions Copied with permission. Expiring 1-2009
Email link