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Powered Parachutes - Articles - PPC Wing Inflation Basics
by By Betty Pfeiffer

" More is not always better but if you do not have enough you can be in big trouble." BFI tries to explain use of throttle control during inflation.

The purpose of this article is to provide a better understand of the forces at play during wing inflation in hopes that PPC pilots will build a healthy respect for their PPC wings.

The PPC wing is composed of a series of cells that must be filled with air to produce the desired shape of the wing. The air enters the mouth of each chamber at the leading edge (front) of the wing. Most current PPC wings have two chambers that form one cell. The cell is the unit between the lines.

Each cell has an upper surface on top, ribs on each side and a lower surface on the bottom. Each rib has a series of cross-ports (holes) that allow the air to move from one cell to the next inside the wing. Air enters the mouth and fills the wing from the trailing edge (back) forward. As the cells pressurize, the air gives stiffness to the wing.

On a calm day, prop-wash is the primary source of initial air to fill the wing. As the wing lifts it is aided by headwind and/or the forward movement of the cart. If you watch a wing inflate you will immediately notice how the center cells seem to fill up before the end cells. This is directly related to the location and size of the propeller. The thrust angle of the propeller can be viewed as the area of most severe turbulence when the wing rises.

Once all the cells are full of air, no more air can enter the wing. As more air hits the mouth of the pressurized cell it is directed around the outside of the wing where it applies the forces necessary to produce lift. The air inside the wing has little movement once it is pressurized, although the wing is always in a state of flux reacting to the conditions of the air around it.

A gust of wind rebounding off ground, trees, or other obstacles and hitting the outside of the wing during the inflation process can adversely affect your take-off. The air will move along the path of least resistance. This can cause cells to be pushed closed, fold under, or channel the air in unusual directions.

Another way of looking at it is this: If the air outside the wing exerts greater pressure than the air inside the wing, you will see the wing distort, lose shape and lose performance. If this happens on one side of the wing only, you will probably experience the wing turning and need to compensate with your controls.

When you apply brakes you are shortening the cell from the tail. John LeBlanc, of PD, compares it to having a full tube of toothpaste then squeezing the closed end. The toothpaste squirts out the opening just as the air does in your wing. If you apply brakes, in other words shorten the cell before it is inflated, less air will be required to fill the cell up. If your end cells are not getting as much air blown into them as the center cells, applying brakes is a good way to help them inflate.

The simple answer is uneven airflow. If all the cells had the same amount of air pushing into each cell from the same direction we would not see erratic inflation. The prop wash blows air in a cone shape with the strongest air towards the outside of the cone shape. If your wing is perfectly lined up and no other factors are considered, you will see the wing rise in a horse shoe shape followed by erratic movements as the air is distributed to the outboard cells. This is where throttle finesse comes into play.

Throttle finesse refers to how you apply power to inflate the wing. To understand the proper technique for your vehicle, engine, propeller, wing combination it is important to understand the following concepts:

Some pilots apply full throttle trying to force lots of air quickly through relatively small openings at the mouth. Although on the surface this seems like a good idea, consider what is happening to all that air that is not going into the mouth. It is pushing against any part of the wing that happens to be in the way. If it happens to push against the bottom surface close to the trailing edge of the wing and if the air finds the path of least resistance to be to move upward, you may have built a wall that does not allow air to move into the cells. If it happens to blow the lower surface against the upper surface, the air entering the cells will have to fight even harder to push the cell open.

If you apply enough throttle to initiate the inflation of the wing, then back off to allow the cells to continue to full inflation before the cart starts to roll, you will experience a more controlled inflation.

Different combinations of vehicle, engine, propeller, wing, local air conditions, pilot weight, and density altitude may work better with a different approach to throttle control. Always keep this in mind when inflating a new wing or flying a different vehicle.

Listed below are some common inflation problems and some possible solutions. This list is far from complete. Use this chart as a starting point to think about what causes the problem and how to deal with each situation.

Problem	Possible cause	Possible solution
End cells are not inflating. The parachute is going overhead.	The end cells are always the last to inflate. Be sure you have given the parachute enough time to fully inflate. Or Air is pushing the end cells under the wing thus closing off the mouth. Or The air pressure inside the wing is not adequate to vent into the end cells	Hold the parachute centered until the problem corrects itself. Or Try pumping the brakes sharply Or Symmetrically hold brakes thus reducing the size of the chambers and assisting the internal air distribution. If necessary release and pop flare.
One side of the wing is not inflating and you are rolling	Sometimes one side of the parachute catches air before the other side has had a chance to inflate. Inadequate air distribution inside or outside of the cells	Always hold the parachute centered overhead. Pump brakes on inflated side of parachute in an attempt to push air through cross-ports into other side of parachute while applying ¼ brakes. Watch your power. Too much will cause the inflated side to turn and pull the cart over.
Wing starts to stand up but builds a wall and will not move	The tail of the wing has formed a seal against the ground. The parachute "cups" the air. Some methods of laying the parachute out are more susceptible to this problem in certain no wind conditions.	Cut the engine and re-set the parachute. Sometimes simply rolling the cart forward will re-set the parachute properly.
The wing lags behind the cart at 45-degree angle and will not come overhead.	You are getting full inflation before the parachute comes all the way up.	Sling-shot the parachute above by: Applying symmetrical brakes, hold two to three seconds until chute drops and then releasing brakes Or Backing off power. As the chute descends quickly go to higher RPM. The chute should slingshot up above the cart. Beware of adding too much power. Your parachute can fly in front of the cart and you may need to apply brakes.
The wing oscillates side to side as the cart builds up forward speed for take off	One side of the parachute was the last to inflate. As the parachute tries to center itself it sometimes will oscillate. or Not enough airspeed, over control by the pilot, or you are in a crosswind. Adding power while wing is oscellating can affect cart stability.	As your wing is 3’ from center start applying the appropriate brake to center the wing (anticipate your controls). Speed up to help increase the pressure in the wing. Be sure you are headed straight into the wind.

Note: Many of these problems can be avoided by using the take off technique appropriate for your system. Many techniques that were required for older wings can cause problems when used with the newer, more efficient wings.

A crosswind take off is very dangerous and should be avoided at all cost! A crosswind is not necessarily a 90-degree situation but is anything that is not a straight headwind. In order to pull it off you must be able to counteract the force of the wind trying to push the wing to one side and at the same time control the direction of the vehicle. Meanwhile you must make sure the downwind portion of the wing is staying inflated.

The secret to pulling off a crosswind takeoff is to always inflate your wing directly into the wind. If you do not have enough room to do that you do not have enough room to takeoff.

Most important decisions affecting inflation are made even before you turn the engine on. These include the method you use to lay out the wing in preparation for inflation, the physical location you choose relative to trees, bushes, and other obstacles, the prevailing wind direction, and the method of routing your lines away from the prop.

In making these decisions you want to use good common sense. You want to stay well away from any obstacles that may cause an unexpected gust of wind to rotor (cause turbulence) into your wing inflation area. You want to lay out your wing in such a way that in the event your cart rolls a short distance backward, your wing will not be constricted from proper inflation.

The bottom line is this: There are many ways to achieve the same result. It is important that you understand and feel confident in your method of laying out your wing. The wise person will constantly be alert to the inflation experiences of other PPC pilots and adjust his/her techniques accordingly.

The tools you have available to deal with problems during inflation are throttle control, ground steering, brake (steering line) input, and the knowledge that you can always abort. The best tool of all is to have a solid plan of action for any problem you can imagine and then to back that plan up with a second option.

American Powered Parachute Wing manufacturers think strong wing construction is essential not only for the longevity of the wing but also for the safety when unexpected damage is experienced. Since powered parachute pilots currently have no feasible backup safety system, the construction, care and maintenance of the wing is tantamount.

Basically we can divide the components of the wing into 2 major groups: Structural components and non-structural components. A structural component is one that will strongly affect your flight if it becomes disabled. Steering lines, line attachments, A lines, A/B lines, and stabilizers are all structural components. A non-structural component of a properly designed wing would be a non-load rib, crossover lines, safety steering lines or one C or D line where a failure would still allow you to safely land.

To determine if the construction on your wing is adequate, take a good look at all the structural components. Imagine the direction the force is being applied and then see if you can visualize what it is doing to the component in question. Pay close attention to any structural components where the stitching has become stretched or pulled apart. If you notice anything questionable be sure to have it checked out by a qualified inspector. (For more details see "Troubleshooting your PPC Wing" Ultra Flight Magazine, May, 1999)

It is our hope that this article has provided you with a better understanding of wing inflation and the vocabulary to ask important questions. As you watch PPC pilots take off, remember the issues addressed in this article and see how they apply. Discuss inflation with other pilots, listen to real life experiences but remember what works with one system may not work with another.

Keep an open mind. Powered Parachute vehicles, engines, propellers and wings are all evolving. Each part of a system affects the unit as a whole. This is an exciting time to be involved with powered parachutes. Through pilots asking questions and understanding the dynamics and forces at play we all help each other to fly safely.

Special thanks to Bill Gargano, Eddie Johnson, Jeff Jensen and Tim LeBlanc for providing valuable input.