Parachute Design/Experimentation

PARACHUTE INFORMATION SHEET

DEFINITION - folding umbrella-shaped device made of lightweight fabric for retarding the speed of a body attached to it by offering resistance to the air and used for making a safe descent from an airplane, dropping supplies from an airplane or slowing down an airplane upon landing; from para = to shield or protect and chute = a fall

HISTORY

???? - parachutes used by Chinese for fireworks displays

1514 - DaVinci designs a cloth pyramid-shaped “fall-breaker”; was trying to invent a portable fire escape (Italy)

1772 - Desforges jumps from a tower using a parachute-like device, breaks his ankle (France)

1783 – Lenormand, generally recognized as first person to make a successful parachute  descent from the top of a tree, wants a portable fire escape (France)

1785 - Blanchard uses a chute successfully for the first time, dropping a dog from a balloon (France)

1797 - Garnerin - first to parachute regularly; jumps 3200’ from balloon with 23’ diameter canvas parachute (France)

1908 - Stevens - first manually-operated chute with harness, backpack and ripcord (U.S.)

1910 - Pino - employs a small pilot chute to pull main chute freely (Italy)

1912 - Capt. Albert Berry, U.S. Army - first to bail out of an airplane (U.S.)

1915 - WWI German pilots use parachutes successfully; U.S. scorns them

1918 - U.S. Army Air Corp begins R&D to perfect parachutes, results in Army Model A

1923 - Parachutes made mandatory for U.S. Army Air Corp personnel

1930s - Robert Goddard uses chutes to recover his rockets

1960s - Apollo command modules splash down after re-entry

Design Trade-Offs

Designing a parachute requires several trade-off decisions related to factors such as: intended use, payload weight, payload characteristics, the desired opening rate and terminal velocity.

Canopies are often made of geometric shapes, such as: squares, rectangles, triangles, circles, pentagons, hexagons and octagons. But the majority are partial spheres or parabolic in shape. Materials vary according to their weight and porosity. But nylon is the primary material used today for the chute, line and harness. Kevlar is being used in high-speed applications.

Operation of a Parachute

Parachute operation involves only a few simple concepts and includes three basic phases: deployment of canopy and lines, inflation of the canopy and descent. After deployment, air rushes into the canopy and forms a ball of air in the top, or crown, which spreads out to the bottom, or skirt, of the parachute until the entire canopy is inflated. Stabilized descent is achieved when a certain velocity is reached and the canopy drag equals the weight of the payload. This stabilized descent is called the terminal velocity.

Technology/Math/Science Concepts

Gravity & Acceleration - Acceleration equals 32 ft/sec² or 1G. This means a falling body travels at 32ft/sec faster at the end of each second than it was falling at the start of each second.

Without air (in a vacuum) the falling body would continue to accelerate forever. However, air causes drag, so that objects reach a maximum speed called terminal velocity. Beyond this speed, they will not accelerate further. For people in the spread eagle position (belly flop position), maximum velocity is approximately 120 mph; in the delta position (legs straight, arms in, head down) maximum velocity is 185-190 mph.

Balance - Garnerin’s 8000’ drop had wild oscillations, which made him very sick. The problem was air rushing out one side, then the other. The solution--put “spill hole” in the top center of the canopy for better balance. Also, unequal line length will cause instability.

Wind Effects – Wind will cause lift in a parachute into the wind.

Coefficient of Drag - Parachutes have a drag coefficient (Cd) of approximately 1.50, while parasheets (chutes made from flat sheets of fabric or plastic) have a Cd of approximately .75.

Design Concepts - The statements below generalize parachute design and performance:

Chute Size 1 - Bigger chutes produce more drag and thus a slower descent.

Chute Size 2 - Doubling the chute diameter will decrease the descent rate by 1/4.

Canopy Material - Lighter material chutes open sooner, giving more drag time.

Line Length 1 - Shorter lines open sooner, have less drag (less canopy area) and are less stable.

Line Length 2 - Longer lines open later, have more drag and are more stable.

Model Parachute Rule - NAR parachute rule is 44 in^2 per ounce of payload. This gives about a 12’’ diameter for loads up to 2 ounces and 24’’ diameter up to 8 ounces.

RECENT TECHNOLOGY

Space Shuttle - The 82-ton solid fuel rockets are recovered after lift-off; 3 chutes 115’ in diameter are used.

Sport chutes - Airfoil-shaped chutes are ram-air inflated giving both drag and lift; typical glide ratio is 2 to 1. Hang gliders are a variation of parachutes.

RAPID - Acronym for Retro-Rocket-Assisted Parachute In-Flight Delivery; used to drop 30-ton tanks from a cargo plane at an altitude of 300’ while moving 300 mph! Uses telescoping probes--the rate of collapse is monitored by microprocessor, which tells the rocket when to fire.

Airplane Parachute - Boris Pupov invented a parachute used to recover hang gliders, ultralights and lightweight airplanes that are in trouble!

Supersonic Parachutes - When the F-111 crew escape module, which weights 3250 lbs., is ejected from the aircraft moving over 1000 mph, it has a descent rate of 32-35 ft/sec, which causes back injuries for crew members upon landing. Sandia National Lab used computer modeling, wind tunnel tests and trial-and-error to design a three-chute system. Each chute is 50’ in diameter and weighs 30 lbs. This chute system develops 8000 lbs. of drag and slows the module descent to 25 ft/sec.