RADIAL REEFING METHOD FOR

ACCELERATED AND CONTROLLED PARACHUTE OPENING

Calvin K. Lee* Aero-Mechanical Engineering Directorate

U.S. A m y Natick Research, Development and Engineering Center Natick, Massachusetts 01760-5017

U.S.A.

Abstract

Future A m y airdrop systems will require aerial insertion of cargo and personnel from low altitudes to minimize ground-fire hazards. A radial reefing method was developed as a potential candidate to meet this requirement. This paper presents the concept, procedure, and full-scale test results of the method. It is found that the radial reefing method shows promise for low-altitude airdrop applications. In addition, the method also improves parachute opening by minimizing canopy enfolding and slumping.

I. Introduction

Opening of a single parachute or a cluster of parachutes is a complicated phenomenon that involves flow and structural interaction between the canopy fabric and the surrounding air-flow field. As the parachute gets larger, the interaction becomes less manageabl~, resulting in opening difficulties. To control the opening of a single parachute, the standard methods of skirt-reefing, pull-down center line, crown chute and secondary chute at skirt have shown some success for certain airdrop applications. Future Army airdrop systems will require aerial insertion of heavy cargo and personnel from low altitudes to minimize ground-fire vulnerability. lr2 New methods and new parachutes need to be developed to meet this requirement. A radial reefing method was developed for this purpose.

11. Method

Fig. 1 shows the inflation sequence of a standard Army 100-ft diameter G-11 cargo parachute using

skirt reefing. The sequential photographs show that the parachute is inflated by the incoming air flowing through the canopy mouth as it moves in a curvilinear trajectory. The growth of the canopy mouth is a slow process, beginning from the horizontal deployment position. If the mouth is large and well formed early in the inflation process, preferably during canopy snatch when the canopy is in the horizontal position (canopy mainly in one-dimensional inlet air flow), the inflation should be accelerated and more positive. This is particularly important and desirable for clustered parachutes that often have irregular and uneven mouth shapes, resulting in the well-known lead- and lag-opening problem. In addition, for large cargo parachutes, such as the 137-ft diameter developmental Army cargo parachute, the large amaunts of canopy fabric often cause canopy enfolding and slumping, resulting in inflation difficulties. ' The radial reefing method was developed to establish a well-formed, stiff, large canopy mouth during canopy snatch and to solve the canopy fabric enfolding problem.

Skirt reefing is commcmly used to control and stage canopy opening. It mainly restricts the skirt opening but it offers no control of the upper canopy. It appears that if the canopy fabric is reefed during opening, the opening should be more manageable and controlled. This is the basic idea of radial reefing .

Fig. 2a s h o w the radial reefing method using a 64-ft diameter and 64-gore etandard Army G-12 cargo parachute to illustrate the method. Five equally spaced

This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

After the G-12 parachute has been radial reefed, the 16 radial reefing lines are grouped together at the center as shown in Fig. 2b. Because of the radial reefing, the suspension lines connected to the

reefing rings (2-ft apart) are sewn inside the canopy on radial No. 1 from point A at the skirt to point A', 8-ft from the skirt. Radial reefing line AA'p, sewn and anchored at point A at the skirt, runs through the four reefing ringe and bends downward at A' toward the parachute confluence point P'. Skipping every 4 gores, the identi- cal arrangement of reefing rings and radial reefing line for radial No. 1 is also constructed on radials No. 5, 9, 13,...and 61 as shown in Fig. 2a. The final result is 16 equally spaced radial reefing lines anchored at the skirt, bent at 8-ft from the skirt and joined at point P. The canopy is now ready for radial reefing .

The radial reefing procedure involves pulling point P of the 16 radial reefing lines downward to point P' so that along gore No. 1 point A is pulled next to point A', resulting in puckering the canopy fabric between points A and A'. Similarly, along gore No. 5, point B is pulled next to point B*, resulting in puckering the canopy fabric between points B and B', and so on for gores No. 9, 13, 17, ... and 61. The net result of this packing procedure is the 8-ft reefing of canopy fabric near the skirt at 16 equally spaced radials as shown in Fig. 2b. The amount of radial reefing is expressed by the radial reefing ratio, r, which is defined as the ratio of the original gore length AA' before reefing (Fig. 2a) to the canopy radius, R, i.e.,

For the current G-12,

AS AA' increases, r increases, meaning more canopy fabric is reefed.

nonradial-reefed radials are slightly loose; the looeeness is kept near the skirt as shown in Fig. 2b. A pyrotechnic cutter is installed at point P to tie points P and P' together so that the canopy remains radial reefed during early opening. To enhance the effect of radial reefing, an 80-lb breaking strength line is uaed to tie the canopy at 3-ft above point A' (Fig. 2b). The radial reefing procedure is now completed. The parachute is then packed in a G-12 deployment bag using standard procedures.

As mentioned earlier, the main purpose of radial reefing is to form a large stiff canopy mouth during canopy snatch and to avoid canopy enfolding. By radial reefing the selected gores, a considerable amount of canopy fabric is puckered, thereby decreasing the excess fabric during early inflation and avoiding canopy enfolding. Between each two adjacent reefed radials, e.g., radials No. 1 and 5 in Fig. 2, the 3 radials and the 4 associated goree are not reefed. Therefore, during canopy snatch, as soon ae the lower part of canopy is exposed to the incoming air etream in the horizontal position, a fabric pocket near the skirt is formed by the 4 gores between radials No. 1 and 5. Similarly, 15 other fabric pockets are also formed simultaneously between the adjacent reefed gores. The looseness of the suspension lines near the skirt and the 80-lb line tie at.the canopy (Fig. 2b) further enhance the formation of these pockets. The 16 fabric pockets together form a large stiff mouth for efficient air flow and early positive canopy inflation. Thie is depicted in Fig. 2c. By increasing the radial reefing ratio, r, the size of the mouth opening and its formation rate can be increased, thereby decreasing the canopy inflation time. Once the large stiff mouth is formed and the canopy is well inflated, the canopy can be disreefed by firing the pyrotechnic cutter at point P (Fig. 2b).

In addition to the radial reefing ratio r, the formation of the fabric pockets is also influenced by the number of gores, n,, between the adjacent reefed

r a d i a l s . This number is determined by t h e t o t a l number of gores of t h e canopy, n,, and t h e number of f a b r i c pockets, np, des i red . These t h r e e v a r i a b l e s a r e r e l a t e d a s follows:

For t h e c u r r e n t G-12, np becomes:

Although a 6-12 parachute has been used t o i l l u s t r a t e t h e d e t a i l s of t h e r a d i a l r e e f i n g method, it is a gener ic method t h a t can be appl ied t o a l l parachutes.

111. Ful l -sca le Tes t s

The r a d i a l r e e f i n g method was inves t iga ted f i r s t using s c a l e model parachutes i n a wind tunnel , and u l t i m a t e l y v e r i f i e d by f u l l - s c a l e t e s t i n g using Army G-12 and G-11 cargo parachutes, and T-10 personnel parachutes.

Fig. 3 shows t h e opening sequence of a G-11 r a d i a l reefed a t r = 25%. A s designed by t h e r a d i a l r e e f i n g method, a t canopy snatch ( p i c t u r e No. 3 from l e f t i n t o p row), a l a r g e s t i f f mouth i s a l ready formed f o r e a r l y and p o s i t i v e canopy i n f l a t i o n . This is more c l e a r l y shown i n Fig. 4 which shows an enlargement of t h a t p ic tu re . The resemblance between Fig. 4 and Fig. 2c i s evident . On t h e o t h e r hand, a t canopy snatch , t h e standard G-11 parachute i s hardly i n f l a t e d ( p i c t u r e No. 3 from l e f t i n t o p row of Fig. 1). A f t e r t h e l a r g e s t i f f mouth is formed f o r t h e r a d i a l r ee fed G-11, t h e canopy continues t o i n f l a t e r a p i d l y and pos i t ive ly . The l a r g e pockets a t t h e s k i r t a r e c l e a r l y shown i n t h e l a s t two rows of t h e p i c t u r e s i n Fig. 3. Comparison of t h e opening between t h e two G-11s is q u a n t i t a t i v e l y shown by t h e measured opening force , F,, i n Fig. 5. The f a s t and e a r l y opening of t h e r a d i a l reefed G-11 is c l e a r l y shown by t h e r ap id rise i n F, immediately a f t e r t h e canopy snatch fo rce , F,, whereas t h e rise i n F, of t h e s tandard G-11 i s much slower. The decrease and inc rease

of F, between t = 9 and 10 s correspond t o t h e d i s r e e f of t h e canopy ( p i c t u r e s No. 3 and 4 i n bottom row of Fig. 3 ) . For t h i s test, t h e r e e f i n g t ime was t o o long. Disreefing a t 4 s should f u r t h e r shor ten t h e opening t ime of t h e r a d i a l reefed G-11.

The r a d i a l r e e f i n g method was a l s o appl ied t o G-12 cargo parachutes and 35-ft diameter T-10 personnel parachutes. S imi la r t o t h e r a d i a l reefed G-11 parachute, a l a r g e s t i f f mouth was a l s o formed a t canopy snatch f o r both of t h e G-12 and T-10. Fig. 6 shows t h e comparison i n F, between a s tandard G-12 and a G-12 r a d i a l reefed a t r = 25%. The acce le ra ted opening and s h o r t e r i n f l a t i o n t ime of t h e r a d i a l reefed 6-12 a r e evident . Fig. 7 shows t h e comparison i n F, between a s tandard T-10, and two r a d i a l reefed T-10's. The comparison shows t h a t a s r inc reases , t h e opening becomes more accelera ted . A t r = 40%, t h e opening f o r c e and its rise r a t e s a r e excess ively high, r e s u l t i n g i n canopy f a b r i c damage. Tes t r e s u l t s show t h a t r values between 25% t o 30% a r e maximum r a d i a l r ee f ing r a t i o s without causing canopy f a b r i c damage (un less f a b r i c s s t ronger than those used on G-12 and T-10 canopies a r e used).

A s expected, because of t h e e a r l y s k i r t opening formation a t canopy snatch, t h e snatch fo rces of t h e r a d i a l reefed canopies a r e higher than t h o s e of t h e standard canopies (Figs . 5, 6, and 7 ) . The r a d i a l reefed canopies a l s o have higher peak opening fo rces than t h e s tandard canopies. Examination of t h e measured opening f o r c e p r o f i l e s and t h e high-speed movies of t h e tests show t h a t t h e opening of t h e r a d i a l reefed parachutes is i n two s t ages . The formation of t h e f a b r i c pockets and t h e t ime dura t ion of t h e l a r g e s t i f f mouth correspond t o t h e time i n t e r v a l between canopy snatch time, t,, and t ime t i n Figs. 5 and 6. Af te r time t,, tke pockets begin t o separa te from each o the r , allowing t h e upper p a r t of t h e canopy t o i n f l a t e . This is ind ica ted by t h e rise i n F, a f t e r

The o v e r a l l opening is t h u s an k e l e r a t e d and con t ro l l ed two-stage opening, avoiding an undesirable s i n g l e sharp peak opening force .

The avoidance of a sharp peak opening f o r c e i s p a r t i c u l a r l y important f o r personnel parachutes.

A s a r e s u l t of t h e accelera ted opening provided by t h e r a d i a l r e e f i n g method, t h e a l t i t u d e l o s s f o r f u l l opening of a r a d i a l reefed parachute is smal ler than t h a t of a s tandard parachute. Fig. 8 shows t h e a l t i t u d e l o s s e s (measured from t h e a i r p l a n e v e r t i c a l l y downward) of a s tandard T-10, and a T-10 r a d i a l reefed a t r = 25% (both p i lo t -chute deployed). It is seen t h a t t h e r a d i a l reefed T-10 f i r s t reaches t h e 20-f t / s v e r t i c a l downward v e l o c i t y a f t e r an a l t i t u d e l o s s of 170', whereas a l a r g e r a l t i t u d e l o s s of 220-ft is required f o r t h e standard T-10. Therefore, r a d i a l r ee f ing enables a e r i a l d e l i v e r y of a payload from a lower a l t i t u d e above ground l e v e l .

The r a d i a l r e e f i n g method was a l s o appl ied t o c l u s t e r e d parachutes. Prel iminary test r e s u l t s from 25% r a d i a l reefed 3-G-12 c l u s t e r s show t h a t t h e opening of each 6-12 i n t h e c l u s t e r was always p o s i t i v e . The s imul tane i ty of t h e openings was not a s good a s t h a t of t h e con t ro l l ed opening method; b u t a s long a s t h e opening is p o s i t i v e f o r each canopy i n t h e c l u s t e r without excessive lead- and lag-openings, t h e p r o b a b i l i t y f o r canopy damage should be g r e a t l y reduced. A s a r e s u l t of t h e p o s i t i v e ind iv idua l parachute opening, t h e a l t i t u d e l o s s of t h e r a d i a l r ee fed G-12 c l u s t e r s f o r s a f e landing was 250-ft a s compared t o 340' f o r a 3-G-12 c l u s t e r with s tandard pull-down c e n t e r l i n e s . More tests a r e being conducted t o f u r t h e r s u b s t a n t i a t e r a d i a l reefed c l u s t e r s f o r low a l t i t u d e a i rd rop app l i ca t ions .

IV. sunrmary

A r a d i a l r e e f i n g method has been developed and success fu l ly t e s t e d using s i n g l e f u l l - s c a l e parachutes. The method involves r e e f i n g of s e l e c t e d equa l ly spaced r a d i a l s using r e e f i n g r i n g s and r a d i a l r e e f i n g l i n e s . Tes t r e s u l t s of t h e method show t h a t :

A. Canopy excess f a b r i c is puckered dur ing e a r l y opening,

thereby e l iminat ing canopy f a b r i c enfolding of l a r g e cargo parachutes.

B. A l a r g e and s t i f f canopy mouth is formed during canopy snatch, thereby allowing e a r l y and p o s i t i v e canopy i n f l a t i o n .

C. The o v e r a l l canopy i n f l a t i o n i s a con t ro l l ed two-stage opening, thereby avoiding a sharp and excess ively high peak opening f o r c e .

D. A s a r e s u l t of t h e accelera ted canopy i n f l a t i o n , t h e a l t i t u d e l o s s f o r f u l l i n f l a t i o n of a r a d i a l reefed parachute is smaller than t h a t of t h e same parachute without r a d i a l reef ing .

Resul ts A and B a r e r e l evan t t o c l u s t e r s of l a r g e cargo parachutes; a l l r e s u l t s a r e app l i cab le t o parachutes designed f o r low-alt i tude deployment. Currently, t h e r a d i a l r e e f i n g method is being f u r t h e r t e s t e d on c l u s t e r e d parachutes.

References

1. Vickery, E.D., Eldridge, M.L., and Vernet, R.A., "Development of a System of S ix Clustered 137-ft Diameter Parachutes t o Recover 60,000 Pounds", AIAA Paper 86-2445, AIAA 9th Aerodynamic Decelerator and Balloon Technology Conference, Albuquerque, NM, October 1986.

2. Watkins, J . W . , "Deployment Optimization and Human Factors Considerat ions f o r Low-Altitude Troop Parachutes", AIAA Paper 91- 0889, AIAA 11th Aerodynamic Decelerator Systems Technology Conference, San Diego, CAI Apr i l 9- 11, 1991.

3. Lee, C.K., "Radial Reefing Means f o r U s e i n Packing and Opening a Parachute Canopy i n a Control led Manner", U.S. Patent being i ssued, U.S. Army Natick Research, Development and Engineering Center, Natick, MA, 1993.

4. Lee, C.K., and Sadeck, J., "Controlled Opening Method f o r Clustered Parachutes", Journal of A i r c r a f t , Vol. 29, No. 2, p. 264, Mar-Apr, 1992.

Figure 2, Photographs showing the opening sequence of a standard 1CQ-ft diatneter G-LI cargo parachute

Figuxe 3. Phodogrzphs showing the opening sequence of a EDO-ft d i a m e t e r G-XX cax:gc parachute r a d i a l reefed at r = 25%

Figure 4. Photagraph shaw:i.ng tila large s t i f f ~ o u t h of a 253 r a d i a l r ea%ed G - 2 2 a t canopy s n a t c h

0 Ibs

t, sec

RADIAL REEFING

--- STANDARD

Figure 5. comparison of the opening force between a standard G-11 and a radial reefed G-11 (r = 25%)

Fo Ibs

t, sec

Figure 6. Comparison of the opening iorce between a standard G-12 and a radial reefed G-12 (r = 25%)

- - - - - STANDARD

r = 25%

- - - - - STANDARD

r = 40%

Figure 7 . Comparison of t h e opening force between a standard T-10 and ( a ) a T-10 r a d i a l reefed a t r = 25%, and (b) a T-10 r a d i a l reefed a t r = 40%

(payload weight = 220 l b s )

Standard T-10 Parachute

Ud Downward

A l t i t u d e Loss , f t

T-10 with Radial Reefing

A l t i t u d e Loss , f t

(b)

Figure 8 . Comparison of t h e a l t i t u d e l o s s between ( a ) a standard T-10 and ( b ) a r a d i a l reefed T-10 ( r = 25%)