US2797885A - Vortex ring parachute - Google Patents

Description

July 2, 1957 D T, BARlSH 2,797,885

VORTEX RING PARACI-IUTE Filed Feb. 11, 1954 la v y I? INVENTOR ATTORNEY Patented July 2, 1957 VORTEX RING PARACHUTE David Theodore Barish, New York, N. Y.

Application February 11, 1954, Serial No. 409,679

6 Claims. (Cl. 244-445) This invention relates to parachutes, and particularly to vortex ring parachutes. For convenience, parachutes will hereinafter be designated as chutes.

To the applicants knowledge, chute canopies as previously designed in the United States have been more or less arbitrarily formed as to shape until the development of the flat circular chute. In this form of device, the panels of the canopy were segments of a circle joined into a generally circular plan-form, and, while possessing a high drag coefiicient (Cd), the canopy had a strong tendency to oscillate. The only appreciable change in this design over a period of years, adding to the stability of the chute, has been the provision of vented pockets at the skirt as in the guide surface chutes. The German inventors during this same interval made various advances in the art, in the three types of devices identified as the ribbon chute, the guide surface chute, and the airfoil chute. These have certain specific advantages in the order of mention in the fact of possession of low opening shock, high stability, and high drag coefficient, respectively. However, it is to be noted that each such advantageous property has been gained at the expense of the other properties.

The introduction of autorotation into the chute field has caused appreciable technical controversy. The general philosophy of the chute designers is understood to have been that a parachute could be approximated by considering it as a disc normal to the direction of motion. From one of the dimensional momentum considerations the maximum drag obtainable would occur when all of the particles in the path of the chute were stopped. This would develop full dynamic pressure on the lower side and produce a Cd equal to 1.0. This was all that they could hope for, except for lowering the pressure below ambient on the downstream side. It had been believed that this could best be done by a Helmholtz type of separation at the canopy lip. Hence a Cd of 1.3 given by some modern chutes which exceeded the drag of a flat plate (Cd of 1.25) has been considered excellent. However, examination of experimental data on modern helicopters in vertical descent shows drag coefiicients based on blade areas as high as 40: i. e., a helicopter whose blade is 4% of the circular area it subtends has a drag roughly the same as that of a chute of the same diameter. The reason for this is explained below.

In autorotation a helicopter enters the so-called vortex ring state. That is, the vortices shed from the blade form a toroidal ring whose central filament is near the tip of the rotor. The action on the ring causes it to move against the relative wind, tending to keep it in the plane of the rotor. The ring produces a strong circulatory field which influences a large mass of air and hence produces large lifting forces on the rotor. Except for viscous effects it would be possible to obtain extremely high lifts. However, the shearing-stresses due to the high velocities near the core of the ring cause energy dis sipation. In the steady state condition, this dissipation is matched by the energy obtained from the newly shed vortices from the rotor.

It will be understood that from the analytical viewpoint this condition is quite diflicult to formulate. In its present state, so far as known, the art is purely empirical. Professor Nikolsky, of Princeton, has derived a relation which, under very stringent assumptions, shows the effect of blade incidence, lift curve slope, solidity, and frictional drag on the rates of vertical descent and rotational speed. This relation indicates that for normal helicopter blades, the drag coefiicient based on total circular area can be raised to about 3.0 by increasing the solidity to 1.0. This would also reduce the rotational speed by a factor of about 3. The relation does not consider the effects of camber, twist, nor interference effects on the characteristics of the rotor.

In rotating devices for parachute-like purposes there have been numerous devices considered. Most of these have used uncambered metal or wooden blades. The

two outstanding problems among many other more or less minor ones have been stowage and pre-rotation. The inventors have used various expedients for coping with these problems, including rolling the metal blades and using their spring tension to implement the unrolling and the initiation of rotation, telescoping the blades and using a powder charge to start the turning, or providing the blades of inflatable rubber or fabric, in which air pressure unrolls the blade tube and shapes it. So far as known, none of these devices has achieved any substantial success because, it is believed, their complex packaging causes them to weigh more than the chute for a given amount of drag, and they also consume far more space. V

The only commercially urged rotating chute presently known to applicant is not pre-rotated and operates in the so-called windmill state. Consequently, its maximum theoretical drag coeflicient is about 1.0.

It will be evident that a rotor operating in the vortex ring state has several unique problems. In this condition the blades are effectively at positive angles of attack. The potential flow factor vector from the blade is inclined forward just enough to counteract the viscous drag. If the. rotation 'of the blade is slowed to a certain value, the drag overcomes the aerodynamic thrust on the blade, and it will stop and begin to rotate in the opposite direction with efiectively negative angles of attack. In helicopters which depend upon centrifugal forces to hold the blades out, this is catastrophic. In chutes it causes a reduction in drag, due to operation in the windmill state rather than the vortex ring state.

It is the principal object of this invention to use the autorotational principle to improve the drag characteristics of a chute, while also securing dependability, low cost, low snatch forces, low opening shock and high stability for a given weight and/ or volume of package, and without introducing any additional materially disadvantageous problems.

In carrying out the invention recognition is taken of the fact that the reversal of rotation as a problem is one whose parameters are minimum rotational speed for the vortex ring state and the angle of incidence of the blades. The invention contemplates that the operative configuration utilizing the principles hereof can be formed of chute canopy panels whose minimum rotation is practically zero, and these, with and, in certain cases, to be explained, without the aid of a small section operative in the windmill state, have no minimum critical speed. Additionally, when the panels are thus shaped they are oscillatorily stable.

The central feature of this invention then is the use of highly eambered, low porosity, illustratively, cloth panels, to operate in the vortex ring state. Research has demonstrated the feasibility of operation when the geometry is such that a chord line which joins the edges of the panel is essentially perpendicular to the direction of motion of the center line of the chute. In other words, prior to the initiation of rotation, the panels have cross sections which are highly cambered symmetrical cate naries. As such they can be initiated into rotation in either direction. As soon as the rotation begins, one edge becomes a leading edge, the other a trailing edge, The action of viscosity causes enforcement of the Kutta condition. This sets up a circulation about the panel, producing drag against the axial motion of the chute. It also produces thrust in the plane of rotation. The balance of this thrust with the viscous and induced drag of the panel determines the rotational speed of the chute.

If it is desired to provide some means for initiating rotation as a windmilling function leading into autorotation, two of many expedients may be resorted to. For instance, if, near the outer edge of a panel, a cut-out is made on one side, the remaining portion will initially have a negative angle of attack. Consequently, it will have a force acting upon it tending to make the panel rotate. When the panel reaches its minimum autorotation speed, the forces on the rest of the panel will cause it to speed up until the equilibrium speed is reached. The cut: out is so designed that at the equilibrium speed the cutout portions are also operating in the vortex ring state. In facilitation of this effect, when necessary to speed up the initial rotation, additional windmilling sections are provided at the outer section of the gaps between the panels.

In the accompanying drawings forming part of this disclosure:

Fig. 1 represents a schematic elevation of a chute exemplifying the invention.

'Fig. 2 represents a side elevation of a detail of the chute of Fig. 1, showing an illustrative device for counteracting the rotational torque of the supporting structure.

Fig. 3 represents a fragmentary section through the marginal edge portions of several panels to disclose the illustrative structure thereof. 7

Fig. 4 represents a fragmentary section taken on line 4-4 of Fig. 2.

Fig. 5 represents a schematic elevation of a modified form of the invention in which the chute has just opened, at a most, favorable solidity factor to establish blossoming of the. chute, i. e. with small gaps between panels.

Fig. 6 represents a schematic elevation of the device of Fig. 5 in autorotation with the gaps between panels onlarged to a degree. adequate, for most eflicient drag characteristics.

The effective canopy 10 of the, chute. according to the invention is comprised of a plurality of substantially segmental panels 11, secured together at their narrowest portions at the central vertical axis of the canopy as at 12, and providing substantially segmental spaces 13 between adjacent panels. The marginal edges of the panels may be connected peripherally together by chords 14 relative to which the panels 11 have a desired camber in the extended condition of the canopy. For this purpose the marginal edges are formed on reinforcing edge chord or the like at 15. The shroud lines 16 attach to the canopy at the intersection of the chords and 14 at the lateral edges of the panels. Reinforcing chords (not shown) may extend along the radially extending edges of the respective panels.

It may be considered that the direction of autorotation of the canopy is from left to right in the side shown in Fig. 1, in the plane of the sheet. The leading edges of all of the panels 11 are continuous from the apex at 12, substantially, to the marginal edge chord 15. Each panel 11 has a small windmilling section 17, partially or wholly removed from the panel toward the trailing edge thereof,

which small section, if retained, is bent about the radial reinforcing chord and extended partially across the space 13 between the panels, toward but spaced from the leading edge of the adjacent downstreamwardly disposed panel 11, in the direction of autorotation.

When the section 17 is completely removed, so as to form a cut-out on one side of a. panel, the remaining portion, in peripheral alignment with the removed section, on blossoming the chute will have an initial negative angle of attack, and consequently the panel will have a force acting upon it tending to make the panel rotate. This initiates rotation, and when the panel reaches its minimum autorotational speed the forces on the rest of the panel will cause it to speed up until the equilibrium speed is reached. It may be noted, as previously observed, that the spaces left by removal of the panels 17, at the equilibrium speed, are also operating in the vortex ring state.

When the section 17 is bent about its radial reinforcing chord it becomes cambered on opening of the chute, and is also at a negative angle such that it initiates windmilling in the same sense as that caused by the negative angle of the portion of the canopy in peripheral alignment with the cut-out 17, as above noted, to augment the windmilling operation prior to autorotation. Each small windmilling section 17 is supported on outer chords 15 extending between panels across the spaces 13 between panels, at the peripheral margin of the canopy, and by inner chords 20 on the radially inward edge of the sections, connected to the inner reinforcing chords 21 connecting the panels. The sections 17 are cambered and have a trailing edge 22 forming a slot with the leading edge of the adjacent downstreamward panel.

The shroud lines or risers 16 converge into a member 23, connected through a thrust bearing 24, with a lower member 25 mounting the load element engaging portions of the assembly. The load element may comprise straps 26 and 27, the lower end of which engages the load, whatever its nature, as will be clear, and at the upper end the straps engage the lower member 25, and, through the thrust bearing, transmit the -load to the shroud lines and to the canopy.

As there may be a small bearing torque from the rotating canopy, this is counteracted in any desired manner. Illustratively, a cloth panel 30 is provided, extending between the upwardly converging load straps 26 and 27, stitched respectively to the strap 27 on a diagonal line 31, and stitched to the strap 26 on an oppositely sloping diagonal line 32 As shown in the section thereof in Fig. 4, the central portion 33. of the cloth panel 30 is twisted about the central axis to generate a torquecounteracting force on the straps and lower member 25 to hold the load against rotation during descent. If desired, this, torque can be adjusted during man-carrying descents by relative movement of the risers 26 and 27.

It is a further important feature of the invention to provide an adjustable canopy in which the solidity can be automatically varied to achieve the utmost efliciency in both phases of operation, one being the quick opening and blossoming, and the other the high drag coefiicients during post-blossoming descent. Reference may be made to Figs. 5 and 6.

In Fig. 5 panels 11 are provided, the peripheral margins of which are connected across the gaps 13' with elastic chords 14', such as of rubber or the like or of elasticised chords, undrawn nylon or the like, which at the moment of blossoming keepthe panels together and permit only narrow gaps 13 or no gaps whatever between panels. The reductionin the effective gap width thus attained enhances the opening and blossoming of the chute. The inertial loads during this phase cause the tension on the risers or shroud lines 16 to dominate over the pressure forces on the panels 11'. After blossoming, however, the other forces become dominant, and the gaps spread to the position schematically indicated in Fig. 6, at which equilibrium becomes established. To facilitate this, the main risers 16 may be coupled to short sub-risers 16 at a point 19 and to the respective edges of the panels. Point 19 is between the skirt and the member 23, and preferably is spaced from the canopy margin or skirt a distance of the order of the width of gap 13. This short coupling of the risers reduces the rotational drag losses which otherwise might tend to reduce the over-all eificiency of the chute. Tests indicate excellent blossoming efi'iciency of the organization just described with only 25% initial porosity.

In considering the panel organizations in their relation to gap area it is preferred to use low porosity cloth, as this does not adversely effect the drag characteristics. These may be typified by nylon cloth or vinyl, or the like. As a matter of fact, the drag coefficients are higher for this type of panel than they are for smooth metallic or wooden panels. The only adverse efliects with cloth panels, as presently developed, in the autorotation stages attaches to too few panels. In experimental work tests have been made of various panel segment organizations, in which the panels ranged from 17 to 60 of definition, and, of course, in varying numbers, and also, of course, with varying gap angles between them. While the tests included various combinations of panel area to gap area, ranging from 8 panels, through 6, 4, 3, and 2 panels, it was found that all of the chutes tested had reasonably good drag characteristics but those incorporating 8 or 6 panels of angular definition between the limits recited were appreciably better as to drag coefl'icients than those of a smaller number of panels, as the latter, if wide enough to establish usable solidity, had too great a chamber.

It is a feature of the invention that the drag of a chute of given diameter is essentially independent of the solidity, i. e. by permitting operation in the vortex ring state the drag remains constant as the gaps between the panels are increased in area, as the rotational speed increases as gap area increases. These interesting functions of the chute are consistent with existing propeller and rotor data.

Although in many cases it may be found desirable to provide either the cut-outs in the panels, or the added semi-panel sections 17 extending into the gaps between panels, or both, for the initiation of windmilling leading into autorotation, with the paneled canopy of this invention this is not essential. It will be observed that in the usual case the panels are symmetrically cambered, and the canopy enters spontaneously into autorotation in one direction or the other.

The only factor of importance in this regard is that with a canopy of fixed panel and gap width, when the solidity is less than 50% there may be some hesitance of the chute to blossom. This may be remedied in whole by the arrangements shown in Figs. and 6, and to a considerable degree by increasing the lengths of the risers 16 beyond the usual and average lengths thereof. In this connection it may be noted that as a rule, the shortening of the risers elfects higher drags but poorer opening characteristics. a

It will be understood that the chute as described can be folded and stowed similarly to conventional cloth chutes. Upon deployment, the canopy opens to the general canopy form shown and the descent begins. If the canopy is instantaneously substantially stationary about its axis of rotation, the flow of relative air upwardly through the segmental openings 13 between panels is accompanied by relative flow across the inclined short panels 17, which react, with a rotational force, urging the canopy in windmilling rotation. As this chute attains a minimum autorotational velocity, the main panels 11 take over, autorotationally, and the descent speed is then greatly reduced due to the large drag forces created by the vortex ring.

The problems of dependability, cost, snatch, and opening shocks, weight, and volume, are favorably solved by the instant invention. Dependability in opening is considered to be excellent if the solidity is greater than 50%. Previously purposed windmilling non-autorotating chutes are known to exhibit low opening forces due to the high spillage of the captured air. This advantage occurs to the presentinvention. The canopies of present chutes represent a large percentage of the total cost of the chute. Since the chute of the instant invention has at least the same drag by removing substantially half of the panels, it is consequently lighter, cheaper, and less voluminous than conventional present chutes.

Having thus described my invention, I claim:

1. An autorotatable parachute comprising a canopy formed of flexible panels each having a relatively wide base forming a peripheral margin and a relatively narrow apex and connected to each other only at the apices and peripheral margins, said panels at least in the inflated condition of the parachute forming generally radial peripheral gaps between adjacent panels without overlap of panels axially of the canopy in said gaps, said panels being cambered to initiate autorotation, whereby to maximize th aerodynamic efficiency of the canopy in rotation with minimization of the parasitic areas of the canopy.

2. An autorotatable parachute asrecited in claim 1 and peripheral edge risers connected to the marginal portions of the respective panels, said panels being subjected to tension in the inflated condition, and the recited camber being controlled solely by the balance of tension of said risers and said panels, and in which autorotation is initiated by spontaneous vortex shedding.

3. An autorotatable parachute comprising a canopy formed of a plurality of generally segmental panels in peripheral relatively spaced relation having a mutual anchorage generally concentric with the vertical axis of the canopy and effecting generally segmental peripheral spaces between panels, said panels being cambered and arranged for autorotation, and relatively smaller panels extending peripherally partially into the space between the contiguous generally segmental adjacent flexible panels for initiating windmilling rotation of the canopy.

4. An autorotatable parachute comprising a canopy having a central axis, a plurality of panels radiating from the axis and connected laterally together only at the axis and at the peripheral margin and defining generally segmental spaces between panels, shroud lines, means connected to the shroud lines for defining a peripheral margin of the canopy relative to which said panels have a camber said shroud lines connected respectively to each of said panels at one side of the peripheral margin thereof, and shroud line extensions shorter than said shroud lines connected respectively to each shroud line and to the respective panels on the other side of the peripheral margins thereof to apply a load to the canopy while permitting the spaces between panels to vary in area.

5. An autorotatable parachute comprising a canopy having a central axis, a plurality of panels radiating from the axis and defining peripheral generally segmental spaces between panels, shroud lines, means connected to the shroud lines for defining a peripheral margin of the canopy relative to which said panels have a camber, portions of said panels bent out of the cambered extent thereof and partially into the space between adjacent panels, and means for connecting the free edges of the portions of the panels to the next adjacent panel to leave an airflow space therebetween to initiate windmilling of the canopy.

6. A parachute as recited in claim 1, in which the camber varies with the radius while the tangent of the chord incidence angle varies in a manner essentially inverse with the radius, said apices being connected along lines essentially parallel to the axis of rotation to provide initial torque due to the twisting of the flexible panel.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS 660,793,

' Avorio Dec. 9, 1930 Tricau Aug. 3, 1937 5 933,149 Synnestvedt Apr. 27, 1948 978,042 Ewing Feb. s, 1955 1,056,062

8 FOREIGN PATENTS Germany June 2, 1938 Germany Aug. 30, 1939 France Dec. 17, 1947 France Nov. 22, 1950 France Oct. 21, 1953

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