Aerospace Archive

Kf

Unit loading, W/CpS

Total porosity, X.

Unit loading, W/CpS

Axial deflections from tangent, 0,(j90 deg —a)

is O

Disreefed

Reefed

Nonreefed

Mass ratio, R_

Reefed

Nonreefed

Axial deflections from tangent, 0,(j90 deg —a)

Mass ratio, R_

Figure 2. - Aerodynamic and design characteristics of parachutes.

nonoscillating system (i.e., the effeetïve-drac coefficient is increased). With increased unit loading, an increase in rate of descent is attended by a higher differential pressure across the canopy which causes the relative porosity of the canopy to increase in proportion to its structural elasticity (fig. 3) (i.e., the effective-drag coefficient is decreased I, o c.

Differential prpntirp, Ap

Figure 3, - Effect of differential pressure on relative porosity of parachute cloth.

Differential prpntirp, Ap

Figure 3, - Effect of differential pressure on relative porosity of parachute cloth.

This behavior places an upper limit on unit-canopy loading for certain types of solid-cloth canopies. The limit is reached when the relative porosity increases to the point where the ratio of inflow to outflow conies to equilibrium before the canopy is fully expanded, and so-called "squidding" takes place. Solid-cloth canopies with inverted-conical skirts are subject to this loading limitation, while circular fiat and conical canopies are not. Canopies of annulate construction, such as the ringslot and ringsail types, have a smaller total variation of drag coefficient with unit loading.

The decline of drag coefficient with increased unit loading of solid-cloth canopies may be corrected by employing nonstandard fabric of low porosity, but the resulting augmentation of both instability and opening shock has created formidable development problems, such as the need for developing multistage-reefing techniques.

2.2.1.4 System-Mass Ratio

Theory supported by test results shows that the opening shock of a parachute is proportional to the ma« ratio (ratio of the mass of air moving with the canopy to the mass of the vehicle plus parachute) and to Fronde nuinher (ref, 55). Empirical data have been evaluated by defining the mass ratio as Rm = pD03/M and representing the opening shock by the F.uler number, defined as En = F0/q , St), where F0 is the measured peak opening force. To make this approach applicable to reefed as well as to nonreefed parachutes, the mass ratio is redefined here as Rm = p\jj3/2/M, where \jj = Cj)S, and the opening load factor Ck = En/CD0 is substituted for Euler number. In the new definition, the characteristic diameter of the canopy is represented by the square root of the effective-drag area, , either reefed or full open. The variation of the opening-load factor with mass ratio for reefed and nonreefed parachutes follows the general trends illustrated in figure 2f. This relationship is considered more dependable than long-used empirical formulas relating opening-shock factor, X (or opening-load factor CjjJ and unit-canopy loading, W/CpS, with dynamic pressure and altitude as the related independent variables. However, the great mass of empirical data in the form of Cg versus W/CpS is still valid and useful within the speed and altitude ranges of the tests that generated the data.

22.1.5 Trailing Distance

Decelerator performance is degraded by the proximity and the size of the towing body. Typical degradation in subsonic decelerator drag caused by body wake is shown In figure 4 as a function of the relative trailing distance and the relative size of the decelerator. Supersonic decelerators exhibit similar behavior, but the effects tend to be more severe for high-drag bodies and for relatively small decelerators (refs. 7, 29, 56, and 57),

Relative trailing distance, x/dg

Figure 4, - Typical effect of trailing distance on decelerator-drag coefficient (subsonic).

Relative trailing distance, x/dg

Figure 4, - Typical effect of trailing distance on decelerator-drag coefficient (subsonic).

22,1.6 Scale Effects

Parachute performance is affected markedly in various ways by chances in the absolute si70. or ".scale,** of the canopy. These scale effects are numerous and complex, and are seldom clearly related to Reynolds number, probably because the ratio of fluid momentum to viscous forces described by Reynolds number includes a characteristic length which has uncertain meaning when applied to a parachute. Generally, the scale cffects embody a combination of macroscopic and microscopic factors (for example, vortex formation and shedding combined with flow through the pores of the fabric). Other aspects of this subject are treated in the discussion of special problems associated with advanced decclerator design in Section 2,2.10 of this monograph,

22 2 Aerodynamic Characteristics of Déployable Wings

The design-relevant aerodynamic characteristics of déployable wings are illustrated schematically in figure 5, (The characteristics of steerable parachutes are similar.) The bulk of currently available performance data is limited to the results of wind-tunnel and free-flight tests of small-scale models with a lifting surface, Sw, of up to = 16.2 m2 (174 ft' ). With the exception of results of parawing tests, the results of intermediate-to large-scale tests of déployable wings have not been published, À comparison of large-and small-scale parawing free-flight performance is made in reference 13. and a structural-optimization study of large-scale parawings is reported in reference 58, which also discusses the aerodynamic characteristics of such decelerators. including the cffects of reefing and porosity on opening loads.

Characteristically, the lift-to-drag ratio (E/D) of a déployable gliding surface can be controlled by varying the ancle of attack, a. over a range of flight attitudes between leading-edge collapse and the stalling point (fig, 5a). The conventional pitch-control system changes the angle of attack by extending or lengthening lines attached to trailinc-edge portions of the win p. Leading-edge collapse will occur at flight angles of attack, a little below the angle of attack for maximum t/l> (fig. 5b), At the stalling point, lift falls off rapidly and the system usually becomes unstable, like a parachute of low porosity. Between the limits of leading-edge collapse and stall, oscillation damping is strong and gliding flight is quite stable (with the pitching-moment coefficient, C^j. varying almost linearly with ancle of attack, as shown in fig, 5c). As the steering control deflection is increased, turn rate approaches an upper limit (fig, 5d);this limit decreases with increasing scale of the déployable wing. In flight, the vertical sinking speed is a minimum near l./l) max (fig. 5f), and increases with controlling deflections that cither reduce I./I) or increase the turning rate. With a sufficient range of L./D modulation in stable flight, a pilot can execute a flared-landing maneuver with a déployable wing to reduce the touchdown velocity.

22. J Performance of Supersonic Decelerators

A wide variety of déployable decelerators (usually called "drogues") have been developed for the purpose of augmenting the drag and stability of a descending vehicle.

Leading-edge collapse

"9

"9

Relative control travel, AS./fi

Angle of attack, a

Relative control travel, AS./fi

Relative control travel, At/i

collapse

Relative control travel, At/i

Coefficient of drag, Cp

Figure 5. - Aerodynamic characteristics of deployable wings.

Of these, only a few perform well in supersonic flow. The most successful supersonic-drogue types are the conical-ribbon, hetnisflo, and parasonie parachutes, and the ballute, a ram-air-inflnted, quash'sotensoid envelope. The parasonie design was derived from a somewhat less stable predecessor called the "hypcrfla." Of the supersonic-drogue types, the conical ribbon has shown the best subsonic performance and. above Mach 1,5, (he poorest supersonic performance.

The general decline in the drag coefficient of parachute drogues with increasing speed above Mach O.N (illustrated in fig. ft) is caused by a combination of wake effects and increasing-inflation instability. In the middynamic-pressure range (where most of the data haw been obtained), inflation instability tends to limit the usefulness of a supersonic-drogue design to a velocity range somewhat below the maximum-test Mach number indicated for each type in figure ft. inflation instability is characterized by alternate opening and squidding of the drogue canopy at high frequency and is attributed to periodic changes in the shock-flow pattern through and around the canopy, coupled with disturbances in the vehicle wake. The drag-coefficient data from which figure ft was developed are from many sources (representing drogues tested in a variety of forebody wakes); these were reduced to a common base by using the total surface area of each drogue type as the reference area. ! ly perflo-drogue data from tests at velocities up to Mach 4,1 were too scattered for meaningful presentation in this figure.

M.trh number, M

Figure 8. - Variation of drag coefficient with Mach number for various drogues.

M.trh number, M

Figure 8. - Variation of drag coefficient with Mach number for various drogues.

The ballute drogue is an exceptionally stable, low-opening-shock device in the hypersonic speed ranee where the parachute drogues exhibit high-opening-shock characteristics and increased inflation instability. Available ballute data are for models equal to or larger in diameter than the towing body; drogue-parachute data are for models equal to or larger than one-half the diameter of the towing body.

The effects of aerodynamic heating on supersonic decelerators are a function of true airspeed, dynamic pressure, and the duration of the heat pulse. Significant heating has been encountered in tests at Mach 3 to 6 in the dynamic-pressure regime of 4788 to 16 760 N/m2 (100 to 350 psf). Protective coatings and Nomex textiles have been used to provide structures of strength adequate for the temperatures generated under these conditions (refs. 12, 15, and 16). However, measured temperatures in general have been much lower than predicted; for example, 367K (200°F) versus 427K to 538K (300°F to 500°F) predicted for a Nomex parasonic drogue deployed at Mach 5.4, and a dynamic pressure of 10 203 N/m2 (213 psf) (ref. 16). All indications emphasize the practicability of using standard nylon textiles in supersonic decelerators for operation at speeds up to Mach 3. The feasibility of deploying large Dacron parachutes of relatively lightweight construction at Mach numbers on the order of 3 has been demonstrated at altitudes where dynamic pressures did not exceed 575 N/m2 (12 psf) (ref. 11). Minor heat damage, which was extended into major rips by canopy buffeting, occurred at Mach 3.3 (ref. 59).