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A compilation of design considerations applicable to spin-recovery parachute sys- tems has been made so thatthe information will be readily available to ...
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SUMMARY OF DESIGN CONSIDERATIONS FOR AIRPLANE SPIN-RECOVERY PARACHUTE SYSTEMS
by Sanger M. Burk, Jr.
Langley Research Center Hampton, Va. 23365
By Sanger M. Burk, Jr. Langley Research Center
SUMMARY
A compilation of design considerations applicable to spin-recovery parachute sys- tems has been made so that the information will be readily available to those responsible for the design of such systems. The information was obtained from a study of available documents and from discussions with persons in both government and industry experienced in parachute design, full-scale and model spin testing,and related systems. This survey indicated that the technology was best defined for tacticaland trainer military airplanes, and was not considered applicable to other classes of airplanes, especially light general aviation airplanes. Even for the military airplanes, however, there are gaps in the tech- nology where one must rely on the judgment of experts based on their related experience. Hence, the present paper is not a handbook for the design of spin-recovery parachute systems, but is simply a summary of the status of the technology and a discussion of approaches that have proven successful, or unsuccessful, in the past. One main conclu- sion evolves from this survey; that is,there are three distinctfields of technology involved: parachutes, airplane spinning, and airplane systems. Specialists in all these fields should be consulted or participate in the design of the spin-recovery parachute system from the very beginning.
INTRODUCTION
The armed services require a contractor to demonstrate by full-scale flight tests the spin and recovery characteristics of certain types of airplanes, such as fighter, attack, and trainer airplanes, as a standard part of the flight demonstration acceptance program. (See refs. 1 and 2.) During these spin demonstrations, the airplane is generally equipped with a tail-mounted spin-recovery parachute system as an emergency recovery device in case recovery from the spin cannot be effected by the airplane control surfaces. The U.S. Navy recently has added a new requirement to the full-scale spin demonstrations "(ref. 2) which states that the emergency spin-recovery device must be tested in a critical spin condition during the spin demonstration tests. Although spin-recovery parachute systems have been used for many years, the technology for system design and qualification
is very inadequatelydocumentedand few guidelines exist in this area. There have been, andcontinue to be, failures associatedwith spin-recovery parachute systems; many of these failures appear to be causedby the lack of understandingof the basic aerodynamic characteristics and mechanicsof the spin and also to lack of experience with spin- recovery parachute systems. This situation exists becausethere has been little conti- nuity in the design teams over the years with respect to spin-recovery parachute systems. The purpose of this paper is to summarize the design considerations for spin- recovery parachute systems so that the information will be readily available to those responsible for the design of such systems. The information that is presented herein was obtainedfrom a study of available documents (refs. 3 to 19), and from discussions with persons experienced in parachute technology, full-scale and model spin testing, and related systems. Personnel from the following organizations were consulted: (1) U.S. Air Force Flight Dynamics Laboratory, (2) U.S. Naval Air SystemsCommand,(3) Cessna Aircraft Company,(4) General Dynamics, Fort Worth Division, (5) Grumman Aerospace Corporation, (6) Irvin Industries, Inc., (7) Ling-Temco-Vought, Inc., (8) McDonnell Douglas Corporation, (9) M. Steinthal and Company,Inc., (10) Northrop Corporation, (11) Pioneer Parachute Company,and (12) NASALangley Research Center. As the information was being compiled, it becameevident that in a number of areas there is no clearly defined basis for determining a best approachfor insuring adequate performance of the recovery system. The present paper doesnot create newtechnology to fill these voids and doesnot presume to make recommendationsin areas where the present technologywill not clearly support such recommendations;hence, much of the paper discusses various aspectsof spin-recovery parachute system design without clear- cut conclusions. It was also apparentthat the technologywas best defined for tactical andtrainer military airplanes and was not considered to be applicable to other classes of airplanes, especially light general aviation airplanes; therefore, the present paper is limited to applications to military airplanes. Although the primary purpose of this paper is to discuss approachesin the design of tail-mounted spin-recovery parachute systems, it was felt that a brief discussion on other spin-recovery devices, such as rockets andwing-tip mountedparachutes, would also be in order, since these devices are alternate systems for accomplishing the same purpose. This information is presented in appendixA.
SYMBOLS
Units used for the physical quantities in this paper are given in the International Systemof Units (SI) and U.S. Customary Units. Measurementsand calculations were madein U.S. Customary Units. Factors relating the two systems are given in refer-
2
angle betweenwing spanaxis andhorizontal measured in vertical plane, positive when right wing is downregardless of spin direction, deg
f_ (^) angular velocity about spin axis, rps
TYPICAL CHARACTERISTICSOF SPINAND RECOVERY
Fully DevelopedSpin The fully developedspin is normally considered to be the critical design condition for a spin-recovery parachute system. Recently, however, it has becomeevident that it may be desirable under some circumstances to deploy the parachute during spin entry where the dynamic pressure may be higher than in a fully developedspin, andconsidera- tions for this condition are discussed in more detail in a later section of the report. In the fully developedspin the airplane is in vertical descentin a fully stalled atti- tude with the angle of attack generally between40° and90° and is rotating about the verti- cal flight-path axis. A sketch illustrating the attitude angles, spin radius, and rate of rotation of an airplane in a spin is shownin figure 2. The motion may be a steady rota- tion or the airplane may be oscillating violently in pitch androll. Oscillations of +30 ° to i45 ° are not uncommon. Any given airplane may have several different spin modes between flat or steep, fast or slow, and steady or oscillatory. In the steady-spin mode the aerodynamic forces and moments acting on the airplane are balanced by equal and opposite mass and inertia forces and moments so that an equilibrium condition exists. These mass and inertia forces and moments are produced by both the spinning rotation and the uneven mass distribution of the airplane about its body axes. As a result, an externally applied force or moment to a spinning airplane often causes the airplane to react like a gyroscope rather than in the normal manner expected in straight and level flight. The effects of these gyroscopic moments can be especially evident and important in spin recovery, as will be discussed in the following section. (Further details on spin characteristics of airplanes can be found in ref. 3.)
Spin Recovery The fully developed spin is primarily a yawing motion and thus the most effective means of terminating it is to apply a yawing moment to oppose the rotation. Considera- tion must be given, however, to the gyroscopic moments which result from the application of forces or moments other than those in yaw, especially since a spin-recovery parachute applies a pitching moment as well as a yawing moment. The gyroscopic moments may be beneficial or detrimental, depending upon the mass distribution of the airplane and the direction of the applied force or moment. Because of the gyroscopic effects, applying a
nose-down pitching moment, for example, to a fuselage-loaded airplane (Iy > IX) in a spin
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results in a yawing moment in the direction of the spin rotation (prospin) andusually causesthe spin rotation to increase rather than causethe airplane to pitch nose down, By contrast, the application of an antispin yawing moment will stop the spin rotation and simultaneously causethe airplane to nosedownout of the spin from its nose-high attitude becauseas the spin rotation is reduced, the gyroscopic nose-uppitching moment is reduced. Thus, in order to explain the action of the parachute for spin recovery, both the yawing momentand nose-downpitching moment applied by the parachute must be consid- ered. The yawing moment, as previously mentioned, is the most effective meansfor both stopping the rotation anddecreasing the angle of attack. The nose-downpitching moment however, is an undesirable byproduct of the parachute which, becauseof the resulting gyroscopic effects, generally will causemost tactical airplanes to spin faster and not recover, and may or may not causethe airplane to nosedown or decrease in angle of attack. If the parachute is properly sized, however, the yawing moment applied by the parachute will stop the spin rotation in spite of the adverse pitching-moment effect. If, on the other hand,the parachute size is too small, it is possible that the airplane would find a new spin equilibrium condition where the spin rate is higher and the pitch attitude is steeper. This newattitude andspin rate should not be considered an improvement over the original spin modebecausethe airplane could continueto spin without ever recovering. Thus, a properly sized parachute that will produce a sufficiently large antispin yawing moment is required in order to effect a satisfactory spin recovery. It also follows from the analysis of the gyroscopic effects that for airplanes with a wing-heavy loading (IX > Iy), the nose-downpitching momentas well as the antispin yaw- ing moment of the parachute will be favorable to spin recovery.
SPIN-RECOVERYPARACHUTE
Design Approach There are three distinctly different branches of technology involved in the design of a spin-recovery parachute system - parachutes, spinning, and airplane systems - and persons knowledgeablein all three of these fields shouldbe brought in on the design from the very outset. In the case of the parachute, such early coordination is particularly important becauseparachutetechnology is a very specialized branch of aeronautics. Expertise in this area is not generally available in an airplane manufacturing firm but can be obtainedfrom government sources or from the potential parachute manufacturer. For a given airplane, the spin-recovery parachute must be designedto recover the airplane from its worst spin condition. (The recovery of an airplane is considered to be completed whenthe spin rotation has beenterminated and the angle of attack has decreased
the Ames 12-foot pressure tunnel on a static model in a flat spinning attitude are pre- sentedin figure 3. The test Reynoldsnumber based on the meanaerodynamic chord of the model wing ranged from approximately 482 000to 3 × 106,but since there was no sig- nificant effect of Reynoldsnumber on the data, the results are presented for a Reynolds number of approximately 1.8 × 106. Figure 3 showsthe wake in terms of contour and profile plots of dynamic-pressure ratios q/q_ of 0.25, 0.50, and 0.75. In terms of full- scale values for a fighter airplane of representative size, the contour plots (fig. 3(a)) are at elevations abovethe airplane of approximately 15, 22, 30, and 38 meters (50, 75, 100, and 125ft). These results indicate that significant reductions in dynamic pressure extend 38 meters (125ft) or more abovethe airplane. Reference4 also presents some results of a wake survey conductedabovea static model mountedin spinning attitudes in a wind tunnel, and the results, in general, are similar to those just discussed. The results of smoke flow tests conductedin the Langley spin tunnel on a model similar to the one tested in the Ames tunnel, and mountedstatically in a spinning attitude, are presented in figure 3(b) as arrows showingthe direction of airflow. These results show a reverse airflow that can suck the parachute down on top of the airplane. Addi- tional tests with a parachute attachedto this model mounted in spinning attitudes and rotating on a spindle in the Langley spin tunnel verified the effects implied by these smokeflow tests in that if the parachute riser was too short, the parachute would be drawn down on the top of the model. Tests on full-scale airplanes also have confirmed these model tests. If the riser was madesufficiently long, however, the parachute would remain out of this reverse flow area. Thus, these results emphasizethe importance of getting the parachute far enoughawayfrom the airplane to avoid adverse wake effects. There is an equally important reason, however, for not making the riser too long since excessive length will causethe parachute to aline itself onthe spin axis; as a result, the parachute produceslittle or no antispin yawing momentwhile it produces an adverse nose- down pitching moment. Therefore, in such cases the parachute may be ineffective in ter- minating the spin. Of course, if the riser is too short, the parachute may not inflate properly or not inflate at all becauseof airplane wake effects. Therefore, careful con- sideration must be given to the selection of the riser length in order to avoid these detri- mental effects. Further information on model spin-recovery parachute testing can be found in ref- erence 5, wherein the parachutetests were conductedin a wind tunnel on a scale model of an airplane fixed at spinning angles of attack; the effects of riser length and canopy porosity on the inflation, stability, anddrag characteristics of the parachute were determined.
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Parachute Requirements Positive and reasonably quick opening(approximately 3 to 4 seconds)of the spin- recovery parachute is necessary for all operating conditions so that the spin may be ter- minated as rapidly as possible to minimize altitude loss. Openingcharacteristics are generally influenced by canopyporosity. Low porosity aids quick opening,but can result in high openingshock loads and an unstable parachute. High porosity may result in poor canopyinflation. Low snatch andopening shockloads are desirable not only from a para- chute load consideration but also to reduce the loads on the airplane structure as well, since the loads are generally of such magnitudethat it is necessary to reinforce the rear of the airplane. In addition, the parachute shouldbe reasonably stable with an amplitude of oscillation of less than approximately +10 °. A stable parachute is required so that it will tend to trail with the relative wind at the tail of the airplane in a spin and thus apply a yawing moment that is always antispin; whereas an unstable parachute because of its large oscillations may apply a yawing moment that varies from antispin to prospin, and thus hinders or prevents recovery. A canopy geometric porosity of 10 to 20 percent has generally been used to achieve the desired stability, and the degree of porosity apparently depends on the particular manufacturer's individual experience.
Parachute Type Experience has shown that either a ring-slot or a ribbon-type parachute will meet all the previously discussed requirements for a satisfactory spin-recovery parachute except the requirement for positive and quick opening. Since this shortcoming can be overcome by the use of inflation aids, these two types of parachutes are preferred for spin-recovery parachutes. The inflation aids may be used singly or in combination and the most generally used ones consist of the following: (1) pocket bands, (2) blow-out cap lightly stitched over canopy vent, and (3) additional vertical tapes sewn across slots in canopy to decrease geometric porosity. Some consideration has also been given in the past to a method which forcibly spreads the canopy skirt by ballistic means (such as used in some personnel escape systems) to aid in the inflation of spin-recovery parachutes. This approach has never been used, however, apparently because of added complexity and also because the parachute would have very high opening shock loads.
Parachute Diameter and Riser Length Determination of the correct parachute size and riser length obviously is very important in the overall design of a recovery system. The riser length controls the posi- tion of the parachute in the wake of the spinning airplane and therefore affects the force that the parachute can apply to the airplane. The minimum parachute diameter and riser length required to terminate the spin of military airplanes must be determined by tests of a dynamically scaled model of the airplane in the Langley spin tunnel. Experimental
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mining the effect of the wake, riser length, and canopyporosity on the ability of the para- chute to inflate andremain inflated.
SPIN-RECOVERYPARACHUTEINSTALLATION AND OPERATION
Parachute Compartment A fundamental requirement in any parachute installation is to locate the compart- ment and the riser attachment point as far aft on the airplane as possible. This approach will reduce the possibility of the riser or parachute striking the airplane and will also give the maximum moment arm for the parachute force to act on. It should be assumed that the angle the riser makes with the fuselage longitudinal axis can be as high as 90 ° if a flat or a highly oscillatory spin mode exists. If the riser is likely to contact the jet exhaust because of the attachment point location, then it must be protected against heat. Additional protection of the riser might be necessary if there is a possibility of its rub- bing against the airplane structure after deployment. Since the riser generally is made of fabric (for example, nylon), abrasions on or nicks in the riser while it is in tension can cause it to fail very rapidly. The parachute compartment also should be designed so that it does not change the spin and recovery characteristics of the airplane by changing the aerodynamic and/or inertia characteristic of the airplane with the installation and thereby invalidate the tests. Recent experiences with high-performance fighter-type airplanes have shown that the type of installations shown in figure 8 have generally met these requirements. Two types of parachute comparhnents in current use are (1) one in which the compartment is perma- nently attached to the airplane and deployment is initiated by pulling the deployment bag from the compartment with a pilot parachute, and (2) one in which the compartment is pulled away and completely separated from the airplane by a pilot parachute which then pulls the compartment off the deployment bag when the riser is fully extended.
Two major requirements for a satisfactory parachute compartment are that it be designed so that (1) the extraction of the deployment bag by pilot parachute or tractor rocket or by forceful ejection can be accomplished regardless of the airplane attitude, and (2) the bag be undamaged during the deployment process. A discussion is presented in appendix B on the methods used and considered to meet these requirements.
Based on the variety of approaches used in designing parachute compartments, there should be enough good design information available so that it is not only possible but highly desirable to develop several basic types which could be adapted to any airplane configuration through the use of suitable interfaces.
DeploymentBag and Packing Methods The deploymentbag may be divided into two or three sections generally dependingon the size of the parachute. If the parachute is small (diameter approximately 4.6 meters (15 ft) or less), the bag is divided into two sections with the canopypacked in one com- partment andthe suspensionlines and riser packed in the other. For larger size para- chutes, the canopy, suspensionlines, and riser are packed in separate compartments within the bag. These arrangements provide an orderly and reliable methodof deploy- ment. Sincethe riser of a spin-recovery parachute is muchlonger than that usedwith other types of parachutes, special care must be taken in packing the riser to insure a reliable deployment. The interior of the deploymentbag shouldbe made reasonably smooth to prevent hangupor friction burns; however, it generally is not necessary to pro- vide a special type of smoothlining in the bagto minimize friction burns becauseof the low extraction speedof the canopyfrom the bag. Selectionof a packing methodfor a spin-recovery parachutewill dependon the size, shape,and location of the parachute compartment. The following packing methodshave been used: (1) hand, (2) vacuum, (3) lace, and(4) mechanical. More detailed information on these packing techniquescan be found in appendixB and in reference 6 (pp. 372-375).
Parachute DeploymentMethods The two basic methodsfor deployingthe spin-recovery parachute from an airplane are the line-first andthe canopy-first methods shownin figure 9. The line-first method is preferred for several reasons, as indicated in the discussion of the method. Line-first method.- In the line-first method (fig. 9(a)), a pilot parachute extracts the deployment bag from the parachute compartment, deploying first the riser, then the parachute suspension lines, and finally, the recovery parachute by pulling the deployment bag off the parachute. The primary advantage of this method is that it provides a clean separation of the deployment bag from the airplane and also insures that the inflation of the spin-recovery parachute canopy will occur away from the airplane; thereby the pos- sibility of parachute fouling on the airplane and the effect of the airplane wake on the parachute are minimized. Furthermore, the snatch loads will be reduced because para- chute inflation will occur after the riser is fully extended. Canopy-first method.- In the canopy-first method (fig. 9(b)), the deployment bag remains attached to the parachute compartment. A pilot parachute extracts the spin- recovery parachute canopy from the bag, then the suspension lines, and finally the riser. The primary disadvantages of this method are (1) the increased possibility of the spin- recovery parachute fouling on the airplane; (2) the high snatch loads that occur because the spin-recovery parachute canopy will become inflated before the riser has become fully extended; (3) the high opening shock loads; and (4) the possibility of the canopy being 11
parachute on both flights deployedwithout fouling, on oneflight it came very close to the airplane tail; thus, the possibility of fouling on subsequentflights was indicated.
PILOT PARACHUTE
Requirements and Types The requirements for a satisfactory pilot parachute are similar to those for the spin-recovery parachute except that it is not necessary for the pilot parachute to be stable. Since the pilot parachute must apply only a force and not a moment to extract the deployment bag, reasonably large oscillations (+15 ° ) of the parachute do not appear to be a handicap. A major factor in choosing a particular type of parachute is the size of the pilot parachute needed to produce a required force. If the diameter of the pilot parachute is no larger than 2.5 meters (8 ft), a solid flat-type parachute may be used despite its high opening shock factor because the force will be relatively small. Since this type of pilot parachute has a high drag coefficient and consequently smaller area, it requires a lower packing volume than some other types. If a larger parachute is required, then ring-slot or ribbon-type parachutes should be used because of their lower opening shock factors.
Pilot Parachute Diameter and Bridle Length The technology for determining the size of the pilot parachute and the length of its bridle line is not well established according to reference 6 (pp. 392-393), and it has been common practice to rely on the judgment of the parachute manufacturer in this regard. A discussion of some important considerations based on past experience in applicable technology, however, may be beneficial and, therefore, this information is presented in the following paragraphs. The size of the pilot parachute is based on a compromise between making it small enough not to exert excessive loads on the extracted item or on the parachute itself at higher speeds that might be encountered in steep spins and making it large enough to extract the deployment bag at low rates of descent that might be encountered in flat spins. Also, for the condition where the pilot parachute remains permanently attached to the recovery parachute, excessive pilot parachute size may prevent the recovery parachute from opening because of too much tension being applied to the top of the parachute canopy when the riser has become fully extended. Thus, the diameter of the pilot parachute is important for the positive and orderly deployment of the spin-recovery parachute. Past experience seems to indicate that the pilot parachute should be sized to provide an accel- eration of 4g to 6g units on the deployment bag at the minimum dynamic pressure that might be expected; and, of course, it should be stressed for the highest dynamic pressure that might be encountered. 13
Althougha size for the pilot parachute may be estimated by considering the preced- ing factors, table I has been prepared to indicate what has beenused. This table is based on limited data and lists the pilot parachute sizes used satisfactorily in full-scale spins. The table showsthat the pilot parachute area in percent of the spin-recovery parachute area varied by a factor of about 3 (from 1.3 to 4.0 percent) with the exception of airplane 8. This airplane, used a large, heavy detachableparachute compartment which would require a very large pilot parachutefor proper system operation. Also it is possible that this particular parachute was sized to operate in a somewhatlower dynamic pressure environ- ment than the other pilot parachutes. There do not appear to be any detailed design guidelines for determining the bridle length of the pilot parachute. Available information on bridle lengths used satisfactorily on airplanes in full-scale spin demonstrations is presented in table I, andthis information indicates that the bridle lengths range from 1.9 meters (6.2 ft) to 17.2 meters (56.5ft). This wide variation in bridle lengths apparently isbased on two different approachesto the problem. Oneapproachis to make the bridle very long in an attempt to minimize wake effects of the airplane. The other approachis to make the bridle very short and assumethat the deploymentof the pilot parachute is a transient condition in that after the pilot parachute is deployed, it will be restrained only for an instant before it begins extracting the spin-recovery parachute package. Althoughthe pilot parachute will be in the wakeof the airplane (that is, region of reduced dynamic pressure for only an instant), there always is a possibility that the parachute may collapse during this instant if it moves into a region of reverse airflow. Although most pilot parachute installations havebeen successful with a short bridle line, this approach might not work for the newer airplane configurations with broad aft fuselages and large all-movable horizontal-tail surfaces. The wake from these airplanes is large not only becauseof their configuration, but also becausesuch airplanes tend to have flat spins which produce a larger wakethan the steeper spins. Thus, a much longer bridle line length might be necessary in order for the pilot parachute canopyto operate in high-energy airflow, and it also may be necessary to eject the pilot parachute away from the airplane with a mortar or deploymentgunto insure that it clears the wake.
Pilot Parachute DeploymentMethods To insure reliable pilot parachutedeployment, regardless of the deploymentmethod, the parachute shouldbe ejected rearward along the fuselage axis to get it into the clean airflow awayfrom the airplane wake in either an erect or inverted spin. There are three basic methodscurrently usedto deploy a pilot parachutefrom spinning airplanes: a deploymentmortar system, a deploymentgun system, and a spring-loaded system.
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