- Email: [email protected]
Impact tests of a graphite-epoxy helicopter tail rotor blade
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE*
JOHNJ. LUCAS
Supervisor, Materials DeveloRment, Sikorsky Aircraft Division, United Aircraft Corporation, Stratford, Connecticut 06602 (USA) (Received: 8 May, 1974)
SUMMARY
Full-scale whirl impact tests simulating tree strikes and low-velocity wrench drop tests have been conducted on a full-scale graphite~glass-epoxy 11 f t dia. two-bladed tail rotor. Damage following whirl impact of ½ in and 1 in dia. hard maple wood dowels and a 1¼ in dia. maple tree branch was limited to the tip cap and tip balance weight retention bolts and was not sufficient to cause a mission abort. Low-velocity impact tests were performed by dropping a 1.6 lb wrench and a 1.9 lb steel projectile from a height of 12 f t on to various areas of the blade. The construction of the blade, which completely encases the graphite-epoxy spar with a glass-epoxy torque tube and aerodynamic shell, proved to be relatively invulnerable to this type of hazard.
ROTOR CONSTRUCTIONAND MATERIALS The rotor tested was an 11 ft dia. two-bladed configuration which is known as Sikorsky's cross-beam rotor system (XBR). The tail rotor (Fig. 1) uses the unique anisotropic properties of high modulus composites to eliminate bearings. The main spar consisted of a unidirectional (0 °) rectangular-shaped A/S graphite-epoxy spar which is continuous from the tip of one blade, through the hub, to the tip of the other blade. A layer of glass scrim cloth is interleaved between each layer of graphite. The spar is approximately 5 in wide x 0.6 in. thick at the inboard end and tapers to 3 in wide x 0.3 in thick at the outboard ends. A light hub can thus be utilised for blade attachment since centrifugal forces from each blade are balanced by means of the opposite blade. Outboard of the 'flex area' the spar is * Presented at the ASTM Symposium on Foreign Object Impact Behaviour of Composites, Philadelphia, Pa, 20 September, 1973. 295 Fibre Science and Technology (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
296
JOHN J. LUCAS
UNIDIRECTIONAL (0° ) GRAPHITE/EPOXY SPAR .HUB / / --PITCH CONTROL HORN Y
.
)
I /
I
+
/ /
/
I
.TORQUE
,
SFER RIB
'"
:
i
FITCH CONTROL CARRIED
~AERODYNAMIC
l] -+45°
IOWA,S;EPOXYOVER
SPAR
FLEX
Fig. 1.
AREA""""'~
Bearingless r o t o r configuration.
encased within a torsionally stiffaerodynamic shell of ___45 ° 1002S glass-epoxy with a combination of aluminium and Nomex honeycomb fillers. The 0 ° graphite-epoxy spar is soft in torsion and allows pitch motion to be achieved by twisting of the spar in the 'flex area'. Pitch control forces and resulting blade pitch motions are accomplished by a control movement at the root end of the +45 ° glass-epoxy torque tube (which is not attached to the spar) and are transferred through the tube to the torsionally stiff aerodynamic shell which starts at the torque transfer rib as shown in Fig. 1. A complete fibreglass shell thus encases the graphite-epoxy spar. The thickness of the glass-epoxy shell (torque tube and aerodynamic cover skin) tapers from approximately 0.1 in at the inboard end to 0.024 in at the outboard end. A combination of stainless steel and polyurethane tape is utilised for protection of the blade leading edge as indicated in the figures. An aluminium tip cap is attached to the outboard end of the spar and covers the chordwise and spanwise balance weights. WHIRL IMPACT TESTS
To demonstrate the ability of a rotor of this configuration to withstand damage from potential tree strikes which could occur if the aircraft were operating at tree-
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
297
top level, the rotor was mounted on Sikorsky's 2000 hp tail rotor whirl test stand for whirl impact tests (Fig. 2). In addition to being able to spin the rotor up to 100 per cent normal operating speed, the test stand is able to precess or revolve about a vertical axis. The obstacles to be impacted were attached to a movable work platform which could be moved towards or away from the rotor as desired. Initially, a ½ in dia. hard maple dowel was attached to the end of a pole extending from the work platform (Fig. 3), and while running the rotor at 1200 rpm the rotor was precessed at 0.5 tad/see into the dowel. Impact occurred as one of the tips passed into the dowel. The impact resulted only in a slight dent in one tip cap. The same tests were repeated using a 1 in dia. maple dowel (Fig. 4) and resulted in minor tip cap damage to one blade (Fig. 5) and tearing of the other tip cap with yielding of the balance weight retention bolts (Fig. 6). Another 1 in dia. maple dowel was then mounted on the work platform and the work platform and dowel were moved into the plane of the rotor for about 12 in at about 6 in/see. The dowel was positioned to impact at about the middle of the stainless steel abrasion strip. No visible or other damage was obtained except for some slight scuffing marks. A maple tree branch was then mounted on the work platform (Fig. 7). The branch had a maximum diameter of about 1¼ in. The rotor was initially precessed into the end of the branch and the tree was then moved about 5 ft into the rotor at 6 in/see (Figs. 8 and 9). No structural damage was experienced except that the polyurethane peeled away in spots (Figs. 10 and 1 l). Pieces of the impacted dowels and branch are seen in Fig. 12. Following each test the non-rotating flatwise and edgewise natural frequency was determined, and following conclusion of the low-velocity impact tests the blade was torn down and the spar ultrasonically inspected. No internal damage was detected by any of these inspections. None of the tip damage experienced would result in a condition which would have caused a mission abort. LOW-VELOCITY IMPACT TESTS
To demonstrate the tolerance of this rotor construction to high-mass, low-velocity impacts, such as wrench drops, a series of tests was conducted by dropping first a steel projectile and then an actual wrench on to the blade from a height of 12 ft (Fig. 13). The projectile was a 1-9 lb torpedo-shaped steel dart with a ] in dia. steel nose and was dropped through a guided tube into the blade in nine different locations (Fig. 14). A torque wrench and socket simulating the largest used on the tail rotor installation (1.6 lb) was then also dropped in similar locations. The only damage incurred visually was a ~ in deep depression in one of the outboard trailing edge areas. The depression was only discernible by touch. At the conclusion of testing the blade was torn down and the spar was impacted directly with the projectiles up to 25 ft lb energy. The spar was then visually and ultrasonically inspected and no damage was detected.
JOHN J. LUCAS
298
Fig. 2.
2000 h p tail r o t o r whirl test stand.
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
Fig. 3.
Precession into ½ in dia. wood dowel.
299
JOHN J, LUCAS
300
Fig. 4.
Precession into 1 m d i a . w o o d dowel.
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
Fig. 5.
Tip cap damage after 1 in dia. wood dowel impact (blaoe (a)).
301
302
Fig. 6.
JOHN J. LUCAS
Tip cap and balance weight retention bolt damage after 1 in dia. wood dowel impact (blade (b)).
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
\
\
\
303
\ N N
\,
\
\
\
\
\
\
\
\
Fig. 7.
Rotor precessed into end of 1¼ in dia. maple tree branch.
\
\
JOHN J. LUCAS
304
\
\
\
\
\
,\ I
\
\
\
\
Fig. 8.
1¼ in dia. maple tree branch being moved into plane of rotor at 6 in/sec.
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAlL ROTOR BLADE
Fig. 9.
Remains of maple tree branch after whirl impact.
305
306
J O H N J. L U C A S
¢o "O
t~
t-
2;
E ,.Q
E
t~
IMPACT
TESTS OF A GRAPHITE-EPOXY
HELICOPTER
TAlL ROTOR
BLADE
307
O
t~
"O t2
...--~
..O O
0)
E t~ 0~ "a
308
JOHN J. LUCAS
o3
E
o3
1
t~
o3
o
a: r~
X
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
Fig. 13.
General arrangement for low-velocity impact tests.
309
310
JOHN J, LUCAS
-tlj Fig. 14. Location of wrench drop and projectile impacts.
CONCLUSION
It is concluded that the graphite and glass-epoxy rotor described herein exhibits good survivability characteristics in the hostile environment for which it was designed. The concern with brittle impact behaviour of thin-walled graphite-epoxy structures is apparently not applicable to thick structures in the configuration evaluated.
JOHNJ. LUCAS
Supervisor, Materials DeveloRment, Sikorsky Aircraft Division, United Aircraft Corporation, Stratford, Connecticut 06602 (USA) (Received: 8 May, 1974)
SUMMARY
Full-scale whirl impact tests simulating tree strikes and low-velocity wrench drop tests have been conducted on a full-scale graphite~glass-epoxy 11 f t dia. two-bladed tail rotor. Damage following whirl impact of ½ in and 1 in dia. hard maple wood dowels and a 1¼ in dia. maple tree branch was limited to the tip cap and tip balance weight retention bolts and was not sufficient to cause a mission abort. Low-velocity impact tests were performed by dropping a 1.6 lb wrench and a 1.9 lb steel projectile from a height of 12 f t on to various areas of the blade. The construction of the blade, which completely encases the graphite-epoxy spar with a glass-epoxy torque tube and aerodynamic shell, proved to be relatively invulnerable to this type of hazard.
ROTOR CONSTRUCTIONAND MATERIALS The rotor tested was an 11 ft dia. two-bladed configuration which is known as Sikorsky's cross-beam rotor system (XBR). The tail rotor (Fig. 1) uses the unique anisotropic properties of high modulus composites to eliminate bearings. The main spar consisted of a unidirectional (0 °) rectangular-shaped A/S graphite-epoxy spar which is continuous from the tip of one blade, through the hub, to the tip of the other blade. A layer of glass scrim cloth is interleaved between each layer of graphite. The spar is approximately 5 in wide x 0.6 in. thick at the inboard end and tapers to 3 in wide x 0.3 in thick at the outboard ends. A light hub can thus be utilised for blade attachment since centrifugal forces from each blade are balanced by means of the opposite blade. Outboard of the 'flex area' the spar is * Presented at the ASTM Symposium on Foreign Object Impact Behaviour of Composites, Philadelphia, Pa, 20 September, 1973. 295 Fibre Science and Technology (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
296
JOHN J. LUCAS
UNIDIRECTIONAL (0° ) GRAPHITE/EPOXY SPAR .HUB / / --PITCH CONTROL HORN Y
.
)
I /
I
+
/ /
/
I
.TORQUE
,
SFER RIB
'"
:
i
FITCH CONTROL CARRIED
~AERODYNAMIC
l] -+45°
IOWA,S;EPOXYOVER
SPAR
FLEX
Fig. 1.
AREA""""'~
Bearingless r o t o r configuration.
encased within a torsionally stiffaerodynamic shell of ___45 ° 1002S glass-epoxy with a combination of aluminium and Nomex honeycomb fillers. The 0 ° graphite-epoxy spar is soft in torsion and allows pitch motion to be achieved by twisting of the spar in the 'flex area'. Pitch control forces and resulting blade pitch motions are accomplished by a control movement at the root end of the +45 ° glass-epoxy torque tube (which is not attached to the spar) and are transferred through the tube to the torsionally stiff aerodynamic shell which starts at the torque transfer rib as shown in Fig. 1. A complete fibreglass shell thus encases the graphite-epoxy spar. The thickness of the glass-epoxy shell (torque tube and aerodynamic cover skin) tapers from approximately 0.1 in at the inboard end to 0.024 in at the outboard end. A combination of stainless steel and polyurethane tape is utilised for protection of the blade leading edge as indicated in the figures. An aluminium tip cap is attached to the outboard end of the spar and covers the chordwise and spanwise balance weights. WHIRL IMPACT TESTS
To demonstrate the ability of a rotor of this configuration to withstand damage from potential tree strikes which could occur if the aircraft were operating at tree-
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
297
top level, the rotor was mounted on Sikorsky's 2000 hp tail rotor whirl test stand for whirl impact tests (Fig. 2). In addition to being able to spin the rotor up to 100 per cent normal operating speed, the test stand is able to precess or revolve about a vertical axis. The obstacles to be impacted were attached to a movable work platform which could be moved towards or away from the rotor as desired. Initially, a ½ in dia. hard maple dowel was attached to the end of a pole extending from the work platform (Fig. 3), and while running the rotor at 1200 rpm the rotor was precessed at 0.5 tad/see into the dowel. Impact occurred as one of the tips passed into the dowel. The impact resulted only in a slight dent in one tip cap. The same tests were repeated using a 1 in dia. maple dowel (Fig. 4) and resulted in minor tip cap damage to one blade (Fig. 5) and tearing of the other tip cap with yielding of the balance weight retention bolts (Fig. 6). Another 1 in dia. maple dowel was then mounted on the work platform and the work platform and dowel were moved into the plane of the rotor for about 12 in at about 6 in/see. The dowel was positioned to impact at about the middle of the stainless steel abrasion strip. No visible or other damage was obtained except for some slight scuffing marks. A maple tree branch was then mounted on the work platform (Fig. 7). The branch had a maximum diameter of about 1¼ in. The rotor was initially precessed into the end of the branch and the tree was then moved about 5 ft into the rotor at 6 in/see (Figs. 8 and 9). No structural damage was experienced except that the polyurethane peeled away in spots (Figs. 10 and 1 l). Pieces of the impacted dowels and branch are seen in Fig. 12. Following each test the non-rotating flatwise and edgewise natural frequency was determined, and following conclusion of the low-velocity impact tests the blade was torn down and the spar ultrasonically inspected. No internal damage was detected by any of these inspections. None of the tip damage experienced would result in a condition which would have caused a mission abort. LOW-VELOCITY IMPACT TESTS
To demonstrate the tolerance of this rotor construction to high-mass, low-velocity impacts, such as wrench drops, a series of tests was conducted by dropping first a steel projectile and then an actual wrench on to the blade from a height of 12 ft (Fig. 13). The projectile was a 1-9 lb torpedo-shaped steel dart with a ] in dia. steel nose and was dropped through a guided tube into the blade in nine different locations (Fig. 14). A torque wrench and socket simulating the largest used on the tail rotor installation (1.6 lb) was then also dropped in similar locations. The only damage incurred visually was a ~ in deep depression in one of the outboard trailing edge areas. The depression was only discernible by touch. At the conclusion of testing the blade was torn down and the spar was impacted directly with the projectiles up to 25 ft lb energy. The spar was then visually and ultrasonically inspected and no damage was detected.
JOHN J. LUCAS
298
Fig. 2.
2000 h p tail r o t o r whirl test stand.
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
Fig. 3.
Precession into ½ in dia. wood dowel.
299
JOHN J, LUCAS
300
Fig. 4.
Precession into 1 m d i a . w o o d dowel.
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
Fig. 5.
Tip cap damage after 1 in dia. wood dowel impact (blaoe (a)).
301
302
Fig. 6.
JOHN J. LUCAS
Tip cap and balance weight retention bolt damage after 1 in dia. wood dowel impact (blade (b)).
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
\
\
\
303
\ N N
\,
\
\
\
\
\
\
\
\
Fig. 7.
Rotor precessed into end of 1¼ in dia. maple tree branch.
\
\
JOHN J. LUCAS
304
\
\
\
\
\
,\ I
\
\
\
\
Fig. 8.
1¼ in dia. maple tree branch being moved into plane of rotor at 6 in/sec.
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAlL ROTOR BLADE
Fig. 9.
Remains of maple tree branch after whirl impact.
305
306
J O H N J. L U C A S
¢o "O
t~
t-
2;
E ,.Q
E
t~
IMPACT
TESTS OF A GRAPHITE-EPOXY
HELICOPTER
TAlL ROTOR
BLADE
307
O
t~
"O t2
...--~
..O O
0)
E t~ 0~ "a
308
JOHN J. LUCAS
o3
E
o3
1
t~
o3
o
a: r~
X
IMPACT TESTS OF A GRAPHITE-EPOXY HELICOPTER TAIL ROTOR BLADE
Fig. 13.
General arrangement for low-velocity impact tests.
309
310
JOHN J, LUCAS
-tlj Fig. 14. Location of wrench drop and projectile impacts.
CONCLUSION
It is concluded that the graphite and glass-epoxy rotor described herein exhibits good survivability characteristics in the hostile environment for which it was designed. The concern with brittle impact behaviour of thin-walled graphite-epoxy structures is apparently not applicable to thick structures in the configuration evaluated.