Accelerative Forces PDF

ACCELERATIVE FORCES Date: 18 Jan 2022 Revision no. 01 ACCELERATIVE FORCES  The human body is well adapted to an existence in an environment with the force of Earth’s gravity.  Modern A/C is highly maneuverable & capable of flying at very high speeds, & during certain periods the occupant may be...

ACCELERATIVE FORCES Date: 18 Jan 2022 Revision no. 01 ACCELERATIVE FORCES  The human body is well adapted to an existence in an environment with the force of Earth’s gravity.  Modern A/C is highly maneuverable & capable of flying at very high speeds, & during certain periods the occupant may be exposed to Accelerative forces of large magnitude. 2 Types of Accelerative Forces  Short duration forces o Act on the body of period of less than 1 second. (Encountered in ejection seat escape).  Long duration forces o Act on the body for period of more than 1 second & may last for several minutes. (Encountered during various A/C maneuvers & during launch & re-entry of space vehicles). Physiological effects of long duration type are produced by: - Sustained distortion of tissues & organs - Alterations in the flow & distribution of blood & body fluids. PHYSICAL CONSIDERATIONS: SPEED: The rate of change of distance (change of distance/ time). SPEED = DISTANCE TIME = m/sec VELOCITY: Describes the rate & direction of movement. (Change of distance & direction/ time / meter/sec). Thus, it is a vector quantity. ACCELERATION: Change of velocity/ time m/sec2. Vector quantity having magnitude & direction.  If change rate of movement along straight line → Linear Acceleration  If change direction of movement → Radial Acceleration  If change rate & direction → Angular Acceleration  If velocity decreasing/ time → Negative Acceleration (decelerate)  If velocity Increasing/ time → Positive Acceleration • In aviation, Acceleration forces are expressed as multiples of the acceleration due to gravity (g) (gravitational constant). • g has the value of 9.81 m/sec2 • The unit of the ratio of an applied acceleration to the gravitational constant is the G. • Thus, the G value of an applied acceleration G is = Applied Acceleration/g • So, the value of an acceleration of 2G is = 2X9.81=19.62 m/sec2. *LINEAR ACCELERATION • Change of speed/ time without a change in direction. • In Aviation, Linear acceleration is produced during: o Catapult-assisted take-off o Arrested Landing o Launch & recovery of spacecraft o This type of acceleration also occurs during take-off, landing, or in level flight when a throttle setting is changed. *Radial Acceleration • Changes of direction of motion without a change of speed. o Aircraft Maneuvers o Centrifuges o when a pilot performs a sharp turn, pushes over into a dive, or pulls out of a dive. * Angular Acceleration o Results from a simultaneous change in both speed and o direction, which happens in: o Spins and climbing turns. Newton’s first law → unless acted upon by a force, a body will remain (Inertia). On determining the effect of acceleration on man, the direction of the accelerative force & direction of the inertial force are described by 3 axes coordinate system. The vertical axis Z is the long axis of the body. Head ward acceleration will displace viscera & blood foot ward & the resultant force is termed +Gz). EFFECTS OF LONG DUARTION ACCELERATION (POSITIVE) *In some high performance combat A/C, occupants can be exposed to up to 9-10 positive G for 10-60 seconds 1.General Effect a. Mobility: Increase in weight i. It is difficult to raise oneself from the sitting posture at +2.5Gz & impossible to do so at +3Gz. (Impossible escape from an uncontrolled A/C). Handle of ejection seat is in front of the face. b. Vision i. Exposure to +4.5 Gz will produce complete loss of vision (black out), while hearing & mental activity are unaffected. ii. At lower level of G forces visual activity reduced + loss of peripheral vision (Grey out). iii. The intensity of the visual symptoms often decreases by 8-12 seconds after the onset of the G force. iv. Normal vision normally returns 3-5 seconds after the maneuver ends. c. Unconsciousness i. After blackout any additional G. force → L.O.C. ii.Exposure of very high G forces, immediate L.O.C. will occur before experiencing blackout. iii.Recovery of unconsciousness after end of exposure is usually slow. 2. Cardiovascular Effect Actually, the effect on vision & mental performance are due to ↓ of blood flow through the vascular beds of eyes & brain. a. Initial hydrostatic effects b. Secondary circulatory changes c. Retinal Circulation d. Cerebral Circulation e. Skin Capillaries 3. Pulmonary Effect - Pulmonary Ventilation & lung volumes o Positive Acceleration causes descent of the abdominal content & diaphragm →↑ Functional residual Capacity - Regional lung ventilation o The increase in the weight of the lung compresses the lower parts, causing closure of the terminal airways & the alveoli become no longer ventilated. - Lung Collapse Tolerance to Positive Acceleration - Heat Stress o Exposure to heat reduces the tolerance to positive G Force. 1c° rise in body temperature reduces the level of acceleration at which blackout occurs by 30-40%. (Mechanism is continuous vasodilation & shift of blood to periphery). - Hypoglycemia o A 50% reduction of the glucose concentration below the resting value, reduces the blackout threshold by about 0.6G. - Alcohol o Ingestion of alcohol reduces the tolerance of positive acceleration. The threshold reduces by 0.1-0.4G. - Hypoxia o If PO2 in alveoli is 70mmHg, the threshold for blackout will reduce by 0.5G. - Empty Stomach o Distension of the stomach increases the tolerance. Drinking 1.5 L of water increases the threshold for blackout by 0.6-1.3 G. o Mechanism: Full stomach prevents the diaphragm from descent → reduce heart/ brain distance. - Intercurrent infection: URTI as an example, markedly reduces tolerance to positive acceleration. - Hypotension Effects of Negative Gz Examples of Negative Gz: - Outside loops & spins - Simple inverted flights *Tolerance of negative Gz is much lower than that of positive acceleration & the symptoms produced by -2 Gz are almost not bearable. *Low levels of – Gz produces serious decreament of performance. Effects: 1. General Effect  Feeling of heaviness & difficult limbs movements are similar like the positive Gz effects.  The specific effect Negative G force occur primarily in the head & neck. i. 1Gz  sense of fullness & pressure in the head ii.2Gz  severe throbbing headache which may persist for hours after the exposure i. 2.5Gz  Even for few seconds  edema of the eyelids & petechial hemorrhages in the skin of the face & neck. Congestion of the mucosa lining in the air passages may cause difficulty in breathing & epistaxis may occur. ii.2.5 to 3Gz  Feeling as if there are popping out of the head. The conjunctiva is suffused & descent of the lower eyelid with excessive lacrimation, reddening & blurring of vision with subjunctival hemorrhage. iii.4 to 5Gz  for longer than 6 seconds causes mental confusion & unconsciousness. 2. Pulmonary Effect Tolerance of Negative G-force - Not well tolerated. The maximum -? Gz force which can be tolerated for several seconds is -5Gz for maximum 5 seconds, -3Gz can probably be tolerated by most individual in the seated posture for 10-15 seconds, whilst 2Gz is acceptable for up to 5 minutes. Transverse Acceleration Gx • Rarely can be seen in aviation. Example: catapult launch, rocket when occupants are seated transverse. The force involved are small relative to man’s tolerance & do not give rise to problem. • Accelerative force is from right to left & the internal force is from left to right. Forward & Backward Acceleration Gy Protection Against Acceleration • There are several methods to raise the threshold to the effects of +G forces. They are based on physiological principles. I. Voluntary Actions a. Avoidance of additive stresses i. Heat & dehydration ii.Alcohol intake iii.Hypoglycemia iv.Hypoxia v.Empty stomach vi.Fatigue vii.Infection viii.Hypotension - Avoidance will maintain the Normal tolerance. b. Muscle Tensing • It has been recognized that straining & tensing of muscles is an effective method of raising the blackout threshold. • (Voluntary sustained contraction of a large number of skeletal muscles could increase tolerance by 2G or more). • Mechanism: Increase venous return to the heart & prevention of pooling of blood in the lower limb, +  intra-abdominal pressure & slight decrease of the heart-brain distance. c. Valsalva Maneuver • Forcible expiration against closed glottis   pressure within the thorax & abdomen  this will be transmitted through the heart & great vessels to the arterial system   perfusion to the brain. d. Anti-G Straining Maneuver (AGSM) also called M-I Maneuver • It remains the principal mean used to increase G tolerance above 5G. • “It is a forced exhalation effort against partially closed glottis while tensing leg, arm, & abdominal muscles to maintain vision & consciousness”. The forced expiration is interrupted briefly once every 3-4 seconds by a rapid inspiration. • The major problem with this maneuver is that it is fatiguing. *Performing of this voluntary action is effective in increasing the tolerance to +? Gz by as much as 2G. II. Posture *The previous procedures provided methods to protect against accelerative forces through Raising blood pressure. *Through changing the posture, we will try to Reduce the height through which the blood has to be pumped to preserve cerebral blood flow. *Any measure which reduces the vertical distance between heart & brain will provide a degree of protection against blackout & L.O.C. *Similarly, a change of posture reduces the pooling of blood to the lower limbs will maintain the circulating blood volume. - Leaning forwards: crouching in the seat harness may reduce the heart to brain column of blood by as much as 8 inches. - Raising the feet - Increasing the back angle: Increasing the back angle to 75º raises the relaxed gray out threshold by 3G. III.Drugs • Theoretically to raise the blood pressure & to increase the vasomotor tone by means of drugs to raise tolerance to +Gz. - Nikethamide - Amphetamine - Adrenaline - Adrenocorticoids - Breathing CO2 IV.Anti-G suit: Pneumatically inflated suit • The most suitable & acceptable method of reducing the effects of positive G forces. Gives protection of around 2G. • It is a trouser-like garment, cut away at the knees to permit greater mobility. Consist, of 5 interconnecting bladders, one on the abdomen, 2 around the thighs, & 2 around calves. • The bladder system is inflated by the Anti-G valve which is mounted in the A/C & controls the flow of air from the engine compressor into the suit. The inflation occurs automatically on exposure to positive GZ. Thank you • G Forces • • • During flight, a pilot may experience a combination of these accelerations as a result of input to the flight controls. These accelerations induce Gforces on the body that may be described as Gx, Gy, and Gz. Gx is described as force acting on the body from chest to back; +Gx is experienced, for example, during the take-off roll as the throttle is advanced. This is the force that pushes the pilot back into the seat as the aircraft accelerates. 44 • • • • -Gx is described as force from back to chest, and it is encountered during landing as the throttle is closed. This force pushes the pilot forward into the shoulder strap. Naval pilots flying from aircraft carriers feel the extremes of this type of G force. During a catapult launch, the aircraft accelerates to 160-plus mph in just under two seconds. During landing, the aircraft will decelerate to a complete stop in just a few feet. Carrier pilots have adapted and successfully functioned with these extreme Gs for decades. 45 • • • Gy is a lateral force that acts from shoulder to shoulder, and it is encountered during aileron rolls. Aerobatic pilots routinely encounter this type of G force and can still safely and precisely maneuver their aircraft. Gz is a gravitational force that is applied to the vertical axis of the body. If it is experienced from head to foot, it is termed (positive) +Gz. This happens when a pilot pulls out of a dive or pulls into an inside loop. -Gz (negative) travels from foot to head, and it is experienced when a pilot pushes over into a dive. 46 47