ACCELERATION Introduction Aircrew can be insulated from most environmental stresses of flight such as heat, hypoxia and noise. However this is not true for acceleration. As a/c become more agile the worse there acceleration stresses. There is unfortunately no anti-gravity device. G-suits are not anti-gravity devices but just alleviates the effects. With new aircraft operating in excess of 8G the human is operating at the limit of his performance. Of particular concern is the potential of for G-induced loss of consciousness or G-LOC
ACCELERATION Forces due to acceleration are vector quantities having proportion both of magnitude and direction. Acceleration occur whenever there is a change in velocity, direction of motion at uniform velocity. Forward acceleration causes a backwards acting inertial force. Centripetal force will be sensed as the resultant centrifugal force. The effect on the body will depend markedly upon its orientation relative to the force vector.
ACCELERATION EFFECT OF ACCELERATION DEPENDS ON: Magnitude: Measured in units of G, this being the ratio of the applied force to the standard acceleration of gravity g, or 9,81m/s². Thus, 5G is equal to an acceleration of 49,05m/s² and the weight of the body is increased fivefold. Direction: The vector direction of acceleration forces is defined according to a three-coordinate system based upon the long axis of the body. Duration: Classified into: –a. Impact acceleration acting for a second or less. –b. Sustained acceleration acting for a second or more.
ACCELERATION NOMENCLATURE FOR VECTORS OF ACCELERATION AND INERTIAL FORCE Direction Description Standard Vernacular Upward s Positive G +GZ Eyeballs down Downwards Negative G -GZ Eyeballs up Forwards Transverse +GX Eyeballs in supine G Backwards Transverse -GX Eyeballs out prone G To right Left lateral G +GY Eyeballs left To left Right lateral G -GY Eyeballs right
Supine G Restraint: In a crash the occupant will continue along his initial velocity vector unless acted upon by a force. This can be done in a controlled manner through the use of a restraint harness or uncontrolled by him striking surrounding structures. Site of action: Sustained acceleration affects all parts of the body but in the dynamic situation of impact differential forces occur. Rate of onset: If the onset time of an impact force is comparable to the natural undamped frequency of the injury mechanism then the system will be excited and overshoot can occur.
ACCELERATION Physiology of sustained +GZ acceleration Effects similar to those of moving from the lying to standing posture. Primary effect is on the cardiovascular system in the form of increase in hydrostatic pressure gradients. 1 GZ BP at brain level is 22 mmHg lower than at heart level. At 5 GZ BP at brain level will fall to 10 mmHg. This effect is immediate and inevitable. The heart and diaphragm fall and the heart to brain distance increases.
ACCELERATION Hydrostatic increase in intravascular pressure below heart level causes vascular engorgement, especially the veins in legs and abdomen, which causes a decrease in the circulating blood volume and decreased venous return. Raised intravascular and transmural pressure causes extravasation of fluid into the tissues with a further slow but progressive loss in circulating volume. The initial fall in BP causes stimulation of the carotid baroreceptors and an increase in heart rate and peripheral resistance with a little restoration of BP at head level.
ACCELERATION Local circulatory demands acts in opposition to the generalised vasoconstrictor response with a redistribution of the available cardiac output with the myocardium and brain receiving disproportionately more blood flow. Owing to the intraocular tension which must be overcome to permit retinal perfusion, the blood supply to the eye fails at about 1GZ lower acceleration than that to the brain. Gradual onset will lead to greyout with tunneling up to blackout and finally G-LOC. With rapid onset retinal Oxygen allows vision to be maintained for 4-5 seconds. G-LOC is frequently followed by fitting, with flailing of the arms and a period of relative incapacitation.
ACCELERATION The gradient of transpulmonary pressure stems from the weight of the lung tissue so that at +5GZ the gradient increases fivefold to about 1cm water per cm vertical distance. Thus for a lung 30cm tall there will be 30cm water difference in transpulmonary pressure from apex to base with gross changes in regional alveolar pressure and regional alveolar ventilation with the following: –The terminal airways close off with ventilation of the overperfused area ceasing. –Absorption of the trapped gas is delayed by the presence of Nitrogen but when 100% Oxygen has been breathed the entire gas volume will be rapidly absorbed and the lung collapses, called acceleration atelectasis.
ACCELERATION If maintained, +GZ leads to vasovagal syncope. This is caused by the continued loss of circulatory blood volume with eventual breakdown of the cardiovascular compensatory mechanism causing a bradycardia, fall in peripheral vascular resistance and hypotension with loss of consciousness.
ACCELERATION Tolerance to +GZ acceleration Anti-G suit: Counter pressure acts on capacity vessels to decrease their transmural pressure and so reduce the pooling of blood and extravasation of fluid. Anti-G straining manoeuvre: Combination of generalised muscle tensing, straining,and a forced expiratory effort. Positive pressure breathing: The application of positive pressure through the aircraft breathing regulator. Reduces muscle fatigue.
ACCELERATION Physical conditioning: Modest weight training of all muscle groups increases the time for which high-G can be tolerated. Centrifuge training: An effective way of teaching an efficient AGSM and so raise G-tolerance. Posture: Crouching forward and raising the rudder pedals assist G-tolerance by reducing the vertical heart-brain distance and improving venous return from the legs.