aviation medicine 航空医学 (I)

choi

(also known as aerospace medicine 航(空)太(空)医学 in Taiwan and /航空航天医学 in China)

After yesterday's posting on hypersonic plane, I was intrigued by how long the manned hypersonic plane X-15 flew hypersonically, because humans can not stand high g-force for long.

(1)
(a) Newton's second law of motion is expressed as F=ma, where F, m and a stands for force, mass and acceleration, respectively.
(b) g-force
https://en.wikipedia.org/wiki/G-force

(i) introduction:

The g-force is "expressed in units of standard gravity

"a force of 1 g on an object sitting on the Earth's surface is caused by the mechanical force exerted in the upward direction by the ground, keeping the object from going into free fall.

"Objects allowed to free-fall in an inertial trajectory, under the influence of gravitation only [in absence of friction], feel no g-force – a condition known as weightlessness. Being in free fall in an inertial trajectory is colloquially called 'zero-g,' which is short for 'zero g-force.' Zero g-force conditions would occur inside an elevator falling freely toward the Earth's center (in vacuum), or (to good approximation) inside a spacecraft in Earth orbit.

(ii) section 1 Unit and measurement: "The unit of measure of acceleration in the International System of Units (SI) is m/s2.[4] However, to distinguish acceleration relative to free fall from simple acceleration (rate of change of velocity), the unit g is often used. One g is * * * is the standard gravity defined as 9.8 metres per second squared * * *

(iii) section 2 Acceleration and forces: "The term g-'force' is technically incorrect as it is a measure of acceleration, not force. While acceleration is a vector quantity, g-force accelerations ('g-forces' for short) are often expressed as a scalar, based on the vector magnitude, with positive g-forces pointing downward (indicating upward acceleration), and negative g-forces pointing upward.

(iv) section 3 Human tolerance
section 3.1 Vertical:
section 3.2 Horizontal: "The human body is better at surviving [horizontal] g-forces that are perpendicular to the spine. In general when the acceleration is forwards (subject essentially lying on their back, colloquially known as 'eyeballs in'),[14] a much higher tolerance is shown than when the acceleration is backwards (lying on their front, 'eyeballs out')

Note:
(i) The en.wikipedia.org does not have a page for inertial trajectory, but does have inertial frame of reference
https://en.wikipedia.org/wiki/Inertial_frame_of_reference
("is a frame of reference in which objects exhibit inertia: they remain at rest or in uniform motion relative to the frame until acted upon by external forces")
(ii)
(A) scalar (n):
"1: a real number rather than a vector
2: a quantity (such as mass or time) that has a magnitude describable by a real number and no direction"
https://www.merriam-webster.com/dictionary/scalar
(pronunciation)
The two definitions sound the same to me.
(B) scalar etymology
https://www.etymonline.com/word/scalar


(2)
(a) During acceleration and deceleration (and only these two phases), hypersonic aircraft experience g-forces that can exceed what humans can tolerate, potentially causing injury or death.
(b) X-!5 to the Edge of Space. NASA, July 2009
https://www.nasa.gov/wp-content/ ... ain_x-15_poster.pdf

Quote (one can increase the scale (%) at the center of the top):

"354,200 ft[:] HIGHEST ALTITUDE ATTAINED FOR A WINGED AIRCRAFT (THAT'S 67 MILES HIGH!) * * *

"8-g[:] THE G-FORCE EXPERIENCED BY X-15 PILOTS ON SOME FLIGHTS DURING REENTRY INTO EARTH'S ATMOSPHERE

" 'X' means 'experimental.' The first 'X' designation went to the Bell X-1, which, on October 14, 1947, became the first airplane to break the sound barrier in level flight.

Please read whole sections whose headings are:
• FLYING AT THE EDGE: " * * * The most dangerous part of a mission was the descent. * * * With only one shot at landing, there was simply no room for error."
• THE CARRIER AIRCRAFT
• FULL-PRESSURE SUIT
• FLIGHTS OF DISCOVERY: " * * * The heart rate of an X-15 pilot ranged from 145 to 185 beats per minute as the adrenaline surged. Chest-to-back acceleration was 2-g up to 4-g. at engine shutdown after nearly 90 seconds thrust, he braced his helmet against a headrest in front of him for the 2-g of immediate deceleration. the rest of the 8- to 12-minute flight continued without rocket power [ie, it was gliding].

Note:
(i) "The most dangerous part of a mission was the descent."

It was not due to g-force. See (3).
(ii) X-15 Carried Aloft by B-52. NASA, Mar 12, 2014
https://www.nasa.gov/image-article/x-15-carried-aloft-by-b-52/
View photo only.


(3) g-force
(a) Kerry Ellis, X-15: Pushing the Envelope. NASA ASK Magazine, issue 40 (Oct 22, 2010)
https://appel.nasa.gov/2010/10/22/40s_x15-html/

two consecutive paragraphs:

"To keep pilots safe from the immense heat and pressure experienced during flight, engineers developed nitrogen cabin air-conditioning for the cockpit. This also helped counteract potential flammability should the engine malfunction. While filling the cockpit with oxygen was an option, the potential for an explosion if it interacted with the engine's flammable propellant was too high a risk. To prevent blackouts from high g-forces and ensure the pilots could breathe within the nitrogen-filled environment, the X-15 team developed an early pressure suit, also pressurized and cooled with nitrogen. The only breathable oxygen available was in the suit’s faceplate.

"Each flight lasted about eight to ten minutes. Around eighty seconds after the X-15 was launched from under the wing of a B-52, pilots would shut down the engines. The rest of the flight was unpowered, and the pilot was essentially guiding a highspeed glider. Precision piloting was critical. If the engine was cut one second too late, or if the pitch was off by one degree, pilots could end up thousands of miles off track. With the only navigation available being line of sight from the X-15’s two small windows, a few ground stations, and non-hypersonic chase planes, overshooting the course meant emergency landing decisions had to be made and communicated quickly."

Note: pitch, yaw and roll
https://simple.wikipedia.org/wiki/Pitch,_yaw,_and_roll

(b) Acceleration in Aviation: G-Force. Aerospace Medical Education Division, Federal Aviation Administration (FAA), 2021 (brochure No 21-0375)
https://www.faa.gov/pilots/safet ... ia/acceleration.pdf

Quote:

"An object in freefall will accelerate at an ever-increasing speed [in vacuum] toward earth until it impacts the earth or reaches terminal velocity, the point at which the force of aerodynamic drag [from air] acting on the object overcomes the force of acceleration induced by gravity." page 2.

"A pilot in a steep turn may experience forces of acceleration equivalent to many times the force of gravity. This is especially true in military fighter jets and high-performance, aerobatic aircraft where the acceleration forces may be as high as 9 Gs [but only momentarily]. Air race pilots in a tight pylon turn also experience high G-forces * * * " page 2.

"Types of Acceleration: Linear Acceleration (page 3) * * * +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.

"This [acceleration] can be particularly hazardous for pilots in the Gz axis. * * * the pilot experiences +Gz [which push blood toward feet]. The heart and cardiovascular system must respond quickly to G acceleration to keep blood flowing to the brain and maintain consciousness. Physiological response to +Gz causes the heart to beat harder and faster with an increased vascular tone [ie, vascular constriction] to keep the blood flowing 'northward' toward the head." page 3.

"One of the first indications of impending disaster may be a progressive loss of vision as the aircraft enters the maneuver. The eyes are extremely sensitive to low blood flow * * * [first] a loss of peripheral vision (tunnel vision) * * * which in turn may be quickly followed by Gray Out and Blackout of the visual fields. * * * The symptoms that result from high G exposure are dependent on the rate of onset of the acceleration." page 4.

"While the effects of +Gz can be profound, the human body is even less well equipped to handle -Gz, which is described as a foot-to-head force * * * Under -Gz, the blood is prevented from flowing back down the jugular veins pin the neck] into the heart, but the arterial blood flow to the
head is enhanced. Once again, the retina of the eye is extremely sensitive, and the visual effect is a loss of vision due to 'Red Out.' If the pilot does not back off the control pressure, loss of consciousness will ensue in short order because the blood does not flow through the brain." page 4.

"The bottom line is that G tolerance for each individual aviator may fluctuate from day to day, and this can lead to disastrous consequences in flight." pages 4-5.

"Most civilian aircraft are not equipped to handle G-protective clothing, a 'G suit.' " page 5.

Note:
(i)
(A) eights on pylons
https://en.wikipedia.org/wiki/Eights_on_pylons

This is because a gigantic pylon is in use. See air racing
https://en.wikipedia.org/wiki/Air_racing
(photo with caption: "Air racing in England: the Red Bull Air Race heat held at Kemble airfield, Gloucestershire. The aircraft fly singly, and have to pass between pairs of pylons.")
(B) traffic cone
https://en.wikipedia.org/wiki/Traffic_cone
(also called pylon)
(ii) greyout
https://en.wikipedia.org/wiki/Greyout
(illustration with caption: "Simulated stages of a greyout")
(iii) Redout apparently sees red. However, I can not find simulation. But see
Advisory Circular: A Hazard in Aerobatics: Effects of G-Gorces on Pilots. FAA, Feb 28, 1984 (AC No 91-61 where AC stands for Advisory Circular)
https://www.faa.gov/documentLibr ... ular/AC%2091-61.pdf
("A condition termed 'redout' may occur. This may be due in part to congestion but may also occur when the lower eyelid, reacting to -Gz, rises to cover the pupil, so that one sees light through the eyelid")
(iv) The terms "eyeballs in," "eyeballs out" and so on are coined by test pilots to describe their feelings or sensation of eyeballs during acceleration. The terms are found to be useful. See Jent K Gillingham and Fred H Previc, Spatial Orientation in Flight. Defense Technical Information Center (DTIC), US Department of Defense, November 1993 (AD-A279 306)
https://apps.dtic.mil/sti/tr/pdf/ADA279306.pdf
("The 'eyeballs' nomenclature is another useful set of terms for describing gravitoinertial [sic] forces. In this system, the direction of the inertial reaction of the eyeballs when the head is subjected to an acceleration is used to describe the direction of the inertial force. The equivalent expressions, 'eyeballs-in acceleration' and 'eyeballs-in G force,' leave little room for confusion about either the direction of the applied acceleratory field or the resulting gravitoinertial force environment. * * * [Sometimes] it may be necessary to describe the coordinate system being used. For most applications, the 'eyeballs' convention is perfectly adequate")
(v) g-suit
https://en.wikipedia.org/wiki/G-suit
(section 2 Design: "trousers are fitted with inflatable bladders which, when pressurized through a g-sensitive valve in the aircraft or spacecraft, press firmly on the abdomen and legs, thus restricting the draining of blood away from the brain during periods of high acceleration. In addition, in some modern very high-g aircraft, the Anti-g suit effect is augmented by a small amount of pressure applied to the lungs (positive pressure breathing), which also enhances resistance to high G")
is not (full, but partial only) pressure suit.
(vi) Skydiving involves 1 g throughout. When the skydiver faces down (+Gx or eyeballs in) and spreads limb to achieve maximum air friction, he will obtain (constant) terminal velocity.
https://en.wikipedia.org/wiki/Terminal_velocity