Wikipedia:Reference desk/Archives/Science/2016 September 19

= September 19 =

Fight or flight
There are many articles which claims the fight or flight response can kill people with underlying heart conditions by causing plaque to become dislodged from an arterial wall and causing a blockage. But have there been any studies which has suggested that adrenaline and the temporary effect this has on the heart can cause permanent damage to even a healthy heart? Clover345 (talk) 00:17, 19 September 2016 (UTC)


 * Cortisol has some fairly serious effects long-term in psychological stress; trying to assess the risk of a single fight-or-flight episode sounds like a hard experiment... haven't looked yet though. Wnt (talk) 02:50, 19 September 2016 (UTC)


 * Every morning, one of the things that helps us wake up is a surge in catecholamines - the body's stress hormones - such as adrenaline. Healthy hearts deal with that, as well as surges in adrenaline and norepinephrine which directly mediate fight or flight response by preparing all the body's muscles, including the heart, for increased exertion.


 * What damages the heart muscle is starvation of blood, and that happens either during occlusion of one or more coronary arteries or by sustained exertion when the body's not getting enough oxygen or enough energy from food.


 * Diseases of the neuroendocrine system such as pheochromocytoma can cause extremely high blood pressure and damage arteries in the heart and throughout the body. Once the lining of an artery's been stretched enough to tear, tissue builds up to heal the torn arterial lining. Arterial plaque forming over these healed tears can limit blood flow through arteries - the reason why it's so important to control one's blood pressure.


 * Of course, high blood pressure can damage the heart for less exotic reasons, such as poor lifestyle choices (no exercise, excessive dietary intake of carbohydrates, untreated diabetes mellitus) and heredity (African-Americans tend to suffer from hypertension more than the general population, for example). These are separate causes from changes in catecholamine levels.


 * So, yes, sustained high levels of catecholamines can damage the heart - but only indirectly by damaging the arteries physically. The direct cause of this damage is untreated hypertension.  Changes in stress hormone levels in the blood don't damage the heart in and of themselves - otherwise, athletes would be at abnormally high risk of heart disease.  The reverse is true, even moderate physical activity, which causes enough stress hormone activity to drive muscles to perform more than usually, lowers the risk in most people of heart disease. loupgarous (talk) 18:12, 20 September 2016 (UTC)
 * True but doesn't adrenaline, for example, as a result of strong fear of something or strong nervousness/anxiety about something normally cause stronger heart beats than when exercising? 2A02:C7D:B942:B800:F425:97C4:9C9B:D3A8 (talk) 22:31, 22 September 2016 (UTC)
 * In animal welfare studies, cortisol is often used as an indicator of stress. Blood cortisol indicates immediate responses to the environment (acute), salivary cortisol indicates a slightly delayed response (perhaps 10 mins), faecal cortisol more delayed (perhaps 24 hrs) and then cortisol in the hair integrates responses over a long period of time.  Chronically elevated levels of cortisol have been linked with shortened life-span in captive elephants (amongst other welfare concerns). DrChrissy (talk) 18:42, 20 September 2016 (UTC)


 * We tackled the issue from different ends of the cascade. Cortisol and ACTH kick off the cascade, so they are reliable indicators of stress. Out of curiosity, in these studies, was proximate cause of death specified for those animals who died sooner than the norm and had chronically elevated cortisol levels?  I'm curious. loupgarous (talk) 19:12, 20 September 2016 (UTC)

Bombardier_Dash_8 one engine taxiing
I recently took a flight on a Bombardier_Dash_8 and was quite surprised when it taxied with only the right side engine running after landing. Here's a video (from someone else) of it in action. How come the asymmetrical thrust doesn't make the plane veer to the left? Pizza Margherita (talk) 03:32, 19 September 2016 (UTC)


 * This practice appeared around 2005-2010 (?) and is widespread, especially for turboprops and for four-engine heavies. It depends on the airport and is part of agreed practices between the operator and the airport: most national regulatory bodies permit it, I don't know any who explicitly forbid it, a few aircraft types don't support it, whether it's done "in general" is up to the airline and whether it's actually done depends on the airport layout and the anticipated taxi time. Obviously long taxis and waits encourage it. Hundreds of pounds of fuel could be saved by eliminating a taxi and long wait for a 4 engine.
 * Rotating a body (in the most general sense of Newtonian mechanics) requires a couple, not just a single force, even one that's off centre. So provided that the only force is from the engine, then it need not cause it to rotate. However if another force is introduced, such as some less than gentle braking, then it will yaw and maybe to an unexpected and surprising degree.  This is a technique for pilots with a light touch!  It does limit the tightness of the turning radius for taxiing aircraft, so some layouts (and gates) may be more accepting of it than others.
 * These are modern brakes, with well-modulated control (subtle movements on the pedals give subtle braking, not sudden grabbing), so there is plenty of control that way. The Dash 8 here is a turboprop, so there's a good wash over the tail, even at low speeds, giving good control from that too - but not for the sort of precise manoeuvring needed around a modern airport. With turboprops, the engines may both be running, but one prop is feathered - this still gives most of the fuel saving, but is a little quicker for a getaway once clear to go. A question about this practice is the risk of causing a serious delay if the aircraft taxis out, then fails in some way. It's unlikely, but we know how congested airports are.
 * The choice of which engine to use may also be dictated by the direction of the turns anticipated. A starboard engine will turn left much more easily, and that could be influenced by the layout at particular airports.
 * To discuss further, Prune is the place for sensible airliner talk. Andy Dingley (talk) 09:11, 19 September 2016 (UTC)


 * Andy, just one minor point about your excellent answer: a rigid body will rotate about its centre of mass under the action of one force if its line of action does not pass through the centre of mass (as is the case when running on a single left or right engine). To keep the plane moving in a straight line, an asymmetric force from the contact force of tyres on the runway (or other steering as you mention) is required to produce an opposing couple about the centre of mass, thus keeping the plane moving in a straight line.   D b f i r s   12:07, 19 September 2016 (UTC)


 * Agreed. See torque. StuRat (talk) 17:33, 19 September 2016 (UTC)


 * I can't make any sense out of that. If the engine delivers a steady force, and the plane moves at a steady rate, then there must be an opposing force of equal magnitude.  I'd expect to find that no closer to the engine than the nearest landing gear. Wnt (talk) 12:12, 20 September 2016 (UTC)


 * Don't forget there's the weathervane effect of the fuselage, the rudder, most aircraft have differential braking (i.e. separate left and right brake pedals), and larger aircraft usually have tiller steering via the nose wheel as well. I.e. you have a good selection of features to keep the aircraft pointing in the correct direction.  A quick search tells me the Dash 8 should have all of those.  When you combine that with the fairly minimal thrust used for taxiing, it's not really such a big deal in terms of the physics.  Pilot skills 101 includes taxiing when the wind is trying to turn you off the tarmac onto the grass, and keeping the aircraft straight on the runway with crosswind effects during takeoff and landing; so a small amount of offset thrust isn't such a big deal.  Modern pilots have it incredibly easy, with ABS brakes, and steerable nose wheels.  Tail draggers are a whole extra set of skills on the ground, in comparison, to the point where the likes of the RAF Battle of Britain Memorial Flight have to keep ancient tail dragger basic trainers around to convert their highly skilled pilots to their WW-II aircraft.   Murph 9000  (talk) 13:37, 20 September 2016 (UTC)


 * Andy's last sentence presumably refers to PPRuNe, the Professional Pilots Rumour Network Internet forum. -- ToE 13:14, 19 September 2016 (UTC)


 * I don't fly multi-engine aircraft, so I don't know very much about taxiing them - but I'm willing to bet that this procedure has more to do with actual real asymmetric thrust - P-factor - than it has to do with saving fuel. The Dash is probably not saving any meaningful quantity of fuel - because the same amount of energy has to be used to taxi, which-ever engine the fuel is flowing into - and in relative terms, turboprops idle efficiently!  P-factor means that the center of thrust is not in the center of the prop - not in the center of either prop  - the thrust is offset to the side of the engine.  Multi-engine aircraft can be particularly susceptible to P-factor because their props are not on the aircraft center-line, so the moment-arm is even longer than a single-engine airplane.  It seems reasonable to believe that in some aircraft, if the yawing moment is powerful enough, it is desirable to keep the thrust coming out of the engine whose center of thrust is most close to the aircraft's center-line.  In other words, the pilot of the Dash may be choosing to use the critical engine only, during ground-operations.
 * I looked in the Airplane Flying Handbook, and I did not see any mention of this procedure in the Ground Operations section of the Transition to Multi-Engine Aircraft chapter. If the answer actually matters, the person you should ask would have to be a multi-engine flight instructor!
 * Nimur (talk) 18:36, 19 September 2016 (UTC)


 * They presumably run less efficiently, the further from their ideal cruising speed they are. Also, using more fuel implies more air pollution, and that might be an issue, too. StuRat (talk) 17:09, 20 September 2016 (UTC)


 * I have an example that might help you to understand. Ever had a shopping cart/buggy with a wheel that's "wonky" and wants to turn the cart one way ?  I find I can sometimes keep it going straight if instead of pushing it equally on both sides, I only push on one side.  Very similar situation of off-center application of force, the plane just manages to simulate the "wonky" wheel. StuRat (talk) 22:50, 20 September 2016 (UTC)

Thanks, all! Pizza Margherita (talk) 04:35, 20 September 2016 (UTC)