Could anybody explain to me ,why the rate of descend is smaller in tail wind ?
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Could anybody explain to me ,why the rate of descend is smaller in tail wind ?
so,why the rate of descend is smaller in tailwind on a power idle descend ?and also why the rate of descend is bigger in headwind?
really need help here.
thank you everybody ,you are all my teachers.and problem solved.
like you guys said ,the airplane cruising in the air no matter in tailwind or headwind,the relative speed to the wind is the same ,but the ground speed in different winds are different, so on a power idle glide which pitching to hold the same airspeed,the glide paths are the same relating to the airmass ,but not to the same to the ground,the one in the tailwind airmass has a shallow gliding angle relating to the ground and the one in the head wind has a deep gliding angle relating to the ground .
same rate of descent no matter what .
thanks again everyone .all amazing teachers .
really need help here.
thank you everybody ,you are all my teachers.and problem solved.
like you guys said ,the airplane cruising in the air no matter in tailwind or headwind,the relative speed to the wind is the same ,but the ground speed in different winds are different, so on a power idle glide which pitching to hold the same airspeed,the glide paths are the same relating to the airmass ,but not to the same to the ground,the one in the tailwind airmass has a shallow gliding angle relating to the ground and the one in the head wind has a deep gliding angle relating to the ground .
same rate of descent no matter what .
thanks again everyone .all amazing teachers .
Last edited by fly4freedom; 28th Nov 2013 at 09:04.
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Exactly, its not the rate that changes, but the angle. Rate is time to descend (or climb), angle is distance to descend (or climb).
Consider your question from the standpoint of a climb...a descent is the same principle, but for some reason I can always explain a climb better than a descent.
For sake of simplicity, we will not consider any (minute) changes to true airspeed in these examples.
1) An aircraft is climbing in nil-wind conditions at 500 feet per minute at 100 knots indicated airspeed from sea level. After climbing for 5 minutes, the aircraft will be at an altitude of 2,500 feet and will have covered 8.3 NM.
Because there is no wind, we can assume the airspeed and ground speed are equal at 100 knots. Our rate of climb is equal to 500 feet per minute (2,500 ft/5 min) and our angle of climb equates to 301 feet per nautical mile (2,500 ft /8.3 NM).
2) This same aircraft is now climbing with a 10 knot headwind at 500 feet per minute and indicating 100 knots from sea level. After climbing for 5 minutes, the aircraft will still be at an altitude of 2,500 feet. However, it will now have covered only 7.5 NM.
Because of the 10 knot headwind, the ground speed reduces from 100 knots to 90 knots. Our rate of climb is still equal to 500 feet per minute (2,500 ft/5 min), but our angle of climb now increases to 333 feet per nautical mile (2,500 ft/7.5 NM). We have covered less ground, but reached the same altitude in the same time.
3) Finally, this same aircraft now climbs with a 10 knot tailwind at 500 feet per minute, and indicating 100 knots from sea level. After climbing for 5 minutes, the aircraft will still be at an altitude of 2,500 feet. However, it will now have covered 9.1 NM.
Because of the 10 knot tailwind, our ground speed increases from 100 knots to 110 knots. Our rate of climb is still equal to 500 feet per minute (2,500 ft/5 min) but our angle of climb has decreased to 274 feet per nautical mile (2,500 ft/9.1 NM). We have covered more ground, but reached the same altitude in the same time.
If you draw these examples out on graph paper using these figures as guides, you will notice the angle for a tailwind is smaller than that for a nil-wind climb, which is smaller than a headwind. Therefore, the angle does indeed change with wind, while the rate (time) does not.
So, the question you may now ask is why did I give my answers about angle of climb in feet per nautical mile rather than as degrees?
Quite simply, flying my airplane I cannot tell what my angle of climb is. I can certainly tell my pitch angle, but this is not angle of climb. Its not even angle of attack. But, I can tell how many feet per nautical mile I will climb in given conditions. That is something I can work with in the cockpit. Angles are not. Indeed, look at any departure or arrival chart into a mountainous region, and it will tell you a minimum climb gradient is required, along with a ground speed and minimum feet per nautical mile to maintain in order to meet that gradient.
Concluding this up for you, the same principle applies to a descent as it does a climb. A headwind will slow the aircraft's ground speed, while a tailwind will increase the aircraft's ground speed. A descent from 2,500 feet at 500 feet per minute will still take 5 minutes, but the distance will change based on the wind.
I hope this all helps and doesn't muddy the water at all!
Consider your question from the standpoint of a climb...a descent is the same principle, but for some reason I can always explain a climb better than a descent.
For sake of simplicity, we will not consider any (minute) changes to true airspeed in these examples.
1) An aircraft is climbing in nil-wind conditions at 500 feet per minute at 100 knots indicated airspeed from sea level. After climbing for 5 minutes, the aircraft will be at an altitude of 2,500 feet and will have covered 8.3 NM.
Because there is no wind, we can assume the airspeed and ground speed are equal at 100 knots. Our rate of climb is equal to 500 feet per minute (2,500 ft/5 min) and our angle of climb equates to 301 feet per nautical mile (2,500 ft /8.3 NM).
2) This same aircraft is now climbing with a 10 knot headwind at 500 feet per minute and indicating 100 knots from sea level. After climbing for 5 minutes, the aircraft will still be at an altitude of 2,500 feet. However, it will now have covered only 7.5 NM.
Because of the 10 knot headwind, the ground speed reduces from 100 knots to 90 knots. Our rate of climb is still equal to 500 feet per minute (2,500 ft/5 min), but our angle of climb now increases to 333 feet per nautical mile (2,500 ft/7.5 NM). We have covered less ground, but reached the same altitude in the same time.
3) Finally, this same aircraft now climbs with a 10 knot tailwind at 500 feet per minute, and indicating 100 knots from sea level. After climbing for 5 minutes, the aircraft will still be at an altitude of 2,500 feet. However, it will now have covered 9.1 NM.
Because of the 10 knot tailwind, our ground speed increases from 100 knots to 110 knots. Our rate of climb is still equal to 500 feet per minute (2,500 ft/5 min) but our angle of climb has decreased to 274 feet per nautical mile (2,500 ft/9.1 NM). We have covered more ground, but reached the same altitude in the same time.
If you draw these examples out on graph paper using these figures as guides, you will notice the angle for a tailwind is smaller than that for a nil-wind climb, which is smaller than a headwind. Therefore, the angle does indeed change with wind, while the rate (time) does not.
So, the question you may now ask is why did I give my answers about angle of climb in feet per nautical mile rather than as degrees?
Quite simply, flying my airplane I cannot tell what my angle of climb is. I can certainly tell my pitch angle, but this is not angle of climb. Its not even angle of attack. But, I can tell how many feet per nautical mile I will climb in given conditions. That is something I can work with in the cockpit. Angles are not. Indeed, look at any departure or arrival chart into a mountainous region, and it will tell you a minimum climb gradient is required, along with a ground speed and minimum feet per nautical mile to maintain in order to meet that gradient.
Concluding this up for you, the same principle applies to a descent as it does a climb. A headwind will slow the aircraft's ground speed, while a tailwind will increase the aircraft's ground speed. A descent from 2,500 feet at 500 feet per minute will still take 5 minutes, but the distance will change based on the wind.
I hope this all helps and doesn't muddy the water at all!
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Precisely. the vertical rate is given in feet per minute.
but a minute of flight will travel different distances depending on the wind.
The rate is always the same, but the distance you travel will never be the same.
but a minute of flight will travel different distances depending on the wind.
The rate is always the same, but the distance you travel will never be the same.
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thank you guys .really appreciated that,i am thankful for your kindness trying to help,but the answers are just not correct.
actually it is not the same, let's say on my plane 737NG ,280KIAS,typical weight,on FLCH mode,still wind rate of descent is about 2200 like mentioned in the training manuel,but some head wind can make it reach about 3000,and some strong tailwind can decrease the vertical speed to less than 1500 feet per minute.
for the past three days i have tried to looking for the answers in the principles of the flight,not any clearly answer. but some principles seem can explain it :
since weight is always the same. and on a power idle descent, one component vector of weight is opposing to lift and the other component vector is opposing to the drag.and headwind contribute some parasite drag parallel to the glide path,so the total drag is bigger ,and lift becomes smaller (some theories in triangle ,square lift +square drag=square weight ),so the angle of gliding is bigger (tag(glide angle)=drag/lift).
actually it is not the same, let's say on my plane 737NG ,280KIAS,typical weight,on FLCH mode,still wind rate of descent is about 2200 like mentioned in the training manuel,but some head wind can make it reach about 3000,and some strong tailwind can decrease the vertical speed to less than 1500 feet per minute.
for the past three days i have tried to looking for the answers in the principles of the flight,not any clearly answer. but some principles seem can explain it :
since weight is always the same. and on a power idle descent, one component vector of weight is opposing to lift and the other component vector is opposing to the drag.and headwind contribute some parasite drag parallel to the glide path,so the total drag is bigger ,and lift becomes smaller (some theories in triangle ,square lift +square drag=square weight ),so the angle of gliding is bigger (tag(glide angle)=drag/lift).
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autopilot mode?
I've never flown a 737 (or anything that big!), but it looks to me like the autopilot mode is set to achieve a set altitude by the time you have got to a set position.
That means it would adjust the rate of descent to allow for the effect of the wind on the groundspeed, so whatever the wind speed you would always arrive at the required altitude at that position.
That means it would adjust the rate of descent to allow for the effect of the wind on the groundspeed, so whatever the wind speed you would always arrive at the required altitude at that position.
I don't fly the 737 but isn't FLCH effectively an airspeed/Mach hold mode? Rate of descent then would depend on weight, airspeed, and thrust, and altitude. When you think you're seeing these differences, is the thrust back at idle? Is the altitude the same in each case?
Sorry, I see you have written it is an idle descent.
There is no aerodynamic reason for the rate of descent to change due to tailwind or headwind. Wind component has no relationship to drag as the aeroplane can't "feel" the wind component, the wind component only affects the resultant track of the aeroplane across the ground.
That means you either aren't seeing what you think you're seeing, or the parameters such as weight, thrust, and airspeed that you think are the same each time actually aren't.
Sorry, I see you have written it is an idle descent.
There is no aerodynamic reason for the rate of descent to change due to tailwind or headwind. Wind component has no relationship to drag as the aeroplane can't "feel" the wind component, the wind component only affects the resultant track of the aeroplane across the ground.
That means you either aren't seeing what you think you're seeing, or the parameters such as weight, thrust, and airspeed that you think are the same each time actually aren't.
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FLY4FREEDOM Are you a pilot? I hope your not making a fuss on flight simulator x about tailwind and headwind?
Now, if you are a real world pilot, here goes.
From your private pilot or instrument rating you should have remembered that for a certain aircraft configuration and airspeed, it yeilds proportional drag and lift. Remember that lift/drag ratio graph in the p.o.h of the c152? At l/d max (since we are talking about idle power descent), it always yeilds a constant rate of descent unless either lift or drag (config) is changed. Then another page showed the effect of winds on glide distances. This is another method to see what your flight path angle and not the rate of descent will look like.
Imagine!?!?!? I think you have the picture now.
The flight path angle for descent just depends on lift to drag ratio (aircraft configuration) and winds aloft at altitude affecting the distance between descent to level.
Now, let me clearly state this, if your kias is constant and the aircraft config is constant, no matter any other factor(except weight), your rate of descent will always be the same.
Assuming the weight is constant, if you had mentioned the use of speed breaks, i could understand your 737 maintaining 280kias and descending at 3000'/min. Then on a different day, same weight at 2200'/min without the speed brakes. If its required to have a higher rate of descent then surely i will have to adjust my airspeed and or lift to drag ratio ~ (speed brakes to maintain constant airspeed with increase rate of descent).
My response to your issue, is that overshooting the 'zero wind' top of descent point for a head wind will require idle thrust and a steep flight path angle of descent. While a tailwind requires undershooting the top of descent for an idle thrust descent, resulting in a shallow flight path angle of descent. This is the only way i can see it happening with constant airspeed and or aircraft configuration.
Who agrees?
Now, if you are a real world pilot, here goes.
From your private pilot or instrument rating you should have remembered that for a certain aircraft configuration and airspeed, it yeilds proportional drag and lift. Remember that lift/drag ratio graph in the p.o.h of the c152? At l/d max (since we are talking about idle power descent), it always yeilds a constant rate of descent unless either lift or drag (config) is changed. Then another page showed the effect of winds on glide distances. This is another method to see what your flight path angle and not the rate of descent will look like.
Imagine!?!?!? I think you have the picture now.
The flight path angle for descent just depends on lift to drag ratio (aircraft configuration) and winds aloft at altitude affecting the distance between descent to level.
Now, let me clearly state this, if your kias is constant and the aircraft config is constant, no matter any other factor(except weight), your rate of descent will always be the same.
Assuming the weight is constant, if you had mentioned the use of speed breaks, i could understand your 737 maintaining 280kias and descending at 3000'/min. Then on a different day, same weight at 2200'/min without the speed brakes. If its required to have a higher rate of descent then surely i will have to adjust my airspeed and or lift to drag ratio ~ (speed brakes to maintain constant airspeed with increase rate of descent).
My response to your issue, is that overshooting the 'zero wind' top of descent point for a head wind will require idle thrust and a steep flight path angle of descent. While a tailwind requires undershooting the top of descent for an idle thrust descent, resulting in a shallow flight path angle of descent. This is the only way i can see it happening with constant airspeed and or aircraft configuration.
Who agrees?
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why the rate of descend is smaller in tailwind on a power idle descend
Rate of descent is dependent on thrust,drag,speed schedule and weight.
However, flight path angle is increased with a HW and decreased with a Tailwind and ground distance from T/D is increased in a tailwind and decreased with a headwind.
If you find in the FCTM a table for Rate of descent changes based on HW or TW ill buy you a couple of Tsingtao
Last edited by de facto; 28th Nov 2013 at 04:17.
It depends on your frame of reference and any constraint.
In a free airmass, moving as ‘wind’, it doesn’t matter which way you are pointing (headwind/tailwind) then the VS will be same - for the same aircraft conditions. If the autopilot VS mode is used, then this constraint changes the aircraft conditions – airspeed.
Once you reference the aircraft’s motion to the ground, then the VS is proportional to the groundspeed and is lower in a tailwind for a given glide path; but this doesn’t prevent you arriving on the ground any softer, in fact due to Aero D and a fickle wind, often a lot harder.
In a free airmass, moving as ‘wind’, it doesn’t matter which way you are pointing (headwind/tailwind) then the VS will be same - for the same aircraft conditions. If the autopilot VS mode is used, then this constraint changes the aircraft conditions – airspeed.
Once you reference the aircraft’s motion to the ground, then the VS is proportional to the groundspeed and is lower in a tailwind for a given glide path; but this doesn’t prevent you arriving on the ground any softer, in fact due to Aero D and a fickle wind, often a lot harder.
If we assume that conditions (windspeed, weight, configuration and IAS all remain constant, then as others have already said, a headwind or tailwind will not affect the rate of descent.
But if you have actually witnessed different rates of descent for situations in which the only changed variable is a headwind compared to a tailwind, then something other than basic aerodynamics must be involved.
You have stated that the conditions include “power idle and pitch to hold the same IAS,”. Using pitch to hold IAS could will cause the rate of descent to change if the wind speed is changing.
If for example the tailwind speed is decreasing as we descend, this will cause the TAS and IAS to increase. When the autopilot senses the increasing IAS it will pitch up to restore the initial IAS. This will reduce the rate of descent. If this is the cause of the changing rate of descent then we should also see changing pitch attitude.
But if you have actually witnessed different rates of descent for situations in which the only changed variable is a headwind compared to a tailwind, then something other than basic aerodynamics must be involved.
You have stated that the conditions include “power idle and pitch to hold the same IAS,”. Using pitch to hold IAS could will cause the rate of descent to change if the wind speed is changing.
If for example the tailwind speed is decreasing as we descend, this will cause the TAS and IAS to increase. When the autopilot senses the increasing IAS it will pitch up to restore the initial IAS. This will reduce the rate of descent. If this is the cause of the changing rate of descent then we should also see changing pitch attitude.
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Autopilot programing apart (VNAV), how does an aircraft "know" if it's experiencing a headwind or a tailwind?
It doesn't.
It doesn't.
I'm not suggesting that the autopilot will know whether or not there is any headwind or tailwind. It will not. But if it has been set to vary the pitch to maintain a constant IAS, it will do exactly that.
If there is a tailwind and the wind speed decreases the inertial speed of the aircraft will initially remain unchanged (or if you prefer the change in inertial speed will lag behind the change in wind speed). This will cause the TAS and IAS to increase. The autopilot will then increase produce a pitch-up motion to restore the initial IAS. No part of this process requires the autopilot to be aware of the tailwind.
As I said in my previous post, the OP has stated that he/she has observed different rates of descent which appear to be linked to headwinds and tailwinds. As previous posters have pointed out, this cannot be explained on the basis of simple aerodynamics. So if we assume that the OP is neither hallucinating nor simply trying to wind us up, we need to look at other factors. The actions of the autopilot may be one such factor.
If there is a tailwind and the wind speed decreases the inertial speed of the aircraft will initially remain unchanged (or if you prefer the change in inertial speed will lag behind the change in wind speed). This will cause the TAS and IAS to increase. The autopilot will then increase produce a pitch-up motion to restore the initial IAS. No part of this process requires the autopilot to be aware of the tailwind.
As I said in my previous post, the OP has stated that he/she has observed different rates of descent which appear to be linked to headwinds and tailwinds. As previous posters have pointed out, this cannot be explained on the basis of simple aerodynamics. So if we assume that the OP is neither hallucinating nor simply trying to wind us up, we need to look at other factors. The actions of the autopilot may be one such factor.
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Although gusts may show some effect, it would be quite temporary.
It would not need to a be a gust.
If we had a continuous decrease in tailwind speed with decreasing altitude, the reduction in inertial speed would lag behind the decrease in wind speed all the way down. The autopilot would select an appropriate degree of pitch up to make the inertial deceleration equal the rate of decrease of wind speed. To make a significant change in ROD we would probably need a high initial wind speed and a considerable rate of decrease, but it would not necessarily need to be a gust.
What would you suggest as the explanation for the reported changes in ROD?
If we had a continuous decrease in tailwind speed with decreasing altitude, the reduction in inertial speed would lag behind the decrease in wind speed all the way down. The autopilot would select an appropriate degree of pitch up to make the inertial deceleration equal the rate of decrease of wind speed. To make a significant change in ROD we would probably need a high initial wind speed and a considerable rate of decrease, but it would not necessarily need to be a gust.
What would you suggest as the explanation for the reported changes in ROD?
