Wind shear
Wind shear
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No 'math corroboration' - 'KISS' - just throw a feather upwards on a breezy day, followed by a brick, and see which one is more affected by the wind.
Oh, alright, its called 'inertia'.
Oh, alright, its called 'inertia'.
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"Affects more" can be taken both ways.
A lighter aircraft (of the same aerodynamic characteristics) will react more to any given disturbance to the air mass (which is what windshear ultimately is; it's the same as gusts in that sense).
But because it reacts more it also reacts faster. Which means that any disturbance which occurs over a finite time is more attenuated by the response of the aircraft, and so the maximum disturbance is less.
Consider two cases for the same aerodynamics. One is a typical airliner, with mass. The other is an airliner but has no inertia (ok, it's magic!)
Both aircraft are on approach, and encounter an "increasing performance" windshear, which means an increase in headwind. The inertialess aircraft will maintain the correct airspeed as prior to the shear, as the increased headwind will cause instantanoeuly increased airspeed, instantanoeusly increased drag and an instantanoeus groundspeed change, rebalancing the airspeed at the original trimmed value.
The "real" aircraft will actually see the airpseed increase due to the shear, and will start to decelerate due to increased drag, but will take a finite time to do so.
Therefore the heavier aircraft's airspeed is more affected - but of course the actual trajectory of the lighter aircraft was more affected.
A lighter aircraft (of the same aerodynamic characteristics) will react more to any given disturbance to the air mass (which is what windshear ultimately is; it's the same as gusts in that sense).
But because it reacts more it also reacts faster. Which means that any disturbance which occurs over a finite time is more attenuated by the response of the aircraft, and so the maximum disturbance is less.
Consider two cases for the same aerodynamics. One is a typical airliner, with mass. The other is an airliner but has no inertia (ok, it's magic!)
Both aircraft are on approach, and encounter an "increasing performance" windshear, which means an increase in headwind. The inertialess aircraft will maintain the correct airspeed as prior to the shear, as the increased headwind will cause instantanoeuly increased airspeed, instantanoeusly increased drag and an instantanoeus groundspeed change, rebalancing the airspeed at the original trimmed value.
The "real" aircraft will actually see the airpseed increase due to the shear, and will start to decelerate due to increased drag, but will take a finite time to do so.
Therefore the heavier aircraft's airspeed is more affected - but of course the actual trajectory of the lighter aircraft was more affected.
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Although BOAC is obviously correct, I guess you want a bit more substance.
Windshear causes a change in airspeed due to the inertia of the aircraft. A change in airspeed influences the lift produced by the wing for the same angle of attack.
A lighter aircraft can "follow" the windshear more easily. In other words it can accelerate or decelerate quicker to adjust for the change in wind/relative airflow.
Windshear causes a change in airspeed due to the inertia of the aircraft. A change in airspeed influences the lift produced by the wing for the same angle of attack.
A lighter aircraft can "follow" the windshear more easily. In other words it can accelerate or decelerate quicker to adjust for the change in wind/relative airflow.
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Windshear
Interesting responses -hmmm
Say your on final in a Citation vs a 747 in the sim
They give you windshear on final, your reaction time, powering through it, your plane's ability to accelerate through it, climb or maintain alt, through this situation is really what we are talking about here.
Some aircraft are lumbering beasts, other more nimble, with a higher power to weight retio an accelerate faster.
A plane that can climb 4000ft/min vs a light aircraft that might do 500ft, will be much better in windshear so I am not sure about the mass theory your expounding. Also a given amount of wind will have a harder time to move a bigger, heavier object, then a smaller light one. so in winshear, I would rather be the brick with power then the feather with power.
It's not about weight and inertia, it's about your ability to power through it, how much excess speed you have that will translate into a climb, performance ect.
I can just see the sim instructors pulling out the calculators explaining this, while the students are crashing the planes in the sim.
SSG
Say your on final in a Citation vs a 747 in the sim
They give you windshear on final, your reaction time, powering through it, your plane's ability to accelerate through it, climb or maintain alt, through this situation is really what we are talking about here.
Some aircraft are lumbering beasts, other more nimble, with a higher power to weight retio an accelerate faster.
A plane that can climb 4000ft/min vs a light aircraft that might do 500ft, will be much better in windshear so I am not sure about the mass theory your expounding. Also a given amount of wind will have a harder time to move a bigger, heavier object, then a smaller light one. so in winshear, I would rather be the brick with power then the feather with power.
It's not about weight and inertia, it's about your ability to power through it, how much excess speed you have that will translate into a climb, performance ect.
I can just see the sim instructors pulling out the calculators explaining this, while the students are crashing the planes in the sim.
SSG
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But what if the wind suffers a sudden decrease?
I always thought that was the greater problem. Let me say first that I fly aircraft with the propeller on top, so it probably does not affect me as much. It is also, hopefully, an excuse for the figures used.
If you are making an approach at 10 kts above the stall speed, and you suffer a sudden drop in windspeed of 20 kts, I assume you will stall. Your ability to accelerate to above the stall speed will depend on two things: power available, and mass of the aircraft - in other words, inertia.
I cannot help but think it would be more difficult to regain airspeed in a heavy aircraft than it would in a light one, different power parameters notwithstanding.
Or am I completely wrong again?
I always thought that was the greater problem. Let me say first that I fly aircraft with the propeller on top, so it probably does not affect me as much. It is also, hopefully, an excuse for the figures used.
If you are making an approach at 10 kts above the stall speed, and you suffer a sudden drop in windspeed of 20 kts, I assume you will stall. Your ability to accelerate to above the stall speed will depend on two things: power available, and mass of the aircraft - in other words, inertia.
I cannot help but think it would be more difficult to regain airspeed in a heavy aircraft than it would in a light one, different power parameters notwithstanding.
Or am I completely wrong again?
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Windshear
From my met book :
"An aircraft encontering windshear tends to maintain its speed over the ground due to its own momentum (the larger the aircraft the more momentum it will have) . If the windshear is due to a reduction in headwind component (or increase in tailwind component) this reduction manifest itself as an energy loss with a reduction in indicated airspeed . Lift is therefore reduced and the aircraft will, without correction, suffer a loss of height/increase in rate of descent/decrease in rate of climb. An increase in headwind component(or decrease in the tailwind component) result in an enegy gain and create the opposite effect . These events become critical when the aircraft is being flown close to the ground during the final stages of the approach or shortly after take off . In the energy loss case the engine reaction time when additional power is applied can be critical .
It may be difficult to accept that a change in the tail/head wind component will (in the short term) change the indicated airspeed rather the groundspeed of the aircraft, however rest assured that it does happen."
So definitely inertia plays against you and the larger the aircraft the more critical it would be . Taking into consideration, of course, the excess power available to "overcome" the critical situation : for example a windshear condition during approach on an empty B747 as opposed to a max landing weight 737 .
"An aircraft encontering windshear tends to maintain its speed over the ground due to its own momentum (the larger the aircraft the more momentum it will have) . If the windshear is due to a reduction in headwind component (or increase in tailwind component) this reduction manifest itself as an energy loss with a reduction in indicated airspeed . Lift is therefore reduced and the aircraft will, without correction, suffer a loss of height/increase in rate of descent/decrease in rate of climb. An increase in headwind component(or decrease in the tailwind component) result in an enegy gain and create the opposite effect . These events become critical when the aircraft is being flown close to the ground during the final stages of the approach or shortly after take off . In the energy loss case the engine reaction time when additional power is applied can be critical .
It may be difficult to accept that a change in the tail/head wind component will (in the short term) change the indicated airspeed rather the groundspeed of the aircraft, however rest assured that it does happen."
So definitely inertia plays against you and the larger the aircraft the more critical it would be . Taking into consideration, of course, the excess power available to "overcome" the critical situation : for example a windshear condition during approach on an empty B747 as opposed to a max landing weight 737 .
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ssg,
Two things needing to be viewed separately in discussion in your post ..
(a) gust response .. which is the thrust of the other posts ... wing loading is a major variable, hence aircraft mass.
(b) operational windshear recovery ... get up and go is the name of the game, as you suggest.
Two things needing to be viewed separately in discussion in your post ..
(a) gust response .. which is the thrust of the other posts ... wing loading is a major variable, hence aircraft mass.
(b) operational windshear recovery ... get up and go is the name of the game, as you suggest.
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I like Mad scientist explanation regarding the time of response, great perspective regarding airframes.
Iīd just like to deepen on this matter and try tio explain it, from a different point of view. I call it the Load Factor(n) point of view.
We all know that n depends on the angle of attack and the coefficient of Lift. With that in mind we can undoubtfully say that for a given gust;
a- The higher the IAS/EAS the higher the correspondinīLoad Factor
b- The lower the Weight the higher the Load Factor and viceversa. This doesnīt mean that a heavy aircraft exposed to a small load factor is going to withstand structurally better than a lighter aircraft, because the influence of the Weight in the stress generated in the wing root tends to increase with the Weight.
This is the approach regarding the wind shear structural influence.
On the abilitty to go around a negative shear with engine power, is quite obvious although we have to take into account not only the HP, lbs... but also the time of response to a given thrust, throttle lever move.
Iīd just like to deepen on this matter and try tio explain it, from a different point of view. I call it the Load Factor(n) point of view.
We all know that n depends on the angle of attack and the coefficient of Lift. With that in mind we can undoubtfully say that for a given gust;
a- The higher the IAS/EAS the higher the correspondinīLoad Factor
b- The lower the Weight the higher the Load Factor and viceversa. This doesnīt mean that a heavy aircraft exposed to a small load factor is going to withstand structurally better than a lighter aircraft, because the influence of the Weight in the stress generated in the wing root tends to increase with the Weight.
This is the approach regarding the wind shear structural influence.
On the abilitty to go around a negative shear with engine power, is quite obvious although we have to take into account not only the HP, lbs... but also the time of response to a given thrust, throttle lever move.
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Windshear
Reading this I need to go back to the books...
I have been in windshear twice..both times it felt like the hand of god was pushing the plane down, both times I landed so hard I hurt the planes in some minor way...
Anyone who has been through this will tell you that it is the weirdest feeling to have the throttles up all the way, engines screaming, airspeed and deck angle as high as your willing to go to keep the wings flying...and you still go down hill.
You had nothing left, you cant pull the nose up higher, you don't have any more power...your along for the ride.
That said, now that I fly jets, I have more power, and higher thrust to weight ratio, a slicker aircraft that accelerates faster, has more climb power, and all techie talk aside of inertia and load factors, when I push the throttles forward, the plane goes up instead of down.
In anyevent, I think we can all agree on the following.
To live through windshear..
1) Best to recognize it and avoid it
1a) If you do get into it, recognize it early and a fast and appropriate response is in order.
2) Hopefully your flying a plane that has the thrust to weight ratio that when you add all available power..
you go up, or level, instead of down.
So in IMHO...it's about performance, pilot and aircraft.
-SSG
I have been in windshear twice..both times it felt like the hand of god was pushing the plane down, both times I landed so hard I hurt the planes in some minor way...
Anyone who has been through this will tell you that it is the weirdest feeling to have the throttles up all the way, engines screaming, airspeed and deck angle as high as your willing to go to keep the wings flying...and you still go down hill.
You had nothing left, you cant pull the nose up higher, you don't have any more power...your along for the ride.
That said, now that I fly jets, I have more power, and higher thrust to weight ratio, a slicker aircraft that accelerates faster, has more climb power, and all techie talk aside of inertia and load factors, when I push the throttles forward, the plane goes up instead of down.
In anyevent, I think we can all agree on the following.
To live through windshear..
1) Best to recognize it and avoid it
1a) If you do get into it, recognize it early and a fast and appropriate response is in order.
2) Hopefully your flying a plane that has the thrust to weight ratio that when you add all available power..
you go up, or level, instead of down.
So in IMHO...it's about performance, pilot and aircraft.
-SSG
Do a Hover - it avoids G
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If you are making an approach at 10 kts above the stall speed, and you suffer a sudden drop in windspeed of 20 kts, I assume you will stall.
A stall is likely because when the wing lost speed it would loose lift, a sink would then happen, triggering the pilot to instinctively pull back, increasing the AoA and so cause a stall.
But wings only stall due to their AoA getting too high not because the airspeed has reduced below the 'stalling speed' which is a rather meaningless term which changes with weight, angle of bank and manoeuvre and even the power used in some circumstances.
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I bow to your vast knowledge, of course, but what if the pilot - poised like a coiled string - does not realise his airspeed has just dropped by 20 knots? Surely the aircraft will still stall?
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No.
The initial response will be a loss of lift due to the loss of dynamic pressure, associated with the drop in airspeed. The airflow will remain attached, therefore it's not a stall.
What happens next MAY be a stall, but it all depends what the pilot (or aircraft) does. If he pulls back to 'regain the lift' he may well stall. If he bunts (perhaps non-intuitive) to try to regain speed the wing will unload further and the plane will not stall. (Of course, it may instead hit the ground; that's the nature of windshear)
The initial response will be a loss of lift due to the loss of dynamic pressure, associated with the drop in airspeed. The airflow will remain attached, therefore it's not a stall.
What happens next MAY be a stall, but it all depends what the pilot (or aircraft) does. If he pulls back to 'regain the lift' he may well stall. If he bunts (perhaps non-intuitive) to try to regain speed the wing will unload further and the plane will not stall. (Of course, it may instead hit the ground; that's the nature of windshear)
