Is this aerodynamic behavior possible?
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Is this aerodynamic behavior possible?
Hi
I am wondering if the following aerodynamic behavior is possible with a Jet airliner aircraft:
The Jet airliner aircraft is flying at 2000 ft or so
Weather calm, wind calm
Aircraft is in level flight
Clean confguration
We put the engines on IDLE
Based on this scenario, is it possible for the IAS to keep increasing despite the engines being on IDLE? I mean by increasing not a few knots but to keep gaining more indicated airspeed. Is it possible with some airliners?
This a debate I am having with someone and I would like to hear professionals give their opinions about it please.
Thanks!
I am wondering if the following aerodynamic behavior is possible with a Jet airliner aircraft:
The Jet airliner aircraft is flying at 2000 ft or so
Weather calm, wind calm
Aircraft is in level flight
Clean confguration
We put the engines on IDLE
Based on this scenario, is it possible for the IAS to keep increasing despite the engines being on IDLE? I mean by increasing not a few knots but to keep gaining more indicated airspeed. Is it possible with some airliners?
This a debate I am having with someone and I would like to hear professionals give their opinions about it please.
Thanks!
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No it is not possible. The only possible scenario this could happen would be in a pronounced, and very very powerful updraft. Holding at 7000' over Genoa with a Cb at one end of the hold, we experienced something like it in a 737. Going one way around the hold, speed was taking off from 210 kts to over 250 increasing with the thrust levers back at idle. Going around the corner in the hold, the speed fell rapidly, thrust levers came up to near full thrust and speed was below 200kts. It was bizarre and we didn't hang around for long before diverting to Milan. But unless you have a peculiar weather phenomenon causing that effect, what you are describing would be a tremendously overpowered aircraft where idle thrust exceeded drag, and as such it would be unflyable and impossible to get down.
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Thankyou I apreciate your explanation
Well one of the argument I heard supporting the on IDLE acceleration is this theory:
"It can if the net thrust at idle is positive as it will often be at low altitude and Mach Nr (MN)."
I wonder if that makes any real life sense?
Thanks
Well one of the argument I heard supporting the on IDLE acceleration is this theory:
"It can if the net thrust at idle is positive as it will often be at low altitude and Mach Nr (MN)."
I wonder if that makes any real life sense?
Thanks
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If we really are talking about level flight in still air, then acceleration at idle power is extremely unlikely. We would require an aircraft in which the thrust from the idling engines exceedes the drag of the entire aircraft in level flight.
But if we have all of this thrust at idle, then other than for take-off, why should we ever want to throttle up to any significantly higher thrust level. This is what Not So Fantastic means when he says that the aircraft would need to be extremely overpowered.
He is also correct in saying that such an aircraft would be very difficult to land. Extending the landing gear and flaps would increase drag, which might be enough to decelerate to landing speed. But how would you achieve the initial deceleration to Vfo and Vlo?
It is quite possible that an idling jet engine might produce more thrust than drag. This would cause the engine to accelerate if it were not attached to the aircraft. But in level flight idling jet engines are never reallistically going to create thrust greater than the total drag of the aircraft to which they are attached.
If they do then they are attached to the wrong aircraft!!!
But if we have all of this thrust at idle, then other than for take-off, why should we ever want to throttle up to any significantly higher thrust level. This is what Not So Fantastic means when he says that the aircraft would need to be extremely overpowered.
He is also correct in saying that such an aircraft would be very difficult to land. Extending the landing gear and flaps would increase drag, which might be enough to decelerate to landing speed. But how would you achieve the initial deceleration to Vfo and Vlo?
It is quite possible that an idling jet engine might produce more thrust than drag. This would cause the engine to accelerate if it were not attached to the aircraft. But in level flight idling jet engines are never reallistically going to create thrust greater than the total drag of the aircraft to which they are attached.
If they do then they are attached to the wrong aircraft!!!
Why do it if it's not fun?
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What about if we're on the back of the drag curve?
Not sure how much you know about this, sorry if you know this already. As you slow an aeroplane down, the wings have to work harder to keep it in the air. The pilot accomplishes this by pointing the nose of the aircraft slightly upwards, increasing the angle of attack on the wings.
A side-effect of this is that, as the wings are working harder, they also create more drag (known as induced drag). This drag requires lots of thrust to overcome it if you want to remain in level flight. So although it's generally true that the slower you fly, the less power you need, there comes a point where the extra drag from the wings actually means you need more power to remain level if you slow down any more.
Now, imagine you are flying along at a very slow speed, such that you are using lots of power to overcome the induced drag from the wings. To recover from this, you would typically lower the nose (to increase the speed), and reduce the power (to prevent the aircraft from climbing). As the speed builds up, the more normal type of drag (known as profile drag) which we are all familiar with increases, and power would gradually need to be added to prevent the aircraft from loosing height.
So, to answer your question, there are cases where you would be able to reduce the power and increase the speed. As for reducing the power all the way to idle, you certainly wouldn't expect the aircraft to remain level at idle power, but it might be that in recovering to a more normal speed, idle power is used for a short period.
All of this is based on light aircraft, but I don't see any reason why a transport jet would behave any differently. You'd need to replace the word "power" with "thrust" everywhere, and if you actually sat down to do the maths there would be some differences, but the net effect would be the same.
Hope that makes sense - it's very difficult to explain without diagrams and graphs!
FFF
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PS - Slingsby, I think you're simple aerodynamics are a little too simple. When I reduce power in my aircraft, it speeds up slightly and starts to descend, which is not what your simple lesson would suggest at all!
Not sure how much you know about this, sorry if you know this already. As you slow an aeroplane down, the wings have to work harder to keep it in the air. The pilot accomplishes this by pointing the nose of the aircraft slightly upwards, increasing the angle of attack on the wings.
A side-effect of this is that, as the wings are working harder, they also create more drag (known as induced drag). This drag requires lots of thrust to overcome it if you want to remain in level flight. So although it's generally true that the slower you fly, the less power you need, there comes a point where the extra drag from the wings actually means you need more power to remain level if you slow down any more.
Now, imagine you are flying along at a very slow speed, such that you are using lots of power to overcome the induced drag from the wings. To recover from this, you would typically lower the nose (to increase the speed), and reduce the power (to prevent the aircraft from climbing). As the speed builds up, the more normal type of drag (known as profile drag) which we are all familiar with increases, and power would gradually need to be added to prevent the aircraft from loosing height.
So, to answer your question, there are cases where you would be able to reduce the power and increase the speed. As for reducing the power all the way to idle, you certainly wouldn't expect the aircraft to remain level at idle power, but it might be that in recovering to a more normal speed, idle power is used for a short period.
All of this is based on light aircraft, but I don't see any reason why a transport jet would behave any differently. You'd need to replace the word "power" with "thrust" everywhere, and if you actually sat down to do the maths there would be some differences, but the net effect would be the same.
Hope that makes sense - it's very difficult to explain without diagrams and graphs!
FFF
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PS - Slingsby, I think you're simple aerodynamics are a little too simple. When I reduce power in my aircraft, it speeds up slightly and starts to descend, which is not what your simple lesson would suggest at all!
FFF, if you're maintaining level flight on the back of the power curve and then reduce power your speed will reduce, not increase - leading to a further & continuing speed loss. Thrust is being used to counter Drag. Remove any amount of Thrust & the now excess Drag will cause a further speed loss. You would have to lose altitude to maintain the speed in this case.
The point about flight on the back side of the curve is that the a/c is no longer speed stable. A speed loss results in more drag and a further speed loss, a speed gain results in less drag & a further speed gain. That's why that part of the curve is called the 'region of reversed command'. On the normal - or front - side of the curve additional speed results in more drag. If thrust hasn't been changed then the additional drag acts to reduce the a/c's speed back to the point where Thrust = Drag.
No matter which side of the curve you're on, less Thrust will result in a speed loss and more Thrust is needed to gain speed. The only options are to lose altitude ie convert height into speed, or reduce drag eg change from a higher drag configuration to a lower drag config.
WRT to your a/c's speed gain on a power reduction, I can think of at least two reasons:
1. A change in airflow around the static &/or pitot sources causing an error.
2. If the a/c is allowed to pitch ND following the power reduction then it will normally pitch further than the steady state pitch attititude needed to retain equilibrium. In doing so its speed will increase beyond the steady state 'on trim' speed. As a result the a/c will pitch NU, then ND etc etc in a diminishing oscillation until it eventually regains equilbrium.
The point about flight on the back side of the curve is that the a/c is no longer speed stable. A speed loss results in more drag and a further speed loss, a speed gain results in less drag & a further speed gain. That's why that part of the curve is called the 'region of reversed command'. On the normal - or front - side of the curve additional speed results in more drag. If thrust hasn't been changed then the additional drag acts to reduce the a/c's speed back to the point where Thrust = Drag.
No matter which side of the curve you're on, less Thrust will result in a speed loss and more Thrust is needed to gain speed. The only options are to lose altitude ie convert height into speed, or reduce drag eg change from a higher drag configuration to a lower drag config.
WRT to your a/c's speed gain on a power reduction, I can think of at least two reasons:
1. A change in airflow around the static &/or pitot sources causing an error.
2. If the a/c is allowed to pitch ND following the power reduction then it will normally pitch further than the steady state pitch attititude needed to retain equilibrium. In doing so its speed will increase beyond the steady state 'on trim' speed. As a result the a/c will pitch NU, then ND etc etc in a diminishing oscillation until it eventually regains equilbrium.
Why do it if it's not fun?
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Hiya Tinny.
Agree completely with your analysis. I was stretching a point, trying to conceive of a situation where Mochel's scenario might work. What I was suggesting wasn't simply a reduction of power - it was a lowering of the nose (to increase speed), accompanied by a short-term reduction of power. Like I said, stretching a point!
As for my aircraft speeding up with a power reduction, it's definitely not 2 - I'm talking about the stable state, after everything has settled down. I like the idea of a change of pressure in the area of the pitot/static tubes causing a change of indicated airspeed - interestingly, I think the PA28 behaves the same way to a lesser extent, and the pitot/static sensors are located in exactly the same place on the two aircraft. Personally, though, I think that the change in airflow over the horizontal stabiliser is responsible for a slight change in trim. (And again, the horizontal stabiliser - all-moving tail-plane with the trim tab doubling as an anti-balance tab - is a very similar configuration to the PA28.) No idea which is correct, and it doesn't really matter, but I think it's interesting nonetheless. (But then I have no life, so I find these things more interesting than I should!)
FFF
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Agree completely with your analysis. I was stretching a point, trying to conceive of a situation where Mochel's scenario might work. What I was suggesting wasn't simply a reduction of power - it was a lowering of the nose (to increase speed), accompanied by a short-term reduction of power. Like I said, stretching a point!
As for my aircraft speeding up with a power reduction, it's definitely not 2 - I'm talking about the stable state, after everything has settled down. I like the idea of a change of pressure in the area of the pitot/static tubes causing a change of indicated airspeed - interestingly, I think the PA28 behaves the same way to a lesser extent, and the pitot/static sensors are located in exactly the same place on the two aircraft. Personally, though, I think that the change in airflow over the horizontal stabiliser is responsible for a slight change in trim. (And again, the horizontal stabiliser - all-moving tail-plane with the trim tab doubling as an anti-balance tab - is a very similar configuration to the PA28.) No idea which is correct, and it doesn't really matter, but I think it's interesting nonetheless. (But then I have no life, so I find these things more interesting than I should!)
FFF
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