Downwind turn discussion
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People here seem to be confusing speed and velocity.
Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both.
An aircraft in a turn is accelerating.
Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both.
An aircraft in a turn is accelerating.
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Hi Megan
You seem confused. On the one hand you say "You are not accelerating". On the other, you say "The only acceleration an aircraft experiences ... is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn" Which is it? No acceleration or 2g?
Let me help. It's the second. That 2g isn't pointed upwards relative to the earth (or we'd be accelerating in a loop), it's pointed at 60 deg from the vertical. That resolves into 1g vertical to the earth which counteracts gravity, and about 0.87g pointing towards the centre of the turn. It's the second one which accelerates us around the turn. (and just FYI, the radius, velocity and acceleration are described by the formula a=v^2/r).
And Fujii is entirely correct in #142
I don't think you've interpreted Gaililean equivalence properly. What it means is that the laws of motion apply no matter what frame of reference you use. There is no correct frame of reference. Using the airmass frame of reference makes the sums easier, sure, but it's quite possible to use the ground, or any other, and it won't change what actually happens. mm_flynn demonstrated that in #130
Paul
You seem confused. On the one hand you say "You are not accelerating". On the other, you say "The only acceleration an aircraft experiences ... is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn" Which is it? No acceleration or 2g?
Let me help. It's the second. That 2g isn't pointed upwards relative to the earth (or we'd be accelerating in a loop), it's pointed at 60 deg from the vertical. That resolves into 1g vertical to the earth which counteracts gravity, and about 0.87g pointing towards the centre of the turn. It's the second one which accelerates us around the turn. (and just FYI, the radius, velocity and acceleration are described by the formula a=v^2/r).
And Fujii is entirely correct in #142
I don't think you've interpreted Gaililean equivalence properly. What it means is that the laws of motion apply no matter what frame of reference you use. There is no correct frame of reference. Using the airmass frame of reference makes the sums easier, sure, but it's quite possible to use the ground, or any other, and it won't change what actually happens. mm_flynn demonstrated that in #130
Paul
Paul, I thought the discussion was in reference to velocity ie in the fore/aft axis. So the question is, what reading would an accelerometer placed in the fore/aft axis read while maintaining a constant airspeed and turning from into wind to downwind? + something, - something, zero? Forget the vertical and lateral.
Maybe I should have spelled it out that I was referring to turns in the circuit in VMC and maintaining healthy airspeed.
Last edited by Geriaviator; 7th Feb 2017 at 11:09.
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Paul, I thought the discussion was in reference to velocity ie in the fore/aft axis. So the question is, what reading would an accelerometer placed in the fore/aft axis read while maintaining a constant airspeed and turning from into wind to downwind? + something, - something, zero? Forget the vertical and lateral.
Still true to say that if the ground speed varies, the aircraft is accelerating. That isn't the same as saying that its airspeed is changing. It isn't though necessarily true to say that if the groundspeed stays the same, the aircraft isn't accelerating.
(I can think of only one, rather extreme, example where the first of those isn't quite true, and that's if we head north/south, in which case the speed of the ground moving under us due to the rotation of the earth changes).
But we're having a physics discussion at this point, not a flying one, and we need to use our terms carefully.
Paul
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That isn't the same as saying that its airspeed is changing
Keep the speed right (IAS), keep the ball in the middle and don't pull further than the light buffet.
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Still true to say that if the ground speed varies, the aircraft is accelerating
Force = mass X acceleration or acceleration = force/mass
From whence does the force commeth?
Force = mass X acceleration or acceleration = force/mass
From whence does the force commeth?
P
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The discussion so far has mostly focussed on still air, or a parcel of air moving at a consistent speed. The advice that in these conditions turning up or downwind makes no difference would tie up with my limited experience - I haven't found the need to "dolphin" around a thermal!
However, what about in gusty conditions? If I turn into wind, as I fly through a gust head-on I would expect the ASI to flicker up (although by the time it registers I'm probably out the other side!). However, if I turn downwind, would I not "overtake" a gust, which should register in a brief drop in airspeed? I would also have thought that it would take longer to pass through the gust, as it's travelling in the same direction. So in these conditions, "average" airspeed would be lower turning downwind that up?
However, what about in gusty conditions? If I turn into wind, as I fly through a gust head-on I would expect the ASI to flicker up (although by the time it registers I'm probably out the other side!). However, if I turn downwind, would I not "overtake" a gust, which should register in a brief drop in airspeed? I would also have thought that it would take longer to pass through the gust, as it's travelling in the same direction. So in these conditions, "average" airspeed would be lower turning downwind that up?
don't accelerometers read one when at rest
Yes, if you are referring to measuring the "g" in the vertical axis, but when placing an accelerometer in the lateral or longitudinal axis they would read zero when the aircraft is at rest.
The three pictures below show the display with the iPhone in three different orientations:
1. Held vertically in front of you +1.0 G
2. Held upside down -1.0 G
3. Flat on a table 0.0 G
An alternative is to use the free iSeismometer app and look at the output of each accelerometer as you rotate the iPhone about all three axes.
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However, what about in gusty conditions? If I turn into wind, as I fly through a gust head-on I would expect the ASI to flicker up (although by the time it registers I'm probably out the other side!). However, if I turn downwind, would I not "overtake" a gust, which should register in a brief drop in airspeed? I would also have thought that it would take longer to pass through the gust, as it's travelling in the same direction. So in these conditions, "average" airspeed would be lower turning downwind that up?
If you think about a thermal - the model for that is a vortex smoke ring - so depending on where you hit it you can find air going up, down, in or out. But the thermal as a whole will be moving at the same speed as the airmass, so if you go through it symmetrically and relatively quickly, you'll spend the same time with air gusting towards the nose as away from the nose.
P
No problem Geri. I should also point out that my instructor is reluctant to teach when the horizon isn't clear until the student has the 'feel' for the right level, climbing and descending attitudes. I feel happy learning with the ASI as backup only.
Maybe this is why I don't understand the influence of the ground on the perception of airspeed.
Maybe this is why I don't understand the influence of the ground on the perception of airspeed.
I'm familiar with Deceleration. Just hadn't encountered Megan's term before. De-acceleration.
Thought it might mean something different. Like returning to a constant speed?
That's what happens when you try to apply logic to English grammar.
Thought it might mean something different. Like returning to a constant speed?
That's what happens when you try to apply logic to English grammar.
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As this has turned into a physics discussion of two frames of reference, I've been having a think, dangerous thing to do!
A ship sailing across an aqueduct displaces its 100tons weight in water, no additional weight added to the aqueduct. The ship sinks 2inches in the middle of the aqueduct and is resting on the bottom, does the aqueduct collapse under the extra 100tons weight? Or does it still weigh the same? Or does it weigh more by the difference in displaced water?
A ship sailing across an aqueduct displaces its 100tons weight in water, no additional weight added to the aqueduct. The ship sinks 2inches in the middle of the aqueduct and is resting on the bottom, does the aqueduct collapse under the extra 100tons weight? Or does it still weigh the same? Or does it weigh more by the difference in displaced water?
It's no longer a distributed load (applied by depth of water x density x g) but a point one, so a different loading case.
Now, what about banging on the side of a 15cwt truck to transport a ton of budgies?
Now, what about banging on the side of a 15cwt truck to transport a ton of budgies?
Where does the displaced water go? If 100tons of water is sloshed over the sides, no change of total weight. If 100tons of water is displaced by raising the level of the aqueduct but all the water is still in situ; net gain of 100tons I would have thought.
If 100tons of water is displaced longitudinally (along the length of the aqueduct) then maybe there would be no weight gain in the section under discussion. Is the length infinite? Does it have to be? My brain hurts.
If 100tons of water is displaced longitudinally (along the length of the aqueduct) then maybe there would be no weight gain in the section under discussion. Is the length infinite? Does it have to be? My brain hurts.