Downwind turn discussion
My last dual flight before solo in a K13, Lasham, 1990 - instructor pulled off the winch launch early, 900 feet, but told me to keep the nose up into a stall, then bank to the left, which precipitated a spin - normal spin recovery, lost 400 feet with just over 1 revolution, so about 600 feet when recovered, just right for a normal circuit. He cleared me to solo after that.
FBW
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BPF
Canadian glider License with Instructor and Aerobatic Instructor ratings.
Sorry for the thread drift but I had to comment.
Back to the thread topic......my 2 cents
I found the thread rather entertaining and as usually happens it forced me to evaluate and consolidate what I knew.
However flying is an inherently practical exercise in hands and feet an coordination. for light aircraft I say again for light aircraft I firmly believe that there inherent low inertia means that all of the physical effects discussed at length in this thread exists to such a small extent that they are effectively theoretical not actual.
The one effect that is very real and regularly kills is the visual illusion.
SSD summarized it nicely on page 3
Turning downwind in a strong wind gives the illusion of skidding during the turn as the groundspeed rapidly increases, and can temp a pilot to pull back to slow down as the ground, which was creeping past before, is now racing by. Look at the ASI.Your airspeed HAS NOT CHANGED, but your groundspeed sure has!
You just can't convince some people BPF. Earlier I made a post re a 777 operating in a 200 knot jetstream and stated
to which Heston replied
Your quoting of SSD has it absolutely correct
Your
is the killer.
For Heston, I never flew a jet, a humble ex helicopter pilot me, and the downwind turn is a killer in helicopters as well, though for different reasons to a fixed wing. As I related earlier we took off at times in 60 knot winds with a climb speed of 75, meaning a GS of 15 into wind and 135 downwind. Most of our copilots only had a bare CPL on joining, and the demonstration of a take off in those conditions, with a bank angle of 60° applied at climb speed to downwind gave an extremely impressive demonstration of the need to fly instruments and ignore the visual. The ASI remained nailed on climb speed, and the ball remained centred throughout.
It matters not if you are in a 777 in the jetstream, or a bug smasher at sea level. The physics are exactly the same.
There is no acceleration in order to change the ground speed...
Oh dear I give up! I hope to God you dont actually fly a 777, megan. Please tell me what does change the groundspeed then?
Look at the ASI.Your airspeed HAS NOT CHANGED, but your groundspeed sure has!
The one effect that is very real and regularly kills is the visual illusion.
For Heston, I never flew a jet, a humble ex helicopter pilot me, and the downwind turn is a killer in helicopters as well, though for different reasons to a fixed wing. As I related earlier we took off at times in 60 knot winds with a climb speed of 75, meaning a GS of 15 into wind and 135 downwind. Most of our copilots only had a bare CPL on joining, and the demonstration of a take off in those conditions, with a bank angle of 60° applied at climb speed to downwind gave an extremely impressive demonstration of the need to fly instruments and ignore the visual. The ASI remained nailed on climb speed, and the ball remained centred throughout.
It matters not if you are in a 777 in the jetstream, or a bug smasher at sea level. The physics are exactly the same.
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You just can't convince some people BPF. Earlier I made a post re a 777 operating in a 200 knot jetstream and stated
Quote:
There is no acceleration in order to change the ground speed...
to which Heston replied
Quote:
Oh dear I give up! I hope to God you dont actually fly a 777, megan. Please tell me what does change the groundspeed then?
Quote:
There is no acceleration in order to change the ground speed...
to which Heston replied
Quote:
Oh dear I give up! I hope to God you dont actually fly a 777, megan. Please tell me what does change the groundspeed then?
If you change the ground speed, you have accelerated. You can accelerate without changing your airspeed. You do it every time you make a turn - the aircraft is accelerating towards the centre of the turn. Acceleration is rate of change of velocity - and velocity (unlike speed) is a vector - that is it has a direction as well as a size. So if we change direction, we accelerate.
That's Physics for you.
But you're quite right about the rest - make your turn whilst ignoring the visual illusions and so on, and the ASI will stay nailed.
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Isn't acceleration an increase in speed relative to the medium that the body is suspended by? Rather than an acceleration relative to something else?
You are accelerating towards the centre of the turn but the centre of the turn is moving, so what ever else you use as a reference is irellevent.
You cannot use two points of reference at the same time.
You are accelerating towards the centre of the turn but the centre of the turn is moving, so what ever else you use as a reference is irellevent.
You cannot use two points of reference at the same time.
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Acceleration = the rate of change of velocity per unit of time
Sorry, thats bollox for you.
The problem with that is that you have NOT changed your airspeed. The airspeed is the medium you require to pay attention to. That is the medium that you are flying in. The physics you quote, is only applicable on the ground, i.e. in the same medium.
Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.
The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.
Thats physics for you...
That's Physics for you.
If you change the ground speed, you have accelerated. You can accelerate without changing your airspeed.
Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.
The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.
Thats physics for you...
My instructor teaches speed by attitude with ASI as a delayed and sometimes erratic backup. The ground doesn't even come into it and I don't notice it in the circuit. Horizon and aiming point are what matters.
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The problem with that is that you have NOT changed your airspeed. The airspeed is the medium you require to pay attention to. That is the medium that you are flying in. The physics you quote, is only applicable on the ground, i.e. in the same medium.
Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.
The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.
Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.
The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.
That force that car passengers experience? Caused by acceleration - whether it's along the line of travel or sideways. (Newton's second law - F=ma). That 2g you feel in a 60 degree banked turn - it's the aeroplane pushing your backside so you accelerate along with it.
I didn't claim for a moment that your airspeed would change. Other things being equal, in a turn, it won't.
But if your groundspeed is changing, you will be accelerating. Even if the airspeed stays the same. And no matter what speed the wind is.
And the idea that the physics is only applicable on the ground will, I suspect, be a surprise to most physicists! They do rather cling to the idea of the physics holding true no matter what frame of reference you use.
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Acceleration = the rate of change of velocity per unit of time
Sorry, thats bollox for you.
That's Physics for you.
The conventional frame for measuring Velocity is the ground (which is very close to inertial after adjusting for a 1 g offset from gravity). So, if an aircraft has changed its ground velocity, it has by definition accelerated.
If the air mass is moving at a constant velocity (with respect to the ground), measuring velocity relative to the air mass will give exactly the same acceleration as measuring vs the ground.
If you do the maths for two cases of a level constant rate turn
1 - flying North at 60 knots and turning to South at 60 knots
2 - flying North at 0 knots (into a 60 knot headwind) and turning to South at 180 knots
You will find the answers are all identical, except the track over the ground is displaced by a constant distance of 60 nautical miles every hour in the second case.
In both cases the air SPEED (not air VELOCITY) will remain constant. The Relative Air Velocity (measured relative to the wing, which for normal rates of turn is effectively an inertial frame) will remain constant. It is this relative Velocity that defines the performance of an aircraft wing.
If the air mass is not moving at a constant speed, the aircraft will require accelerations to maintain the relative air velocity, hence the importance of the aircraft's inertia when exposed to gusts and shears.
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Thanks MM. There you go Paul.........apologies, I am not a physicist. Got a well earned B in my A level though.....I am teasing a bit, obviously, and Paul, I admire your argument based on pure physics. I am sure everyone is now totally bored and baffled with the physics, but the scenario is a serious one. If anything this thread could achieve, is that it gets folks thinking about it......
You wake up, not sure where you are. You see that you are in a small, windowless room. Apart from your chair, the room is empty.
On the floor, you find a small model aeroplane with a battery-powered motor driving counter-rotating propellers. A simple instruction sheet tells you that when the motor is switched on and the 'plane is hand-launched, the controls are fixed so that it will fly perfect circles until a timer turns the motor off after 30 seconds.
You launch the model and watch, pleased and impressed, as it flies circles around you.
After the flight has ended, you notice that there is actually a window blind in the wall behind you. You open the blind and are amazed to find that you are in fact inside a carriage being pulled by a train along a perfectly straight, smooth track at 60mph.
You consider the fact that the model aircraft was flying in a parcel of air which was moving across the ground at 60mph. To an observer not on the train, it was, in effect, flying in a 60mph wind. This is a speed which is about five times greater than its own flight speed, yet the model did not seem to be affected at all! It did not, for example, exhibit any signs of 'stalling' or losing height when it was turning 'downwind'.
You come to the conclusion that the model was simply unaware of its location and speed relative to the tracks, as were you until you looked out of the window.
On the floor, you find a small model aeroplane with a battery-powered motor driving counter-rotating propellers. A simple instruction sheet tells you that when the motor is switched on and the 'plane is hand-launched, the controls are fixed so that it will fly perfect circles until a timer turns the motor off after 30 seconds.
You launch the model and watch, pleased and impressed, as it flies circles around you.
After the flight has ended, you notice that there is actually a window blind in the wall behind you. You open the blind and are amazed to find that you are in fact inside a carriage being pulled by a train along a perfectly straight, smooth track at 60mph.
You consider the fact that the model aircraft was flying in a parcel of air which was moving across the ground at 60mph. To an observer not on the train, it was, in effect, flying in a 60mph wind. This is a speed which is about five times greater than its own flight speed, yet the model did not seem to be affected at all! It did not, for example, exhibit any signs of 'stalling' or losing height when it was turning 'downwind'.
You come to the conclusion that the model was simply unaware of its location and speed relative to the tracks, as were you until you looked out of the window.
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And the idea that the physics is only applicable on the ground will, I suspect, be a surprise to most physicists! They do rather cling to the idea of the physics holding true no matter what frame of reference you use.
The ground doesn't even come into it and I don't notice it in the circuit.
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Well, if you really want your head to hurt.....
As I'm sitting typing this, I am in fact accelerating at 1g upwards, pushed by the force from my chair. Relative to me, the inertial frame of reference is falling towards the centre of the earth at 1g, as I would be (following a straight line in space time), were it not for the earth getting in the way.
But then we'd be having a physics conversation, not a flying one. And we'd be using Einstein's physics, rather than Newton's. The latter works quite well for our purposes, and the former might make the PPL exams a bit tricky.
Paul
As I'm sitting typing this, I am in fact accelerating at 1g upwards, pushed by the force from my chair. Relative to me, the inertial frame of reference is falling towards the centre of the earth at 1g, as I would be (following a straight line in space time), were it not for the earth getting in the way.
But then we'd be having a physics conversation, not a flying one. And we'd be using Einstein's physics, rather than Newton's. The latter works quite well for our purposes, and the former might make the PPL exams a bit tricky.
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The thinking power being brought to bear on the acceleration topic brings to mind a question I've had in my mind for years. It's really an astronaut question, and I came close to being able to ask Chris Hadfield at his presentation, but he sure is popular with the crowd! Anyway:
The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...
When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?
The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...
When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?
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The thinking power being brought to bear on the acceleration topic brings to mind a question I've had in my mind for years. It's really an astronaut question, and I came close to being able to ask Chris Hadfield at his presentation, but he sure is popular with the crowd! Anyway:
The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...
When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?
The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...
When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?
The shuttle direction change is an acceleration which requires force. The astronauts must have some force applied for them to stay with the shuttle's new direction.
Zero G occurs when you accelerate downwards at 9.8ms^2. Google the vomit comet for another example.
PaulisHome, hate to tell you but Heston, as are you with
are incorrect. You are not accelerating. There are two frames of reference, in both of which Newtonian physics apply. One frame is the earth, the other is the airmass, and for an aircraft in flight the relevant frame is the airmass. The only acceleration an aircraft experiences when making a turn, while maintaining a constant airspeed, is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn. Have a read up on Galilean invariance. eckhard in his post re the model in a train exemplifies exactly the Galilean invariance.
But if your groundspeed is changing, you will be accelerating. Even if the airspeed stays the same. And no matter what speed the wind is.