Aviation Mythology and Misconceptions
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Maybe I miss the point, but AFAIR the "danger" of the down-wind turn materializes only when the pilot tries to maintain a certain track over ground during the turn, with the tailwind first making him faster (relative to the ground) and becoming a crosswind pushing him "outwards" (again, relative to the ground), which might tempt him to overbank and stall the airplane (and/or trying to compensate with rudder only and crossing his controls to avoid an "excessive" bank angle).
So AFAIK the danger of the down-wind turn does not lie in the turn itself (with airplane unaware of wind direction), but in the reaction it might provoke.
So AFAIK the danger of the down-wind turn does not lie in the turn itself (with airplane unaware of wind direction), but in the reaction it might provoke.
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Wily
It is your visual association with the ground that is subconsciously messing up your circuits.
The downwind turn thing is easily put to bed.
ASW helicopters spend a lot of time flying in circles at low level (200ft and below)
The proof that this is a perceived and pilot induced issue is that on many occasions I have been pinging away in total darkness/IMC, carrying out orbits/circuits at 100kts in 40kts of wind at 200ft and below.
No problem until the sun comes up, and then suddenly it all feels uncomfortable on the downwind turn. Your mind overides what the instruments are telling you because the visual system is so strong.
The fact that when all you can see are flight instruments everything feels exactly the same as a zero wind day and your powers are steady is difficult to reconcile with the visual skidding over the ground and groundspeed changes etc.
You fly relative to the air, not the ground.
The only time the ground speed is of any relevance is at the moment you touch it.
Incidentally, you are venturing into the silly with the rotorblade thing. retire gracefully with some credibility.
It is your visual association with the ground that is subconsciously messing up your circuits.
The downwind turn thing is easily put to bed.
ASW helicopters spend a lot of time flying in circles at low level (200ft and below)
The proof that this is a perceived and pilot induced issue is that on many occasions I have been pinging away in total darkness/IMC, carrying out orbits/circuits at 100kts in 40kts of wind at 200ft and below.
No problem until the sun comes up, and then suddenly it all feels uncomfortable on the downwind turn. Your mind overides what the instruments are telling you because the visual system is so strong.
The fact that when all you can see are flight instruments everything feels exactly the same as a zero wind day and your powers are steady is difficult to reconcile with the visual skidding over the ground and groundspeed changes etc.
You fly relative to the air, not the ground.
The only time the ground speed is of any relevance is at the moment you touch it.
Incidentally, you are venturing into the silly with the rotorblade thing. retire gracefully with some credibility.
The proof that this is a perceived and pilot induced issue is that on many occasions I have been pinging away in total darkness/IMC, carrying out orbits/circuits at 100kts in 40kts of wind at 200ft and below.
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I believe it's Anti Submarine Warfare, but as an ex-GA fixed-wing pilot don't quote me!
Anyway Tourist please don't inject pesky logic and rationality into it, I was just sitting down with a bowl of popcorn and beginning to enjoy myself...
Anyway Tourist please don't inject pesky logic and rationality into it, I was just sitting down with a bowl of popcorn and beginning to enjoy myself...
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This thread seems to have become stuck in a groundhog day style loop.
Aviation Mythology and Misconception(s)- was it intended to become the singular?
Aviation Mythology and Misconception(s)- was it intended to become the singular?
its to do with the need to energise acceleration and momentum against inertia during the turn downwind, and then turning the excess speed back into height during the turn into wind.
BTW, I have landed a Ram Air Parachute over 1000times and flown conventional Gligers extensivley. Does that count as a big enough appeal to authority?
As to my experience in holding in high winds, we are talking about holding at 240kts odd in 100kts of wind. If turning downwind had any appreciable effect I would have noticed.
Please outline how you think the fact that the air is moving in relation to the ground effects an aircraft that isn't TOUCHING the ground.
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Hawk:
Yeah that one bugs me too.
Eck:
Sorry Eck but the engines still do work because even though the aircraft is stationary the engine still blew a mass of air over a distance. Hence willl05's comment about standing behind a running engine to test the theory. Ultimately the 'work' is being 'wasted' in a futile attempt to turn terra firma and the atmosphere in opposite directions but work is still done and power is still produced.
It really just comes down to Newton's 3rd law about action and reaction.
[myth] If an aircraft is not moving (stopped on a taxiway for example), then its not producing any power.
Eck:
One definition of Power is that it is the 'rate of doing work'.
Work = Force x Distance
If the aircraft's engines do not apply their force (thrust) through a distance, then no work is being done. Ergo, no power is being produced.
Work = Force x Distance
If the aircraft's engines do not apply their force (thrust) through a distance, then no work is being done. Ergo, no power is being produced.
It really just comes down to Newton's 3rd law about action and reaction.
Last edited by oggers; 8th Nov 2011 at 20:30.
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Oggers is correct. Work is being done by the engine on the mass of air it is accelerating (ie. it is being subjected to a force). Work done by an aircraft engine is work done on the air it is accelerating, not the aircraft to which the engine is attached.
Power, Joules per second, is being produced by the engine - it is burning fuel of high energy content.
Power, Joules per second, is being produced by the engine - it is burning fuel of high energy content.
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They are seriously considering extending the retirement age to 70 for airline pilots
Well, they will have to do something.
At present, they are going to steal my licence at 65, but not give me a pension until 67. Who is going to pay, for this enforced unemployment?
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So AFAIK the danger of the down-wind turn does not lie in the turn itself (with airplane unaware of wind direction), but in the reaction it might provoke.
Grrr....
The danger in a downwind turn is provided by the windshear, not the pilot's perspective.
If you take off on a runway with 90o crosswind of 5 knots, your tail wind component is zero (and you are not yet a part of the moving airmass).
If you now turn downwind and climb to 500ft, into a screaming 40 knot tail wind, the aircraft does not have time to react to that sudden wind shear ( by accellerating), and loses nearly 40 knots of airspeed. And in a light aircraft, that can be fatal - both in terms of height loss or airspeed loss.
So yes, a downwind turn can be fatal - but the lurking spectre of doom is the windshear close to the ground, and not the turn itself.
Try doing some gliding, to find out what windshear can do to an airframe.
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If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope. If it turns downwind it needs to get some kinetic energy from somewhere, quickly.
An aircraft with it's engines running on the ground with the parking brake set may be doing some work but it's efficiency is zero.
HTH.
An aircraft with it's engines running on the ground with the parking brake set may be doing some work but it's efficiency is zero.
HTH.
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If a hang glider is flying at 20kt in zero wind, it has x kinetic energy. If it suddenly encounters a 20kt tailwind (windshear), it now needs 4x kinetic energy to maintain the same airspeed. Newton says it cannot change inertial velocity instantly, and so a quick dive to convert potential energy is in order.
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I maintain both camps are correct depending of what you are flying and how you fly it...
No aircraft (that has mass) can change ground track instantly as that would imply instant acceleration.
So it seems obvious that a sudden yaw into wind differs from a sudden yaw downwind. In both cases the aircraft will continue on the original track for a few seconds until the aircraft responds to the new forces on it, the track changes and it either accelerates or decelerates to regain it's original air speed. During that time the airflow over the aircraft is different.
It's equally obvious that this effect dissapears if you make your turns co-ordinated and slow enough that the aircraft has plenty of time to accelerate or decelerate and maintain air speed.
Inertia is very real. Consider a glider decending through strong wind shear on approach. One camp will say the changing head wind doesn't matter because the aircraft will accelerate or decelerate to maintain a constant air speed. The other camp will tell you that due to inertia a glider can't accelerate fast enough to do that and can suffer a loss of airspeed. I'm in the latter camp.
No aircraft (that has mass) can change ground track instantly as that would imply instant acceleration.
So it seems obvious that a sudden yaw into wind differs from a sudden yaw downwind. In both cases the aircraft will continue on the original track for a few seconds until the aircraft responds to the new forces on it, the track changes and it either accelerates or decelerates to regain it's original air speed. During that time the airflow over the aircraft is different.
It's equally obvious that this effect dissapears if you make your turns co-ordinated and slow enough that the aircraft has plenty of time to accelerate or decelerate and maintain air speed.
Inertia is very real. Consider a glider decending through strong wind shear on approach. One camp will say the changing head wind doesn't matter because the aircraft will accelerate or decelerate to maintain a constant air speed. The other camp will tell you that due to inertia a glider can't accelerate fast enough to do that and can suffer a loss of airspeed. I'm in the latter camp.
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If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope. If it turns downwind it needs to get some kinetic energy from somewhere, quickly
Not true.
An aircraft does not feel wind at all. Wind is totally irelevant to the aircraft, and the turn downwind makes no difference whatsoever.
The problem is the shear (change of wind). If the glider turns downwind while descending through a shear (20kt tail increasing to 40kt tail), now the glider has lost 20kt of airspeed, and will need to lose height to regain speed (trading potential for kinetic).
The above scenario can happen, but is unusual. The big problem for a glider is final approach into 40kt of wind, and then flying into the lee of a hill or trees, so that the wind suddenly reduces to 10kt. Now the glider has lost 30kt of airspeed, and needs to readjust by closing the airbrakes.
Just remember. Wind has no effect on aircraft flight, but a rapidly changing wind (shear) does.
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Now the glider has lost 30kt of airspeed, and needs to readjust by closing the airbrakes.
You can try retracting the brake and stuffing the nose down but it will still take time and height to accelerate. While you trade height for speed you descend through even more wind shear. Might not be able to recover your speed fast enough. It's one reason why you carry extra speed on approach in windy weather.
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'tis true Silverstrata, Newton says so!
As the stationary mass of the glider is turned through 180 degrees it must be accelerated by 40 knots if it is to stay airborne. This requires an input of energy. So a gentle turn whilst trading height for speed is necessary. To believe that an aircraft is excused from the laws of motion can be dangerous.
As the stationary mass of the glider is turned through 180 degrees it must be accelerated by 40 knots if it is to stay airborne. This requires an input of energy. So a gentle turn whilst trading height for speed is necessary. To believe that an aircraft is excused from the laws of motion can be dangerous.
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Lancman
Newton was a clever chap, but Einstein trumps him.
You are completely, utterly wrong when you say:
"If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope"
Kinetic energy is all relative to some frame of reference, hence Einsteins "relativity"
The relevent kinetic energy to an aircraft in flight is that between the aircraft and the air.
That is the only valid frame of referance.
This is easy to see if you consider the aircraft relevant to the earth, or to the sun or to the spiral arm of the galaxy.
The kinetic energy is all about the difference in velocities of the two objects. It is not a quantifiable absolute, rather it is quantifiable between two objects.
In a crash, yes, the energy between the ground and the aircraft becomes relevent suddenly.
You notice that all those that believe this downwind effect to be true are the ones with the highest reliance on the visual external cues. Visual clues which we know are very susceptible to errors in the aviation environment.
The same problem comes up when yacht racing in a river/tidal estuary near the shore. As long as you sail by the water not the shore, everything is fine.
Wings don't get confused by visual effects, and neither do instruments. Trust them.
"To believe that an aircraft is excused from the laws of motion can be dangerous. " is undeniably true, but not as dangerous as a lack of understanding of those basic laws of motion Lancman
Newton was a clever chap, but Einstein trumps him.
You are completely, utterly wrong when you say:
"If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope"
Kinetic energy is all relative to some frame of reference, hence Einsteins "relativity"
The relevent kinetic energy to an aircraft in flight is that between the aircraft and the air.
That is the only valid frame of referance.
This is easy to see if you consider the aircraft relevant to the earth, or to the sun or to the spiral arm of the galaxy.
The kinetic energy is all about the difference in velocities of the two objects. It is not a quantifiable absolute, rather it is quantifiable between two objects.
In a crash, yes, the energy between the ground and the aircraft becomes relevent suddenly.
You notice that all those that believe this downwind effect to be true are the ones with the highest reliance on the visual external cues. Visual clues which we know are very susceptible to errors in the aviation environment.
The same problem comes up when yacht racing in a river/tidal estuary near the shore. As long as you sail by the water not the shore, everything is fine.
Wings don't get confused by visual effects, and neither do instruments. Trust them.
"To believe that an aircraft is excused from the laws of motion can be dangerous. " is undeniably true, but not as dangerous as a lack of understanding of those basic laws of motion Lancman
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Airbrake controls the glide angle. Elevator controls speed. At least that's what I was taught.
Airbrake controls the glide angle. Elevator controls speed. At least that's what I was taught.
The airbrakes place the glider on an artificially steeper glideslope. So if you lose airspeed due to windshear, you can close the airbrakes and maintain the same speed (because the artificially steep glideslope will now allow the glider to accellerate).
If you had to lower the nose every time you lost airspeed in a glider, you would soon find yourself surrounded by sheep, and separated from the airfield by a barbed wire fence.
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