Aviation Mythology and Misconceptions
Joined: Sep 2011
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From: Vienna
The danger in a downwind turn is provided by the windshear, not the pilot's perspective.
(And will hopefully find the time for a gliding trial lesson just for fun and educational purposes next season.)
Joined: Jun 2001
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From: Boldly going where no split infinitive has gone before..
'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
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
Newton formulated the concept of frames of reference. An objects momentum is dependent on it's velocity which is relative to the frame of reference.
The aerodynamics of an aircraft are dependent on it's velocity relative to the air around it. The most useful frame of reference, therefore, is one in which the wind velocity is zero.
The aircraft has no idea whether the air around it is moving relative to the ground.
Tell me: you are currently on a planet rotating at around 1000mph, orbiting the sun at around 6000mph, and orbiting the galactic centre at around 300 000mph.
What would you say your momentum was?
Answer? The correct answer is, "Relative to what?"
By the way, do the math-what is the difference in acceleration between a glider going from 20 knots north to 20 knots south, and one going from stationary to 40 knots south.
Hint:- this requires a correct understanding of the tem acceleration.
Moment in is not exclusivley relative to the Earths surface or anywhere else.
Joined: Mar 2006
Aviation Qualifications: PPL
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From: Vance, Belgium
@Lancman, barit1, cwaters
Truly amazing in an aviation forum.
But a common misconception that is worth an explanation.
Since 1632 and what is called "Galilean relativity" (no need for Einstein here), it is known that the laws of physics are the same in all inertial frames.
This means that the measurements of interacting forces, accelerations, variations of speed, momentum changes remains unchanged when switching to another inertial frame
although measurements of distance, speed, kinetic energy are changing through frame switching.
Let's look at this glider flying 20 kt in a 20 knot headwind and turning 180° .
A turn always requires an acceleration ; this is the centripetal acceleration.
In the frame of reference of the air, the glider turn changes its speed from +20kt to -20kt.
The integral of the centripetal acceleration has completed a momentum change of -40kt (x the glider mass).
In the ground frame of reference, the glider turn changes its speed from 0kt to -40kt.
The centripetal acceleration has also completed a momentum change of -40kt.
So yes, measurements of accelerations, momentum changes and forces give the same results in the 2 frames.
And that's untrue for kinetic energy.
Kinetic energy alone is irrelevant ; it can vary from zero to infinity just by changing the frame of reference.
The actual meaning of the kinetic energy in a specific frame is that "it measures the work that the mobile object can do through interacting with a still object of the frame".
It also measures the work necessary to give it its speed through an interaction with a still object of the frame.
So kinetic energy is only relevant in the frame of reference of the objects that the mobile interacts with.
The common misconception here is generated by these facts:
1. the ground frame has a special meaning to humans since it is the frame of the objects our feet interact with.
2. the concept of inertia is objectively handled in physics through the measurement of mass, but is subjectively felt in all day life as the work required for increasing the speed of a mobile object.
3. the "inertial work" can only be measured in the frame of the interacting object, but humans tend to consider that it is an absolute value measurable in the ground frame of reference.
Back to turning into head wind/down wind.
In summary, one can state:
1. In itself, turning into head wind or down wind makes no difference ; the airplane only interact with the air and so it is the frame of reference of the air that matters.
2. Turning whilst crossing a windshear does make a difference ; turning into increasing relative wind increases the total energy of the plane (temporary increase of air speed or height gain),
and turning into decreasing relative wind decreases the total energy of the plane (temporary decrease of air speed or height loss).
A turn into decreasing relative wind is a potential hazard at low altitude.
3. Flying perfect circles with reference to the ground makes irregular circles in the air (with non-zero wind). The rate of turn is lower when flying headwind and higher when flying downwind.
Thus flying a ground pattern at low altitude and allowing very high rate of turn when turning downwind is also a hazard.
Luc
Originally Posted by Lancman
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.
But a common misconception that is worth an explanation.
Since 1632 and what is called "Galilean relativity" (no need for Einstein here), it is known that the laws of physics are the same in all inertial frames.
This means that the measurements of interacting forces, accelerations, variations of speed, momentum changes remains unchanged when switching to another inertial frame
although measurements of distance, speed, kinetic energy are changing through frame switching.
Let's look at this glider flying 20 kt in a 20 knot headwind and turning 180° .
A turn always requires an acceleration ; this is the centripetal acceleration.
In the frame of reference of the air, the glider turn changes its speed from +20kt to -20kt.
The integral of the centripetal acceleration has completed a momentum change of -40kt (x the glider mass).
In the ground frame of reference, the glider turn changes its speed from 0kt to -40kt.
The centripetal acceleration has also completed a momentum change of -40kt.
So yes, measurements of accelerations, momentum changes and forces give the same results in the 2 frames.
And that's untrue for kinetic energy.
Kinetic energy alone is irrelevant ; it can vary from zero to infinity just by changing the frame of reference.
The actual meaning of the kinetic energy in a specific frame is that "it measures the work that the mobile object can do through interacting with a still object of the frame".
It also measures the work necessary to give it its speed through an interaction with a still object of the frame.
So kinetic energy is only relevant in the frame of reference of the objects that the mobile interacts with.
The common misconception here is generated by these facts:
1. the ground frame has a special meaning to humans since it is the frame of the objects our feet interact with.
2. the concept of inertia is objectively handled in physics through the measurement of mass, but is subjectively felt in all day life as the work required for increasing the speed of a mobile object.
3. the "inertial work" can only be measured in the frame of the interacting object, but humans tend to consider that it is an absolute value measurable in the ground frame of reference.
Back to turning into head wind/down wind.
In summary, one can state:
1. In itself, turning into head wind or down wind makes no difference ; the airplane only interact with the air and so it is the frame of reference of the air that matters.
2. Turning whilst crossing a windshear does make a difference ; turning into increasing relative wind increases the total energy of the plane (temporary increase of air speed or height gain),
and turning into decreasing relative wind decreases the total energy of the plane (temporary decrease of air speed or height loss).
A turn into decreasing relative wind is a potential hazard at low altitude.
3. Flying perfect circles with reference to the ground makes irregular circles in the air (with non-zero wind). The rate of turn is lower when flying headwind and higher when flying downwind.
Thus flying a ground pattern at low altitude and allowing very high rate of turn when turning downwind is also a hazard.
Luc
Last edited by Luc Lion; 15th November 2011 at 19:19.
Joined: Nov 2002
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From: Heart of Europe
Nicely put Luc Lion and for the non-believers try to read a bit of reports on glider and small aircraft accidents in Samedan Switzerland.
Nice place to go btw. but tricky with the winds involving narrow turns to downwind in the valley.
So, yes there are reported and causal accidents due to climbing turns into downwind not having enough speed. But then from eyewitness accounts some of the guys turned in 30 deg. bank turns to downwind at some 5 kts above stall speed ...
The one I just found (in German): www.bfu.admin.ch/common/pdf/1833.pdf
Plus a nice one from my (oh so easy flyable) turboprop times: Approaching LSZH we experienced a nice windshear taking away about 12 kts of IAS. Reported to tower which gave the information to the followin 737. The skipper obviously aware asked about type of aircraft reporting the windshear. Was given "ATR 72". He then reported 17 - 18 kts speed loss at some 2'000 feet AAE.
Shoud answer both:
1st - no not a myth but can kill you if lacking IAS!
2nd - yes inertia plays a vital role.
Nice place to go btw. but tricky with the winds involving narrow turns to downwind in the valley.
So, yes there are reported and causal accidents due to climbing turns into downwind not having enough speed. But then from eyewitness accounts some of the guys turned in 30 deg. bank turns to downwind at some 5 kts above stall speed ...
The one I just found (in German): www.bfu.admin.ch/common/pdf/1833.pdf
Plus a nice one from my (oh so easy flyable) turboprop times: Approaching LSZH we experienced a nice windshear taking away about 12 kts of IAS. Reported to tower which gave the information to the followin 737. The skipper obviously aware asked about type of aircraft reporting the windshear. Was given "ATR 72". He then reported 17 - 18 kts speed loss at some 2'000 feet AAE.
Shoud answer both:
1st - no not a myth but can kill you if lacking IAS!
2nd - yes inertia plays a vital role.
Joined: Jan 2007
Posts: 116
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From: UK
look at dynamic slope soaring, blows away all these stupid myths about downwind turns etc, its all about constant changing of kinetic and potential energy and converting low level windshear/ gradient on the rear of the slopes into useful energy.
. Go back to your flight sim or Hello magazine.
Joined: Aug 2009
Aviation Qualifications: Military
Posts: 9,346
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From: Texas
Note the note at the bottom of this section: - "Compare VOR and ADF systems to detect possible map shifts". You cannot do a map cross-check, if the VOR or ADF is u/s or not fitted !!
The 1996 Croatia USAF CT-43 crash is what happens, when people use a (map-shifted) RNAV FMC approach, instead of the raw NDB/ADF.
The 1996 Croatia USAF CT-43 crash is what happens, when people use a (map-shifted) RNAV FMC approach, instead of the raw NDB/ADF.
If I can find it, I'll post a link to the public released USAF investigation.
Joined: May 2008
Posts: 171
Likes: 13
From: Sheffield
#79 Yes Tourist - but Newton suffices
Don't disagree with the general drift (
) of your post but Newton suffices. While Einstein extends our knowledge we don't need him for flying aircraft.
NEWTON Inertial frame of reference - Wikipedia, the free encyclopedia
Contains this
In air that is moving at constant velocity you have a good frame of reference for flying speed. It is a rubbish frame of reference for ground speed.
My hang gliding instructor used to say 'the ground is a dangerous place, stay away from it'.
) of your post but Newton suffices. While Einstein extends our knowledge we don't need him for flying aircraft.NEWTON Inertial frame of reference - Wikipedia, the free encyclopedia
Contains this
The motion of a body can only be described relative to something else - other bodies, observers, or a set of space-time coordinates. These are called frames of reference. If the coordinates are chosen badly, the laws of motion may be more complex than necessary.
My hang gliding instructor used to say 'the ground is a dangerous place, stay away from it'.
Last edited by 911slf; 15th November 2011 at 21:08. Reason: insert reference to #79
Joined: Jun 2001
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From: Boldly going where no split infinitive has gone before..
its got every thing to do with downwind turns and change of energy state. Go back to your flight sim or Hello magazine.
Dynamic Soaring is possible bymutilising the different velocities of two air masses.
It is not possible in a steady wind. The acceleration an aircraft goes through in a turn is identical whether or not it does so in a wind.
Joined: Jun 2001
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From: Boldly going where no split infinitive has gone before..
Oggers,
Dynamic soaring involves using a vertical sheer layer. In the video, a model glider is utilizing the lea of a hill, with a strong wind blowing in the free stream.
You fly above the hill with the wind, then dive into the lee. This is a positive shear and so you gain airspeed. Before the speed bleds off, you turn and climb(with the models they half loop) The free wind is now a head-wind and you again have a positive shear and gain airspeed. Repeat ad-Infinitum. Truly impressive speeds (300mph+) have been achieved by model this way.
BUT you need two winds of different speeds. It IS irrelevant to the discussion of down-wind turns in constant winds.
Dynamic soaring involves using a vertical sheer layer. In the video, a model glider is utilizing the lea of a hill, with a strong wind blowing in the free stream.
You fly above the hill with the wind, then dive into the lee. This is a positive shear and so you gain airspeed. Before the speed bleds off, you turn and climb(with the models they half loop) The free wind is now a head-wind and you again have a positive shear and gain airspeed. Repeat ad-Infinitum. Truly impressive speeds (300mph+) have been achieved by model this way.
BUT you need two winds of different speeds. It IS irrelevant to the discussion of down-wind turns in constant winds.
Joined: Jan 2001
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From: Home
pilot999
"Go back to your flight sim or Hello magazine"
It is still irrelevant to downwind turns as wiz says.
911slf,
yes, but it helped to make the point that you should not mock others knowledge of physics unless your own is adequate. Which his is obviously not.
"Go back to your flight sim or Hello magazine"
It is still irrelevant to downwind turns as wiz says.
911slf,
yes, but it helped to make the point that you should not mock others knowledge of physics unless your own is adequate. Which his is obviously not.
Joined: Nov 2005
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From: Zulu Time Zone
You fly above the hill with the wind, then dive into the lee. This is a positive shear and so you gain airspeed. Before the speed bleds off, you turn and climb(with the models they half loop) The free wind is now a head-wind and you again have a positive shear and gain airspeed. Repeat ad-Infinitum
