Downwind turn discussion
So imagine an aircraft flying north at 75kias in no wind. To turn it through 180deg and fly south at 75kias we have to provide an acceleration in a southerly direction capable of changing its velocity by 150knots.
A reading of how an INS works may give an understanding.
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There is no acceleration in order to change the ground speed...
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There is no acceleration in order to change the ground speed, because the aircraft is operating with respect to the airmass, not the earth below.
My car requires acceleration to change groundspeed, and it operates with respect to an airmass, (it's just not supported by it, or propelled within it - but it still sees it as drag or propulsion).
My airplane requires acceleration down the runway, and is propelled in the the airmass, but still must accelerate relative to the ground to increase both groundspeed and airspeed.
When I open the throttle, my 'plane will accelerate, but not instantly so, the propeller operates inefficiently while it overcomes the inertia of the aircraft to accelerate ithen things get better, as the propeller operates at greater efficiency. My car is a little better, and will press me into the seat, (no tire to ground inefficiency) but that's as it accelerates me inertially - I'm personally, not affected by the airmass.
I continue to cling to the belief that mass and inertia play a role in how the 'plane responds while moving in and with the airmass, at least at slower airspeeds.
My airplane requires acceleration down the runway
Have a read on INS and in particular the frame of reference by which it works.
http://www.courses.netc.navy.mil/cou...14009A_ch7.pdf
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Turning downwind with 74 k climb speed meant our ground speed went from 14 to 134 knots. No acceleration applied to the airframe or pilot, as any accelerometer on the aircraft would tell you
Since the INS works by measuring accelerations, in the example you quote, if there was no acceleration when you turn downwind then the INS would instantly be lost.
Im sorry, but this is basic Newtonian physics (again as the document you link to explains) which you learn at school when you are about 14 years old (in the UK anyway). But I'll let you off, because you aren't that old are you?
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I just don't believe this thread!
Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!
Model flyers, leaning into a 20 knt wind, may be forgiven for not knowing that as far as an aeroplane is concerned there is no wind once it is off the ground and in the air. Aeroplane pilots should know better.
Good grief have you guys never flown a level constant bank angle 360 and noted no change in airspeed all the way round? Sure, you won't describe a circle over the ground unless you are in still air, but that because your groundspeed will change throughout due the wind.
This is REALLY basic stuff!
Two things you should be aware of:
1) Wind sure affects groundspeed, so watch your navigation, and of course it can make the transition between ground and air and back again a bit tricky.
2) All of the above applies to a steady wind only. Gusts, and wind gradients (or wind shear) have a very real change of airspeed effect which is dependent on the inertia of the aeroplane. But those are not steady-state wind conditions relative to the aeroplane.
Oh, and there's a third one, very relevant if you are flying low. 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!
There are some posts on this thread that surprise me in that they have been posted by pilots I thought knew these basics, then some.
Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!
Model flyers, leaning into a 20 knt wind, may be forgiven for not knowing that as far as an aeroplane is concerned there is no wind once it is off the ground and in the air. Aeroplane pilots should know better.
Good grief have you guys never flown a level constant bank angle 360 and noted no change in airspeed all the way round? Sure, you won't describe a circle over the ground unless you are in still air, but that because your groundspeed will change throughout due the wind.
This is REALLY basic stuff!
Two things you should be aware of:
1) Wind sure affects groundspeed, so watch your navigation, and of course it can make the transition between ground and air and back again a bit tricky.
2) All of the above applies to a steady wind only. Gusts, and wind gradients (or wind shear) have a very real change of airspeed effect which is dependent on the inertia of the aeroplane. But those are not steady-state wind conditions relative to the aeroplane.
Oh, and there's a third one, very relevant if you are flying low. 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!
There are some posts on this thread that surprise me in that they have been posted by pilots I thought knew these basics, then some.
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Well said SSD, but yes, pretty frightening that you actually had too. This thread I think shows quite clearly that basic groundschool during PPL theory training is a must. The lack of basic knowledge appears staggering. In fact, potentially fatal. We appear to have a number of pilots in the air, that do not understand the basics of what keeps them up there. Beyond belief.
Last edited by maxred; 1st Feb 2017 at 16:39.
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I just don't believe this thread!
Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!
Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!
Another great misconception is what clouds do in the wind. You know those nice puffy summertime cumulus clouds? Did you know you can get out of the wind by flying upwind of them - they shelter you from it. Yes really!
SSD and maxred 
Heston, you seem to have changed your tune with your last post. As SSD says,
which is what I've been saying all along, as in,
PS: You'll need to study the principle of INS a little closer.
A little bedtime reading from a highly trained individual.
http://www.australianflying.com.au/n...-downwind-turn
Regrettably it doesn't surprise me at all.
as far as an aeroplane is concerned there is no wind once it is off the ground and in the air
Turning downwind with 74 k climb speed meant our ground speed went from 14 to 134 knots. No acceleration applied to the airframe or pilot, as any accelerometer on the aircraft would tell you, for the simple reason an aircraft is operating, as I said, with respect to the airmass, not the earth.
A little bedtime reading from a highly trained individual.
http://www.australianflying.com.au/n...-downwind-turn
Avoid imitations
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No, it's a problem because of inertia!
If there's no inertia, there's no problem. The larger the mass of the aircraft, the more of a problem there is. Tiny objects, such as small insects, don't suffer a problem.
If there's no inertia, there's no problem. The larger the mass of the aircraft, the more of a problem there is. Tiny objects, such as small insects, don't suffer a problem.
Extrapolating from Shy T.
Inertia ?
A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].
It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.
Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.
IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.
mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)
Inertia ?
A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].
It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.
Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.
IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.
mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)
PS: You'll need to study the principle of INS a little closer.
An INS has a unique point of reference, that being a known, fixed starting point - effectively the point on the ground at which the system was initialised. Everything that subsequently happens to the aircraft, as far as the INS is concerned, is measured as an acceleration / deceleration in one of the 3 axes. Whether that's taxi, take-off, climb, cruise, turning or whatever.
Put simplistically, it's the aggregation of the effect of all the accelerations over time relative to the starting point on the ground that provides the position information.
As an example, you fly at a constant airspeed on a constant heading in a zero wind condition so your airspeed and groundspeed happen to be the same. At this point the INS detects no acceleration or deceleration while in this zero wind situation, so a constant velocity relative to the reference point on the ground is recorded.
A headwind develops, so your groundspeed decreases, but your airspeed remains the same. The accelerometer in the INS does measure a deceleration, relative to the reference point on the ground, however, and can therefore compute that the groundspeed has changed. The aircraft's velocity WRT the air is unchanged, but the aircraft's velocity WRT to the ground has indeed changed.
So an INS knows nothing about what the air is doing, other than accelerating or decelerating the aircraft in space, relative to the reference point on the ground.
FBW
The accelerometer in the INS does measure a deceleration, relative to the reference point on the ground
The start point is not a frame of reference, INS works by using a global or body frame of reference. What I think you're trying to say is the INS tracks the position and orientation of the aircraft relative to a known starting point, orientation and velocity. The known starting point may not be the point of departure, but an update from a positive fix during flight to remove accumulated errors.
Last edited by megan; 2nd Feb 2017 at 07:36.
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Extrapolating from Shy T.
Inertia ?
A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].
It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.
Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.
IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.
mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)
Inertia ?
A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].
It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.
Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.
IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.
mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)
Warning: thread drift to answer piperboy84 question.
TBH there's are a few shortcomings in descriptions of how they work in some posts I've seen here ...
however FWIW:
INS/INAS 101: Yes, it is an amazing bit of kit, FWIW they've been around since the 60's, if not earlier ... they do "drift" away from an accurate position/velocity but these days you generally don't "recalibrate" the INS itself in flight ( though some systems do or did allow it....oh, that was fun......) but they're normally only one of several data sources into the nav systems on modern airliners, along with GPS and ground based radio aids so they're not usually the only source of info/data on position etc.
....Next week we will discuss Schuler tuning.....
and back to the thread: I think SSD had it in one, back permalink #47.
Sound like an amazing bit of kit, is it subject to errors and if so how do you recalibrate it enroute?
however FWIW: INS/INAS 101: Yes, it is an amazing bit of kit, FWIW they've been around since the 60's, if not earlier ... they do "drift" away from an accurate position/velocity but these days you generally don't "recalibrate" the INS itself in flight ( though some systems do or did allow it....oh, that was fun......) but they're normally only one of several data sources into the nav systems on modern airliners, along with GPS and ground based radio aids so they're not usually the only source of info/data on position etc.
....Next week we will discuss Schuler tuning.....
and back to the thread: I think SSD had it in one, back permalink #47.
Last edited by wiggy; 2nd Feb 2017 at 08:26.
Leaving aside all the theoretical arguements about this subject - everyone seems to agree that rapidly changing wind/shear does affect IAS. This, of course, is because the rate of change of the wind/gust exceeds the rate at which the inherent momentum of the aircraft ie. it's inertia can change. So,now, why is it that the "one parcel of air" concept can totally ignore inertia? Is it because the rate of change is usually so minor at usual speeds?
Perhaps we are straying into the old Bernoulli versus Newtonian argument where neither theory is totally and or wholely at work?
I would also like someone to explain to me the cause of the large change in VSI indications at CONSTANT IAS when turning into or out of a very strong wind with respect to the TAS that I cited earlier. The only cause I can come up with is the large inertial change in speed of the aircraft wrt to it's own TAS.
Perhaps we are straying into the old Bernoulli versus Newtonian argument where neither theory is totally and or wholely at work?
I would also like someone to explain to me the cause of the large change in VSI indications at CONSTANT IAS when turning into or out of a very strong wind with respect to the TAS that I cited earlier. The only cause I can come up with is the large inertial change in speed of the aircraft wrt to it's own TAS.
Last edited by Meikleour; 2nd Feb 2017 at 09:56. Reason: amplification query
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I'd just got through tearing my hair out about the lack of physics and flying knowledge shown here when I came across SSD's post (#47).
I can glue it all back on now. He has nailed it.
All those who aren't physicists - just read that and try to internalise it.
Paul
I can glue it all back on now. He has nailed it.
All those who aren't physicists - just read that and try to internalise it.
Paul
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I haven't had time to read all the above posts, but people are obviously getting rather worked up by the debate.
This debate has been going on for decades:
I remember an AAIB report touched on this matter. If I recall correctly, c. 1990 a C150 flown by a father with his son as a passenger, crashed when turning 180 degrees from a headwind to a tailwind. The flight was part of a low-level navigation competition in Hampshire and the aircraft was flying slowly so as be precisely on time. Tragically both were killed in the crash. The AAIB report, if I recall correctly, gave credence to the theory of the proposing side of this argument. Perhaps somebody will find the AAIB report? It generated a huge debate.
Whatever, as a practical (as opposed to a theoretical) pilot, when I do a similar turn, my eye is always steadfast on the ASI!
This debate has been going on for decades:
I remember an AAIB report touched on this matter. If I recall correctly, c. 1990 a C150 flown by a father with his son as a passenger, crashed when turning 180 degrees from a headwind to a tailwind. The flight was part of a low-level navigation competition in Hampshire and the aircraft was flying slowly so as be precisely on time. Tragically both were killed in the crash. The AAIB report, if I recall correctly, gave credence to the theory of the proposing side of this argument. Perhaps somebody will find the AAIB report? It generated a huge debate.
Whatever, as a practical (as opposed to a theoretical) pilot, when I do a similar turn, my eye is always steadfast on the ASI!