Downwind turn discussion
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Downwind turn discussion
Elsewhere, thread drift has wandered to a discussion of the characteristics of a downwind turn at a slower airspeed. Passions smoulder on two sides:
One being that the wind has no affect on turn performance nor indicated airspeed, as the aircraft is moving in a parcel of air. The airplane does not know that the air is still or moving, just that it is flying with indicated airspeed, which is not affected by wind. What it was doing into the wind, it will be doing out of the wind once turned 180 degrees. Albeit with a slewed turn, and differing groundspeed.
The other side mentions characteristics of inertia of the aircraft, with the belief that it could be possible for the change from into the wind to out of the wind could occur faster than the inertia of the aircraft [in space, not the parcel of air] could be overcome by the turn acceleration. The possible result being a temporary reduction in indicated airspeed resulting from the turn out of wind, while the inertia of the aircraft is overcome and accelerated in space back to the original IAS, while groundspeed increases too.
Thoughts form the group? How is this concept taught and learned?
One being that the wind has no affect on turn performance nor indicated airspeed, as the aircraft is moving in a parcel of air. The airplane does not know that the air is still or moving, just that it is flying with indicated airspeed, which is not affected by wind. What it was doing into the wind, it will be doing out of the wind once turned 180 degrees. Albeit with a slewed turn, and differing groundspeed.
The other side mentions characteristics of inertia of the aircraft, with the belief that it could be possible for the change from into the wind to out of the wind could occur faster than the inertia of the aircraft [in space, not the parcel of air] could be overcome by the turn acceleration. The possible result being a temporary reduction in indicated airspeed resulting from the turn out of wind, while the inertia of the aircraft is overcome and accelerated in space back to the original IAS, while groundspeed increases too.
Thoughts form the group? How is this concept taught and learned?
When turning my slow and light (and thus, very low on inertia) craft in pronounced steady wind, I certainly feel the effect of that wind, though I could not describe it.
But the wording of the question makes me doubt: "downwind" is one stage of the traffic circuit, are you discussing the effects of wind on how to fly the circuit? What exactly is meant by "a downwind turn", if not "a turn into the downwind section of the circuit"?
But the wording of the question makes me doubt: "downwind" is one stage of the traffic circuit, are you discussing the effects of wind on how to fly the circuit? What exactly is meant by "a downwind turn", if not "a turn into the downwind section of the circuit"?
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To clarify Jan, the term "downwind" is not relative to position in the circuit as much as having been flying directly into the prevailing wind, to now be flying away from it.
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A few years back I was in a ground school being taught by a freshly minted instructor. He was explaining that on downwind at an airspeed of 100kts if there was a tailwind of 20 kts the aircraft would have an airspeed of 120kts. During the break he joined me outside for a smoke, I took a cigarette paper and threw it into the wind and asked him if that wind was 10kts what is the papers airspeed? He said 10kts, I gave him a raised eyebrow and went back inside. After the lunch break (where he did not join myself and the other students) he came back into class looked over and just nodded with a sheepish look on his face. He proceeded to review the mornings session and correct any misapprehension any of us may have had.
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I remember being confused by this when learning to fly.
I was taught your 'other side' which accounts for inertia, though it was not described in these terms, which might have helped my understanding of what I was being told. I couldn't see it, arguing the 'moving parcel of air' view and I recall my argument used the example of circling a hot air balloon.
When I eventually figured out by myself what the instructor was trying to convey I did at least acknowledge the possibility but maybe I've never made a sufficiently abrupt turn down wind but I cannot say I have ever noticed the effect of inertia in that situation.
I was taught your 'other side' which accounts for inertia, though it was not described in these terms, which might have helped my understanding of what I was being told. I couldn't see it, arguing the 'moving parcel of air' view and I recall my argument used the example of circling a hot air balloon.
When I eventually figured out by myself what the instructor was trying to convey I did at least acknowledge the possibility but maybe I've never made a sufficiently abrupt turn down wind but I cannot say I have ever noticed the effect of inertia in that situation.
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Wind shear effect? As you turn from into wind to downwind, the effect of sudden gusts changes. Flying into wind, airspeed is likely to increase with sudden gusts. Flying downwind, sudden gusts tend to decrease airspeed. The pilot of an aircraft flying close to the stall in a steep turn may well learn this quite suddenly.
Pilot DAR: You might be interested in this observation. Many years ago I would routinely fly into Narita, Japan in B747s. In the winter it was common to have descent tailwinds of in excess of 100kts. The aircraft would usually be operated with the speed mode lockes onto 300kts. The first half of the arrival procedure, down to about 20,000ft. would have the full benefit of the entire tailwind. Next the arrival called for a 90 degree turn which resulted in the loss of the entire tailwind component. The aircraft would continue at 300kts. BUT the VSI rate of descent would increase hugely during, and for a time after the turn was complete, before returning to its usual value. Patently, inertia was at play here as the aircraft had to dive more steeply to recover/maintain the IAS. A rather unusual set of conditions no doubt but the "independent in its own block of air" view did not hold here.
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When I started gliding, it made sense to me that my airspeed is relative to the air, so surely by and large I would be carried along with any fluctuations. All this talk of wind gradients didn't make sense.
My first landing in a strong headwind dissuaded me of this notion. If any doubt remained, turning downwind off a practice cable break in a decent cross pushed the message home
My first landing in a strong headwind dissuaded me of this notion. If any doubt remained, turning downwind off a practice cable break in a decent cross pushed the message home
Jan
I know you'll all know this but just in case I'll offer up here what I offered in the other place (where TBH I think discusion on windshear and gusts and parcels of air may be confusing the matter):
Once upon a time UK CFS Bulldog students were demo'd the downwind turn "problem" when they started Low Level Nav flying/nav exes, 250 feet aal dual, perhaps, but not sure, 90 knots ish IAS). The training was nothing to do with coping with parcels of air or windshear; you can fly round at 45 AOB or more all day in your parcel of air at low level quite safely as long as the fuel allows and as long as you don't care where you end up, the point of the exercise was to make folks aware that it gets more complex when you need to turn to accurately to overfly a waypoint on the ground.
e.g. You are flying downwind, and it's a strong wind 30 knots plus, tailwind initially... you are going to turn so as to be on a new heading as you overfly a fixed point on the ground. You start off by using your "standard" 10 knot day angle of bank, standard IAS, standard 10 knot day amount of anticipation.
.... Of course the tailwind becomes an increasing beam wind and half way around the turn you see you are sliding sideways over the ground and are now struggling to get around to overfly the fix, something you really want to do that to get set up accurately for the next leg of the navex...so....the temptation is bound to be to tighten the turn ....."just a bit more bank"...."just a bit more pull / loading ( more g /AOA)"....."oh, darn, It's really windy and I'm not quite getting there even with 60 AOB, so perhaps I'll take just a bit more.......
I suspect "wrapping" up a turn at low level has killed more pilots than parcels of air or windshear ( on "light" types)
Same problem/logic can apply to display flying ( without pointing fingers but e.g. trying to stay inside a fixed line such as a display boundary) and no doubt elsewhere.
Flying close to the (already much increased) stall speed in a (steep) turn is never a good idea, whatever the wind, whatever the gusts.
I know you'll all know this but just in case I'll offer up here what I offered in the other place (where TBH I think discusion on windshear and gusts and parcels of air may be confusing the matter):
Once upon a time UK CFS Bulldog students were demo'd the downwind turn "problem" when they started Low Level Nav flying/nav exes, 250 feet aal dual, perhaps, but not sure, 90 knots ish IAS). The training was nothing to do with coping with parcels of air or windshear; you can fly round at 45 AOB or more all day in your parcel of air at low level quite safely as long as the fuel allows and as long as you don't care where you end up, the point of the exercise was to make folks aware that it gets more complex when you need to turn to accurately to overfly a waypoint on the ground.
e.g. You are flying downwind, and it's a strong wind 30 knots plus, tailwind initially... you are going to turn so as to be on a new heading as you overfly a fixed point on the ground. You start off by using your "standard" 10 knot day angle of bank, standard IAS, standard 10 knot day amount of anticipation.
.... Of course the tailwind becomes an increasing beam wind and half way around the turn you see you are sliding sideways over the ground and are now struggling to get around to overfly the fix, something you really want to do that to get set up accurately for the next leg of the navex...so....the temptation is bound to be to tighten the turn ....."just a bit more bank"...."just a bit more pull / loading ( more g /AOA)"....."oh, darn, It's really windy and I'm not quite getting there even with 60 AOB, so perhaps I'll take just a bit more....... I suspect "wrapping" up a turn at low level has killed more pilots than parcels of air or windshear ( on "light" types)Same problem/logic can apply to display flying ( without pointing fingers but e.g. trying to stay inside a fixed line such as a display boundary) and no doubt elsewhere.
Last edited by wiggy; 31st Jan 2017 at 21:11.
FWIW.
Noticeable in a light a/c when turning say LH Base Leg (from Downwind) & the wind is regular & brisk on the runway heading, that that turn left actually exceeds 90 degrees in the air because the wind is pushing you to the right.
With reduced speed for the circuit, if careless, it could be as low as stall +30%. Yet there's a need to turn more steeply than in still air - just to maintain what is optically the familiar square pattern over the ground.
Steeper turn, at low speed is said to be a classic stall/spin scenario where one is too low to recover.
I always watch the ASI like a hawk in all stages of approach/landing & get down to safe flap speed before the turns to give extra margin over stall speed.
In stiff wind be aware of this optical illusion 'trap'.
mike hallam.
Noticeable in a light a/c when turning say LH Base Leg (from Downwind) & the wind is regular & brisk on the runway heading, that that turn left actually exceeds 90 degrees in the air because the wind is pushing you to the right.
With reduced speed for the circuit, if careless, it could be as low as stall +30%. Yet there's a need to turn more steeply than in still air - just to maintain what is optically the familiar square pattern over the ground.
Steeper turn, at low speed is said to be a classic stall/spin scenario where one is too low to recover.
I always watch the ASI like a hawk in all stages of approach/landing & get down to safe flap speed before the turns to give extra margin over stall speed.
In stiff wind be aware of this optical illusion 'trap'.
mike hallam.
If you have a physicist friend, ask him about 'frames of reference'.
Depending on your latitude, you have a velocity (relative to the centre of the globe you are airborne above) of between 450 and zero metres/second, plus the vector sum of your groundspeed. Now, does turning downwind at the North Pole, and doing it at the Equator have a different effect? Discuss (as my lecturer used to say).
The bright ones will catch on, the rest go by whatever book they trust.
Wind gradient is very important, (as is the optical illusion of speed over the ground), but a totally different effect from 'inertia'.
Depending on your latitude, you have a velocity (relative to the centre of the globe you are airborne above) of between 450 and zero metres/second, plus the vector sum of your groundspeed. Now, does turning downwind at the North Pole, and doing it at the Equator have a different effect? Discuss (as my lecturer used to say).
The bright ones will catch on, the rest go by whatever book they trust.
Wind gradient is very important, (as is the optical illusion of speed over the ground), but a totally different effect from 'inertia'.
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Wiggy, yes, been there, done that CFS "low level intro", at 250 feet in the old Bulldog. My staff instructor watched me fly my "give back" to him then quietly asked what height we ex-Heli pilots were used to flying at. I correctly answered along the lines of "down to 50 feet agl, in training areas, and no minimum height if carrying out a concealed approach or departure". He politely asked if I would mind climbing a bit then, as he wasn't used to being that low. Oops! I'd been doing my stuff much lower than he was used to. I'd just gone down to what I felt was a comfortable height.
The best way I can think of to illustrate this to a student is to place an object (pen) on a piece of paper on a flat surface (table).
The pen is the aircraft, the paper is the "parcel" of air the aircraft is flying through.
I move them both across the table with the pen move faster than the paper, thus representing what is happening when flying in a moving parcel of air.
I then stop the movement of both the pen a paper, letting go of the pen, I then tell the student to watch what happens if the speed of the "parcel" of air changes. I jerk the paper and the pen gets "left behind".
I then explain this is what happens to an aircraft when the parcel of air in which it's flying changes direction or speed. The result is manifested in a momentary increase or decrease in IAS while the aircraft catches up.
The pen is the aircraft, the paper is the "parcel" of air the aircraft is flying through.
I move them both across the table with the pen move faster than the paper, thus representing what is happening when flying in a moving parcel of air.
I then stop the movement of both the pen a paper, letting go of the pen, I then tell the student to watch what happens if the speed of the "parcel" of air changes. I jerk the paper and the pen gets "left behind".
I then explain this is what happens to an aircraft when the parcel of air in which it's flying changes direction or speed. The result is manifested in a momentary increase or decrease in IAS while the aircraft catches up.
What I posted in the thread Pilot DAR refers to.
Flying helicopters in the offshore world it was not uncommon to have 60 knots of wind when taking off from a platform. Climb speed in our particular aircraft was 75 knots and the turn to downwind while holding climb speed was visually spectacular if not seen previously. The point is, the aircraft doesn't care what the wind is, and if you are flying by reference to instruments you would have no idea what the strength of the wind is, or indeed, if there is any wind, save for the fact that you already have 60 knots airspeed prior to commencement of the take off.
One example we used to use by way of explanation was, imagine a Piper Cub flying a constant rate turn at 90 knots and such an altitude that its inboard wheel was resting on the bonnet (hood) of a convertible car, and the car drivers job was to maintain the position of the aircrafts wheel.
Now consider what the car driver experiences when no wind is blowing, and when say 60 knots of wind is blowing. Also what does the aircraft/pilot experience in both cases.
Aircraft inertia only comes into the discussion when talking with reference to gusts, which is not the subject under discussion.
Flying helicopters in the offshore world it was not uncommon to have 60 knots of wind when taking off from a platform. Climb speed in our particular aircraft was 75 knots and the turn to downwind while holding climb speed was visually spectacular if not seen previously. The point is, the aircraft doesn't care what the wind is, and if you are flying by reference to instruments you would have no idea what the strength of the wind is, or indeed, if there is any wind, save for the fact that you already have 60 knots airspeed prior to commencement of the take off.
If you have a physicist friend, ask him about 'frames of reference'.
Now consider what the car driver experiences when no wind is blowing, and when say 60 knots of wind is blowing. Also what does the aircraft/pilot experience in both cases.
Aircraft inertia only comes into the discussion when talking with reference to gusts, which is not the subject under discussion.
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Fascinating topic and posts. Just so I'm on the same page, the terms gusts and windsheer are interchangeable and mean the same thing?
And, 27/09's pen and paper demonstration method is limited to simulating horizontal sheer (gusts) and not vertical sheer?
And, at what point in the sheer does the inertia come into play ?
And, 27/09's pen and paper demonstration method is limited to simulating horizontal sheer (gusts) and not vertical sheer?
And, at what point in the sheer does the inertia come into play ?
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I agree that the aircraft does not know what the air in which it is flying is doing, to a point. I have been taught that at the slow side of the aircraft speed scale, where wind speed is faster, inertia can begin to play a role in pilot perception, and then performance.
Though I have nowhere near Megan's experience flying helicopters, during my training, the balling out I received turning downwind after takeoff was memorable. In the SW300, I was climbing nicely after translation, and with lots of room under me (though not established in the normal [airplane] circuit yet), I gently turned crosswind then downwind. My instructor lit into me for that, explaining that the inertia of the helicopter might be overcome by the change in apparent wind direction relative to its mass, and would not accelerate fast enough with the wind to maintain the IAS and the rate of increase of groundspeed, and re-enter translation He went on to demonstrate the effect of allowing the helicopter to be caught by and apparent (to the helicopter) increase in tailwind, and begin to settle as more power had to be added to maintain the climb. It was a memorable demonstration. He is a 20,000 hour helicopter pilot, so I listened to what he said.
I accept that the effect is probably minimal to none with an airplane flying a normal circuit or at normal speeds, as it does simply move with the air. My experience with STOL equipped Cessna floatplanes has been that it is possible to get into an alarming situation turning downwind during a low speed climb, when trying to evade rising terrain. I agree that gusts, terrain effects, and increasing windspeed with altitude would be factors in the perception of decreasing performance. If the pilot could climb ahead safely, they probably would have. If they have chosen a low speed climbing turn, terrain is probably a factor in their decision. then they are flying with visual reference to that terrain, and may try to fly more slowly than they should in the climbing turn. Though a steady wind may not be affecting performance singularly, the combination of effects of wind and perception can get a pilot in trouble. I have experienced this in low powered skiplanes and floatplanes, where tight climbing turns in confined areas resulted in the sensation (or stall warning) of degraded climb performance at that speed. I've known two pilots who have reported to me after the accident that as they climbed in a turn, the wind caught them from behind, and they settled into the trees. In both situations, my senses allowed me to believe that they experienced a combination of terrain effect, and increasing windspeed as they climbed - but they still crashed in a downwind turn, where a climb ahead would have worked fine.
When dissected in physics, I'm sure there are purist answers which differ. From my training and experience, I fly and train others that this is a cautionary, or avoid if possible situation.
Flying helicopters in the offshore world it was not uncommon to have 60 knots of wind when taking off from a platform. Climb speed in our particular aircraft was 75 knots and the turn to downwind while holding climb speed was visually spectacular if not seen previously.
I accept that the effect is probably minimal to none with an airplane flying a normal circuit or at normal speeds, as it does simply move with the air. My experience with STOL equipped Cessna floatplanes has been that it is possible to get into an alarming situation turning downwind during a low speed climb, when trying to evade rising terrain. I agree that gusts, terrain effects, and increasing windspeed with altitude would be factors in the perception of decreasing performance. If the pilot could climb ahead safely, they probably would have. If they have chosen a low speed climbing turn, terrain is probably a factor in their decision. then they are flying with visual reference to that terrain, and may try to fly more slowly than they should in the climbing turn. Though a steady wind may not be affecting performance singularly, the combination of effects of wind and perception can get a pilot in trouble. I have experienced this in low powered skiplanes and floatplanes, where tight climbing turns in confined areas resulted in the sensation (or stall warning) of degraded climb performance at that speed. I've known two pilots who have reported to me after the accident that as they climbed in a turn, the wind caught them from behind, and they settled into the trees. In both situations, my senses allowed me to believe that they experienced a combination of terrain effect, and increasing windspeed as they climbed - but they still crashed in a downwind turn, where a climb ahead would have worked fine.
When dissected in physics, I'm sure there are purist answers which differ. From my training and experience, I fly and train others that this is a cautionary, or avoid if possible situation.
I have been researching my own low level windshear incident with the assistance of GPS, Altitude, TAS and GS among other items at one second intervals from my recorder.
Much of the discussion of windshear is based on the blithe assumption that it's laminar - simplifies the equations
There's vortices lurking down there. When the air drops some 60 metres in 5 seconds, you're in the toilet bowl
Much of the discussion of windshear is based on the blithe assumption that it's laminar - simplifies the equations
There's vortices lurking down there. When the air drops some 60 metres in 5 seconds, you're in the toilet bowl
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I used to have the student fly a low level downwind turn In two ways. First fly it looking out at the ground- result invariably a loss of airspeed. Second fly the same turn looking only at the instruments- result no loss of airspeed. The problem in this scenario is the visual effect of the apparent gain in speed as you turn downwind. HOWEVER, I have observed that if it is a climbing turn onto the downwind, that is climbing with an increasing tailwind component, maintaining airspeed does need a more nose down attitude to maintain airspeed.
