Downwind Turns
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Downwind Turns
Aaaargh! Not that old chestnut!
I'm interested if anybody thinks there are real dangers in turning downwind at low level in windy conditions. I'm familiar with the theory, an aeroplane flies in a parcel of air and it doesn't matter if that parcel of air happens to be moving over the ground, but I still think there are very real dangers.
I think probably the biggest factors are the pilots *perception* of increasing airspeed speed due to the increasing groundspeed leading to the temptation to pitch up or reduce power, and the *perception* of the aircraft slipping as it turns across the wind leading to the temptation to unbalance the aircraft with innapropriate rudder input.
If it is a climbing turn to downwind there will be a tendency to lose airspeed because of the increasing tailwind component due to wind gradient.
(I also have a pet theory developed after spending many airline standby hours standing in a field flying a radio controlled model glider, that a gust is sharper than a lull, therefore:
When flying into wind the relatively sharp gust results in a momentarily increased airspeed, but the gentler lull results in a smaller loss of airspeed.
When flying downwind the relatively gentle lull causes a small gain in airspeed, but the sharper gust causes a bigger loss of airspeed.
It certainly felt like this was the case with the model - and no I wasn't being fooled by the winds effect on the models groundspeed, I was thermal soaring and tended to trim the model for just over min sink speed and then let it look after itself in pitch as much as possible. I freely admit this could be complete bollox, I haven't been able to find any information to support my little theory, and even if it is true it may not be particularly relevant to full size aircraft. Any thoughts?)
I'm especially interested to hear the views of John Farley, or anyone else who has spent a lot of time at high alpha, near the ground, on windy days, and maybe glider pilots are more likely to have noticed these effects? Are there any accident reports or studies that have looked into the effects of turning downwind at low level in strong winds?
THAI TUN
I'm interested if anybody thinks there are real dangers in turning downwind at low level in windy conditions. I'm familiar with the theory, an aeroplane flies in a parcel of air and it doesn't matter if that parcel of air happens to be moving over the ground, but I still think there are very real dangers.
I think probably the biggest factors are the pilots *perception* of increasing airspeed speed due to the increasing groundspeed leading to the temptation to pitch up or reduce power, and the *perception* of the aircraft slipping as it turns across the wind leading to the temptation to unbalance the aircraft with innapropriate rudder input.
If it is a climbing turn to downwind there will be a tendency to lose airspeed because of the increasing tailwind component due to wind gradient.
(I also have a pet theory developed after spending many airline standby hours standing in a field flying a radio controlled model glider, that a gust is sharper than a lull, therefore:
When flying into wind the relatively sharp gust results in a momentarily increased airspeed, but the gentler lull results in a smaller loss of airspeed.
When flying downwind the relatively gentle lull causes a small gain in airspeed, but the sharper gust causes a bigger loss of airspeed.
It certainly felt like this was the case with the model - and no I wasn't being fooled by the winds effect on the models groundspeed, I was thermal soaring and tended to trim the model for just over min sink speed and then let it look after itself in pitch as much as possible. I freely admit this could be complete bollox, I haven't been able to find any information to support my little theory, and even if it is true it may not be particularly relevant to full size aircraft. Any thoughts?)
I'm especially interested to hear the views of John Farley, or anyone else who has spent a lot of time at high alpha, near the ground, on windy days, and maybe glider pilots are more likely to have noticed these effects? Are there any accident reports or studies that have looked into the effects of turning downwind at low level in strong winds?
THAI TUN
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THAI TUN
Hi.
I have exactly the same view as you with regard to r/c models. I mainly fly mine over the sea and have learned the hard way that when it is gusty (when is it not so far as a model is concerned?) then you really need to watch it with such turns. I agree the balloon when turning into a gusty wind and the sink when turning downwind and a gust hits you up the backside.
At full scale speeds there are plenty of very experienced guys who rubbish the whole idea and back up their views with the standard theories that you have touched on. Rufus Heal wrote a very convincing article in Flyer a while back explaining that when full sized aircraft crashed turning downwind in the circuit it was for other reasons.
I have a very trusted mate whose maths are of the standard to question the detail of Einstein’s theories (I am not jesting and nor are the Unis he works with) and who is also the only bloke to get a Harrier (not Kestrel or P1127) back onto an airfield following a donk failure. He says there is no such thing as the downwind turn problem in a steady wind. But he says gusts are quite a different matter. And as we have said, when is there a wind that has no gusts.
It is never a problem with a jet borne Harrier because the lift forces are being generated by the exhaust gas velocities which are enormous compared to any wind gust speeds and so the distortion of said lift system by any gust is negligible. But be careful of turning your tail to wind as it is like reversing into your own hot air which will loose you thrust at 1000lb per 10deg C rise in the temperature of the air entering the intake.
Cheers
John
Hi.
I have exactly the same view as you with regard to r/c models. I mainly fly mine over the sea and have learned the hard way that when it is gusty (when is it not so far as a model is concerned?) then you really need to watch it with such turns. I agree the balloon when turning into a gusty wind and the sink when turning downwind and a gust hits you up the backside.
At full scale speeds there are plenty of very experienced guys who rubbish the whole idea and back up their views with the standard theories that you have touched on. Rufus Heal wrote a very convincing article in Flyer a while back explaining that when full sized aircraft crashed turning downwind in the circuit it was for other reasons.
I have a very trusted mate whose maths are of the standard to question the detail of Einstein’s theories (I am not jesting and nor are the Unis he works with) and who is also the only bloke to get a Harrier (not Kestrel or P1127) back onto an airfield following a donk failure. He says there is no such thing as the downwind turn problem in a steady wind. But he says gusts are quite a different matter. And as we have said, when is there a wind that has no gusts.
It is never a problem with a jet borne Harrier because the lift forces are being generated by the exhaust gas velocities which are enormous compared to any wind gust speeds and so the distortion of said lift system by any gust is negligible. But be careful of turning your tail to wind as it is like reversing into your own hot air which will loose you thrust at 1000lb per 10deg C rise in the temperature of the air entering the intake.
Cheers
John
Last edited by John Farley; 15th Feb 2004 at 21:04.
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Downwind Turns
Go fly a Chopper and give the downwind turns a try, at low speed. (Hint: pls don’t attempt it at low altitude)
It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible.
The aircraft is not able to accelerate to overcome the loss of headwind component/increase in tailwind component. Think of it in terms on wind shear. Only here the wind is constant, and the aircraft changes velocity.
NO, this is not demonstratable in a B747 at FL350 holding in 100kt winds, but believe me, try it in a Chopper/gyrocopter or microlite with 30kts of headwind and a 30KIAS.
Watched a very spectacular prang of a Christan Husky at Wonderboom, Pretoria, about 15yrs ago, attempting a steep downwind turn. Literally crashed and burned, very sad, pilot died.
It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible.
The aircraft is not able to accelerate to overcome the loss of headwind component/increase in tailwind component. Think of it in terms on wind shear. Only here the wind is constant, and the aircraft changes velocity.
NO, this is not demonstratable in a B747 at FL350 holding in 100kt winds, but believe me, try it in a Chopper/gyrocopter or microlite with 30kts of headwind and a 30KIAS.
Watched a very spectacular prang of a Christan Husky at Wonderboom, Pretoria, about 15yrs ago, attempting a steep downwind turn. Literally crashed and burned, very sad, pilot died.
Last edited by Avi8tor; 15th Feb 2004 at 16:23.
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There was a detailed report in the RAeS mag some years ago on this topic ... I found it interesting reading .. shouldn't be too difficult to track down a copy.
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Do not forget that in the early 90's there was a helicopter on a rig to rig flight, of just minutes, that turned downwind at night (I think) after departing a North Sea oil rig. All perished. Verdict was the loss of performance during the turn downwind. Relative speeds are important. It blows its arse off in the North Sea and these guys were probably in visual contact at night (at what may have been 4 pm) with their destination in a helicopter that really performs.
It's a real phenomena. Don't underestimate it in slow flying/underpowered aircraft or in low level/ low visibilty conditions.
It's a real phenomena. Don't underestimate it in slow flying/underpowered aircraft or in low level/ low visibilty conditions.
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In many years of aircraft carrier operations, where the airport moves as well as the air -- often at speeds well in excess of 30 knots -- I have NEVER heard of anyone running into difficulty in the downwind turn! The main reason is that over open water there are virtually no speed clues, so the turn is made ON INSTRUMENTS.
If it's windy, it's more likely to be gusty. If it's gusty, your stall margin is reduced when your airspeed is instantaneously reduced in a gust (or its let-up). If you don't compensate in a turn by increasing airspeed, that stall margin is reduced even more. If the stall margin goes to zero, so may your altitude.
After 30+ years of light to heavy (Areonca Champ and Schleicher kA-6 to AH-1W to 747-400), zero- to four-engine airplane and helo flying, nobody will convince me the 'downwind turn demon' is anything other than the pilot improperly substituting ground references for instrument references for airspeed and bank angle.
If it's windy, it's more likely to be gusty. If it's gusty, your stall margin is reduced when your airspeed is instantaneously reduced in a gust (or its let-up). If you don't compensate in a turn by increasing airspeed, that stall margin is reduced even more. If the stall margin goes to zero, so may your altitude.
After 30+ years of light to heavy (Areonca Champ and Schleicher kA-6 to AH-1W to 747-400), zero- to four-engine airplane and helo flying, nobody will convince me the 'downwind turn demon' is anything other than the pilot improperly substituting ground references for instrument references for airspeed and bank angle.
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Thanks for the input guys.
Avi8tor, "It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible." I think you've got it wrong there, imagine the parcel of air is the air inside a railway carriage, as the train trundles along at a constant speed you could fly a paper aeroplane around inside it, and it would behave no differently than it would if the train were stationary.
john_tullamarine, I phoned the RAeS library but they didn't have any articles indexed under 'downwind turns', do you have any more details?
Mark
Avi8tor, "It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible." I think you've got it wrong there, imagine the parcel of air is the air inside a railway carriage, as the train trundles along at a constant speed you could fly a paper aeroplane around inside it, and it would behave no differently than it would if the train were stationary.
john_tullamarine, I phoned the RAeS library but they didn't have any articles indexed under 'downwind turns', do you have any more details?
Mark
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'Parcel of air'
Hmmm…. I am a little shocked that people battle with this basic concept. At no stage did I mention the turn being executed with reference to a ground feature. The result WILL be the same if the said manoeuvre was carried out on instruments. If it has nothing to do with inertia and ground speed, then explain wind shear!?!
The block of air theory is all good and well, till we come to overcoming inertia. If we were flying into a 60kt head wind with 60kt TAS, we would all agree that the groundspeed is zero. If this headwind component drops to zero, the IAS would drop to zero. This is due to inertia.
In wind shear conditions, headwind component turn into tailwind component; the aircraft is not able to accelerate fast enough, with spectacular results. It requires that aircraft accelerate from 0kts groundspeed to 120kts groundspeed(60kts w/v and then 60kts TAS) in a matter of seconds (if the head/tail wind is 60kts). This phenomenon is not in question. If you have any doubts, the FAA has some good reading on the topic.
Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.
QED
The block of air theory is all good and well, till we come to overcoming inertia. If we were flying into a 60kt head wind with 60kt TAS, we would all agree that the groundspeed is zero. If this headwind component drops to zero, the IAS would drop to zero. This is due to inertia.
In wind shear conditions, headwind component turn into tailwind component; the aircraft is not able to accelerate fast enough, with spectacular results. It requires that aircraft accelerate from 0kts groundspeed to 120kts groundspeed(60kts w/v and then 60kts TAS) in a matter of seconds (if the head/tail wind is 60kts). This phenomenon is not in question. If you have any doubts, the FAA has some good reading on the topic.
Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.
QED
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I believe this is the old wives tale, that does not go away.
WRT the A-320, it may affect ground speed mini, perhaps that is why you are seeing and increase, although, I cannot say what would cause it in a 76.
Spent 12000 hrs in anything from J-3 Cubs to DC-8, never seen anything remotely like it.
Have done slow flight in the J 3 going backwards over the ground, but even wehen I turned, I did not fail out of the sky.
GS and IAS are to entirely different creatures.
The windshear scenario does not apply, windshear is an abrupt change in wind over a very short distance, it would take some interesting (not to say impossible) maneuvering to achieve this change while turning.
As far as the a/c that crashed into the lake, stallspeed increase with angle of bank, so if the a/c was close to stall, it could very well perpetrate an into the drink, because it stalled. Or, as an aircraft is turned, the pitch has to be increased due to loss of vertical lift component, failure to do so, may cause a/c to go swimming. Then again, perhaps he caught the wingtip, or was flying in ground effect.
The only scenario I can imagine, is one that really cannot be considered flying. See if you can guess it
WRT the A-320, it may affect ground speed mini, perhaps that is why you are seeing and increase, although, I cannot say what would cause it in a 76.
Spent 12000 hrs in anything from J-3 Cubs to DC-8, never seen anything remotely like it.
Have done slow flight in the J 3 going backwards over the ground, but even wehen I turned, I did not fail out of the sky.
GS and IAS are to entirely different creatures.
The windshear scenario does not apply, windshear is an abrupt change in wind over a very short distance, it would take some interesting (not to say impossible) maneuvering to achieve this change while turning.
As far as the a/c that crashed into the lake, stallspeed increase with angle of bank, so if the a/c was close to stall, it could very well perpetrate an into the drink, because it stalled. Or, as an aircraft is turned, the pitch has to be increased due to loss of vertical lift component, failure to do so, may cause a/c to go swimming. Then again, perhaps he caught the wingtip, or was flying in ground effect.
The only scenario I can imagine, is one that really cannot be considered flying. See if you can guess it
Hmmm…. I am a little shocked that people battle with this basic concept.
Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.
An aircraft making a 180 degree turn in nil wind also undergoes a change in momentum of 120 knots (times the aircraft mass), from +60 knots to -60 knots. In your example, the aircraft also undergoes a change in momentum of 120 knots (times the aircraft mass), from 0 to -120 knots. From the point of view of the aerodynamics, there's no difference -- same momentum change, same force in both cases.
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Avi8tor, sticking with steady winds and ignoring the added complication of gusts or windshear for the moment:
If inertia were groundspeed related then my paper aeroplane flying round in circles in a railway carriage travelling at a constant speed, would be affected by it's groundspeed, but it isn't.
I'm no physicist, but I think momentum/inertia are relative to whatever you choose as a point of reference, and the appropriate point of reference for studying the behaviour of an aircraft flying in a parcel of air, is the parcel of air. The momentum of the aircraft relative to the ground may suddenly become of interest though, if the aircraft contacts the ground.
THAI TUN
If inertia were groundspeed related then my paper aeroplane flying round in circles in a railway carriage travelling at a constant speed, would be affected by it's groundspeed, but it isn't.
I'm no physicist, but I think momentum/inertia are relative to whatever you choose as a point of reference, and the appropriate point of reference for studying the behaviour of an aircraft flying in a parcel of air, is the parcel of air. The momentum of the aircraft relative to the ground may suddenly become of interest though, if the aircraft contacts the ground.
THAI TUN
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Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.
As others have pointed out, this happens with or without gusts or wind shear. As I pointed out earlier, gusts (or wind shear, which is simply a gust of a particular magnitude) affect the airplane because they are sudden. You are correct that the airplane's inertia causes the airspeed to vary significantly during the gust/shear.
However, a coordinated turn is not a sudden maneuver. A 180 degree turn may take anywhere from 30 seconds to a minute or more. In any steady-state wind, the control and power response is sufficient to accomplish the required change in momentum/energy vector in a controlled manner with respect to the air mass, and with no regard for the ground.
Only if the pilot changes his frame of reference at an inappropriate time (i.e., fixates on the ground) or if the air flow around the airplane changes suddenly and uncontrollably (i.e., a gust or wind shear) is there any "problem" in completing the turn. Again, if power and indicated airspeed are not increased in the turn, stall margin is reduced, and the effect of any gust or wind shear is exacerbated.
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Some interesting points being raised here. To share a vivid example of tailwind effects, I fly light aircraft in gusty winds from a local field in Australia most weekends. The field is shared with a variety of homebuilt aircraft and gliders including a guy who flies a gyrocopter. One morning it was blowing a particulary brisk 30 knots straight down the strip when the gyro pilot taxied out, lined up and gave it full power down the strip. The machine became airborne in no time and climbed like a missile to about three hundred feet when ( no doubt inspired by the spritely performance) he cranked it around to fly downwind back down the strip ....when it promptly fell like a stone and smacked back into the ground about three feet from where he started the take off roll. Blades and bits of gyrocopter fly in every direction and he extracts himself with a puzzled look on his face.
Obviously the old gyro is more vulnerable under these conditons but turning downwind at low level in higher than usual winds certainly has it's risks. I can only compare it to ships losing helm ability in a following sea. It's all fluid ain't it.
cheers all
Vic
Obviously the old gyro is more vulnerable under these conditons but turning downwind at low level in higher than usual winds certainly has it's risks. I can only compare it to ships losing helm ability in a following sea. It's all fluid ain't it.
cheers all
Vic
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Hi Fellas,
After near 40 years of flying I still don't know the answer. I do know that after many years of very low level work that when planning an escape route in gusty conditions it will be into wind where possible. Otherwise increase the power and airspeed for the downwind turn.
After near 40 years of flying I still don't know the answer. I do know that after many years of very low level work that when planning an escape route in gusty conditions it will be into wind where possible. Otherwise increase the power and airspeed for the downwind turn.
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Obviously the old gyro is more vulnerable under these conditons but turning downwind at low level in higher than usual winds certainly has it's risks. I can only compare it to ships losing helm ability in a following sea. It's all fluid ain't it.
It is very possible that the pilot did the same thing that other pilots have done in the past -- used the ground as the frame of reference for his turn when there was a significant wind. If it was an open gyro with minimal or no instruments, the probability is even higher!
The "following sea" analogy is an interesting one, worthy of some further thought. As a sailor and kayaker as well as a pilot, I've fought following seas in many circumstances, but haven't quite been able to correlate them to flying yet. Maybe in a helo, when the gust factor is higher than the forward airspeed...
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Intruder has it.
Fly the air you are in, not the surface you are over.
I do literally hundreds of downwind turns, 100' to 200' often climbing and at MAUW most days.
If you are careful, keep your airspeed, do not load it up in the turn too much, and KNOW THE DIFFERENCE BETWEEN STALL BUFFET AND LOW LEVEL TURBULENCE, you should be OK.
Increase margins for rough air, like over trees, it eats performance.
Put yourself in a position where you are obliged to tighten the turn to avoid an obstacle whilst operating in or near the region of reversed command, then you are going down....
Fly the air you are in, not the surface you are over.
I do literally hundreds of downwind turns, 100' to 200' often climbing and at MAUW most days.
If you are careful, keep your airspeed, do not load it up in the turn too much, and KNOW THE DIFFERENCE BETWEEN STALL BUFFET AND LOW LEVEL TURBULENCE, you should be OK.
Increase margins for rough air, like over trees, it eats performance.
Put yourself in a position where you are obliged to tighten the turn to avoid an obstacle whilst operating in or near the region of reversed command, then you are going down....
Nobody has yet mentioned Kinetic Energy, the energy of movement. Equal to half x Masss x V squared. This is relative to your frame of reference.
With reference to the ground (which is the thing you are most likely to hit), a helo travelling at 30 kt in nil wind has energy related to 30 squared, or 900 somethings of energy. When it is doing 30 kt into a 30 kt headwind, it has zero groundspeed, and zero kinetic energy - it can't hit anything, so it can't be damaged by that energy.
If it is doing 30 knots with a 30 kt downwind, i.e. 60 kt groundspeed, the KE is related to 60 squared, or 3600 somethings of energy - seriously bigger than before, and a lot more energy to dissipate if it wants to avoid hitting something.
So, we watch as the chopper hovers into the 30 kt wind. Zero KE. We watch it start a turn out of the wind, and if it goes slowly, we see it fly crosswind at about 30 kt groundspeed, and eventually downwind at 60 kt groundspeed.
Where did the extra 3600 units of energy come from to do this? Did it extract the energy from the airflow? Did the temperature of the airflow decrease to supply the energy? Did it move slower, having given up some of it's "oomph"? No, and no. It got hotter (from being whacked by the rotor blades) and it moved faster, because the blades ADDED energy to the airflow. However, if it did somehow extract energy from the airflow, how does it "give it back" when it turns back into wind?
From practical experience gained over 35 years and 12,000 hours of flying, mostly helicopters at very low level, it sure as heck seems that the energy has to come from the engine. If you don't have plenty of power, you don't turn downwind. And the IAS drops off the clock, too. So, any turn out of a strong wind is gradual, and accompanied by a power increase, and do it slowly to allow the aircraft time to build up the KE.
Turn back into wind, and lower the power.
Yes, most of the manoeuvering is done with reference to ground features - powerline inspections require that. But once the turn out of wind is commenced, it is not relative to the ground features - we are not trying to follow a road or such, we are just trying to hold height, hold airspeed, and NOT fall from the sky.
When the speed of the air and the speed of the aircraft are similar, the theories of parcels of air don't seem to work. The paper aeroplane doing slow circles in a train moving at any speed, is actually moving backwards relative to the earth. Any helicopter pilot who does that at low level on a regular basis is inviting a call from his maker.
With reference to the ground (which is the thing you are most likely to hit), a helo travelling at 30 kt in nil wind has energy related to 30 squared, or 900 somethings of energy. When it is doing 30 kt into a 30 kt headwind, it has zero groundspeed, and zero kinetic energy - it can't hit anything, so it can't be damaged by that energy.
If it is doing 30 knots with a 30 kt downwind, i.e. 60 kt groundspeed, the KE is related to 60 squared, or 3600 somethings of energy - seriously bigger than before, and a lot more energy to dissipate if it wants to avoid hitting something.
So, we watch as the chopper hovers into the 30 kt wind. Zero KE. We watch it start a turn out of the wind, and if it goes slowly, we see it fly crosswind at about 30 kt groundspeed, and eventually downwind at 60 kt groundspeed.
Where did the extra 3600 units of energy come from to do this? Did it extract the energy from the airflow? Did the temperature of the airflow decrease to supply the energy? Did it move slower, having given up some of it's "oomph"? No, and no. It got hotter (from being whacked by the rotor blades) and it moved faster, because the blades ADDED energy to the airflow. However, if it did somehow extract energy from the airflow, how does it "give it back" when it turns back into wind?
From practical experience gained over 35 years and 12,000 hours of flying, mostly helicopters at very low level, it sure as heck seems that the energy has to come from the engine. If you don't have plenty of power, you don't turn downwind. And the IAS drops off the clock, too. So, any turn out of a strong wind is gradual, and accompanied by a power increase, and do it slowly to allow the aircraft time to build up the KE.
Turn back into wind, and lower the power.
Yes, most of the manoeuvering is done with reference to ground features - powerline inspections require that. But once the turn out of wind is commenced, it is not relative to the ground features - we are not trying to follow a road or such, we are just trying to hold height, hold airspeed, and NOT fall from the sky.
When the speed of the air and the speed of the aircraft are similar, the theories of parcels of air don't seem to work. The paper aeroplane doing slow circles in a train moving at any speed, is actually moving backwards relative to the earth. Any helicopter pilot who does that at low level on a regular basis is inviting a call from his maker.
Where did the extra 3600 units of energy come from to do this? Did it extract the energy from the airflow? Did the temperature of the airflow decrease to supply the energy? Did it move slower, having given up some of it's "oomph"? No, and no. It got hotter (from being whacked by the rotor blades) and it moved faster, because the blades ADDED energy to the airflow. However, if it did somehow extract energy from the airflow, how does it "give it back" when it turns back into wind?
Whirl a 1 kg stone attached to a string around your head so it has a velocity of, say, 6 m/s. It has a KE of 18 J, constant around the circle. Now take it on to a bus travelling at 6 m/s, and continue to twirl it in the same way. It now goes from a KE of zero when it is moving backwards with respect to the twirler, to a KE of 72 J when it is moving in the same direction as the bus.
Does it behave differently? No it doesn't, and if you don't believe me, try it next time you're on a moving walkway in a deserted airport.
So how does it get and give back that extra energy?
I can't comment on the math, it is interesting reading but not too practical when your in the saddle and about to turn downwind.
What I believe in and what I used to drum into students is this:
The aircraft doesn't know or care where the ground is or how fast it is going over the ground. All it knows is the air coming at it, and it will perform according to that. Forget the ground, set the power and attitude to give sufficient airspeed and performance and voila!, one shouldn't hit the ground. Scan the bloody instruments!
In my opinion, all turning downwind accidents are in some way related to the pilot letting the ground come back into the basic performace equation.
We are flying AIRcraft not GROUNDcraft eh?
What I believe in and what I used to drum into students is this:
The aircraft doesn't know or care where the ground is or how fast it is going over the ground. All it knows is the air coming at it, and it will perform according to that. Forget the ground, set the power and attitude to give sufficient airspeed and performance and voila!, one shouldn't hit the ground. Scan the bloody instruments!
In my opinion, all turning downwind accidents are in some way related to the pilot letting the ground come back into the basic performace equation.
We are flying AIRcraft not GROUNDcraft eh?