T/O Downwind -v- power available ?
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T/O Downwind -v- power available ?
Hello Rotorheads,
As I have progressed up into the realms of Turbine power, I have been shown, and then been allowed to execute a downwind and/or crosswind T/O to see what this entails and feels like, whilst I am very aware of the problems associated with this type of T/O, is there any rule of rough calculation which equates wind speed to power available that would allow for this sort of T/O, I hasten to add I always T/O into wind but one of the FI's has shown me downwind and crosswind T/O's in the B206, and said with the power increase over piston engines you are safe to consider this , can any of you confirm this?
As I have progressed up into the realms of Turbine power, I have been shown, and then been allowed to execute a downwind and/or crosswind T/O to see what this entails and feels like, whilst I am very aware of the problems associated with this type of T/O, is there any rule of rough calculation which equates wind speed to power available that would allow for this sort of T/O, I hasten to add I always T/O into wind but one of the FI's has shown me downwind and crosswind T/O's in the B206, and said with the power increase over piston engines you are safe to consider this , can any of you confirm this?
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The wind direction really has little effect on the rotor performance in a hover, any wind is good wind if you don't take off. The rotor really doesn't know which direction the wind comes from, and tail rotor power increase is nearly always less than the performance gain from the wind speed.
In a downwind takeoff, three factors complicate the matter (let's use 15 knot winds as an example):
1) The aircraft must accelerate from minus 15 knots velocity through zero velocity. At the zero velocity point (where the aircraft is scooting along at 15 knots groundspeed) the power will be at the maximum for the procedure. (If you were foolish enough to have been at max power when in the steady downwind hover, at this point you will settle and ground contact is a real possibility.) As you then continue to accelerate, the power will decrease (or the climb will begin) and you are off.
2) The takeoff distance (when compared to still air) will be the sum of that needed to get to zero plus that needed for a normal take off. With a 5 degree nose down as "normal" you will take you about 110 feet just to get to zero knots airspeed. Additionally, the acceleration will seem very flat relative to the ground and obstructions, since you are always 15 knots faster, so your climb angle will be considerably shallower than a still air takeoff.
3) If you have a power failure anywhere along the takeoff, you will not be able to stop the aircraft prior to touchdown, because that would require that you decelerate to zero, then continue decelerating to a rearward 15 knots. Count on needing to run it on at about the wind speed.
That was all for a dead downwind takeoff. For a cross wind, the issue is much cleaner. Let's assume the same 15 knot wind.
Since the rotor doesn't know where the wind is coming from, the performance is better from the start, it takes much less power to hover, so your acceleration for takeoff is greater, if you chose to use the extra power. The distance to takeoff is less than that of a zero wind takeoff, so you are ahead there, too. The only two complications are:
1) You gave up the chance to shorten your takeoff by about that 110 feet if you had taken the headwind.
2) If you have a power failure, you will not have the advantage of that extra 15 knots of airspeed on touchdown, plus you must compensate for the cross wind on touchdown, thus complicating the touchdown a bit.
All in all, the rules might be:
1) Take a headwind component as often as possible, always if it allows good takeoff paths and barrier clearance.
2) If dictated by the scenario, take a cross wind with the assurance that it is better than still air.
3) As a last resort, take the down wind if you have good power margin above the "steady" hover and if the takeoff path allows extra distance for barrier clearance and allows a running touchdown if the engine quits.
I have been trying to boil down a rule of thumb, but its a complex situation. If you have a 15 knot wind, and a helo that climbs at 500 FPM at 45 knots:
At 20 knots, the upwind allows a 40% climb gradient (40 up at 100 across), still air allows 7% and downwind allows 4%. In other words, for a slow speed climbout, you have 10 times the climbout gradient into the wind as down wind! Rule of thumb might be, if there are barriers, don't go downwind.
[ 03 September 2001: Message edited by: Nick Lappos ]
In a downwind takeoff, three factors complicate the matter (let's use 15 knot winds as an example):
1) The aircraft must accelerate from minus 15 knots velocity through zero velocity. At the zero velocity point (where the aircraft is scooting along at 15 knots groundspeed) the power will be at the maximum for the procedure. (If you were foolish enough to have been at max power when in the steady downwind hover, at this point you will settle and ground contact is a real possibility.) As you then continue to accelerate, the power will decrease (or the climb will begin) and you are off.
2) The takeoff distance (when compared to still air) will be the sum of that needed to get to zero plus that needed for a normal take off. With a 5 degree nose down as "normal" you will take you about 110 feet just to get to zero knots airspeed. Additionally, the acceleration will seem very flat relative to the ground and obstructions, since you are always 15 knots faster, so your climb angle will be considerably shallower than a still air takeoff.
3) If you have a power failure anywhere along the takeoff, you will not be able to stop the aircraft prior to touchdown, because that would require that you decelerate to zero, then continue decelerating to a rearward 15 knots. Count on needing to run it on at about the wind speed.
That was all for a dead downwind takeoff. For a cross wind, the issue is much cleaner. Let's assume the same 15 knot wind.
Since the rotor doesn't know where the wind is coming from, the performance is better from the start, it takes much less power to hover, so your acceleration for takeoff is greater, if you chose to use the extra power. The distance to takeoff is less than that of a zero wind takeoff, so you are ahead there, too. The only two complications are:
1) You gave up the chance to shorten your takeoff by about that 110 feet if you had taken the headwind.
2) If you have a power failure, you will not have the advantage of that extra 15 knots of airspeed on touchdown, plus you must compensate for the cross wind on touchdown, thus complicating the touchdown a bit.
All in all, the rules might be:
1) Take a headwind component as often as possible, always if it allows good takeoff paths and barrier clearance.
2) If dictated by the scenario, take a cross wind with the assurance that it is better than still air.
3) As a last resort, take the down wind if you have good power margin above the "steady" hover and if the takeoff path allows extra distance for barrier clearance and allows a running touchdown if the engine quits.
I have been trying to boil down a rule of thumb, but its a complex situation. If you have a 15 knot wind, and a helo that climbs at 500 FPM at 45 knots:
At 20 knots, the upwind allows a 40% climb gradient (40 up at 100 across), still air allows 7% and downwind allows 4%. In other words, for a slow speed climbout, you have 10 times the climbout gradient into the wind as down wind! Rule of thumb might be, if there are barriers, don't go downwind.
[ 03 September 2001: Message edited by: Nick Lappos ]
Vfr,
Orthodox UK mil teaching discounts the windspeed, as Nick Lappos mentioned - all downwind departures are treated the same. A hover check downwind is carried out and the power noted, then the max power available is confirmed. If the difference between the 2 is 20% or greater, then the power margin is considered adequate for a downwind transition. However, all the comments appended by NL are agreed - we only carry out downwind transitions if there are over-riding (that's not flying across Yorks, by the way...) operational factors or considerations.
Orthodox UK mil teaching discounts the windspeed, as Nick Lappos mentioned - all downwind departures are treated the same. A hover check downwind is carried out and the power noted, then the max power available is confirmed. If the difference between the 2 is 20% or greater, then the power margin is considered adequate for a downwind transition. However, all the comments appended by NL are agreed - we only carry out downwind transitions if there are over-riding (that's not flying across Yorks, by the way...) operational factors or considerations.
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Great bunch of info there,
I'd add that while turbines do have more power then pistons they actually use that power in working situations. That power margin difference you mentioned is nice for a lightly loaded ship but not for one doing work near mgw.
An 11,ooo pound ship slinging a 9,000 pound load has lots of power but most of it is already in use. You may only have a small percentage of power left to stop you or direct your flight path.
In this case the downwind/ into the wind decision you make will be more critical due to the inertia that you're trying to control.
A rule of thumb for power required I was told a while ago was that a downwind approach/ departure requires 2% additional NG. It may not be exact but it does make you look at the problem prior to becoming involved in it.
I'd add that while turbines do have more power then pistons they actually use that power in working situations. That power margin difference you mentioned is nice for a lightly loaded ship but not for one doing work near mgw.
An 11,ooo pound ship slinging a 9,000 pound load has lots of power but most of it is already in use. You may only have a small percentage of power left to stop you or direct your flight path.
In this case the downwind/ into the wind decision you make will be more critical due to the inertia that you're trying to control.
A rule of thumb for power required I was told a while ago was that a downwind approach/ departure requires 2% additional NG. It may not be exact but it does make you look at the problem prior to becoming involved in it.
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NL ,T&B & T,
Thank you for that very interesting break down of my seemingly simple question, I have noted the feeling of what could be described as drifting with power when in the down wind T/O which was and is a very errie feeling, I did notice that height gain was indeed very slow, the cross wind situation was better and the B206 seemed to act very similar as when into wind, but I have taken in what you have all said, and I fully understand the problems that could and would be associated with down wind T/O's, thank you all, I envy your experience, thank you for sharing it>
My Regards
By the way Thud, only a Yorkist or a lad from Lancashire would understand the Over riding bit, I think!!
Thank you for that very interesting break down of my seemingly simple question, I have noted the feeling of what could be described as drifting with power when in the down wind T/O which was and is a very errie feeling, I did notice that height gain was indeed very slow, the cross wind situation was better and the B206 seemed to act very similar as when into wind, but I have taken in what you have all said, and I fully understand the problems that could and would be associated with down wind T/O's, thank you all, I envy your experience, thank you for sharing it>
My Regards
By the way Thud, only a Yorkist or a lad from Lancashire would understand the Over riding bit, I think!!
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And a good bit of advice, load up your Jetranger to max gross and then go for a flight, you may not feel it has so much power any more (try to do it before things cool off much more).
This will help you get a feel for when downwind departures are and are not acceptable.
This will help you get a feel for when downwind departures are and are not acceptable.
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mr Vfr man,
regarding downwind takeoffs in a Robbie, a good rule of thumb is that if you have 2" in hand in the hover from your 5 minute rating then you should be able to do it safely(however if the engine fails in the early climbout, things are looking v.bad)
also I agree with heloteacher about the 206, if you have got 4 heavy punters in it as well as you then you have to be real careful to keep torque below 100% while hover taxying.
if you want a bit of power then try a super puma, they really have got power
stay lucky
regarding downwind takeoffs in a Robbie, a good rule of thumb is that if you have 2" in hand in the hover from your 5 minute rating then you should be able to do it safely(however if the engine fails in the early climbout, things are looking v.bad)
also I agree with heloteacher about the 206, if you have got 4 heavy punters in it as well as you then you have to be real careful to keep torque below 100% while hover taxying.
if you want a bit of power then try a super puma, they really have got power
stay lucky
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Let me establish an argument here to keep things lively -
It takes no more power to take off down wind than it does to hover in still air. That means that the rules of thumb presented above (good ones, I think) are for you to use if you don't know what the capability of your aircraft is, and you have to judge by what you are seeing right now on a windy day.
The power rise due to downwind takeoff is never more than the power increase experienced between a windy day and a still air day.
The transition needed for the private pilot to move to the next stage is to begin to understand the performance of the machine against the charts, so the power needed on a given day is known before the liftoff to a hover. Then the downwind takeoff is a choice, not a chance.
It takes no more power to take off down wind than it does to hover in still air. That means that the rules of thumb presented above (good ones, I think) are for you to use if you don't know what the capability of your aircraft is, and you have to judge by what you are seeing right now on a windy day.
The power rise due to downwind takeoff is never more than the power increase experienced between a windy day and a still air day.
The transition needed for the private pilot to move to the next stage is to begin to understand the performance of the machine against the charts, so the power needed on a given day is known before the liftoff to a hover. Then the downwind takeoff is a choice, not a chance.
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Um...sorry to butt in but here's my two penny's worth.
Don't.
If you went in Clear Area, you can get out Clear Area by back tracking.
If you went in Helipad, you can get out Helipad.
If you're going to approach to land downwind, ask why.
If you have to take off downwind, ask why.
Even during Police Ops I avoid downwind landings unless it's life or death & even then there has to be no viable alternative.
At worst I'll take a cross wind, but I haven't come across a good reason to take off downwind in my very humble 12 years flying.
It can be done & reasonably safely....but why? The machine prefers being into wind, chances of successful reject, after donk, are much better into wind (not least because the tail hangs lower downwind).
Into wind is the safest option (I don't think anyone can argue with that? Except Chinook drivers?)
Cheers!
Don't.
If you went in Clear Area, you can get out Clear Area by back tracking.
If you went in Helipad, you can get out Helipad.
If you're going to approach to land downwind, ask why.
If you have to take off downwind, ask why.
Even during Police Ops I avoid downwind landings unless it's life or death & even then there has to be no viable alternative.
At worst I'll take a cross wind, but I haven't come across a good reason to take off downwind in my very humble 12 years flying.
It can be done & reasonably safely....but why? The machine prefers being into wind, chances of successful reject, after donk, are much better into wind (not least because the tail hangs lower downwind).
Into wind is the safest option (I don't think anyone can argue with that? Except Chinook drivers?)
Cheers!
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A general rule would be to calculate your Out of Ground Effect(OGE)+5% hover performance calculation or equivalent for your machine, and ensure you have that performance. Due to your movement across the ground you will not have In Ground Effect(IGE) performance i.e. ground effect benefits, when you are at zero airspeed therefore requiring OGE performance.
OGE +5% should allow you to hover in free air and overcome light turbulence without exceeding the power limits. Light turbulence is defined as updraughts and downdraughts not exceeding 2-300' per minute (met office). Therfore you should be able to climb vertically in still air in the hover and acheive a rate of climb of 300' per minute. Alternately you could increase your aircraft weight by 5% and maintain a hover but drift up and down with the turbulence.
If you haven't calculated OGE +5% then for SA342 a rule of thumb is 10% torque in hand before attempting the departure. Regards!
OGE +5% should allow you to hover in free air and overcome light turbulence without exceeding the power limits. Light turbulence is defined as updraughts and downdraughts not exceeding 2-300' per minute (met office). Therfore you should be able to climb vertically in still air in the hover and acheive a rate of climb of 300' per minute. Alternately you could increase your aircraft weight by 5% and maintain a hover but drift up and down with the turbulence.
If you haven't calculated OGE +5% then for SA342 a rule of thumb is 10% torque in hand before attempting the departure. Regards!
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Nick,
I would agree that the transitioning pilot needs to know the charted performance parameters inside and out of the aircraft they are flying.
But I think it does require more power for a downwind takeoff.
Let me throw this out for consideration.
On a zero wind, standard day aircraft "A" loads up to a specified weight and sits at a hover.
The aircraft is transitioned to forward flight slowly and maintains a three foot skid height along the way.
Just prior to ETL the aircraft settles a little. We'll call it one foot per second. The rate of acceleration applied by the pilot up to this point (with power) means that the aircraft will take 2 seconds to go from the onset of that settling into ETL and clean air. Only having lost 2 feet, the aircraft can accomplish this maneuver with no additional power applied.
Aircraft "B" is identical in all respects but is doing the takeoff with a ten knot tailwind.
I believe his settling rate would be the same but this aircraft will be in that settling rate for a longer time, lets say 4-5 seconds
as it fights it's way to the clean air just ahead of the disc.
Requiring more power. (to not hit the ground)
I would agree that the transitioning pilot needs to know the charted performance parameters inside and out of the aircraft they are flying.
But I think it does require more power for a downwind takeoff.
Let me throw this out for consideration.
On a zero wind, standard day aircraft "A" loads up to a specified weight and sits at a hover.
The aircraft is transitioned to forward flight slowly and maintains a three foot skid height along the way.
Just prior to ETL the aircraft settles a little. We'll call it one foot per second. The rate of acceleration applied by the pilot up to this point (with power) means that the aircraft will take 2 seconds to go from the onset of that settling into ETL and clean air. Only having lost 2 feet, the aircraft can accomplish this maneuver with no additional power applied.
Aircraft "B" is identical in all respects but is doing the takeoff with a ten knot tailwind.
I believe his settling rate would be the same but this aircraft will be in that settling rate for a longer time, lets say 4-5 seconds
as it fights it's way to the clean air just ahead of the disc.Requiring more power. (to not hit the ground)
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Acorn,
The OGE power plus 5% is a very high margin indeed. The difference between OGE and IGE is about 15% and if you add 5% on top of that you lose about half your payload. That is a very conservative rule.
No performance is lost because of the downwind at all. The ground effect does not change because of movement of the air or the ground, that is an ancient piece of pilot lore. Think of the big bombers, with 1 of 4 engines running, limping back across the channel, 1/2 wing span above the waves. They had no trouble gaining ground effect!
For all the ppruners who would like to try, I invite you to make a careful downwind landing and note the power (use manifold pressure or engine turbine temp) needed to complete a careful approach. Compare downwind to that needed for still air, there is no difference. Ground effect is not lost because the aircraft is translating, that effect is nil.
Regarding the safety of downwind takeoff or landing, it is clearly less safe, due to barrier clearance, failure tolerance and ground covered (roofus shows great wisdom, why not back up across the clear area to the rear?)
On the other hand, I have had to do hundreds of downwind landings, and dozens of downwind takeoffs in my career, and knowing how is part of our craft.
The OGE power plus 5% is a very high margin indeed. The difference between OGE and IGE is about 15% and if you add 5% on top of that you lose about half your payload. That is a very conservative rule.
No performance is lost because of the downwind at all. The ground effect does not change because of movement of the air or the ground, that is an ancient piece of pilot lore. Think of the big bombers, with 1 of 4 engines running, limping back across the channel, 1/2 wing span above the waves. They had no trouble gaining ground effect!
For all the ppruners who would like to try, I invite you to make a careful downwind landing and note the power (use manifold pressure or engine turbine temp) needed to complete a careful approach. Compare downwind to that needed for still air, there is no difference. Ground effect is not lost because the aircraft is translating, that effect is nil.
Regarding the safety of downwind takeoff or landing, it is clearly less safe, due to barrier clearance, failure tolerance and ground covered (roofus shows great wisdom, why not back up across the clear area to the rear?)
On the other hand, I have had to do hundreds of downwind landings, and dozens of downwind takeoffs in my career, and knowing how is part of our craft.
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Tgrendl,
Your thought experiment is a good one, and clearly illustrates the issue. However, the settling you describe is caused by the aircraft running into the ground vortex created by its main rotor wash. In a steady, no wind hover, the rotorwash spreads outward evenly in all directions, at about 8 to 12 knots on average, as a thin sheet close to the ground. On takeoff, the aircraft moves into this outward spreading air, and it causes the air ahead to roll up and interact with the forward part of the rotor disk. This causes a loss of efficiency, and thus the settling. It also causes the lateral stick shift and the ruffling prior to ETL.
A great paper was prepared by a Boeing team during their development of their YUH-61A UTTAS (the one that lost to the Black Hawk) which defined this effect.
In any case, that interchange between the aircraft and its wake takes place up wind, down wind and even in sideward flight.
In the case you describe, the only difference between the still air aircraft and the up wind or downwind aircraft is the speed at which the ground is moving. All the aerodynamics stays the same.
The power to land downwind is exactly the same as the power to land in still air. The negative point is that you did not gain the power that is inherent in landing up wind, where you never have to make your aircraft slow down below windspeed.
Your thought experiment is a good one, and clearly illustrates the issue. However, the settling you describe is caused by the aircraft running into the ground vortex created by its main rotor wash. In a steady, no wind hover, the rotorwash spreads outward evenly in all directions, at about 8 to 12 knots on average, as a thin sheet close to the ground. On takeoff, the aircraft moves into this outward spreading air, and it causes the air ahead to roll up and interact with the forward part of the rotor disk. This causes a loss of efficiency, and thus the settling. It also causes the lateral stick shift and the ruffling prior to ETL.
A great paper was prepared by a Boeing team during their development of their YUH-61A UTTAS (the one that lost to the Black Hawk) which defined this effect.
In any case, that interchange between the aircraft and its wake takes place up wind, down wind and even in sideward flight.
In the case you describe, the only difference between the still air aircraft and the up wind or downwind aircraft is the speed at which the ground is moving. All the aerodynamics stays the same.
The power to land downwind is exactly the same as the power to land in still air. The negative point is that you did not gain the power that is inherent in landing up wind, where you never have to make your aircraft slow down below windspeed.
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Nick,
Happy labor day to you and to all on the board.
Two quick questions and then the thread. Is it possible to go to your plant and get a tour without causing too much grief for the workers?
And is the RAH66 scheduled sometime to get to ADTA at Rucker as a J serialed aircraft?
The thread,
I believe and can intuitively agree with all you said except the part about rotor wake interaction and groundspeed/airspeed.
It seems like in this example you would be in this region longer because it's taking you longer to accelerate through it. (result of the ten knot tailwind)
The relative airspeed over the disc would be the key here regardless of groundspeed.
Continuing to scrape my knuckles in the dirt when I walk,
Tom
Happy labor day to you and to all on the board.
Two quick questions and then the thread. Is it possible to go to your plant and get a tour without causing too much grief for the workers?
And is the RAH66 scheduled sometime to get to ADTA at Rucker as a J serialed aircraft?
The thread,
I believe and can intuitively agree with all you said except the part about rotor wake interaction and groundspeed/airspeed.
It seems like in this example you would be in this region longer because it's taking you longer to accelerate through it. (result of the ten knot tailwind)
The relative airspeed over the disc would be the key here regardless of groundspeed.
Continuing to scrape my knuckles in the dirt when I walk,
Tom
Just to add fuel to the flames.... With a conventional helicopter, it often takes less power to hover downwind than into wind. Blade vortices/downwash impinge on the tail rotor making it less efficient when into wind?
The above remarks about crosswind takeoffs are strictly accurate, but from a practical point of view there can be problems. Some types have very benign handling qualities when crosswind, others can be a real handful (or foot-full!) If operating at high weights, the ensuing pedal 'dancing' as you transition while applying full power, can lead to inadvertant overtorqueing.
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Nick, I'm not attacking you, just responding to all your posts at once.
You said, "It takes no more power to take off down wind than it does to hover in still air. "
I don't agree with this. When you take off with a tail wind, you must accelerate your helicopter first to zero airspeed, then beyond. Hovering in still air requires less power than accelerating and climbing from the still air hover.
If you meant to compare power required to take off with a tail wind to the power required to take off from a still air hover, I wouldn't argue. I think if there is a difference it would be small.
You also said, "any wind is good wind if you don't take off."
I always thought that in wind speeds slower than translational lift speed, the power required to hover with any relative wind would be greater than that for still air.
Finally, you said, "...make a careful downwind landing and note the power...needed to complete a careful approach. Compare downwind to that needed for still air, there is no difference."
The power required to approach along the same path, with the same speed gradient will be greater if you have a tailwind.
Practically, you can do the approach with the same power requirement down to zero airspeed by shallowing your approach angle with a tailwind.
What really amazes me in this discussion is the strength of the tail winds that have been mentioned. We have a published limitation of 15 kts for downwind takeoffs and landings. Even without that limitation there would still have to be considerable mitigating circumstance before I'd consider any downwind approach/landing in 15kt or stronger winds.
Matthew.
You said, "It takes no more power to take off down wind than it does to hover in still air. "
I don't agree with this. When you take off with a tail wind, you must accelerate your helicopter first to zero airspeed, then beyond. Hovering in still air requires less power than accelerating and climbing from the still air hover.
If you meant to compare power required to take off with a tail wind to the power required to take off from a still air hover, I wouldn't argue. I think if there is a difference it would be small.
You also said, "any wind is good wind if you don't take off."
I always thought that in wind speeds slower than translational lift speed, the power required to hover with any relative wind would be greater than that for still air.
Finally, you said, "...make a careful downwind landing and note the power...needed to complete a careful approach. Compare downwind to that needed for still air, there is no difference."
The power required to approach along the same path, with the same speed gradient will be greater if you have a tailwind.
Practically, you can do the approach with the same power requirement down to zero airspeed by shallowing your approach angle with a tailwind.
What really amazes me in this discussion is the strength of the tail winds that have been mentioned. We have a published limitation of 15 kts for downwind takeoffs and landings. Even without that limitation there would still have to be considerable mitigating circumstance before I'd consider any downwind approach/landing in 15kt or stronger winds.
Matthew.
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Nick + tgrendl,
The requirement for the additional power is to ensure you are able to overcome the additional settling described as 1' per second. A rate of climb capability of 2-300' per minute is more than adequate for that purpose. OGE +5% in temperate climates at heights below 3000' i.e UK would rarely limit your AUM performance. Should the temperature or density altitude preclude OGE performance then you would be in the realms of appropriate into wind techniques from the outset.
With regards payload, an SA342 in UK with 340 KGS of fuel would carry 2 pax with about 50 KGS spare. OGE +5% performance during Spring through to Autumn would be achievable to 3-4000' at AUM. The 5% thrust margin equates to being able to increase the aircraft AUM by 95 KGS and still hold a free air hover without being able to compensate for turbulence. Therefore the reduction in AUM would not affect routine UK operations. Assuming the same generic calculations are used across types the same would be so although this I cannot confirm.
As an asside, with regards tail rotor effectivness downwind, assuming the downwash from the main rotors is therefore blown away from the tail rotor, if the tail rotor is optimised to utilise downwash as in the case of the Lynx then the loss of translational lift from downwash would cause an increased power requirement downwind. Regards
[ 04 September 2001: Message edited by: ACORN ]
The requirement for the additional power is to ensure you are able to overcome the additional settling described as 1' per second. A rate of climb capability of 2-300' per minute is more than adequate for that purpose. OGE +5% in temperate climates at heights below 3000' i.e UK would rarely limit your AUM performance. Should the temperature or density altitude preclude OGE performance then you would be in the realms of appropriate into wind techniques from the outset.
With regards payload, an SA342 in UK with 340 KGS of fuel would carry 2 pax with about 50 KGS spare. OGE +5% performance during Spring through to Autumn would be achievable to 3-4000' at AUM. The 5% thrust margin equates to being able to increase the aircraft AUM by 95 KGS and still hold a free air hover without being able to compensate for turbulence. Therefore the reduction in AUM would not affect routine UK operations. Assuming the same generic calculations are used across types the same would be so although this I cannot confirm.
As an asside, with regards tail rotor effectivness downwind, assuming the downwash from the main rotors is therefore blown away from the tail rotor, if the tail rotor is optimised to utilise downwash as in the case of the Lynx then the loss of translational lift from downwash would cause an increased power requirement downwind. Regards
[ 04 September 2001: Message edited by: ACORN ]
vfrpilot
Don't be put off by DW transitions, sometimes they are the only option - but always think far enough ahead so you have as many options as possible....
Is it safer to t/o into wind but over a substantial infrangible object - or depart downwind over a clear area?
Personally, I'd always go for the clear area.....with only one engine, one can sometimes lose the big picture!
With all the other good points raised, don't forget the downwind wind limit for B206 - 17kts
Must admit, bit surprized to hear our EMS pilot restrict his 2-eng, dw approaches to "life and death" tasks only! That's not meant as a side swipe, just surprized, that's all.
Don't be put off by DW transitions, sometimes they are the only option - but always think far enough ahead so you have as many options as possible....
Is it safer to t/o into wind but over a substantial infrangible object - or depart downwind over a clear area?
Personally, I'd always go for the clear area.....with only one engine, one can sometimes lose the big picture!
With all the other good points raised, don't forget the downwind wind limit for B206 - 17kts
Must admit, bit surprized to hear our EMS pilot restrict his 2-eng, dw approaches to "life and death" tasks only! That's not meant as a side swipe, just surprized, that's all.
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I have often found that I am able to to do an out of wind approach that has a far better approach angle than a directly into wind one. For example approaching to a particular wharf coming from one direction requires a verticle descent of some 100', where as approaching from over the ocean has an approach angle of almost 0. Even though the sea brezze is almost always behind me it still uses far less power if handled carefully. What I am saying is (as with all aspects of RW flying) flexability is the answer.
One word of caution though!! Approaching with a even moderate down winds with a heavy aircraft may see you not slowing as fast as you expect. Add to this a higher than normal rate of descent and you have the following situation. Hi ROD, Low-Nil forward IAS (Due to the tail wind), High power. You guessed it the result is Vortex Ring (sometimes called power setteling), and at low alt you may be unable to recover before the ground.
Also watch out for flap forward it can surprise you with how much cyclic is required to overcome it, and if you are flying an aircraft with limited Tail Rotor authority or one that is susceptible to LTE (as in the B206) it may start to weather cock and swap ends on you without you being able to do anything at all.
One word of caution though!! Approaching with a even moderate down winds with a heavy aircraft may see you not slowing as fast as you expect. Add to this a higher than normal rate of descent and you have the following situation. Hi ROD, Low-Nil forward IAS (Due to the tail wind), High power. You guessed it the result is Vortex Ring (sometimes called power setteling), and at low alt you may be unable to recover before the ground.
Also watch out for flap forward it can surprise you with how much cyclic is required to overcome it, and if you are flying an aircraft with limited Tail Rotor authority or one that is susceptible to LTE (as in the B206) it may start to weather cock and swap ends on you without you being able to do anything at all.