Downwind turns equal disaster??
The Original Whirly
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Right then, it seems like the problem with the downwind turn in strong gusty winds is that you can't judge your airspeed by reference to either your inaccurate ASI or the ground, doesn't it? So what do you do?
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From 16 Flight Maneuvers, http://www.av8n.com/how/htm/maneuver...-downwind-turn
"Summary: Changing Headwinds and Tailwinds
For ground-reference maneuvers, a steady wind has a direct effect.
For other maneuvers, a steady wind has no effect on the airplane or on the pilot in the cockpit. However, the maneuvers will appear different to ground-based observers.
In the presence of windshears, you can gain or lose energy due to the albatross effect. In real life, this means for instance that you will get slightly better performance climbing into the wind. This gives you a reason to turn downwind a little later than you otherwise would.
For any maneuver that doesn't depend on a ground reference, a steady wind has no effect on the maneuver. For example, a standard-rate turn to upwind is just the same as a standard-rate turn to downwind. You can't even determine the magnitude or direction of the wind without using a ground reference.
If you want to calculate the energy in the ground-based frame of reference, you must account for the airplane being batted by the wind. "
"Summary: Changing Headwinds and Tailwinds
For ground-reference maneuvers, a steady wind has a direct effect.
For other maneuvers, a steady wind has no effect on the airplane or on the pilot in the cockpit. However, the maneuvers will appear different to ground-based observers.
In the presence of windshears, you can gain or lose energy due to the albatross effect. In real life, this means for instance that you will get slightly better performance climbing into the wind. This gives you a reason to turn downwind a little later than you otherwise would.
For any maneuver that doesn't depend on a ground reference, a steady wind has no effect on the maneuver. For example, a standard-rate turn to upwind is just the same as a standard-rate turn to downwind. You can't even determine the magnitude or direction of the wind without using a ground reference.
If you want to calculate the energy in the ground-based frame of reference, you must account for the airplane being batted by the wind. "
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Nick...
Does that mean that from the cockpit you cannot determine the nett effect or their is no nett effect ??
And obviously nobody has told the atsb that a rotor can't tell where the wind is coming from.
Ambient wind conditions can have significant and differing effects on a tail rotor equipped helicopter's performance. Engine power is delivered to a transmission system, which drives the main and tail rotors The power required to drive the transmission system is determined primarily by the amount of drag being produced by the rotors and the power available is determined by the power output of the engine(s). The difference between the power available and power required is known as the power margin. If the power required to drive transmission exceeds the amount of power available from the engine, then the main and tail rotor speed will decay, or droop. When the speed of the main rotor droops significantly, the main rotor loses lift and the helicopter descends. Wind blowing over a main rotor provides translational lift that can significantly reduce the power required to drive the helicopter's transmission system. Wind may also assist a helicopter to maintain heading, which also reduces the load demand on the transmission and therefore reduces the power required to drive the transmission. Conversely, a wind from an adverse direction may increase the load demand on the transmission and, in turn, the power required from the engine. Therefore, the wind may cause a net effect which, depending on its strength and direction, will reduce or increase the power required for a tail rotor equipped helicopter to maintain flight.
Does that mean that from the cockpit you cannot determine the nett effect or their is no nett effect ??
And obviously nobody has told the atsb that a rotor can't tell where the wind is coming from.
Ambient wind conditions can have significant and differing effects on a tail rotor equipped helicopter's performance. Engine power is delivered to a transmission system, which drives the main and tail rotors The power required to drive the transmission system is determined primarily by the amount of drag being produced by the rotors and the power available is determined by the power output of the engine(s). The difference between the power available and power required is known as the power margin. If the power required to drive transmission exceeds the amount of power available from the engine, then the main and tail rotor speed will decay, or droop. When the speed of the main rotor droops significantly, the main rotor loses lift and the helicopter descends. Wind blowing over a main rotor provides translational lift that can significantly reduce the power required to drive the helicopter's transmission system. Wind may also assist a helicopter to maintain heading, which also reduces the load demand on the transmission and therefore reduces the power required to drive the transmission. Conversely, a wind from an adverse direction may increase the load demand on the transmission and, in turn, the power required from the engine. Therefore, the wind may cause a net effect which, depending on its strength and direction, will reduce or increase the power required for a tail rotor equipped helicopter to maintain flight.
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Headwind, tailwind, groundspeed......
Example:
Distance 120NM, zero wind, speed 120 ( speedfactor 2, Groundspeed same).
Outbound 120/2 = 60 min.......inbound same = 120 minutes total flight time.
Distance 120NM, 30 headwind (dead on), speed 120, groundspeed 90 ( factor 1.5) = 120/1.5 = 80 minutes outbound.
Inbound same numbers except for tailwind ( groundspeed 150 = factor 2.5 ). 120/2.5 = 48 minutes = total flight time 128 minutes
Where did the 8 minuttes come from ? ;-)
Example:
Distance 120NM, zero wind, speed 120 ( speedfactor 2, Groundspeed same).
Outbound 120/2 = 60 min.......inbound same = 120 minutes total flight time.
Distance 120NM, 30 headwind (dead on), speed 120, groundspeed 90 ( factor 1.5) = 120/1.5 = 80 minutes outbound.
Inbound same numbers except for tailwind ( groundspeed 150 = factor 2.5 ). 120/2.5 = 48 minutes = total flight time 128 minutes
Where did the 8 minuttes come from ? ;-)
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You had a headwind factor effecting you for longer than the tail wind factor. The quickest return round trip is with nil wind, unless you can get a tail wind both ways.
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Dead on Nigel.....it toke a couple of days to get it myself...thought I could baffle you guys, but I'm way behind :-)
So why doesn't anybody care about circling minimas for helicopters IFR........we are talking JAR law based upon inproper information (laws of physics) according to this discussion so far.
Any thoughts ?
So why doesn't anybody care about circling minimas for helicopters IFR........we are talking JAR law based upon inproper information (laws of physics) according to this discussion so far.
Any thoughts ?
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I've just been rereading this thread and scratching my head.
From what a lot of you guys are telling me. Once I leave the ground I can manouver at will, upwind, downwind, crosswind, as slow or as fast as I like and it's not going to affect my aircrafts performance AT ALL ( don't tell me "maybe just a little bit" "hardly so you would notice"... I'm talking science..it either exists or it doesn't) as long as I don't look at the ground because I am part of the air, part of the sky, its all one etc etc...... then
the same people tell me I will however have a problem with wind shear, mountain waves, bumps, eddies, rubber duckies etc....
Now make up your minds. Am I one with the wind or not. You can't have it both ways. If my inertia no longer exists(relative to the Earth) because I'm flying well then it doesn't exist. You can't say .. well it's not there if you turn the aircraft , but if the wind turns you could be in deep do do
And at this point I'm not interested in how the aircraft does or doesn't behave I want to know the scientific law, the rule, the principal, thought out and handed down by some musty old fella with a long white beard in some recent or not so recent time in history.
Anybody help???
From what a lot of you guys are telling me. Once I leave the ground I can manouver at will, upwind, downwind, crosswind, as slow or as fast as I like and it's not going to affect my aircrafts performance AT ALL ( don't tell me "maybe just a little bit" "hardly so you would notice"... I'm talking science..it either exists or it doesn't) as long as I don't look at the ground because I am part of the air, part of the sky, its all one etc etc...... then
the same people tell me I will however have a problem with wind shear, mountain waves, bumps, eddies, rubber duckies etc....
Now make up your minds. Am I one with the wind or not. You can't have it both ways. If my inertia no longer exists(relative to the Earth) because I'm flying well then it doesn't exist. You can't say .. well it's not there if you turn the aircraft , but if the wind turns you could be in deep do do
And at this point I'm not interested in how the aircraft does or doesn't behave I want to know the scientific law, the rule, the principal, thought out and handed down by some musty old fella with a long white beard in some recent or not so recent time in history.
Anybody help???
overpitched
I reckon forget about inertial reference frames and windshear and its effect and if a 60 KT headwind suddenly turns into a 60 KT tailwind. Any windshear has unpredictable efffects anyway and it sounds as if you are pretty confused by people taking everything to the n'th degree. Stick to the basics mate.
If you turn downwind maintain a safe AIRSPEED (unless you have HOGE power available). EXPECT the ground speed to increase.
Don't use the GROUNDSPEED as your judge of AIRSPEED.
Scan/maintain the attitude that gives you the airspeed you need, set the power you need to hold height, and keep checking the ASI.
The goal here is to not spud in if you turn downwind isn't it? Keep your AIRSPEED on and you won't. Simple.
How about you think it all out and hand us down the guru stuff?
I reckon forget about inertial reference frames and windshear and its effect and if a 60 KT headwind suddenly turns into a 60 KT tailwind. Any windshear has unpredictable efffects anyway and it sounds as if you are pretty confused by people taking everything to the n'th degree. Stick to the basics mate.
If you turn downwind maintain a safe AIRSPEED (unless you have HOGE power available). EXPECT the ground speed to increase.
Don't use the GROUNDSPEED as your judge of AIRSPEED.
Scan/maintain the attitude that gives you the airspeed you need, set the power you need to hold height, and keep checking the ASI.
The goal here is to not spud in if you turn downwind isn't it? Keep your AIRSPEED on and you won't. Simple.
How about you think it all out and hand us down the guru stuff?
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Coyote
By telling me I have to "manage my airspeed" on a downwind turn you are agreeing with me...... ie the helicopter has to accelerate to maintain airspeed. If somebody has a different idea to that I just want to know the scientific principal is all, so I can do a bit of further reading.
Now the reason I think I'm right is that while in flight the rotor itself is doing just what we are discussing. each blade in turn goes upwind crosswind downwind crosswind etc. While you are hovering its all good but as forward airspeed of the aircraft increases the upwind and downwind components get larger creating dissymetry? of lift. Theoretically speaking I can't see why there would be one rule of motion for an aircraft turning downwind but a different one for a rotorblade doing the same.
You are not going to tell me that retreating blade stall is caused by the retreating blade trying to turn downwind with visual reference to the ground are you ???
By telling me I have to "manage my airspeed" on a downwind turn you are agreeing with me...... ie the helicopter has to accelerate to maintain airspeed. If somebody has a different idea to that I just want to know the scientific principal is all, so I can do a bit of further reading.
Now the reason I think I'm right is that while in flight the rotor itself is doing just what we are discussing. each blade in turn goes upwind crosswind downwind crosswind etc. While you are hovering its all good but as forward airspeed of the aircraft increases the upwind and downwind components get larger creating dissymetry? of lift. Theoretically speaking I can't see why there would be one rule of motion for an aircraft turning downwind but a different one for a rotorblade doing the same.
You are not going to tell me that retreating blade stall is caused by the retreating blade trying to turn downwind with visual reference to the ground are you ???
Last edited by overpitched; 24th Feb 2004 at 16:06.
Charlie S Charlie - typo error on my part - it should be 20 kts IAS, 40 kts GS. I could have just edited the post but I thought I ought to own up!
Coyote - "managing your airspeed" just means maintaining it by maintaining your attitude. As mentioned before, the natural tendency is to use groundspeed as a reference and keep it constant - this will lose you airspeed as you turn downwind.
Overpitched - if you transition forwards, backwards or sideways - at 10 to 15 kts of airflow across the disc the rotor will experience translational lift and an increase in thrust. That is why the rotor doesn't care which way the wind is coming from, it just responds to airflow across it.
Coyote - "managing your airspeed" just means maintaining it by maintaining your attitude. As mentioned before, the natural tendency is to use groundspeed as a reference and keep it constant - this will lose you airspeed as you turn downwind.
Overpitched - if you transition forwards, backwards or sideways - at 10 to 15 kts of airflow across the disc the rotor will experience translational lift and an increase in thrust. That is why the rotor doesn't care which way the wind is coming from, it just responds to airflow across it.
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Turning downwind
Went water-skiing on the river last weekend. Couldn't figure it out, every time the boat turned downstream we sank into the water because the boat slowed down. It all makes sense now, obviously it was inertia.
Fact is we didn't sink during the turn. Boat speed never varied. Never expected it to either. Wasn't that the current was not strong enough as it was kinda quick in a few places.
Well - if we had sank into the water on the downstream turn I suppose we could have blamed inertia, couldn't we?
Fact is we didn't sink during the turn. Boat speed never varied. Never expected it to either. Wasn't that the current was not strong enough as it was kinda quick in a few places.
Well - if we had sank into the water on the downstream turn I suppose we could have blamed inertia, couldn't we?
I've been following the thread trying to work out what's what. Haven't managed it yet - too complex.
However, one thought. In an RFM / POH / whatever, what is performance related to / graphed against ? Airspeed or groundspeed ?
And if you also remember that
Performance = Power + Attitude
does it help ?
However, one thought. In an RFM / POH / whatever, what is performance related to / graphed against ? Airspeed or groundspeed ?
And if you also remember that
Performance = Power + Attitude
does it help ?
Overpitched
Windshear is what it says, the two bits of air moving in different directions at the same time. It's like jumping from the up elevator onto the down elevator - you will land on your ar*e. Nobody on this thread has suggested that turbulence, windshear etc will not affect you.
The question I think is being asked is, 'in a steady, non-turbulent horizontal air flow, will a turn downwind have any effect on the flying characteristics of the helicopter?' - The answer is 'No it won't!'
Cheers
TeeS
Windshear is what it says, the two bits of air moving in different directions at the same time. It's like jumping from the up elevator onto the down elevator - you will land on your ar*e. Nobody on this thread has suggested that turbulence, windshear etc will not affect you.
The question I think is being asked is, 'in a steady, non-turbulent horizontal air flow, will a turn downwind have any effect on the flying characteristics of the helicopter?' - The answer is 'No it won't!'
Cheers
TeeS
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Ok TeeS
Tell me why not. When you fly in a constant direction from a parcel of air moving in one direction into a parcel of air travelling in a different direction thats called windshear and it affects you,
but on the other hand when the air moves in a constant direction and the aircraft travels in one direction and then travels in another direction that is called a downwind turn and it DOESN'T affect you....... seriously??? is that what you are telling me ?????
Now picture the rotor blades again. Advancing blade traveling upwind... lots of lift, retreating blade traveling downwind... less lift. You will remember it from basic training , it is one of the aerodynamic limitations of a helicopter. Now why would that happen on a blade(wing) but not the 2 wings attached to a plane. And if it happens to a plane why would it not happen to a helicopter.
River, water skiing, sinking boats, what crap is that???
Tell me why not. When you fly in a constant direction from a parcel of air moving in one direction into a parcel of air travelling in a different direction thats called windshear and it affects you,
but on the other hand when the air moves in a constant direction and the aircraft travels in one direction and then travels in another direction that is called a downwind turn and it DOESN'T affect you....... seriously??? is that what you are telling me ?????
Now picture the rotor blades again. Advancing blade traveling upwind... lots of lift, retreating blade traveling downwind... less lift. You will remember it from basic training , it is one of the aerodynamic limitations of a helicopter. Now why would that happen on a blade(wing) but not the 2 wings attached to a plane. And if it happens to a plane why would it not happen to a helicopter.
River, water skiing, sinking boats, what crap is that???
The Original Whirly
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overpitched,
I think I can explain it...
That's basically right. Except that if you get below 15-20 kts AIRSPEED, you will lose translational lift, and therefore require more power. You could do this by either slowing down, or due to losing a headwind. But it's AIRSPEED we're talking about, remember. If that doesn't change, neither will the performance.
Yes, but for different reasons. If a downdraft in the mountains is dragging the aircraft down, it'll require more power to keep flying. In the event of windshear or sudden changes of direction, the ASI won't be accurate since the pitot tube can't cope (read earlier in thread for details) so you'll need to keep the ATTITUDE of your aircraft in order to maintain you AIRSPEED ...not always easy in the mountains, so you can come to grief. Bumps, eddies, mountain waves...all same thing. But this isn't due to complicated rocket science or aerodynamic principles relating to each blade being different...if it was I'd need a diagram and a lot of thought and probably wouldn't be posting about it in case I got it wrong. 
It's simply due to downdrafts dragging you down, updrafts giving you a bit of free lift, and so on.
Well, I just re-read that and I think it's clear. But to summarise...
Performance won't alter unless you go below translational lift airspeed. Up and down drafts may affect your performance, for fairly obvious physical reasons.
Hmmm...if it's not that simple, someone had better tell me.
I think I can explain it...
From what a lot of you guys are telling me. Once I leave the ground I can manouver at will, upwind, downwind, crosswind, as slow or as fast as I like and it's not going to affect my aircrafts performance AT ALL
the same people tell me I will however have a problem with wind shear, mountain waves, bumps, eddies, rubber duckies etc....
It's simply due to downdrafts dragging you down, updrafts giving you a bit of free lift, and so on. Well, I just re-read that and I think it's clear. But to summarise...
Performance won't alter unless you go below translational lift airspeed. Up and down drafts may affect your performance, for fairly obvious physical reasons.
Hmmm...if it's not that simple, someone had better tell me.
Overpitched - imagine you are in a steady 5 foot hover with no wind, the hover attitude is set with cyclic, the hover height with lever and the heading with pedals; you are in equilibrium with all forces balanced to keep you in a steady position over the ground.
Now consider 2 different scenarios:
1. A wind of 20 kts suddenly appears from the dead ahead and you, the pilot, do nothing with the controls - eventually the aircraft will be translating across the ground at 20 kts in the same attitude, at the same height and on the same heading. (purists will tell me the disc will flap back and pitch the nose up as the gust hits, and they are right, but I am trying to illustrate a concept).
2. A wind of 20 kts suddenly appears from the dead ahead so you correct the tendency to drift back with forward cyclic, correct the tendency to climb (due to ETL) with lever and compensate for all the yaw changes (ETL on the TR and reduced power required) with pedals. If the wind stays constant, you will be in a state of equilibrium again (still over your position on the ground) but with a different attitude, power setting and pedal position.
In situation 1 you are "going with the flow" as you are when you turn at constant speed, power and attitude in a moving mass of air - ignoring the resulting movement across the ground.
In situation 2 you are maintaining ground position and having to compensate for the changes in airflow to prevent the wind blowing you across the ground. If you now switch the wind on and off, you are in a constant state of change with respect to attitude, power and heading which is the same as encountering turbulence or windshear.
Now consider 2 different scenarios:
1. A wind of 20 kts suddenly appears from the dead ahead and you, the pilot, do nothing with the controls - eventually the aircraft will be translating across the ground at 20 kts in the same attitude, at the same height and on the same heading. (purists will tell me the disc will flap back and pitch the nose up as the gust hits, and they are right, but I am trying to illustrate a concept).
2. A wind of 20 kts suddenly appears from the dead ahead so you correct the tendency to drift back with forward cyclic, correct the tendency to climb (due to ETL) with lever and compensate for all the yaw changes (ETL on the TR and reduced power required) with pedals. If the wind stays constant, you will be in a state of equilibrium again (still over your position on the ground) but with a different attitude, power setting and pedal position.
In situation 1 you are "going with the flow" as you are when you turn at constant speed, power and attitude in a moving mass of air - ignoring the resulting movement across the ground.
In situation 2 you are maintaining ground position and having to compensate for the changes in airflow to prevent the wind blowing you across the ground. If you now switch the wind on and off, you are in a constant state of change with respect to attitude, power and heading which is the same as encountering turbulence or windshear.