Pitch Attitude in Turns
Thread Starter
Pitch Attitude in Turns
The 'Hovering Downwind thread has stirred up a bit of theory and, as usual, a bit of argument. This got me thinking about another helicopter flight characteristic which I've never had fully explained; so here goes.....
When a helicopter is flown at a constant angle of bank through a 360 deg turn, the pitch attitude changes in order to maintain IAS constant. This effect is only noticeable when the wind is blowing, it doesn't happen in calm winds.
I've got my own ideas, but I'll wait for some of you theorists to explain it before I embarass myself. J
When a helicopter is flown at a constant angle of bank through a 360 deg turn, the pitch attitude changes in order to maintain IAS constant. This effect is only noticeable when the wind is blowing, it doesn't happen in calm winds.
I've got my own ideas, but I'll wait for some of you theorists to explain it before I embarass myself. J
When a helo turns during cruise, if it turns in the same direction of the advancing blade (i.e. advancing blade anticlockwise, turn to the left), that section of the rotor has further to travel, being on the outside of the turn. Therefore the speed of air over that section of blade increases and consequently, so too does the lift. If nothing is done to compensate, the helo pitches up and so the pilot has to pitch it in the opposite direction to maintain IAS.
Vice versa for turns in the opposite direction.
[Someone is now going to tell me that even though the lift vector increases, drag increases at the square of velocity and so the controlling factor is drag and so the effects are exactly the opposite!!!!].
Vice versa for turns in the opposite direction.
[Someone is now going to tell me that even though the lift vector increases, drag increases at the square of velocity and so the controlling factor is drag and so the effects are exactly the opposite!!!!].
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TC
assuming constant Rotor rpm, why would the airspeed over the advancing blade increase at constant IAS?
may I suggest it is because in the turn (in direction of rotation)the advancing blade travels further and therefore generates lift for a longer time that side?
assuming constant Rotor rpm, why would the airspeed over the advancing blade increase at constant IAS?
may I suggest it is because in the turn (in direction of rotation)the advancing blade travels further and therefore generates lift for a longer time that side?
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this effect can easily be seen with student pilots,
if you dont tell them it will happen, the nose always pitches up in a left turn in a conventional anticlockwise turning rotor a/c, and the nose always pitches down in a right turn, because they dont anticipate the effect and therefore dont correct for it.
regards
CF
if you dont tell them it will happen, the nose always pitches up in a left turn in a conventional anticlockwise turning rotor a/c, and the nose always pitches down in a right turn, because they dont anticipate the effect and therefore dont correct for it.
regards
CF
I think the effect TC is trying to explain is roll induced TQ spike where a roll to the retreating blade (left in US helos, and to the right in French/Russian) will produce a TQ "spike" during the roll rate. I believe this is caused by the fact that the reduction in drag on the retreating side (AoA reduced to roll aircraft) is less than the increase in drag on the advancing side (AoA increased to produce roll rate) due to the exponential effect of velocity (Vsquared). Thus the over-all drag of the system increases whilst rolling and this produces a TQ requirement to overcome. When stabilised in the turn, over-all drag is roughly the same as pre turn, thus TQ goes back to previous value. The velocity increase/decrease induced by the roll itslef would be extremely insignificant (even Vsquared) given the roll rate and distance travelled, thus I would be very suprised if this had anything to do with pitch up/down.
I think what Jellycopter is refering to though, is nose pitch changes during a stabilised constant turn, not during roll in or roll out. My initial thoughts would be that the rotor has absolutely no idea which way the wind is going, and is flying through a packet of air. Only the groundspeed makes us visually aware of the wind direction and speed, but the rotor system does not have such perception. Thus in a constant turn, the rotor causes no pitch up or down as it flies 360 degrees, because it doesn't feel wind speed or direction. I think that any pitch up or down tendancy is pilot induced as a reaction to the apparent ground slide/speed change during the turn.
I think what Jellycopter is refering to though, is nose pitch changes during a stabilised constant turn, not during roll in or roll out. My initial thoughts would be that the rotor has absolutely no idea which way the wind is going, and is flying through a packet of air. Only the groundspeed makes us visually aware of the wind direction and speed, but the rotor system does not have such perception. Thus in a constant turn, the rotor causes no pitch up or down as it flies 360 degrees, because it doesn't feel wind speed or direction. I think that any pitch up or down tendancy is pilot induced as a reaction to the apparent ground slide/speed change during the turn.
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Camp Freddie: There is another problem being revealed here.
In level flight, the student is generally taught to maintain attitude by referring to where the horizon crosses the canopy bow (in R22). They learn to keep the horizon in one spot, just near the compass. In reality, they should be using a spot on the windshield directly ahead of their eyes, but with a clean windshield, there is no reference point. (Try a blob of Blutack, specially in a 47 where there is no canopy bow. No, put the blob on the INSIDE!!)
In a correct turn, they would roll in, keeping the horizon on the same imaginary point. But the canopy bow is off to the side, bringing in a parallax error. Left turn, the dopey students see the canopy point is below the horizon. So, they pull the nose up and the nose is high. Right turn, nose is low.
This error is in addition to any aerodynamic reason that may exist.
In level flight, the student is generally taught to maintain attitude by referring to where the horizon crosses the canopy bow (in R22). They learn to keep the horizon in one spot, just near the compass. In reality, they should be using a spot on the windshield directly ahead of their eyes, but with a clean windshield, there is no reference point. (Try a blob of Blutack, specially in a 47 where there is no canopy bow. No, put the blob on the INSIDE!!)
In a correct turn, they would roll in, keeping the horizon on the same imaginary point. But the canopy bow is off to the side, bringing in a parallax error. Left turn, the dopey students see the canopy point is below the horizon. So, they pull the nose up and the nose is high. Right turn, nose is low.
This error is in addition to any aerodynamic reason that may exist.
Thread Starter
Helmet Fire has got the idea of the effect I'm talking about; for clarity, I'll rephrase my initial question:
When established in a constant AoB turn at say, 90 kts, in balance (in trim), as the helicopter turns from a downwind to an into-wind heading, a nose-down pitch is required to maintain IAS. Conversely, when it turns from an into-wind heading to a downwind heading, a nose-up pitch is required to maintain IAS. Why?
The effect is only apparent with wind. When the wind is calm, the same pitch attitude can be maintained throughout the turn to maintain IAS.
Also HF, it's not a visual 'grounspeed' effect. I first noticed this when instrument flying!
Ideas? J
When established in a constant AoB turn at say, 90 kts, in balance (in trim), as the helicopter turns from a downwind to an into-wind heading, a nose-down pitch is required to maintain IAS. Conversely, when it turns from an into-wind heading to a downwind heading, a nose-up pitch is required to maintain IAS. Why?
The effect is only apparent with wind. When the wind is calm, the same pitch attitude can be maintained throughout the turn to maintain IAS.
Also HF, it's not a visual 'grounspeed' effect. I first noticed this when instrument flying!
Ideas? J
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hey Mr Delimit,
I am aware of the visual effect you are are talking about i.e. the a/c attitude in turns appears to be different depending on the direction of turn, this is discussed during basic training here in the UK.
However I disagree however that this is because the "dopey" student is pulling up in a left turn.
The effect is still evident where it can be clearly seen that the student is not making a correction, and I have observed the nose pitching up in a left turn whilst instrument flying with students which would tend to agree with this theory.
this type of question always amuses me, cos there are many things in helicopter aviation that I know to be true, but someone will always say they are completely wrong, which can be confusing for a newbie observing the discussion, for example in a conventional helicopter, I know that if you lower the collective in the cruise without cyclic input that the nose will drop and the airspeed will increase and the altitude decrease, but I have met people who absolutely deny that this happens !.
regards
CF
I am aware of the visual effect you are are talking about i.e. the a/c attitude in turns appears to be different depending on the direction of turn, this is discussed during basic training here in the UK.
However I disagree however that this is because the "dopey" student is pulling up in a left turn.
The effect is still evident where it can be clearly seen that the student is not making a correction, and I have observed the nose pitching up in a left turn whilst instrument flying with students which would tend to agree with this theory.
this type of question always amuses me, cos there are many things in helicopter aviation that I know to be true, but someone will always say they are completely wrong, which can be confusing for a newbie observing the discussion, for example in a conventional helicopter, I know that if you lower the collective in the cruise without cyclic input that the nose will drop and the airspeed will increase and the altitude decrease, but I have met people who absolutely deny that this happens !.
regards
CF
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Hi jellycopter,
I can't say that I've ever experienced the effect you describe. Certainly in instrument flight, it is very easy to demonstrate that pitch attitude=same airspeed @ constant bank angle regardless of the direction or speed of the wind.
Of course, I fly at 70kt IAS - maybe at 90kt things are different.
I can't say that I've ever experienced the effect you describe. Certainly in instrument flight, it is very easy to demonstrate that pitch attitude=same airspeed @ constant bank angle regardless of the direction or speed of the wind.
Of course, I fly at 70kt IAS - maybe at 90kt things are different.
You blokes have got too much time on your hands!
Must say I haven't noticed this one.
What are you using to measure your pitch attitude? AIs aren't exactly flash in turns a lot of the time, although I guess if it doesn't happen in nil wind you have proven the AI must be OK, if that's what you're using.
Must say I haven't noticed this one.
What are you using to measure your pitch attitude? AIs aren't exactly flash in turns a lot of the time, although I guess if it doesn't happen in nil wind you have proven the AI must be OK, if that's what you're using.
Thread Starter
Arm out the Window,
Too right I've got too much time on my hands......40 kts winds for the last two days plus the season of merriment means I'm not flying as much as I'd like and PPrune is getting more attention than I normally give it.
Anyway, yes, I do use the AI as a pitch indicator for this effect. I first noticed it when teaching a student instrument flying in a Gazelle. We were doing 180 turns parallel to the wind and on my demo I used 'x' degs nose down and noted this for the student's benefit. When he had his go, he selected exactly the same attitude but his airspeed wandered. This got me thinking.......
I've seen, and demonstrated this effect on Gazelle, Puma, AS350 and A109 so it does exist. They can't all have crappy AIs.
My angle on this is as follows; we fly the helicopter with reference to air data ie IAS. Therefore, we don't care whether we're into wind or downwind - we just fly the numbers. However, simple physics of momentum is related to kinetic energy ie GROUNDSPEED. Therefore taking a 20kt day and a 90kt cruise; into wind you have 70 kts ground speed, downwind 110kts. Momentum = 1/2MVxV (V Squared; couldn't find the proper symbol). So, during our level 90 kt turn from say, into-wind to downwind, we need a means of increasing our momentum from 1/2M70x70 to 1/2M110x110; and vice-versa. This is acheived by accelerating (nose-down pitch) towards the downwind heading and decellerating towards the into wind.
If we keep the exact same attitude throughout the turn, momentum carries us over the target speed when turning into wind, for example.
Hey guys; I'm no physicist. They say a little knowledge is a dangerous thing. Have I got it completely wrong?
The effect DOES exist - try it when you've "got too much time on your hands".
HNY, J.
Too right I've got too much time on my hands......40 kts winds for the last two days plus the season of merriment means I'm not flying as much as I'd like and PPrune is getting more attention than I normally give it.
Anyway, yes, I do use the AI as a pitch indicator for this effect. I first noticed it when teaching a student instrument flying in a Gazelle. We were doing 180 turns parallel to the wind and on my demo I used 'x' degs nose down and noted this for the student's benefit. When he had his go, he selected exactly the same attitude but his airspeed wandered. This got me thinking.......
I've seen, and demonstrated this effect on Gazelle, Puma, AS350 and A109 so it does exist. They can't all have crappy AIs.
My angle on this is as follows; we fly the helicopter with reference to air data ie IAS. Therefore, we don't care whether we're into wind or downwind - we just fly the numbers. However, simple physics of momentum is related to kinetic energy ie GROUNDSPEED. Therefore taking a 20kt day and a 90kt cruise; into wind you have 70 kts ground speed, downwind 110kts. Momentum = 1/2MVxV (V Squared; couldn't find the proper symbol). So, during our level 90 kt turn from say, into-wind to downwind, we need a means of increasing our momentum from 1/2M70x70 to 1/2M110x110; and vice-versa. This is acheived by accelerating (nose-down pitch) towards the downwind heading and decellerating towards the into wind.
If we keep the exact same attitude throughout the turn, momentum carries us over the target speed when turning into wind, for example.
Hey guys; I'm no physicist. They say a little knowledge is a dangerous thing. Have I got it completely wrong?
The effect DOES exist - try it when you've "got too much time on your hands".
HNY, J.
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Is it Groundhog Day? I know we've been here before.
A helicopter in flight doesn't take "momentum cues" from the ground. It doesn't know or care if there is a 0kt wind or a 50kt wind, as long as that wind is constant. Wind GUSTS will of course have an effect.
However, a steady "upwind" or "downwind" doesn't exist as far as any aircraft, balloon, bird, soap bubble, or vampire bat is concerned until you start interacting with the ground.
A helicopter in flight doesn't take "momentum cues" from the ground. It doesn't know or care if there is a 0kt wind or a 50kt wind, as long as that wind is constant. Wind GUSTS will of course have an effect.
However, a steady "upwind" or "downwind" doesn't exist as far as any aircraft, balloon, bird, soap bubble, or vampire bat is concerned until you start interacting with the ground.
Thread Starter
FW207,
Your absolutely right when speaking 'aerodynamically'.
However, when you state "Wind GUSTS will of course have an effect." you've blown your own argument out of the water. If the aircraft is flying in a 'packet of air' and it's IAS can vary because of wind gusts, it is its inertia (or momentum) which permit this.
Just because something leaves the ground, doesn't mean that it leaves the Laws of Physics behind.
J
Your absolutely right when speaking 'aerodynamically'.
However, when you state "Wind GUSTS will of course have an effect." you've blown your own argument out of the water. If the aircraft is flying in a 'packet of air' and it's IAS can vary because of wind gusts, it is its inertia (or momentum) which permit this.
Just because something leaves the ground, doesn't mean that it leaves the Laws of Physics behind.
J
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when you turn there is an increased coning angle (a 60deg bank is equiv to 2g)
When the coning angle increases the rrpm increases due to the conservation of angular momentum. (Spin yourself round on that office chair you spend too much time in and pull your legs in and out)
As rrpm increases so does coefficient of lift 1/2rasVsquared
as V is increased by it's square the only other thing you can control is angle of attack so you reduce it.
if the wind you turn into is strong enough to make a negative effect on V even with the increased rrpm you will have to add pitch.
arbitrary numbers ....
at 100% tip speed 700km/h
at 103% tip speed 721km/h
may explain the increase in TQ
flying 360 at a constant angle of bank with just have the same effect as flying straight with gusts will it not...
When the coning angle increases the rrpm increases due to the conservation of angular momentum. (Spin yourself round on that office chair you spend too much time in and pull your legs in and out)
As rrpm increases so does coefficient of lift 1/2rasVsquared
as V is increased by it's square the only other thing you can control is angle of attack so you reduce it.
if the wind you turn into is strong enough to make a negative effect on V even with the increased rrpm you will have to add pitch.
arbitrary numbers ....
at 100% tip speed 700km/h
at 103% tip speed 721km/h
may explain the increase in TQ
flying 360 at a constant angle of bank with just have the same effect as flying straight with gusts will it not...
Avoid imitations
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4ero,
I might have missed your point , but provided the rotor isn't in autorotation, the NR only increases momentarily because the governor controls it (unless it's a manual throttle and the pilot doesn't correct it).
However, I think that an increased coning angle would result in a decrease in EFFECTIVE rotor thrust for the same angle of attack of the blades. A compensatory increase in the amount of rotor thrust will require more rotor torque.
Jellycopter,
I tend to agree with your observation that there is a requirement for a pitch nose down to maintain airspeed. When I was a young sprog of a pilot, one of our (Puma) squadron QHIs used to make us practice flying low speed (50kts) figures of eight across a field when we were carrying out GH in strong winds. If an accelerative attitude wasn't adopted as the aircraft turned downwind, the airspeed would reduce quite markedly . I have always put this effect down to the inertia of the aircraft (negative wind gust/windshear). That's two heretics
I might have missed your point , but provided the rotor isn't in autorotation, the NR only increases momentarily because the governor controls it (unless it's a manual throttle and the pilot doesn't correct it).
However, I think that an increased coning angle would result in a decrease in EFFECTIVE rotor thrust for the same angle of attack of the blades. A compensatory increase in the amount of rotor thrust will require more rotor torque.
Jellycopter,
I tend to agree with your observation that there is a requirement for a pitch nose down to maintain airspeed. When I was a young sprog of a pilot, one of our (Puma) squadron QHIs used to make us practice flying low speed (50kts) figures of eight across a field when we were carrying out GH in strong winds. If an accelerative attitude wasn't adopted as the aircraft turned downwind, the airspeed would reduce quite markedly . I have always put this effect down to the inertia of the aircraft (negative wind gust/windshear). That's two heretics
After suggesting that other people had too much time on their hands, I've also been giving this concept a bit of thought. How does this sound?
The helicopter weighs quite a bit and has significant momentum.
It flies because of aerodynamic forces that depend on the airspeed, but is also influenced by its momentum - to use those wildly aerobatic little fixed wing a/c as an example, they very obviously use both the aerodynamic and ballistic (momentum-based) characteristics to carry out those wild-arsed manoeuvres that they do, with angles of attack and IAS all over the place as the machine responds to a combination of aerodynamic forces and its own momentum.
Back to the helicopter; if you had a very light machine holding a small steady angle of bank in a constant wind, I reckon it would go around without much attitude changing required.
Your everyday heavyish machine with a decent angle of bank probably experiences an observable combination of aerody and momentum effects as it turns.
If it's moving upwind and turns downwind, the momentum could cause a small loss of airspeed requiring more nose up, and vice versa when going the other way.
Does that sound reasonable?
The helicopter weighs quite a bit and has significant momentum.
It flies because of aerodynamic forces that depend on the airspeed, but is also influenced by its momentum - to use those wildly aerobatic little fixed wing a/c as an example, they very obviously use both the aerodynamic and ballistic (momentum-based) characteristics to carry out those wild-arsed manoeuvres that they do, with angles of attack and IAS all over the place as the machine responds to a combination of aerodynamic forces and its own momentum.
Back to the helicopter; if you had a very light machine holding a small steady angle of bank in a constant wind, I reckon it would go around without much attitude changing required.
Your everyday heavyish machine with a decent angle of bank probably experiences an observable combination of aerody and momentum effects as it turns.
If it's moving upwind and turns downwind, the momentum could cause a small loss of airspeed requiring more nose up, and vice versa when going the other way.
Does that sound reasonable?
Shytorque - I think your QHI was trying to make an old CFS point about maintaining airspeed when manoeuvering close to the ground and turning downwind - the natural tendency at low level is to use groundspeed as your visual cue and consequently lose IAS as you turn downwind.
Jellycopter - there was another thread last year that talked about this phenomenon and I fully concur that it does happen (there, 3 heretics). Lots of clever sounding people banged on about inertial reference frames and much greek flute music was expounded. However, I think you have got it right, I'm just not clever enough to prove why.
Jellycopter - there was another thread last year that talked about this phenomenon and I fully concur that it does happen (there, 3 heretics). Lots of clever sounding people banged on about inertial reference frames and much greek flute music was expounded. However, I think you have got it right, I'm just not clever enough to prove why.
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Crab,
Yes, the idea WAS of course to make a pilot aware of the ASI (I used to teach it myself in a later life - both rotary and fixed wing). I'm certain we are just talking about the phenomenon known as windshear.
This "inertia effect" can be understood more easily by considering extremes (now there's a good CFS technique). At one end of the scale a fully laden 747 is very prone to the danger of windshear at low IAS because of it's very large inertia. On the other hand, a tiny insect has absolutely no problem with maintaining airspeed because it "goes with the flow" due to it's miniscule inertia. Tiny insects probably never suffer from the danger of turbulence induced structural failure either - they just go up and down (and round and round) with the peaks and troughs....
Always an interesting discussion.
Yes, the idea WAS of course to make a pilot aware of the ASI (I used to teach it myself in a later life - both rotary and fixed wing). I'm certain we are just talking about the phenomenon known as windshear.
This "inertia effect" can be understood more easily by considering extremes (now there's a good CFS technique). At one end of the scale a fully laden 747 is very prone to the danger of windshear at low IAS because of it's very large inertia. On the other hand, a tiny insect has absolutely no problem with maintaining airspeed because it "goes with the flow" due to it's miniscule inertia. Tiny insects probably never suffer from the danger of turbulence induced structural failure either - they just go up and down (and round and round) with the peaks and troughs....
Always an interesting discussion.
Shytorque - it had nothing to do with windshear and everything to do with external visual cues overshadowing instrument indications - the pilot perceived the same groundspeed when he looked outside (a good thing at low level) but missed the airspeed dropping off as he turned downwind. The classic demo started at between 30 and 40 kts on a 15 to 20 kt day so that as the IAS bled off on the downwind turn, ETL was lost and a RoD built up (low IAS + RoD = potential VRS).
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FW207,
Your absolutely right when speaking 'aerodynamically'.
However, when you state "Wind GUSTS will of course have an effect." you've blown your own argument out of the water. If the aircraft is flying in a 'packet of air' and it's IAS can vary because of wind gusts, it is its inertia (or momentum) which permit this.
Just because something leaves the ground, doesn't mean that it leaves the Laws of Physics behind.
J
Your absolutely right when speaking 'aerodynamically'.
However, when you state "Wind GUSTS will of course have an effect." you've blown your own argument out of the water. If the aircraft is flying in a 'packet of air' and it's IAS can vary because of wind gusts, it is its inertia (or momentum) which permit this.
Just because something leaves the ground, doesn't mean that it leaves the Laws of Physics behind.
J
A steady-state wind is in every way "invisible" to a flying craft (or creature) until it's time to land. A gust of wind is a changing state.
If you are riding a train traveling at a constant velocity, it takes the same effort to walk toward the front or the back of the train, and it is the same effort if the train is standing still. Only if the train is slowing down or speeding up will there be a difference.
Now what if you hover a helicopter inside of a giant hot-air balloon? Does the balloon acquire the mass of the helicopter?
