Pitch Attitude in Turns

[jellycopter]

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

[Thomas coupling]

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!!!!].

:ugh:

[Billywizz]

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?

[Camp Freddie]

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

[helmet fire]

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.

[Disguise Delimit]

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.

[jellycopter]

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

[Camp Freddie]

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

[Flingwing207]

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.:E

[Arm out the window]

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.

[jellycopter]

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.

[Flingwing207]

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.

[jellycopter]

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

[4ero]

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...

[ShyTorque]

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 ;)

[Arm out the window]

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?

[[email protected]]

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.

[ShyTorque]

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.

[[email protected]]

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).

[Flingwing207]

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.

JErrr... A wind gust means an acceleration. Inertia is meaningless until you apply an accelerative force to an mass, reguardless of the speed that mass is traveling.
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?:8

[4ero]

if you are trying to maintain a certain path over the ground then you will definatley need these changes in attitude

If not then you won't, but you will be blown down wind.


could also be down to flying with your arm on your leg. turn left move leg in and the arc described by your knee pivoting at your hip pulls back slightly, turn right, move leg out and it moves the cyclic forward...oh but what about pedal input too?

[Arm out the window]

I think Jellycopter is talking about an established steady turn, and saying that attitude changes are required to maintain a constant IAS throughout if there's wind blowing, so hand positions and so on shouldn't have an effect.

[NickLappos]

This chestnut crops up about once a year, and it is a very hardy nut, indeed. The idea that the aircraft changes trim in its steady turn is simply wrong. I have trimmed several different helos into steady turns of 30 degrees of bank, and let them turn hands off for several turns. No pitch attitude change, nada. No climb no descent, no nothin.

The concept of the downwind turn dies very very hard. The people who see the earth as somehow the stable universal platform see the wind and its effect on the inertial platform of the aircraft. Why are they unable to see the rotation of the earth's surface, which is over 1,000 mph (so if you turn up-rotation, you are going 1,000 mph slower than when you turn down-rotation!!)

How about the 40,000 MPH trip the earth is taking through the universe?

Ppruners, where do the set of blinders stop or start? There is no such thing as a "downwind turn" affecting the aircraft's trim.

[Thomas coupling]

NL: I should have stated in my observations atleast, but the 'effects of pitch up or down can only be observed with trim switched off, otherwise the SAS/stab will do its job and cancel the attempted break from trimmed flight:confused: :confused:

Jellycopter never mentions "trim"???

[Rotorbee]

I think in the november issue of FLYING is a very good article from Peter Garrison. I think it is called "Realy the last article about the downwind turn" or something similar. It explains it all.

[4dogs]

Folks,

My spin on this issue: nil wind or steady wind - absolutely no relevance. Gusts invoke all sorts of rotor effects and are a different story.

If you enter a turn at constant body angle and torque, the tilting of the thrust vector into the turn will reduce the total lift and the aircraft will descend. I suspect that the introduction of cyclic pitch will induce a small element of flapback as well. If you wish to remain level, the total lift must be restored by increasing the thrust vector with collective pitch, which most certainly will induce some flapback. Left unattended, the body angle will increase and the airspeed will decrease. If you wish to maintain airspeed, then a compensatory reduction in body angle is required.

In my experience, different amounts of pitch reduction are required for left or right turns - I always presumed that the difference was a consequence of mast tilt and rigging issues.

Try as I might in some 5000 hours of instructing, I could never detect any effect that could be attributed to the wind whilst flying on instruments.

Stay Alive,

[NickLappos]

4dogs,
I don't think this thread was addressing the natural need for more power (or lower speed) to generate the extra load factor needed in a turn.

You are right, the natural imbalance of the single rotor helo makes the left and right banked turn different. The tail rotor thrust requires a natural right sideslip, and horizontal tail down/upload change the lateral trim significantly. It has been my experience that the left turn needed slightly less bank for a given turn rate, and the right, more bank. Also, the pitch attitude that you trim to is different in each direction, but this varies with the particular helo and its CG, based on how the horizontal tail behaves in the turning flow.

Low speed turns are more affected, since the turn rates are higher, and the radius is smaller, so the turning sideslip is greater.

[4dogs]

Nick,

Actually, I thought it was.

Jellycopter's original post said:

"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."

My analysis was that it had nothing to do with the wind but probably everything to do with the additional power required to maintain level and to maintain IAS constant.

I therefore think, for all of the reasons you state, that we are in heated agreement about why the pitch attitude must change to achieve the stated data point.

Stay Alive,

[NickLappos]

No dogs, the ficticious effect he describes is clearly due, in his very question,to the wind and its direction. Of what use would his observation about wind be if the question asked about the obvious, well researched and well understood retrim needed in loadfactor?

[pa42]

It is worrisome that certificated pilots may get through the system believing that bank angles (absent ground reference maneuvers) must be varied, or do vary, when a wind is blowing.

Let us be very clear (and, therefore, undiplomatic and confrontational) that basic physics dictates that the untethered helicopter is NOT connected to the ground and DOES NOT have the faintest wind influence on bank angle or pitch in turns. Nada. Zip. No way!

IF anybody holds on to the notion that bank or pitch must be varied according to wind, please advise, I shall gin up a reading list for you in the tens of thousands of words refuting this superstition.

Students: if your instructor tries to convince you that a constant-rate turn requires changes in bank or pitch depending on upwind or downwind position in the turn, immediately get a new instructor. The one you have is seriously out of contact with both reality and with the aeronautical literature. Don't let this intellectual virus fester in your own mind!

True, most of the other posters on this thread have indicated the fallacy of pitch/bank variation, BUT BUT BUT you have been far too gentle and oblique. This obnoxious weed in the garden of pilotage must be stamped out firmly and vigorously ASAP lest it poison another generation of young inquiring minds!

And yes, I believe the superstition has also been overdone in the realm of low-altitude downwind turn dangers. Pilots DO get terminated losing ETL and the like turning downwind, but it's because they've not had enough discipline in maintaining IAS in spite of conflicting visual clues (and, of course, unfavorable wind shear!), not because the helicopter "knows" it's getting increasing tailwind.

Free dual instruction for unbelievers (you supply the helicopter).

Dave
Chief Flight Instructor
one of those mutitudinous Florida schools

[jellycopter]

Nick L,
ficticious effect

I didn't think I'd find myself writing this but; 'Simply not true Nick'. I have observed this effect many, many times. Not only when flying myself but also observing others. Crab labels us 'heretics', and he may well be correct. Nonetheless, it DOES occur! My question that prompted this thread is WHY?

Nick, you also wrote:
QUOTE]I have trimmed several different helos into steady turns of 30 degrees of bank, and let them turn hands off for several turns. No pitch attitude change, nada. No climb no descent, no nothin.[/QUOTE]

I am not speaking of trimming a helicopter into a turn. Infact, trimming into turns, except in specific circumstances, is frowned upon in the UK military with whom I flew helicopters for nearly 20 years.

It is my understanding that 'trimming' sets up a bias within the helicopter's cyclic system to 'hold' the cyclic, and in unaccelerated flight, hold the rotor disc in a given position relative to the airframe. Therefore, assuming a helicopter is trimmed accurately into a turn, one would not expect to see any pitch attitude change. I'd suggest you consider flying the helicopter manually, without trim or auto-stabilisation and, in my experience, you'll see that you need to adjust the pitch attitude to maintain the IAS.

PA42
Students: if your instructor tries to convince you that a constant-rate turn requires changes in bank or pitch depending on upwind or downwind position in the turn, immediately get a new instructor.

I didn't mention anything about rate of turn. (However, I do concur that turn rate would vary if IAS was to vary for a given AoB. Alternatively, you could recommend that students consider getting an instructor that has the accuracy and sensitivity of control over their helicopter that they can demonstrate such effects reliably and convincingly, time and time again.

Rotordog,

Thank you for your PM.

J

[NickLappos]

jellycopter,

Given that you and me and most other ppruners can fly, and we can actually fly by hand, the issue under discussion is whether the WIND is the source of the retrim effect. The belief that as you travel up or down wind you need to retrim is ficticious. The need to trim any helicopter, and fly it precisely, is not at issue!

The longitudinal trim of the helicopter is subject the the phugoid mode, where the pitch axis has an oscillation of perhaps 5 seconds. In a turn where you have upset the longtudinal trim of the helo, you have naturally upset this phugoid mode, so you can expect to see the nose work itself up and down in a divergent manner at perhaps 2 to 8 seconds (depoending on many factors, mostly the effectiveness of the horizontal tail). Thus, as you turn, you have to pump the cyclic longitudinally because the nose is slowly bobbing up and down. In windy days you have more turbulence, so the mode might be more active throughout the maneuver.

If I can simply state the main point of this thread (hey its your question, if I have it wrong, correct me!), it is the Assertion:

The wind direction makes the longitudinal axis retrim as the aircraft goes upwind and down wind in forward flight.

I contend that this is not true.

PS, the autopilot CANNOT invisibly fix the aerodynamics of the aircraft, it would show this "retrim" were it a fact. Autopilots are just dumb but precise pilots of their own right, and they cannot change the inherent stability of the aircraft, they can just use the controls to help quell the basic aircraft.