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Traverse flow, coning and co


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Traverse flow and MR coning make the helicopter roll to the right during accelaration and requires left cyclic in order to keep the helicopter level. According to R.W. Prouty traverse flow (or nonuniform induced velocity distribution) at low and MR coning at high speeds.

If you look at the lateral displacement of the cyclic vs airspeed different articles report a S-Shaped curve (French pilots call it the "Bosse du manche"). Obviously besides the two mentioned forces they must be something else to counteract them. What is this other thing (aerodynamic force, Main rotor head rigging??).

 

Who can help me on this one?

 

Best regards & happy new year

Jonathan

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I've never heard of anything other than cyclic (pilot inputs) being used to counter transverse flow or any dihedral effects due to M/R coning. Possibly a SCAS/SAS might minimize it, but I don't think there is a way to aerodynamically counter these aerodynamic effects. Transverse flow and Dissymmetry of lift's "blowback" were always pounded into my head during my initial training because there wasn't any way to counter them except through the pilot's anticipation and corrective inputs in the controls.

 

YMMV

Edited by Linc
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Many thanks for all the replies. I will try and be more precise.

 

If you look at the picture attached I have depicted 3 areas (A,B,C).

- In area A left cyclic is required to maintain the helicopter level. This is mainly due to the non uniform airflow (or traverse flow / inflow roll).

- In area B left cyclic is also required to main the helicopter level. This is mainly due to coning.

 

My question is what effect is in between area A and B. If you look at the chart cyclic to the right is required to maintain the helicopter level (area C).

 

So what causes area C? When accelerating from A to B, what causes the helicopter to roll to the left and therefore requires right cyclic to maintain a level flight?

 

cyclicpath2.gif

CyclicPath2.bmp

Edited by HumblePilot
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Nothing causes it to roll left, just that the effect of the transverse flow is minimized by the increased airspeed and it only has its maximum effect at A. After A, its effect decreases as increased airflow due to airspeed reduces the ratio of the difference of induced flow below what was experienced at point A (there is less aerodynamic effect inducing a right roll in a clockwise rotating system). At a point after C, the dissymmetry of lift causes significantly more advancing blade upflap (and retreating blade downflap to a lesser degree) and resultant roll to increase requiring more cyclic input to counter it.

 

It is possible that there could be some design adjustment of the M/R Pitch control system to cause cyclic inputs to be input a couple degrees off to the right to induce a more fore and aft cyclic control path during the full range of forward flight.

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Nothing causes it to roll left, just that the effect of the transverse flow is minimized by the increased airspeed and it only has its maximum effect at A. After A, its effect decreases as increased airflow due to airspeed reduces the ratio of the difference of induced flow below what was experienced at point A (there is less aerodynamic effect inducing a right roll in a clockwise rotating system). At a point after C, the dissymmetry of lift causes significantly more advancing blade upflap (and retreating blade downflap to a lesser degree) and resultant roll to increase requiring more cyclic input to counter it.

 

It is possible that there could be some design adjustment of the M/R Pitch control system to cause cyclic inputs to be input a couple degrees off to the right to induce a more fore and aft cyclic control path during the full range of forward flight.

Okay,

 

I have to pause a moment, because I'm getting turned around, because the minute you quoted the French pilots I started mixing up counter-clockwise and clockwise rotating systems, which is easy because I'm only the slightest bit dyslexic. Not for reading, but for remembering things (never diagnosed, but it seems to spring up in the oddest places). Anyways, that's my excuse and disclaimer for this discussion right now.

 

Your confusion, as it appears to me at this moment is that you're using a text discussing a counter-clockwise rotor system to make sense of a diagram referring to a clockwise rotor system. Is it not true that the bosse du manche you referred to is depicted in your diagram? Aerospatiale always made clockwise turning rotor systems, so the diagram should be interpreted in relation to the clockwise rotor system. Hover has increased right cyclic to counter the left translating tendency (usually countered by mast tilt and pilot input). Point A is the removal of the counter-translating tendency input as the aircraft is transitioned to forward flight (leveling the wings, as a tail rotor mounted below the main rotor plane will cause a clockwise-rotored helicopter to hang right skid low). Point C is the left roll induced by transverse flow effect in a clockwise rotating system (countered by right cyclic) and Point B is the left counter to the right rolling tendency as the clockwise-rotating system nears retreating blade stall and the no-lift areas of the retreating (right side) blade increases and the upflap of the advancing blade (left side) increases due to dissymmetry of lift.

 

I feel so much better since this was going to bring me to a work stoppage today until I figured out why I was such a dumbass and putting the wrong information out while I was confusing the two systems, and since I don't get to fly today to figure it out in the aircraft. I may still be wrong, but I feel much better about my answer.

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Hey Linc ... I really appreciate the time and energy that you are spending on my problem.

 

Basically I'm always talking about US-Type helicopters (Rotor turning anti-clockwise) and not French ones (I just used the term "Bosse du manche" because I believed most pilots can then associate it with the S-shaped cyclic movement). The picture depicted in the attachment in my previous post is related to an anti-clockwise turning rotor.

 

To be honest I prefer your first reply that brought me up with the following idea (I might be right or wrong):

  • The figure depicted in the attachment assumes level flight at a given altitude above ground.

  • We also know that the power required for a level flight at different airspeeds corresponds to the power curve.

  • We also know that in forward flight an increase in collective will make the helicopter roll to the right due to traverse flow. The pilot corrects with left cyclic. In the opposite way a reduction in collective will make the helicopter roll the left. The pilot corrects with right cyclic.

So putting one and one together I get the following scenario:

  1. Translating from Hover (OGE) to a forward flight regime the helicopter will be subject to strong effect of nonuniform induced velocity. The helicopter will tend to roll to the right and the pilot will apply left cyclic (Point A ).
     
  2. Gaining up to let's say around 50kt (ca.bottom of the power curve) the power required is less than at 20kt. So the pilot reduces collective to a minimum. By reducing the collective he implicitly "counteracts" or "neutralizes" the right roll (Point C )
     
  3. After passing 50kt power requirements increase again meaning the pilot has to increase his collective setting that makes the helicopter roll again to right (besides nonuniform flow and coning effect) which requires left cyclic to maintain a level flight (Point B ).

What is your opinion?

 

Jonathan

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Jonathan,

 

It doesn't gibe with the nature of counter-clockwise rotor systems.

 

In a counter-clockwise rotor system, the anti-torque causes a right drifting translating tendency, which although minimized by a slight left tilt of the rotor mast still requires left cyclic in the cockpit to counter. That's why the diagram strikes me as one of the clockwise turning rotor's cyclic path. Since, at a hover the cyclic position is indicated as being right of center (countering the left drift translating tendency of a clockwise rotor).

 

As the aircraft begins to transition to forward flight, merely repositioning the cyclic to center will not level the aircraft, so the pilot actually transitions to the right of center until approaching ETL. This would be point A but on the opposite side for the type of helicopter (U.S., counter-clockwise rotor) you are referencing.

 

The counter-clockwise rotor will induce a right roll as the aircraft transistions through ETL and the transverse flow has its greatest effect requiring a left input to counter and then as airspeed continues, the cyclic will tend back towards the right to counter the left roll as the upflapping advancing blade imparts more and more roll as the dissymmetry of lift effect increases with increased forward airspeed.

 

Transverse lift has its greatest effect as the aircraft transitions through ETL, after ETL, the increased airspeed results in increased airflow and actually minimizes the induced flow difference. This would also be why the power requirement decreases to the Maximum Endurance power state where total drag is at its lowest value.

 

Consider the following from the Rotorcraft Flying Handbook:

As the air pressure differential increases with an increase in angle of attack, stronger vortices form, and induced drag increases. Since the blade’s angle of attack is usually lower at higher airspeeds, and higher at low speeds, induced drag decreases as airspeed increases and increases as airspeed decreases. Induced drag is the major cause of drag at lower airspeeds.
So, transverse flow effect would not necessarily increase again as the airspeed increases or even with the power increases required to increase airspeed and maintain altitude, since it is a temporary condition as the rotor disk transitions from the recirculated air (at a hover and airspeeds below ETL), with the associated high induced flow, to clean air with lower induced flow since the air is acted upon less before reaching the rear half of the rotor disk.

 

For those reasons, I say that the attached diagram describes the path of a clockwise rotor's cyclic rather than a counter-clockwise rotor's. If you inverted the image, I would agree that you were describing a U.S.-designed helicopter with a counter-clockwise turning M/R. The diagram was what kept messing me up as I tried to reconcile it to a counter-clockwise M/R, so my previous explanations are suspect which was why I posted to counter myself. Granted, I could be faulty now, but I feel more confident of this answer even more now that I've refreshed myself looking through my aerodynamics reference material.

Edited by Linc
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Hallo Linc

 

If you check the attachment in this posting (extracted from Helicopter aerodynamics, Chapter 5, R.W. Prouty) you will notice that the lower figure with the cyclic displacement is more or less consistent with the previous figure. I don't believe that R.W. Prouty wrote articles for a French public. ;)

But anyway in regard to my original question the direction in which the rotor turns is secondary, because the directions of cyclic displacement are just opposite. The aerodynamic effects remain the same.

 

Again R.W. Prouty states that in a anti-clockwise turning helicopter left lateral cyclic is due to

  1. traverse flow (at low speeds)
  2. Coning (at high speeds)

In your reply I especially like this passage:

 

The counter-clockwise rotor will induce a right roll as the aircraft transistions through ETL and the transverse flow has its greatest effect requiring a left input to counter and then as airspeed continues, the cyclic will tend back towards the right to counter the left roll as the upflapping advancing blade imparts more and more roll as the dissymmetry of lift effect increases with increased forward airspeed.

 

This could be an excellent explanation for the going from Point A to Point C (cyclic movement to the right) which was in fact my original question. So the scenario could be the following:

  1. Hover to Point A: cyclic towards the left mainly due to traverse flow
  2. Point A to C : cyclic towards the right due to flapping up advancing blade
  3. Point C to B : left towards cyclic mainly due to coning

rwprouty2.gif

RWProuty.bmp

cyclicpath2.gif

CyclicPath2.bmp

 

 

Jonathan

Edited by HumblePilot
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The counter-clockwise rotor will induce a right roll as the aircraft transistions through ETL and the transverse flow has its greatest effect requiring a left input to counter and then as airspeed continues, the cyclic will tend back towards the right to counter the left roll as the upflapping advancing blade imparts more and more roll as the dissymmetry of lift effect increases with increased forward airspeed.

 

OK, now isn't the right roll strictly the effect of transverse flow effect (~20kts)? I thought ETL (16 - 24 kts) causes the left yaw and pitch up tendancies due to the more efficient rotor system.

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Jonathan,

 

But it doesn't explain the right cyclic at a hover in your diagram, and it especially doesn't explain the continuing left cyclic at B in your diagram. When the aircraft is already experiencing an increasing left rolling moment induced by the upflapping advancing blade, why would the pilot input left cyclic which would exacerbate the left roll?

 

I'll check it out tomorrow when I fly again. In the meantime, maybe somebody else's experience agrees with the diagram?

 

Pogue,

 

Pitch up is due to building dissymmetry of lift creating "blowback" requiring an increasing forward cyclic input. Both effects are encountered during ETL, but remember that ETL isn't a phenomenon in and of itself but a descriptive of this airspeed range during which the rotor begins to enter clean air and these effects manifest. So, it isn't ETL causing anything, it is the transverse flow effect and the dissymmetry of lift and where these effects take place in the rotation of the rotor that requires specific inputs by the pilot.

 

Dissymmetry of lift will continue to effect the rotor system as the helicopter increases airspeed, but transverse flow effect drops off as the airspeed increases and the induced flow reduces, since the faster the aircraft flies the less the front half of the rotor disk has time and opportunity to work on the air and create greater induced flow for the rear half.

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OK, now isn't the right roll strictly the effect of transverse flow effect (~20kts)? I thought ETL (16 - 24 kts) causes the left yaw and pitch up tendancies due to the more efficient rotor system.

 

 

Hello Pogue

 

I'm very glad to know that other pilots have some concerns of problems that are not well documentated. Thanks a lot for participating.

  • The Left yaw is in my opinion due to the tail rotor going into translation. As you speed up the helicopter from a hover you will have a surplus of power pedal due to the tail rotor going into translation.

  • The pitch up tendancy as been described by Linc (Blowback).

By the way some of my low time students got haunted by these two effects because they requires more or less a correct timing and coordination of pedals and cyclic.

 

However don't mix these effects with my original question which addresses a roll effect in depency of speed. As a CFI you will typically encounter these effects with students on their very first flight. During cruise they might unintentionally gain speed by lowering the nose keeping the helicopter level. After a few seconds the helicopter will start rolling without any cyclic input. They usually then ask the question: "Whas that me" ... or .... "Hey I didn't do that".

 

By the way I have found another figure out of R.W. Prouty which depicts the roll effect of low (traverse flow) and high (coning) speeds. However what's inbetween is left to the reader to guess and that is the answer I'm looking for.

 

RWProuty3.gif

RWProuty3.bmp

Edited by HumblePilot
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Hello Linc

 

Again many thanks for your commitment.

 

But it doesn't explain the right cyclic at a hover in your diagram
I don't see here anything special. In both figures the cyclic is aft centered when hovering. So that is in my opinion OK.

 

 

..., and it especially doesn't explain the continuing left cyclic at B in your diagram. When the aircraft is already experiencing an increasing left rolling moment induced by the upflapping advancing blade, why would the pilot input left cyclic which would exacerbate the left roll?
According to R.W. Prouty left cyclic at Point B is due to coning. Coning effect reduces the efficiency of the back part of the rotor disc and will induce a right roll, that is correct by left cyclic input . The only answer that sounds plausible in this context is that the coning effect is greater than the upflapping effect.

 

RWProuty%20Coning_NUIV.GIF

 

By the way I found in Army field manual (...I guess your employer) a figure with the cyclic displacement which is consistent with the two others:

 

US%20Army%20FM.GIF

Edited by HumblePilot
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Odd, because there is also a downflapping moment on the retreating blade which is attempting to match the lift produced by the advancing blade, and as the aircraft gets faster, the rotor system loses lift on the left side as the no lift areas increase in size and the blade tip approaches its Mu limit. Prouty seems to be making the argument that induced flow has greater effect as the aircraft accelerates. This is contrary to the discussions of induced drag (induced flow) which decreases as the aircraft accelerates and the entire rotor system approaches closer and closer to equal access to fresh air across the rotor disk.

 

The Army manuals no longer include that figure referencing cyclic travel. Possibly because they've determined it to be inaccurate in the aircraft the Army operates (or else irrelevant for instructing pilots how to fly)? I've scoured the oldest version I have available (1988), as well as the most recent draft (which is being developed to more closely match the FAA Rotorcraft Flying Handbook) and can't find a similar figure.

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Prouty seems to be making the argument that induced flow has greater effect as the aircraft accelerates. This is contrary to the discussions of induced drag (induced flow) which decreases as the aircraft accelerates and the entire rotor system approaches closer and closer to equal access to fresh air across the rotor disk.
That induced drag decreases as the aircraft accelerates is clear. "Equal access to fresh air" ... that is the point I believe can't quite true (If you just think that advancing blades are operating at very low or even negative AOA and that the retreating blades at high AOA). So during a level cruise regime you will have an asymetrical and nonuniform distribution of induced flow that must be compensated somehow... and I believe that is the point that Prouty is addressing.

 

 

 

The Army manuals no longer include that figure referencing cyclic travel. Possibly because they've determined it to be inaccurate in the aircraft the Army operates (or else irrelevant for instructing pilots how to fly)? I've scoured the oldest version I have available (1988), as well as the most recent draft (which is being developed to more closely match the FAA Rotorcraft Flying Handbook) and can't find a similar figure.
Sorry about that... I'm obvously out of date. But looking at our discussion I bet the guy who rewrote the book didn't know the answer himself and probably dropped the theme in order to avoid long discussions :)

 

 

OK ... Lets get back to the original question....

 

What is aerodynamically happening between point A and Point C? Why does the cyclic path go to right (or at least stops going to the left)?

 

cyclicpath2.gif

 

 

It would be great if anybody qualified in aerodynamic engineering (University labs, Robinson, McDonell, Schweizer, Aerospatial...) would join in.

Edited by HumblePilot
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Maybe this will help.

GYROSCOPIC PRECESSION

 

Gyroscopic precession is a phenomenon occurring in rotating bodies in which an applied force is manifested 90 degrees later in the direction of rotation from where the force was applied.

Although precession is not a dominant force in rotary-wing aerodynamics, it must be reckoned with because turning rotor systems exhibit some of the characteristics of a gyro. The graphic shows how precession affects the rotor disk when force is applied at a given point:

A downward force applied to the disk at point A results in a downward change in disk attitude at point B, and an upward force applied at Point C results in an upward change in disk attitude at point D.

Forces applied to a spinning rotor disk by control input or by wind, gusts will react as follows:

This behavior explains some of the fundamental effects occurring during various helicopter maneuvers.

For example;

The helicopter behaves differently when rolling into a right turn than when rolling into a left turn.

During the roll into a left turn, the pilot will have to correct for a nose down tendency in order to maintain altitude. This correction is required because precession causes a nose down tendency and because the tilted disk produces less vertical lift to counteract gravity.

Conversely, during the roll into a right turn, precession will cause a nose up tendency while the tilted disk will produce less vertical lift.

Pilot input required to maintain altitude is significantly different during a right turn than during a left turn, because gyroscopic precession acts in opposite directions for each.

 

 

 

Not sure how to send the drawing?

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Maybe this will help.

GYROSCOPIC PRECESSION ......

 

Hallo Hal

 

Thanks for your contribution.

I would be very happy if you could be more precise about your answer. I guess Linc, I and the community that is following this case understand the concept of gyroscopic precession. Could you please explain at which point and where you believe gyroscopic precession acts within the context of the question. Please try to apply your idea to the question.

 

Thanks

Edited by HumblePilot
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That induced drag decreases as the aircraft accelerates is clear. "Equal access to fresh air" ... that is the point I believe can't quite true (If you just think that advancing blades are operating at very low or even negative AOA and that the retreating blades at high AOA).
Jonathan,

 

Don't chop my quote up. I said, ""the entire rotor system approaches closer and closer to equal access to fresh air across the rotor disk." I never said it got there, since I believe that would be near impossible. Prouty assumes that this differential induced flow will continue or increase. I see no proof that the induced flow increases, in fact, everything I see says that as the aircraft increases airspeed the induced flow is decreased, partly through the aircraft's speed decreasing the time the front half of the rotor has time to play with the air that the rear half of the rotor will see and partly through the rotor disk's increasing tilt forward. I think Prouty discounts this, since his diagram shows the disk approaching level at increased forward airspeeds when on my aircraft it is significantly lower in relation to the inclination of the nose of the helicopter.

 

There is also sporadic reference to the fact that high speed tests don't match up with some of the theoretical explanations of what the aircraft does. I think there may also be issues to deal with in an undampened rotor system (no stability system) that may more readily explain the phenomenon you're searching for an explanation for.

 

Have to go get ready to fly...maybe I'll change my tune when I get back.

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I will try to be as smart as everyone else. Lets first define the two items you questioned; 1st Transverse Flow Effect is defined as ;

 

TRANSVERSE FLOW EFFECT

In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. This is due to that air being accelerated for a longer period of time as it travels to the rear of the rotor system.

The downward flow at the rear of the rotor disk causes a reduced angle of attack, resulting in less lift. Increased angle of attack and more lift is produced at the front portion of the disk because airflow is more horizontal. These differences between the fore and aft parts of the rotor disk are called transverse flow effect. They cause unequal drag in the fore and aft parts of the disk resulting in vibrations that are easily recognizable by the pilot. The vibrations are more noticeable for most helicopters between 10 and 20 knots.

So, what does this mean to us pilots? Well, the result is a tendency for the helicopter to roll slightly to the Right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots. (Assuming a counterclockwise main rotor rotation, reverse for a clockwise rotation).

You can recognize transverse flow effect because of increased vibrations of the helicopter at airspeeds just below effective translational lift (ETL) on takeoff and just passing through ETL during landing.

To counteract transverse flow effect, a cyclic input will be needed to correct the rolling tendency.

 

and M/R Coning that in itself it completly seperate from Transverse Flow. M/R Coning deals with variables such as acft weight and altitude and aerodynamic forces within the regions of the main rotor blade. You must have a complete understanding of the Drive/Driven/ & Stall Range of that specific rotor system. Coning will change base on hover and forward airspeeds.

 

Form a dumb old country boy...

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Hallo Linc

 

Sorry for chopping your quote. It wasn't my intention to offend you.

 

In what concerns Prouty He's not saying that induced velocity increases. He just tries to explain the Point A with the traverse flow and Point B with coning which seems to me plausible. My question relates to what is inbetween.

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Here are Transverse Flow and Gyroscopic Precession together;

 

Transverse flow effect is an aerodynamic effect encountered when the helicopter moves (typically forward) through the air.

 

In a hover, the air above the rotor disk is being pulled down from above and is equally distributed around the rotor disk. The air is descending from above, which has the effect of reducing angle of attack.

 

As the helicopter starts moving into undisturbed air, a portion of the disk is in clean, unaccelerated air, while the remaining portion of the rotor disk is still working on descending air. The part of the disk working on clean air therefore sees a higher angle of attack than the portion of the disk which is working on descending air. This causes a difference in lift between the section in clean air and the section in descending air. The result is that the portion in clean air develops more lift, and the disk tilts as a result. Which way the disk tilts depends on a couple factors:

 

Gyroscopic precession will cause the extra lift to be seen approximately 90 degrees later in rotor rotation, so the pilot will experience either a right or left roll, depending upon whether the rotor of the helicopter rotates clockwise or counter-clockwise. Typically, an American helicopter would roll to the right during the takeoff roll, while a French or Russian helicopter would roll to the left.

 

Which way the helicopter rotor disk is moving in the wind will determine which part of the disk has higher lift. For instance, hovering sideways, or hovering stationary in a crosswind, the clean portion of the disk might be on the left or right, rather than the front of the rotor disk. Again, depending on which direction the rotor rotates, this might be seen as a nose pitch up or nose pitch down, a roll, or something in between.

 

As the helicopter accelerates into a higher airspeed, more and more of the rotor disk will be in clean air and the lift differential will decrease. At some higher airspeed the effect will disappear. In a typical single rotor helicopter, the effect can be felt to start at around 5-10 knots, increases in magnitude to a maximum around 20 knots, and decreases above that until it is almost completely gone by 40-60 knots of airspeed.

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Hallo Hal

 

... nobody is dumb .... and every contribution is welcome :)

 

With your statement you are basically talking about Point A (Traverse flow effect that will require left cyclic) and Point B (coning effect that also requires left cyclic). Now if you look a the figure you will see that something (whatever it is) causes a cyclic movement to right (opposite to traverse flow and coning).

 

The quiz question is what is this something?

Edited by HumblePilot
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What if you are looking at it from the wrong angle???

 

We all agree that point A (left cyclic input) is caused by something.....transverse flow effect. We also agree that point B (left cyclic) is caused by something......coning. What if point C (neutral cyclic) is caused by nothing???

As soon as the helicopter moves faster than a certain airspeed, the effects of transverse flow effect are minimized. The disc now moves in close to equally "clean air" over its entire surface. You have no rolling tendency anymore, thus the left cyclic input needed to correct that tendency is no longer necessary. Then when approaching a certain airspeed again, the effects of coning comes into play, you get a new rolling tendency and again left cylic is needed to compensate for it. Between these two events you have no rolling tendency, so no left cyclic is needed to compensate for it.

 

Point C is where most helicopters spend most of their life, so it makes sense that the engineers rigged the controls to be as close to neutral as possible at that point. Both for pilot comfort (would be strange to sit in cruise flight with a lot of left/right cyclic in all the time) and for CG concerns (neutral cyclic means more to play with both ways).

 

maybe...?

Edited by flyby_heli
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Don't worry the brain too much about this, Humble. In the real world, you do with the controls whatever is needed to get the desired effect. The stick will be moving around to counter wind effects, passengers shifting in their seats, minor heading changes, attempts to hold a steady speed or altitude, and so on. There is an overall movement in a general direction, but it is made up of a squillion little moves all over the place. In most choppers, you will end up with forward right stick in cruise flight. Look at where the R22 trim control puts the cyclic.

 

And it is TRANSVERSE (an adjective), not traverse (a verb). Picky picky.

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