Dissymmetry of lift, flapping and blowback...

[Golden Rivet]

The CAA recently set this as an engineering question

What is the advancing blade on a helicopter doing

a Increasing in lift
b Going to the highest point
c Increasing in drag

Its option B that is causing the confusion. For the advancing blade which direction is the blade flapping ? After reading Mr Coyles book it is evident that for the advancing blade, two mechanisms are occuring.

Cyclical change of pitch which gives the advancing blade a lower angle and less lift and the retreating blade a higher angle and thus more lift. This is accomplished via the mechanics of the pitch change system. The overall effect here is for gyroscopic precession to put the advancing blade at its lowest point at 180 deg rotor azimuth.

The second mechanism is flapping to equal out the lift on opposite sides of the rotor disc. The advancing blade flapping up and thus reducing in lift and the retreating blade flapping down and thus increasing in lift. The two effects, on opposite sides of the disc help keep the lift equal between the two halves.

These two seem to contradict each other somewhat and after reading the description of flapback/blowback it turns it on its head.

Could someone please explain which way the advancing blade is flapping

Many thanks

GR (fixed wing !)

[Lu Zuckerman]

To: Golden Rivet

I may be totally wrong and risk the wrath of NickLappos and Shawn Coyle but this is what I think is happening. The upward flapping of the advancing blade and the downward flapping of the retreating blade occurs during translational lift flight or up to approximately 20 knots. This will result in a roll to the right. The pilot can remedy this situation with cyclic input. If there is a tendency for blowback the pilot can introduce forward cyclic. The lift differential (Dissymmetry of lift) is countered by pitch flap coupling. When the advancing blade flaps up due to the increase of lift the pitch flap coupling will extract pitch from the blade reducing its’ lift. Conversely if the retreating blade flaps down pitch is increased due to pitch flap coupling thus equalizing the lift.

In addressing this situation you must deal with the individual blades and not the disc.

Duck for incomming.

:E :E

[MightyGem]

There are two situations to be considered here, Flapping to Equality
and Inflow Roll.

Flapping to Equality.

As the aircraft moves forward, start at a blade tip in the 6 o'clock
position(viewed from above) on the disc. As this blade starts to move
around the disc, due to the forward motion of the aircraft, airspeed
starts to increase. This causes more lift so the blade starts to
rise.

As the blade passes the 3 o'clock postion (US type helos), the
airspeed is at the maximum, induced flow is increased(due to blade
rising), angle of attack is at the minimum, and rate of flap up is at
maximum.

As the blade moves towards the 12 o'clock, airspeed is starting to
decrease, along with rate of flapping up, but the blade is still
rising.

At 12 o'clock, the airspeed is normal and the blade is at it's
highest point.

As the blade moves further, the airspeed is now starting to decrease
leading to less lift, so the blade starts to flap down.

At the 9 o'clock, airspeed is at a minimum, rate of flap down is at
maximum, induced flow is decreases and angle of attack is at maximum.

As the blade moves back towards the 6 o'clock, the airspeed is
starting to increase, rate of flapdown is decreasing.

Back at the 6 o'clock, the airspeed is normal and the blade is at
it's lowest point.

During translation from the hover, the above produces flapback and is
countered by moving the cyclic forward.

Inflow Roll.

This time, we start off in the 3 o'clock position. As the blade
moves forward, induced flow is decreasing, so angle of attack is
increasing, so the blade flaps up.

At the 12 o'clock, induced flow is at a minimum, angle of attack is
maximum, as is the rate at which the blade is flapping up.

As the blade moves towards the 9 o'clock, induced flow is starting to
increase, so angle of attack is starting to decrease, but the blade
is still flapping up.

At the 9 o'clock, induced flow is still increasing, angle of attack
is decreasing, the blade is at it's highest point.

As the blade starts to move towards the 6 o'clock, as the induced
flow is still increasing and angle of attack decreasing, the blade
starts to descend.

At the 6 o'clock, induced flow is at a maximum, angle of attack is
at a minimum, and the rate of flap down is at a maximum.

As it starts to move towards the 3 o'clock, induced flow starts to
decrease, angle of attack starts to increase but the blade is still
flapping down.

As the blade gets back to the 3 o'clock, it is now at it's lowest
point.


Inflow roll wants to roll the aircraft to the right, opposed by left
cyclic. As this is being combined with flapback, theaircraft wants
to pitch up and right.

In answer to the question, as increase in lift gives an increase in
drag, all three answers could be right.


Phew, now I need a rest!

[Golden Rivet]

Cyclical change of pitch gives the advancing blade a lower angle and less lift and the retreating blade a higher angle and thus more lift. This is accomplished via the mechanics of the pitch change system. The overall effect here is for gyroscopic precession to put the advancing blade at its lowest point at 180 deg rotor azimuth.
The above explanation from Mr Coyles book seems to contradict your description MightyGem, and seems to go against the way the disc should be tilted for forward flight ?

[heedm]

The advancing blade has less pitch but is moving down, increasing angle of attack, and it's seeing a higher airspeed. Flapping to equality is when the flapping (moving down) is sufficient to balance lift on either side. Of course, the opposite happens to the retreating blade.

I can't see any conflict here.

[ShyTorque]

A isn't true because of flapping to equality.

B would occur (flapback) if there was no pilot or automatic trim input but not where the aircraft is in controlled and steady flight.

I think answer C is most correct :8

[vorticey]

an increase in air speed, makes more lift and drag and would make the blade fly to a higher point, if the pilot did nothing. but that would change the disc attitude (flapback) and the helicopter would slow down. the pilot must hold the disc level (slightly forward) with cyclic, to fly strait and level, so we cant let this happen. all these things that are mentioned earlier are compensated for, by the pilot. in forward flight there is less pitch on the advancing blade to stop it increasing in lift with the extra air speed , and more pitch on the retreating blade to produce the same lift at the reduce air speed (on that side, in forward flight).

advancing blade - faster air flow, less pitch, less induced drag but higher parasitic drag.
retreating blade - slower airflow, more pitch, more induced drag but less parasitic drag. same lift! theorfore same total drag (if lift is uneven the disk tilts and you start to fly that way)

i dont think its any of the answers but questions like that dont usualy consider the pilots reactions so i would have locked in (D) all of the obove!:ok:

[Dave_Jackson]

There can only be one sure answer to the CAA question; " What is the advancing blade on a helicopter doing " :(

It's 'advancing'.

[Jcooper]

Vorticey- "advancing blade - faster air flow, less pitch, less induced drag but higher parasitic drag.
retreating blade - slower airflow, more pitch, more induced drag but less parasitic drag. same lift! theorfore same total drag (if lift is uneven the disk tilts and you start to fly that way)"

Blades do not have parasitic drag since they are lift producing bodies and parasitic drag deals with non lifting producing bodies. Perhaps you meant profile but then I believe that the decrease in AOA and the increase of velocity would be close to equaling out...dont know the exact numbers.

[Nomadd]

Ideally an aerofoil section should not have the parasitic drag but in reality it will surely have some parasitic drag.

Parasitic drag is the sum of profile drag and interfernce drag.

Profile drag is sum of Skin friction and pressure drag

1. Usually the aerofoil is designed to give the greatest decrease in Skin friction drag at low lift coefficients or in other words at high speeds

2. Pressure drag arises due to overall presure didtriburion of an aircraft, it the diffrence between forces caused by high pressure on the forward portion and low pressure on the aftportion of the aircraft

3. interference drag is caused by regions of turbulence at junctions

All the three factor above may be small for heli blades, will they be increasing more than reduction in induced drag ?

Hard to answer that i suppose.:confused:

[NickLappos]

The problem is the question is badly flawed. In trimmed level flight, the advancing blade is doing none of those things, because the cyclic controls have angled the swashplate (because there is a pilot somewhere around) to make the blade produce roughly the same forces (lift) and moments (control) as it advances or retreats.
In an uncontrolled helicopter rotor, the advancing blade gets increased speed (the sum of forward spoeed and rotational speed) and it has an increase in lift and drag. This makes it rise, and peak at the full forward position. The handy-dandy pilot forsees this because he pushes the cyclic forward as the nose starts up (the nose up is a product of the blade rising and the disk tilting backward).

In Canada, there is no pilot, and the blade flaps uncontrolled, causing the helo to crash, of course.

The correct answer is (z) Why ask a government flight examiner to write a test on aerodynamics when we have Dave Jackson? ;)

[PPRUNE FAN#1]

Nick, you put it so simply and elegantly.

I'm always amused when pilots talk about how a blade flaps as it goes round and round the mast. I suppose that in a rotor system without cyclic control, the advancing blade would surely flap upward.

But in a real helicopter, the advancing blade will not flap up- otherwise how could forward flight be maintained? Obviously, the flapping that the books talk about *must* be in equilibrium. That is, the advancing blade "flaps" up exactly the same amount as the retreating blade "flaps" down: none. Why? Primarily because the pilot is pushing forward on the stick, making the pitch of the advancing blade lower and the retreating blade higher.

Thanks to Nick for putting it in a way that makes it so clear.

[Dave_Jackson]

Hi Nick

Talking of aerodynamics, how about a discussion on Sikorsky's 'Reverse Velocity Rotor Concept' (http://www.brazd.ru/books/ahs58/book/58af089.pdf)?

` . ` . ` . ` . ` . http://www.aero-space.nasa.gov/curevent/news/vol2_5imgs/innovative3.jpg

You might wish to assess the craft's ability to transition from forward flight to autorotation before saying yes. ;)

[ShyTorque]

No, Dave.

Leave that one for the Eng-Tips forum. :E

[Dave_Jackson]

ShyTorque,


:ok:

[NickLappos]

I posted a web page with the film of the rotor blade in high speed flight.

http://www.s-92heliport.com/rotor.htm

[Golden Rivet]

Many thanks for all the answers. Video of rotor blade especially good.

GR

[vorticey]

is this old footage nick. is the blade made of wood or fiberglass? or is this footage of a helicopter blade going supersonic? is there resent footage like this? surely the pilot could see this happening by looking at a portion of the disc (it wouldnt look flat) please expain!:confused:

[NickLappos]

vorticey,

That blade has an aluminum spar, and is shown in high speed level flight. I doubt that the track looked too bad, as the movement at the tip would not be apparent to the crew. Each blade would pass through about the same space, so the tip path would look reasonably sharp, it just would not be flat, but we usually don't measure that do we?

This is my guess, perhaps some ppruner could translate the German narration, which might shed some light (although it might have been made by someone who was misinformed, ofcourse).

I know of no more recent such footage, although the weight and size of tv cameras should make this kind of video very easy to get. Certainly the size of the deflections we must induce in our fatigue tests of blades make me think these motions are fairly normal, even on today's blades.

[Thomas coupling]

Same footage, dubbed in English was shown on an HORIZON programme in 1989!! I still have the footage.

Can't recall anything more recent.

NL: Correct me if wrong, Vorticey: rotor blades cannot and nust not go supersonic, because of the stresses involved. hence one of the reasons why we are limited in fwd speed.

[Dave_Jackson]

It will be interesting to see if the CarterCopter is able to take its blades supersonic. Particularly, since they will probably go from subsonic, to supersonic and then back, each revolution.

[Flight Safety]

I worked up this time index table for the first complete rotation of the rotor blade in the video clip. The clip includes 2 complete rotations. If anyone desires, they can use this time index table to determine where the rotor blade is at a given time in the clip. You can see that the rotation is counter-clockwise.

The table starts the time index at zero degrees, where the leading edge of the rotor blade first visually aligns with the tail rotor disk. The ending time index occurs when the leading edge again aligns with the tail rotor disk. The table's values are calculated and assume a constant rate of rotation.


Degrees Time Index
-------------------------------
0 3.50 (start index)
10 3.76
20 4.02
30 4.28
40 4.54
50 4.81
60 5.07
70 5.33
80 5.59
90 5.85
100 6.11
110 6.37
120 6.63
130 6.89
140 7.16
150 7.42
160 7.68
170 7.94
180 8.20
190 8.46
200 8.72
210 8.98
220 9.24
230 9.51
240 9.77
250 10.03
260 10.29
270 10.55
280 10.81
290 11.07
300 11.33
310 11.59
320 11.86
330 12.12
340 12.38
350 12.64
360 12.90 (end index)

Enjoy.

[Dave_Jackson]

Flight Safety,
Using your plotting, the tip is at its maximum height when the blade is at 270-degrees. This would make sense since the lift on a retreating blade is concentrated near the tip. The tip is lowest a 0-degrees and this may be the combined result of the lift moving inward on the blade while the tip is springing back down.


Nick,
Basically, the text says that the camera is mounted on the leading edge of the blade (presumably, this means outboard of the flapping hinge). It also says that if the blade were not allow to flex, the rotorhead, gearbox and/or fuselage would be destroyed.

I have difficulty with this statement about flexure; when applied to future rotorcraft. It would appear that an absolutely rigid rotor will cause little more vibration then do the wings of a plane ~ after blade-to-blade, rotor-to-rotor and rotor-to-fuselage aerodynamic interactions have been addressed, and the tail rotor has been discarded.

[Aser]

I have found this translation in another forum:
""The rotortips absorb the turbulence with their flexibility. This recording was made, during flight, by a camera mounted to the leading edge of the rotorblade. At every turn you can see the tailrotor. If the flexible rotorblades wouldn't absorb the the forces, they would be transmitted to drive mechanism and fuselage and the vibration would destroy the helicopter.""

Regards.

[Trimstring]

Greetings all!

First post from a CPL-H (FAA) student:

Had an interesting discussion today on what should be basic aerodynamics, leaving a couple of unanswered questions.
I have looked in my books (Wagtendonk, Coyle, RFH) and searched the Rotorheads forum but haven't found a clear answer so far.

I'm sure someone here will come up with a reasonable explanation:

1. FLAPPING AND BLOWBACK

Statement 1: "Flapping will automatically and perfectly balance the dissymmetry of lift between the retreating and advancing side. The blowback is a side-effect of the flapping caused by the blade reaching it's highest position over the nose and lowest over the tail."

Statement 2: "Flapping is only partially eliminating dissymmetry of lift, the remaining difference in lift causes the blade to start rising on the advancing side and the blade to reach its highest position over the nose and result in blowback."

Hopefully one of the statements is close to the truth. I would very much appreciate a clue to which one is most correct and why.


2. DISSYMMETRY OF LIFT AND FLAPPING

If Statement 1 above is the most correct, can someone describe the mechanics behind the blades flapping the exact amount necessarry to achieve equal lift on both sides?


Looking forward to read your responses!

[krobar]

I'd venture saying that statement 1 isn't 100% true. Flapping will not balance dissymmetry of lift perfectly. You'de still have a rolling tendency according to the FAA RFH.
The way I understand it, the advancing blade would still provide more lift as it rises towards the 12 o clock position, causing the left roll in a anti-clockwise system.
Happens in the R22. Thus the trim pulling thingy.
By no means an expert answer, more part of the discussion.

[Gaseous]

Statement 1 is pretty close to the truth as I understand it, although the word perfectly would be best left out. The pilot balances minor errors in the system with a little cyclic input.

The mechanism of this is that when the blade has a faster relative airflow, it generates more lift and so flaps up.

As the blade flaps up, the angle of attack is reduced so the lift is reduced.

Low airspeed on retreating side, less lift. Blade flaps down so angle of attack is increased to increase lift. this more or less balances things out.

Blowback is what you describe. (Flapback in UK) Pilot counters it by applying forward cyclic.

Simple innit.

(actually there are a few more things to throw into the equation but thats basically it- unless someone shoots me down in flames)

Edit to remove misleading and irrelevant reference to pitch horn placement.

[delta3]

Statement 1 : false if it means blade lift is equal.

Equilibrium

For the sake of simplicity think of the rotor as a theatering rotor. That means hinges are free to move. As a pilot you know the rotor-disk settles quite fast in a given situation. In this situation forces of aerodynamic and inertial origin are in equilibrium. Flapping means blades go up and down when going around exactly to achieve this equilibrium. In each state an overall trust (= time average over one revolution) results that is almost perpendicular to the disk. That does not mean the lift of each blade is the same, because the lift is only part of the equilibrium, the inertial forces that makes the blades go up and down account for the other part. In many other threads intrincancies of this dynamic system have been discussed. In forward flight the forward blade will produce much more lift while maintaining equilibrium, that is not roll the disk and as such the heli.
So 'neutralising' the dissymetry of lift does not mean each blades produces the same lift, it means aero- and inertial forces are in equilibrium.

Blow back

Changing one parameter, namely increasing the wind speed, while keeping all other parameters (such as cyclic input) equal creates a new equilibrium. It creates more lift an the forward blade, less on the retreating blade and this makes the whole disk tilt backwards (again this is a simplification, it may not go precisely backwards but at a certain angle, but again that is a long story)

Statement 2 : correct if interpreted in the above sense.

It is important to make a difference between stable flapping (that means flapping of a stable disk) and a change, that is a transient between two stable positions.

Delta3

[gadgetguru]

the visuals are coming...(I promise), just not for a while yet

[helmet fire]

Welcome Trimstring,

You may need to learn to search a little better, as this topic has been discussed at length. Both statements are partially correct, but both are incomplete, and statement 1 oversimplifies to the point of innaccuracy.

For the detailed explanation, delta 3 is good. For a simple explanation (though suffering simplification): flapping evens out dissymetry of lift due to the pitch horn placement. As the blade flaps up, pitch angle is reduced, thus angle of attack, thus lift. Reverse for retreating side. This, coupled with cyclic corrections to keep the aircraft level is cyclic feathering. So dissymetry of lift is overcome by flapping to equality and cyclic feathering.: ie the equilibrium described by delta 3.

Blowback, or Flapback is very accurately described by delta 3. Not to be confused with inflow roll.

It is discussion like these where we really miss Lu!!

[MortenT]

Is it not the change in relative airflow that change the angle of attack, and not the pitch angle?

Look at several helicopters for example schweizer 300, or cobra and so on, they have the pitch link mounted at the trailing edge, which would make an upflapping blade get an even higher pitch angle thus increasing angle of attack and lift, and make dissymetry of lift even worse?

[Trimstring]

I found some similar threads from the past but the one I read seemed to drift a bit from the questions I had.

My understanding of this has so far been based on what MortenT says, that the flapping works based on changing the relative airflow because of the up/downward movement. This is the description most textbooks give but it does not seem to be the explanation you guys are giving.

Why is this way of explaining it not correct?

The concept of the flapping movement changing the actual pitch angle of the blade is something I have read is only used to limit flapping as an alterntative to the delta3 hinge(Wagtendonk, page 92).

And when it comes to the S300 which is the helicopter I am training in, I was told that it has a "flapping neutral" pitch horn. This would mean that the pitch links are attached to the blades in line with the flapping hinge and therefore the flapping movement should not change the pitch angle during flapping.


Bonus question: What is the aerodynamic reason for having to apply right force to the cyclic in the R22 in forward flight?

[Gaseous]

Well spotted, MortenT. I have altered my original post to make more sense. Too much alcohol. It doesnt matter which side of the blade the pitch horn is on as the effect depends on whether it ends inboard or outboard of the flapping hinge and there are heads with both designs. forget pitch horns in this context.

[[email protected]]

Well spotted trimstring - it is indeed the change in relative airflow that causes the AoA to change and therefore the amount of lift the blade produces. The delta 3 hinge (or other arrangements that mimic the effect) is most commonly found on tail rotors to limit the amount they can flap - it does reduce pitch as the blade flaps.

Bonus question - flapback (blowback) is one part of flapping to equality caused by differential airspeed between the advancing and retreating sides of the disc (maximum at the 3 and 9 o'clock position) and is overcome with forward cyclic as the aircraft accelerates.
Inflow roll (transverse flow) is also flapping to equality but caused by different amounts of induced flow between the front of the disc and the rear. As the disc is tilted and moves forward, the air entering the forward part of the disc from above has less time to be influenced by the disc than the air that has to cross the whole disc and reach the back of the disc. The angle that the air reaches the blades at the front is therefore closer to the horizontal than the air reaching the back. This means that the Induced Flow (vector diagram) is reduced at the front giving a greater AoA and increased lift. So the blade flaps up until the AoA is reduce again by the new relative airflow (the rest is the same as for flapback regarding flapping) The difference is that the blade high point is in the 9 o'clock (for american rotation) and the aircraft wants to roll towards the advancing side of the disc. This is countered with lateral cyclic. Although the effect is most noticeable during the transition to forward flight, it is still there at all speeds above that and pilots subconciously correct with lateral cyclic.
Since the R22 has no trim capability, Frank introduced a system (a weight if memory serves) that reduces the lateral cyclic load felt by the pilot in the cruise. I think that you apply left cyclic in the R22 and not right.

[NickLappos]

The idea that rotor dynamic hinge placement will make more stability is only partly correct. The pop-corn texts that discuss this and make such sweeping statements as those in the first thread are so woefully short of the facts as to make them misleading.

Flapping is a way of partially relieving the unbalancing aerodynamic forces, but it cannot make a perfect response (as in "automatically and perfectly balance") because the flapping itself introduces pitching and rolling moments that upset the aircraft. Regardless of the flapping, if the pilot or stability system does not correct the controls, the helo will pitch and roll, flip upside down and destroy itself. Also, the position of the flap hinge (percent hinge offset) and the geometry of the hinge (alpha one and delta three) will decide what the rotorcraft does when a flap occurs. Remember (or learn, please) that the lift of the blade is only one of the forces it delivers to the aircraft. There are powerful moments that the blade delivers, and these moments are often more powerful than the lift. When a blade flaps, it delivers these moments, and a control force is generated. Think about it, most real helicopters can maneuver at zero G, where the blade lift is zero. Where does that control capability come from? Do not let the limited controllability of a teetering rotor define for you the typical helicopter control paradigm.

Forget that bunk implied in either statement above, mostly, the hinges are there to help the rotor structure withstand the unbalanced forces of flight, and the pilot is there for stability. All else is mostly rubbish.

[baobab72]

Hi folks
Can someone explain to me in plain english why lowering the collective requires aft cyclic and viceversa, cos i am having an hard time to visualize the mechanics behind it.
Also and correct me if i am wrong, during a hover you normally pull, depending on the circumstances, takeoff power which is whatever you get from the table plus a factor of .9 in the r22. As you accelerate down the runway during the takeoff roll, as you reach 50kias you establish the 60kias attitude and raise the collective to takeoff power, if you have any left, which should give you about 500fpm, while when at 10% roc prior to your assigned altitude, 50ft at 500fpm, you reduce the pitch and set the power to 21 inches for the r22 which should equal approximately 70kias or if you want to go at the best perf speed of 83 you set mcp.
Am i correct?

Many thanks

Baobab72

Many thanks

[Sir Korsky]

I'm guessing you're referring to maintaining a constant pitch attitude with power changes/collective movement. It's all due to dissymmetery of lift. There is an excellent thread on this somewhere.

[Aluminium Mallard]

Hi Baobab,

It's not mechanics as much as aerodynamics :)

It's caused by flapback. Dissymmetry of lift causes flapback... dissymetry of lift increases with forward speed and also increased angle of attack i.e. pitch angle from collective.

For power requirements you pull what you need to whist keeping within the limitations. There is no hard and fast rule to what power you pull doing different things as it varies greatly with changes in weight, temperature and height.

AM

[Arm out the window]

Not to mention increased or decreased downwash on the tailboom and aerodynamic surfaces aft of the mast as opposed to those in front, so the effect will also be noted in the hover.

[baobab72]

Hi
So if i get it right, as airspeed increases the horizontal component of the relative wind on the advancing blade is comprised of the rotational velocity plus the aircraft velocity, while the horizontal component on the retreating blade is comprised of the rotational velocity minus the aircraft velocity, resulting in a greater velocity thus lift on the advancing blade side than on the retreating, however due to gyroscopic precession the increase in lift is sensed 90 degrees away - at the 12o'clock position, causing the rotor disk to flap back, and the nose to pitch up. As the nose pitches up a portion of the thrust vector is riderected forward and away from the forward direction ultimately resulting in a decrease in speed which must be compensated for by a forward movement of the cyclic.
So this applies to forward flight and explains the tendency of the helicopter to picth up as you accelerate.
How is it related to changes in pitch with the collective, since the rrpms are kept constant so even though the pitch is increased, the rotational velocity should remain the same and thus the dissimetry of lift? Or am i missing something? Or better what i am missing cos the facts prove me wrong!

Thanks

Baobab72

[[email protected]]

Baobab - you got it mostly correct apart from this bit however due to gyroscopic precession the increase in lift is sensed 90 degrees away

There is no precession or 'sensing' it is just flapping.

As the blade comes over the tail on the advancing side it begins to see more airspeed so it begins to flap up - by the time it is at the 3 o'clock position it has reached its maximum airspeed and therefore the maximum rate of flapping up.

From the 3 o'clock to the 12 o'clock the airspeed begins to reduce and so does the rate of flapping up until it runs out of momentum at the 12 o'clock position and the blade it at its highest point.

It is important to recognise that the blade has essentially the same airspeed at the 6 and 12 o'clock positions which is why, anywhere else in that arc, it has an increased airspeed and therefore wants to flap up.

[AnFI]

Baobab:

You do need to be able to answer questions along the lines of Crabs answer if you need to pass exams however if you want to understand how this works it is useful to understand this:

Flapping (there are 2 definitions for this depending on reference plane) is NOT responsible for nullifying D of Lift) flapping, if it is employed, results in Attitude Change, it is this Attitude Change that provides the cue to the pilot to move the cyclic forward,holding the cyclic forward results in flatter pitch on the advancing side and coarser pitch on the retreating side - nullifying DoL. ... (no knee jerk new friend crab .. please)

Since the advancing side is running lower pitch and the retreating side coarser pitch - when the lever is raised this will have a proportionally greater effect on the advancing side (2 deg plus 1deg is bigger than 6 deg plus 1) so this will have an asymmetric effect resulting in a rate of Attitude Change for the disc. (Nose up)


Simples

[[email protected]]

Imagine the aircraft trimmed into straight and level flight at 60 kts - we are speed stable with a specific attitude and power selected to maintain that equilibrium.

Then raise the lever to add 2 degrees of pitch to all the blades - the advancing side has the highest airspeed and the retreating side the lowest - simple application of the lift formula tells you that the advancing side will flap up more due to higher V squared and the result (just like with blowback/flapback) is that the nose pitches up because that is what the disc has done.

Lowering the lever is the opposite - I believe it is called Instability with Angle of Attack.

Both effects are overcome by the pilot moving the cyclic in the appropriate direction to maintain the desired attitude and speed.

Note this is nothing to do with downwash on stabilisers or fuselage - those are separate effects dependent on aircraft type and design.

Many aircraft have adjustments applied within the control runs (mixing unit if you have one) that offsets these effects to reduce pilot workload.

[AnFI]

Crab - I think you are coming around to my way of thinking?


"the advancing side has the highest airspeed" and therefore must have the lowest angle of attack because of the cyclic position "and the retreating side the lowest {airspeed}" and because of the forward cyclic a higher angle of attack...

pretty eh?

[[email protected]]

I don't think I have ever questioned the fact that AoAs are always bigger on the advancing side than the retreating side - most basic textbooks have diagrams of the AoA distribution across the disc.

You do seem a bit hung up on the cyclic causing dissymmetry of lift though - what about a helicopter in a still air hover with all the AoA equal around the disc - no advancing or retreating side.

Then a gust of wind comes from the 12 o'clock - suddenly you have a difference in airspeed between the right side and the left side - guess what? the disc will flap back as it is aerodynamic effect that has caused this - BUT it is CORRECTED with cyclic (just like inflow roll).

The rotor is affected by aerodynamic inequalities (airspeed or inflow angle) and we use the cyclic to put the disc back where we want it. Now there will be transient effects caused by accelerations whilst moving the controls (this is what Frank Robinson was alluding to I believe) but cyclic movement doesn't cause flapback and inflow roll - it is the dissymmetry of lift which is a result of making the helicopter move in our chosen direction.

[AnFI]

"the fact that AoAs are always bigger on the advancing side" I am sure you intended to write; smaller

I think we are getting there.

Using your example of the gust: I say the attitude change caused by the DoL is the cue to the pilot to move the stick to the position where the DoL is cancelled by assymetric pitch.

Most of these effects are generally not noticed(able) by the pilot since all the time he is making continuous corrections to the position of the cyclic such that there are no (enduring) dissymetries from any cause (Cliff recirculation, greater induced flow at the rear, or speed differences between advancing side and retreating side). If the pilot fails to hold the stick where it is required then an Attitude change will result. Most pilots are able to react such that attitude does not significantly change even in quite extreme gustiness, but gusts aren't really the point - the point is for any steady state condition the pilot is holding the cyclic in a position where all half cycle dissymetries are cancelled by 'Dissymetry of Pitch'



In your example the pilot is allowing the Dissymetry to last sufficiently for the rate of attitude change to result in a changed attitude - which of course he (eventually) will have to rectify, also with the cyclic.


The points about inertial effects and non teetering heads whilst true are partially a red herring since it is the cyclic which must be moved to counter-match any attitude changing effects which last for half a cycle.

[[email protected]]

Yes, I did mean to write smaller regarding AoA on the advancing side.n Doh!

Wrt the rest - you are just arguing from an unconventional standpoint - I am explaining why there is a dissymmetry of lift and you are stating that the pilot negates it with cyclic.

Both viewpoints are valid but pretty much everyone knows that you control a helicopter with the cyclic and collective - you are overcoming the aerodynamic factors using the controls.

Why you believe that your standpoint gives a deeper understanding of helicopter P of F is a mystery.

PS did you agree with my comments about advance angle and control orbits on the other thread?

[AnFI]

Yes I do agree.

Why so important?
Because the fundamental beauty of the helicopter is not being well understood and that is sad. It is sad when helicopter pilots refer to helicopters as angry palm trees and don't really know what they are doing.

It guides the pilot's style of control if he understands what he is controlling and why: Large displacements of cyclic to deal with Dissymetries which are big (and generally slow to develop) and small ultra fine displacements to create rates of attitude change to adopt required attitudes.

It is a major change in thinking to think of Flapback as the cue to the pilot to move the cyclic to counter any need for 'Flapping to Equality', as it is to think of the pilot function as selecting cyclic position such that effective lift is equal around the disk - don't YOU agree? The feedback mechanism is elegant and helps the pilot with the 'feel' of the function.

By not understanding how these fundamentals work for the pilot we run the risk of creating helicopters which must handle the way we mis-understand they should work. eg whether a cyclic commands a new attitude or a rate of attitude change.




Incidentally the various definitions of flapping don't help much and cause confusion - eg if you use one common definition that Flapping is the movement about the Flapping hinge (ie Angular change measured against the Hub Plane) it is most unhelpful since a (nil wind) hovering helicopter will show flapping in any chosen direction depending on CoG. And a helicopter in forward flight, if the fuselage is relatively horizontal, will show flapping in the opposite direction to that prescribed by the (flawed) classical Dissymetry of Lift argument.

When the Earth was the center of the universe by Papal decree astronomers had to go through agonisingly convoluted complication with their clear results in order not to fall fowl of the established body of opinion and avoid having their heads chopped off - we should be smarter than that in 'helicoptering'.

[[email protected]]

I have read your last post several times and still don't understand what your standpoint is or how relevant it is to pilot training.

I don't know your experience but I have instructed for over 20 years most things from basic effects of controls through NVG, IF, formation, USLs, SAR, and many more and given that instruction to basic students all the way to the operational front-line including teaching trainee instructors.

In all that time and amongst the many, many pilots I have flown with, not one has failed to grasp how to fly a helicopter based on the theory of P of F that is in mainstream use. Control action is a function of demonstration and practice, not whether or not you know what is happening at the pitch links in great detail.

I just don't understand why you feel you have some magic secret ingredient that will suddenly open the eyes of students and make them fly better.

I have been brought on in my career by some truly outstanding individuals and I have tried to pass on their attitudes and knowledge - I don't know what you measure your instructional success by but the way we teach in the UK Mil (and in most other places in the world) seems to work just fine.

Do please try to explain your philosophy of instruction because you advocate a different direction without validating its quality.

[ShyTorque]

I've been trying to follow this thread but it seems I have have no real idea how a helicopter flies, or how to fly one. I did have few lessons in the late 1970s but it's obviously much more complicated than I thought.

So where can I get some proper lessons, where someone can explain all this properly, I wonder?

[Arm out the window]

I vote for 'look out the front and arrange the view in an eye-pleasing manner using whatever hand and foot movements seem to do the trick.'

[Ascend Charlie]

Hey, Arm, we went to the same school but you musta forgotten the words to "How to Phly a Phantom " - "arrange the knobs and dials in a pleasing and eye-catching manner".

I'll let you off this time, but the Pedant Police will be called in for any future violations.

[AnFI]

Shy: "much more complicated than I thought."
- simpler than you thought - i think!

Since all these effects really become one.

and it's really just AscendCharlie's broomstick - it works anyway even if you don't actually know what you're doing.

[Arm out the window]

I'll let you off this time, but the Pedant Police will be called in for any future violations.

If they turn up, my response will be "Seagull 500." :ok:

[[email protected]]

Hmmm. AnFI - I think we are now stuck in the same feedback loop that we have been on other threads - where I ask you to clarify your instructional qualifications and experience and explain your viewpoint clearly and then you answer with obtuse comments like the ones above.

PM me if you don't want to put it on here.

[ShyTorque]

Shy: "much more complicated than I thought."
- simpler than you thought - i think!
Since all these effects really become one.
and it's really just AscendCharlie's broomstick - it works anyway even if you don't actually know what you're doing.

No, every time I read something like that I find it more complicated. Not sure I'll ever understand this rotary wing stuff. :\

[Ready2Fly]

@Shy
Don't worry ... it will all come with time and experience. One day it will all come together in one piece and open your eyes.


P.S.: ;)

[[email protected]]

You have gone very quiet AnFI!

[[email protected]]

Still waiting....................................

[DOUBLE BOGEY]

Holy **** ANFI - I am not sure what I have being doing all these years sitting on my furry seat cover with no idea what the bades are doing.

All I have been doing is rotating the nose, pulling up a bit of collective and putting the cyclic into like a, sort of, 3rd gear position like a car. Can you please explain why I have to do this. Is it because of the "Charlie Broomstick" effect??

DB

[AnFI]

DB: "no idea what the bades are doing."

do you mean blades or 'babes'?;)

either way I am sure "no idea" is slightly overstating your confusion - but then you don't actually need to know much in order to have quite a fruitful career, as you have proved. After a beverage or 2 pilots can pronounce blades as bades but to spell it that way too must take an extra night cap.:zzz:


"Still waiting" ...really!? - I have answered Cb on experience and background - he is making it circular - it is not relevant - only the merit of the points in their own right has importance here - besides he published my pm previously.

[[email protected]]

So, still hiding behind bluster then AnFI - you might have some credibility if you actually came out with your credentials (unless they don't exist) but you keep making confusing posts in answer to straightforward questions from newbies.

[AnFI]

It must be more confusing for the newbie to receive your self admittedly erroneous post.

And furthermore I do not want to derive credibility, as you think I should, from credentials.

And I still maintain that the view I am espousing is a simple one:

In a nutshell: If for (almost) any reason the lift distribution around the disc is not equal then the ability of blades to flap would equalise it but in practice really just provides the cue that the cyclic position must be changed (and the magic bit) to a place where that inequality no longer exists, and no longer requires flapping to equalise the lift.

This encompasses a variety of effects: Speed inequality (DofLift), inflow roll, cliff recirculation etc

So more simply stated : the pilot holds the stick where he needs to (providing unequal pitch) so that flapping is not needed to equalise lift.

If you don't agree or don't think that's an insight then I'd be happy to hear an alternative explanation.


(Of course there is the red herring of hinge offset and pitch-flap coupling - but they are unnecessary complications at this level)

[[email protected]]

No, strangely enough, it's not an insight to say that the pilot has to move the cyclic in order to control the helicopter!

Is that your magic secret??????FFS

Many thousands of instructors have taught even more thousands (or millions) of people to fly a helicopter by moving the cyclic so that the aircraft goes in the direction you want it to - try 'select and maintain an attitude' as simple method of overcoming flapback, inflow roll - it works and doesn't hide behind some pseudo-science ramblings about flapping.

My error on the R22 lateral spring was an honest one - I don't think I am beyond being corrected, unlike you.

[DOUBLE BOGEY]

Mr CRAB, an error, tut tut. I suggest you give yourself a damn good thrashing. I did.

You get the same effect as reading "ANFIs World Of Helicopter Magic Insights" without the mental strain of trying to workout the more difficult PoF concepts like "Ascendingbroomstick" effect!!

DB

[AnFI]

A) no ... the trim direction was not the only error ... the direction of the swash plate movement was the other. With those two errors in mind I would think it was fairer to say your posts were more confusing to the 'newbie' wouldn't you concede?

B) I said: if you had an alternative view point please make it.


DB ... of course you are right you can just yank the sticks around and it works pretty well.


I hope this is a place where a straight conversation on the point can be genuinely made?

[DOUBLE BOGEY]

DB ... of course you are right you can just yank the sticks around and it works pretty well.

Thank Christ for that.

[[email protected]]

No, your direction of swash plate movement is specific to a helo that has a 90 degree advance angle (like the R22) - my swash plate movement was of a generic helicopter without any advance angle or jack offset, I did explain that clearly but again you weren't listening.

When I make a mistake, I a. admit it and b. try to explain how that mistake came about - you don't either admit or explain, just BS.

[AnFI]

and for completeness the third fundamental error which made your posts confusing was "the fact that AoAs are always bigger on the advancing side" yes you did admit you were wrong about that too, but it does add up to confusion. (AND We were talking about the R22 where the swash plate moves not in the direction you stated)

Of course if I am wrong about what I say I'd be delighted to correct my simple and effective model and admit any errors... I like discovering (my) errors it means improvement and learning.



(It might help to describe these things with respect to the Zero pitch (change) plane - but I doubt it!)

[army_av8r]

Is there still a question to be answered here??? I tried to follow along to provide some insight to the OP's question. But now im lost as to the real topic here. it is my belief that the question was just worded poorly. I also believe that it is referring to dissymmetry of lift. one question to ask yourself is "At what point does it become the Advancing Blade?" The way i teach is once it passes over the tail it is the advancing blade, and begins to see an increase in relative wind, this increase "peaks" at the 3 or 9 clock position(dependent on type) and then begins to lose this extra airspeed until passing the nose. SO... if we assume straight and level - unaccelerated flight at say "50 knots". then we can dissect what is happening. lets assume the pilot doesnt make any inputs. If the helicopter airspeed increases due to external factors to 60 knots, the advancing blade will have an increase in Velocity due to this increase in speed, this produces a potential for more lift. The Potential increase in lift will cause the blade to flap up (to equality) as it travels around the advancing side of the aircraft and it reaches its peak at the nose and begins to flap down from this high point all the way to the tail which will be its low point. This causes a nose pitch up moment and the aircraft will try to slow itself down to its trimmed speed over a series of "ups and downs". Now this is a perfect world account of whats happening. this is not what actually happens in real life. first of all, no pilot will just allow the aircraft to pitch up when they sense an increase in speed. and few helicopter rotor systems exhibit this perfect stability. but in terms of the original Question, it is NOT increasing in lift(because it flapped to equality). it is NOT increasing drag for the same reason. but it IS "Going to the highest point". sorry this went so long... but it is freshman level aerodynamics, and many pilots dont have a good grasp on it.

[AnFI]

Do you actually disagree with me anyway?

I had thought you believed that Flapping was responsible for nullifying the lift dissymetry arising from speed differences etc Whereas I believe that the dissymetry of pitch is fundamentally responsible for cancelling dissymetries and that does represent a change in thinking on the topic.

Anyone actually disagree?

Please stick to genuine discussion of the points.

[AnFI]

...further to that look at what holding the stick further forwards achieves - it achieves a flatter pitch on the advancing side and an increased pitch on the retreating side - this eliminates the need for flapping to equality to nullify that unequal lift due to unequal speeds.

[army_av8r]

without getting into the weeds. for a given airspeed, there is a corresponding cyclic position(fore/aft) that allows the aircraft to remain at that airspeed. your average pilot, will continually adjust this "trimmed" position to maintain the desired cruise speed or fuselage attitude. if the aircraft builds up excessive speed, and the pilot doesnt notice it, it will naturally flap to equality, and try to recover to its trimmed speed.(based on cyclic position) so, at any point in flight, you can say that due to FLAPPING, the Pilot must reposition the cyclic forward or aft to maintain the desired attitude/airspeed. one factor is a direct result of the other. and yes, forward cyclic will wash out the flapping tendency of the rotor system so that it may accelerate to the new desired airspeed.

[[email protected]]

ArmyAV8r - you are wasting your time with the troll known as AnFI - his new and original take on helicopter P of F is nothing more than a deliberately obtuse argument - we say chicken before the egg and he argues egg before the chicken.

He has no new information, no mystical insight and certainly no technical knowledge above and beyond yours or mine, no matter what he might claim - we say flapping occurs due to inequalities of lift and is corrected by the pilot with cyclic - he says moving the cyclic prevents flapping from occurring.

He likes to think this makes him cleverer than the rest of us and that he has 'discovered' some hitherto unknown secret of aerodynamics.

AnFI's P of F is like one of those secret weight loss technique books that promises new and original methods, guaranteed to give results - however, after pages and pages of guff and bluster, turns out to be eat less and do more exercise.

He did the same with VFR flight in poor weather conditions on a previous thread.

And still he is too ashamed to put his instructional qualifications up for public scrutiny - much like many of the weight-loss quacks.

[AnFI]

yup av8r, on the button, Flapping gives the Flap-back cue which gives the pilot the cue to move the cyclic which eliminates flapping by running the appropriately asymmetric pitch. That moves the thinking on - a little bit, which I think Cb is coming round to gradually.



Crab - that is an outrageously rude and uncalled for personal attack which has nothing to do with the merit of the discussion - I'll probably get in trouble again for YOUR rudeness. "troll, obtuse, ashamed, weight loss quack" - have some dignity man - you are wrong to be so rude and you should admit that error too - perhaps even apologisehttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/confused.gif

[DOUBLE BOGEY]

ANFI just let us know, are you a Flight Instructor (H) and if so what qualifications do you hold.

You should appreciate that your posts are a bit child-like in explanation to most of us on this forum and read a bit like Born Again Christianity. Like you stumbled upon God and need to share this amazing news with the rest of the world.

More importantly, the PofF for rotorcraft recognise that all descriptions are only applicable to a finite blade chord segment somewhere along the blade span, at one point around the disc, for one assumed set of conditions. That is why they are "Principles" and actually do not really reflect what is happening as the blade(s) travel around the head.

However, I have to say your marvel and wonder at how this actually all works is commendable and maybe Crab should cut you some slack and just enjoy your utter enthusiasm, for a subject that most of us have long forgotten, for what it actually is. The seemingly miraculous wonder of rotary flight!!

How bout it Mr Crab!!

DB

[AnFI]

Thanks DB.

Yes the helicopter is deeply beautiful and maligned by the confused and i am very enthusiastic about the Principle and Practice of Flight.

The reason for the baby language was that i was previously 'accused' of over-complicating.


Whilst: "PofF for rotorcraft recognise that all descriptions are only applicable to a finite blade chord segment somewhere along the blade span, at one point around the disc, for one assumed set of conditions" - can be true, in this case it is not true since a half cycle integral of 'Flapping' for a whole blade can be observed in the real world as as change of plane of rotation.

The CAA for example use 2 definitions for Flapping and Crab has used a 3rd definition elsewhere - without agreement on terminology it is easy to see how folks can get emotional.

The credentials are off limits I am afraid.


Crab purports to be an upstanding member of HM forces and an Honourable fellow - his behaviour does not match that - and he accuses me of being a troll!


(But would you kindly give me the definition of flapping which you use.)

[[email protected]]

AnFI - Having read back through this thread, and your lovely PM, I have to agree that you are enthusiastic about your P of F and you may know more about aerodynamics than me but, if that is the case, you have a huge difficulty in explaining yourself in clear terms.

That shortcoming would lead me to believe your PPrune handle of AnFI, apart from being grammatically incorrect (vis - a flying instructor not an flying instructor) means you don't have any instructional qualifications - backed up by your assertions that The credentials are off limits I am afraid.

If you really as good as you say you are, come out with the truth and enlighten everyone, in clear and unambiguous terms.

The P of F which I use to try and explain things to newbies like baobab (where we started in post#40) is a simple explanation of things that occur and not in any way mathematical proof of the complexities of a rotor system. That P of F has been tried and tested over many years in many parts of the world because learning and understanding what actually happens in a rotor system requires degree level maths and a lot of study.

The simple concepts of flapping to equality and dissymmetry of lift are taught from the viewpoint of what happens to the blades if an inequality is introduced - without adding the complication of needing to move the controls to create that inequality. Once the effect is understood, then the action of moving the controls to compensate for the effect can be considered, such that you get the idea that either your autopilot or your right hand reacts to a change in attitude which has been brought about by an inequality of lift.

If you insist on saying that keeping the cyclic in the correct place ensures that there is no inequality, you are technically correct but ignore a huge chunk of understanding that is required by the student.

[ShyTorque]

The credentials are off limits I am afraid.

Why is that? If I was a student, I would certainly want to know the credentials of any tutor or lecturer. Without that information there can be no credibility. Refusing point blank to state one's credentials to instruct when requested reduces that credibility even further. Any instructor would know that.

[AnFI]

Thanks Crab I am pleased that you now agree.

"understanding what actually happens in a rotor system requires degree level maths and a lot of study." Yes I have found that it helps.

Grammar/decode - debatable.

I'm not going to keep re-stating it anymore - sorry if it's not clear - maybe a re-read?

Thanks for being a decent chap in the end.


( Regarding how students are taught: Control is a (feedback) loop - it needs closure - DoL, Flap to Eq, Fl back, Cyclic Pitch change; resulting in a loop-closing elimination of the DoL.

This is a basic understanding of how to fly and is not taught in the basic art at the moment.

The alternative which DB identifies well is the 'wiggle the stick around to achieve a pleasing view' which seems to work quite well, to a point)

[AnFI]

Quite right Shy, under the hypothetical circumstances you proffer.

"Why is that? If I was a student, I would certainly want to know the credentials of any tut ...... " ANSWER because this forum is not my student. This is a discussion forum. Logic should stand in it's own right. The Pope's credentials trumped Galileo's but we moved on eventually.

[[email protected]]

The Pope's credentials trumped Galileo's but we moved on eventually ha ha ha ha, your pomposity and egotism know no bounds AFI - like your knowledge of grammar, your espoused superiority in P of F is just hot air;)
And you clearly haven't ever taught a real helicopter student in your life.

Your PM used the words 'wind up' twice but if you think you have been winding me up then you are too easily satisfied:ok:

[ShyTorque]

because this forum is not my student.

Bull****! Having read your input, it appears that you think everyone is your student! Yet it seems you know little about practical flight instruction, despite your chosen username which logically makes folk think you are a flight instructor.

I doubt that many students could follow your discussion because your logic seems to become more esoteric and obscure in every post.

[DOUBLE BOGEY]

Anti - the PoF bear no relevance to the "stick wiggling" that actually gets done.

Think of a car going around a corner. Does the driver need to know the forces in play on tyres, camber, suspension etc. of course not. We "wiggle" the steering wheel, poke occasionally at the pedals and the job gets done.

Anyway I am going to explore your talents seeing as you have them. The following pesky subjects have always confused me:

1. Can you please explain to me the coanda effect like on the MD902 tail boom?

2. Also the role of Coriolis acceleration in the tip path plane during forward flight?

3. Inflow roll (is this lunch or a real effect)

I have only got an "O" level in maths!

Thanks. DB

[AnFI]

this is too long and boring

[AnFI]

this is too long and boring - very sorry

[DOUBLE BOGEY]

ANFI this is my joke for you:

Is "Flapback" something that only affects old female helicopter pilots??

And

Does "Hooks Joint Effect" only affect Chinese "Herricopper Pirrates"??

Or

Is "Inflow Roll" caused by eating lunch with the helicopter doors off??

Chill ANFI and I like the "Inflow Troll"

[Arm out the window]

The origins of the schism are lost in the mists of time, but it is thought to have happened around the year 2013.

The mystical anfi faction, similar in some ways to the mysterious sufi and totally unlike them in others, were of the view that the rotor disc, holy in its role as the giver of lift, responded to offerings from the lowly pilot in the form of control inputs that would cause elegant and beautiful flapping, after which all would be well.

This angered the followers of the crab faction, as it flew in the face of (no pun intended) their belief that the flap came before the dissymmetry of lift, or was it the other way around? In any case, they questioned the right of the anfi to speak on these matters, and moved away in haste making the finger-snapping sign of the holy crustacean to ward off evil.

Were the ancient texts to be taken literally, or should believers make their own interpretations? Was the gyroscopic effect real, or just some kind of simplistic explanation for the actions of the rotor blade, moving as it did in mysterious ways? Could this simple yet wonderfully flexible thing even be said to form a 'disc', as so many of the prophets would have them believe?

And so, we tell the story today. Please put offerings in the (swash) plate as it comes around, and so endeth the waffle.

[DOUBLE BOGEY]

Arm out the Window - outstanding posting and much better read than PofF

DB

[AnFI]

very funny high priest :D

full of mystical hidden wisdoms

[[email protected]]

And lo, the origins of the schism were revealed to the faithfull - in the mists of time were gathered the old and wise discussing the ways to avoid angering and being smitten by the hateful god of CFIT.

Yeah and verily did the wise and knowledgeable agree that paying sacrifice to the deities of godown and also slowdown was the wisest choice for those so blighted by the cruel nimbostratus, the god of lowering cloudbases and RADZ, the imp associated with diminishing visibility.

Yet there was a dissenter, a heretic amongst the high circle of ppruners, he alone claimed to possess mystical knowledge that would protect even the lowest of the low timers against the wrath of CFIT.

Speak! speak! The heretic was urged but all that emanated from the self-proclaimed visionary were ramblings in ancient tongues so confusing that none were able to decipher them.

Many times was the heretic asked to prove his worth by demonstrating his powers but, time and time again the masses were considered unworthy - surely they could just take his word as proof of his knowledge.

And so it came to pass that sacrifices to the gods of godown and slowdown, along with penitent acts in worship of the demi-gods of turnaround and justland became the truth, the whole truth and nothing but the truth to be passed down from generation to generation.

No more was heard from the heretic for many eons until one wise teacher tried to pass onto a youngling padawan, some secrets of the disc of rotor and how it's writhing and contorting could be controlled through the use of the sword of flapping to equality and the shield of dissymmetry of lift.

Once more the heretic's plaintive cries were heard from the wilderness - 'take the feathering stick of cyclic and repel the rotor' he pleaded, 'that will lead you to the path of wisdom and understanding for you are in the darkness'.

The masses responded in unison 'yes we already frickin knew that!'

The End

[DOUBLE BOGEY]

Sorry, Crab, are you saying that Anfi is Jesus. Or is he the devil??

[[email protected]]

I know which one he thinks he is;):ok:

[ShyTorque]

This is beginning to remind me of the circular discussions we had some years ago, with the late Lu Zuckerman.

Lu, who was not a pilot, was convinced that no-one else here knew how helicopters were controlled. He consistently argued that the cyclic controls merely provided forces to precess the rotor blades like a gyro - he just couldn't get it that aerodynamics were the key force.

RIP, Lu. Some entertaining (if not utterly frustrating) times.

[MightyGem]

Just fly the aircraft. Don't think too hard about how it all works, cos it'll just turn your brain to mush. And for goodness sake don't look up through the roof at the rotorhead spinning round. :eek: