Hanging one side low

[Jeep]

Why does a helicopter with a fully articulated head (AH64) hang one side low? What does the panel think? Tail rotor drift (lateral tendency), tail rotor roll, something else? I'm interested in your views.

[MightyGem]

Jeep, how's the course going? I'm surpised that you've got the time and energy to Pprune:D

Because the t/r being lower than the main rotor forms a couple. Move the t/r level with the m/r(which it tends to in forward flight), and things level out. Well that's my theory anyway.:confused:

[TwinHueyMan]

From what I understand, the tail rotor wants to push/roll the helicopter a certain way (to the right on CCW rotating blade helicopters), so the pilot has to counteract with the opposite direction cyclic (left in CCW helicopters). This explains why Hueys (for example) hover left skid low, and A-Stars hover right skid low.

Just what I've picked up over time..

Mike

[vorticey]

in a hover, the helicopter will hang left skid low on an anticlockwise blade rotation heli.
so you put the fatty on the right to balance the tail rotor roll.

[vorticey]

aswell, if you have 1 passenger and they want to go on one side (to take photo's) the lateral balance will be noticable in hover and in flight. and while in flight if you centre the balance ball the aircraft will level but you will be crabing in the wind. i think the hover balance ball reading would be the correct in flight setting for the cabin being strait into wind??
any idears?
:eek:

[SASless]

Now let's really confuse the issue....in the Alouette III, the mast is inclined to correct that....and the aircraft flies a half ball off....and most operators put a piece of yarn on the canopy for trim indications.....guess pilots cannot get used to flying one wing low or something. Also the attitude indicator shows a wing low attitude when in level balanced flight.

[Rob_L]

ON the SA365C they tilted the artificial horizon to compensate for the built in lean!!!!!!!

[flygunz]

Jeep
What a question to ask, as if you didn't know!!;)
MG has the UK answer sewn up there, you an A2 yet MG or is all that planking getting in the way?
However, in the States as every patriot will tell you, if it doesn't say its a coupling effect in 'their' fundamentals of flight, then it aint true, dont forget me old mate, you need a reference for everything you say!!!:p
If you look in F of F page 5-26 para 1, the US explanation. Sorry to be so sad!
email on its way.

[Arm out the window]

Pretty simple concept - main rotor torque wanting to twist the body of the helicopter in the opposite direction is countered by a fan on the end of a stick - your tail rotor - which provides a sideways force.

In the absence of another force pushing the other way laterally, the machine will drift sideways. To counter that, we put in some lateral cyclic, tilting the lift vector opposite to the direction the tail rotor wants to push.

This, as mentioned above, creates a couple; high (talking about vertical position, not magnitude) main rotor force one way and lower tail rotor force the other way.
That makes the helicopter roll until this couple is exactly balanced by another one, that is, the couple between gravity pulling down on the centre of gravity, and the upward force on the 'suspension point', ie the centre of the mast.

Something like that, anyhow!

[Nick Lappos]

Arm out the window has it almost perfectly right. For US convention helicopters (rotor turns CCW from the top):

The tail rotor is pushing the aircraft to the right, because it has a pure thrust. The aircraft does not lean to the left until the pilot sees the right drift and cancels it with some left bank.

The height of the rotor head has nothing to do with it, the height of the tail rotor relative to the head has nothing to do with it either. The height of the tail rotor relative to the cg of the aircraft (the vertical cg) is important, however. A high tail rotor cancels some of the needed left roll, a low tail rotor, on the centerline, needs more left roll.
Articulated, semi-rigid and rigid rotors all have the left roll tendency, the type of head has very little effect on the issue.

[vorticey]

cant you balance it with weight in the oposite side?

you- >The height of the rotor head has nothing to do with it, the height of the tail rotor relative to the head has nothing to do with it either.

are you sure? if you raised the tail rotor above the rotor hub would'nt the effect be to change the low side over?

:)

there was a problem in my last post the part begining with 'you' was to nick lappos.

[ATPMBA]

A helicopter with CCW rotor rotation will usually hover with the left skid low. Tail rotor thrust is to the right, known as translating tendency, the copter wants to move to the right. The pilot applies left cyclic pressure to stop the movement, the horizontal component of lift to the left cancels out the tail rotor thrust to the right.

Some manufactures will offset the mast to the left by one or two degrees to counteract the translating tendency, Enstrom uses an offset while Schweizer does not.

[GLSNightPilot]

Well, you could put the lateral CG way over to the right side, & it might hover level, but you'd still have lateral CG issues in cruise flight. Why bother?

I don't see how putting the tail rotor above or below the main rotor could reverse the thrust of the tail rotor. It's thrust that tries to move the helicopter laterally, regardless of the height of the center of thrust. Having the tail rotor above the main rotor is not uncommon.

[CyclicRick]

Bloody simple really;
It's a fat pilot plus lunch box.
If he/she is a thin pilot then the fat pilot left his/her lunchbox in the cab on which ever side it leans.
If pilot has eaten lunchbox then it's the left over tupperware box plus crisp packet and a half eaten apple core (leans less).
If pilot is thin and lunchbox is not present then it must be something else in the boot i.e. crate of beer left over from last do.
If crate is empty the the glass bottles are too thick, change brewery.

[vorticey]

it doesnt change the tailrotor thrust, just the way it rolls the cabin.:)

[paco]

You could have left the fuel hose in.....

Phil

[Q max]

Two attitudes here:
Fuselage attitude
Disc attitude

Disc must run at an attitude to cancel translating tendancy

Fuselage attitude will be such that cg is directly below thrust unless modified by t/r thrust not being in the same horizontal plane as the disc (in which case a cg/thrust couple is required).

Rotor head matters (a bit)
In Rigid and Fully Articulated cases another (much over debated) couple also modifies the fuselage attitude - which does not apply in the pendulous type rotor heads.




Incidentally Nick is the only person I have seen correctly qualifying CW and CCW with "when viewed from above/below" - they are meaningless terms without. - Similar to: "captain the left hand engine is on fire"

[Randy_g]

Seems to me, anytime I look up through the skylight the blades are turning CCW on my Bell. Now if you are on the other side of the equator does it spin the opposite way ??? :D :D :D

Cheers

http://randyspics.tripod.ca/gifs/naughty.gif Randy_G

http://randyspics.tripod.ca/gifs/bear_eating_picnic_md_clr.gif

[Nick Lappos]

Q Max,
You bring an interesting increase in depth to the discussion, and are some what correct, in that the type of rotor head does affect the angle, and the disk angle can be different than the fuselage angle, but the idea that the thrust must pass through the CG is actually quite a bit off, unless you are flying a teetering rotor helicopter. The old idea that thrust is the only product of the rotor is brought about by the fact that all the old training manuals assume a teetering rotor, and were written for a simpler time!

A few threads ago I discussed the moment produced by an articulated rotor, and illustrated the concept of how any cyclic flapping produces a powerful moment that is transmitted through the mast to help control the aircraftr. The moment produced is quite large, and for articulated and rigid systems, produces a clear mismatch between the line of action of the rotor thrust and the cg.

In a nutshell, most helos tend to have a fuselage tilt that approximates the disk tilt (usually a bit less fuselage tilt, due to the head moment due to flapping). Rigid rotors have the most, and teetering rotors have the least.

Thanks, Q Max for opening the discussion!!

[GLSNightPilot]

Vorticey, I've flown several models over the years, certainly not every one ever built, but I've never flown one that hovered with the rotor OR the cabin rolled any direction but against the tail rotor thrust, to one extent or another, unless a strong crosswind countered it.

[MightyGem]

Flygunz, I think you may have me wrongly ID'd. I was a QHI in my
former life but never had aspirations to be an A2. As for planking,
the last plank I flew was a Chippy in '83

Regards Dave B

[flygunz]

What a wally, sorry MG we do know each other but I had you clocked as someone else, the fun part of pprune:D

[SASless]

GOSH !!! I love it when Nick starts with the explanation of all the angles of the dangles....and the thrusts of the hinges....but really appreciate the concept of the Brand X rotor system being simple...must be why I keep finding myself herding them around the sky.....matches my intellectual abilities to the rotor system ! That or the 0330 get up this morning with only Decaf in the rig chowhall got me ! Did you guys ever update that lovely little blue book that Sikorsky used to produce on aerodyamics for helicopters ? Wish I could find mine...but it got lost in one of the 157 moves I made while following my helicopter career.

[Nick Lappos]

SASless,
sorry about how I start to get off into angles and stuff, but it is math that serves as the best smokescreen when real understanding is in short supply!;)

That Blue Book is still around, and does need an update! Perhaps a nice retirement task for me, as my sailbaot glides across the Pacific, the Satdish locked on a commsat and me cruisin the net as that nice bikini-clad shipmate makes another cuppa coffee. :D I could distill a few years of Pprune into a nice set of Q and A's!

[GLSNightPilot]

Maybe I'm confused about all this, but it has always been my understanding that the forces acting on an aircraft in flight (and I consider a hover to be flight, it's off the ground) act around the plane(s) of the center of gravity, not the plane of the rotor, or the skids, or any other plane. It might be possible to make the opposite skid hang low if the tail rotor were below the CG, but I still don't see what having the tail rotor above or below the main rotor has to do with it. ISTM that the tail rotor imparts thrust, countering torque, & we counter that by putting in a combination of pedal & cyclic, to keep the nose straight, & the cyclic input rolls the fuselage around the CG, & we end up with a balance of forces, more or less, changing by the microsecond, resulting in a tilt into the t/r thrust. Does this sound right, Nick, or anyone?

[Nick Lappos]

GLSNightPilot,
You are exactly right, the relationship to the CG is what counts. If the Tail rotor were much higher than the CG, the roll attitude in a hover is less. That is why a centerline tail rotor helo has about 4.5 to 5 degrees degrees left roll, while a high tail rotor helo (S-76) has about 3.5 degrees.

Somehow, the idea that the height above or below the main rotor is important has been surfaced from time to time. It is not correct.

Nick

[vorticey]

you said to GLSNightPilot,
You are exactly right, the relationship to the CG is what counts. If the Tail rotor were much higher than the CG, the roll attitude in a hover is less. That is why a centerline tail rotor helo has about 4.5 to 5 degrees degrees left roll, while a high tail rotor helo (S-76) has about 3.5 degrees.

so the roll degrees reduces as the tail rotor moves towards the main rotor (this is what your saying) doesnt this reinforce what im saying? if the tail rotor is level the roll will be '0'.
:confused:

[handyandyuk]

Hey Nick,

How about spending your pre retirement time on writing a book for us low hours types? Something slightly less basic the the PPL guides and including usefull stuff to practice while building hours?

Some of us need all the help we can get :D

[Q max]

Nick GLS and vorticey (but especially Nick)

Respectfully I say: you are wrong (close but wrong)

For simplicity first consider only a pendulous type rotor head:

In the horizontal plane the disc must create a force equal and opposite to the t/r force - or you'll translate (right, right?)
(NO fuselage roll required yet)

In the lateral vertical plane an additional couple is created if the t/r is not in the same plane as the disc - the couple which opposes this the cg/vertical thrust couple. This is created by the cg 'stepping out' from under the point of suspension. I.E.:Fuselage is rolled by an ammount to the left.

(consequenses of this are odd: ie disc will be 'tilted to the left' but fuselage will tilt either left if t/r is below plane of rotation or right if t/r is above plane of rotation (no-one's made one like that tho') , zero FUSELAGE tilt if t/r in plane of rotation)

It is absoloutely not a function of t/r thrust height w.r.t. cg UNLESS you look at the INERTIAL scenario (ie accelerations - for instance if 'weightless' when t/r thrust above/below cg WOULD definately created a roll acceleration). This does not apply in the steady state hover. Cg 'height' does have a bearing on degree of roll since the 'restoring force' will be greater per degree of tilt for a cg which is further away from the disc.

So to answer the question: Yes you can make the fuselage level by messing around with the cg position - but the disc has still "gotta do what a disc's gotta do" ie tilt left.

Now consider the other rotor heads (breifly).
They all make a direct rolling moment on the fuselage (if there's a differance between disc plane and head plane) - so if the disc is tilted there is the additional consideration of how much that will tilt the fuselage BUT but a disc has still "gotta do what a disc's gotta do" ie tilt left.



(left/right when viewed from behind, terminology watered down for readability, some simplifications)

[vorticey]

GOOD STUFF, I TOTALLY AGREE

[Q max]

vorticey - thanks - really nice to have a comment like that
(... positive agreement so rare around here) - you've made my day:)

[CRAN]

Nick and Q,

I have just read both of your posts a couple of times and with the exception of a couple of points in Q's last post I think you are both arguing the same point. Nick is taking the standard mathematical approach in flight mechanics of referring the forces and moments back to the aircraft CG while Q is using the hub.

When balancing forces you balance in the three directions and the line of action of forces is very important (i.e its point of application). The point of application of a moment on a body is irrelevant as long as the axis about which is occurs is known. Therefore, whether you choose to take moments and do the force balance about the hub or the CG the answer will be the same. Provided you stick to your convention and consider all of the forces.

Taking moments about the CG (as Nick has) means that weight cannot provide a moment as there is no moment arm. MR side-force and TR thrust both produce moments about the CG and hence the perpendicular distance is important.

Taking moments about the hub (as Q has) means that the MR side force cannot provide a moment as again their is no moment arm. However, the tail rotor thrust provides a moment as does the weight, so in this case it is the perpendicular distance of the TR thrust from the hub and the CG from the hub that is important.

If you adopt either system and follow either convention through thoroughly then they will both give the right answer. It is common to take moments through the CG since is simplifies the maths significantly as we are fundamentally dealing with inertial forces in aircraft. The acceleration due to gravity (9.81 m/s/s) and any additional maneuver accelerations.

Fundamentally, Q, the height of the MR and TR from the CG are important even in steady state hover because the weight and inertial forces can be idealised as acting at the CG, but your view of taking moments about the hub is correct providing you follow the same convention all the way through your analysis.

I also agree that in a teeter-rotor helicopter with the TR hub at the same height as the MR in a perfectly trimmed helicopter with no other external forces acting on it (CG directly below hub centre) then the helicopter would hover skids level. However, this is because both FORCES, (TR Thrust & MR Side force) are equal and opposite and have the same moment arm about the CG.

Hope this helps
CRAN


:)

[SASless]

Nick,

Forget the Q & A....and Bikini.......my goal is Bora Bora....42' pilothouse cutter.....T&A....and no "kini".....surrounds. I intend to carry a tail rotor blade and shovel with me.....walk around every port of call with them over my shoulder....until the third person in a row asks me what they are.....then I will dig a hole...throw both of them in....kick the sand over them...and settle right there! The third question would confirm the locals did not know what a helicopter is nor what hard work is....and that is the kind of place I wish to retire to.

[Q max]

Sure it works with either (any) origin.

Yes. It is more conventional to take cg as the referance.

I think (unimportant) that in this case it is more instructive to take the hub center as the referance - as it's not really an inertial problem - maybe you can see the benefit of it?*

Incidentally do you know the other useful unit for describing gravitational strength: gravity; 9.81 Newtons/kilogram


* you wrote:
"Fundamentally, Q, the height of the MR and TR from the CG are important even in steady state hover because the forces are actually** created about the CG, but your view of taking moments about the hub is correct providing you follow the same convention all the way through your analysis."
- My 'zero at the hub is 'nicer' for this bit - since 'cause' and 'cure' become two independant functions. Whereas in 'yours' if you knew how high the TR was from the CG you still wouldn't know if it were going to tilt left or right yet.


**I'm not sure I like the referance to "actually created about the CG" - since they are 'actually created' about anywhere you define.

- Slavery to convention ;)

[Nick Lappos]

For all who attempt to answer the basic question, it does not matter where you sum the forces and moments at, as long as you are consistent with that reference point. If you do, you will realize that the height of the tail rotor above or below the main rotor hub has nothing to do with the hover angle. There is a misconception that the aircraft "hangs" by the rotor hub, so all forces are summed there. This is fundamentally in error, and will lead you to determine some bizarre findings.

[MightyGem]

Jeep, since you started this discussion you've been conspicuous by your absense. So what's your theory??:D:D

[Jeep]

Head above the parapet, shields up!

MG:

The maths, physics and aerodynamics are way above my pay scale, but I have benefited from the answers given. I asked the question originally because since becoming a student again, my long held belief that an aircraft hung low on one side because I was sitting on that side no longer holds true, as I sit behind or in front of my other crewmember. On asking an old sweat IP he tells me that the AH64, hangs left wheel low, not because of tail rotor drift and roll, but the balanced centrifugal forces acting through the fully articulated head.

So firstly I posted the question, secondly I watched the aircraft in the hover and though even though it sits 5 deg nose-up the TR is about level with the mast, which is higher (more level in the hover) than I’ve seen on my previous types because they were designed to be level in forward flight; my reasoning being that with the moveable stabilator on the AH64 the attitude can be controlled more throughout the speed ranges, so the hover attitude has been optimised for missile launch constraints/sensor sweeps etc.

So with a TR almost level with the MR, yet still hanging left wheel low, it must be to do with the TR drift, requiring a tilted disc – all normal. Because of the fully articulated mast, with offset hinges and a static mast, I reasoned the MR Head is canted off, so the body of the aircraft will tilt.

I enjoyed the discussion about c of g and disc relationships, as I had not thought of it anything other than the height of the discs before, but all are important. But when someone asks me why it hangs low, I shall tell them – “tail rotor drift (or the US equivalent), fully articulated head, static mast and balanced forces of c of g, wind, rotor heights and the way it was designed for maximum effectiveness to get hardware down range.

Now if only I could get a board plan, diagrams and some simple maths.

Nick:
Do hurry with that blue book, it may well be a best seller.

MG - PS after 4 weeks of flying and academics, it is calming down a bit. I start the bag on Friday, so I’m in the calm before the next storm. I shall let you know.

[Q max]

Nick sez:
"you will realize that the height of the tail rotor above or below the main rotor hub has nothing to do with the hover angle"

That has got to be wrong.

... sure, for rotor heads with flapping hinges at a distance from the hub center there is a direct couple which will try to force the fuselage (hub) to adopt a similar attitude. The disc attitude is determined by the need to combat translating tendancy. So, yes rotor head type has a bearing on the subject - no dispute there.

If you eliminate the rotor head influence by choosing a pendulous type rotor head eg B206 teetering head (semi-articulated, semi-rigid) then you will see more clearly that TR height above or below the rotor hub is EXACTLY what does influence whether the fuselage is forced to roll right or left by the TR thrust.

I know it sounds counter intuative - but just try and look at it freshly again:
If the TR is above the hub it will attempt to roll the fuselage 'the wrong way' ie to the right.

It follows that with the TR in the plane of MR (ie at 'the same height') there IS NO rolling tendancy due directly to TR thrust (or MR's need to complete the couple).

Yes because the disc must 'tilt' to counter translating tendancy there is a force, at the head, which will attempt to tilt the fuselage to comply....


Rushed response...


I hope you can just say : "yes on that point you are right."

......... Otherwise I might have to say it!

[GLSNightPilot]

Something that comes to mind: do designs with no tail rotor hover with one or the other side low? I've never flown one, & didn't pay that much attention to that when I had to ride in them - I was too scared to have time to think about it.

I still don't believe you can treat a 206 as freely hanging from the head - there's a lot of coupling present, as you can see when you put the rotor disk forward to take off, or do a turn with forward motion.

[CRAN]

Yes NOTAR helicopters will hang one skid low, as the NOTAR system still produces a side force on the boom. They will hang left skid low.

Co-axials, intermeshers and tandems won't.

CRAN

[[email protected]]

Blimey Jeep and you an A1 an'all, fancy not knowing simple stuff like that!!!

No, I am taking the p*** mate, good luck with the bag, it doesn't sound much fun- especially for an old knacker like you!!!!

Flygunz, you are just too anal for remembering the page ref in the PoF book, I will just go and look in the copy PD gave me to see if you are right!

[sycamore]

The reason the fuselage hangs low is primarily determined by the position of the tail rotor in relation to the a/c`s CofG,Longitudinal,lateral and vertical. The moments derived by the t/r in producing "t/r drift" or "translating effect" have to be opposed by the moments produced by the rotor,and it doesn`t matter what type of rotor it is,rigid,semi-rigid,teetering,etc,those factors relate to sensitivity,response,and control power.If you are in a hover at fully fwd.c of g,and note the a/c attitude,then go to fully aft and hover, the a/c attitude will be more "left side down"(for CCW rotor rotn.)If you can now change the lateral CofG,you will now see an even greater variation in attitudes,and finally ,if you can find the vertical CofG and change it from it`s normal position,usually fairly high on modern helos,and use depleted uranium to lower it sufficiently(even as an u/s load-rigidly attached) and always using the same a/c weight,you can draw a matrix of attitude/weight(K)/CofG(lat/long/vert.).From the known a/c attitudes ,the position of the t/r relative to the a/c CofG can be ascertained and should show that an aft,high and left CofG will give the most adverse left roll!Of course ,if you are French and dress the other way,then you`ll roll right,and if you`re Oz then use a mirror-This ,of course could all be b*****ks,as this Chilean red is slipping down too easily,and I might have it all wrong!!!! :cool: :cool: :cool:

[Haga Martins]

I´m helicopter pilot in Brazil and CFI.
When I´m teaching a new student, they keep asking me why a Robinson helicopter takes off the right side first.
I lot of friends here, explain that the reason is ´cause the main tank is at the right side, so the aircraft is havier in this side. But I really dont believe in it. The tank is so near to the main rotor shaft that I dont believe that the small amount of weight can change so drastically th CG.

When you have to taxi the helicopter and you are in a low fuel condition, the R66 for example, seems to have this condition maximised.

At second, I thought It could be as a result of tail rotor thrust, so to compensate this force, the main rotor shaft is tilted a little to the right. But this is just a guest. And I couldnt find anything in the maintenance manual that could validate my guest.

This morning I was in a Helicenter here and I was watching a Squirrel (Eurocopter AS-350) landing and it was easy to see the same thing happening.

Well, I still think that it has something to do with the engineering stuff, not just a matter of weight.
Can someone help me with this and explain why the helicopter takes off and land one side first?

[paco]

Or rather one skid hanging low, depending on which way the rotors spin. Helicopters using anticlockwise rotation will have the tail rotor thrust slightly out of alignment with the main rotor, because they are set to be level with each other in the cruise. As a result, the TR thrust line is below the level of the MR hub and will produce a couple that makes the left skid lower in the hover - the reverse for clockwise rotating rotors.

Should an anticlockwise rotating helicopter end up hovering right skid low, you've left the refuelling hose in :)

Phil

[Helicopter ASI]

From the Helicopter Flying Handbook (FAA-H-8083-21B) Chapter 9 states:
"Helicopters usually hover left side low due to the tail rotor thrust being counteracted by the main rotor tilt."

Left side low implies a anticlockwise rotation of the main rotor.

[Sloppy Link]

Or rather one skid hanging low, depending on which way the rotors spin. Helicopters using anticlockwise rotation will have the tail rotor thrust slightly out of alignment with the main rotor, because they are set to be level with each other in the cruise. As a result, the TR thrust line is below the level of the MR hub and will produce a couple that makes the left skid lower in the hover - the reverse for clockwise rotating rotors.

Should an anticlockwise rotating helicopter end up hovering right skid low, you've left the refuelling hose in :)

Phil
Brilliant. Hours wasted trying to understand that just in case the feared Trapper asks me and you explain it it half a dozen lines. I hate you.

[Buitenzorg]

Brought to you by your friendly FAA (https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch02.pdf)

Page 2-15, Translating Tendency

[MightyGem]

Hmm...am I the only one who thinks an instructor should know that?

Maybe he should go here:
https://www.pprune.org/rotorheads/630689-principles-flight-books.html

[JimEli]

Or rather one skid hanging low, depending on which way the rotors spin. Helicopters using anticlockwise rotation will have the tail rotor thrust slightly out of alignment with the main rotor, because they are set to be level with each other in the cruise. As a result, the TR thrust line is below the level of the MR hub and will produce a couple that makes the left skid lower in the hover - the reverse for clockwise rotating rotors.

Should an anticlockwise rotating helicopter end up hovering right skid low, you've left the refuelling hose in :)

Phil

really?

https://www.pprune.org/rotorheads/64917-hanging-one-side-low.html#post614480

[Ascend Charlie]

You have two opposing Moments.
The first is the T/R thrust, acting to the right and lower than the rotor head, and the other half of the moment is the horizontal component of the main rotor thrust, which has been set at an angle to oppose the drift effect. This first moment tries to roll the aircraft to the left.

Opposing that, once the aircraft rolls a little bit, is the CG, which is no longer in line with the vertical component of the rotor thrust. It acts straight down, and is off set to the right from the main rotor thrust, acting straight up. This moment tries to roll the aircraft right.

The two moments fight a little, the more the T/R moment tries to roll to the left, the more the CG moment is offset and opposes that roll.

(
https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/1968x1206/t_r_roll_fee5181986c377910b0dc4419c950fcd828a4a07.jpg

[rotarywise]

I can't believe we're up to 50 posts on something as simple as tail rotor roll. As I recall, it took Uncle Dennis about 5 mins on day 2 at Ternhill, shortly before he rode his bike into the lake..

[SASless]

Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?

Memory serves me the Alouette III had a 3.5 Degree tilt to it.

[SASless]

Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?

Memory serves me the Alouette III had a 3.5 Degree tilt to it.

[Evil Twin]

If you’re a flight instructor and you don’t know why this happens, perhaps you should be hanging up your instructor wings

[Ascend Charlie]

...but hang them left wing low...

[typerated]

https://live.staticflickr.com/822/41306619801_f690842a95_b.jpg

and Kaman did something completely different with offset transmission ans servo flaps.

[Bravo73]

I can't believe we're up to 50 posts on something as simple as tail rotor roll.

The thread is only up to 50 posts because 2 threads have been merged together.

[cattletruck]

When I first flew the R22 20 years ago it hung left skid low. Then I flew it a couple of years ago and it hung right skid low. Now I only fly the AS350 and my conscience remains clear.

[RedhillPhil]

Hanging one side low sounds like my testicles:E

[[email protected]]

The right roll in ACs diagram (opposing TR roll) is a result of the arm between the action of the TR thrust and the vertical position of the C of G. All forces act around the C of G whether it be vertical, horizontal or lateral. I believe that was the point Nick Lappos was making 18 years ago.

[HissingSyd]

The right roll in ACs diagram (opposing TR roll) is a result of the arm between the action of the TR thrust and the vertical position of the C of G. All forces act around the C of G whether it be vertical, horizontal or lateral. I believe that was the point Nick Lappos was making 18 years ago.
That is not how physics works. The C of G is only relevant as the point through which the weight (gravity) can be considered to act - hence the name. Otherwise there are just the equal and opposite couples and there is nothing whatever to say about forces acting around any point.
I am amazed by the the amount of discussion this has generated. When I was teaching this to student pilots in the 70s it was a five minute item at the end of the lesson on the design and function of tail rotors/fenestrons. The summary is a two-sentence paragraph with a diagram very like Ascend Charlie's.

[[email protected]]

That is not how physics works. The C of G is only relevant as the point through which the weight (gravity) can be considered to act - hence the name. Not sure you are right about that - you need a point for a force to work around or opposing forces to create a couple - a pivot point essentially, and that is the C of G.

Force x arm = moment

I've taught the same as AC to student pilots since the 80s but it is a simplistic and easily explainable version of the truth - as Lofty Marshall used to say- P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.

[HissingSyd]

Not sure you are right about that - you need a point for a force to work around or opposing forces to create a couple - a pivot point essentially, and that is the C of G.
No, you do not. There is no pivot with a helicopter in the hover. A couple is a couple is a couple.
Force x arm = moment
And every couple has its moment, it does not have a pivot.

This is a matter of static equilibrium and pretty straightforward. If things begin to move then it starts to get complicated and inertia brings the CoG into consideration. Then:
P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.

[JohnDixson]

SAS: “Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?”

The S-64 with 3 degrees left. If SA still owned the design,I’d not be surprised to learn that the ensuing discussions were still ongoing.

[MarcK]

Not sure you are right about that - you need a point for a force to work around or opposing forces to create a couple - a pivot point essentially, and that is the C of G.

Force x arm = moment

I've taught the same as AC to student pilots since the 80s but it is a simplistic and easily explainable version of the truth - as Lofty Marshall used to say- P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.
I think the pivot point is the rotor hub. That's why the weight (considered at the CG) can provide an offsetting force.

[Ascend Charlie]

Rotor thrust is acting from the hub, not the CG. When the T/R thrust tilts the fuselage left, the CG is no longer under the hub, and a restoring moment is created. The 2 moments are continually playing games with each other.

[HissingSyd]

Rotor thrust is acting from the hub, not the CG. When the T/R thrust tilts the fuselage left, the CG is no longer under the hub, and a restoring moment is created. The 2 moments are continually playing games with each other.
You clearly understand what is going on, but what you have written above could still be confusing. An accurate explanation must use the terminology correctly.
When introducing levers in elementary science, a physics teacher as I was, will only use moments about a fixed pivot/fulcrum. That is as far as Galileo went.
This makes the calculations easy to understand and completely hides the existence of the couples in equilibrium. If someone's education goes no further they will continue to think in those terms when faced with rotational effects and be forever looking for a pivot. That is not your situation, but it is that of many that have posted on this topic, including Nick Lappos.
A couple is a rotational effect, a vector, and its size is a moment. They are not the same thing once you advance from the simplifications of elementary science.
If you replace 'moment' with 'couple' above all is well, except that 'are continually playing games with each other' is not a decent substitute for 'are in equilibrium' or 'are in balance'.
I'm sorry to be picky, but these things matter to me. :8

[Ascend Charlie]

Well, I also have a science degree with a major in physics, but when dealing with helicopter pilots, I reduce things to the minimum to help understanding. They are simple folk, as you might be aware...

However, the degree was 50 years ago, and I had forgotten the subtle differences between moments and couples.

So, to put things right, we have a couple of couples coupling in the back of the helicopter. Better?

[HissingSyd]

Well, I also have a science degree with a major in physics, but when dealing with helicopter pilots, I reduce things to the minimum to help understanding. They are simple folk, as you might be aware...

Maybe, but some simplifications are possibly counter-productive.

However, the degree was 50 years ago, and I had forgotten the subtle differences between moments and couples.

Much the same as mine, but I have been back to school for a second career since. :)

So, to put things right, we have a couple of couples coupling in the back of the helicopter. Better?

Every couple has its moment, as I said above.

[212man]

If someone's education goes no further they will continue to think in those terms when faced with rotational effects and be forever looking for a pivot. That is not your situation, but it is that of many that have posted on this topic, including Nick Lappos.

I think Nick's education did progress a little further...…..:eek:

[HissingSyd]

I think Nick's education did progress a little further...…..:eek:
I am certain that it did.

Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.

[SASless]

Every couple has its moment, as I said above.

Some wind up being a climatic event too which is not always a good thing.

[JohnDixson]

Ah,”simple folk”. Hard to place Mr. Lappos in that group. Second in his class at Georgia Tech,but more to the point, over three decades or so at Sikorsky,I’ve been at any number of meetings with Nick and our resident PhD’s and in those discussions dear friends, Mr Lappos won/loss record remains unbeaten. His main problem was that he was often more than a tad ahead of everyone on the subject at hand,so there was some misunderstanding as a result. Count me in that crowd on occasion.

[HissingSyd]

Ah,”simple folk”. Hard to place Mr. Lappos in that group. Second in his class at Georgia Tech,but more to the point, over three decades or so at Sikorsky,I’ve been at any number of meetings with Nick and our resident PhD’s and in those discussions dear friends, Mr Lappos won/loss record remains unbeaten. His main problem was that he was often more than a tad ahead of everyone on the subject at hand,so there was some misunderstanding as a result. Count me in that crowd on occasion.
I am sitting here isolating myself, so I am up for the challenge, in a friendly way.:)

While I have been lurking here, I have read back over a number of threads to which NL has contributed and I have assured myself of his very high credentials. I, too, have been impressed by his lucid explanations and I understand how he came to earn guru status. But gurus can be mistaken. This is just plain wrong:

You are exactly right, the relationship [of the tail rotor] to the CG is what counts. Somehow, the idea that the height above or below the main rotor is important has been surfaced from time to time. It is not correct.

[[email protected]]

This is just plain wrong: and your experience of testing and designing helicopters is????

Don't get me wrong, I'm not equipped for an academic argument with you on physics but I have flown helicopters for 38 years and the height of the TR from the MR really makes no difference to the hover attitude.

[HissingSyd]

and your experience of testing and designing helicopters is????
Nil, of course, but I did once read a lot of information from Westland, Boscombe Down and what I think is now the US DCIT. I also had some access to the Sikorsky manual mentioned in the parallel thread.
Don't get me wrong, I'm not equipped for an academic argument with you on physics but I have flown helicopters for 38 years and the height of the TR from the MR really makes no difference to the hover attitude.
Just to clarify that you are thinking of the design position of the tail rotor relative to the main rotor? NL himself says this about that: :)
A high tail rotor cancels some of the needed left roll, a low tail rotor, on the centreline, needs more left roll.
The attitude of a helicopter in the hover is tail low. In level forward flight it is more or less horizontal. The vertical distance from the line of the tail rotor thrust to the rotor head changes. It is that separation that is most relevant to tail rotor roll, although it is not the sole contribution, as a rehearsal of this thread will point out.

[Ascend Charlie]

When I said that we helicopter drivers are "simple folk" I wasn't casting nasturtiums at Nick or John. I was referring to the way that things need to be simplified for students to understand them, but the simplification makes the student believe the wrong thing.

Many years ago, Nick put out a list of Helicopter Urban Myths, and among the myths were the misconceptions such as:

"The rotor system IS a gyroscope", rather than "it sometimes behaves LIKE a gyroscope."
"There is an area of high pressure air under the hovering disc, and this makes a cushion of high pressure air that the aircraft sits on". Bong! Wrong!
"Flapping to equality is happening all the time, so the advancing blade is flapping UP to equalise the lift."
O. M. G.......

[JimEli]

I am certain that it did.

Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.

Are your observations limited to a teetering rotor head like the R-22 or generalized to include the articulated/hingeless head with significant hinge offset? It seems an H-60/S-92/EC-145 arraignment doesn't put the T/R thrust line very far below the MRH in hover.

[[email protected]]

OK Syd, a genuine question for my education:

We have been discussing couples that create rotation of a body - how do you identify the axis of that rotation? Is it along the line between (in this case as in ACs diagram) MR head and TR and, if so, where along that line?

My point about height of TR having little effect is that I have flown helicopters with both high and low TR (of both clockwise and anti-clockwise rotation and differing head designs) yet the roll angle in the hover is always between 2 and 4 degrees with that upper limit being on a semi rigid MRH with a large effective hinge offset.

[Ascend Charlie]

Interesting question, Herr Krebs.

The left-rotation couple has the rotor hub at one end, and the tail rotor at the other - well above the CG. No idea where the axis would be.
The right-rotation couple has the rotor hub at one end, and the CG at the other end. I don't think the axis could be at the extremity, at the CG.

But does it really matter, other than giving you something to think about, and divert your mind from the lack of Lou Rawls and the closure of Dan Murphy's?

[HissingSyd]

IBut does it really matter, other than giving you something to think about, and divert your mind from the lack of Lou Rawls and the closure of Dan Murphy's?
Good morning everyone. It is only the exceptional circumstances I find myself in that have tempted me out of my shell. I hope I can be forgiven for raising a bit of diverting controversy.

I have been away looking for more information and I now have some doubts about the cause of the roll in the hover.

I am sure about the physics: the balance/equilibrium of forces and couples in the hover; the need for correct terminology; the absence of a pivot.

I looked back at what Nick Lappos had said in earlier threads and he is telling us that the torque applied to an articulated or semi-rigid rotor head by a tilted rotor disc is big. I was ignorant of this in the 70s and it is something I learned here. I could not find that he explicitly said this was enough to cause the hover roll and he also said that the position of the tail rotor was significant in determining the degree of the roll. I am also largely ignorant of the very sophisticated design of modern helicopters - the last I had some small knowledge of was the Lynx.:hmm:

Here is where I stand at the moment. There is tail rotor drift and this is corrected by tilting the rotor disc laterally. The mechanism by which the disc is tilted is a combination of design choices and pilot control. The design choices will be a compromise between providing for the hover and cruise. In nearly all cases tilting the disc will result in a roll couple, which is from the torque on the rotor head and/or the offset between the tail rotor and main rotor. As the roll progresses the roll couple is balanced by the couple formed between the vertical component of the rotor thrust and the weight acting through the CG.

My real doubts are about where the roll couple comes from. The hover is a state of static equilibrium and in comparison to most helicopter aerodynamics it should be relatively easy to analyse. However, I am aware that there are a lot of things contributing to the forces and couples and that Nick Lappos may know of interactions that have completely escaped me.

Lastly, I think that the simple explanation may be true for teetering rotors, but I am even questioning that.:)

[cattletruck]

Bah! All you experts with thousands and thousands of hours arguing the merits of which side they hang.
Last time I parked one near the hanger with the wind tumbling over the roof it was hanging left, then right, then left, then right and so on until it finally touched that hard allergic grey stuff.

Fair weather flyers the lot of you.

(tongue-firmly-in-cheek :E)

[HissingSyd]

We have been discussing couples that create rotation of a body - how do you identify the axis of that rotation? Is it along the line between (in this case as in ACs diagram) MR head and TR and, if so, where along that line?
I am ok on this one, I hope. A couple is a vector - it has direction and magnitude. The direction for a couple is parallel to the axis of rotation. It does not have a point of application as such. - it is acting on the whole body. I think that this is actually a tricky thing to conceptualise. I might also point out that this is statics, with forces and couples in equilibrium. With dynamics the situation is much more complex.

I am pretty sure that it is ok to apply statics to a hovering helicopter, but I am even having some doubts about that.:)

[HissingSyd]

Many years ago, Nick put out a list of Helicopter Urban Myths,
I do not know how long ago that was, but when I started teaching PoF in the 70s they were still quite widespread. Even Shawberry was turning out QHIs with traces of them. Their teaching materials seemed to be derived ultimately from the US helicopter manual of the 50s which was rife with them.

I tried to eliminate those I new of and met with a little resistance from the beefers as a result.:ugh:

However, this thread has made me consider the awful possibility that I might have fallen for one of them regarding tail rotor roll.:ouch:

[[email protected]]

Syd, thank you for your answers but surely there has to be an axis of rotation if the body rotates.

Consider just the translating tendency of the TR thrust before the disc is tilted to compensate - that lateral force must act either through or at some distance from the vertical C of G. If it acts through it the body will simply move laterally but if it acts at some distance (ie TR vertically separated from the vertical C of G, it must also cause a rotation.

If my sentence is correct then applying lateral cyclic gives a force acting at the rotor head which, again, if not aligned with the vertical C of G, will produce a rolling moment as well as a lateral force to oppose TR drift.

Thoughts?

[[email protected]]

As far as rotor design goes - with a teetering head, the fuselage just hangs under the rotor so you tilt the disc and the fuselage obediently follows it. When you add hinges (physical or virtual) you allow the blade flapping to exert a force on the rotor head which is fixed to the fuselage - the results are the same but the mechanisms are different.

The Lynx, with it's titanium flapping forging had an effective hinge offset of around 17% if memory serves (I used to instruct on it). The bigger this offset, the quicker the fuselage reaction to any cyclic input and the greater control power you are deemed to have. Having rolled, looped and back-flipped it, I know it is VERY responsive.

[HissingSyd]

Syd, thank you for your answers but surely there has to be an axis of rotation if the body rotates.
The essence of a helicopter in the hover is that it does not move or rotate - it is in static equilibrium - and there is no axis of rotation. It means that the sum of linear forces is zero and the turning effects (couples/torques) are balanced.

When analysing this you must be sure that you have identified all the linear forces and all the couples. It is my ability to do the latter that has thrown doubts in my mind.
If my sentence is correct then applying lateral cyclic gives a force acting at the rotor head which, again, if not aligned with the vertical C of G, will produce a rolling moment as well as a lateral force to oppose TR drift.
That is true and is one couple I am pretty sure of. But what is it balanced by? :)

[HissingSyd]

As far as rotor design goes - with a teetering head, the fuselage just hangs under the rotor so you tilt the disc and the fuselage obediently follows it. When you add hinges (physical or virtual) you allow the blade flapping to exert a force on the rotor head which is fixed to the fuselage - the results are the same but the mechanisms are different.

The Lynx, with it's titanium flapping forging had an effective hinge offset of around 17% if memory serves (I used to instruct on it). The bigger this offset, the quicker the fuselage reaction to any cyclic input and the greater control power you are deemed to have. Having rolled, looped and back-flipped it, I know it is VERY responsive.
All that is familiar, but it is describing the dynamic response and analysing that is way beyond my pay grade.

[Ascend Charlie]

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/252x300/bkballs_862447f2345aca9ad66797d513aacf918fabb9e9.jpg
The subject is "hanging low", but the BK seems to have them both at the same level.

[[email protected]]

The essence of a helicopter in the hover is that it does not move or rotate - it is in static equilibrium - and there is no axis of rotation. It means that the sum of linear forces is zero and the turning effects (couples/torques) are balanced. But those couples/torques must be acting around a point to have created the rotation until equilibrium is reached.

[SASless]

AC....BK used mine as a model!:oh:

Well...size and weight perhaps.... but not cast in Brass!

[HissingSyd]

But those couples/torques must be acting around a point to have created the rotation until equilibrium is reached.
Here is a secret. Physicists prefer to deal with steady state conditions because they are relatively easy to analyse. They ignore how the steady steady was arrived at. Sometimes you can fudge the transition from one steady state to another by assuming that all states in between are also steady states. In the case of a helicopter you might be going from resting on the ground to the hover. Following your own dictum, the pilot sees what the aircraft does and makes control movements to achieve what is required from moment to moment. The helicopter changes its attitude quite slowly and we can say that it never really rolls around any point as such. Nice, eh?

[[email protected]]

So are you saying that I am correct when the aircraft is moving but you are correct when the aircraft is static?

[HissingSyd]

So are you saying that I am correct when the aircraft is moving but you are correct when the aircraft is static?
When the aircraft is moving all bets are off as far as I am concerned. What happens does depend on the position of the CG but also in the Centre of Inertia, which is probably as far as this forum should go.

[212man]

When the aircraft is moving all bets are off as far as I am concerned. What happens does depend on the position of the CG but also in the Centre of Inertia, which is probably as far as this forum should go.
No - don't rock the boat! But if you did, which point would it be rotating about?...….. ;-)

[HissingSyd]

No - don't rock the boat! But if you did, which point would it be rotating about?...….. ;-)
Don't forget I am also ex-RN. Are you posing a problem of ship stability with CG and Centre of Flotation? ;-)

[MarcK]

Try not to think of this as rotation about a point in 3-space. In the static equilibrium case, you can pick any point you want, and the most logical (giving simplest equations) is the rotor hub. Rotor disc is tilted with respect to the hub. Fuselage is hanging with respect to the hub, pushed one way by the tail rotor, and the other way by the CG.

[Ascend Charlie]

AC....BK used mine as a model!https://www.pprune.org/images/smilies2/eusa_silenced.gif

Well...size and weight perhaps.... but not cast in Brass!

Sassy, I thought that having b@lls of brass was a requirement for army pilots?? Well, tin anyway...

[JohnDixson]

Gentlemen,scholars, engineers, aviators, five pages of discussion about a fairly simple free body diagram but without concurrence as to a solution,and now,we are to proceed into an allegorical treatment of the relationship between the MBB Pendab and Army Pilot physiology?

[[email protected]]

It would seem that physics isn't much use where helicopters are concerned since they never stay still once running so static diagrams don't help.

ISTR the people that investigated the 2 Sea King dynamic rollovers (one at St Magwan and the other brilliantly reproduced at Boscombe Down) on level ground were very interested in rolling moments from MR and TR and especially their position relative to the vertical C of G:)

[HissingSyd]

ISTR the people that investigated the 2 Sea King dynamic rollovers (one at St Magwan and the other brilliantly reproduced at Boscombe Down) on level ground were very interested in rolling moments from MR and TR and especially their position relative to the vertical C of G:)
I wondered whether to mention dynamic rollover. In this case there is a pivot and it is critical to what happens. Is this thread-drift too far now?

[onehunglow]

Can't believe I missed this thread!

[bryancobb]

Regardless of direction of rotation...
Regardless how high or low the TR output shaft is...
Regardless of how passengers are arranged...

ALL SINGLE ROTOR HELICOPTERS have a vertical propeller creating horizontal thrust that blows the aircraft sideways. The correct term is "translating tendency."
THE ONLY WAY TO STOP the sideways movement of the helicopter is to tilt the rotor tip-path-plane a little bit to the opposite side and create an equal amount of sideways thrust the other way.
All helicopters' fuselages will tilt along with and in the same direction as the rotor's tip-path-plane, some head designs more than others.

[[email protected]]

Nobody calls the rotor a 'vertical propeller' and translating tendency is a term of US origin, it is better understood when called tail rotor drift, with tail rotor roll the description of the fuselage attitude after the rotor had been tilted to counter the drift.:ok:

[bryancobb]

Nobody calls the rotor a 'vertical propeller' and translating tendency is a term of US origin, it is better understood when called tail rotor drift, with tail rotor roll the description of the fuselage attitude after the rotor had been tilted to counter the drift.:ok:

A Tail Rotor is in fact a vertical propeller that produces horizontal thrust. A Main Rotor is in fact a horizontal propeller that produces vertical trust. If you are wound too tightly to accept that description,
then you are part of the reason beginners find it difficult to understand the rotors, how they work, the job they do, and the side-effects they cause.

Bryan

[bryancobb]

Nobody calls the rotor a 'vertical propeller' and translating tendency is a term of US origin, it is better understood when called tail rotor drift, with tail rotor roll the description of the fuselage attitude after the rotor had been tilted to counter the drift.:ok:

If your springy-thingy is wound too tightly to accept my characterization, then you are part of the reason beginners have a tough time understanding helicopter rotors, their design features, how they work, the job they do, and the side-affects of it. A Tail Rotor IS in fact a vertical propeller that produces horizontal thrust, and a Main Rotor IS in fact a horizontal propeller that produces vertical thrust.

If a beginning student's first exposure to rotor systems is through this viewpoint, and as their depth of understanding grows, they continue to process ideas through this idea, they will fully grasp all the other complex principles like flap/hunt/feather, transverse flow, driven/driving/stalled regions, dissymmetry of lift, retreating blade stall, and all the other tough to grasp subjects.

Bryan

[SASless]

Oh dear me....next thing we know there will be colored pencil drawings required and the mandate that only CFS terminology be allowed.

[PDR1]

It just shows where people don't actually understand a thing - they can only parrot what was on a training slide.

PDR

[Ascend Charlie]

only CFS terminology be allowed.

Too true,Sassy, and you must use the Vu-Graph for the slides, turning it off before removing one slide and putting the next one on.

[eagle 86]

I'm watching Ascend Charlie.
E86

[[email protected]]

Bryan - perhaps it is you who is wound too tight:) I didn't say the MR and TR were not propellers in the strict definition of one - just that nobody in the rotary world calls them that.

Talking about propellers when you mean rotors is surely a quick way to confuse newbies.:ok:

I've only been teaching helicopter stuff for 31 years so I probably don't have a clue.......................................

CFSH has quite a good reputation worldwide but its easy to knock if you haven't experienced it.:)

[Same again]

I've only been teaching helicopter stuff for 31 years so I probably don't have a clue................

It is more likely that you have been teaching CFSH stuff for 31 years.

[bryancobb]

Bryan - perhaps it is you who is wound too tight:) I didn't say the MR and TR were not propellers in the strict definition of one - just that nobody in the rotary world calls them that.

Talking about propellers when you mean rotors is surely a quick way to confuse newbies.:ok:

I've only been teaching helicopter stuff for 31 years so I probably don't have a clue.......................................

CFSH has quite a good reputation worldwide but its easy to knock if you haven't experienced it.:)

Well THIS 34 year helicopter guy (87-17 Royal Blue Ft. Rucker) will continue to refer to the rotors as propellers
sometimes, as well as air-pumps, egg-beaters, paddles that thump the air, a "a big fan that keeps the pilot cool."
LOL Because if it stops...watch the pilot start sweating.

In your defense, I work as an engineer in advanced composites for Meggitt, a pretty large aerospace manufacturing
company. A few of my peers at work are wound a little too tight also and they used to give me grief for my red-neck
analogies. After several years there (in my 6th) they have learned that I can often "get-through" to someone when
they can't.

Bryan

[Ascend Charlie]

I'm watching Ascend Charlie.
E86

Oh jeez, I was talking when the teacher was standing behind me. I'm in trouble now.

The CFS books, I think it was the AP3456A, had some glaring faults, as do the FAA helicopter instructional documents. But if you show it on a Vu-Graph, it seems to make sense, even if it's horsefeathers.

[JimEli]

I looked up the acronym for CFSH online. I'm sure it stands for "Chicken Follicle-Stimulating Hormone".

[[email protected]]

It is more likely that you have been teaching CFSH stuff for 31 years and Prouty, Wagtendonk, Coyle and others. The CFS H teaching is a method of teaching not the syllabus and uses the best of the best techniques gleaned from all areas of education.

You do the course and then start learning your trade - the quality of instruction comes down to the character and personality of the instructor not the need to use the right colours.

AC are you referring to the OverHead Projector when you say vu-graph?I'm too young to know what one is otherwise:)

Jim Eli - it's all about Pelicans not chickens:)