View Full Version : Etl vs transverse flow
baobab72
1st Sep 2013, 16:54
Hello everyone
Is it a correct statement that transverse flow effect occurs before etl during a normal takeoff?
Many thanks
Baobab72
Overdrive
2nd Sep 2013, 07:28
Yes, correct.
Transverse flow - or inflow roll as we call it in UK mil - is present across the whole speed range but it is most pronounced during the transition.
As you tilt the disc to go faster, you create an imbalance of induced flow between the front and rear of the rotor disc which leads to a roll towards the advancing side of the rotor due to phase lag.
IIRC the R22 has a pull-operated counter balance weight arrangement to relieve the lateral forces in the cruise due to inflow roll - in a real helicopter you can just use the trim;)
Soave_Pilot
2nd Sep 2013, 12:34
R22 has a pull-operated counter balance weight arrangement
and where would that be?
in a real helicopter you can just use the trim
y'all dont miss a chance! :}:}
On the end of right arm ? or is it a trick question :uhoh:
On the R22 I last flew (20 years ago) there was (I'm pretty sure) a knob (no jokes please) that when pulled, offset the cyclic forces in roll either by means of s spring or a balance weight - I am happy to be corrected since it was a long time ago:ok:
Ready2Fly
2nd Sep 2013, 17:01
You remember correctly:
http://www.pprune.org/rotorheads/299441-r22-longitudinal-bungee-spring-lateral-force-trim-knob.html
:ok:
Rt trim force is required to overcome the force required to increase pitch in the FRONT portion of the disk (needed to create high pitch at the retreating side to overcome 'Dissymetry of Lift')
But the lateral trim spring exerts a LEFT cyclic force to counter the inflow roll/transverse flow because the aircraft wants to roll towards the advancing side of the disc due to dissymmetry of lift.
army_av8r
4th Sep 2013, 13:24
Transverse flow effect or "inflow roll" occurs due to the unequal distibution of induced drag across the disk. The front half sees less induced flow, the rear sees more. This unequal drag/ unequal lift causes a roll toward the advancing side (right in a Bell). Its most noticable as you transition from the hover(high induced flow across the entire disk) to forward flight. Once the initial transition is made and the roll is countered by that lateral cyclic, you will not see much of a need to continue to correct for it. As for dissymmetry of lift(if you want to call it that) it causes the aircraft to pitch up, due to blowback, and would tend to "slow" the helicopter if the pilot did nothing. The way we counter blowback is by applying more and more forward cyclic as the airspeed(and dissymmetry of lift) increase. all of this assumes a perfect world, textbook model, and does not account for other forces at work. so your result may be complicated by other interferences. but in the end, ETL is when the disk has a greatly reduced induced flow due to forward movement in the airmass. Transverse flow shudder occurs just before you get to ETL and is basically when "half" the disk is through ETL.
but holding the stick in a more forward position to counter D o Lift surprisingly requires force to the right. (sounds weird but think it through ... the stick is displaced in the forward direction but the FORCE required is to the RIGHT ... strange but true
( the pitch link of the blade at the front , where pitch is being increased, is found on the left hand side , so this is where the upward force from the swashplate is required hence a force to the right required from the cyclic stick)
No - there is some right swashplate movement required to tilt the disc forward in order to make the aircraft accelerate BUT that is still forward cyclic.
THEN the dissymmetry of lift caused by differences in induced flow makes the aircraft roll RIGHT - this is the effect which is most noticeable in the transition but is present across the whole speed range and is countered by LEFT cyclic.
Flapback/blowback makes the disc want to rise at the front and that is countered with more FORWARD cyclic (right swashplate movement).
Go and try a transition yourself - from the hover, displace the cyclic forward slightly and hold it - you will see that initially the aircraft descends (if you don't move the collective) then it pitches nose up (flapback) and rolls to the advancing side (inflow roll/transverse flow). It doesn't take a rocket scientist to see that these effects are countered with FORWARD cyclic for flapback and LEFT cyclic for inflow roll (in a helo with an R22 direction of MR rotation).
Flapback/blowback makes the disc want to rise at the front and that is countered with more FORWARD cyclic (right swashplate movement).Check please...
and
Stick Force and Stick Dissplacement are surprisingly not the same thing...
No, stick displacement is the amount you have to move it - stick force is the effort required to hold it there -
Now your point is?????????
How do you (in AnFI world) counter flapback and inflow roll - clearly differently to the rest of us:ugh:
" :ugh: " !! why are you so rude?
"and your point is?" ..... simply that you are wrong (:rolleyes:)
You say: "R22 has a pull-operated counter balance weight arrangement to relieve the lateral forces in the cruise due to inflow roll" But no, it is D o Lift which requires the stick to be displaced further forward - holding the stick in a more forward position causes a higher pitch on the retreating side. The place where that pitch is being increased to achieve that is at the front, the pitch link which relates to the blade at the front is on the left hand side and it is there that the upward force must be made - requiring a FORCE to the right - and THAT is what the trim is helping with.
You also said: ".....countered with more FORWARD cyclic (right swashplate movement)."
That is also incorrect : forward stick tilts the swashplate forwards.
and as for: "No, stick displacement is the amount you have to move it - stick force is the effort required to hold it there - "
the point about that is that I take it that you believe both displacement and force would be in the same direction - but t'aint necessarily so: holding the stick in a more forward position may require a force to the right - ie not in the direction of the displacement.
I am not rude to you and I would prefer if you were not so rude to me.
AnFI - I have come to the conclusion that either you know just enough P of F to be dangerous or you are a troll.
Either way, you keep confusing straightforward answers that I try to give to people like baobab who are just learning about helos and want to understand.
For all I know, you might be a brilliant aerodynamicist and an outstanding instructor (although you have never given details of your flying and instructional experience) but much of your brilliance must be lost in translation because it usually comes across as gobbledegook.
I won't bother arguing the point any further except to say - get into an R22 and use the lateral spring force knob, you will find it offsets the amount of force you use to counter inflow roll and that force (and the displacement) is to the LEFT to prevent the aircraft rolling to the RIGHT.
We will only p*ss each other off and end up getting moderated which will lose the thread and any sensible help that baobab might have received.
So much anger!
The trim in the R22 is a cyclic right trim. It produces a tension that pulls the cyclic forward, and to the right somewhat. This relieves the pilot of some fatigue when cruising.
The reason is that, when you displace the cyclic forward, lowering the nose, and causing you to accelerate, the aircraft rolls to the left.
This is because the advance angle on the R22 is some 11 degrees off 90. So for each movement forward of the cyclic, it produces a corresponding forward and left result in the disc. Hence you subsequently require right cyclic as you progress further forward.
Note that this has nothing to do with either ETL or Inflow Roll!
Exo - you are clearly cleverer than me! I have checked the Robinson maintenance manual on line and it agrees that it is a right lateral trim.
I was told the previous info about it being the other way 20 years ago by a CAA examiner and I haven't flown the R22 since.
So, AnFI is correct about the direction of the force but not about its reason - I had forgotten how strange the phase lag and hinges are on the robbie - could it be the mysterious 'wee-waa' that Lu Zuckerman always banged on about?
So AnFI, I apologise for being rude but you are still wrong about inflow roll - I shouldn't have used the R22 as an example as my memories of it are clearly muddied and flawed - however, I am completely correct in which way the aircraft wants to roll due to the phenomenon.
Please re-examine the piece about DoLift requiring the highest rate of Pitch increase at the front, the pitch link for which is on the left - requiring a stick force to the right.
Yes things are sometimes not clear when they don't accord with your world view so it is particularly worth re-examining them with an open mind.
Incidentally (Crab or others) in which direction does the swashplate of a Lynx move with fore-aft cyclic stick? - just curious
Since the gobbledegook was actually yours: "I won't bother arguing the point any further except to say - get into an R22 and use the lateral spring force knob, you will find it offsets the amount of force you use to counter inflow roll and that force (and the displacement) is to the LEFT to prevent the aircraft rolling to the RIGHT." then I have to accept your kind assertion that I "might be a brilliant aerodynamicist and an outstanding instructor". I think you would be highly enriched and pleased by re-reading some of the assertions I have previously made which you have rubbished so mercilessly. Feel free to PM if I can help with that. And incidentally your appology is slightly grudging in that you say "but you are still wrong about inflow roll" - that is simply not true - I don't even mention inflow roll !
I say again I have the highest regard for what you do and I do not wish to receive weight from the credentials which you seek but rather from the points I make in their own right. Best wishes. AnFi
You also said: ".....countered with more FORWARD cyclic (right swashplate movement)."
That is also incorrect : forward stick tilts the swashplate forwards.
Sorry AnFI but forward cyclic tilts the disc forward but the swashplate right.
In order to obtain a disc forward attitude, the minimum pitch must be on the right hand side - this gives maximum rate of flapping down and a low point 90 degrees or so later (at the front in this case) This is phase lag.
Phase lag on the R22 seems to be greater than 90 degrees since forward cyclic is clearly giving an element of left roll - I had missed that part so we were arguing from different directions.
Baobab started the thread with a question about transverse flow which I answered - then I made the mistake of including the lateral trim in the R22 as part of my argument; on that part I was wrong but I thought your argument was not only about the R22 ( about which I was wrong) but also about inflow roll (about which I am correct).
So, some mis-communication there and me jumping to some incorrect conclusions for which I apologise unreservedly.
However, you might want to review the relationship between the cyclic movement and the swash-plate.
The Lynx doesn't have a swash plate, it has a spider arrangement but it does essentially the same thing - every other helicopter I have flown has had a swash plate that tilts towards the advancing side to achieve a disc low at the front. I have flown aircraft with different arrangements of jack positioning and pitch change advance angle but they have all been to exert a minimum or maximum pitch angle 90 degrees ahead of where the lowest or highest point of flapping is required.
Friends now:ok:
Sure friends now :ok:- we are after all brothers of the order of helicopter pilots !
[ ... and anyway I am sure someone else (please) will correct you with that swashplate orientation issue:
"Sorry AnFI but forward cyclic tilts the disc forward but the swashplate right."]
...and i really mean it you'd love a session of ground school on PoF - it would genuinely enrich your life - promise, money back guarantee - and you can teach me how to spell (again)!!
Arm out the window
6th Sep 2013, 22:15
Sorry AnFI but forward cyclic tilts the disc forward but the swashplate right.
In order to obtain a disc forward attitude, the minimum pitch must be on the right hand side - this gives maximum rate of flapping down and a low point 90 degrees or so later (at the front in this case) This is phase lag.
erm ... I'm slightly hesitant to jump in to such a good banging 'discussion', but consider this:
To be lowest at the front of its orbit, a blade needs to be at its lowest angle of attack about 90 degrees before, where the rate of flapping down will be greatest. So, when the blade's out to your 3 o'clock, pitch needs to be lowest.
However, pitch control to the blade comes from a horn and pitch change link setup leading the blade by (not quite) 90 degrees. Therefore, to give that maximum 'pitch down' command in the right spot, the swash plate will need to tilt forward, telegraphing its punch, so to speak, so the blade flaps down fastest out to the right and reaches its lowest point at the front prior to starting to travel up again.
A good old simple setup like a Bell 205 has the pitch command coming through the stab bar, leading the blade by exactly 90 degrees, whereas the Robbie is offset by 18 degrees or so, I believe, to create a good compromise between dissymmetry of lift and the effects of hinge offset (or some such thing that kind of makes sense when I read Frank Robinson's discussion of it, but drops out of my brain ASAP afterwards:))
There is a good discussion of it somewhere in the PPrune archives that should settle this issue if cups of tea and good lie downs are had ...
Arm out the window
6th Sep 2013, 22:57
Here's an excerpt of Frank Robinson's explanation of various R22 features from a post on PPrune - very good of him to take the time, I thought. The last line of the first bit clears up the swashplate tilt bizzo pretty clearly:
R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.
Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.
Aotw - you are completely correct and I know where the confusion between myself and AnFI has occurred:
On all the helos I have flown, all (bar the R22) have hydraulic jacks and are considerably bigger and heavier. As I said in my last post, they all use a combination of jack position and advance angle to ensure that max or min pitch is achieved 90 degrees before the desired high or low point.
Often, half of the angle is taken out with the jack positioning and half with the advance angle such that pure forward cyclic would tilt the swashplate to the half-past one position and the 45 degree advance angle obtained by putting the pitch change arm ahead pf the feathering axis would complete the job.
What I forgot, in my hazy recollection of the R22, is that the pitch change horn is mounted pretty much 90 degrees ahead of the feathering axis so that the swashplate does tilt forward with the cyclic.
This is the problem with arguing about helicopters since there are so many different designs which achieve the same effect in different ways.
Hopefully AnFI will agree that if there was no advance angle on a rotor system and the pitch change arm was exactly on the feathering axis, then to achieve a blade low at the front would require the swash plate to be tilted to the right - this was the picture in my head of a basic control orbit.