transverse flow vs translation lift - threads merged

[johndrew]

I am doing some studying before hoefully hopping over the pond to Pensylvania to learn to fly a helicopter.
Could someone explain in normal language what transverse flow effect is?
I believe it is something to do with the airflow at the aft side of the rotor disc inducing drag and producing less lift but I can't quite get my head around it.

[vorticey]

transverse flow effect or in flow roll happens when just starting off from a hover to foward flight.
when hovering, there is a column of air coming down through the rotors and u use power to hover in the down draught.
as you move foward, the front of the disc starts cuting clean air (air that hasnt been effected by the rotors yet) it gets more lift because the induced air flow is recuced (downward flow). so now the front is getting more lift than the back so the blade will fly up as it turns to the left (on robbo) producing a high blade on the left. this makes the helicopter roll right unless the pilot corrects it with left cyclic. and because the induced air flow is less the helicopter with start to lift up with the extra lift, so you can push it over to get some more speed instead.
;)

[Irlandés]

Hi John,
the key to understanding things like Transverse Flow Effect is to have a good understanding of 'prerequisite' topics such as Induced Flow and Gyroscopic Precession. You can read up on all of these at Paul Cantrell's website at...

http://www.copters.com/helicopter.html

and specifically on TFE at...

http://www.copters.com/aero/transverse.html

This webpage is excellent if you haven't gotten round to buying any texts yet (and excellent if you have!). :D

And just when you get your head around that, you may find there's more (or less) to Gyroscopic Precession than meets the eye. But that's another kettle of fish... :rolleyes:

Best of luck!
Irlandés

[ShyTorque]

Please don't mention gyroscopic precession while Lu Z can't answer.

His head may well explode with frustration :D

[The Nr Fairy]

ShyT - go on, please - explain it to me ? I've got a webcam ready ! :D

[Aladdinsane]

transverse flow will also happen anytime the collecive is raised in forward flight for the same reasons as stated above.

[ShyTorque]

I think c) is more correct.
The induced flow is GREATER and AA less at the rear compared to that at the front of the disc, not the reverse.:ok:

There is no increasing / decreasing effect at the front of the disc, only a start point for comparison.

[540DEGREE TorqueTurn]

:eek: :E :E :E :E :E :E :E

[imabell]

there are two correct answers there if you dont look at the big picture. in nil wind the induced flow is the same value across the disc. as you introduce airflow over the dics the induced flow is decreased at the front of the disc relative to the airflow more than the rear due to the action of the blades but never the less the induced flow is reduced over the entire disc.

transverse flow effect is a product of this imbalance causing the machine to roll slightly right in a conventional helicopter. as the disc wants to flap back due to aerodynamic influence at the front of the disc the reaction is 90 degrees later in the plane of rotation causing the inflow roll.

[MightyGem]

These may help:

Flapback

http://img.photobucket.com/albums/v465/MightyGem/Flapback.jpg

Inflow Roll

http://img.photobucket.com/albums/v465/MightyGem/Inflow.jpg

[ETL2GO]

heres a question thats got me a little bamboozled??

some texts say that pilots often confuse the vibrations associated with transverse flow with the onset of effective translational lift...
also that transverse flow isnt noticeable in underslung teetering systems..
thats all good...but why does the R22 get the shakes around early ETL and at loss of ETL...? I was quite content believing it was the indication of ETL...???
:ugh:


any comments?

[RVDT]

Translational lift - http://cybercom.net/~copters/aero/translational.html (http://cybercom.net/%7Ecopters/aero/translational.html)

Just to add to that - ever notice why there is no vibration when it happens when you are out of ground effect?

When in ground effect you are flying through your own downwash which you are pushing in front of you. This is an "indicator" that you have achieved translation.

Transverse flow - http://www.aviationtrainer.com/Transverse_Flow_Effect.htm

[[email protected]]

Transverse flow doesn't produce vibration, the onset of ETL does. They often occur at the same time but transverse flow is most noticeable in the early stages of the transition from the hover. A good demo can show the seperate effects of the two but you will only get vibration with the ETL part.

There is no roll-up vortex to fly through in OGE so the vibration from said vortex doesn't occur.

The explanation of the transverse flow vibration in RVDT's second link doesn't hold water - though the rest of the text seems sound.

ETL2GO - you are right - the R22 experiences vibration through the onset and loss of ETL just like any other helicopter.

Transverse flow is a factor across the whole flight envelope not just the transition which is why the R22 has the litlle pull-up knob to offset cyclic lateral loads in forward flight.

[helisphere]

That is not true that transverse flow is not noticable in underslung, teetering rotor systems. Underslinging a rotor puts the rotor cg in plane with the teetering hinge, and this minimizes the difference in coriolis force between the two blades so that one blade does not try to accelerate at the same time the other is trying to decelerate. This eliminates the need for drag hinges, but it does not eliminate the fact that there will be very uneven drag forces on the individual blades in the speed range where tranverse flow occurs. This uneven drag will cause vibration in any type of rotor system.

[ETL2GO]

Thanks folks
I remember noticeable inflow roll more so in the 300 I soloed in a number of years ago, than in the R22..
the explanation of the translation vibrations (IGE) makes perfect sense
cheers..

[bluestack]

you know how it goes....

a student asks you a question that gets you thinking yourself....

Transverse flow in a nutshell - lower induced flow at the front of disc due to more horizontal airflow creates more lift, translated through 90degress for gyroscopic precession = more lift left portion of the disc = roll right..

Translational lift - disc moves out of vortices and into cleaner air resulting in more lift producing area at front versus rear of disc still in "vorticed air" therefore disc blows back

Maybe a stupid question - but if gyroscopic precession affects induded flow and angle of attack under so called transverse flow - why does it not affect the front portion of the disc under translational lift 90 degrees later, as control inputs are already put in earlier to get equal lift across the disc.

Or does it all happen at the same time and we break the phenomena down seperately for explanation and actually translational lift is a part of the lowering of induced flow in transverse flow effect ???

cheers

[daver_777]

Hi there,

I believe that flap back or blow back as you call it has more to do with the relative velocities of the blades rather than translational lift.

If you break it down you see once you start moving the advancing blade with have a greater relative velocity than the retreating (due to adding and subtracting the airflow velocity) this increase in velocity causes a increase in lift/rotor thrust and the advacing blade flaps up (in the case of moving foward it begins its upward movement 90 degrees before the front of the disc to arrive at its highest point over the nose of the aircraft)

So precession has taken place. This can also be seen when hovering with some wind over the disc, if a climb or descent is initiated the disc with flap back with a climb and foward with a decent (again due to the different relative velocities of the advancing and retreating blade)

As for translation if the front of the disk was in 'clean air' you still get the roll to the right with the higher inflow angles at the rear of the disc.

Just my thoughts

Daver_777

[[email protected]]

Inflow roll (transverse flow) and flapback are both examples of the rotor disc flapping to equality - with inflow roll there is a difference in inflow angles and thus induced flow between the front and rear of the disc - with flapback there is a difference in Vsquared and thus lift between advancing and retreating blades.

The flapping (and especially the damping that reduces the flapping) is due to the aerodynamic forces and not to gyroscopic precession - this is proved by the fact that phase lag changes from 90 degrees with density altitude and rotor head design - a gyroscope wouldn't experience these changes but that is what is taught by many schools because it is sometimes easier to understand.

Translational lift is due to a change in the inflow angle and thus induced flow across the whole disc as a result of moving - forward to keep it simple but in any direction since the disc doesn't know or care where the air is coming from.

[nodrama]

Daver 777 Some rotor systems are said to act like gyroscopes because their phase lag is 90 degrees. Note the words act like.
Phase lag isn't gyro precession, it is due to the inertia of the blades. Depending on rotor design, phase lag can be anywhere from 75 - 90 degrees.

[[email protected]]

No drama - you will have a hard time convincing our cousins from across the pond about that since precession is what they are taught, including the US military.

[nodrama]

Ok cousins.....place your arm across your chest, then swing it outwards at speed. At the same time, raise your arm.

Bet it doesn't reach the top of the 'raise' at the same point you started to raise it.

Is that due to gyroscopic precession :confused:

[daver_777]

To Nodrama

It seems i may have inadvertantly opened a whole new can of worms here, I was led to believe throughout my training that phase lag is the phenomenon where "a cyclic input causes the disc attitude to change, the blade reaching its highest and lowest 90 degree later than the point where the maximum increase and decrease of pitch are experienced" and that this 90 degree delay was due to the principle of gyroscopic precession.

I was also told that phase lag was always 90 degrees and that it was the advance angle to overcome this problem that could be varied.

From what your saying clearly this doesn't seem to be the case, could you please elaborate or point me in the direction of some decent notes? As i'd quite like to know both sides of the story

Cheers

Daver_777

[[email protected]]

Daver - as I said earlier, many have been taught the precession explanation because it it easy to understand - I think if you do a search on Pprune you will find threads that explain the aerodynamic version of events properly - if you can't find anything then one of us will post an explanation.

[nodrama]

One extract I found. Not as technically accurate as I would like but it's a start while I look for other references:

'Phase lag will cause the extra lift to be seen approximately 90 degrees later in rotor rotation in semi-rigid two bladed rotor systems. Phase lag is a separate phenomenon from gyroscopic precession (http://en.wikipedia.org/wiki/Gyroscopic_precession), which applies only to rigid systems. Rotor systems are not rigid systems since all helicopter rotors are designed to "flap" up or down as they change position around the rotor arc. This flapping counter-acts dissymetry of lift in forward flight. Phase lag is a property of all rotating systems acted upon by a periodic force. For systems hinged at the axis of rotation ( in our case, a semi rigid flapping type rotor head) the phase lag is 90 degrees. For systems that are hinged at some distance from the axis of rotation (such as a fully articulated rotor head) the phase lag is less than 90 degrees.'

Sorry ELT2GO, we seem to have drifted.

[tony 1969]

Hi Guys we have been through all this before! a search on the forum will find all the answers.
My tuppence worth: It is definately not gyroscopic precession,
Crab@saavn- The way I understood it was -Flapback is due to a speed change, whereas flapping to equality is a cyclic pitch change, surely not the same thing. Inflow roll is the difference in lift due to difference in induced flow not flapping to equality
Confused ? I usually am

[[email protected]]

Tony - the mods have helpfully added the posts from a previous thread at the beginning of this one which may help (after the initial confusion of 'that wasn't there earlier').

However, flapback and inflow roll are both examples of flapping to equality - which is exactly what it says on the tin - there is a difference in lift (whether through speed, induced flow or AoA) between one side of the disc and the other which causes the blades to flap. Aerodynamic damping (as a blade flaps up it's induced flow starts to increase so the AoA reduces and the extra lift is lost) stops the blade continuing to rise or fall. The blades inertia means that the effect of the lift change increases and decreases gradually.

[helicfii]

As far as vibrations during ELT or transverse flow are concerned- there will always be vibrations present whenever there are differences in drag across the span of the blade which are moving inboard/outboard as the rotor is turning. Anything other than a calm wind hover will produce vibrations in the rotor disc.

To ask a student to try to distinguish an ETL vibration from transverse flow vibration seems a bit ridiculous to me- you might be kidding yourself if you really believe that these vibrations occur separately and only at different times.... RW aerodynamics are far too dynamic to deconstruct them and then try to distill some type of "rule" that will apply in all instances.

How the rotor disc behaves, however, gives the pilot an indication of which, of many, phenomena he is experiencing. And he is always experiencing more than one at any given time!

And what about Hooks Joint Effect? :eek:

[[email protected]]

Helicfi - I'm afraid I have to disagree, inflow roll and flapback do not (in the normal course of events) produce vibration - there is no mechanism for production of vibration when the blades are simply flapping. However, with the onset of ETL, the roll-up vortex that is created by the outflow of the downwash in the hover over the ground, is encountered by the blades - that is what causes the vibration during transition.

[flame_bringer]

Been reading about the helicopters aerodynamics lately and i came accross this discrepancy :
a decrease in lift in the aft portion of the rotor disk occurs in hovering conditions or slow speed forward motion , when the helicopter conducts a forward motion at a low airspeed typically between 10 to 15 knots the airflow across the aft portion of the disk is accelerated for a longer time than the fore portion , which results in the air moving more vertically in the aft portion than the fore portion and consequentally a decrease in the angle of attack on the aft rotor portion and a decrease in lift and gives the helicopter a tendency to pitch the nose up .

hows the air accelerated for a longer time ? and what does that got to do with altering the angle of attack of the blades ?
please anyone explain
Thanks in advance

[thecontroller]

This explains it.

Transverse Flow Effect (http://www.dynamicflight.com/aerodynamics/transverse_flow_eff/)

and maybe...Transverse flow effect - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Transverse_flow_effect)

[Shawn Coyle]

Flame Bringer:
What book did that explanation come from???

[choppertop]

That first link doesn't really explain the phenomenon. I'll have a go and prepare myself for a thousand insults... lol...

The horizontal flow, created by moving forwards, is accelerated for a longer time at the rear of the disk because the airflow has travelled the diameter of the disk (front to back), the molecules of air towards the rear of the disk having had more time under its vertically downward influence. With more flow downwards than sideways (think of that vector diagram now) the angle between the relative airflow and blade chordline gets smaller (reduced angle of attack). The reduced angle of attack here means less rotor thrust (on that vector diagram the total reaction vector would lean further away from from the vertical 'best efficiency' position). So now there is less rotor thrust at the back of the disk, compared to the front and the rear of the disk wants to drop. The result of this is seen 90 degrees later in the blade revolution, giving you the distinctive roll to the right (in a counter-clockwise rotating blade system).

Note: Your text book says this gives the helicopter a tendency to pitch the nose up but this is incorrect.

How did I do? :O

[paco]

Hmmm....

During forward movement, the air going into the rear of the tilted disc has a more pronounced perpendicular flow because it has had time to accelerate and become part of the induced flow, which means that the induced angle is more, the angle of attack is less and the thrust is reduced.

The airflow into the disc is more horizontal at the front, however, so there is less induced flow and more angle of attack, and more lift, so the disc rises at the front. Phase lag means that the maximum upwards blade displacement occurs to the left, and the maximum downwards displacement to the right, which tilts the disk to the right to change the direction of the thrust vector to the advancing side.

Phil

[choppertop]

Isn't that what I said, boss?! lol

[delta3]

So, If I get it right : because the induced speeds is greater at the rear of the rotor, it starts vibrating....


WhenI look at the graphs of the link, imho, coning looks more responsible for asymmetry than induction speeds.

So applying the wisdom of the reference, a coned rotor should vibrate like hell....

d3

[Torquetalk]

Try teaching it in an R22: It might go something like this :-

1) Brief it
2) Go out and fail to demonstrate it (the aircraft will roll the wrong way)
3) Go back to the briefing room and talk about couples instead :ouch:

[delta3]

Torquetalk,

thx, at least something I understand

d3

[delta3]

The reasoning in the reference 1 is pure non sense.

Because of coning the blades will see an extra variation in angles, and there is nothing wrong with that, other than the fact that a coned rotor blows back more , and typically will phase shift a bit more, to be taken into account in the delta3's in the rotor rigging to maintain controllability.

The vibration at the low speeds comes from the fact that the rotor ingests some vortices from the air it is blowing in front of the machine and that the heli is progressively overtaking. Also only a IGE effect, try it OGE...

d3

[flame_bringer]

Hey guys , actually it didnt mention anything regarding what happens after the transverse flow effect i included the '' pitch the nose up '' part on my own as per how i understood it .
but i guess i was mistaken and it should roll accoring to the gyroscopic effect theory .
and i read it on the Flight simulator 2004 in the helicopters section in the learning centre .

[busdriver02]

Killing the reference to gyroscopic precession is an ongoing battle to this IP. I freaking HATE when guys talk about it. What it comes down to is that the Hawk's (and most US combat helos) phase lag angle is just about 90*, so it's easier and more understandable to simply call it gyroscopic precession, no matter that it isn't remotely correct. Trying to explain to a non-engineer that the aerodynamic forces involved can completely overwhelm the gyroscopic forces seems a stretch too far I guess.

[thecontroller]

Try teaching it in an R22: It might go something like this :-

1) Brief it
2) Go out and fail to demonstrate it (the aircraft will roll the wrong way)
3) Go back to the briefing room and talk about couples instead


I seem to remember it's quite easy to demonstrate in an R22, from the hover you put the stick forward a little bit and keep it there, you will get flapback, push it forward a little more and the aircraft will start to accellerate and get ETL, you get inflow roll (to the right), caused by transverse flow effect.

Although I'm sure the actual aerodynamics are much more complicated than the simple explanation we learn.

[choppertop]

you get inflow roll (to the right)

... although you might have to 'help' this along a bit, ahem... for clearer demonstration purposes ... lol

[delta3]

Would not it be useful to just talk more about coned rotors as complete systems instead of always going back to all the nuts and bolts it is composed of.

The precession thing has been a very interesting topic on this forum, remember the lively discussions with Lu,
but a rotor is a lot more than that, and going each time through half of an advanced physics course to explain something is imho not always the most productive educational way. It also has a danger that by omitting the other half, wrong conclusions are drawn.

I would follow the suggestion of thecontroller and talk more about aggregate things

I would only go once thru the details of precession and blade speed differences and flap etc to explain the underlying mechanismes, but I would summarize it at an aggregate level as follows

- a rotor blows back, just like an umbrella
- the more its coned the more it blows back
- the higher the speed to more it blows back
- to compensate blow back we put the cyclic more forward, if we don't the nose goes up.
- retreating blade stall makes blow back worse
- coning provokes a blow back delay in the system resulting in a sideways tilting of the rotor that is ironed out by the rigging, so not to worry to much about.

m2c
d3

The student can then forget the details.

[Torquetalk]

I seem to remember it's quite easy to demonstrate in an R22, from the hover you put the stick forward a little bit and keep it there, you will get flapback, push it forward a little more and the aircraft will start to accellerate and get ETL, you get inflow roll (to the right), caused by transverse flow effect.


Then you had more luck than me: I was also looking to demonstrate the effect before the main rotor flies through its vortices and found any effect indiscernable; dwarfed by the massive roll to the left as the tail rotor flies through its own vortices. It suddenly shed light on why two senior instructors at Bristow had different opinions on the direction of roll: one had thousands of hours in Robinsons.

[MightyGem]

I once asked a rotary test pilot at Boscombe why the Lynx rolled away from the advancing side as it accelerated. He couldn't answer. He said that sometimes that they have absoloutly no idea why things happen as they do.