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transverse flow vs translation lift - threads merged

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transverse flow vs translation lift - threads merged

Old 29th Aug 2002, 23:07
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transverse flow vs translation lift - threads merged

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.
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Old 30th Aug 2002, 00:17
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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.
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Old 30th Aug 2002, 09:29
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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...


and specifically on TFE at...


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

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...

Best of luck!
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Old 30th Aug 2002, 17:43
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Please don't mention gyroscopic precession while Lu Z can't answer.

His head may well explode with frustration
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Old 31st Aug 2002, 05:28
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ShyT - go on, please - explain it to me ? I've got a webcam ready !
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Old 1st Sep 2002, 00:12
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transverse flow will also happen anytime the collecive is raised in forward flight for the same reasons as stated above.
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Old 10th Jan 2007, 22:55
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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.

There is no increasing / decreasing effect at the front of the disc, only a start point for comparison.
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Old 11th Jan 2007, 02:50
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Last edited by 540DEGREE TorqueTurn; 11th Jan 2007 at 05:21.
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Old 11th Jan 2007, 03:56
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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.
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Old 11th Jan 2007, 07:39
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These may help:


Inflow Roll

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Old 30th Jun 2007, 04:51
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transverse flow vs translation??

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...???

any comments?
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Old 30th Jun 2007, 05:29
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Translational lift - http://cybercom.net/~copters/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/Trans...low_Effect.htm
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Old 30th Jun 2007, 05:54
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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.
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Old 30th Jun 2007, 06:05
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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.
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Old 1st Jul 2007, 00:20
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makes sense

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
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Old 27th Mar 2008, 00:13
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transverse flow versus translational lift

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 ???

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Old 27th Mar 2008, 01:05
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Flap back

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

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Old 27th Mar 2008, 06:48
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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.
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Old 27th Mar 2008, 09:50
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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.
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Old 27th Mar 2008, 11:27
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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.
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