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:

Phase lag
 

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

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.

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, 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.

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

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.

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:

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.

Transverse flow effect ??
 

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

This explains it.

Transverse Flow Effect

and maybe...Transverse flow effect - Wikipedia, the free encyclopedia

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

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 but this is incorrect.

How did I do? :O

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

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

Transvers flow
 

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

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:

Torquetalk,

thx, at least something I understand

d3

Now back to my serious mode
 

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

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 .

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.