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HeliTorque :: View topic - Secondary Effects of Yaw - Why?
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HeliTorque Forum Index » Flight Dynamics

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Secondary Effects of Yaw - Why? Goto page Previous  1, 2, 3, 4, 5  Next
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Heliwhore
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PostPosted: Mon Feb 16, 2009 12:53 pm    Post subject: Reply with quote

Yep, that was my point in the other thread. At constant speed, we get a disym of lift just by increasing the pitch, ie flap-back. I agree this is WIDE open to argument. I'm not going back into the formula again, it hurt my head last time.
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PostPosted: Mon Feb 16, 2009 1:05 pm    Post subject: Reply with quote

I'll take a quck stab at this one having just drawn a couple of piccies.

By definition Lift Force acts at 90 degrees to the relative airflow, Drag force acts in line with the relative airflow.

Taking the Left Pedal case

The lift force generated by the vertical stabiliser acts to weathercock the nose back into wind because the resultant of the drag and lift it produces acts to pull the end of the tail to the left (imagine lift produced by it pulls left at 90 degrees to where the airflow is coming from and drag acts backwards). Hold this thought.

The Mast fairing on an R22 is in effect a big stabiliser when the airflow is from the front it just makes drag, which if we think about it wont be great in magnitude but will act to pitch the nose up in forward flight (the force being higher than the vertical C Of G).

Now if we apply left pedal we show the same relative airflow to the mast fairing and the fuselage and lift produced by them will act at 90 degrees to the relative airflow and any drag in line with it. The magnitudes of these forces we can only estimate but the lines of action are fairly clinically defined. Then if a greater force is generated above the Vertical C of G then below a roll will result to the left and vice versa. More fuselage behind the longitudinal C of G will cause a stabilising effect and in front will be destabilising in yaw.

If a descent starts (and it would need to be fairly quick I would have thought) this would change where the RAF is coming from and the Lift and Drag vectors would change accordingly.

Some proof of this can be seen if you think about the actions required in the event of a door popping open , which pedal gets applied and why ?

I said the other day I'd get vertical C Of G in somewhere but it is important, more so than we give it credit for.

If you have held the controls still the fuselage is now pointing left of where you started and the rotor thrust will be more aligned with pointing that way and the relative airflow will be coming from more in front of you again. If you try and maintain track with the cyclic it will be going right and rolling you that way.

PS I wrote this while HW was off referring to his FIE and have taken half an hour to hit send, while I drew some pictures.


Last edited by veeany on Mon Feb 16, 2009 6:07 pm; edited 1 time in total
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PostPosted: Mon Feb 16, 2009 1:11 pm    Post subject: Reply with quote

I think you're pretty much on the case there Veeany. In all of this discussion I think you were the only one to bring in Cof G. I take my hat off to ya.
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PostPosted: Mon Feb 16, 2009 1:21 pm    Post subject: Reply with quote

I never even thought about vertical C of G! Be interesting to know where the vertical C of G position is...

Sarah

[edit]Edited to correct for momentary brain fart[/edit]
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PostPosted: Mon Feb 16, 2009 1:37 pm    Post subject: Reply with quote

Sarah

I am in the process of writing up all this stuff to try and stop us teaching some stuff thats been handed down to us by the hierarchy which is just untrue.

One of the articles I am working on is the importance of the Vertical C Of G, the C Of G position as a whole (in 3 axes) is so hugely important and yet gets overlooked in most teaching (probably because its now how we get taugh to teach things). It you know roughly where it lies in your aircraft you can explain all kind of things easily (tail rotor roll and drift, roll in a low G situation as a couple of examples).

The next thing I am going to write up is a look at the miconception that for a ROTOR BLADE as whole L= CL 1/2 rho V squared S, which a lot of people seem to believe in.

GS
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PostPosted: Mon Feb 16, 2009 1:42 pm    Post subject: Reply with quote

Quote:
miconception that for a ROTOR BLADE as whole L= CL 1/2 rho V squared S, which a lot of people seem to believe in.


Oh go on expand. Thats just a teasing comment.
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PostPosted: Mon Feb 16, 2009 5:51 pm    Post subject: Reply with quote

We did continue the lift stuff here but split it off after a few more posts cos I'd hijacked another thread again

The lift on a blade thread is here http://www.helitorque.com/portal/modules.php?name=Forums&file=viewtopic&t=3068&highlight=
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PostPosted: Tue Feb 17, 2009 9:32 pm    Post subject: Reply with quote

Just going back to the original topic, definitely rolls left with a momentary left pedal input. Tried it today just to check I wasn't going mad.

Sarah
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PostPosted: Tue Feb 17, 2009 9:45 pm    Post subject: Reply with quote

So, momentary roll left, then settles into right roll?
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PostPosted: Tue Feb 17, 2009 9:53 pm    Post subject: Reply with quote

Well not exactly, only because I leveled the aircraft - the bank angle got pretty steep pretty fast and because the nose also dropped I didn't really want to wait and see what happened next! Laughing Laughing

I suspect had I not moved the controls it would have just continued to roll / pitch down further. I feel like I'm auditioning to become a test pilot! Laughing

Sarah
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PostPosted: Tue Feb 17, 2009 9:59 pm    Post subject: Reply with quote

Well, that quite nicely settes that one, Cheers WG. Now don't get carried away with this test pilot business, but I've heard a 22 will fly inverted..... Twisted Evil
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PostPosted: Tue Feb 17, 2009 10:02 pm    Post subject: Reply with quote

Twisted Evil Bagsy not trying THAT one! Rolling Eyes

Sarah
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PostPosted: Thu Feb 19, 2009 9:29 pm    Post subject: Reply with quote

HW,

Just thought you'd like to know, after further experimentation that the helicopter rolls right on application of left pedal in forward flight if the cyclic is positioned in the direction of travel. i.e. you're flying towards a mast on the horizon but you're out of balance so you can see the mast at your two o'clock.

So you were right about that bit! But then it makes sense because you're compensating for the out of balance condition by applying more right cyclic than you would normally in balanced flight.

That's it now, my test pilot days are over... Laughing

Sarah
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PostPosted: Fri Feb 20, 2009 10:28 am    Post subject: Reply with quote

Oooo Sad And theres me waiting for your demonstration of rotor-streaming Twisted Evil
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PostPosted: Fri Feb 20, 2009 11:09 am    Post subject: Reply with quote

Thanks WG. Looking through my FI notes, I can't find any reference to demoing the secondary effects of yaw. So I guess I've always just compensated with cyclic.
That said, it's now something I am going to add to my future repertoire.
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