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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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WhirlyGirl
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PostPosted: Fri Feb 20, 2009 11:51 am    Post subject: Reply with quote

Hehe, ok.

Erm, rotor streaming. No thanks!

Sarah
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PostPosted: Fri Feb 20, 2009 1:03 pm    Post subject: Reply with quote

ok, I googled 'rotor streaming' and I'm none the wiser...
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PostPosted: Fri Feb 20, 2009 1:34 pm    Post subject: Reply with quote

It's a term I've heard from an experienced examiner regarding a test pilot who was sent up to about 10,000' to see what happens if you allow vortex ring to develop beyond the incipient stage! All I can say is it sounded very scary.

I don't know much more about it than that.

Sarah
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PostPosted: Fri Feb 20, 2009 10:23 pm    Post subject: Reply with quote

Vortex Ring discussion continued on another thread, as requested!

Sarah
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PostPosted: Fri Feb 20, 2009 10:30 pm    Post subject: Reply with quote

Well, today on approach to Sola in the 92 I flew slightly out of balance.

Left pedel = left roll and vice versa. I can only think that the "opposite roll" would come from when you push (for example) left pedal in and to keep the same track right cyclic is added, creating a right roll.

HH
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PostPosted: Fri Feb 20, 2009 10:34 pm    Post subject: Reply with quote

Haggis,

That's exactly what I tried the other day - on the previous page I posted that by keeping on a certain track and flying out of balance the direction of roll is indeed opposite due to the excessive amount of opposite (to pedal) cyclic applied to keep flying in said desired direction.

The pure left pedal input without any cyclic correction does indeed result in a roll in the same direction as the yaw. Interesting topic this, I have enjoyed the comments and it just goes to show how we overlook these simple principles when we fly every day (in balance!!). It was also good to hear the examiner comments on what makes this happen, which was the original poster's question.

Thumbs Up

Sarah
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PostPosted: Fri Feb 20, 2009 10:52 pm    Post subject: Reply with quote

I may not have seen it as pronounced in the 92 as other helicopters due to the tilted tail rotor, left pedal also causes nose down attitude and right pedal nose. With left pedal and nose down it would probably fall into the turn easier then to the right.

I think natural habit is to apply lateral cyclic, as you said to maintain track.

I am liking this topic!!!

HH
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PostPosted: Sat Feb 21, 2009 8:43 pm    Post subject: Reply with quote

Ok, let's take it a step further. I have actually noticed that when I demo this to the left the left roll is more pronounced than when I do it to the right. Might it have something to do with the direction the blades are rotating? Or is it maybe more to do with the horizontal stabiliser being on the right? Or perhaps that the tail rotor is mounted on the left hand side?

My brain is not working well this evening (as WhirlyGuy has discovered as I dished out the lovely dinner he'd cooked onto the wrong plates, and poured the wrong drink out into wrong glasses!) but I'm thinking perhaps as the aircraft rolls left the airflow now pushing underneath the horizontal stabilizer because the helicotper is out of balance might increase the rate of roll? I'm probably talking utter b0ll0x and inventing new principles of flight here. Just having one of those "I wonder why that happens" moments!

Perhaps I'm just enjoying this thread way too much and want to find an excuse to continue discussing it! Laughing

Sarah
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PostPosted: Sun Feb 22, 2009 10:54 am    Post subject: Reply with quote

I don't know the answer, but in my usual style, I'm willing to have a go.

Theory 1:

I've not taken much notice of how high the tail rotor sits on a 22 at 60-70kt. But the horizontal stabiliser is very good and tends to keep the tail down (ie level cockpit during flight, unlike the 300).

I THINK that the tail rotor is below the level of the main rotor in flight, and so we would still be getting the "tail rotor roll effect" we were talking about previously, although not as pronounced as we have now eased off on left pedal.

If we boot a load of left pedal then it would add to the tail rotor thrust, and therefore just add to the roll. If we boot a load of right pedal, we are not using as much thrust as we are not counteracting torque so the roll would not be as much.

Theory 2:

When the nose pitches down, it raises the tail surfaces above the C of G and so creates more of a lever arm to roll us left.

Does the nose pitch down as much when you use right pedal?

If not, then we are just not getting the same lever arm as the tail has not risen so high.

I prefer theory 2 myself, but requires knowing if the nose pitches as much or nae......
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PostPosted: Sun Feb 22, 2009 11:18 am    Post subject: Reply with quote

I agree with theory one. You are in the cruise with a good chunk of left cyclic in because of tail rotor roll.

HH
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PostPosted: Sun Feb 22, 2009 11:26 am    Post subject: Reply with quote

Hmmm, problem there is that we have actually moved cyclic right as we increase speed, counter-acting the effects of left roll (thats another thread for some time later). But I agree that we must be opposing some sort of rotor thrust as we have not got a cambered vert stabiliser like the 44 etc.
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PostPosted: Sun Feb 22, 2009 11:38 am    Post subject: Reply with quote

With regards to airframe tilt in forward flight, unless the airframe changes its shape the relationship between the point of application of the force(s) generated by the tail rotor and the main rotor about the center of gravity must remain unchanged.

What changes is the magntude of those forces but they do not move around the center of gravity they remain fixed in relation to it, so their moment arms remain the same and its the moments that change due to the change in the size of the force.

The magnitude of the forces are controlled in essence by the flying controls and what the air around the helicopter is doing at that moment in time.
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PostPosted: Sun Feb 22, 2009 1:23 pm    Post subject: Reply with quote

Quote:
With regards to airframe tilt in forward flight, unless the airframe changes its shape the relationship between the point of application of the force(s) generated by the tail rotor and the main rotor about the center of gravity must remain unchanged.


Absolutely, but by applying a new force to the front of the airframe ( airflow/drag ) we have effectively changed the C of G, as it is no different than adding mass, and this C of G will vary according to the airspeed at the time. So I would now argue that we have changed the moment simply by changing its point of origin.
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PostPosted: Sun Feb 22, 2009 1:51 pm    Post subject: Reply with quote

I think we must get away from the idea that the C of G moves unless it really does, the C of G is so fundamentally important to how all the forces interact it must be considered incontrovertible.

What you have done is added a resultant force which will have a point of application (as yet undetermined).

I don't know where the concept came from that the C of G moves within the aircraft because of an applied force but I've heard it from a lot of instructors over the years and yet it is just untrue.

I can however see why someone would use it to perhaps make an explanation work, but it makes life much more complicated when trying to look at multiple forces.

It also then potentially confuses the issue that mass is a force (which at first glance it looks like it is).
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PostPosted: Sun Feb 22, 2009 8:17 pm    Post subject: Reply with quote

In my defence, I've never actually heard anyone teaching that the CofG changes, so this is lunacy derived by myself.

I had a very long blurb here which I have deleted after thinking it through.

My point was that it had effectively changed the CofG, which was as you pointed out Embarassed technically incorrect(edit: instead we can refer to it as the centre of force). By adding another (resultant) force, surely we have still achieved the same end ie altered the point at which the moment originates.

......and no, mass isn't a force......it just illustrated my argument well.
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