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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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rotor-rookie
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PostPosted: Tue Dec 16, 2008 9:44 am    Post subject: Secondary Effects of Yaw - Why? Reply with quote

Hi,

I came across the forum on google - hope you can help me. I'm studying for my POF exam (PPL) and I think I'm doing ok but I just can't understand why the secondary effects of yaw in forward flight are that the helicopter rolls and then the nose pitches down.

It's just been bugging me and I can't seem to find a clear explanation anywhere. Can anyone explain it to me?

Cheers,

rotor-rookie
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PostPosted: Tue Dec 16, 2008 10:40 am    Post subject: Reply with quote

Hey rotor-rookie welcome to the forum, later on some of the more experienced pilots will put a more concise (is that right english FF?) explanation than i could so i'll leave it to them.
Very cool how it all works though, you'll love it when you get it, shows how independent our limbs have to work in order to be a helicopter pilot. Surprised
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PostPosted: Tue Dec 16, 2008 2:25 pm    Post subject: Re: Secondary Effects of Yaw - Why? Reply with quote

rotor-rookie wrote:
.........yaw in forward flight are that the helicopter rolls and then the nose pitches down. It's just been bugging me........


It should bug you since the nose pops UP and rolls RIGHT in a counter rotating system.

I'll try to give you a "bare bones, before morning coffee" explanation. Laughing

Three aerodynamic effects are in play: I'll give you both US and UK designations:
Traverse Flow (US) or Inflow Roll (UK)
Effective Translational Lift and Gyroscopic Precession (same)

As the copter moves forward to about 20kts, additional "air" flows over the advancing blade creating more lift BUT due to Gyroscopic Precession, the "effect" is not felt until the blade has rotated 90 degrees. Therefore, the nose pops up and you need to hold the cyclic forward to "fly through" the Effective Translational Lift. As speed increases, the effect of ETL is negligible.
Think of it like a clock: Increased lift at 3 but not felt until at 12
NOTE: Translational Lift ( ETL) can even occur while in a stationary hover if the wind is blowing at about 15 kts!

Secondly, the airflow pattern over the front half of the rotor disc plane changes in forward flight and more lift is created from "9 to 3". This is the result of the angle of attack increasing between the relative wind and pitch angle on the blades, and the increase of air flowing "through" the front half of the rotor disc. Simply put, the advancing air gets to the front half of the rotor disc before the back half. You lose the vertical column of air (induced airflow) and take on horizontal air (relative wind)

Again, since more lift is created over the front half, the "effect" is not felt for 90 degrees. Therefore the copter rolls right (counter-rotating system).
Think of it like a clock: Greater lift from 9 to 3, but not felt until at 6 to 12.

That is the short explanation. You can look up these aerodynamic effects for a more detailed one with nice color pictures, and cute vector diagrams.
A more detailed explanation would require talking about "blade flapping".
For that, I need a pot of coffee.

Hope this makes sense to you, (and me). I'm useless before coffee in the morning. (it is morning in the US).

Helicopter
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PostPosted: Tue Dec 16, 2008 4:34 pm    Post subject: Reply with quote

rr - a good book that helped me was 'the principles of helicopter flight' by Norman Bailey if you fancy a read Very Happy

this is it on amazon

http://www.amazon.co.uk/Helicopter-Pilots-Manual-Principles-Techniques/dp/185310759X/ref=pd_sim_b_4/278-0578149-8961959
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PostPosted: Thu Dec 18, 2008 7:50 pm    Post subject: Reply with quote

Hi everyone,

Thank you for the replys. I wasn't talking about Inflow Roll actually (alhtough I have read about that too), what I meant was when I was shown something in effects of controls where the instructor put the helicopter out of balance, e.g. left pedal without correcting on the other controls to show me the secondary effects, which were that it rolled to the left and the nose dropped as a result.

My question was, why does this happen? What is the aerodynamic reason for it to roll as result of applying the pedal. I forgot to ask at the time and I haven't flown for a couple of weeks so just thought I'd ask on here before I forget about it. Sorry I wasn't that clear about the question to start with.

While we're on the subject, what is the reason for the nose trying to come up as you increase power in forward flight? Is it that there's more flapback with more pitch on the blades, or is it the downwash pushing down harder on the tail? Or is there another reason that this happens?

There are so many effects! I just want to understand why they all happen!!!

Thank you,

rotor-rookie
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PostPosted: Thu Dec 18, 2008 8:05 pm    Post subject: Reply with quote

Putting a helicopter of of balance in forward flight simply increases drag on the fuselage

Left pedal, nose left, right roll and vice versa. As for nose pitching down, well I would say that's just an effect of the drag increase and being out of balance.

Many good PofF books out there, just takes time finding one thats suits your learning style best.

Happy Days, HH
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PostPosted: Sat Feb 14, 2009 11:49 am    Post subject: Reply with quote

OK, a couple of issues here I feel.
As we accelerate, we get flapback occuring. To overcome this we move the cyclic further forward just to maintain the disk at the current attitude.
If we now push left pedal the aircraft will roll RIGHT due to the the fusilage being draged sideways through the air (hence the ball falling over to right side).
Now we are travelling slightly sideways through the air, the airflow (flapback) is coming slightly from the side, so not as much of our original forward cyclic correction is required. If we do not remove this forward correction, the disk flaps forward - nose goes down.

Now, nose up with increase in power. People argue this point a lot. Theres no doubt that the extra down wash pushes on the horizontal stabilizer, and that must contribute a certain amount.

Heres the controversial theory:

[/list][/code]
Cl = Coefficient of Lift
p = rho (density)
V = velocity
s = surface area of blade

As the air density is constant in our example, as is the surface area and the half, we can replace them with k (constant).

If V = 3, then the advancing blade is 4(V+1), the retreating blade is 2(V-1). So Cl needs to be adjusted accordingly around the disk plane to evn the lift on both sides of disk. (ie cyclic pitch change).
However, if Cl is increased by 1 (collective pitch change) then the advancing side is gaining lift x 4, but the retreating side is gaining lift x 2. Now we are in a flapback situation, and therefore nose pitch up.

Cat thrown decidedly amongst pidgeons, thrower ducks and waits for feathers..........
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PostPosted: Sat Feb 14, 2009 8:08 pm    Post subject: Reply with quote

Hello heliwhore,

Thanks for your reply. Would the helicopter not roll left with left pedal? I'm sure it did this when my instructor demonstrated it. Helicopter was out of balance and I've asked him since and he said that while we were flying along out of balance (i.e. airflow on the right), the advancing blade reaches it's highest point at the aircraft's 3 o clock position making the right hand side of the aircraft pitch up and therefore causing it to roll left. Don't know if that makes sense...

But your reason for the nose dropping makes a lot of sense.

Thank you,

rotor-rookie
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PostPosted: Sat Feb 14, 2009 8:56 pm    Post subject: Reply with quote

If the aircraft is travelling very slowly (I have no official speed here) perhaps 20kt or less, and you jammed the other controls when you put in left pedal, then you might once again get flapback from the side which might roll you left. I think this is the only way I could explain your instructors reasoning.
As the drag on the airframe is so great at higher speeds, it overcomes this effect and basically drags the fusilage behind the rotor system. Hence, as I said previously, the aircraft rolls right and the ball falls over to the right. It can't be otherwise I'm afraid Wink
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PostPosted: Sun Feb 15, 2009 11:15 am    Post subject: Reply with quote

In the demonstration he was freezing the other controls as it was ex 4 and he was just trying to show me the secondary effects of yaw. It definitely rolled to the left with left pedal.

With the right roll you describe, as you apply the left pedal what are you doing with the other controls? Just trying to understand your explanation and why it would go right. The drag thing makes sense I just need to picture what the cyclic is doing at the time you apply the pedal.

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PostPosted: Sun Feb 15, 2009 11:48 am    Post subject: Reply with quote

I'm not sure how much more I can offer here. I also don't know how your instructor achieved this either.

If you are doing this display and just want that aircraft to keep tracking where you want to go, then you use a little right cyclic.

If you are locking the controls, then the aircraft still rolls right, although not as much, but the aircraft does a slow slipping turn to the left. When you think about how we use the ball to detirmine weather we are in balance, if you have too much left pedal in, the ball is on the right. The ball is only a spirit level(in reverse) and orienting itself to the vertical. The only way to keep ball in centre at this point would be to move cyclic left, and to roll left, you need cyclic further left.

Once again, the only other way I can think of doing as you say would be to do the manouvre at low speed. This would be especially true if you're flying something with a very low hanging tail rotor, then we are getting into the same realms as the tail rotor roll that we talked about else where.

You still haven't said what speed this was demo'd at, as it could just be flapback from the side as said previously.....

I would love to hear from any other instructors on this....
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PostPosted: Sun Feb 15, 2009 12:01 pm    Post subject: Reply with quote

I am wih HW, I can't see how he managed to roll left with left pedal
without other control imputs. Did he demo it whilst initiating a descent out of interest?

HH
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PostPosted: Sun Feb 15, 2009 12:07 pm    Post subject: Reply with quote

He was flying at 70 knots, in balance. He applied left pedal, didn't move the cyclic from the forward position (as far as I know), ball went out to the far right, air speed indicator dropped to zero (think this is because the pitot tube was no longer into the airflow) and the aircraft rolled left (quite significantly) and the nose dropped.

Maybe I should ask him to show me again so I can make sure I have got this right.

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PostPosted: Sun Feb 15, 2009 12:21 pm    Post subject: Reply with quote

Honestly no idea, never achieved it, anyone? ? ?
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PostPosted: Sun Feb 15, 2009 3:58 pm    Post subject: Reply with quote

Errrrm i'm a little confused ... i agree with HW and HH drag of the fuselage would cause right roll. errr... what about as you turn the tail, the disc that was dipped into the oncoming 70kt wind is now (if cyclic kept as was) dipped to the left (the way the heli is now pointing) allowing wind to flap disc up causing left roll!!!

ok i'm clutching at straws here - it's just a thought???

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Last edited by animalsticks on Sun Feb 15, 2009 4:05 pm; edited 1 time in total
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