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HeliTorque :: View topic - NOTAR - High or Low Pressure?
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HeliTorque Forum Index » Flight Dynamics

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NOTAR - High or Low Pressure?
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WhirlyGirl
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PostPosted: Thu Jul 06, 2006 11:33 am    Post subject: NOTAR - High or Low Pressure? Reply with quote

This one's for you SS (and anyone else who flies a NOTAR)...

Would anyone mind explaining whether a NOTAR uses high or low pressure air ducts to balance fuselage torque reaction? I understand that the propulsion is through high velocity jet flows, but a question I came across asked about the pressure, and I can't find a mention of that anywhere. The answer said high, but a few people I have spoken to would be inclined to disagree.

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SilsoeSid
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PostPosted: Fri Jul 07, 2006 12:18 am    Post subject: Reply with quote

WG,

You couldn't have been listening when I showed you around Wink
Low pressure, high volume.

On lifting into the hover, left pedal is applied. Not only does this move the 'drum' to point to the left, this yaw input also increases the pitch on the Notar blades, (between the engines) thereby increasing the volume of air within the tail boom.



As the flow of air downward from the blades is increased, the air coming out of the 2 slots that run along the right hand side of the tail boom, shapes this passing air resulting in the coanda effect. (Boundary layer attachment) This produces an aerofoil shaping of air around the tailboom with the low pressure area to the right, higher to the left, resulting in an anti torque force yawing to the left.




Hope thats's what you were after.

Low pressure, high volume.
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PostPosted: Fri Jul 07, 2006 6:42 am    Post subject: Reply with quote

Thanks SS,

I actually went with my instrincts and put low pressure in the test (so I WAS listening to your briefing SS, which was incidentally about 2 years ago now! Wink ), but it seems that the school marked it wrong anyway!! I was a little confused by my the result which is why I posted the question here. The CPL tutors where I'm doing this interim course are all fixed wing so they didn't even realise it was a dodgy answer on their mark sheet. Sad

When we had the helicopter tutor in, he told us it WAS low pressure and that the feedback question was in fact wrong, but I thought I'd get a third opinion just to confirm. Oh it's so frustrating - why can't they just get it right. I only hope the CAA have corrected that one, becuase if it comes up in my exam I am sticking to my answer, low pressure.

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PostPosted: Fri Jul 07, 2006 4:53 pm    Post subject: Reply with quote

Wot's the actual question? Didn't you read that stuff I sent you? Smile

Tangential slots, which are located at 70 and 140 degrees, inject low-pressure air at 250 fps onto the outer surface of the tailboom. This produces about two-thirds of the required anti torque force.

cheers

phil
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PostPosted: Fri Jul 07, 2006 5:52 pm    Post subject: Reply with quote

Hi Phil,

Yes, thanks Phil - your material is excellent. I did promise though to post this on the forums all the same, following a day at ground school this week when we were told that the answer was invact high pressure (according to a feedback question), and to highlight the fact that there is a dodgy question in floating around, which I was told today COULD actually be in the exam! I was advised to appeal if this one comes up, and I suggest anyone else who comes across it does the same.

Quote:
The hughes NOTAR balances fuselage torque reaction by:

a. Using a cambered tail fin.
b. Hydraulic assistance.
c. Low presure air ducts.
d. High pressure air ducts.


It's quite clearly answer C, as everything but when the assement came back marked it stated D. I think it's outrageous that something like this would even be in the question bank! And perhaps they should update that to an MDH NOTAR while they're at it...

WhirlyGirl Cool
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TOT
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PostPosted: Fri Jul 07, 2006 9:22 pm    Post subject: notar Reply with quote

from the company

:It's definetely low pressure in the tail boom. only around 2 psi. It was designed that way to reduce risk incase of a loss of pressure. If it was a high pressure system the boom and subsequant warning systems would have to be redesigned.
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PostPosted: Fri Jul 07, 2006 9:58 pm    Post subject: Reply with quote

Hi TOT,

Nice to see you on the forums. Wink

The thing is, I was told today that there is a reasonable probability that a question like this will come up in the exam as it's part of the question bank. I will answer "low pressure" no matter what, as that is the correct answer, but it would be rather disappointing if they marked it wrong.

According to the school they regularly have students appealing because there are a number of bad questions in the system - it's worrying that this happens at all! I think the questions should be checked to make sure they are all correct in the first place!

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PostPosted: Sat Jul 08, 2006 2:34 am    Post subject: Reply with quote

If it's any help, two of the people involved in the question bank came to see me at the Aero Expo and they are aware that there are problems. I've emailed them on that question. The questions and learning objectives now don't have to go through committee and are very much more flexible for change - those schools that rely on feedback take note! Interestingly, one of them is a slight acquaintance from the AAC. I think there will be an improvement soon, not that that helps people taking their exams in the near future.

I am making a push to get the wrong answers returned to you for review, like they do in Canada.

Is it from the JAA question bank or the school's? And are the school sure they have the right answer?

Phil
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PostPosted: Sat Jul 08, 2006 7:35 am    Post subject: Reply with quote

paco wrote:
I am making a push to get the wrong answers returned to you for review, like they do in Canada.


Now that's a good idea. I know a lot of people don't care what they got wrong once they've passed, but I would have actually been interested to know where I went wrong so I could at least learn from it.

paco wrote:
Is it from the JAA question bank or the school's? And are the school sure they have the right answer?


It's was a question in the school's feedback paper. I don't think it's a JAA one - the school had it down as "high pressure", but when the heli tutor came in he told them it was definitely low. So whether that means it's only wrong on their master sheet or whether it's wrong in the actual question bank I don't know.

Thanks for your help all!

WhirlyGirl Cool
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James T Lowe
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PostPosted: Sat Jul 08, 2006 1:27 pm    Post subject: Re: notar Reply with quote

TOT wrote:
:It's definetely low pressure in the tail boom. only around 2 psi. It was designed that way to reduce risk incase of a loss of pressure. If it was a high pressure system the boom and subsequant warning systems would have to be redesigned.


Just thinking as well, and I've virtually no knowledge of this, other than SilsoeSid's kind explanations!

If the system were high-pressure, it'd compromise safety too - surely one of the ideas of NOTAR is to reduce the risk to personnel around the rear of the helicopter from tail rotors. Instead replace it with high pressure air jets/flows - yeah right. Great idea guys... !! Wink Very Happy
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PostPosted: Wed Jul 19, 2006 2:35 pm    Post subject: Reply with quote

The question is not in the central database, but the learning objectives have now been adjusted to reflect the problems with this question.

phil
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PostPosted: Thu Dec 07, 2006 10:51 pm    Post subject: Reply with quote

here is a neat picture of the coanda effect using a faucet and spoon
http://upload.wikimedia.org/wikipedia/en/thumb/7/75/Coanda_Spoon.jpg/367px-Coanda_Spoon.jpg
somehow after i saw this picture it all made sense.
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