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HeliTorque :: View topic - Induced Flow in the Free Air Hover
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HeliTorque Forum Index » Flight Dynamics

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WhirlyGirl
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PostPosted: Wed Jul 05, 2006 6:28 pm    Post subject: Induced Flow in the Free Air Hover Reply with quote

Hi guys,

I'm sitting CPL helicopter principles of flight on Monday, and this dodgy question came up today which caused quite a debate in our POF class. I told my fellow studes I'd post it up on the forums to see what the experts say, so here goes...

Quote:
Q. In a free air hover how does Vi vary along the blade?

a. It is greater at the tip because of tip vortices
b. It is greater at the root because of the demarcation vortex
c. It is less at the tip because of tip vortices
d. It is less at the tip because of recirculation



I answered "a", which I found to be the only logical answer, from what I know about induced flow, but I was told this was wrong. I won't tell you which was the supposedly right answer to give you a chance to voice your opinions, but we had a lengthy discussion about it and couldn't agree with the one they had said was correct.

Help would be greatly appreciated!

WhirlyGirl Cool
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Last edited by WhirlyGirl on Tue Jul 11, 2006 3:38 pm; edited 1 time in total
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James T Lowe
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PostPosted: Thu Jul 06, 2006 7:19 am    Post subject: Reply with quote

vi? I thought that was a file editor.... Confused Wink
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PostPosted: Thu Jul 06, 2006 7:32 am    Post subject: Reply with quote

C Wink
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PostPosted: Thu Jul 06, 2006 8:32 am    Post subject: Reply with quote

Vi = Incuded Flow

Can anybody explain this?? Head Banging

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PostPosted: Thu Jul 06, 2006 9:04 am    Post subject: Reply with quote

Promise me a beer at WSM..... Wink

Velocity induced by the trailing vortices on the ith control point
(m.s-1)

velocities induced at a collocation point. the different components are the velocities induced by the near wake, by other tip vortices and the self-induced velocities.

I need a sit down now... Very Happy
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PostPosted: Thu Jul 06, 2006 9:30 am    Post subject: Reply with quote

Thomas Coupling wrote:
Promise me a beer at WSM..... Wink


That goes without saying...

But I though induced flow was greater at the rotor tip, and that vortices (or recirculation) add to induced flow, not reduce it.



Image courtesy of copters.com

Maybe I'm totally off track.

WhirlyGirl Confused
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PostPosted: Thu Jul 06, 2006 9:59 am    Post subject: Reply with quote

I'm with you WG - I would have gone for A.

TC:

Quote:
Velocity induced by the trailing vortices on the ith control point
(m.s-1)

velocities induced at a collocation point. the different components are the velocities induced by the near wake, by other tip vortices and the self-induced velocities.

Is that your reason for C being the correct answer? Can you repeat it in English please?
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PostPosted: Thu Jul 06, 2006 10:00 am    Post subject: Reply with quote

I found this page online which looks a bit more like it could back up answer C...

http://www.rnk.co.uk/himan/documents/Exercise_4.htm




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PostPosted: Thu Jul 06, 2006 10:02 am    Post subject: Reply with quote

In this diagram one would assume the vortices are reducing the induced flow, however this conflicts with the first diagram.

What's the bet that this question comes up in my exam? Twisted Evil

Just want to be clear in my mind of the answer and more importantly why it is the correct answer.

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PostPosted: Thu Jul 06, 2006 10:26 am    Post subject: Reply with quote

WhirlyGirl wrote:
What's the bet that this question comes up in my exam? Twisted Evil


I wouldn't focus too much on one question just now, annoying though it is. It is only a single possible mark and may not even show up. Your time would be better spent revising more possible questions with "resolved" answers - increasing your point scoring capability...

if it does crop up just mark the one you have been told is correct.

(Incidently did they not give you a reason as to why you were wrong?)
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PostPosted: Thu Jul 06, 2006 10:46 am    Post subject: Reply with quote

You're right, I just thought I'd ask on here becuase it's frustrating when you get stuck on something and can't explain it. The tutors available were all fixed wing (it is the interim course I'm on) so they didn't know a great deal about induced flow!

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PostPosted: Thu Jul 06, 2006 10:55 am    Post subject: Reply with quote

I completely understand your frustration - I'm the same!

Having looked it up in Wagtendonk, pg. 31(which I am led to believe is the publication all the PoF questions are based on) I would stick with your instincts and go for A until someone gives you a good reason as to why that is not the correct answer.

But then you're on the interim course and the may be a different question pool?

I'm definitely not helping any more so I'll just shut up...

Good luck!
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PostPosted: Thu Jul 06, 2006 11:20 am    Post subject: Reply with quote

Intentionally Blank wrote:
I'm definitely not helping any more so I'll just shut up...


Not at all! It's good to have someone to yarn to! Very Happy

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PostPosted: Thu Jul 06, 2006 12:51 pm    Post subject: Reply with quote

Purely what I understood... Isn't Recirculation higher speed airflow so the efficiency of lift is reduced since the air is already travelling faster as it becomes available to the m/r so it's less 'use'?

Open to correction I know, but feel free..

FF

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PostPosted: Fri Jul 07, 2006 4:36 pm    Post subject: Reply with quote

If you're stuck on a question, the technique is to choose the answer with the most words (it works about 85% of the time!), but I would have chosen c) myself.

Phil
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