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HeliTorque :: View topic - Lift Formula Debate
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HeliTorque Forum Index » Flight Dynamics

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WhirlyGirl
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PostPosted: Mon Feb 16, 2009 5:46 pm    Post subject: Reply with quote

Consider it split! You might wish to find a more appropriate title...

Discuss away Wink

Sarah
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PostPosted: Mon Feb 16, 2009 5:50 pm    Post subject: Reply with quote

WhirlyGirl wrote:
Quote:
Loving to hear someone giving the whole gyroscopic precession in rotor systems a hard time, hallelujah. (at least I think thats where you're going).


I'm looking forward to that one... Twisted Evil Twisted Evil Twisted Evil

Sarah


Oh, I'm not! Vee is a tough customer.

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veeany
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PostPosted: Mon Feb 16, 2009 7:00 pm    Post subject: Reply with quote

Ladies and Gents

An open invite to criticise the diagram stuff, it will only be any good if its right, if you disagree or think of a better way to present some of it please be open it is my intent to be able to use it a resource for training for all who want to use it, and a lot of work is going into it, I'd sooner someone tore it to pieces now and I am a big boy so I won't get offended.

I am also more than happy to explain how bits work if it looks wrong.

GS
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PostPosted: Wed Feb 18, 2009 5:48 pm    Post subject: Reply with quote

This is an interesting discussion on the lift formula debate. J. Gordon Leishman has a detailed discussion on rotorcraft theory in his book Principles of Helicopter Aerodynamics. It’s an engineering text book for those looking for formulas beyond L = CL * rho * V^2 * S/2. However, for flight instruction I think the basic lift equation will suffice. A bunch of calculus and differential equations would scare the average student away. Hell, around here it would scare the average CFI away. They wouldn’t have a clue about derivatives or integrals.
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PostPosted: Thu Feb 19, 2009 2:00 am    Post subject: Reply with quote

Are you going to allow Isaac Newton to have a say in this one? Deflecting air downwards pushes the wing/ blade upwards and backwards.

The old lift equation doesn't work too well when you see that a flat plate can fly, or that your flat-blade ceiling fan can push air around.
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veeany
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PostPosted: Thu Feb 19, 2009 8:35 am    Post subject: Reply with quote

AC

In the words of NASA no, thats one of the the wrong lift theories according the Glenn Research Centre at NASA http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html

The 2 Dimensional Lift formula holds just fine for any shape of object at a given reynolds no. its just CD and CL that change.

The inflow angle and its effect on angle of attack will bring the effect of downwash into the result.

Its the equal transit time theory that doesn't hold flat plates or symmetrical airfoils.

Bear in mind that I don't think we've even started on the reasons why aerodynamic forces are produced yet, just the use of the lift formula.
How the net aerodynamic force gets produced is usually a surprise to most pilots.

And I do think that what we are talking about is too much for PPL teaching but handy to know about for instructors who get asked awkward questions.
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animalsticks
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PostPosted: Thu Feb 19, 2009 12:56 pm    Post subject: Reply with quote

this is great stuff

hey vee how about lining diagrams up of a cross-section of blade showing longer and shorter arrows to demonstrate what the graph is describing.
should show angle of attack changes relative to airflow pressures along the length of the blade and corresponding lift achieved - feather away Very Happy
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veeany
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PostPosted: Thu Feb 19, 2009 1:17 pm    Post subject: Reply with quote

Animal

Just clear up for me what you are after.

I don't catch on quick to start with !
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PostPosted: Thu Feb 19, 2009 1:59 pm    Post subject: Reply with quote

you mentioned possible ways of making it easier to understand for peeps like me - i thought perhaps accompanying pictures showing your findings 'as it really happens' on the blade.
i guess these would have to be drawn unless your cameras pretty quick!


Maybe cross section diagrams under the results on the graphs that adjust accordingly - could be difficult to make the pictures adjust to the results though,
i'm thinking like the pictures in the training manuals that explain raf, lift, drag, using a slice of the wing/blade and longer and shorter arrows for bigger and smaller results,

not sure if this is a clearer explanation?
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veeany
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PostPosted: Thu Feb 19, 2009 2:08 pm    Post subject: Reply with quote

You mean like the one at the top of the page ?

I do need to put some short narratives with each one to explain what is what, it is easy for me to forget that because I wrote it I can read it , whereas at the moment it is just a collection of arrows !
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PostPosted: Thu Feb 19, 2009 2:19 pm    Post subject: Reply with quote

yeh, only using diagrams that show the results in the graph plotted on a blade so a student can visualise what is actually happening to a blade without having to interpret the results in their head - just a thought Very Happy
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PostPosted: Thu Feb 19, 2009 2:31 pm    Post subject: Reply with quote

How about I make each blade force diagram a link to a more detailed

Heres the forces, and here is a vector diagram for the 0, 1/4, 1/2 and 1 Radius positions kind of page ?
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PostPosted: Thu Feb 19, 2009 2:37 pm    Post subject: Reply with quote

yeh exactly, a kind of step between ppl learning and the stuff you guys play with Very Happy
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PostPosted: Thu Feb 19, 2009 11:27 pm    Post subject: Reply with quote

Veeany,

That's a great NASA illustration, I will use it with my students.

But I wasn't referring to a skipping stone concept, more in the line of there being a net downward deflection of air by the airfoil, so there has to be a net upward reaction on the wing. Instead of referring to lower upper surface pressure and higher pressure on the underside pushing the wing up, it is simply a Newtonian reaction. Same with the way a spinning ball deflects air downward or sideward in flight to get a curved flight path - though now we are getting Coanda into the debate.
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PostPosted: Fri Feb 20, 2009 8:34 am    Post subject: Reply with quote

AC

Yes now we are getting into Coanda, and viscous effects. If its alright with you I think we'll split the how does lift get produced into another thread which I'll start now.

http://www.helitorque.com/portal/modules.php?name=Forums&file=viewtopic&p=141980#141980

If we decide it more appropriate back here I am sure Sarah can reassemble us at some point.

But to answer what I think you are getting at here, Its still taken into account by the traditional lift formula (but is hidden from view) by the magnitude of CL and CD which are different for the same angle of attack at different reynolds numbers, they reynolds number is what brings in the viscosity to the result.

The CL/CD diagrams I've produced here are for a Reynolds no. of 1,000,000

Gary
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