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Old May 02, 2011, 09:49 AM
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Question
Zipper CG using PW51

I'm rebuilding a couple of my older BirdWorks Zippers. I've cut new cores with the PW51 airfoil. What is the suggested CG for this wing. The span of the wing is 50" and I'm using the same planform as the original EH airfoil that Steve Hendrix used to use.

Thanks,
Ed
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Old May 03, 2011, 04:39 AM
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The CG is 40mm from the LE at the root.
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Old May 03, 2011, 07:51 AM
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Ed,
Also airfoil has nothing to do with balance. You may be interested in some of my Excel spreadsheet calculators and an accompanying article called "Sailplane Calc" that I wrote for RCSD that touches on this a little.

You can find this at www.TailwindGliders.com

Just some free aerodynamic advice.

Curtis
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Old Mar 24, 2015, 04:50 PM
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Quote:
Originally Posted by Curtis Suter View Post
Ed,
Also airfoil has nothing to do with balance. You may be interested in some of my Excel spreadsheet calculators and an accompanying article called "Sailplane Calc" that I wrote for RCSD that touches on this a little.

You can find this at www.TailwindGliders.com

Just some free aerodynamic advice.

Curtis
So, you would CG a PW51 flying wing at the same balance point as a tailed aircraft with an RG15 foil?
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Old Mar 24, 2015, 06:17 PM
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Wow! That thread has whiskers on it but I do remember the reply to my post. I never responded to the sender because his reply struck me as being so off base that I saw no reason to question what seemed like a very strange assertion.. I am not sure what qualifications the responder has but I believe he is simply wrong. My own experience with various airfoils contradicts his statement.

For example, an airfoil that Bill Evans used on his Simitar series has the CG at 10 percent of Mac. The good old Clark Y CG would not fly at the Evans CG location.

When you get advice on the Internet, you never know if the advice is from a qualified person.

Ed
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Old Mar 24, 2015, 06:38 PM
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Well in any event I had good reason for unearthing this thread - I've finally gotten together that PW51 Zipper wing and fuse and am about ready to CG it! Has the 40mm CG served you well on the 48" PW51 wing, at least to start?
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Old Mar 24, 2015, 07:05 PM
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Sorry to report that it's been too long for me to remember but that sounds like a good start.
Ed
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Old Mar 24, 2015, 07:37 PM
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Thanks-
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Old Mar 24, 2015, 09:36 PM
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Research the term static margin and how it is derived. Yes, even a flying wing has a static margin.
Certainly don't take my word for the fact that an airfoil doesn't make a significant difference in Balance point. Purchase this must have book http://www.amazon.com/Model-Aircraft.../dp/1854861905

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Old Mar 24, 2015, 09:38 PM
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Quote:
Originally Posted by screamin' eagle View Post
So, you would CG a PW51 flying wing at the same balance point as a tailed aircraft with an RG15 foil?
Of course not, but I would use a similar static margin.

I never balance my models using only the MAC for a tailed airplane, but you could in a flying wing.

Static Margin is the answer.

Curtis
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Old Mar 24, 2015, 09:38 PM
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Zipper who stated the CG actually own the rights and produces the Zipper with the PW51 and the original Eppel section if you really wanted it ( horrible trim issues though) Short answer CG is spot on
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Old Mar 25, 2015, 06:10 AM
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Everybody's flying stile is different and unless the wing is made in a mould, they are never exactly the same. So 40mm is a good starting point that suits most people.
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Old Mar 25, 2015, 07:07 AM
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Theoretically Curtis is right, the CG will not depend on the airfoil, just on the geometry and the chosen static margin.
But at lower Re the neutral point - on a plank at 25% chord - where the pitching moment should be constant, is not! like the theory points out, constant.
This is important for flying planks, which typically fly at very low static margins around 4% and flying with flap deflection all the time. I did some simulation in Xfoil on the PW51 and some real measurements (flap deflection , CG) from planks with the PW51 and it seems to me, that the neutral point is more likely to be at around 23,5% - as far as I remember - for the PW 51 and higher Re´s. Another result was, that the static margin seems not to be constant during flight as the neutral point moves with different Re and flap deflection necessary to trim the plank´s speed......so the CG´s actually flown are a kind of compromise having enough stability in the various regions (Re) during flight - sometimes more, sometimes less. If you leave the typical flight regime, like in DS, you will probably have to use another CG, than on the slope.
It was one of the design goals for the PW 51 to reduce this arbitrary behaviour, which is one of the big differences to the EH 2/9.0 airfoil.
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Old Mar 25, 2015, 10:39 AM
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Peter,

Thanks for chiming in. BTW my aeronautical experience is as a full scale pilot and what I've read over the last twenty or more years in the books I've purchased on aerodynamics and some spreadsheets I've written to make these complex things easier. Nothing as awesome as you've done!
I recall that the pitching moment is 25% plus or minus 2% but the 25% is close enough for a test flight then actual fine tuning of the balance point is always preferred with actual flight testing. The idea is not the theorectical info other than to get the model in the air safely during it's initial flight.

Curtis
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Old Mar 25, 2015, 01:16 PM
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Quote:
Originally Posted by EBerris View Post
...................... I am not sure what qualifications the responder has but I believe he is simply wrong. My own experience with various airfoils contradicts his statement.
If you are referring to the reply from Zipper, he is about at qualified as qualifed gets. And your own experience you refer to, was that with planks?
Quote:
For example, an airfoil that Bill Evans used on his Simitar series has the CG at 10 percent of Mac.
Are you sure of this? I have never heard of a plane flying with such a radically forward CG postion. Typographical error perhaps?
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When you get advice on the Internet, you never know if the advice is from a qualified person.
Ed
That is true. However, with some time and experience on these forums, one can accumulate an list of reliable and experienced designer/builders. It enables you to sort the gold from the dross.

If you would like to do some reading on the PW51 airfoil, there are a couple of extensive threads here on these forums. Just Google the name. In my own experience, I have adopted the habit of bench setting them up @ 21% MAC. Most often, they fly off the bench settings.
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