RE: Re: Formula for percentage arcs anyone?

From: <leop_at_lyradev.com>
Date: 31 Oct 2013 20:47:51 -0700

Mike,
  
 John Barker's formula for the radius of an arc from the chord and camber height is correct:
  
 r = 0.5(c^2/(4h) + h)
  
 Another way of stating the formula is:
  
 r = c^2/(8h) + h/2
  
 I like to use h/c, or the "percentage arc" times 100 to get:
  
 r = (c/2)( 1/(4(h/c)) + (h/c) = 0.5c(.25/(h/c) + (h/c))
  
 If anyone wants to see the simple derivation, let me know (off list or whatever) and I will send it out.
  
 LeoP
  
  
 

---In Indoor_Construction_at_yahoogroups.com, <indoor_construction_at_yahoogroups.com> wrote:

 Hi John.
 
 Are you sure this formula is correct?
 
 I used this to convert Bill Gowen's A6 plan ribs to arcs.
 They convert very closely to 6% and 3% for wing/stab.
 
 I had misplaced my 6% template and made one up after printing it out in Profili. Immediately afterwards I found the template I had lasercut using this formula. I compared them and they do not match - My lasercut one is far taller.
 
 Then to make matters worse, I drew a 5" radius arc and it is even taller than the lasercut template at the referenced 2.25" chord for the Gowen A6.
 
 Regards.
 Mike Kirda
 P.S. Taking down the arcs spreadsheet until I can verify this formula.
 
 
 
 --- In Indoor_Construction_at_yahoogroups.com mailto:Indoor_Construction_at_yahoogroups.com, "John Barker" <john.barker783@...> wrote:
>
> Mike,
>
> Don has shown how to do the arc in Autocad but if you want a formula to
> solve the problem then one can be written from the geometry of intersecting
> chords and I give it below.
>
>
>
> Radius of Arc = 0.5(C^2/4h + h) where C is the chord of the arc and h is
> the height at the centre of the camber.
>
> Example: A 6" chord with a 5% camber height at the centre.
>
> Then camber height, h = 0.05 x 6 = 0.3".
>
> Radius = 0.5(6^2/(4 x 0.3) + 0.3) = 15.15".
>
>
>
> John Barker - England
>
>
>
>
>
> From: Indoor_Construction_at_yahoogroups.com mailto:Indoor_Construction_at_yahoogroups.com
> [mailto:Indoor_Construction_at_yahoogroups.com mailto:Indoor_Construction_at_yahoogroups.com] On Behalf Of
> mkirda_at_...
> Sent: 04 September 2013 05:04
> To: Indoor_Construction_at_yahoogroups.com mailto:Indoor_Construction_at_yahoogroups.com
> Subject: [Indoor_Construction] Formula for percentage arcs anyone?
>
>
>
>
>
> Greetings.
>
> I am trying to reproduce a 5% airfoil in AutoCAD and I'm not having a lot of
> luck. Everything seems to default to degree arcs, which isn't a problem,
> however I cannot find a formula to convert between the two.
>
> Anyone have the formula handy?
>
> Regards.
> Mike Kirda
>

Received on Thu Oct 31 2013 - 20:47:51 CET