[NSRCA-discussion] Odyssey CG

[Ron Van Putte]

Calculating the proper CG of an airplane has almost become like  
religion: "My religion is better than yours."  There are no 'right'  
answers, but there are many 'wrong' answers.
If you tell me what a particular elliptical wing and stabilizer  
combination looks like, I could tell you where I think the CG of an  
airplane having an these elliptical planforms should be.  It involves  
integrating the moment-producing capability of each chordwise strip  
across the wing/stab span and the length between the wing and the  
stabilizer.  However, there are ellipses and then there are other  
ellipses.  If that sounds weird, consider that a rectangular wing has  
a MAC at the 25% chordwise point.  It is pretty well known where the  
MAC of this wing is located.  Now consider the same wing that's swept  
something more than a trivial number of degrees, say 30 degrees.   
What's the MAC?  Well, it's certainly not the same number.  It's more  
like the previous MAC divided by the cosine of the sweep angle.
The elliptical wing has a MAC that's somewhat inboard of the  
midspan.  How much inboard of the midspan the MAC is located is  
determined by the shape of the ellipitical wing.  Is it short and  
stubby like the Spitfire, or long and slender?
I could go on, but by now almost everyone is either bored or looking  
for a squabble.

Ron Van Putte

On May 1, 2007, at 9:23 PM, Nat Penton wrote:

> Tom
> Thanks for the input. I use a CG formula from Gordon Whitehead's  
> book "How
> to Make Scale Models Fly". Since there are only two variables, %  
> stab and
> stab moment arm, for pattern airplanes it reduces to 36% MAC. I  
> have never
> found a need to move the Cg after test flying. If anyone is  
> interested in
> the formula they can let me know.
>
> My opinion is that 95% of trimming is best taken care of in the shop.
>
> BTY I question Sutherland's determination of the MAC of an  
> elliptical wing.
> Maybe RVP can shed some light.      Nat
>
> ----- Original Message -----
> From: "Tom Simes" <simestd at netexpress.com>
> To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
> Sent: Tuesday, May 01, 2007 1:09 PM
> Subject: Re: [NSRCA-discussion] Odyssey CG
>
>
>> On Tue, 1 May 2007 10:40:01 -0800
>> Tom Simes <simestd at netexpress.com> wrote:
>>
>>> On Tue, 1 May 2007 14:13:54 -0400
>>> "Ken Velez" <kvelez at comcast.net> wrote:
>>>
>>>> Anthony I think the 3 inches will not work for you that is not 30%.
>>>> That holds true if it was a straight wing like in a trainer. In a
>>>> taper wing you have to figure out the center of the wing  
>>>> compensated
>>>> for the taper and them measure and figure out your 30%. There is a
>>>> few different ways to do this but I think the easiest is to add the
>>>> root length to the tip and the tip length to the root. Any way I
>>>> attached the figure so you can see it, If you have any questions
>>>> just ask. Hope it helps.\\
>>>
>>> I agree with Ken, I believe the good Dr. is referring to 30% aft of
>>> the leading edge on the Mean Aerodynamic Chord (or MAC) as  
>>> opposed to
>>> 30% aft of the LE on the root chord. Here is a good explanation  
>>> of the
>>> the method Ken was referring to:
>>>
>>> http://www.lferc.com/Tips/mean_aerodynamic_chord.php
>>
>> Just for the archives, here are a couple more good links on  
>> deriving a
>> resonable CG range for a model by finding the Mean Aerodynamic  
>> Chord (or
>> MAC):
>>
>> http://www.airfieldmodels.com/information_source/ 
>> math_and_science_of_model_aircraft/formulas/ 
>> mean_aerodynamic_chord.htm
>>
>> Here's a good drawing explaining the various measurements as well  
>> as a
>> web based tool to figure it all out for you:
>>
>> http://www.geistware.com/rcmodeling/cg_calc.htm
>>
>> (just enter data in the top 5 boxes and hit the calculate CG button)
>> Tom
>>
>> ===================================================================== 
>> =
>>   "Z-80 system stack overflow.  Shut 'er down Scotty, the system's
>>         sucking mud" - Error message on TRS 80 Model-16B
>>
>> Tom Simes                                        
>> simestd at netexpress.com
>> ===================================================================== 
>> =
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