Replica CoG

brinesharks · 24 July 2009, 11:49 PM

After some discussion with Nick, it occurred to me that none of the drawings or rigging manuals I have for Sopwith/Nieuport/SE5a aircraft provide a Centre of Gravity (CoG) range. That's fine if you have a factory constructed machine with all original engine, etc, however it's a concern for replica builders who are making changes (eg. different engine).

Can anyone suggest a way of determining the correct CoG? From my basic understanding of it, calculating the CoG for a monoplane is a complex job, and no one I know (nor any of my references) can tell me how to do it for a biplane (sesquiplane!).

Does anyone know the correct location CoG for a Nieuport 17 (and Nick might be interested in the Snipe)?

jumpinjan · 26 July 2009, 01:58 PM

To calculate the CG you need to find the center of pressure (lift). For a good airfoil, it should be almost 30% from the leading edge (That will vary for the type of wing used. A "plank" wing, the CP is about 14% from the leading edge). The CG should be ahead of the CP for best stablity, but it could be very slightly behind or ahead.
With wings with sweep back, it could be derived by plotting the wing on paper. With a Biplane, the top wing gives the majority of lift and the CP could be plotted on paper as well.
Jan

Jeff Brooks · 26 July 2009, 02:46 PM

I went through this a while back. not sure it ia all correct, but there is a lot of discussion of it here.
The Biplane Forum: Weight & Balance
http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf

these calculations were baised on 25% cord, but in the dialog I had on the biplane forum, I should have used 23%.

brinesharks · 26 July 2009, 11:58 PM

Jeff - thanks for the detailed response. I've seen the calculations for monoplanes but no biplanes.

I'll try the calculations on the Nieuport and post the results (a review of my calculations is always good!).

BTW - why 23% chord?

Nick · 27 July 2009, 04:39 AM

Many thanks for posting that Jeff, much appreciated.

Cheers, Nick

John McKenzie · 27 July 2009, 05:53 AM

Hi all, . With all bi-planes , there exists what is known as " interferance " . whereby the low preasure above the bottom wing is to some extent countered by the high pheasure under the top wing . The amount /effect of this interference , depending on amongst other things , the gap between the upper and lower wings and stagger . That the high preasure under a wing is of a lower order , than the corresponding lower preasure above ,that is , the upper surface contributes more towards the lift than the bottom ,..means that the "interferance has a greater (bad ) effect on the lower wing than the top , and hence ., the efficientcy of the upper section will be greater than that of the lower section and this should be factored into the equasion along with the (if different ) lift coeficients of top and bottom wings .....This brings the overall centre of lift forward from that described above , hence in this case produceing an aft C.G.....For machines with no stagger there is no effect of the above calcs , to concern the C.G. , however it obviously effects all other calcs,. .....For a thiplane , there exists similar but more complex lift distribution between the 3 wings as against 2 wing interference .( aside from area and section lift coeficients etc,. which apply in all cases .)....This was all known about in 1917 , though the calculations in actuality may have been slightly flawed .....

....Regards John

John McKenzie · 27 July 2009, 06:09 AM

P.S....With respect to nieuport wing celule , .The upper section is a high lift (&drag) section , whereas the lower is /almost flat bottomed .(depending on model ) lower lift ,higher speed . .It would appear that the lower wings existance is mainly due to the need for it as a structural part of the bracing for the top wing ,and the section mainly streamlines the spar , adding of course to the lift by its section ,rather in the same reasoning as the section over Fokker axil box ..ie as it has to be there , might as well use what lift that can be got from it .This is a good case for the lift coefficient of top and bottom wings having an effect on for/aft position of C.P.aggregate...JM

brinesharks · 27 July 2009, 03:35 PM

Quote:

Originally Posted by John McKenzie

P.S....With respect to nieuport wing celule , .The upper section is a high lift (&drag) section , whereas the lower is /almost flat bottomed .(depending on model ) lower lift ,higher speed . .It would appear that the lower wings existance is mainly due to the need for it as a structural part of the bracing for the top wing ,and the section mainly streamlines the spar , adding of course to the lift by its section ,rather in the same reasoning as the section over Fokker axil box ..ie as it has to be there , might as well use what lift that can be got from it .This is a good case for the lift coefficient of top and bottom wings having an effect on for/aft position of C.P.aggregate...JM

My choice of a Nieuport is starting to look a lot more complex aerodynamically! Stagger, different chords, inverse taper, sweep back and different aerofoils....

I have a rough idea of where a 'sensible' CoG range is for this type of plane. I also have the Redfern plans which show a slightly different aerofoil but the CoG range at least gives me something to check my calculations against.

Thanks for you input John.

Joe Perkel · 28 July 2009, 02:54 AM

Quote:

Originally Posted by brinesharks

My choice of a Nieuport is starting to look a lot more complex aerodynamically! Stagger, different chords, inverse taper, sweep back and different aerofoils....
.

Bryan,

Somewhere, (the Nieuport 17 datafile perhaps), there is an image of a N-17 having lost its left lower wing in flight. The flying wires held and the pilot landed the aircraft intact.

It's an interesting image, and it makes one think about the relationship between the main wing and the smaller chord sub wing.

brinesharks · 28 July 2009, 03:11 AM

Quote:

Originally Posted by Joe Perkel

Bryan,

Somewhere, (the Nieuport 17 datafile perhaps), there is an image of a N-17 having lost its left lower wing in flight. The flying wires held and the pilot landed the aircraft intact.

It's an interesting image, and it makes one think about the relationship between the main wing and the smaller chord sub wing.

Hi Joe!

I had heard of this from written accounts. I guess it's how they found out the lower wing failed due to flutter during steep dives. The upper wing looks quite robust on the Nieuport and it could have easily been made a Parasol in my opinion. Scary to make the conversion during fligt