Sopwith Baby Project

Maxim08 · 2 February 2008, 09:46 AM

Joe,
Regarding the teardrop trailing edge. Our museum made some dies for rolling this a couple of years ago. It was too late for the Strutter but defined as necessary for the Camel project. The difficulty was not in rolling the shape but in drawing the final form so that it comes out straight. At this time it is still an unresolved challenge. On the other hand, the museum is finishing the overhaul of the SE and not focusing on the Camel so there is still time.
Something to keep in mind. (The drawing of the tubing, not the time factor!)
Regards,
John

Joe Perkel · 2 February 2008, 10:14 AM

Quote:

Originally Posted by Maxim08

Joe,

Regarding the teardrop trailing edge. Our museum made some dies for rolling this a couple of years ago. It was too late for the Strutter but defined as necessary for the Camel project.

Regards,
John

John,

So what was done for the Strutter, "off the shelf?"

I can see you as a museum wanting to stick to all details however minute but, I am wondering if this may be one of those items just not quite worth the trouble for me, so therefore maybe just go "off the shelf."

For the moment, I need to pin down the aft radius so I can draw out the airfoil section tangent from each end. But which radii to choose for the trailing edge is bothering me a bit,... the original AP 121 section, or something off the shelf like that from Wicks or Spruce Aircraft Supply???

Thanks for your input John!

Joe Perkel · 2 February 2008, 03:53 PM

A wee off topic but just too cute to pass up. Came across this today in research perhaps old news to many but a surprise to me.

This is a prototype PV7 "Grain Kitten", apparently powered by a 35 hp "Gnat" motor, intended for shipborne launched Zeppelin interceptor. Never panned out (Under powered / 12k ceiling) so never produced but, nonetheless an authentic WWI bird just "screaming" for someone to reproduce in either authentic wood or Al tube!

I think maybe the Flitz Biplane Index may be influenced by this

Joe Perkel · 2 February 2008, 08:37 PM

Here's a head scratcher for me, put the brakes on me tonight but good!

Has anyone else had or seen a fit discrepancy between ribs and spars? To my knowledge, all spars are perpendicular to the ground plane,...is this not correct?

So why this forward 4 degree forward angle in the aft cut-out here? The forward spar cut-out specifies a 1 degree aft angle!

The text along the top is as specified in the drawing. The Red spar section is also as positioned in the drawing 90 degrees to ground but, the yellow spar section I rotated in a trial fit to match the 4 degrees specified above??? This just doesn't seem right.

I'm both exhausted and thoroughly confused as I write this,...please help!

Dogtail2 · 3 February 2008, 12:45 AM

Joe

Looks to me that unless the spar is rotated, the cut out will get way too close to the bottom of the rib. (red spar bottom rear edge). Rotating leaves 'meat' top and bottom.

Joe Perkel · 3 February 2008, 06:50 AM

Quote:

Originally Posted by Dogtail2

Joe

Looks to me that unless the spar is rotated, the cut out will get way too close to the bottom of the rib. (red spar bottom rear edge). Rotating leaves 'meat' top and bottom.

And that is what's causing me a fit! It would seem that I should not allow that oddball cut-out but, yet there is the discrepancy between the cut-out angle on the drawing and the spar section detail with respect to it's position (90 deg face to "C" plane).

So far, I can "confirm" that I cannot change the dimensions of the spar as per the below passage from the AC 43.13-1B

Quote:

1-41. SPAR REPLACEMENT. Owner-produced
spars may be installed providing they are made
from a manufacturer-approved drawing. Also,
a spar may be made by reference to an existing
spar providing sufficient evidence is presented
to verify that the existing spar is an original
part, and that all materials and dimensions can
be determined. The dimensions and type of
wood used are critical to the structural strength

of the aircraft. Care should be taken that any
replacement spars accurately match the manufacturer’s
original design.

So this eludes to precisely as you point out, rotation of the spar to the 4 degrees. Still, not quite a good fit between the spar caps.

Questions to be answered then.

1) Rib profile re-check?

2) Can I "trim" into the end caps to fit the spar if my rib profile checks out ok?

2) Grain orientation

3) Pup & Strutter comparison?

Thanks for the input!

j ferguson · 3 February 2008, 07:14 AM

Joe,
it surely looks as though there's something wrong with your spar profile. I would think that the sides would be parallel and the top and bottom milled to different angles to fit within the ribs.

Clearly, neither of the profiles you show here would fit. as to whether the face should be vertical with respect to some aircraft datum or perpendicular to either the top or bottom of the rib at that point, seems to me for a spar of the size shown to likely be more a choice by the designer of how to describe the structural geometry than how to best carry the load.

keep at it, john ferguson

Machinbird · 3 February 2008, 08:33 AM

Joe, AC 43.13-1B applies to repairs made to certified aircraft. You are working on an experimental aircraft, aren't you?
You obviously have conflicting data, and reason will be the first tool in resolving the problem. The rib web cutout will have a small effect, you will never want to to notch the rib cap strips, nor will you want to notch the spar. It all has to fit.
Look up and down the wing stations for more clues to the problem. Check other sources. Then design as best you can.
You can always test your design with sandbags to confirm the soundness of your work.

Joe Perkel · 3 February 2008, 09:16 AM

Quote:

Originally Posted by j ferguson

Joe,

it surely looks as though there's something wrong with your spar profile. I would think that the sides would be parallel and the top and bottom milled to different angles to fit within the ribs.

keep at it, john ferguson

John

I Agree with your assessment, suspecting now the problem may in fact be the "rib" profile as opposed to the Spar which was pretty straight forward dimension wise, although conflicting info as to orientation.

In order to interpolate the curve through coordinate points, I had to choose a radius for the trailing edge to come off tangentially, I think I may have gone wrong there. Still, it looks like I may still have a fit problem anyway??,..will have to see in several days at my next opportunity.

Thanks John!

Quote:

you will never want to to notch the rib cap strips, nor will you want to notch the spar. It all has to fit.

Look up and down the wing stations for more clues to the problem. Check other sources

Sid,

You're confirming for me that little nagging voice that says,.."that aint right!"

Will be looking for those other clues!

Thanks Sid!

j ferguson · 3 February 2008, 11:53 AM

Joe,
since you're pretty confident that the spar profile is right, I think I would assume that the sides ARE vertical and include the edges in your point geometry for curve-fitting to arrive at the rib geometry. Clearly you will need to move the profile up and down to get a best fit. Do this with both spars if the front spar top or bottom is sloped.

think about it. If they could have the slope on top and bottom any angle they want, why wouldn't the sides be vertical? You have to start somewhere.

I seem to remember that there are several methods for fitting curves to data points and it would be nice to know if Sopwith specified one.

good luck with this, John