In this context I don't know what "vintage" means. Are we to be looking at the Original 1940 Sparky or the published date of Don Srull's plans?
In late 2022 I learned that Don Srull was ailling, so I thought I'd build one of the many plans I have with his name on them. I choose the AMA Plan #414 as I've been having a ball flying my Benny Boxcar (Park480 size). I'm sorry to say I think Don Srull passed away August 2023.
As is typical of me it has taken 3 or 4 years to get where I'm currently in the build. Yes, it still isn't finished. Another thing is that as a kid I was horrable at coloring inside the lines! In my mid 60's that is still true in that I can't build to plans. So this build really is my take on Don's 2X Sparky plans!
The first thing I did was change the curved outlines from pieced parts to laminated hoops. I also changed the spars from blade types to cap style with wandering shear web.
Oh, the other major change is I plan to use a Speed 400 6v on a Ludwig 5.9:1 gearbox with 2 cell lipos as the power system.
Wow, one post into this thread and I have to make a few corrections. Don Srull died in October 2022. I think his death was the impetus for this build. The date on the AMA Electric Sparky AMA plan #414 is July 83.
Please don't do as I did. As a manufacturing engineer at some point in my life I know that the quality and repeatability of a process is highly dependent on the tooling used. Here I really didn't want to make proper forms to bend the strips into the proper shape needed. I choose to just stick a bunch of pins into the building board and hope that the kinks in the laminates don't line up.
I hate diamond LE's as I think they result in the rib splitting prematurely at the slightest impact. Here I've changed the LE to make contact with the ribs at 90°. I also choose to make the LE out of 2 layers to control warping and aid in the straight assembly of the wing. And last I tried to use finger joints to blend the LE into the tip hoops.
I've also changed the I beam web to a wandering web in hopes to add some resistance to rotation (flutter). The I beam works great at resisting bending loads. But not so well against rotational loads. I noticed that Don added another bay to the rear drag spar to try to add some rotational stiffness to the wing structure when going to 64".
QuoteI really didn't want to make proper forms to bend the strips into the proper shape needed. I choose to just stick a bunch of pins into the building board and hope that the kinks in the laminates don't line up.
This brings back memories. I decided to use a laminated TE/tip bow for a 18"WS HE71 and had quite a few failures. I eventually found that the only successful approach for me was to use shaped form and do a max of 3 strips(1/32) at a time. It still required care but would work.
The wing is looking quite neat - interesting use of the wandering web - almost a sine wave web, for the spar.
Good luck with it.
John
I have to admit I had to look up HE71. Wow, there isn't a straight line on that Heinkel.
Quote from: OZPAF on Apr 13, 2026, 03:35 AM...
The wing is looking quite neat - interesting use of the wandering web - almost a sine wave web, for the spar.
Good luck with it.
John
Thank you. I've been building with the wandering web since I was a wee lad. In fact my father said it was a sign of sloppy workmanship when I was trying to build I beams. It wasn't until I started to look at some F1C ships that I realized there was something to be gained by them!
LOL, I don't think the engineers at Boeing would agree as the amplitude is so low compaired to the period. I have to admit I didn't know about the sine wave web until I first saw them being used on the fuselage hoops for the B-787 (crica 2006). If we are to call this by a geometric name maybe a triangle web. I still like wandering web myself.
https://patents.google.com/patent/US5919543A/en
Beautiful work. Well done.
Thanks all that info on Sine Wave Spars Konrad.
Wandering spar webs it is :)
I have heard of it being used in some of the latest F5J models and probably F3F and F3B as well.
Actually for models - a double wander per rib bay would be interesting but the benefit may not be worth the effort.
John
Quote from: OZPAF on Apr 14, 2026, 03:41 AMThanks all that info on Sine Wave Spars Konrad.
Wandering spar webs it is :)
I have heard of it being used in some of the latest F5J models and probably F3F and F3B as well.
Actually for models - a double wander per rib bay would be interesting but the benefit may not be worth the effort.
John
I want to be clear. Conceptually the sine wave spar is superior to the classic I beam if done properly per the Boeing Patent.
I know of no F3F manufacture that is carrying the fibers across the web/cap interface. So in practice the Sine Wave Spar is far inferior to what else is out there as the bonding surface is just "line contact". Most other F3F spars use a very large surface contact area to bond the upper and lower spar caps.
Back to Sparky! It is time to air my dirty laundry. As an engineer we know that the stiffness of a I beam goes up by the square of the distance between the load elements (spar caps). So when I redrew the ribs I place the spar caps at the very outside of the airfoil. This is great but for the fact that this wing is film covered. This covering sags between the ribs. So the airfoil is only true to the intended shape on the ribs. Anywhere else and the film covering results in the very poor performing cat back style airfoil!
I was involved in another open bay project making adjustment for film sag based on my talks with Don Stockhouse of DJ Aerotech. Key to this is understanding that the sag acts a lot like a catenary curve.
So I figured I could add a 1mm rib cap strip to go over the spar and I'd fix my original error. Well much to my horror 1mm only lifted the film to clear about 20% of the spar. Yikes, I was getting a real visceral feel for how a catenary curve works! So, time to go back and add another 1mm cap strip for a total of 2mm. This one extra 1mm lifter the other 80%. So I can say that 80% of my wing's span has an airfoil that is within 1mm of the intended shape. Or to say that my wing now has an airfoil that is much more inline with the intended performance than the 2% (rib thickness) I and most designers wing's actually end up with
QuoteI know of no F3F manufacture that is carrying the fibers across the web/cap interface. So in practice the Sine Wave Spar is far inferior to what else is out there as the bonding surface is just "line contact". Most other F3F spars use a very large surface contact area to bond the upper and lower spar caps.
I noticed the coverage associated with the 'Teeth" in the patent.
That's an interesting display of the covering sag although I'm not so sure i would prefer this approach to say 2 riblets per rib bay. These riblets could be strip ribs(delicate I know).
However it's always interesting to see new approaches.
John
Quote from: OZPAF on Apr 14, 2026, 11:24 AMQuoteI know of no F3F manufacture that is carrying the fibers across the web/cap interface. So in practice the Sine Wave Spar is far inferior to what else is out there as the bonding surface is just "line contact". Most other F3F spars use a very large surface contact area to bond the upper and lower spar caps.
I noticed the coverage associated with the 'Teeth" in the patent.
That's an interesting display of the covering sag although I'm not so sure i would prefer this approach to say 2 riblets per rib bay. These riblets could be strip ribs(delicate I know).
However it's always interesting to see new approaches.
John
To be clear I don't like film/fabric covered wings. Mainly because of the increase in the separation bubble by the increased pressure rise in the field of the sag. This is associated with a significant increase in drag.
What I show is a "fix" for the issue I and many designer introduce with the spar being at the very outside of the airfoil. That is the "Cat Back" airfoil with the trip point being right where it will encourage the separation bubble. Adding half ribs/riblets/ false ribs will not address this issue unless there is a strip that actually goes over the spar to lift the covering off the spar.
With most carbon tubed wing this isn't a problem as the film rarely makes contact with the tube spar. false ribs are often added to address the increased pressure rise in the mid-field of sag. I try to show this with the span-wise circular arrows. (I'm not convinced that a lot of small span-wise vortices is much better than the one large span-wise vortices).
But what I have seen is that these false ribs almost always result in wake turbulence where they end. (again often near where the separation bubble begins. I've noticed that wings with false ribs stall a lot earlier at high coefficients of lift. such as we have with the inboard wing tip when coring out a low level thermal.
So any performance we might get from minimizing the mid rib sag often has too high a price in low speed handling near the stall. Note that what I'm doing with the 2mm cap strip is trying to get an airfoil that is close to the design shape over 80% of the span by deliberately distorting the other 20%.
I rarely know what to do with the Comet or Guillows tips. This build is no different.
Here I choose to bring the tip hoop up so that both the top and bottom of the wing meet in the middle of the rib height.
I also placed the hoops so that they meet the tip vortex at a lower angle of attack (added washout). I will also add some linear twist (washout) in the wing as I hope to be spending a lot of time in tight turns coring out thermals. This is to delay the inboard wing from stalling early.
Time to clean up some loose ends on the wing before going onto the other parts of the Sparky structure.
First the I mis-spelled Don Stackhouse of DJ-Aerotech. Many of you might remember that Don spent some time explaining the concept of film sag compensation in my old HIP thread covering my build of the DJ-Aerotech Chrysalis lite F3-RES. I've been trying to apply these concepts to this Sparky build (a bit after the fact). Don used a round dowel as the LE to again try to make a smooth transition from the structure to the film. Here I try to maintain this smooth transition by breaking the top edge of the leading edge. ( I hope this shows). {I'm reading that some designs actually like to see this discontinuity as it thought that it might act as the turbulator at very high CL values.}
As we were earlier discussing design and process particularly with the Sine Wave Spar I thought I'd close that with this photo. You can clearly see the spar failed at the glue joints between the web and carbon caps. This joint failed way too early as there is no damage to the surrounding parent material. This is exactly what the Boeing patent was trying to address. What failed wasn't the idea of the Sine Wave Spar but rather the processes used in the implementation of the Sine Wave Spar. I should note that Aeroic has changed to an "industrial adhesive" and that this has upped the yield point of failure. But the failure point is still far below that of the parent material. That is the spar caps and web depart far too early when the spar is loaded.
Yes, my wandering web is highly dependent of the bond strength between the web and spar caps. It is generaly understood that aliphatic glue is much stronger than the parent balsa wood. So I think I'll get the full benefit of the wandering web even when using line contact.
Thanks for that Konrad. Drag due to the covering sag is an interesting topic.
The sine wave spar failure was also interesting - showing also a lack of consideration of the shear stress at the web/flange joint.
John