Sparky, Don Srull AMA plan 414 64"

[Konrad]

In this post I hinted as to what my next project would be. It is a Don Srull 2 times enlargement of a Comet 32” Sparky. This is roughly based on Ed Lidgard's 41 Sparky. The impetus for me to build one of these was the great time I’m having flying the Old timer RC Benny Boxcarr.This model will be built based on my recollection of the SAM speed 400 old timer Texaco event. This will be using tech I’ve forgotten for over 2 decades. It will be 6:1 brushed 6V speed 400 on 7.4 volt 2 cell lipo. I’ll have to remember how to seat brushes, time for spark-less commutation. And learn how to climb on the wing.

I will be changing the structure to meet my desired performance and still look like a Comet Sparky. First I’ve changed the spars from a dual blade type to a squiggly web I beam type (loosely based on the Boeing sine wave spar). Next I did away with the fish mouth Leading Edge (LE) we see far too often with balsa ribs. This guarantees that the diamond will split the rib at the slightest tap! It also means the the wing will follow any warp/stresses in the LE. I’ve also changed the LE to a double laminated one. This should add stability to the LE as the forces in the wood might counter act each other, minimizing long term warping. And to save weight, and that I like them, I’m using a 5 layered 1/16 laminated tip hoops The lamination allow me to make a stepped splice with the laminated LE.

I’m also trying to raise the wing tip to aid in aerodynamic efficiency. This gave me the option to try to sand in some wash out to the tip.

BTW; All the balsa wood came from Aloft. While not contest grade it is good, dare I say great, sport class wood. Far too often the wood we see from places like Midwest look to be a close kin to Oak!

 

[Hank GB Z]

Hey Konrad,

Looking good. I like that you’ll put the brushed motor in the nose. I also like the wandering sheer web design. Like the common mans sein wave spar.

Hank

 

[Konrad]

That's going to be the challenge. The plans show an "Leisure geared 05" on 6 cells. As I recall the deepest gears on that system was 3.5:1. I loved what the Leisure 05 did for us the sport electric flier with those replaceable brushes. I feel it was a bit heavy. I flew Astro motors in the day.

But I have my doubts that a Speed 400 (Mabuchi 380) is really a good replacement for the Leisure 05 . It's not, but I saw a SAM class with these Comet doubled sized kits, Sparky and Phantom Fury and love the idea. I'll be using a Ludwig 5.9:1 gear box, with the lighter lipos and modern folding prop I think the climb should actually be better than the as shown Leisure 05 set up.

With the much lighter geared Speed 400, lipos and modern radio, system integration will be a whole lot different than that shown on the plans. I think everything will be in the nose. But I like real tail heavy old timer set up (lifting stabs). So I'm not sure how to arrange the fuselage internals and hatch access. I've decided to make the wings and tails first to be used while as I mock up the fuselage.

I don't know how SAM will define the outline of the model. I see that on Ed's Model Builder plans there is no ventral fin. I like that as the dihedral is a bit excessive for RC! I do see where ED has the stab tips slanted down 45°. I think this was done to use the stab as part of the 3 point ROG requirement, and has no aerodynamic benefit. So I'm thinking the smaller vertical fin area will be a benefit for the RC guided Sparky. But the fuse will have the hard points for the ventral fin if flight testing should show the need or if the SAM rules will drive it to keep the look of the Comet Sparky.

Based on how my Benny Boxcar trimmed out, I'll place the CG way further aft than what Don shows on his AMA plans. I will set the wings to zero-zero. Both the Benny boxcar and Sparky have flat bottom airfoils with a modest Philips entry. This and the elimination of the ventral fin are the extent of my planed aerodynamic changes. I will move the Comet location for the landing gear aft at least one fuselage bay to aid against the ground loop. This will align a bit more with where Ed put his mono pole gear.

If anyone has flown a Sparky in the SAM speed 400 classes please let me know what you think of the Sparky and my use of Don's plans as a bases for the model.

 

[cedar]

I don't usually look at this thread, but it surprised me to see that someone else is building a Sparky. I had one way back in the 80's. Yes, it was powered by a geared can motor with 6 nicads. At the time I was the only guy in the club that would dare to bring an electric to the loud field. It was a long time ago, but I do remember that it flew fairly well. By today's standards it was too heavy and underpowered. I think the speed control was an automotive switch and a Futaba S48 servo. Thanks for the flashback.

 

[Konrad]

Yep, one needed to be on the bleeding edge of technology to get any kind of performance from an electric. I was flying in the Denver area often at fields above 6K feet. In the thin air power to weight was critical. I quickly learned to use the Sanyo SCR cells and Astro motors. I loved to see the jaws drop when I would throw out a glider and then rotate into the vertical going out of sight (well almost). This was not your typical Goldberg Electra. I first did this with the Astro Challanger using geared Astro 05 with FAI winds. I think we were drawing a whopping 35 amps! I made my own speed controllers using Buz 10 Mosfets maybe 8 in parallel! The automotive relays didn’t hold up for long. I was using Bantom midget servos and maybe a few Futaba 20’s (sub micro).

Now at that high an altitude the I.C. Engines were at a disadvantage. I recall racing a K&B 0.40 (8011) powered Scat Cat with a 10 cell Electro Streek and beating it.

It was fun changing folks ideas as to what was possible!

 

[cedar]

We could keep up the old stories for a long time. So much change in the hobby that now there are nights when there are no I.C. engines making noise. I am just about finished with a little plane called "Handful". A 1997 Andy Clancey design. Brushed gws gear motor and about 30 watts on a 2 cell. I too have a drawer full of Astro motors. They are my all-time favorites.

Be sure to let us know how the Sparky turns out. I don't remember what mine weighed but I am pretty sure yours will be much lighter. It should be a nice flyer.

 

[Konrad]

When I look at a product I often ask myself, what did the tooling to make this look like? Such is the curse of having been a machinist and tooling engineer!
Proper tooling and fixturing is key to making consistent and high quality products. But unfortunately there is often a high up front cost.

Here I'm showing how not to make a laminated hoop tip. Well, not if you want a high quality product. The problem was that I didn't want to take the time or spend the material to make a form to bend the balsa strips around. I choose to make my "form" out of pins. This is a very poor way to do it as the pins result in stress riser that will kink the wood fibers. To minimize this I use a sacrificial strip as backing when forming the hoop.

I soak the balsa strips in water for about a 1/2 hour. I then poor hot water over these prior to bending these over the form (pins). I let this dry about 80% (wet damp). I then separate the strips and apply aliphatic glue. I reform the strips over the form and let dry over night.

I lift the hoop off the board and cut off the sacrificial backing. Clean up the hoop with some 120 grit sand paper and try to not notice the few kinks in the strips. With any luck no two kinks line up.

The hoop tip is about 1/3 the thickness of the classic pieced tip. It is about twice as strong as there are no spliced joints. Because there is a continuous fiber path I'm able to use 4 lb to 6 lb (contest) balsa for a weight of about 1/4 that of the pieced tip. The use of contest grade balsa makes it easy to form the strips. It also means that they are easy to kink. The only real down side of making hooped tips is that it takes a day or two soaking the wood and allowing the hoop and glue to dry. The classic pieced tip only take 20 minutes to cut out and glue if using CA glue.

 

[Konrad]

I like composite structures. Here I'm going back to one of the earliest, the lamination. I showed earlier how I don't like to make laminated hoops. Now I'll go into some detail on how I try to tie this into the leading edge (LE).

With the wing I made the LE out of 2 layers of 3/32 balsa. It is hoped that any stresses in the individual pieces with counter act each other resulting in a beam (LE) that is less likely to warp. With the stab I made the LE the same thickness, but from 3 layers of 1/16 balsa. I did this hoping that the added layers will result in a beam that is less prone to impact damage. The stab will be lower closer to the weeds so I suspect it might get hit more often than the wing.

Like the wing tip I tried to take advantage of the laminations to make finger joints (staggered glue lines) bringing the LE into the tip hoops. As both the stab's hoops and LE are made of 1/16 material it was actually a bit easier than with the mismatch wing's 3/32 and 1/16 materials.

 

[Hank GB Z]

That’s a nice bit of building. I would have just cut the laminations at an angle and cut a matching angle on the LE piece.

Hank

 

[Konrad]

That’s a nice bit of building. I would have just cut the laminations at an angle and cut a matching angle on the LE piece.

Hank

That would give me more glue contact area. But it would have made it more dificult to fit the LE pieces. I say this as I kinked the LE at the center line rather than cut them in a effort to carry some fibers across the center line.

 

[Konrad]

The secret to my builds is the use of the sanding bar. In my youth I often hated sanding. But that could be traced to dull sandpaper and not understanding the purpose of a sanding block.

Here the plans call out 1/4 thick materials. But the 1/4" x 1/2" is shaped to follow the airfoil profile. By the time we reach the elevator hinge line the airfoil is 3/32" thinner. This means that I'll have to plane down the TE stock to maintain the stab's airfoil.

This 3/32 change in the 1/4 x 1/2 stock means a lot of sanding. To protect the ribs I like to mask them.

 

[Konrad]

I meant to show this earlier, but the razor plane was at my other shop.

While not as versatile as the sanding block the razor plane makes quick work of sizing balsa stock. For example the shaping of the hard 'C' grain triangle stock to be used as the Sparky's elevator would have taken 1/4 of an hour even with fresh sharp sandpaper. With the razor plane it was done in less than a minute. And with a sharp iron (razor) the finish really is close to ready to cover. Yes, I'll finish sand the triangle stock. I have yet to plane in the hinge relief.

I mention the razor plane and other tooling as it looks like Aloft is trying to move into the kit manufacturing in a big way. I'm of the opinion that a lot of the success and pleasure of a build can be tied to the tooling and fixturing used in the build. I'd like to see Aloft carry some of these hobby tools.

The quality of the tools is critical to the outcome. I've used some plastic planes (Master Airscrew ?) with dismal results. The one I'm showing has an aluminum body. I got this one 40 years ago in Germany. I don't know if it is still available or what actually is in the market place.

But good tools make just about any job a pleasure.

On the subject of tools @Hank GB Z what are you using as a building board?

 

[Hank GB Z]

Hey Konrad,

Right now I’m using 12” sq pieces of drywall I got from my local Ace Hardware. My old bench top from IKEA is a mess and needs to be re-surfaced and fixed it’s on my to do list but for the size of models I’m building the drywall is working ok. Would be better to tape the sides but that would make the top have a slight ridge around the edge.

I agree with you about tools. Always makes a huge difference.

Hank

 

[Konrad]

Thank you Hank.

WOW, I must really not want to make a form!

When this dries I'll use this laminated outline to make a fleshed out fin closer to what was found on the 1941 Sparky.

 

[Wayne]

Did you run out of T-pins?

 

[Konrad]

Not even close. But these are low grade T pins as they are ferrous and staining the wood as it dries.

I tried to raid the wife’s stash of stainless steel T pins. But she wouldn’t have any of it, seeing how many get bent!

 

[Konrad]

Well, the good news is that all those lamination dried. What really surprised me was that the hoop came off the plans with no issues. I feared that with all those pin holes the hoop and plans would be permanently glued to the building board.

Now I like to rip my strips to be about 2 mm oversized. This allows for any mismatch while forcing the strips around the form. I like to clean up the hoop with sandpaper after they've dried to about 80% clean up. This gives me just a bit more material thickness to work with during the build.

I'm showing what I might do to try to flesh out the fin. I'm showing my classic Gollywock fin. You should be able to see that the ribs are bent over a central spar. I think I'll do this for Sparky 64".

 

[Hank GB Z]

are you building a golly wok too?

Hank

 

[Konrad]

That’s another of those builds lost for decades builds. But yes, I still have the materials. I think I even have the contest rubber in an air tight container in the fridge.

Now that I think about it I think the stab’s trailing edge needs to be replaced. I think the rats used it to sharpen their teeth.

 

[Hank GB Z]

I've always wanted to build one. Thought about buying this kit from time to time.

Half scale Jabberwock/Gollywock combo short kit w/plans | J&H Aerospace

Hank