Author Topic: Brodak Yak-9 Build (Read 2431 times)

Colin McRae · « **on:** November 22, 2023, 06:25:48 PM »

I have been wanting to build a 'smallish' warbird model and use a four-stroke engine. So, I decided on a Brodak Yak-9 kit that will be powered with an OS 26 FS. The Brodak Yak-9 also qualifies for OTS since it is a replica of the original Sterling kit design.

I was hoping to get some great 'lessons-learned' guidance from builders that have build-experience with this particular model. I am somewhat of a rookie builder, so I am open to all suggestions and recommendations.

The only thing I am not going to change is my OS 26 Surpass FS engine choice. I really want to do a CL warbird w/ a FS engine. Absolutely love that FS sound.

Thanks in advance.

Steve Berry · « **Reply #1 on:** November 22, 2023, 06:30:33 PM »

You may want to look into getting the dihedral and wing mount landing gear kit for the Yak-9 through Brodak.

https://brodak.com/yak-9-wing-gear-dihedral-conversion.html

It raises the leadouts to get the vertical CG closer to where it needs to be, which will give better handling.

Between the Yak-9 and F-51, the Yak flies better, so good choice.

Steve

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Colin McRae · « **Reply #2 on:** November 22, 2023, 06:35:33 PM »

Quote from: Steve Berry on November 22, 2023, 06:30:33 PM

You may want to look into getting the dihedral and wing mount landing gear kit for the Yak-9 through Brodak.

https://brodak.com/yak-9-wing-gear-dihedral-conversion.html

It raises the leadouts to get the vertical CG closer to where it needs to be, which will give better handling.

Between the Yak-9 and F-51, the Yak flies better, so good choice.

Steve

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Thanks Steve, I would love to do the conversion, but our club somewhat bumpy CL circle is not particularly friendly to wing-mounted landing gear. So, I will use aluminum profile-mount gear. Not the kit-provided wire gear.

Steve Berry · « **Reply #3 on:** November 23, 2023, 09:33:49 AM »

Quote from: Colin McRae on November 22, 2023, 06:35:33 PM

Thanks Steve, I would love to do the conversion, but our club somewhat bumpy CL circle is not particularly friendly to wing-mounted landing gear. So, I will use aluminum profile-mount gear. Not the kit-provided wire gear.

You can still use the dihedral part of the kit to adjust the vertical CG, and use the aluminum gear.

Steve

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Ken Culbertson · « **Reply #4 on:** November 23, 2023, 10:12:29 AM »

The Yak-9 was my slow combat plane during a year of friendly warfare while stationed in Korea. My counterpart, one of the Wolf Pack pilots flew the Mustang. McCoy 35's at high noon! We had quite a spectator audience for the duels. The Yak outperforms the Mustang but "it's the pilot, not the plane" so I only won my share. I concur with the dihedral, it needs it for stunt. Good thing the BX kept both in stock. They didn't last long!

Ken

Colin McRae · « **Reply #5 on:** November 23, 2023, 11:37:26 AM »

Thx for the recommendation on the dihedral conversion kit. I do want a good stunter, so it sounds like it is needed. But I was also planning on using the Brodak wing jig for the entire wing. Sounds like that may not work. But I can work around that.

Then that brings up a related question. I was also planning to build it w/ operable flaps. If the wing has dihedral, does that mean forget operable flaps? I also understand it flies well with or without operable flaps. Since it only has a 40" wing, maybe operable flaps are overkill. I just want it to fly well.

Steve Berry · « **Reply #6 on:** November 23, 2023, 11:42:41 AM »

You can still do the flaps. Most use some form of a lucky box for the horn to ride in. It allows side to side movement of the horn arms, but no up/down slop. Also, the dihedral isn't extreme, so we aren't talking extreme angles. It's been done many, many times by some of the top fliers in the world, so it does work.

All that said...the wing isn't much bigger than a Ringmaster, if it's bigger, at all. So stationary flaps aren't going to hurt it. The plane needs the wing area.

Steve

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Colin McRae · « **Reply #7 on:** November 23, 2023, 01:28:44 PM »

Quote from: Steve Berry on November 23, 2023, 11:42:41 AM

You can still do the flaps. Most use some form of a lucky box for the horn to ride in. It allows side to side movement of the horn arms, but no up/down slop. Also, the dihedral isn't extreme, so we aren't talking extreme angles. It's been done many, many times by some of the top fliers in the world, so it does work.

All that said...the wing isn't much bigger than a Ringmaster, if it's bigger, at all. So stationary flaps aren't going to hurt it. The plane needs the wing area.

Steve

A Ringmaster has a 42" wing and 382 sqin area. The Yak-9 a 40" wing and 323 sqin area.

The OS 26 FS (with stock muffler) is heavier than an OS 25LA (with stock muffler) by 1.5 oz. My current plan is to move the engine back a bit to keep it from being too nose heavy. If I were to guess at this point the model will come in around 30 oz with a correspond wing loading of 13.4.

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Motorman · « **Reply #8 on:** November 23, 2023, 08:55:57 PM »

The wing/fuselage joint needs to be strong as there's not much holding it on the bottom. I would use some hard balsa for the top of the wing box and fiberglass cloth over the joint. Also I made my inboard landing strut longer so the lead outs don't catch on take off.

Dennis Toth · « **Reply #9 on:** November 24, 2023, 07:28:20 AM »

Colin,
The Yak 9 is OTS legal with the nose mounted gear and flat wing. You can use coupled flaps but you lose 5 bonus points (https://pampacl.org/wp-content/uploads/2018/10/pampa-ots-rules.pdf). If you are planning to go glow, I would suggest using 3/4oz fiberglass at the fuse wing joint. Also if the kit uses short plywood doublers you might want to consider replacing them with 3/23" birch ones that go back 2" past the leading edge. For the fixed flap version leave the flaps flat and square that the trailing edge. This makes them more like having 10% ish flap deflection as the ship starts into maneuvers (old tip from Lew Andrews). Set the CG around the 1 1/2" back of the leading edge (at the fuse joint) to start, you might go back another 1/8 - 1/4" as you get it trimmed. Use heavy wheels because the low wing puts the vertical CG way above the wing they will balance this out some.

If electric mount the battery as low on the fuse as you can and use a reverse rotation setup. HobbyStar offers 4 & 5 cell 1800mah packs that fit the nose area (only ~75mm long) like an IC fuel tank.

Best, DennisT

Colin McRae · « **Reply #10 on:** November 24, 2023, 09:05:45 AM »

Quote from: Dennis Toth on November 24, 2023, 07:28:20 AM

Colin,
The Yak 9 is OTS legal with the nose mounted gear and flat wing. You can use coupled flaps but you lose 5 bonus points (https://pampacl.org/wp-content/uploads/2018/10/pampa-ots-rules.pdf). If you are planning to go glow, I would suggest using 3/4oz fiberglass at the fuse wing joint. Also if the kit uses short plywood doublers you might want to consider replacing them with 3/23" birch ones that go back 2" past the leading edge. For the fixed flap version leave the flaps flat and square that the trailing edge. This makes them more like having 10% ish flap deflection as the ship starts into maneuvers (old tip from Lew Andrews). Set the CG around the 1 1/2" back of the leading edge (at the fuse joint) to start, you might go back another 1/8 - 1/4" as you get it trimmed. Use heavy wheels because the low wing puts the vertical CG way above the wing they will balance this out some.

If electric mount the battery as low on the fuse as you can and use a reverse rotation setup. HobbyStar offers 4 & 5 cell 1800mah packs that fit the nose area (only ~75mm long) like an IC fuel tank.

Best, DennisT

Thanks Dennis, all good suggestions.

The Brodak kit is an upgrade from the original Sterling one. It has fuselage ply doublers that go past the LE as you suggest. Also, the motor mount hardwood is twice as long as the original Sterling design. So good there.

And I have already decided to go glow and use an OS 26 Surpass four stroke. (Being a retired mechanical engineer, I need pistons and crankshafts to power my models!!)

Ken Culbertson · « **Reply #11 on:** November 24, 2023, 09:35:49 AM »

I have two suggestions if you are going OTS legal. Get some 1/8" x 1/32" CF strips to glue to the TE of the wing flaps. , movable of not. This will keep them from warping which they do quite easily. Second, if you are considering electric find some one that can make you a box configuration for Li-Ion. Any configuration from 2s to 6s can be made by stacking them instead of placing them end to end. This makes them fit nicely in that short nose. Something that will help with the wing joint is to use 5/16" square, or triangle if you can find it, balsa fillets solidly glued in. It is much like a saddle but stronger and easier to flair into the joint. Fiberglass OVER the fillet. If my memory is working today, the way the original gear was mounted created a gigantic oil soaking hole at the LE. We hand launched most of ours, so they had that hole sealed. I think the Brodak moved it higher. In either case it would be better to use bolt on aluminum than through the fuselage wire. It is a great era appropriate plane. Flys well if flown fast and never over controlled. Better than both it's Mustang and Ringmaster siblings. If you want to turn really tight make sure your pilot figure is wearing a parachute and is stall certified.

Missed your post on going IC.

Colin McRae · « **Reply #12 on:** November 24, 2023, 09:56:59 AM »

Quote from: Ken Culbertson on November 24, 2023, 09:35:49 AM

I have two suggestions if you are going OTS legal. Get some 1/8" x 1/32" CF strips to glue to the TE of the wing flaps. , movable of not. This will keep them from warping which they do quite easily. Second, if you are considering electric find some one that can make you a box configuration for Li-Ion. Any configuration from 2s to 6s can be made by stacking them instead of placing them end to end. This makes them fit nicely in that short nose. Something that will help with the wing joint is to use 5/16" square, or triangle if you can find it, balsa fillets solidly glued in. It is much like a saddle but stronger and easier to flair into the joint. Fiberglass OVER the fillet. If my memory is working today, the way the original gear was mounted created a gigantic oil soaking hole at the LE. We hand launched most of ours, so they had that hole sealed. I think the Brodak moved it higher. In either case it would be better to use bolt on aluminum than through the fuselage wire. It is a great era appropriate plane. Flys well if flown fast and never over controlled. Better than both it's Mustang and Ringmaster siblings. If you want to turn really tight make sure your pilot figure is wearing a parachute and is stall certified.

Missed your post on going IC.

Thanks Ken, again, great suggestions.

Electric

For a warbird replica, a model that makes no noise is a foreign concept to me! Must be a FS.

Colin McRae · « **Reply #13 on:** November 24, 2023, 12:10:25 PM »

Quote from: Ken Culbertson on November 24, 2023, 09:35:49 AM

I have two suggestions if you are going OTS legal. Get some 1/8" x 1/32" CF strips to glue to the TE of the wing flaps. , movable of not. This will keep them from warping which they do quite easily. Second, if you are considering electric find some one that can make you a box configuration for Li-Ion. Any configuration from 2s to 6s can be made by stacking them instead of placing them end to end. This makes them fit nicely in that short nose. Something that will help with the wing joint is to use 5/16" square, or triangle if you can find it, balsa fillets solidly glued in. It is much like a saddle but stronger and easier to flair into the joint. Fiberglass OVER the fillet. If my memory is working today, the way the original gear was mounted created a gigantic oil soaking hole at the LE. We hand launched most of ours, so they had that hole sealed. I think the Brodak moved it higher. In either case it would be better to use bolt on aluminum than through the fuselage wire. It is a great era appropriate plane. Flys well if flown fast and never over controlled. Better than both it's Mustang and Ringmaster siblings. If you want to turn really tight make sure your pilot figure is wearing a parachute and is stall certified.

Missed your post on going IC.

I have already sealed up the hole on the fuselage that was intended for the stock wire landing gear setup and going to use bolt-on aluminum landing gear. Concerning the aluminum profile-mounted gear, what I have doing in the past, and is simple, is to use two glued-in brass tubes through the fuselage (flush w/ the fuselage doubler) for the gear 4-40 mounting bolts to pass through. So, when tightening the gear bolts, the alum gear bears up against the tubes nice and tight without crushing the fuselage.

Concerning using woven fiberglass cloth for the wing to fuse joint for added strength, others recommended 3/4 oz cloth. I will have to buy some, but I don't see 3/4 oz readily available at hobby shops. But I do see 0.6 oz and 1 oz weight. Is one weight recommended over the other for such a small wing? And what is the best resin product to use for the cloth that is sandable?

Steve Berry · « **Reply #14 on:** November 24, 2023, 12:34:32 PM »

Quote from: Colin McRae on November 24, 2023, 12:10:25 PM

Concerning using woven fiberglass cloth for the wing to fuse joint for added strength, others recommended 3/4 oz cloth. I will have to buy some, but I don't see 3/4 oz readily available at hobby shops. But I do see 0.6 oz and 1 oz weight. Is one weight recommended over the other for such a small wing? And what is the best resin product to use for the cloth that is sandable?

Personally, I'd go with Z-Poxy finishing epoxy (for laminating) as it's nice & thin, and sands easily.

For the fiberglass, the .6oz stuff should work fine. If you're at all worried about it, use the 1oz. Just be sure to feather out the edge so you don't build in a stress riser inadvertently.

Steve

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Ken Culbertson · « **Reply #15 on:** November 24, 2023, 12:39:24 PM »

Quote from: Colin McRae on November 24, 2023, 09:56:59 AM

Electric For a warbird replica, a model that makes no noise is a foreign concept to me! Must be a FS.

I felt quite the same once. That was *until* I tried Electric and got over the rather extreme learning curve. Somehow selecting your RPM on a digital display instead of tweaking a needle valve till it sounded just right was just not manly. You get over the sound in time.

Ken

To your last post - Either .6 or 1oz will be fine. I tend to use thinner glass because it is easier to finish over. I have always apply it with thinned (a drop or 2 of denatured alcohol) 15min Epoxy but I have read here that heating it may be better than thinning it. Finishing resins like Z-Poxy also work and are meant to be brushed. Just keep in mind that they are for finishing, easy to sand, and are not for strength. That is why I use regular. I have also tried using slow cure CA applied with a smoothing stick. Seems to penetrate the sheeting better, then add a coat Epoxy/Z-Poxy after it is stuck down.

Like the brass tubes. I use aluminum for the same thing.

Colin McRae · « **Reply #16 on:** November 24, 2023, 01:29:42 PM »

Quote from: Ken Culbertson on November 24, 2023, 12:39:24 PM

I felt quite the same once. That was *until* I tried Electric and got over the rather extreme learning curve. Somehow selecting your RPM on a digital display instead of tweaking a needle valve till it sounded just right was just not manly. You get over the sound in time.

Ken

To your last post - Either .6 or 1oz will be fine. I tend to use thinner glass because it is easier to finish over. I have always apply it with thinned (a drop or 2 of denatured alcohol) 15min Epoxy but I have read here that heating it may be better than thinning it. Finishing resins like Z-Poxy also work and are meant to be brushed. Just keep in mind that they are for finishing, easy to sand, and are not for strength. That is why I use regular. I have also tried using slow cure CA applied with a smoothing stick. Seems to penetrate the sheeting better, then add a coat Epoxy/Z-Poxy after it is stuck down.

Like the brass tubes. I use aluminum for the same thing.

Again, thx for the build recommendations as I am a novice builder.

Concerning electric vs glow, at our club field, I notice RC'ers (electric) packing up and leaving because they used up the charged batteries they brought. Maybe 4 flights.

With my gallon jug of fuel, I can fly as many times as I want. (But then I spend a couple of hours at home cleaning up my models after a day of flying.) Pros & cons I guess.

Colin McRae · « **Reply #17 on:** November 26, 2023, 05:01:18 PM »

I do think I will build the Yak-9 to be OTS legal, therefore not use the dihedral modification to address the vertical CG issue. But I read somewhere a wing trim tab can also be used to compensate for the CG issue to level the wing during flight.

Do others have experience with this method specifically on the Yak-9 kit? I would like to better understand the pros/cons of a wing trim tab, and its effect on how the Yak-9 model will stunt.

Paul Smith · « **Reply #18 on:** November 26, 2023, 06:46:12 PM »

I got a used Yak-9 years ago and used it for OTS, Fox race and Fox Stunt combat.
It really wasn't much of a plane compared to better designs of the time. The extreme low wing is the disadvantage. And of course, thin airfoil and small wing area. A Flite Streak or a Sig Skyray is a better bet.

If I really wanted to build a Yak-9 and make it fly better, I might put the controls external with the bellcrank and leadout guide at the same elevation as the crankshaft. That should resolve the issue of hanging out better than dihedral or a big trim tab.

The subject of a Yak-9 with dihedral and wing gear jogged my memory. Several year ago a scaled a Yak-11 racer and it served me will for a long time. It looked scale and stunted well enough for its purpose.

Dennis Toth · « **Reply #19 on:** November 27, 2023, 07:49:23 PM »

Paul,
Moving the bellcrank up on the fuse will not improve the vertical CG as only the leadout at the wing tip sets this. With both the Sterling Yak and Mustang the only thing that helps is to mount the leadouts on top of the wing tip former and as high up as you can get it. Second this is to use heavy wheels to get some weight below the wing. Use a light tank and engine. Some have used tabs or wedges to lift a little on the outboard wing.

Best, DennisT

john e. holliday · « **Reply #20 on:** November 27, 2023, 09:23:29 PM »

As I stated on the other thread, build the Yak according to the kit. Keep it light and straight as the old time pattern is not as suveer as the AMA pattern.

Paul Smith · « **Reply #21 on:** November 28, 2023, 03:56:48 PM »

Quote from: Dennis Toth on November 27, 2023, 07:49:23 PM

Paul,
Moving the bellcrank up on the fuse will not improve the vertical CG as only the leadout at the wing tip sets this. With both the Sterling Yak and Mustang the only thing that helps is to mount the leadouts on top of the wing tip former and as high up as you can get it. Second this is to use heavy wheels to get some weight below the wing. Use a light tank and engine. Some have used tabs or wedges to lift a little on the outboard wing.

Best, DennisT

The idea of raising the bellcrank to the leadout level is to eliminate the stress on the leadouts if you only move the leadout guide. Yes, you can just move the leadout guide and it will work for a while.

Colin McRae · « **Reply #22 on:** December 06, 2023, 04:34:06 PM »

Have another build question.

On the trailing edges of the operable flaps and elevator, is it better to round off or taper the TE; or keep it square.

(I thought I read somewhere that a square TE functions better aerodynamically. But don't remember where I read it. I also notice a lot of RC models have square trailing edges.)

Thanks in advance.

Steve Berry · « **Reply #23 on:** December 06, 2023, 06:18:21 PM »

I've always just left mine either square or rounded over, but have yet to really taper them.

Steve

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Ken Culbertson · « **Reply #24 on:** December 06, 2023, 09:08:29 PM »

Quote from: Colin McRae on December 06, 2023, 04:34:06 PM

Have another build question.

On the trailing edges of the operable flaps and elevator, is it better to round off or taper the TE; or keep it square.

Square. I was a taper fan most of my life. They simply look better. On advice of council (the guys at the circle) I changed. Squared or "slab" flaps have advantages. They react faster, not much but still faster. They do not warp as easily as tapered. Shaving or sanding in the taper releases tension in the grain and causes them to warp. And finally, it makes adding the ribs in the lightening holes easier.

Ken

Ken Culbertson · « **Reply #25 on:** December 07, 2023, 08:05:24 AM »

Quote from: Ken Culbertson on December 06, 2023, 09:08:29 PM

Square. I was a taper fan most of my life. They simply look better. On advice of council (the guys at the circle) I changed. Squared or "slab" flaps have advantages. They react faster, not much but still faster. They do not warp as easily as tapered. Shaving or sanding in the taper releases tension in the grain and causes them to warp. And finally, it makes adding the ribs in the lightening holes easier.

Ken

Question - if flat flaps offer a faster response, why not the same for the elevator? Would that have the same effect as a fence?

ken

Colin McRae · « **Reply #26 on:** December 07, 2023, 08:35:18 AM »

Quote from: Ken Culbertson on December 07, 2023, 08:05:24 AM

Question - if flat flaps offer a faster response, why not the same for the elevator? Would that have the same effect as a fence?

ken

I have the same question. I was inquiring about both the flap and elevator TE. To square or not to square. And I am talking about my relatively small Yak-9. (Maybe on this particular model intended for OTS it makes little if any difference.)

Colin McRae · « **Reply #27 on:** January 03, 2024, 06:42:57 PM »

Admittedly slow but some progress on my Yak-9 build.

Completed the structure on the fuselage. Just needs rounding and sanding.

Completed the elev/stab. I decided to just use the kit cloth hinges as the surfaces are only 1/8" thick, and I was worried about using nylon pin hinges and getting them properly centered and aligned.

Got the flaps/joiner wire together. I increased the joiner wire diameter to 3/32" music wire. The 1/16" kit joiner wires seemed weak to me. I am going to make the flaps operable. Not sure as yet what hinge design I will go with. The kit calls for cloth type on the flaps. But I may go with nylon pin type since the flaps are thicker balsa. Either should work fine on this 40" smallish model.

Next I will start on the wing fabrication. Also planning on using CF pushrods w/ ball joints.

Based on some simple engineering calculations along with some algebra, I moved the heavier OS 26 FS engine mounts back 11/16" (plus shortened the nose by the same amount) for obtaining the desired CG vs a lighter OS 26 LA. (I did the same procedure on my recent Skyray 35 build (i.e.: Fox 35 weight vs heavier OS 25 LA w/ stock muffler, and I did not have to add tail weight to CG balance.) Hopefully it will work out the same on the Yak-9)

kenneth cook · « **Reply #28 on:** January 06, 2024, 06:40:01 AM »

I personally never found the Yak to fly better with flaps, in fact mine flew worse. These were all original Sterling Yak-9's as I'm not familiar with the Brodak version. I believe your losing a bit of wing area when operated in my opinion. In addition, due to the wing's trailing edge having taper, as the flaps move up or down, they bind. They need to come away from the trailing edge . When attached via hinges, they no longer can do this creating a very hard bind. Cloth hinges allow for this movement but after a while they begin to flutter in flight. Using pin hinges makes the problem worse as well. The hinge next to the jointer is trying to pull out of the wing when the flaps operate. On one of my Yak-9's, I used a wishbone like pushrod off of the bellcrank to operate the flaps so that they utilized no joiner. This worked with the exception of one problem, as the bellcrank went through it's travel, one flap would travel a bit more. I ended compensating this with a Kwik-Link but for all the difficulties in making this operate, my later Yak's used fixed flaps.

Ken Culbertson · « **Reply #29 on:** January 06, 2024, 07:59:49 AM »

Quote from: kenneth cook on January 06, 2024, 06:40:01 AM

I personally never found the Yak to fly better with flaps.

I am with you on this one. "Back in the Day" I also built several and all but one was built fixed. We used them for slow combat and the flaps didn't help, in fact they were lethal. My opponent was always the Mustang and it was flapless too. The one I built with flaps slowed down so much in corners that I never got a decent pattern out of it. I guess with the larger sizes of the OTS pattern it might work with something more robust that a McCoy 35. The flap angle never caused any problems I can remember. I used full span cloth hinges on both the flaps and stab. Guess I was sealing and didn't even know what that was!. Still, with all of it's flaws, I really enjoyed my Yaks. Has anyone ever figured out why the Sterling Yaks outperformed their Mustangs?

ken

Bob Hunt · « **Reply #30 on:** January 09, 2024, 03:57:12 PM »

I thought this photo belonged in this thread. That's my brother, Jimmy, kneeling down next to my father's Yak 9 at a local Combat contest in the New Jersey area in 1953. Jimmy is now 85 years old! The Yak is long gone...

Memories - Bob Hunt