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General control line discussion => Open Forum => Topic started by: Chris Fretz on November 11, 2019, 03:24:58 PM
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So I've been building a Brodak 38 Special, I thought it would be cool to have a Biplane to stunt with. Not too many to choose from really (maybe there is a reason for this) I was putting the lower wing, stab and tail on yesterday when I sat back and thought to myself "man this thing is tiny!" Well anyway I plan on putting a LA46 in it since... well I really like them. I'm surprised it suggests a 35-46 really. I've read on here a lot of people using the LA46 said it flew good and the engine was over kill. So those of you that used a LA46 in the 38 Special what prop and line length did you use? It also sounds like it isn't very good in the wind also. I'm sure it will be fun to attempt to trim out. Thoughts on this bird?
Chris
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Top wing not glued in yet.
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520 inches of wing and extra drag.
The 46 shouldn't be a problem.
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I had an OS .46 in mine. Don't expect tight corners or tight anything for that matter. Big, smooth shapes or you'll kill all your airspeed and stall it. I flew mine like a BiSlob most of the time anyway. .015x60 is what I used on mine.
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I had one. Put a Fox 40 coffin case first generatio, serious great power engine.. Great flying machine I got really proficient at doing the pattern at 90 MPH. I never considered the model a serious anythng but that \Fox is a real stump puller and it was more fun than should have been legal. Outside maneuvers were particularly exhilarating and my sense of timing was never better than with that model. Dan Banjock flew it once. he did admit that it was a breathtaking event. This from a guy who flies the slowest Dyna Jet in the world. I eventually sold it to a friend who never flew it but has it hanging from the ceiling in his shop with his admonition of 'someday'
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We need a Biplane event at Brodaks!! ;D
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I think the .46 would be a great choice.
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Chris,
Looking forward to flying one myself....
I had a "pre-Brodak" kit. It was probably the worst cut kit I've had the displeasure of opening. Essentially, I chucked all the misshapen ribs, most of the wing wood, the tail--and should have replaced all of the fuselage parts, but decided to splice, patch and fill there.
The fuse on mine came out heavy after trying to salvage some of the kit parts. Trying to decide if I should chuck the fuse and build another. But I'm pleased with how the scratch built wings and tail turned out.
The Model Aviation article talks about its lineage, and that it needed some decent horsepower up front. I remember a modified Royal in Dixon's own plane. So I drilled the holes for the .46LA. If you haven't read the article, you should go into the MA archives and check it out.
Hope yours flies great!
The Divot
"When a wise man told me to leave my mark on the world, it didn't occur to me that he was not talking about chipped asphalt...."
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Chris,
Looking forward to flying one myself....
I had a "pre-Brodak" kit. It was probably the worst cut kit I've had the displeasure of opening. Essentially, I chucked all the misshapen ribs, most of the wing wood, the tail--and should have replaced all of the fuselage parts, but decided to splice, patch and fill there.
The fuse on mine came out heavy after trying to salvage some of the kit parts. Trying to decide if I should chuck the fuse and build another. But I'm pleased with how the scratch built wings and tail turned out.
The Model Aviation article talks about its lineage, and that it needed some decent horsepower up front. I remember a modified Royal in Dixon's own plane. So I drilled the holes for the .46LA. If you haven't read the article, you should go into the MA archives and check it out.
Hope yours flies great!
The Divot
"When a wise man told me to leave my mark on the world, it didn't occur to me that he was not talking about chipped asphalt...."
Looking good! I was lucky enough to have a laser cut kit.
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I put a 4in bellcrank in mine, what did you guys use? Did anyone put bellcrank stops on theirs or not really needed? I'm concerned it is going to be pretty nose heavy but not sure yet.
Chris
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I had an OS .46 in mine. Don't expect tight corners or tight anything for that matter. Big, smooth shapes or you'll kill all your airspeed and stall it. I flew mine like a BiSlob most of the time anyway. .015x60 is what I used on mine.
I was hoping for a nice corner. Ohhhhh well. What happened to yours?
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I was hoping for a nice corner. Ohhhhh well. What happened to yours?
Engine quit directly overhead, fell like a homesick anvil, you know the rest......
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I put a 4in bellcrank in mine, what did you guys use? Did anyone put bellcrank stops on theirs or not really needed? I'm concerned it is going to be pretty nose heavy but not sure yet.
Chris
Bellcrank stops are always a bad idea. They let the plane hang on one line and can create leverage inside the plane that result in sadness.
It is better to limit bellcrank travel with the correct line spacing at the handle.
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Steve Millet of DVIII built one years ago and used a C/L ST-34. I think he used a 4" bellcrank too, and messed around w/ vertical CG adjustments as well as I remember. The ST-34 hauled it pretty good and it flew a respectable pattern w/o him having to struggle to do it.
Doug
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Engine quit directly overhead, fell like a homesick anvil, you know the rest......
Yep, all too well!
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I have had one for years. Randy Smith Tower 40, never tried an LA-46 because well, if it ain't broken, don't fix it. The LA-46 will be more than enough power and if you use a tongue muffler, should balance OK. I have a 4 ounce clunk tank on mine, plenty for the 40 but marginal for my 46's as I run them in a 4x2x4 break style with an APC 11x5 prop. I think a Brodak 40 would be a good choice as well.
I installed a 4" bellcrank and limited the elevator travel to around 30 degrees. Before this modification it would stumble and stall at even the suggestion of a square maneuver, Ringmaster style. This modification made all the difference. Squares still cant' be 'banged" but are respectable at least, good enough to win first place at a local profile stunt contest a few years ago. 8)
Did you limit the travel with bellcrank stops or in the control system?
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Paul's right, common wisdom will tell you that there are better ways to go about this. However in this case, handle, pushrod and horn adjustments just weren't enough. Since all the controls are out in the open the risks to giving it a try were minimal. If it were a full bodied stunter I would have gone different route.
In practice, I seldom hit the stop but when I do, it is hardly noticeable, much less so then the stall that accompanied a full elevator deflection prior to its installation. I decided to keep the stop and people continue to comment about how nice this small biplane flies. I'll post a pic of how it was made. This stop can be easily removed of you find it unnecessary. 8)
Bellcrank stop consisting of a rectangular plywood plate mounted at the bellcrank pivot and screwed onto the inside fuselage with 2 wood screws. Fuel tubing is sliced and fitted over the edges so if the bellcrank does hit the stops it gives some cushion as opposed to hitting a hard piece of 1/16 plywood.
I'd like to see your set up with the bellcrank stop if you could post a picture! I think I get what you're explaining though.
Thanks!
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I could let you borrow one of my incidence meters if you run out.
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I could let you borrow one of my incidence meters if you run out.
No Hudak I've acquired quite a few of them from swap meets and if I need another I still have a new one in the package. Thanks though! Its crazy how cheap RC guys will sell them.
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Hi Chris,I have no experience at all with that model,but it's refreshing to see someone building a model just for the fun of it and not worrying about that it won't be a "contest ship". I like to build fun stuff just for the hell of it. Cheers,Skip
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I fly a sig ultimate profile with a max 60 the one thing I find is you have to fly it thru the corners Don’t try to hit a sharp it won’t like it. I have a 38 special too I would use the la 46 in it they run good you all ready know that have fun fly it profile at a contest it will be fun
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Hi Guys I just started to scratch build a 38 Special a couple of weeks ago so this has been an interesting read for me. I have both wings built the fuselage and other parts are cut out. My original plan was to build this as an electric plane but reading everything here says to use an ic engine. My questions are has anyone built or seen the plane as an electric plane what is the finish weight of the plane before the engine is installed and if you know what the electric set up would be good for this plane. I have the Brodak electric conversion kit a rim fire motor capable of pulling a 48oz plane around an castle creations lite 50 speed control and a Hubin Fm9 timer and a 3s battery. I also have the LA46, the fuselage is not cut out for the power yet so now is the time for me to make the decision on what I'm going to power the plane with any suggestions, ideas, or experiences are much appreciated.
Thanks Den W
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De'Warch,
Sounds like a fun project. I can give you the weights of all the components from my build so you can see how one specific build is going. The hardware choices and fuse construction seem to be the killer for me. You can check out the notes and see my options to get the weight back under control from my original concept. And, since mine is not done, some of these weights are estimates which I hope are conservative. For example, I don't know that I'll need an ounce of tail weight, or that it will take 6 oz. to cover and finish, but it's my best guess right now.
Good luck with your project,
The Divot
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Years ago Jerry Raimo built one with an RC LA-46 and 3-line set-up for throttle control. He had a lot of fun flying that when NVCL used to do demo's for the cub scouts, Udvar-Hazey, etc.
Check the metal gear in your kit, a lot of that gear was not correctly tempered and would collapse on hard landings. Jerry was always having to rebend his.
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Hi Chris,I have no experience at all with that model,but it's refreshing to see someone building a model just for the fun of it and not worrying about that it won't be a "contest ship". I like to build fun stuff just for the hell of it. Cheers,Skip
Thanks Skip! But I must confess... I'm itching to build a stunter!
Formally
Years ago Jerry Raimo built one with an RC LA-46 and 3-line set-up for throttle control. He had a lot of fun flying that when NVCL used to do demo's for the cub scouts, Udvar-Hazey, etc.
Check the metal gear in your kit, a lot of that gear was not correctly tempered and would collapse on hard landings. Jerry was always having to rebend his.
Wow a 3 line would have been a cool idea! I've never tried one though. Is there a way test if it's tempered?
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Is there a way test if it's tempered?
Sure! Pancake it in on asphalt. If it collapses, it wasn't properly tempered; if it springs the plane back into the air, it was. ;D
Generally, if it is properly tempered you shouldn't be able to easily bend it, in fact, it is more likely to crack than bend. There are guys on here that have a lot more knowledge on this than me, hopefully they'll speak up.
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If you are forming the gear yourself, you can tell a soft temper by how easy it is to bend. If you already have formed gear from the kit, then take a different approach and try to simulate a Rockwell hardness test. Take a spring-loaded centerpunch and put a divot (like that technical term, eh?) in the gear down near where the wheel goes. (In other words, in a low stress area.) Notice how big/small the divot is. Then do this again on a known piece of good stuff, like a piece of 6061-T6 or 2024-T3. Compare. If the divot in your parts is much bigger than the calibration piece, then you probably want to plan on upgrading. If you can matchdrill accurately, you can always upgrade the gear later with better stuff if you find it is yielding in normal use.
Now since this is just model airplanes, you could take one of your gear legs and try to bend it in your hands. Unless you've got some impressive grip strength, it should be pretty tough to bend. If not, then you probably aren't going to be happy with it.
Sometimes you will get parts with the mill marks still on it. On a part as small as a gear leg you'll never see enough of the label to make any sense. What you can tell is if the mill mark is red it is likely 6061. If blue, likely 2024. This can be helpful if you are grabbing cutoffs and drops from the local metal supply shop and they aren't too careful about maintaining the scrap bin. You are much more likely to find 6061 material associated with model airplanes than 2024 simply due to the difference in cost. Architectural grades as found in Home Depot and elsewhere are going to have great corrosion resistance--but really poor strength. I would not recommend using it, even in thicker sections. It still bends and you are carrying around the extra weight all the time....
Remember that each alloy has a minimum bend radius requirement. (Go find one of the published tables.) The stronger/harder the alloy, the bigger that radius needs to be to avoid cracking. Always deburr and smooth your part before bending if you are trying for a minimum radius. Polish the edges even. And lay out the part with the grain running perpendicular to the planned bends. Yes, rolled aluminum sheet has a grain orientation. It is not completely isotropic. Don't use a brake or tool that has a sharp edge.
I attached a few numbers for guys that want to compare strengths.
The Divot
Yield Strength of Some Applicable Aluminum Alloys, Sheet form
6061-O 8 ksi [12]
6061-T4 21 ksi [16]
6061-T6 40 ksi (35) [35]
2024-T3 50 ksi (41) [39-40]
Sources: engineering texts; (McMaster-Carr); [Aircraft Spruce]
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Thanks Dave for the weight summery of your build . I weighed mine last night and the weights are very similar, I now have an idea of what I have to work too.
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Dave Hull: great explanation! Thanks!
Can you bend to a tighter radius with heat or should you ever even bend without heat? Years ago I got a flat gear blank from Sig and had to heat it up to be able to bend it. Without heat I thought it would crack.
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Steve Millet of DVIII built one years ago and used a C/L ST-34. I think he used a 4" bellcrank too, and messed around w/ vertical CG adjustments as well as I remember. The ST-34 hauled it pretty good and it flew a respectable pattern w/o him having to struggle to do it.
Doug
He has a .46LA with hemi head in it now... and really doesnt like flying it lol.
Sent from my SM-G965U using Tapatalk
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Scott,
“Can you bend to a tighter radius with heat or should you ever even bend without heat? Years ago I got a flat gear blank from Sig and had to heat it up to be able to bend it. Without heat I thought it would crack.”—Scott Richlen
You can heat aluminum to bend it—but you may not get the result you are trying to achieve. There are a few issues to consider.
First, before we talk about bending it, understand that there is the hot strength issue. The strength of aluminum starts to go away at a relatively low temperature. If I recall correctly, for aircraft the military requires that you progressively derate the handbook values of aluminum for temperatures above 140F. That’s not very hot, in a practical sense. A car hood sitting in the midday sun will get that hot. And it’s why you don’t have the forward surfaces of triple-sonic aircraft made out of aluminum. It would be like rubber as loads are applied. So, for the question at hand, heating up your gear leg will reduce the yield strength, and it will take less force to bend it--but it may still fail while you are bending it unless you have reached a hot enough temperature to cause the precipitates to go back into solution.
Second, for something like a landing gear you are going to want the high yield strength in the finished part so it can do its job without permanently deforming every time you bounce a landing. If you could anneal the material to make it easy to bend or easy to make a tight bend, then what you are left with is a soft gear. The material will work harden some as it is bent, but it will not be close to the original heat-treated properties. For aerospace, if a small bend radius is critical, they would form the part in the O-condition and then send it to a certified heat treater to have it properly brought back to the correct temper. Shops that do a lot of forming work have ovens next to the presses and form tools. They may reanneal it multiple times before achieving the final shape. Then they go back in the heat treat oven for solution HT and aging. You are not going to be able to do that in your kitchen oven. It must be done hotter, for a long period of time, and be quenched.
Third, many alloys will naturally age harden, but they only “come back” so far. Since this is not at a “solution heat treat” temperature, you’re not going to get much back.
Fourth, trying to hit a high enough temperature to cause enough annealing to make the part malleable without overheating and melting it (oxyacetylene) can be tough because the metal does not go through obvious progressive color changes like steel does. Heating too fast, in too localized and area, or on small thin parts can lead to a meltdown.
So while you can take your torch (propane or oxyacetylene) to your parts to make them easier to bend, you are not going to have an optimum result for a lightweight landing gear where strength matters. Instead, if you buy material that is exactly the alloy and temper you want, cut the blanks aligned properly with the material grain, and bend it properly using the minimum (or larger) bend radius, you should not have any problems. If you do not have access to a sheet metal brake and find the blanks are difficult to bend (ie. takes a lot of force) then secure the part in a padded jaw vise (I use a pieces of radiused hardwood). Next, clamp a piece of hardwood to the part to use for leverage. I use a piece of ¾” by 2” by 18” oak that was handy. I use it also when bending titanium gear, which takes much more force but is certainly possible.
Minimum Bend Radius for Likely Gear Applications
For .090” thick stock--
6061-T4, T6 .16” (all dimensions to inside of bend)
2024-T3 .38”
7075-T6 .50”
For .063” stock—
6061-T4, T6 .09” (all dimensions to inside of bend)
2024-T3 .22”
7075-T6 .31”
References:
1. Principles of Heat Treating of Nonferrous Alloys, ASM Handbook Vol.
2. Heat Treating of Aluminum Alloys, ASM Handbook Vol. 4, NIST 1991
https://materialsdata.nist.gov/bitstream/handle/11115/192/Heat%20Treating%20of%20Aluminum%20Alloys.pdf?sequence=3&isAllowed=y
3. Minimum Recommended Bend Radius for Maximum Strength, American Machine Tools Co., Chicago IL
https://americanmachinetools.com/bend_radius.htm
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Dave Hull:
Thanks for the additional clarification. One more question: you said " they would form the part in the O-condition ", what is "O-condition"?
Thanks!
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Scott,
Aluminum uses an "O" code to denote the fully annealed condition.
As a side note, you may have heard of aircraft being built with "ice box" rivets. That is, the larger rivets which were made from solid aluminum, which would have to be "set" or driven using air hammers and bucking bars were too hard to be driven without either breaking the rivet, or damaging the structure. So they annealed them to the O-condition to make them softer and easy to drive. But to keep them in the annealed state, they had to store them on the manufacturing floor in ice chests. At room temperature, they would naturally age to a harder condition and again be undriveable. Not a disaster, they just sent them back into the oven for another annealing.
Solid rivets are much less expensive than structural self-plugging blind rivets so they would be preferred. As long as the cost to install them could be kept low. Dry ice was not that expensive.
I was at an aircraft components fabricator a couple of years ago in NY state. They had annealing ovens right next to their press stations so that as they formed the parts, the operator could re-anneal right there at that work station. It seemed pretty efficient, and it looked like they had the parts volume to afford to set up the stations with the right equipment.
Good luck with your metalwork,
The Divot
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From previous research on available formed aluminum gears, they were all 6061T6, and from my 35 years of machine shops, most sheet aluminum you'll find in commercial shops (i.e., not aerospace) will be 6061T6. Highway signs also are likely to be 6061T6, so... LL~ Steve
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Hi Chris, I had one for over ten years and then I moved to VA! While flying at Fentress NAS about 8 years ago , I landed my 38 Special in some tallish grass and the fuselage simply whip lashed itself in half behind the lower wing. The bird was powered by a really great running LA .40 which I later sold to an older friend from PA who then died! I have no idea what happened to my .40 but it was quite adequate to fly this model. I used 60' lines eyelet to eyelet. By this measure the line are slightly longer than instant use flying lines from any source. I think that the bell crank was a 3" affair but it flew well nevertheless! I do think 3 line throttle control would be spiffy to build into this plane.
You will enjoy this model I am quite sure! Its a really fun flyer!
Phil Spillman
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Anneal 6061 aluminum, heat to 800 degrees ,let soak for 2 hours, slow cool 50 degrees per hour to 550 pull from oven and let cool to room temperature. needless to say there aren't a whole lot of modelers that have such an oven. But I did for 20 years.
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It isn't a terrible sin to just bend 6061T6 as it comes...IF you use a decent radius at the bends. Near as I could tell from the research previously mentioned, that's the way they're manufactured. Shocking, I know! FWIW, I've seen the LG struts that come with the 38 Special (aren't they the same as a Firecat?), and I thought they were both too thin and too short to be practical. But if you beef up the LG, then the wing loading goes up. Ewww!
The only "38 Special" I am aware of here in the PNW was built (beautifully) from a kit by Pete Peterson. It has a Magnum XLS .36 for power and as I recall, it's pretty light in the high 30 oz range. When I asked Pete why I've never seen him fly it, he said it just didn't fly very well. I'd bet it would be fun for a sport flier, IF you don't ask it to do the whole pattern or at least, not with hard corners. I'd go with the .46LA and 11x4.5 TT Cyclone prop or 11-5 APC as a starting point. D>K Steve
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I've seen Pete's fly, made the venture for the engine. It flew great, if flying level is the only requirement. It stalled at every opportunity and is not stunt worthy, for any competition at least. Obviousliy a lot of of drag and vertical CG is fussy. Cute tho'🙂
Edit for sp.
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Must have been before I limited the elevator travel.
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Sometimes competition excellence dos no matter a fig, in my case here is a couple of pictures of the 38 Special I built and flew several times about 5 years ago. My wife loves Bipes and asked me to build her one. I picked the 38 Special and modified it a bit. Flew it for her with an old FP .40 I had on hand. Now, I am not much of a builder, or flyer for that matter, but the look on her face as it flew was worth all he effort I had put into the ship! We enjoyed it that day and it hangs in the playroom as a memento of that day and all the days that we have shared each other's hobbies.
Joe Just
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It isn't a terrible sin to just bend 6061T6 as it comes...IF you use a decent radius at the bends. Near as I could tell from the research previously mentioned, that's the way they're manufactured. Shocking, I know!
Yes, just bend it, you might get some flakes, but it's OK that way.
Brett
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Yeah, there's a lot of poof of that. A few flakes here and there aren't a big deal....in more ways than one! LL~ Steve