daddyrabbit1234

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I'm building my second 72" piper cub and I need your expertise. This last one was electric with floats if you remember. This one is going to have a Super Tigre .40
(1) Do I set CG with the fuel tank empty or full?
(2) How close does the fuel tank need to be to the engine?
(3) What % fuel will be best? This engine has never been fired.
(4) What size prop would be best to fly scale? It's got a Master Airscrew 10x6 now.

I'll post pics soon. I got some hinges glued in tonight.
Thomas

52larry52

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I'am not Jester_s1 but let me give you some input on your Cub anyway. 1) liquid fueled (glow, gas, etc.) planes are always balanced dry (no fuel). Of course being a high wing, it's done right side up, a low wing would be done inverted. 2) the fuel tank on your Cub will naturally be positioned between the firewall and the backside of the dash panel. Height, not for and aft positioning, is most important. As close as possible position the center of the tank (where the lines come out and thru the firewall) at the same height as the carburetor of the engine. This will be determined by how you mount the engine, upright, inverted, or sideways. I my opinion, sideways laid over at a 90* angle with a pitts muffler is best with a J-3 cub. You can let the head of the engine stick out in the air stream for cooling and have a fax engine head on the other side of the cowl for a realistic looking cowl. 3) Super tigers like low nitro content, 5% or no more than 10% will work. If you are new to glow fueled engines, and I think you are, then a Super Tiger 40 is a bad choice for your first cowled scale glow project. They make great power but require a tuning ability that can stump most novice glow flyers. Not "user friendly" compared to say an O.S. LA or FX/AX. A .46 LA with a Pitts muffler will be plenty of power for a 72" Cub and will be easy to tune any not overly costly. A .46 FX/AX is way more than this plane needs. BTY an OS LA does not like to be mounted inverted, but is very happy on it's side. FX and AX series are fine inverted. If you still go with the Super Tiger (you'l be sorry) they require a lot of break in time before flying. Follow the directions from the maker. 4) prop size...10-6 or 11-6 on the Super Tiger or the OS 46 LA, and 11-6 or 12-6 on the OS 46 AX/FX. Another great engine selection for a Cub would be a 4 stroke in the .55 to .65 range. This will cost more than a 2 stroke but they sound great and will pull the plane very nicely. Let your wallet be your guide.

JohnBuckner

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Good Morning, ringing in here just to say I absolutely concur with just about everything 52Larry52 has posted, indeed lots of good information there. About the only thing I differ in and that's just me is I hate box mufflers (pitts) and avoid them for stock ones on the that size airplane even if the engine mount has to be rotated to an odd positon or a tunnel cut in the firewall for a stock muffler, agine that's just me

If you are new to float planes I just want to add one point about CG. Now since this is the size and may be the type of float plane where it is desired to fly both on wheels and floats. The very best way to acheve easy change outs between floats and wheels is to set up the airplane on wheels first and yes even fly it and balancing it at what ever position it flys best. Yes always balance with empty tank. Now mark this CG, This will be the exact same CG that you want the airplane to balance at when floats are installed.

In the majority of that type of airplane on twin floats when you install the floats from wheels it will normally come in tail heavy or that CG position has moved aft from where it is at when the wheels are installed. In this case the trick is to not go back to the airframe and start moving stuff or adding weight to get the CG back to that same spot when it was on wheels. The reason is if you do that then every time you just want make a changeover your will have to do it all over agine i.e. adding/removing weight or moving stuff.

The solution is to simply add weight (yes and this is one of the rare times I suggest using lead) to the floats and normally right up in the tip of the bows. I just cut in a little hole in the top deck of the bow and drop in the lead then some epoxy or ca. and cover over with a patch of monocoat. Use enough lead to move the CG back forward to where the airplane flys best when on wheels. In this way change outs can be done very quickly and you actually will never have to balance agine.

John

jester_s1

I think this is the first time someone has asked me a question directly. I'm flattered.

These two knowledgeable guys have pretty much covered it. I'll agree with John that the Pitts muffler is not necessary. Yes, it would look the best, but you're probably not trying to build a scale showpiece here. My .40 size Cub (80 inch wing) had a tunnel cut in the bottom of the fuselage for the standard muffler to fit into. The engine was set with the head at a 45 degree angle down so the muffler would be pointed straight down. The whole thing was hidden by the cowl and worked fine.
I'll disagree on prop selection. 10x6 is a good prop for a .40 on a smaller slippery plane, but it won't pull a Cub very well. 11x6 is too much prop; it won't let the engine get into its power RPM. So I'll suggest an 11x5 to start with. And yes, ST engines are built to not need much nitro. I had a .51 and currently have a .90 that run fine on 10%, which is what I use for everything else. But if I were buying fuel just for a Super Tigre, it would be 5% or no nitro. The .40 will power the plane appropriately for flying off of wheels, but I think you'll find that it will be marginal for water if you plan to add floats. Takeoffs require more power off of water and you also have the floats to pull through the air.
Break in on Super Tigres does take some time. I personally don't find tuning them to be that hard assuming there are no air leaks. Go over the engine before you mount it to make sure all bolts are tight. Locktite the backplate bolts and carb mount bolts, but not the head bolts. Retighten the head bolts after you've broken in the engine as they stretch a little when they are new. If you've ever played guitar, it's the same thing that happens with new strings. Buy the cheapest fuel that the hobby shop has that contains some natural castor (you need it for cooling during the break in) and do the break in procedure as the manual suggests letting it cool completely to ambient temperature between runs. It will probably take an hour of running for the ring to seat, so don't expect a reliable idle or consistent power until that happens. You can break in the engine mounted to the plane if you want to, but it's easier to use a test stand. ST engines have a quirk in that they tend to have rich midranges when the top and bottom end is set properly. My .90 is slobbering rich at 1/2 throttle, but the engine still runs and makes stump pulling power at the top so it's really not a problem for me. OS and Thunder Tiger engines are easier to tune right across the whole throttle range, so consider one of them for your next project.
Since you've flown a Cub before, I probably don't have to tell you to wait for a fairly calm day for you first flight. But I'll do it anyway- wait for a fairly calm day for your first flight. If you find that you have power to spare and the plane flies comfortably around 1/2 -2/3 throttle, you may have enough juice to go with floats if you want to. But I suspect you'll find that it requires nearly full throttle to fly smoothly and not be on the verge of a stall everytime some wind turbulence hits it or through the turns. There's nothing wrong with that as that's pretty much how a full scale Cub flies, but floats will require more power.

daddyrabbit1234

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John, Thanks for the info. The first cub I built is electric with floats. I built this plane with a G60 motor, 70 ESC and a 5s battery. This plane is setup and flies great. The 60 electric motor is a beast of a power plant and I love it. The cub I'm building now is the same type airframe but it'll be my land plane with only the wheels being modified to large 4" bush plane wheels. I'm building this plane so I wont have to switch back and forth between water and land flying. I'm wanting to use the super tigre 40 on this plane because its new and It was virtually given to me. After this build I plan on building an 80" L4 with a 10cc evolution gas engine.

daddyrabbit1234

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Thanks Jester, You the man! I'll be flying this plane strictly with wheels. The first cub I built will be only for water flying. I have to drive about 60 miles one way to my nearest hobby shop and Monday I'll be making that trip. I'll purchase a few 11x5 props as well as some 5% fuel as you have instructed. What is your suggestion for electronics setup? I already have several BB standard digital servo's in my tool box.

jester_s1

It doesn't take much in the servo department for a Cub. Those standard BB servos you have will work fine. If you find that you need to buy some, I use the Hitec 425's for planes this size when I don't need digital.
The first few years I was in the hobby, I really had no use for scale models or Cubs in particular. My Cub was a freebie after a moron nosed it in at the airfield. It's the hardest plane I have to fly, but there's a certain pleasure in getting it right and making it look easy for the peanut gallery.

52larry52

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Both John Buckner and Jester_s1 are correct in offering an inverted @ 45* angle for the engine. I didn't think of that option at the time but it does work well especially because you are kit building and can build the exhaust tunnel as part of the construction not as a modification on an ARF. Easier to do it up front not after the fuselage is already ARF built. More work but you don't have to spend $35 or so on a special muffler. Your choice. Jester gave you more details on what it takes to break in a Super Tiger. If you want to go through that, "knock yourself out". I have, in my old lazy years, taken to the "smooth and easy" route in doing things and an O.S. would sure be smoother and easier. But if your into pain and frustration go ahead and use that almost "free" S.T. I have several Super Tigers in various planes (not in any Cubs) and learned to tune them and so can you, it's just not the easy route. Either way, have fun.

52larry52

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daddyrabbit1234, just for info,what 40 size Cub kit are you building? Great Planes, Goldberg, or some other?

daddyrabbit1234

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Hi Larry, The cub is an ARF that I get from a friend. Not sure what brand but he thinks it's a world model. All the glueing,brace's, covering and landing gear are top quality. This will be my 4th plane. I started about a year ago with a 55" foam 182 Cessna, Then a large scale ep P-47 thunderbolt, next a 72" cub with float's and now this one. I understand what your saying about tuning the super tigre. I welcome a good challenge because I've found that is usually where I have my best learning experience's.

daddyrabbit1234

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What do I use with my receiver? ubec or sbec? I would like to be able to use 2s lipo as the power source.

52larry52

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OK daddyrabbit, It helps to know exactly what your working with. I had the impression you were stick building a kit so the stock muffler with a built in tunnel made some sense, but cause it's an ARF you are putting together I will go back to laying the engine over at a 90* angle with a pitts muffler as the best choice. "Smooth and easy" and well worth the $35-$45 price of the muffler. Is the 3rd photo the one you put on floats or the one your working on? I have 6 glow powered and 3 electric Cubs in my hanger plus two more under construction. Glow power range from .20 size to 1.20 Saito powered 1/4 scale. Some are ARF's, some are kit built. Included in the mix is a 72" World Models J-3. That's the one I have the O.S. .46LA with the pitts muffer in so I know that combo works well. Slimline made pitts mufflers for Super Tigers also so you could use a similar setup using the Super tiger .40. Don't invert it, your only asking for more trouble. KISS means "keep it simple stupid" and that goes along with "smooth and easy". Enjoy.......Oh hay, it looks like you have a great shop to work in too. Lots of good planes can come out of there!

daddyrabbit1234

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The cub in the pic is the first one I put together and it has since been fitted with float's. I found a super tigre .61 I'm trying to get. If I can get it I'll probably put it on instead. Now I'm working on building an engine stand so I can run some engines in.

52larry52

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Super Tigers are heavy. A .61 is not a good choice for that 72" ARF. That would work in 80"-82" ARF such as a hanger-9 or Great Planes 1/5 scale J-3. I just rebuilt a 82" G.P. J-3 for a club mate and used an O.S. 61 FX with a Slimline pitts. Very Nice flying Cub with gobs of take off power yet I balanced it with no added ballast. Power is good (you can use the throttle to adjust it in flight) but too much additional weight is not good, it is not adjustable. The plane becomes a "lead sled"! I think a .61 S.T. in a 1/6th (72") Cub would require a fair amount of lead in the tail to achieve balance. When you do something like that you have the additional weight of the oversize engine plus the additional weight of the tail ballast needed to set the C/G balance. You add double the weight of the upgraded engine and adversely impact an otherwise nice flying airplane. Yes it will fly but will require faster takeoff and landing speeds and faster cruising speed to keep it in the air. If you want to make a hot rod J-3 out of this, that's OK. Clip one bay off each wing and power it with a Thunder Tiger Pro .46 or an O.S. .46 AX. That combo will go just as fast the stock winged one with the .61 (MAYBE FASTER) and be more aerobatic. That takes us back to "KISS" doesn't it. Decisions, decisions!!! P.S. you will also need a larger fuel tank to feed that S.T. .61, or have much shorter flight times!

daddyrabbit1234

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I also bought an O.S. .40 LA today and found a brand new pitts muffler for it that was priced right. I'm hoping santa will bring me that 80" L-4 I've had my eye on. That may be a good candidate for the .61 motor.

52larry52

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Who sells an 80" L-4?? I would be interested in one of those. ARF ?

jester_s1

Larry's right, Cubs don't need that much power. All you'll do with a .61 is overstress the airframe and make the plane heavier.

On your battery question, you're headed down a bad path. There is no good reason whatsoever to use a 2s lipo to power a glow engine plane. You have to remove lipos to charge them, which is a pain to do. You have to buy a bec which is added expense, complexity, and an additional failure point, Don't make things more complicated than they need to be. A 6v NiCd will work now as well as it has for 40 years. If you feel the need to save weight or want something that doesn't self discharge, go LiFe. They don't need a regulator or a BEC and give you all the benefits that Lipos do without the fire hazard that requires you to remove the battery to charge it.

daddyrabbit1234

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How does a nicd or life battery plug into the receiver without a bec? This is why I called you in on this thread. To keep me straight on this build. And I know you will.

daddyrabbit1234

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My friend has it. I'll see what brand it is.

jester_s1

All a BEC is is a regulator that lets you pull power from the flight battery of an electric model to power your receiver. It keeps you from needing two batteries in the model at different voltage, hence the term "battery eliminating circuit." So you'll be replacing the battery eliminator with a battery. Just plug the thing in to your switch and plug the switch in to your receiver.

One thing I'll tell you that you didn't ask is to use good quality switches. The little $2 things that come with most entry level radio gear aren't very good. They'll lose the spring tension that makes the electrical connection in a year or two and don't have any redundancy. A good quality DPDT (double pole/double throw) switch will set you back $15-$20, but it will be robust enough to last 4-5 years in normal conditions. And being DPDT, it is essentially two switches so that if one does lose spring tension and fails, you have a backup built in. Use good batteries too. Tenergy and some other cheap brands are priced right, but they are noticeably poorer quality than Sanyo cells. I buy mine from www.hangtimes.com.

daddyrabbit1234

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Thanks jester, I got a couple of the suggested batteries and a switch on the way. On another note I got my engine stand built today. I gut the Super Tigre .40 on it and ran 3 tanks of fuel thru it. Since it has never been fired I ran it rich and let it 4 stroke and slobber. Tomorrow I hope to run it in some more and maybe lean it up some and start the fine tuning. It runs great. I wanted 5% fuel with 18% castor but the closest thing the hobby shop had was Omega 5% with a 17% blend of castor and synthetic. I cant wait for the OS 40 and the Super Tigre 61 to arrive.

jester_s1

You should be run in enough to be able to get a reliable idle and enough top end to fly the plane. You still have to run it rich for the rest of that first gallon of fuel though. If you don't, the cylinder will glaze from the heat and the ring won't fully seat (at least no time soon) which will mean you won't ever get the power that you should get or as good an idle as the engine is capable of. If you have a tach, 1500 rpm rich on the ground should keep it cool enough in flight and still give you enough power to keep the plane up.

daddyrabbit1234

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I ran a couple more tanks thru the engine today. I leaned it up a little and it idles decent but it's far from being ready for me to fly. I'm gonna finish running this gallon of fuel thru it before I lean it out anymore. By then it should be ready for some fine tuning.

jester_s1

If you can get it to idle low enough not to pull the plane then it's broken in enough to fly. Just keep it extra rich for the rest of that gallon and put in on a plane that you're confident landing deadstick.

daddyrabbit1234

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I ran 3 more tanks thru it today at a mid range speed with mix still being rich "plenty of smoke and slobbering oil". I leaned it up toward the last tank and it will almost idle. I've got enough left of the gallon for about 4 more tanks. Today on the last tank the engine performance changed dramatically and it ran super sweet. It revs from near idle to top end like a chainsaw then back down to near idle is steady and smooth.