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BalsaUSA 1/4 Piper J3 Cub servo selection advice

Old 03-21-2023, 12:40 PM
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Masterguns
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Default BalsaUSA 1/4 Piper J3 Cub servo selection advice

Howdy,

I have the above model & need your suggestions for the servos. Current specs to be installed:

Spektrum Avian 6362-250Kv : Same as Rimfire 1.60 Brushless Electric RC Motor
Spektrum Avian 120 Amp Brushless Smart ESC Speed Control 6S - 12S w/BEC
Spektrum 22.2V 4000mAh 6S 50C Smart G2 LiPo Battery IC5 (onboard x2)
Master Airscrew Wood-Maple - 18x8 Propeller (subject to change....suggestions?)
Several feet of Solartex & custom paint & Callie Graphics vinyl decals
Robart 1/4 Scale Cub Gear
Kavan 5" (125mm) Baloon Wheels with air valve (possibly, but they're MUCH lighter than a set of Dubro's of the same size)
Might go with the Pheonix Models 1/4 Super Cub aluminum wing struts

Should analog suffice or should I spend the extra cash & purchase digital? Personal preference or is there an absolute requirement for digital on a plane this size?

That's all I can think of that's going to add the most weight on top of the factory supplied kit components. I think the weight of the basic kit parts is negligible when compared to the aftermarket gear listed above. I'd estimate the built weight to be around 16lb+/-. I guess I'll find out, however, I need to decide before the covering is applied. What do you think based upon what's listed & what you that know what this kit consists of? Thanks!

S/F,
Masterguns

Last edited by Masterguns; 03-21-2023 at 12:46 PM.
Old 03-21-2023, 08:27 PM
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mongo
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does the kit mfgr make any servo recommendations?
if so, i would start with those specs, and really, as long as the speed/torque specs are met, analog v digital becomes a matter of personal preference.
Old 03-22-2023, 04:00 PM
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Originally Posted by mongo
does the kit mfgr make any servo recommendations?
if so, i would start with those specs, and really, as long as the speed/torque specs are met, analog v digital becomes a matter of personal preference.
I just got the kit this afternoon. I've learned that BUSA doesn't put much info about all the specs & the accessories you'll need for this kit in their ads. Yes, I know this isn't an ARF, but I'm surprised that you'll need much more than what they advertise than I'd anticipated to build this into a respectable model if you don't already have such things. Thankfully I do. You need to be familiar with what's in the instructions before hand & are able to decipher them. I'm not in a rush and maybe I'll wait for the Sig kit to restock if I don't get this one completed beforehand. I still look forward to the build, however. I've not built a kit this large before & it'll be a welcome challenge.

They only suggest in the instructions to go with a servo that has a minimum of 100oz/in torque. These instructions are confusing in some areas, the pics aren't that great & there are typos on the references. I went with the HiTech 5645 Standard Digital High Torque Metal Gear x5 (HS-5645MG at HH site). Those are 143oz/in @4.8v. They should work nicely. Thanks for your reply!

S/F,
Masterguns
Old 03-23-2023, 09:27 AM
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Originally Posted by Masterguns
I just got the kit this afternoon. I've learned that BUSA doesn't put much info about all the specs & the accessories you'll need for this kit in their ads. Yes, I know this isn't an ARF, but I'm surprised that you'll need much more than what they advertise than I'd anticipated to build this into a respectable model if you don't already have such things. Thankfully I do. You need to be familiar with what's in the instructions before hand & are able to decipher them. I'm not in a rush and maybe I'll wait for the Sig kit to restock if I don't get this one completed beforehand. I still look forward to the build, however. I've not built a kit this large before & it'll be a welcome challenge.

They only suggest in the instructions to go with a servo that has a minimum of 100oz/in torque. These instructions are confusing in some areas, the pics aren't that great & there are typos on the references. I went with the HiTech 5645 Standard Digital High Torque Metal Gear x5 (HS-5645MG at HH site). Those are 143oz/in @4.8v. They should work nicely. Thanks for your reply!

S/F,
Masterguns
The Hitec D645MW is an updated version of the HS5645 that offers far better performance and usually at a lower price. They have been available for well over 2 years now and I'm not quite sure why Hitec keeps the 5645 in their lineup. The differences are quite impressive.
Old 03-23-2023, 09:51 AM
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Originally Posted by Truckracer
The Hitec D645MW is an updated version of the HS5645 that offers far better performance and usually at a lower price. They have been available for well over 2 years now and I'm not quite sure why Hitec keeps the 5645 in their lineup. The differences are quite impressive.
Fair enough. Cheaper depending on where you obtain them & if they're not on back order. I'm looking at them on amainhobbies since they show in stock at a better price than HH (back ordered anyway). Which is potentially better for the specs I mentioned for this model...the D625 or 645 that you referenced in my other thread? Thanks.

S/F,
Masterguns
Old 03-23-2023, 10:02 AM
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Originally Posted by Masterguns
Fair enough. Cheaper depending on where you obtain them & if they're not on back order. I'm looking at them on amainhobbies since they show in stock at a better price than HH (back ordered anyway). Which is potentially better for the specs I mentioned for this model...the D625 or 645 that you referenced in my other thread? Thanks.

S/F,
Masterguns
Either would work just fine in that airframe. I've flown several 1/4 Cubs (SIG, BUSA, Hanger 9) with far less servo performance than these servos offer and with no issues. The D625 / 645 has become my go-to servo when performance and economy are important. They out perform anything I've had in that price range. Choosing between the 625 / 645 is pretty much whether you want a faster / slower servo. Just personal preference and whether the particular servo is up to the task torque wise which both are in this case.
Old 03-23-2023, 12:38 PM
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Originally Posted by Truckracer
Either would work just fine in that airframe. I've flown several 1/4 Cubs (SIG, BUSA, Hanger 9) with far less servo performance than these servos offer and with no issues. The D625 / 645 has become my go-to servo when performance and economy are important. They out perform anything I've had in that price range. Choosing between the 625 / 645 is pretty much whether you want a faster / slower servo. Just personal preference and whether the particular servo is up to the task torque wise which both are in this case.
Which is the faster, more smooth servo? Thanks.
Old 03-23-2023, 01:29 PM
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wana be careful here...

this rabbit hole gets real deep real quick.

ya wind up spending way too much time looking for the fastest-smoothest-most powerful-best cost...

and not ever deciding to just buy what ya need and evaluate it in flight.

mongo
Old 03-23-2023, 04:10 PM
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Originally Posted by Masterguns
Which is the faster, more smooth servo? Thanks.
The 625
Old 03-24-2023, 07:13 PM
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Default Servo size?

I have flown several large models (53cc gassers and 1.20 glow) with analog servos with no issues for many years. The Digital servos will hold a tighter center at the expense of higher current draw. You would probably notice the performance improvement of digital servos when flying extreme 3-D aerobatics, or in helicopters. No need to spend the extra money just to get digital servos, although most of the larger servos made now are digital. You will never notice the difference in a J-3 Cub. Since you are going electric, be sure the battery eliminator circuit can pump out at least 10 amps if you go the digital route. A stalled digital servo will easily pull 5+ amps. You may want to consider higher voltage rated servos that can take 6.0 to 8.4 volts without damage, Then you could use a separate 6.6 volt LiFe battery for the Rx and servos, with an Opto ESC for the motor fed from a separate LiPo battery. This could save you some brown-outs and subsequent loss of control. The weight penalty would only be about 4 ounces for the LiFe battery.
Old 03-24-2023, 07:47 PM
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Originally Posted by LLRCFlyer
I have flown several large models (53cc gassers and 1.20 glow) with analog servos with no issues for many years. The Digital servos will hold a tighter center at the expense of higher current draw. You would probably notice the performance improvement of digital servos when flying extreme 3-D aerobatics, or in helicopters. No need to spend the extra money just to get digital servos, although most of the larger servos made now are digital. You will never notice the difference in a J-3 Cub. Since you are going electric, be sure the battery eliminator circuit can pump out at least 10 amps if you go the digital route. A stalled digital servo will easily pull 5+ amps. You may want to consider higher voltage rated servos that can take 6.0 to 8.4 volts without damage, Then you could use a separate 6.6 volt LiFe battery for the Rx and servos, with an Opto ESC for the motor fed from a separate LiPo battery. This could save you some brown-outs and subsequent loss of control. The weight penalty would only be about 4 ounces for the LiFe battery.
Digital servos in the same torque spec range as similar analog servos donít draw any more current. Modern digital servos offer zero disadvantages compared to analog servos and digitals offer many advantages. Some people are stuck in the past and donít know how well the current crop of digital servos work. Oh yes, modern digitals no longer sing like the old ones did. Many people complained about that but modern processors eliminated that complaint. Look around at what companies are offering. Many familiar analog servos are gone from product lineups and will soon be gone completely.

I also disagree about not being able to feel the difference between a quality digital servo and an analog servo on a CUB or any airplane for that matter. I firmly believe that any average flyer would feel the difference given an honest side by side comparison. Iíve seen it happen several times over the years and with some real hard headed non believers. Oh well, be stuck with 20+ year old technology, its not my airplane. Iíll continue to enjoy the improvements that come along and Iím a very hard headed 74 year old that is often accused of living in the past Ö. but now with servo tech.

Last edited by Truckracer; 03-24-2023 at 07:56 PM.
Old 03-27-2023, 07:36 AM
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Still more questions...

What are thoughts on placing the rudder & elevator servos on the outside of the fuselage inches away said control surfaces like most foamies are OOB these days? I would have more positive control, yes? I want to avoid setting up those 48" rods from the cabin to the control surface. Just like the ailerons are mounted. Thanks!

S/F,
Masterguns
Old 03-27-2023, 07:48 AM
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Originally Posted by Masterguns
Still more questions...

What are thoughts on placing the rudder & elevator servos on the outside of the fuselage inches away said control surfaces like most foamies are OOB these days? I would have more positive control, yes? I want to avoid setting up those 48" rods from the cabin to the control surface. Just like the ailerons are mounted. Thanks!

S/F,
Masterguns
Cubs have long tails and easily get tail heavy. For that reason, most are set up with forward servos.
Old 03-27-2023, 03:35 PM
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If the fuselage is long enough to require a 48" long 4-40 pushrod, the rod will weigh about 42 grams, or about 1.5 ounces. The plastic push rod guide tube will weigh about a half ounce. Assuming a tail mounted servo will use a rod less than a foot long, then the reduction in weight for each push rod will be about 3/4 of the total 2 ounce weight, or about 1.5 ounces, leaving a push rod weight of about a half ounce. Deleting 1.5 ounces of pushrod which had a center of mass 18 inches aft of the original servo location yields a reduction of 27 ounce-inches of tail heavy moment. Neglecting the additional weight of the 3 ft servo leads, moving each 2-ounce servo three feet towards the tail adds 36" x 2 ounces, or 72 oz-in of tail heavy moment, for a net increase of 45 oz-in per servo relocated. With three servos, the plane will be more tail heavy by a moment of about 135 ounce-inches. Since you are using electric power, the weight forward of the firewall will probably be less than a gas engine would have been and the plane will tend to be even more tail heavy. If an early model gas engine with a heavy flywheel such as a Zenoah G-38 or a Quadra 35 was to be used, then the tail weight would have been less cumbersome. Remember the Balsa USA Cub was designed when these heavier gas engines were the "go to" engines. Assuming the distance forward of the CG to where a balance weight could be readily attached is about a foot, the plane may need about 11+ ounces of lead to balance out moving the servos aft.

Therefore, I suggest trying to estimate the cg location as equipment is added and make the servo and battery locations based on achieving the correct CG while adding the least amount of ballast. If the servos are to be rear mounted, then also know that the servo leads will need to be a larger wire gage to avoid unacceptable voltage loss due to the long servo lead. Also some type of an RF choke or voltage buffer may need to be added to reduce radio noise. This is essentially adding electronic filters or wrapping the servo lead through and around a circular permanent magnet 3 or 4 times. Look up how the giant scale 3-D fliers handle this issue.

Any change always seems to have a circular impact of necessitating endless compensatory changes. So maybe there there actually was a good reason for creating the Military Standard Specification (Mil-Spec) # MILTP4AC, which stands for "Make it like the plans for a change".
Old 03-27-2023, 10:00 PM
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Originally Posted by LLRCFlyer
If the fuselage is long enough to require a 48" long 4-40 pushrod.....
Okay, I'll continue with the manufacturer recommendations. They call for the 48" NY-ROD semi-flex pushrods & their 36" cable. I'm not using a pull-pull or such & I'm not buying the cable. Do y'all have a preference on the sleeve? I'll connect one rod to the rudder & one to the elevator. I've got some high torque servos now. The 48" NY-ROD's I'm finding out are hard to find. Do you have a recommendation for a good alternative for another brand? Any thoughts on the Du-Bro Lazer Rods? The control horns look a tad small to my eye for a model this size. Shouldn't matter because my servo torque is prolly more than enough provided the spec BUSA called for back when this kit was designed? Pitch them & go a size or two larger? I've neither built nor flown such a big model before...at least nothing larger than a 2.1m.

I'm still trying to gather as much knowledge on this thing as possible especially. The hardware pack is lacking & showing its age. I hope to have this kit completed by mid-June. I think I'm going to use the Castle Creations Creation BEC Pro 20 Amp 20A Battery Regulator 2S - 12. Comments? Can I forgo using a capacitor, as well? I'll be using some variety of any one of the endless Spektrum rx I have. I'll prolly use one of their SPM4651T DSMX SRXL2 remote serial rx if it can possibly provide a bit of extra reliability. Y'all keep the feedback inbound. Thanks.

S/F,
Masterguns
Old 03-28-2023, 04:22 AM
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Originally Posted by Masterguns
Which is the faster, more smooth servo? Thanks.
It's a Cub, not a pattern plane. Or a jet. Geez.
Old 03-28-2023, 05:50 PM
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Originally Posted by rgburrill
It's a Cub, not a pattern plane. Or a jet. Geez.
Point well taken and appreciated but some of us are very picky about out CUBs! I donít treat or fly my CUBs any different than any other airplanes I own and a CUB doesnít have to fly like a slug.
Old 03-29-2023, 05:00 AM
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Default Pushrods

Hi Masterguns,

Concerning the pushrods, the Du-Bro Lazer rods are very nice rods for smaller models with shorter pushrod lengths. Du-Bro did a good job of reducing the temperature elongation issues that were so common years ago. There are still some length changes due to temperature variation, but they have it down to where it is negligible in average 2' lengths. But in 4' lengths, you might notice some minor trim changes being required between winter and summer flying. My main concern would be about strength, as the Lazer rods (as far as I know) are only set up for 2-56 threaded rods, not 4-40. The Balsa USA Cub specifies up to a 1.08-2C or a 1.20 4-C engine, These produce about 2.5 HP. If your electric installation will be much over 2 HP, my personal opinion is that 2-56 threaded hardware could be marginal. I prefer 4-40 threaded hardware when servo torque exceeds 100 ounce-inches and I believe your servos will be in the 140 ounce-inch range. It is true the actual load on the servo will normally be limited to whatever the aerodynamic loads produce. However, if there is any binding or control surface flutter, the servo will crank out torque up to its rated value. Therefore, I would use an outer Nyrod tube and run a 4-40 rod (threaded at one end) through the tube. The thermal coefficient of expansion for the steel rod is less than for a Ny-rod installation. I would also recommend using two elevator servos, one for each elevator half. That adds some redundancy such that if one servo quits, half elevator control is still available from the other servo to get it safely back on the ground. You are probably already using 2 aileron servos, so why not have the same redundancy for the elevator? By mixing the elevator channel to an extra channel to drive the second servo, the sub-trims can be individually adjusted as needed for each elevator to eliminate any aileron effect if the elevators are not perfectly even.

Although marginal, the Du-Bro Lazer rods would probably work. If you do decide to use them, I would suggest using the outer holes of really long servo arms and control horns to produce maximum pushrod travel for the desired control surface deflection. This would provide some mechanical advantage to reduce the stress on the pushrod and fittings. This, along with the reduced vibration levels of an electric installation might make Lazer rods with the 2-56 threaded fittings workable. However, if it was my plane, I would install 4-40 steel rods and not have to worry about it.

Hope you enjoy the build.
Old 04-01-2023, 06:41 AM
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Originally Posted by Truckracer
Point well taken and appreciated but some of us are very picky about out CUBs! I don’t treat or fly my CUBs any different than any other airplanes I own and a CUB doesn’t have to fly like a slug.
Originally Posted by rgburrill
It's a Cub, not a pattern plane. Or a jet. Geez.
Thanks to all that've been hookin' & jabbin' along with me with all these questions.
I'm not here to learn about pattern, sport or jets or apply those genres as they relate to the flying style of my Cub. I have a few. Attention to detail, recruit! As I previously stated this is my first giant scale kit build & I'm wanting to learn what's reasonable & prudent to use or not to use. I'm here to ask questions & obtain as much knowledge as I can. Again, attention to detail. I'm sure folks can appreciate wanting to learn all they can about something they're wanting to do right or anything else that might be new to them for that matter. I like to have & maintain things as nice as I can get them. That said, these are the next items I'm wanting to buy...

Size & vendor for those quite small, scale bolts & hitch pins?
Which width pinking tape are y'all using for 1/4 scale?
Wing tube & wing tube sleeve diameters? ---Perhaps overkill with the BUSA examples since the wing struts are functional on their Cubs, but it's another point of contact that I want redundancy. I also want to keep wight to a minimum. Carbon fiber tubes?
Multiplex wing quick connect adapter parts?

S/F,
Masterguns


Last edited by Masterguns; 04-01-2023 at 07:06 AM.
Old 04-01-2023, 09:24 AM
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I hope I didn't come on too strong in previous posts. It wasn't my intention to disrespect anyone but just to point out that quality equipment can contribute to a plane that is pure joy to fly and not just OK.

As far as hardware goes, hobby sources such as Micro Fasteners is a good one. You can move on from there to an industrial supplier like McMaster Carr that has a selection of just about anything hardware you could ever imagine. Please visit their sites to see what they have to offer. Buying from McMaster offers extremely quick service and prices at a fraction of the hobby suppliers though I do use both sources. You will find that many times you can buy 50 or 100 of something from McMaster for a price close to what a dozen or less of the same part would be from a hobby source. Excellent company.

I use a 1" wing tube in my Cubs and tubes and sockets are available from a number of sources. For aluminum which would be fine for the CUB I buy them from TnT Landing gear. You can find carbon from many sources but it is much more expensive and frankly not needed unless ultimate strength and light weight are important. A very weak area on all CUBs is the cabin top. I have seen more than a few CUBs destroyed when a wing tip gets into the ground from what might have seemed like a minor incident and the whole top of the fuselage gets twisted and ripped off. Build that area strong keeping this problem in mind and a wing tube helps tie the area to the wing. Wing struts work well in positive "G" flight loads but do very little to support negative "G" loads and landing forces as these negative loads tend to pull the top of the fuselage outward and apart. To prevent this I also attach the wing panels to the tube with a bolt from below. Some people object to the non scale nature of a wing tube. My thoughts are that I'd rather have the tube along with its benefits rather than a short lived airframe. CUBs are great fun airplanes but they do have their weak areas that we can improve on to help their longevity.
Old 04-01-2023, 03:50 PM
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Default Which width pinking tape are y'all using for 1/4 scale?

The full scale Piper Cub has a 1/2" wide reinforcement tape applied to the fabric over each rib. Then the rib stitching is done on 3.5" centers (for aircraft <170 mph). Then the top surface 2" wide pinked tape is applied. Assuming you want the surface tapes to be scale and your Cub is 1/4 scale, then the pinked surface strips would be 1/2" wide. If you want to go ultra scale, know that the rib stitching does slightly show through the surface tape on the real aircraft. If you want to see how the actual rib stitching and taping is done, take a look at this video on Youtube.

https://duckduckgo.com/?q=rib+stitch...%3DdpoJT60_pRU

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