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Servos? How big is necessary?

Old 11-09-2020, 06:35 AM
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Default Servos? How big is necessary?

Greetings. Been involved with models since big enough to hold a razor blade. But have had a few years off due to the usual “life stuff” that happens. I am way behind on radio gear although I do have an older Spektrum radio I used to fly my Raptor helis with.

I am building a BUSA Smoothie XL and am wondering how stout the servos need to be, other than throttle. Putting a G38 on it and wondering what prop sizes are appropriate for that engine/airplane? I inherited the engine brand new from my father-in-law and have zero experience with gas engines.

Any help will be appreciated by this aging modeler.

Chumpy77
Old 11-10-2020, 09:39 AM
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I always liked the Hitec servos, as they've been affordable, reliable, and easy to get for me, and Hitec has great service. Their newer line of the "D-Series" servos are real nice, and mostly affordable for sport planes such as your Smoothie. The D645MW's are a decently strong servo with about 180oz or torque at 7.4v, and nearly 160oz at 6v's. A decent value at about $40 each. There's also the older technology HS645MG that are only rated up to 6v(will also take 6.6v LiFe batteries), and have a torque rating of about 133oz... and run about $30-$32 each. They would work decently in your Smothie as well, and probably would have plenty of power for sport flying. So these are just a few ideas, and where I start if someone inquires about servos. There's literally hundreds of possibilities, but the above servos are all mainstream and available from most every supplier.

As for your prop, I'd go to the manual for size recommendations, but I'd also venture a ballpark size of about 18x8-18x10 for that engine. Xoar brand props are the cream of the crop for woodie props, and again they're affordable without breaking the bank. Good value for a nice looking prop that performs well. I'm not a fan of MasterAirscrew they don't perform nearly as well, and that plastic/resin they're made of is just pretty ugly... just my opinion is all. Those belong on a small trainer that will get beat up, as the MA props are pretty durable, whereas the woodies will break if you even nick the ground.
I wouldn't go the length of putting a carbon fiber prop on that engine, as the crank-shaft really isn't set up for it.. It's a single bolt hub, and those don't play nice with c/f props.. and props tend to slip.. whereas a woodie prop allows the knurling to bite and they stay tight forever until you want them off. The Xoar line of props though are works of art, and actually have attractive finishes, like fine furniture.. haha.

So yeah, there's a little bit to study, especially that engine manual. You'll be mixing your gas/oil, so you'll need a gas can. Usually a 2.5 gallon can is plenty for these smaller engines/planes. Most hand crank pumps are gas compatible.. and make sure you gasoline compatible fuel line(Tygon), and a gas compatible stopper in the tank in the plane.

Good luck with it. This may just get you started with food for thought.
Old 11-10-2020, 10:58 AM
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Thank you. I had wondered if the D645 servos would be sufficient. Perfect.

I inherited the G38, but no manual, just the bare engine. I have no idea where. He got it. Thank you for the explanation of wood prop versus carbon/polymer props. Good info for sure.

Chumpy77
Old 11-10-2020, 11:52 AM
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Here's a link to the manual. It covers the whole Zenoah G-series family of engines, and the 38cc is within. .

zene23a-manual.pdf

Yeah there's many props available in several different materials. The resin props are a bit more durable no doubt, and can usually withstand a scrape or two on the grass or asphalt even... If you're not sure of your flying abilities and there's any chance you'd scrape a prop, get a resin MasterAirscrew, but for the most part the larger planes are easier to fly and land, and any prop strikes are rare, especially when you get experience, or experienced help. Maybe start with a MasterAirscrew, but definitely go to the Xoar woodie once you're confident. Falcon brand is another beautiful prop... but the Xoar Beechwoods are a work of art... and dress up most any plane I think.

As I mentioned, these type engines don't do well with carbon fiber props, simply the fact that carbon fiber in that form is way too hard for the knurling on the prop-hub/backplate to bite into, whereas a wooden prop, and yes even the resin MasterAirscrews are softer, and you "crush" these props into the knurling when you tighten them.

Notice most larger engines have multi-bolt hubs... those are what's needed to use a carbon fiber prop and be safe. If you mount a carbon fiber prop on a single bolt hub(like the G-38), the chances are very high that it will slip/loosen/toss that prop. Hope that makes sense. Have fun with your build. You're going to like it... as well as those Hitec servos. You'll do well I'm sure.



I have a G-38, and a G-62 for sale if you're interested.
Old 11-12-2020, 01:26 PM
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Thank you for the manual!!! You are a great help to me. I appreciate your willingness to share your knowledge.

Chumpy77
Old 11-12-2020, 02:55 PM
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The Hitec servos suggested will certainly fly the airplane and provide a safe margin of power. That said it really does boil down to your expectations. The last time I tried to save money on servos I installed some JR analog servos with specs very close to the Hitecs suggested here in a large scale bipe. Although it flew just fine, it lacked the crispness that going with digital servos provides. After flying the bipe for a year I decided to refinish it and install all new MKS 777HV servos. Although not finished yet, I'm expecting to be happier with the performance.
Old 11-12-2020, 06:50 PM
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The Hitec D625MG and D645MG servos offer a measure of performance much above what is typical for servos in their price range. I've been using them for several years now in sport planes where their torque and speed is adequate. I no longer recommend any analog servos for any application as there are so many better choices within the same price ranges. From there, there are many, many more higher performance and more expensive servos available for most any purpose. I've found excellent servos in Hitec, Futaba, Savox and MKS brands and there are many more. So many choices for quality products.
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Old 11-27-2020, 02:30 PM
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Can anyone tell me if the D in front of the number mean that servo is digital? example D645, D485
Thanks in advance
Old 11-27-2020, 07:53 PM
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The Hitec "D" series servos are definitely digital. That's not what the "D" means though. I think I read somewhere the D stands for Dynamic.. I can't confirm that, but I'm pretty sure that's it.

The D-series is the newest line of servos though, with several features that make them perfect for a wide range of use. One feature is the range of voltage they accept, from 4.8v up to 7.4v, another being the soft-start feature, when powered up they don't jolt into place, rather they gently and solidly center themselves. Theres' other features as well that bring them current and up to date, you'll have to go to the site and read it... but yeah anything starting with a D (example D755) will be in that series.

Hitec has several other digital servos in the lineup too. The D-series is just one line of several, there's several analog servos as well. So there's a servo for every application, as well as price range. From about $8 up to a few hundred bucks and more. Look at the Hitec servos with a four digit number, with model# beginning with a "5"(example 5645), or 7(example 7955), as those types of numbers will also be digital, but not a D-series servo. Anything with a three digit number will be an analog servo(example 645, 311, 485

Best info you can get is by just studying and browsing the Hitec site, or any mainstream retail outlet description... It will tell you much more detailed info then I am, and more accurate as well. It's all there if you just search it out.
Old 11-28-2020, 07:07 AM
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Thanks DGRANT, the reason I ask is I have a 7008 Futaba receiver and I'm told I need digital servos for that receiver. I look at Tower Hobbies web page and it says the Hitec 645 is a digital servo so I bought 6of them. Now a friend says they are not. So I'm looking at buying the d645's Once again thanks,
Old 11-28-2020, 08:51 AM
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Towers description is incorrect, obviously misleading. The Hitec site lists the hs645 in the analog/sport page... interestingly they don't disclose that in their own description though. The hs645 is definitely analog though... and been around a long time. Very decent sport servo though... just not up to the snuff like the more precise digitals. I've probably still got a few dozen analogs in various older planes.
My newer planes are the 79** series, and my newest giant's are the D series in various arrangements.

Yeah if you can get the D's do it.
Old 12-30-2020, 09:18 PM
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DGrant, do you still have the G-38 for sale? If someone has spoken for it I will respect that.
Building a Smith Miniplane and the servo discussion is helpful to me too.
Thank you guys.
Old 12-31-2020, 12:02 PM
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Originally Posted by Mbrunhoeber
DGrant, do you still have the G-38 for sale? If someone has spoken for it I will respect that.
Building a Smith Miniplane and the servo discussion is helpful to me too.
Thank you guys.
Yes, still have it. I'll pull it down and check it out. I haven't had the G-38 out of the box in years, so I just want to have a good look at it before I strike any firm deals to make sure it's decent and complete, or what it might be worth.

I've still got that real nice G-62 as well. That'ones in good shape, looks near new, I'm not sure if it's ever been ran, minus the muffler. but has a Bennet mount, and upgraded ignition.

I'm not planning to use either one in the future that I know of. I'm flying bigger planes these days.

I'll get back to you that G-38.
Old 12-31-2020, 05:45 PM
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Okay thank you.
Happy New Year!

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