As is usual around here, one of these caseSTUDY article regarding servos for a model begins with a query and this time is no different. Fellow recently wrote asking my opinion regarding servos for a really sweet model Hangar 9 miniature reproduction of the far-ahead-of-its-time Fokker D.VII fighter.
FWIW, the 87" wingspan works out almost perfectly to 1/4 scale because the original sported a wingspan of 29' 2" (or 350 inches) and 350/87=4.02 or 1:4.02 or 1/4 scale. Note; the scale prop at 262cm works out to 110 inches, so a shade over 27" diameter will get you pretty close to scale.
Hangar 9 are some of the best in the business at producing RC models that look really good and fly superbly, and this one is no exception. So let's delve into the question and answers!
Q. I've recently purchased a Hangar 9 Fokker D.VII and in looking over their recommended servos, the A6380, which makes 200oz-in at 8.4V, I'm wondering if this is really enough torque and if not, what would you suggest, instead, in the ProModeler range of servos.
A. That's a large, faithful reproduction of a full scale aircraft not often seen at the field. And it's a remarkably easy to fly model. However, if you don't mind, I'll begin by first answering questions you've not asked.
Engines
First, they say it's suitable for 30-60cc engines but honestly, there's a world of difference in mass and performance between these two ends of the power range.
If it were mine I'd be leaning toward the 60cc instead of a 30cc Why? A couple of reasons. One, because I suspect you will end up with a fair bit of nose weight before it balances properly with the smaller engine (meaning you may as well install a larger/heavier engine in the first place).
And two, because there's practically no such thing as too much power with a large and boxy airframe (meaning draggy and relatively non-aerodynamic), which scale models like this one offer.
Major point being, it's only going to go so fast no matter how much power you throw at it and because there's a lot of drag to slow it down if you're not careful, then a lot of power is good to save your ass when you make a mistake. Trust me. Also, and this is important, more weight will be greatly appreciated before it ever takes wing, believe me, more engine beats added lead every time. Especially, when it allows swinging a larger propeller to move more air (amongst many prototypes, the V 22 variant even flew with a four-bladed propeller).
Servos - flight controls
Second, and to the heart of your question; servo wise, this isn't a very demanding model (for the same reason you can't add too much power). Basically, it's going to be relatively slow. That, and the scale dimensioned control surfaces aren't huge. So yes, 200oz-in are enough power.
Point being, even our bog standard DS180DLHV is probably enough servo - but - I'd guide you toward the generally more capable DS270DLHV, instead 'if' the added price doesn't scare you off. Why? Principally because they're a bit stronger and more in keeping with my advice regarding the larger engine. Incidentally, both the DS180 and the DS270, like the recommended Spektrum A6380, feature an all-metal gear train, which is important with large heavy models like this one.
However, if you can stand the difference in price, the DS255BLHV is what I would use if this were my model. This, despite it a little more than the DS270DLHV, and offering a shade less torque, and being waaaaay the heck faster (transit time) than is needed for this class of model. Why?
For one simple reason. While all our servos have all-metal gears, facts are all metals aren't the same strength. In the case of the DS255BLHV, all metal translates to all-steel, and steel is 'mo betta in a world of big single-cylinder gassers, which vibrate like hell.
- Even a Ferrari F1 transmission uses all-steel gears - there's nothing better
So this recommendation has zero to do with being a bit stronger than what they (Hangar 9) recommend (255oz-in vs. 200oz-in), or with it being a heck of lot faster than what is required, or simply because it's more expensive than the others (to include the ones they recommend). Instead, this recommendation has everything to do with better metal in the gear train, which is both more durable and will far better withstand the vibration the engine will generate, which is pure hell on servos.
So this addresses servos for the flight controls. Next, lets talk throttle servo alternatives.
Servo - throttle
For throttle, a DS90DLHV is all you need, but if you can stand the price difference, then get another DS255BLHV for the very same reason. Required? Nope? Moreover, while a throttle only needs like 10oz-in of torque, I'm never going to make a weak sister of a servo. So the 90oz-in is the least I'd recommend for the throttle.
Note; if you were somehow fitting a twin cylinder engine, then the DS90DLHV would be a great choice because twins run so smooth, but you can't use a twin because it would look stupid with the jugs hanging out in the breeze (the original BMW was an inline engine). So with a single-cylinder paint shaker powering the prop, then servos with all-steel gear trains are simply going to be more durable. By a long shot!
Look, I'm not intimate with your wallet so I can't speak toward the budget implications of what I am saying, which is why I'm showing you good (DS180 or DS270) and better (DS255) in terms of servo recommendations. However, I am very familiar with how vibration can destroy anything and everything on this kind of model. Like what? Things like hinges, interplane struts, wheels vibrating on the axles, the servo gear trains, scale detail, like machine guns that'll fall off in flight, etc. Trust me on this.
Note; before I move on, if you've got more money than God then the recommended servo for the guy who wants our best is the DS505BLHV. Do you need this much performance? Nope. But since I've mentioned good and better, I may as well mention best, too, take my meaning? Our DS505 is a sweetheart!
By the way, breaking the code regarding what our part numbers means is easy;
- DS = Digital Servo
- 505 - Torque in oz-in
- BL = Motor Type (DL is an iron core like in the A6360 and BL means brushless)
- HV = High Voltage (actually a range from 5V to 8.4V)
To learn more about RC servo motors, click the link to review an article in which we cut motors apart on a lathe to show you inside (meaning what makes the difference between three different type motors, DL, CL, and BL - arranged in order of desirability and increasing price). So with servos covered, let's yak about servo horns.
Servo horns
Servo horn wise I recommend the same ones regardless of which of our servos you select; get a 5-pack of these PDRS101 heavy duty polymer arms (we have aluminum arms if you just want the bling but you don't need them, trust me). You'll also want one PDRS105 for the throttle and because the D.VII uses pull-pull cables for the rudder, get one PDRS102 while you're at it.
Note; maybe if you find time to review this article regarding gasser throttle servo you'll find it's a profitable use of your time. So now, next let's move onto powering the control avionics (battery pack), plus switch harness, and servo extensions.
Receiver battery pack and servo extensions
The manual says two 9" and one 36" extension but double check in case that' a misprint (you definitely don't want to mess with a Y-harness for this application even though we offer one of if not 'the' nicest one available), trust me.
Anyway, we have 10" instead of 9", which won't matter. And our extensions are 20AWG wire (the heaviest that can be commercially crimped into the connectors). And instead of PVC, we have them made with super flexible and abrasion resistant silicone jackets. In short, there are none better at any price.
And with regard to the control avionics battery pack, I recommend a single B2S5000 LiIon or B2S6000 LFP.
Honestly, if you can afford the difference in price, then convenience-wise the 6000 is hands down better because you don't need to put it at storage voltage at the end of the day. If your engine uses an ignition module, then you need a battery for that as well either a B2S1500 or B2S2500 will be suitable. More on this later.
Anyway, facts are you can charge one of these LFP packs and leave it sitting for a year before going to fly and it'll still deliver the charge. They're also safer. And thus, these are all I fly in my personal models.
Note, there's a world of difference between a LiFe pack and an LFP (LiFePO4, or Lithium-Iron-Phosphate, also known in the trade as an A123 cell). The former we won't offer due to long ongoing issues with them. If other guys want to put their names to them, we're fine with that.
Two batteries vs. one battery
Why not two batteries? Because batteries are very reliable. Do police cars have two? Nope. What about ambulances? Nope? What about full-scale aircraft? Nope? What about satellites in space? Nope, one battery. In fact, I can't think of a single application that installs and carries around the dead weight of a spare battery.
Major point being, if someone is selling you the idea you need two batteries, then you might be wise to wonder why. Especially when reputable manufacturers of hobby-grade RF equipment are just fine with using just one. That's all I'll say on the subject.
However, while I don't recommend two batteries, I do recommend two switches. Why? Two simple reasons, first to provide a parallel switch in case one takes a crap on you (so you don't crash because switches in our experience are anything 'but' reliable). Second, because the connector for a switch is rated at 5A and having two in parallel means you can draw 10A (5+5=10) and 10A is enough. 5A may be enough, depends. I sleep better with a 10A connection between my battery pack and receiver for it to draw from.
So what will leave you in a bad spot are radio switches and for these, I recommend you use two in parallel instead of one. No, we don't offer a radio switch but 'I' use bog-standard Nobel type slide switches ($10 type). These are widely available from hobby shops, Amazon, etc. Anyway, don't believe the snake oil salesmanship that has you buying expensive switches because switches fail, all of them do! Protect yourself with statistics!
By this I mean what protects you is mathematics, not a fancy all-singing, all-dancing switch that supposedly can't fail, but mathematics, instead. Mathematics as in, the odds of both switches failing on the same flight are freaking astronomical. The statistics protect your investment, not BS about switches that don't fail in such a way as to crash your model.
In summary of batteries and switches; don't buy into the fear trade where some companies advise using two battery packs. Don't take me wrong, we'll be perfectly happy to sell you two packs - but - in my own models? One pack, two switches (and when I can avoid a switch altogether, I do). Basically, instead, I use two short extensions to connect the pack to the receiver before flight. This is made easy by the fact I have our packs built with parallel leads (meaning two leads).
Of course, short extensions won't be convenient in this D.VII model. It's really only convenient with 3D models where the design is optimized for electric and thus, the entire front part of the fuselage lifts off to expose access to the forward structure (for propulsion battery swaps). Otherwise, two switches.
Wrapping this up, and circling back around to engines; it's my opinion an old school Zenoah G-62 is a lovely running boat anchor for this model. Don't take my words regarding boat anchor as disparaging because instead, they're meant lovingly.
FYI, I own two and wouldn't part with them. Also, because I own a couple large WWI models myself (Nieuport 11 and Fokker Dr.1 the Triplane), and because they also have relatively short noses, they need the added weight of a larger engines. This makes these G-62 engines a great engine choice.
Note; these things have lost favor with a certain part of the market, which means they're readily available on the used-market for cheap. Word to the wise shopper, eh?
Anyway, the G-62 is one of the two recommended engines (the other being a 33cc engine they produce) and I like the fact the G-62 have self-contained ignition (magneto generated) meaning no separate battery as is needed for an ignition module equipped engine like their 33cc engine recommendation. Keeps things simple in my view so there's a lot to recommend the Zenoah G-62 engine in my book.
That, and it'll swing a 22" prop versus the ridiculous looking toothpick-like 18" prop, which is all the 33cc engine can swing. Yes, the 22" is a far cry from 27" required for scale looks - but - it's way closer to the real thing.
Moreover, if you really want to do it right, then both Mick Reeves and Volgelsang offer belt reduction units for these large Zenoah singles that let you swing the really big ass prop that'll look just right. Tell Mick I sent you and that plus a buck will buy you a cup of cheap coffee (my way of saying there's nothing in it for me to refer you to him).
Anyway, a visit to Mick's website will definitely get your juices flowing with regard to things you can do with this model of the gorgeously executed Fokker D.VII - not least of which is, 'I' wouldn't use the supplied CA-type hinges and would instead turn to Robart for a set of their superb #322 scale-like steel-pinned hinges, which again, I have no economic interest in mentioning. It may be worth stripping the UltraCote, which is shiny and just looks wrong in favor of fabric and paint . . . but I digress, yet again!
On gyros and tail wheels
Before I forget, one last thing. This model doesn't have a tail wheel and instead, uses a scale-like skid. However, unless you're intent on competing, I'd sacrifice the skid in favor of a tail wheel. A little creative modeling lets you embed one within the skid and you'll hardly see it. The tail wheel is simply more practical, trust me.
Another thing is this; I'd add a gyro for the rudder. There are those who say this is cheating. I say bunk . . . do whatever helps fly the aircraft better! Facts are while Manfred von Richthofen loved it, and it's widely considered the best of the war, directional stability is not its strong suite. The vertical fin and rudder are kind of small in modern terms and engine torque will be tough to deal with. Me? I'll accept help whenever it's offered and a gyro fits my definition of welcome help. Enough said?
Toward that aim, Horizon offer a neat receiver in their AS3X line with built in 3-axis gyro, it's worth consideration. There are other gyros on the market, an Aura8 is a nifty one I own several of. Yes, this bit about gyros and tail wheels is just my opinion (and opinions are like bellybuttons in that we all have them), so this is mine. Heads up because the idea is to have fun, not be fighting with a model that's less than optimal in directional stability near and on the ground during take off and landing, take my meaning? Just saying.
Anyway, if you have any further questions, reach out to me via email: info@promodeler.com - or - if it's urgent, you may always call; 407-302-3361 because I love bullshitting about airplanes more than anything.