Bypassing gear controllers to deliver steady DC current

It’s a beautiful day and you’re out flying your giant scale P-51 model. Totally enjoying the rush of performing high speed passes and victory rolls – just like the full scale aircraft returning from a successful sortie – suddenly a landing gear door is ripped off and as you catch sight of it, your heart sinks. Fortunately, a pal watches as it flutters to the ground and it’s recovered. But what happened?

That’s when you discover the servos aren’t holding the doors closed tightly enough and at high speed a blast of air got beneath one and ripped it right off – yes, by effectively a hurricane force wind. And no, not kidding because GPS shows the model hitting 120mph in level flight (Cat 3).

And it’s going even faster in a dive, so call it Cat 4. And it’s not that the servos itself is giving way under load. Instead, the issue is the servo can rock on the rubber isolation mounts juuuust enough to let air catch the lip of the landing gear door.

Moreover, rigid mounting the servos is bad juju for its longevity because of engine vibration. So this requires another solution. Fortunately, it turns out to be easy. Keeping the servos from rocking on their mounts is just a matter of adding ProModeler transverse output shaft stabilizers.

But then another issue raises its head. When using powerful servos, the control box which powers the retract gears and the sequencing of the inner gear doors doesn’t output enough current to supply the servos without tripping and going offline. If this happens in flight, then the issue becomes, how to extend the retracts since the control box also manages extension/retraction as well as sequencing the inner gear doors?

Turns out ProModeler have a nifty solution in an HV2SV-X2, an inexpensive wiring harness allowing you to divorce servo power from control signal. With this in place, the control box merely sends the signal to the servos, which operate normally using a dedicated power source. As for the servos? They’re dumb and don’t even know how they’re being powered separately!

Powered separately? Yup, we’re going to use a dedicated power source for these servos and this then puts an end to current draw knocking the controller offline.

This brief article shows you how to get this done in a few easy steps. You’re going to need . . .

• 2) ProModeler Transverse Output Shaft Stabilizer kits
• 1) ProModeler HV2SV-X2
• 1) ProModeler XT30-switch
• 1) ProModeler B2S850 battery
• 1) ProModeler 200mm XT30M-XT30F extension

Mechanical rocking – lost motion

With regard to servos mechanically rocking side-to-side on the rubber mounts due to the force of the air, the brute force fix involves hard mounting the servo by eliminating the soft rubber isolation mounts. Problem with this is it exposes the servo to damaging engine vibrations (perhaps leading to premature failure). Is there a better way?

Yes, there’s an engineered solution called a transverse output shaft stabilizer kit. This photo shows ProModeler part number PDRS60-T. Once installed on a servo, then it can’t rock side-to-side (whilst still being support by soft rubber isolation mounts). End result is stability whilst maintaining the best possible anti-vibration protection.

Installation is easy because the M3 screw securing the servo arm to the output shaft is replaced with a threaded M3 stud. The stud has a smooth shaft which fits into a bearing. The bearing is captured by a polymer bridge assembly which fits transversely over the servo.

This bearing allows rotation without friction and the bridge supports the servo from rocking whilst the rubber isolators do their job of protecting the servo from vibration. The only caution is selecting the proper thread for the shaft when ordering. While your ProModeler servos use M3 machine threads, we offer M2.5 as well for off-brand servos. Heads up!

What’s included is the output shaft stud (M3 or M2.5), the appropriate bearing, plus the transverse stabilizer that bridges the top of the servo.

The bridge may be secured to structure with a pair of #2 screws. These are not included because the screw type, e.g. wood or machine-thread, is to be determined by the user depending on the mounting structure.

Since we’re discussing the use of the transverse mounting kit, it’s reasonable to suppose we offer a longitudinal mounting kit . . . and you’d be right!

This is part number PDRS60-L (note; L is for longitudinal versus T for transverse). Installation is similar in that you need a pair of #2 screws and depending on the structure to which it’ll be secured, these are either wood thread or machine thread, e.g metric M2 threaded bolts, or imperial threaded 2-56 bolts.

Divorce servo power from the sequence control box

Now let’s turn our attention to the servo’s electrical needs. To keep the current draw for powerful servos from knocking the sequence control box offline (thus leaving you in a lurch), we’re going to divorce, or isolate, the power from the box (or a receiver) and instead make it easy to use a dedicated source – like a 2S battery pack.

Since this particular application (Hangar 9 P-51 Mustang) uses two servos for the inner gear doors, then for this purpose we’re going to use a special harness that let’s you divorce the power for the two servos from the sequencer box and power them independently, instead (yes, we have a single-servo version, also, it’s HV2SV-X).

Anyway, the harness has four leads, two for the servos, two going to the control sequencer (where the servos once were plugged in), plus one for the dedicated power source.

Since the landing gear sequencer control box is dumb, it doesn’t have a clue whether there are servos connected, or not. And even less of a clue if there’s power flowing through it for these two servos (there’s not).

Electrically, the servos continue to receive their pulse signal for operation from the sequencer (yellow wires).

However by using this special harness, now the power for the servos comes from an independent power source (via the XT30 connector) instead of being limited by the control box’s ability to deliver current.

This means now these two servos can now draw as much power as they need for operation without affecting the sequencer because the XT30 wired directly to a 2S pack far exceeds the needs of a pair of servo (by a country mile).

What constitutes a lot of current? Pretty much any modern day HV servo if it makes any power consumes a lot of DC-current. This was happening with a pair of 180oz-in servo which at full song can consume nearly 2.5A each. However when the customer decided to use 180° servos (because he could optimize travel and take the load off the servo by transferring it to the bearing supporting the 25T splined output shaft), the this made for a real issue because of how much current these puppies can consume.

All ProModeler servos disclose these details within the Specifications. And the pertinent number is current at stall because this is the worst case scenario. Look at the last line of column 4 of 5 for a 2S LiIon battery pack and as you can see, at full song they can each suck down more than 3A. No, not continuously, just briefly.

However, divorcing power draw from the controller to its own independent source effectively (and economically) puts an end to the risk of excessive current draw knocking the sequencer offline, thereby hanging you out to dry with electric-powered retracts that are inop!

XT30-type 20A switch harness

So the next consideration is being able to turn off the power to the servos because they will happily idle the battery pack capacity down to zero and ruin the pack (remember, servos are dumb). Dumb owner plus dumb servos are a match made in heaven (not).

Saying don’t be stupid, and when you’re done flying, remember to switch off the independent battery power to these servos!

The switch harness is constructed with a heavy duty 20A toggle switch with 150mm leads of 16AWG wire on each side. Total length is about 350mm . . . call it 14 inches, give or take.

The reasonable man might wonder if we offer something similar in a DuPont style (colloquially known as JR-type connectors in the sport) and the answer is . . . yes we do!

However, electrically the difference between the two types of switches are the leads are 20AWG instead of 16AWG, and the switch itself is a slide type versus a toggle type, and it’s rated at 10A, instead of 20A.

Thing is, ProModeler packs are available wired with dual DuPont leads plus an XT30 lead, so you have some interesting alternatives when it comes to powering servos and accessories. Using a pair of these 10A switches still delivers up to 20A but with redundancy!

Also, note; these packs range in capacity from 650-6000mAh but for this application, the tiny B2S850 is about right.

Extension – 200mm XT30 Male to XT30 Female

Last thing, just in case you need it, we can also offer you a 200mm extension (about 8″ long). Comprising high strand count copper wire, the 16AWG wire lead is covered with the good stuff, a super supple silicone jacket.

It’s equipped with an XT30 male on one end, and XT30 female on the other. And to make life easier, the red/black leads are covered with expanding mesh PET in black, and finished off with a bit of black heat shrink to keep it nicely in place.

Summary

Sorting the retract sequence control box going off line due to inadequate power handling capability is easy. We just divorce the servo’s signal leads from the power leads using an inexpensive HV2SV-X2 power adapter. This let’s the retract sequencer continue to direct the servo operation as before whilst at the same time they now draw their power from an independent power source (in this case a 2S850mAh battery pack).

So merely powering the servos independently, and thus bypassing the sequencer’s control box (for which only signal is required) sorts the issue satisfactorily and, as our cousins across the pond would say . . . Bob’s your uncle!

In closing allow me to add this article is what’s referred to as a living document. This means, unlike with a magazine article, once it’s published I can and do add to it as I learn more.

This also means your thoughts and opinions can affect what others see. Major point being, if you have questions we’ve not answered to your satisfaction, or maybe you have photos to share you believe may help get the point across better, then kindly email or call me at 407-302-3361 because the goal is to help each other.