An open letter to model airplane designers

Contents

1 Introduction
2 Pocket-style
3 Root rib pocket type
4 Plate-style surface mounts
5 Hidden linkage in-wing mounts
6 Alternatives – CNC-machined
- 6.1 Dimensional drawing
- 6.2 Alloy mounts in 4-sizes
7 Side-mount specialty servos
8 Final thoughts
- 8.1 Last thing

Introduction

I have grown to hate anything other than plate-mounted servos for wing and stab control surfaces. Yet models are still brought to market – some pretty expensive ones, too – with old school servo installation. Like this one.

Lovely model, superb servo, plus sophisticated servo arm, cheap ass pocket install – sigh

Basically, I’m speaking of servo installations, where the designer actively inconveniences the customer. Sure, very easy to install, but you have to live with the damn thing and that’s not so fun.

So a very convenient installation, but one with geometry issues and which, leaves the servo sticking out of the surface like a sore thumb. This designation to include our own just because hiding them is a trivial exercise for the talented aircraft designer. Why do they do it?

And note; they’re especially hateful when sticking out of the surface and into the breeze because they add to the induced drag. Frankly, as an engineer, it not only drives me batty, it actually offends me!

So these days, when it’s so easy to hide the servos, I wonder why this is. Also, purely as consumer – as a modeler like you – well, I’m totally fed up!

Hence this open letter to model aircraft designers.

Since a picture is worth a 1000 words, several photos will detail what I mean. And honestly, it’s so easy to do better that I beg of you aircraft designer-types, come one guys . . . please up your game! And honestly? We (your customers) deserve better!

So we’re going to pick apart three types of mount designs, and offer up an inexpensive solution (because any idiot can find problems). After all, greatness lies in resolutions to problems (and genius within elegant solutions).

First up, the most egregious offender, in my opinion (although opinions are like bellybuttons in that we all have one). So this is mine. It’s the old school pocket style mount first used +40 years ago!

Pocket-style

It seems like the most common design, especially with the no-name imported el cheapo models, is the ‘drop in pocket’ type design.

This mount style features an upright exposed servo installation, where the servo is just dropped in from above and secured with mounting screws. Advantages are it’s quick and easy when installing the servo. So you save five minutes and it’s a pain in the ass forever after.

Interestingly, it’s found on some very expensive and otherwise superbly made models, too. Anyway, it presents several disadvantages.

Let me explain what I don’t like. Maybe you’ll agree.

Neutral position of linkage is offset – fore and aft – linkage rod end translates side-to-side

While the pocket style mount is the absolute cheapest possible servo mount for the manufacturer, and very, very easy for the builder to produce, and even easier when the modeler is installing the servos, it’s rather less than considerate of the modeler’s desires for aesthetics (in terms of hiding the servo).

So the defect that springs to mind first, is it looks cheap. And the aerodynamics suck. Gets worse because my real bitch has to do with the linkages.

Eyeball the angle of the above linkage rod at neutral. Now visualize how it only straightens perfectly into the airflow at the very extremes of throw! Only at full down and full up does the arm align with the airflow. Otherwise, it’s sort of at an angle increasing drag.

Big deal? Nope! It maybe amounts to 1%, maybe less. Means it’s a nothingburger for many but for me, as ProModeler? Considering we live and die seeking to eek out a 1% improvement in anything, this bothers me.

But there’s more. Reason I find this type installation to be so much bullshit (especially these days when models are so expensive) is because existing alternatives – costing at most pennies in terms of wood – resulting in a cleaner more aerodynamic installation. And, importantly, with better linkage geometry (and looks).

Recapping; my objection isn’t just because the servo is exposed (visible). Or because it makes cleaning and polishing the wing harder. Or even because it causes more induced drag (although I don’t like that, either).

Instead, what sucks is the servo arm – in swinging side-to-side as the servo arm rotates back and forth – results in the linkage rod making side-to-side motion. This is called translation.

Happens because as the ball link moves fore/aft, it also swings in and out on an arc. So it moves closer toward wing root at the extremes of travel (full up or down aileron) and then further out again toward the tip as it approaches neutral.

Two issues with this; first, increased wear on the plastic linkages. Second, and added to which, the off angle forces (both thrusting and pulling on the linkage) perhaps explain the popularity of the captured ball dual-horn setup on the aileron.

This, presumably to preclude the ball linkage itself from popping off. Yet interestingly, a simple washer works perfectly well for the ball on the arm.

Root rib pocket type

Next in the line of mounts created by ~~idiots~~ inconsiderate designers is how they sometimes make you install the servo through a root rib. Problem is this makes servo installation (and maintenance) rather more onerous.

Access requires removing the stab. And as if that weren’t bad enough, because some modelers would prefer stabs permanently mounted, this type of servo installation makes them scream in frustration.

Again, big deal? Maybe not. But added to which, servo arm installation and adjustment (especially the cross mounted screw used for backlash compensating), also becomes inconvenient – grrrr!

Total pain in the ass side mount servo install is accessed through the root rib after removing the stab

Plate-style surface mounts

The plate type mount circumvents the deficits of both previous methods. Moreover, they’re easily implemented.

These are done very cheaply with bits of laser cut plywood. And they work superbly because installation and servo arm adjustment is done outside of the wing-structure. This is especially useful when securing the bolt on the clamp (backlash compensating screw) on the arm.

it doesn’t get more convenient than mounting the servo and the servo arm whilst holding everything in your hands! What could be easier?

It doesn’t come more simple than a plate mount installation – suitable for ailerons, flaps, and stabs

However, beyond hiding the servo, the chief benefit of this installation is the servo linkage doesn’t transcribe an arc, or exhibit an accelerating wear pattern. Why not? Simple, it’s because the rotation of the servo’s output shaft and the hinge line are parallel. This means you get a beautiful linkage setup with no hinky motion artifacts of geometry – none whatsoever!

So once the servo is installed, the arm sticks out of the mounting surface and only the linkage (perfectly aligned with airflow) presents an obstruction. It’s a very clean look.

If screw heads bother you, what the heck, use countersunk types!

The hatch mounted servo with PDRS32-25T servo horn uses a ball link equipped pushrod

Hidden linkage in-wing mounts

Next are those mounts that place the servos sideways to the ribs. This, so they may hide the linkage rod, and which I also find to be so much bull.

Especially when moveable mounts – like plate types – can easily and cheaply be used to do the job of giving ready access to the servo!

End result is you end up with a huge hatch on the bottom just to grant access to the ball driver.

Internal side mounted servo can be tricky to reach with tools and especially securing bolts

Anyway, every time I see this style I always wonder, what the heck are you thinking? Making me approach the screw heads sideways to the servo mount tabs? This is nuts!

After all, servos are universal, so the mounting tabs are a decades old design feature. E.g. long ago established. And by a long time, I mean decades, so designers? Do your design-thing for the existing products, darn it!

Anyway, while the purpose of the above is admirable (hidden linkages), it comes with too high a price because it’s too much of an inconvenience for the modeler to deal with in terms of installation and maintenance.

Especially as alternative mounting methods exist!

Hidden linkages offer advantages in terms of drag and better looks

Alternatives – CNC-machined

So an alternative to cheap plywood mounts are alloy mounts which are CNC-machined from a solid billet of aircraft aluminum. The benefit of these over plywood has to do with never delaminating over time (don’t laugh, it happens, especially with low bidder plywood).

Securely installed with four rubber isolators, the mount has four tapped holes for mounting

And yes, of course simply wicking thin CA into the plywood sorts this, but what if you don’t catch it in time? Point being, inexpensive alloy mounts totally eliminate this risk to your model of ply mounts weakening with age and/or due to the heat of the sun.

Also, with machine threads for mounting, installation can be quite simple. Added to which, because the threads are metal instead of wood, then moving servos for maintenance brings no associate cost with regards to thread strength in wood.

And if you’d rather mount clear through the aluminum mounts and into the wood, then if the threads interfere run a drill through first. This will clear them out of the way – so you have alternatives.

Dimensional drawing

So modern CNC-machined alloy side mounts sort the issue of installing servos within the structure for the benefit of hidden linkages. Here’s the print if you’re curious – and they’re 7075-T6.

3-view dimensioned print and computer render of ProModeler standard class side mount

Anyway, these alloy side mounts aren’t rocket science. We offer them, so do others. Means just installing the servo to the mount, and then simply placing the mount with pre-installed servo into place and securing it to the structure with screws and presto, reduced bullshit!

Anyway, we offer them in four sizes. These encompass the range of products from 23mm long (submicros), 29mm long (micros), 36mm long (minis), and 40mm long (standard) servos.

Alloy mounts in 4-sizes

And because there’s no patent protection on these things, anyone can make them and we don’t hide the print, either. We literally don’t care if someone wants to copy ours and offer to make your life easier (and for others) using our design. KYO!

The smallest ProModeler servo can be fitted to confined quarters using a submicro mount

Side-mount specialty servos

Note: we do actually make a servo that side-mounts directly. It is a highly specialized product created expressly for very thin wing installations.

It’s our DS115CLHV, which at 8mm is actually thinner than a cigarette!

Cigarette for scale, ProModeler DS115CLHV is an 8mm thick wing-type specially servo

Final thoughts

Allow me offer a few final thoughts. What’s the right servo, mount, or servo arm for you? It’s a reflection of yourself, your goals, your dreams, and your budget.

We work hard put a better grade of product in your hands. But nobody can make you buy them. This you decide for yourself.

Still have questions? Feel free to reach out – we’re readily available;

Telephone: 407-302-3361
Email: info@promodeler.com

. . . then maybe together we can suss out what’s best for you!

I’ll close with this, one thing’s certain, best is a race that’s never finished. Best right now? ProModeler, but already others have more marketplace buzz. Deserved? Yes, they worked hard for it.

Are other servos worth your money? Not our decision. Your decision, and yours alone. Anyway, if you end up trying our servos and like them, then do us the honor of taking one to a club meeting.

Maybe pass it around. Take along a 1.5mm Allen driver and open it up for show-and-tell. This article shows how they go back together;

On rebuilding damaged DS505s

So link’s to an article are a pair of servos a guy beat the ever living dog shit out of. Because they had steel bushings, the cases weren’t damaged and all he was on the hook for were a set of gears (he sent them in, we don’t charge labor).

So we took photos as we disassembled and cleaned them, put them back together and lubed before buttoning them back up and returning them to the customer. Virtually all servos come apart and go back together the same way and once you’ve seen one, you’ve seen them all. It’s not rocket science, believe me.

Saying there’s nothing to be afraid of in opening one up at your club meeting. Also, please, consider showing them to a pal. But most of all, kindly grace us with a photo sharing your thoughts.

What for? To put on the website, and within articles like this. Basically, for telling the next guy what ‘you’ like about our servos. So what we’re Jones-ing for are photos like this next one because your favor is priceless and can’t be bought. We know this. So do others.

Last thing

Have you enjoyed this? If you like reading and are interested in learning more, then maybe find time to review more articles like this;

. . . and hundreds more. Best part? They’re all free!