Aileron Servos on my Guillows Sopwith Camel

Figuring out the aileron servos for the Guillows Sopwith Camel 801 build (see the separate post here) has been a very interesting journey. So I’ve created this separate post to write it up. Hope it helps.

Direct Drive mounting of the Aileron servo

The out of the box plan has moveable ailerons, but only for static display. They are not intended to be functional. So I want to have ailerons and I want them on top and bottom wing. This discussion has gotten so interesting I’ve move it to a separate post which you can find here.

Both Cliff Harvey with his Guillows Spitfire, and Tim McKay who had a similar experience with the Guillows Zero, pretty much proved that the control of ailerons is going to be essential to making this plane behave nicely when it flies. So I have to solve two problems 1. Getting ‘control’ out to the end of the wings. 2. Getting the control up from the bottom wing to the top wing.

First I’m going to mount servos out on the wing. I’m actually thinking that because this is a biplane which will have 4 ailerons, the torque required for each of them is much smaller as its divided by 4.

So I’ve been doing more thinking and shopping and found some absolutely amazing tiny 1.7g AFRC D1302 digital servos that have torque of 0.15 kg/cm (0.014 Nm) which is exactly what is required for ailerons of this size according to at least 2 different surface area/torque calculators I have found online.

So I am going to do an experiment – using direct drive of the ailerons using these wonderful little digital servos.

Calculations

Trying to figure out exactly how much torque is actually required for a servo isn’t easy. What seems to have happened is that many years ago someone figured out that a 9g servo was “good enough” – and was pretty light compared to the weight of other components so everyone just kept using 9g servos.

But like most other areas of technology in this ever-changing world we live in, the technology of servos is improving in leaps and bounds. This is being driven by other uses of servos such as robotics. To me this means it makes sense to ask two simple questions:-

  1. How much torque is required for a particular purpose?
  2. What is the smallest servo I can use to give me that torque?

For the Sopwith Camel, I have a target weight in mind of 180g. If I don’t meet that, the absolute worst case I want is < 250g. This means weight is important, and if (for example) I can shave off 20-30g or more by switching out 4x or even 6x 9g servos for some much smaller digital servos, I’m all for it.

I did a lot of research (i.e. Googling) about this, with not a lot of results. I have basically found 2 web sites that seem to have usable and understandable formulas for calculating torque.

There is a calculator at Radio Control Info. This is ok but doesn’t explain its formula, so I am suspicious of it. I use it to validate my own calculations. This one is also very strange as it asks for all the input parameters in metric and then gives the results in “oz-in”. This is confusing and annoying.

Minnesota Big Birds has a page with a fully documented torque calculation formula by Chuck Gadd. I love this. The rational and mathematics are out there in the open and explained and I can put this into spreadsheet and do my own calculations and even tweak it. This is their formula:

Torque (oz.-in) = 8.5E-6 * (C2 V2 L sin(S1) tan(S1) / tan(S2))

                        Where:

§  C = Control surface chord in cm

§  L = Control surface length in cm

§  V = Speed in MPH

§  S1 = Max control surface deflection in degrees

§  S2 = Max servo deflection in degrees

Of course this is also frustrating because it uses “Miles per hour” (remember miles? – something from the old British Empire I think), and gives the results in oz-in instead of Nm or even kg-cm. But having the formula it’s pretty easy to update it to use modern metric units. Miles/hour to km/hour is easy – Just need to multiply by 0.621371^2, so that just changes the constant at the front. The result can be converted to km-cm by multiplying by 0.07200778893234 or 0.00706155 to get Nm which is actually the proper metric unit for torque. Most servos are sized in kg-cm (or oz-in, I’ll ignore that), so the new formula is

Torque (Nm) = 0.0000000231750630904477 * (C2 V2 L sin(S1) tan(S1) / tan(S2))

Torque (Kg-cm) = 0.000000236319937054984 * (C2 V2 L sin(S1) tan(S1) / tan(S2))

Lastly there is a pdf thoroughly documenting servo torque requirements by Andy Meysner of the Southern Ontario Glider Group, which is the one I like the best so far. It refers to the other two, but this explains how the formula works in details and it calculates using metric inputs, and gives the results in Nm. as it says:

Servo torque is usually specified in oz-in or kg-cm. To obtain the torque in oz-in or kg- cm, multiply the result in N-m by 141.6 or 10.2 respectively

https://soggi.ca/wordpress/wp-content/uploads/2020/09/ServoTorqueCalcArticle_App.pdf

The whole formula assuming a servo arm, control rod and control horn on the control surface is:

Ts = V2 x L x C2 x sin(αh) x tan(αh)/ (4 tan(αs))

(assuming Cd – drag coefficient = 1.0 and p = 1.2. Read the article for details)

V = Airspeed in m/s (multiply km/hour by 0.277778)

L = Length of the control surface in meters (3.5 cm = 0.035 meters)

αh is the rotation angle of the control surface from neutral in degrees

αs is the rotation angle of the servo arm in degrees measured from the servo arm position at 900 to the pushrod – I’m assuming αh= αs (I hope this is valid but I think it should be)

But here is where it gets interesting, Andy’s article also explains in detail how the calculation is done and as part of the calculation, it shows the calculation for the torque on the control surface itself as an intermediate step. This is fascinating because this would be the torque required for a direct drive servo like the one I plan to use on the ailerons on the Sopwith Camel. The formula for this would be:

T = ((Cd ρ V2 C L sin(αh))/2) C / 2

(again Cd = 0, p = 1.2)

Note that C appears multiple times, so this can be simplified to

T = ρ V2 C2 L sin(αh)/4

For the ailerons of the Sopwith Camel, which has:

  • Chord = C = 3.5 cm or 0.035m
  • Length = L = 14 cm or 0.014m
  • Speed = V = 50 km/hour (I’m making an assumption here)
  • Control surface deflection = servo deflection = 20 degrees

So the results for the Sopwith Camel ailerons are:

Torque kg-cm: 0.14 – which is less than the 0.15 spec for the 1.7g digital servos I’m using

Interestingly I did the calculations for the all the control surfaces and this is the result:

SurfaceChordLengthDeflectionSpeed
Km-hour
Torque
[Big Birds]
Torque
[Meysner]
Ailerons3.5 cm14 cm20 degrees50 km/hr.05 kg/cm
.005 Nm
0.05 kg/cm
0.005 Nm
Elevator3 cm18 cm20 degrees50 km/hr
Rudder4.5 cm8 cm20 degrees50 km/hr

Which was a very long winded way to show that the 0.15 kg/cm digital servos at 1.7g that I plan on using have approximately 3X the required torque for the Sopwith Camel Ailerons!

Installation

I’ve started installing the first of possibly 4 aileron servos directly inside the wing next to the ailerons. They will be direct drive which cuts out a lot of weight and inefficiency. These will be 1.7g digital servos. It seems to make sense. Stay tuned.

This image has an empty alt attribute; its file name is 20210702_123640.jpg

Wires

Running the wires from the aileron to the fuselage is an interesting challenge. I see most people end up putting holes in the middle of the ribs. It struck me while I was doing this that I was weakening the weakest part of the rib by doing this. Why not, I thought, run the wires along the spar, which is the strongest part of the wing, and would make for the easiest place to run the wires while also making them less visible if I use tissue paper for the covering (which I haven’t decided on yet). This is how it looks.

Guillow’s Sopwith Camel 801 RC Conversion

I just opened [20 June 2021] my “new” Sopwith Camel kit from Guillows. I say “new”, but I bought this off eBay because at the time I ordered it, the Guillows website was Out of Stock. So the “new” kit I bought from eBay is actually pretty old. It’s well preserved, but the paper is yellowing and the box doesn’t have the “Laser Cut” sticker that the new models have. So this model is die-cut, the old way, pre the modern days of laser cut balsa.

This is quite fun because some of the things included in the box are things you will never see any more. Like a paper order form for ordering replacement parts by sending the form and a cheque (remember cheques?) to Paul K. Guillow Inc. Box 229, Wakefield, MA, 01880. Luckily all of the wooden pieces are well preserved, except for the plywood containing some of the slats and frame pieces. I’m soaking this in water and trying to dry it flat.

Review of Instructions

Having opened the box, the first thing I did was read the instructions. There were some very interesting things in these instructions, not all relevant to building the plane, but I thought I’d just note them as I go.

Copyright 1973

The copyright on these plans is 1973, and the copyright on the little “Catalog” included in the box, is 1974. Now this doesn’t prove the kit is that old, but it is certainly interesting. I haven’t found anything dated more recently than 1974.

It’s also very interesting that included in the kit was an “order form” for ordering replacement parts.

U-Control

The kit is made for rubber powered free flight or “U-Control”. I didn’t know what that was, but I did figure it out. It is what used to do when I was a teenager building and flying “control line” planes – flying around in a circle controlled by wires and a little handle. You tilt your hand to make the plane go up and down. The kit even includes the handle (but no wires) and I didn’t know what it was at first. It’s for control line flying!

This confused me because there is a control horn and control rods, but only for an elevator. The hinge ailerons were never intended to be functional, which they will be when I convert this to Radio Control.

Wooden Wheels

There are some absolutely gorgeous solid wooden wheels included in the box. They seem to be made of some kind of hardwood – maybe cherry (I’m not really sure). They might be a little heavy for a flying model, but they will look great so I might use them, I’m not sure yet.

Turned wooden wheels out of the box

Planning for the Radio Control

This is just some notes. Kind of “thinking out loud” as I read the plans and try to figure out what I need to do to build the plane for RC.

Ailerons

The out of the box plan has moveable ailerons, but only for static display. They are not intended to be functional. So I want to have ailerons and I want them on top and bottom wing. This discussion has gotten so interesting I’ve move it to a separate post which you can find here.

Electronics access

The electronics – receiver (with built in ESC) and 2S Lipo battery will need to go in the front of the plane. Access is going to be a challenge. Even for accessing a battery, let alone if I need to plug or unplug a servo from the receiver for any reason (likely there will be lot’s of reasons). It’s going to be difficult to make the wing removable (as Cliff Harvey did on his Spitfire), because the bottom wing is connected to the top wing. It will be difficult to build an access hatch on the top of the plane (as Tim McKay did on his Zero), so I’m going to do this in 3 parts.

  1. The battery will go at the front. I’ll build an access hatch in the side of the plane between B1 and B2. The battery I plan to use is a 2S 850 mAh battery that is 52mm x 28m m x 15mm. It will fit nicely across the plane if I build a little shelf where the fuel tank would have been for the glow-plug (gas) engine. The 50g will mostly be in front of the centre of gravity and should mean I may not need any other weight. This will go at the bottom in a shelf between “upper side keel A6” and “lower side keel A8”.
  2. The receiver (with built in ESC) will also go at the front. This will be in a 2nd shelf above the battery to allow the receiver to connect to the motor. I may build a drawer for this so the receiver can slide in and out. (not sure if this is overengineering, we shall see what happens when I get to it. The receiver will at the rear of this space up against B2 which should put it directly over the CoG. I might make the access panel for this on the opposite side of the plane from the battery access. The power wire from the receiver thought does need to be able to easily feed down to the lower compartment so it can easily be connected/disconnected to the battery. The receiver I think I’m using weighs 6.4g so if it’s close to directly above the CoG the impact should be negligible.
  3. I’m planning to install an NX3 Gyro/flight stabilizer to hopefully protect the plane from my amateur attempts at piloting. This must be mounted at the CoG. This weights.
  4. The servos for the elevator and rudder will probably be mounted between B3 and B4. I’m going to use 3.7g digital servos, and ideally I want to be able to replace these later if whatever I try first doesn’t work. Access to this part is tricky because the space between B3 and B4 is right below the “P13” plastic piece for the cockpit which goes back to B4.
  5. I’m going to try to make the cockpit/pilot plastic assembly removable and attached with magnets. I’m not sure if this is doable yet. Stay tuned.
  6. I might have thought putting an access panel at the top of the fuselage would make sense, but this will not be easy, so the alternative is a drawer again – sitting on “side keel A8” between B3 and B4. This will have the servos sitting quite low, but the horn at the top will sit above the level of A8 which will line it up quite well with the bottom of the elevator and rudder for connecting a rod to the control horns. That will make connecting a new servo wire tricky if I decide to switch servos as I’ll need to fish it though the space between B2 and B3 to get it to the receiver. This also puts around 8g of weight around 120mm behind the CoG, but with a 50g battery at the front I’m thinking it will be ok.
  7. The alternative would be to put the servos for elevator and rudder between B2 and B3. The only way to do this would be to make the P13 plastic cockpit (with guns, pilot and windshield) completely removeble. This would give access to the receiver and servos from the top, but would require figuring out how to attach P13, because the standard build has the struts for the upper wing feeding through P13 and having P13 glued onto the fuselage. Maybe some judicious triming and some magnets might do the trick. If I do this, the receiver and servos mount under the pilot and removing the pilot/cockpit assembly gives easy access to everything, but because it’s underneath the main wing, working in that space might be difficult.
  8. So there is another option. This one came up from watching the recent video from Tim McKay about his Beechcraft Staggerwing Foamboard model. Tim made the bottom wing removable by putting slots in the bottom wing for the wing braces. It’s probably not that simple for the Sopwith Camel, but I did think – what about putting magnets on the bottom of the wing braces? Then the bottom wing could clip off and on. It probably needs to be fixed more firmly at the fuselage (like rubber bands), but this would make it doable.

Building

This is notes from building. I’m not doing a detailed build log but just noting important things I find as I go.

Wing Blocks

The instructions say to block up the leading edge when building the wings. From what I can see this doesn’t make sense. So I’m blocking up the trailing edge and the rear spar instead. This gives me a much better match with the wing cross section shown on the plan. After building and sanding the lower wing using this approach, I’m very happy with it.

Wing Carbon fiber

I’m adding some 1mm carbon fiber rods to the wings for strength at very little cost in weight. I’ve put one behind the leading edge by putting a groove behind the leading edge and gluing the rod into the groove. I will add a second rod on the 2nd spar that the aileron servos will be mounted to. I decided to do this because I had to trim out a slot in the spar for the servo and that weakened the spar. So having to strengthen it anyway, I decided to take it all the way from the fuselage out to the wing tip.

Covering

The kit comes with some pretty basic white/translucent tissue paper. I am still considering using this, but I have two other options. I like the idea of building a “naked” model with all the structure visible. This is kind of driven by the wheels – if I want to keep those beautiful wooden wheels visible, I think I should do the same with the rest of the model. What holds me back is the electronics, because it will also be visible and that might make a naked model kind of ugly. So I’m also considering:

  1. Coloured tissue. I picked up some very nice dark green tissue paper from Michaels. I think it might look quite nice and be very close to a realistic look.
  2. I’ve ordered some silkspan. I like the idea of covering in cloth because that’s what (I understand) was used on the real planes. I think it was canvas. So I might try silkspan. I could do a naked model with the silkspan which will mostly hide the electronics, since the silkspan isn’t transparent, or I could paint it.

My plan at this point is to build 3 test panels and do the 3 different coverings and see how it comes out. I’ll update when I’ve done that.