Aileron Servos on my Guillows Sopwith Camel

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

There is also a YouTube video with a demo of the working ailerons (still under construction) on my Tim the Plane Man channel.

Direct Drive mounting of the Aileron servo
Showing how the servo arm is built into the edge of the aileron.

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.

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 cm25 degrees50 km/hr.043 kg/cm
.004 Nm
.044 kg/cm
.004 Nm
Elevator3 cm18 cm20 degrees50 km/hr.088 kg/cm
.008 Nm
0.033 kg/cm
.003 Nm
Rudder4.5 cm8 cm20 degrees50 km/hr.087 kg/cm
0.008 Nm
.035 kg/cm
.003 Nm

[I’m not sure why there is such a discrepancy between the Big Bird and Meysner numbers for elevator and rudder while the aileron numbers are so similar. I think I might have an error in my spreadsheet somewhere – more digging required].

This 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!

It also means that I should probably be able to use the same 1.7g servos for elevator and rudder, although I had been planning to use some 3.7g servos I have for those controls. I’m not sure what I will do about that yet (2021-07-30).

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. See my YouTube video that shows how it works.

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.

The next thing is to figure out how to connect the wires to the receiver. I had been planning to have 2 channels and a Y cable on each channel, a bit like running a 2 channel mix driver for ailerons on a single wing plane, but using the Y to split the signal so the upper and lower wing each get the same signal.

Rob Mawer on the Guillow’s Facebook group suggested another option – use 4 channels with separate wires to each servo. This wouldn’t add much more weight, but would make it easier to centre and trim each servo if they are on separate channels. It makes the model setup in the transmitter a little tricky, but with OpenTX, it should be doable. Right now I’m thinking of this as a backup plan, but that might change when I build the upper wings. Stay tuned!

Upper wings

Well I thought the upper wings were going to be easy. Just use a Y connector to drive the upper wing from the same receiver channel as the lower wing. I would do a 2 channel aileron mix in the Jumper T-Lite, OpenTX makes it so easy. But after I posted on YouTube and Facebook, this was the first thing people called into question.

Rob Mawer on Facebook groups pointed out that centering and trimming the two ailerons might be tricky if the two ailerons were not exactly aligned. He suggested using a 4 channel design so each servo could be centered and trimmed separately. The OpenTX setup would be very custom, but doable.

This also makes me think that using the Y to split the signal on the same side might not be as smart as running the upper wings with one Y on one channel and then the lower wings on a second Y on a second channel. This would mean the ailerons are centered and trimmed as per a normal single channel arrangement, but it would have to be done twice, once for the lower wings and once for the upper wings. I kind of like this idea.

Cliff Harvey RC Planes on YouTube suggested that ailerons on the upper wings might not even be necessary, the ones on the lower wings might be enough. I’m definitely drawn to this idea, it makes things much simpler.


Irwin Weisbrot on YouTube suggested that I could mechanically link the lower aileron to the upper which would mean I would not need a servo in the upper wing. This is attractive since it is very similar to how the real plane was built, and if I got it right it would look very “scale”, but it would double the thrust that the servo would be working against and therefore the amount of torque required. My calculations right now have a lot of ‘fudge factor’ and the two different methods give so different results, that I’m not convinced this would be safe. I might keep this in my back pocket.

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.

The copyright notices on the plans and the enclosed catalog say 1973 and 1974. The stamped model number on the kit pieces say “801-1”. I can’t be sure but I think this kit is about 50 years old.

Luckily all of the wooden pieces are well preserved, except for the plywood containing some of the slats and frame pieces which is a bit warped. I’m soaking this in water and trying to dry it flat. This worked out fine.

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

Errata and other comments on the instructions.

The Guillows instructions and plans are incredibly detailed and generally very complete and accurate, but I’ve found a couple of minor problems, so I thought it might be worth mentioning them in case it helps you.

  1. I am very sure now that the leading edge of the wings should not be blocked up when building them. I built both my wings by blocking up the trailing edge and rear spar and it “just makes sense” – based on the balsa pieces and the target wing profile. This was proven out by how easy it was to slot the lower wings into the L1 pieces on the fuselage when I got that built. See my video about the wing construction on Tim the Plane Man on YouTube.
  2. The instructions say to soak the L1 pieces because they need to curve when fitting on the fuselage. This should read L6 – it’s the L6 piece that will be curved and needs to be soaked. Minor typo but important to know.

Credits – other builds of this model

Guillows Kit No – 801 Sopwith Camel

There are a lot of people who have built this model over the years and posted details of their build online. These are some of those I have referred to in doing my build.

radfordc https://www.rcgroups.com/forums/showthread.php?t=305930
Warren Jones https://www.rcgroups.com/forums/showthread.php?t=79733
Bill G http://www.wattflyer.com/forums/showthread.php?t=39480
John Cole  https://www.rcgroups.com/forums/show….php?t=2320877

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 B2 and B3, but the sliding forward into the front of the plane. 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. The receiver will at the front of this space between B2 and B3. 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, so in front of the receiver between B1 and B2.
  4. The servos for the elevator and rudder will probably be mounted upside down 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. As I build out the model, this is looking like the best choice.
  6. I’ve decided to make the P13 plastic cockpit (with guns, pilot and windshield) completely removable. This gives access to the receiver and servos from the top. With some judicious trimming where the P3 slots around the struts and some magnets I can make this work. 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, I’m going to have to make the top wing removable (and held on with rubber bands).

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. This is what I am doing.
  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. Silkspan will wait for the next build – likely the Dancing Wings Hobby S17 Fokker DR1.

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.

Painting the plastic parts

I agonized over this, but in the end it’s pretty simple. Standard acrylic hobby paint works fine on the plastic parts, but you must do several coats. The first coat will look horrible, streaky, weak. Persist! 2nd coat the plastic will still show through. The 3rd coat will look great.

The one exception was the cowling. I wanted it to look awesome, so I used spray paint. Rustoleum “Painters Touch 2X Ultracover Gloss” colonial red which I bought from Rona. I also painted the wheel covers with this.

Painted plastic parts for the Guillows Sopwith Camel 801
The cockpit and pilot. I think it looks awesome.

Electronics

Specifications

The motor is the 1811-3800KV from Rayspeed RC on AliExpress, the specs say a 5030 prop will produce 205 g of thrust at 7.4 volts and will draw 7.5A. I chose this motor because it weights only 10g, which is crazy because most of the other motors I’ve found that will give me the 200+g thrust I”m looking for weight 20g or more, and weight is critical for this build.

The receiver is the RX444 from AEORC. I bought it off BangGood, but they also have their own site and you can order direct. The RX44x series receivers give you the choice of protocols, so if you want Futaba get the RX442, if you want DSMX its RX444, FrSky is RX445 for the D16 and RX447 for the D8. This receiver has a built in 2S 15A brushless ESC, so I save even more weight here. The receiver weights 7.1g and I don’t need an ESC, probably saving another 5-10g. The 15A ESC is more than double what I need to power the motor.

I am planning to use a flight stabilizer to try to protect the plane from my inexperience as a pilot. I’ve ordered the Radiolink Byme-A from AliExpress, which has good reviews as a flight stabilizer, but importantly weights only 4.5g.

I am using 2x 1.7g AFRC D1302 digital servos  mounted in the wings (see my detailed post about this here) for the ailerons. My initial plan is to only put working ailerons on the lower wings. If I find I need more authority on the ailerons, I’ll think about putting them in the upper wing too. This saves at least 10g.

The rudder and elevator are controlled by 2x 3.7g digital servos I bought from Wishiot on Amazon. I don’t think I need servos this big, but I’m kind of going overkill just in case the aileron servo experiment doesn’t pan out. These awesome little digital servos generate 0.5 kg/cm torque which should be about 5x what I need. Saved 10g compared to using “standard” 9g servos.

The battery is a HHZ brand 2S 7.4V 850mAh 20C LiPo from Amazon. It weights 50g which is about the heaviest thing on the plane. If I need to save more weight, this will be the place to do it, but I’m starting with this targeting 185g for the total weight of the plane, with an absolute top limit of 200g.

Installation

The motor is screwed directly on the plywood firewall. It fits perfectly and the drive shaft protrudes out through the cowling with just enough room to mount the prop. I added a washer under one of the mounting arms to give (I hope) about 5 degrees down thrust and right thrust.

The receiver mounts at the top of the fuselage under the removable cockpit/gun mount/pilot piece, at the very front between B2 and B3. I’ve left a space between B1 and B2 for a gyro/stabilizer to be mounted at the CoG.

The elevator and rudder servos are mounted behind the pilot between B4 and B5. I found that to get the control rods to run cleanly and fairly straight, it made most sense to mount them upside down. Access will be from below, so I need a new hatch on the bottom between B4 and B5.

The aileron servos are mounted in the wings. I explain this in detail in this post.

I built a battery box, so that I could slide the battery in from an access hatch on the side of the plane between B2 and B3, but then slide it forward so it actually sits under the receiver at the very front of the plane between B1 and B2. I’ve made the box positon adjustable using velcro, so I can move the battery back if necessary to adjust the centre of gravity.

I have installed a switch for the power. Because installing the battery is a bit fiddly, I want to be able to slide the battery in, get it seated and then close the hatch before powering up the electronics. I’ve ordered some pre-wired switches from BangGood, they come with JST style plugs that will plug directly into the power wires on the receiver and the battery with no soldering required, I love that! Here is a picture of how the switch looks: