F5 RES-Raven design & build

[CTR]

Hi all, Neil Pritchett here.

As some of you know, I started flying RES earlier this year coming from  F3K where my ageing bones were suffering! Entered 4 national RES comps and managed quite well with my own design which evolved during the year.

I fully agree with the ethos of making RES a good, modest cost, enjoyable class; but went my own way on design as I considered the kit prices to be creeping up too much and most designs were more suited to weather conditions on the continent.

Hence the birth of RES Raven, which is the settled design that I have used in the last 3 national comps, and several ISA & an Aylesbury contests, again with decent results.

Some of you have asked me to provide a kit of cut parts which I’m going to produce in the next month or so. I don’t intend to become a kit manufacturer as I’m retired from running my own business and don’t need the stress! However, this small batch is okay.

In an attempt to encourage more pilots to design/build their own, I’m going to do a build log of my RES Raven design here if that’s allowed.

I’ll await ‘the powers that be’ to confirm if it’s allowed, and if so will start posting my design philosophy & build process.

Happy flying.

[PeteMitchell]

I can't think there could be any objection to a build log Neil.

Maybe you would have a wider audience if you posted in the 'Builders Workshop' forum.

Where ever, it will be good to see and read.

[CTR]

First, a bit about my design decisions and findings:

My early encounters with the class; it looked like some of the current designs were barely fit for my purpose. The typical appeal for light weight seemed to result in cases of serious flutter and/or difficulties in returning after following a thermal any distance downwind. I also looked at various kits and chatted with others regarding quality and their build experiences.

I suspected I could come up with my own design which would be a better fit for my criterion; namely Low cost, cheap power train, 550g or less, good thermal response and tough enough for typical UK weather conditions. Not to mention the satisfaction of flying my own design.

Fuselage ideas:

I’m not too keen on carbon booms because they’re difficult to ‘field repair’ , cost more than balsa builds and they dislike shock loads compared to traditional materials. Result, a balsa & lite ply fuselage with sufficient space for RC gear, battery of choice etc. Also the cross section should be predominantly rounded for minimum wetted surface and torsional stiffness; also it’s my preference for looks!

the wing should bolt on the top of the fus’ for easy changes, and the tail feathers need to be fully removable without any spikey bits sticking out for easy transport.

Wings:

For relative ease of build, I’ve avoided a fully elliptical plan because the benefit is marginal in this class when compared to increasing tapered panels approximating an elliptical plan. The leading edges of each panel could also be made strongly and ‘ding proof’ with straight carbon rod and strip.

Airfoil choice was originally ag35 to ag37 which was easy to build because of the ‘flat’ underside. I then experimented with the ag24 to ag27 series with greater success; so I settled on that. The sections transition throughout the span. The polyhedral wing needed to divided into 3 sections, with a wide centre spoiler and compact transportation. The spoiler chord has become thinner and increased in width in the latest version through experimentation. The centre spoiled doesn’t have any noticeable effect on tailplane authority.

Tailplane:

The fin has a couple of 2mm carbon rod spars which go into aluminium tubes in the fuselage. A single M3 screw goes through the fus’ into the bottom of the fin to hold it all together. The stabiliser is sandwiched between fin and fuselage by the same components.

Electronics:

Low cost motor choice was determined by diameter & being able to get to 100M in 15 sec. This leaves 15 to push out in the direction of expected thermals! 23mm dia. results in a relatively small nose section and complements a 32mm spinner. 8x5.5 prop or 9x5 work well with the motor size. The motors used so far cost between £17 and £34 so unlikely to break the bank. A 20A esc is ample. In fact I’m using 15A in my models. 
batteries are 450 to 550mAh 3s which gives 2 flights with safety margin, and 3 flights for new batteries with full capacity.

Tail servos are typically 9g to 11g. 5g 20x8mm is used for the spoiler. A DLG style Z spring made from 0.5mm piano wire is used for spoiler closing. Tail servos are in the fus’ forward of the wing for easy access. 2mm carbon rods are used to the control surfaces. The rods go through holes in the fuselage formers without any guide tubing. This saves tail weight and I’ve never had one fail, so feeding a new one through the fuselage isn’t necessary. The former spacing is adequate for guiding the rods without them buckling.

all the electronics, with the exception of the spoiler servo, fits in the nose which makes for easy wiring.  The nose length was made to balance the Raven without needing additional weight.

There’s ample space under the wing at the C of G for ballast if needed. You could easily fit 300g but I’ve not had to use more than 100g in windy conditions.

Next instalment will be the fuselage build. Watch this space if you’re interested!

[CTR]

Spent the last couple of days laying out the laser cutting files for the Raven. The great thing about laser cutting is that it allows very close nesting of the parts, especially ribs. Each part will be engraved with its part number except the wing sheeting which is only 1/32 balsa, too thin to ensure the engraving doesn’t show through.

I’ve also incorporated all the minor mods learned from the season’s flying and the various revisions (4 previous builds). These make for easier construction and include some jig and alignment features for accurate building.

The photo is the CAD nesting file of all the balsa and ply parts needed. This includes the pre-cut wing sheeting too.

The jig design for the wing has been completed. This ensures accurate washout, polyhedral angles and symmetry.

Next job is to cut the fuselage parts for the build log. I hope the weather warms up a bit because my workshop is lacking heating!

For those of you who have requested kits, balsa has been ordered and should be with me next week. I use Balsa Cabin because they can supply ‘C’ grain (for ribs) and a selection of weights needed for different parts in the kit. These will be cut after the build log is complete.

Watch this space!

[CTR]

Weather forecast for tomorrow is a bit warmer! Should be cutting the RES Raven parts for the build log at last.

I will be straight on with the build and post photos as it progresses. I hope to complete it this week. Previous fuselages have taken two days once cut; one for the construction and the second for the shaping & sanding. Covering comes later but I’m sure builders will want to use their preferred material for that.
Coverings and glues will not be included in the kits. No electronics are included either, but all construction materials are, including about 37 ply & more than 150 balsa laser cut parts, carbon & wood spars, strip and sheeting. Aluminium & carbon tube, screws, piano wire, pushrods, hatch magnet etc will be provided too. The wing jig will be provided which is cut from 3mm Medite; each part is laser cut and numbered.

There’s  no need for a full size plan as the slot- together design of the fuselage, tail feathers and the accurate wing jig will ensure build accuracy. The scaled drawings will be emailed out and are for reference to the parts and where they all fit. There’s plenty of notes on the drawings to help understand the more design specific detail of construction.

This final rev4 one includes all the latest mods, most of which are to make the build easier and more accurate without the use of a fuselage jig. I’ve also taken the opportunity to do another version of the stabiliser. This one is interchangeable with the previous ones which makes it easy to do flight comparisons. It has an airfoil ribbed structure which in theory should be aerodynamically better. However, I’m not convinced that it will be noticeable in practice!

Watch this space!

[martynk]

Are you cnc or laser cutting the parts yourself? If so, I would be interested to know what toolset you are using. (Hard and soft)

[CTR]

I have a professional 100W RF laser cutter which I use for all my cutting. The great thing about laser cutting is that internal corners have a very small radius, approximately 0.12mm. I also use a concentric, high pressure air assist when cutting. The result is a cut edge without blackening and significant soot. This means there is no need to clean up the edges for glueing. 
I’m also careful to set the correct kerf width for each different material. It’s a bit different from router kerf which is only determined by the radius of the cutter. Basically, the slower you laser cut, the wider the kerf. Maybe ranging from 0.1mm to 0.2mm in woods. The lens focal length also has an effect too. Shorter lengths result in thinner kerf, but only when used for materials less than 2mm thick.

[CTR]

Progress today: 

Wing and stabiliser jigs cut and assembled. Parts are designed to be snug fits so no glue is needed. This means they can be packed away easily after use; ready for any future repairs! A set of jig parts will be included with the kit.

I hope to get the fuselage parts cut tomorrow, then assembled. The nose formers are laminated and aligned using a balsa cut jig and the 8mm wing spar tube; so it makes sense to make the fuselage before the wing centre section. The photo of the cad file shows the front end to give an idea of how it will turn out. The fuselage nose photo shows how the rounding of the section is to be done. This is a rev3 one which was part way through build. I’ll still complete it as functionally it’s identical to the rev4, just not quite so easy to build.

More to come!

[CTR]

Fuselage (plus some other parts in the nesting) cut today. Each part is engraved with the a reference as you can see in the photo. This is shown on the drawings too. Also note the selection of Balsa, harder for the nose pieces, lightweight for the tail end and ‘C’ grain for the ribs etc. I’ve maximised the useful amount of spare 1/8” and 3/16 sheet by tight nesting too. With laser cutting you only need to leave one or two mm between parts.

I don’t tag the inner holes etc in the parts as these are thrown away anyway. Less work when removing the parts ready for assembly. They drop into the bottom tray of the laser; just need to remember to empty them from time to time to avoid a fire!

Should be starting the fuselage construction tomorrow. I expect it to take more than the usual day because of the photo log! Hopefully that will be the start of the more interesting bit for those of you persevering with this thread!

[CTR]

Okay, here we go!

As pictures paint a thousand words, here’s loads of photos:

first release the ply formers an balsa parts needed for the fuselage. Glue the front and rear side parts together; making sure the left and right parts are matched up because they are different. This is important for the side thrust to be correct. If you use tape like I do, make sure you tape both sides so they remain straight.

the remaining 10 photos in this post are details of the tail block. Accuracy with this part will pay off later. Make this up while the fus’ sides are drying. First, sand the middle piece (1/8” balsa) F13a, down to 3mm thick, then glue it to one of the two side pieces (3/16” balsa) F13. When dry, cut out the small bridge bits to allow the aluminium tubes to be fitted. Select the F14 & S5 pieces and hold them and the F13 pieces together. Measure the total which will be about 24mm. Cut 2 pieces of 3mm O/D tube ant 1 piece of 4mm O/D tube to the same length (or a bit longer, but NOT shorter). Clean up the tube ends and make sure the 2mm carbon rod will slide into the 3mm tubes. Press the 3mm tubes into the outer locations and use part S5 to set the protruding height of the tubes from the stepped side of the F13 block. Apply thin CA to hold their location. Next, file a 0.5mm deep grove into each F13 piece to allow the 4mm diameter tube to be inserted on the centreline, and with the same protrusion as the 3mm tubes. Glue the other F13 piece on to complete the lamination of the tail block. Set it aside to dry.

next post will cover assembly of the nose formers and shaping of the F13 tail block.
 

 

[CTR]

Nose formers etc:

Use the 8mm spar tube to align the parts. Start with gluing the 2 balsa jig parts together. Leave them on the tube but make sure they’re not glued to it! The ‘hump’ in the parts is for the motor wires to pass the motor.

Glue and Fit F3 and F2 onto the jig, not gluing it to the jig. It’s not difficult to dig out the balsa jig if they do get stuck. Next select the motor mount F1 that matches your motor of choice (2 mounts are included) and glue that to F2.  Make sure it is in the correct rotation so the motor wires will line up with the ‘hump’! The centre hole in F1 is a snug fit on the tube to ensure it is concentric. Use small clamps to hold the assembly together, then slide it off the tube.

while the nose laminate is drying, grab the F13 tail block and plane & sand it to taper down to 3mm at the tail end (nothing off the leading end of the block).

Now we’ve completed those parts we can start gluing up the fuselage.

[CTR]

Fuselage sides & formers are next in the assembly process:

Before we do, use the fuselage sides to mark each side of block F13 where the pushrods will exit for rudder and elevator. Once marked, use the 2mm carbon pushrod to grove the balsa for the pushrod exits.

Decision time: Will you use the carbon pushrods without guide tubes as I do, or do you want to be more traditional and use guide tubes?

Advantage of not using tubes is tail weight saving. Disadvantage is that it would be a more complicated job if you ever needed to replace a pushrod! I personally have never needed to replace one on a RES model. You will need to increase the pushrod hole size in the formers if you want to use tubing. Do this now.

 

[CTR]

Fuselage glue-up:

Carefully align & glue the F5 wing seat parts to the fuselage sides. Make sure they are glued on the insides (sides with the engraving showing).

insert former F6a into F6 as shown in the 2 photos, then insert the servo tray to make the assembly in the 4th photo. These are a good fit and will hold together without glue at this stage. Take the left fuselage side and insert the F6 assembly into the correct slots. CA these parts together.

Add F4 in front of the servo tray and glue it while using an engineers square to make sure it is at 90 deg. Do the same with F8a & F10, but make sure the face engraved with ‘Front’ is facing the nose.

Here’s a confession; F8a was engraved on the wrong face The benefit of doing a final prototype assembly is to seek out & fix these errors! 

Next is to glue the remaining F11 & F12 formers to the side, but instead of using a square, use the angle template as shown in the photo. Make sure the template in on the nose side of the formers (image 6), yes, I know it’s upside down! Too late to rotate it now.

Finally for this stage, glue the F13 tail block in place.

Take the right fuselage side and fit it to the former tabs. Only glue from F4 to F10 (wing TE former) at this time. Now you can add the F5 doublers to the main hatch area. The top edges of these will be sanded to match the fus’ sides later after sheeting the bottom.

you’ll notice we haven’t used a fuselage jig, and don’t need to. There’s a small hole in the top of F6 and fine notches in the top edges of each former to the tail end. Use a piece of thread and pass it through the hole in F6 and wrap it found F4 to anchor the front end. Knot it or use a piece of tape. The tail end of the thread is put through the rearmost aluminium tube in F13, pulled taught and held with tape. Now’s the time to glue the tail end making sure the former notches are in line with the thread. Don’t worry if there is a slight discrepancy along the fuselage as this will be corrected when the bottom sheeting is applied. The thread is left in place until the bottom sheeting is completed; resulting in a straight fuselage without a jig!

Add F4 and the motor mount assembly to complete the formers. You will notice that the motor mount has side & down thrust designed in, hence the different left & right fus’ sides.

More in the next day or two.

[PeteMitchell]

Great see the pics and read all this Neil.

One thing I don't understand is your wing jig.

Most other designs seem to use wing rib cut off tabs and a simple alignment strip to ensure accurate building etc,

Just what decided you to use your method ?

[CTR]

Good question Pete.

Later when I get to the wing you will see there is quite an area of sheeting. That makes it difficult to have sensible tabs. Also, the tabs would need to be cut off following the airfoil shape. That’s not so easy and often results in ‘flats’ in the rib profile. The main wing spar is 8mm diameter which doesn’t leave a lot of material in the ribs above & below. Easy to distort of brake if only supported by tabs near the ends.

the full form jig should ensure accurate washout too. For example, the even thinner wing under development has 0.2 to 0.5deg washout at different points. Not a lot dimension wise with a tip cord of 150 mm.

Having said all that, if someone comes up with a better idea for an accurate jig to match my design, I would be happy to adopt it. There’s quite a bit of material and cutting time in the full jig!

[CTR]

And there’s more!

First a note of warning: In the previous post covering the motor mount assembly, do not use CA! Use aliphatic which gives time for the jig and parts to be removed from the wing spar. Otherwise you’ll have a very funny bulge in the wing!

Time to continue the fuselage by add the F8 wing nut plates in the locations cut out in wing seat doublers. Then add the doubler F9 on top of the rear one, noting that the longer pointy bits face forward! When dry, insert the M4 nuts and fix with a drop of thin CA.

Glue the two pieces for the servo hatch, SH1 and 2, together and set aside to dry.

Next job is to add the 1/8” square balsa strips to the top edges of the fuselage sides, wing TE to tail block. Careful design means both pieces come from one 3’ length.

Follow that by adding the 3/8” triangle strips to the lower edges of the Fuselage sides. A couple of notes here; the total length means you need to join strips. Make sure you use a tapered joint for strength not a butt joint. 1” or 25mm will be long enough. Also, you need to sand or file a curve in the hypotenuse of the triangle (longest side) at the nose end of the strip so the triangle fits snuggly to the radius of the motor mount. I found that a half round file was good for this. Do this for all four pieces at the nose ends. The tail ends of the strips will need tapering to the centreline of the fuselage too.

triangular strip is notorious for inaccurate sizing! Sand the strip to match the fuselage sides when dry. This is quite easy because the brown laser edge of the fuselage acts as a witness to not sand too much!

There are two more triangular strips to add on the top edges from nose to F5; don’t forget the motor mount end shaping! The curvature of the fuselage sides is gentle enough that saw slits in the strips should not be necessary; but if your wood is on the stiff side, then add razor saw cuts part way through as required.

After all the strips have dried, add F14 to the underside of the tail block, then add the bottom sheeting BS1 to BS12. Make sure the engraving is inside the fuselage (BS1 is angled for side thrust). I use masking tape to hold the sheeting while setting (no pin holes).

On the home run for the fuselage with the top sheeting etc. to go.

 

[CTR]

Fuselage just about done, all except the sanding & shaping.

Before applying the top sheeting, the carbon pushrods must be installed if you haven’t used guide tubing.

the nose end of the top sheeting is part CG1 and dust be the right way round. The engraving is on top and is sanded away when shaping the fuselage. Next in line is the motor access hatch then CG2 followed by the Servo hatch. That all covers up to the wing LE. Those of you with sharp eyes will see that CG2 was short by 1.6mm! This has been corrected for future kits.

The rear top sheeting pieces are 1/16” balsa and sequentially numbered. Again, masking tape is your friend!

The bottom sheeting is 1/8” balsa, transitioning to 1/16” towards the rear. Sand about 200mm if the 1/8” to taper it down to 1/16” for a smooth transition.

The servo hatch is made up of 6 pre-cut balsa pieces, and a 1/16” ply finger grip.

The fuselage cross section shaping is fully round at the nose. The underside is fully rounded along the full length. The easiest way to do this is by planing the corner off at 45 deg. to the stage where a 3mm width of the triangular strip is exposed. Follow this by sanding a full radius. Note that more of the triangle will be visible towards the nose. When sanding the top nose section, but the hatches in place first. This will result in a much better shape.

The top edges from wing to tail are rounded to the point where the corner of the 1/8” strip is just visible, no more!

There’s a couple of photos of the shaping for guidance. The tail underside shaping will include sanding/filing the aluminium tubes to be flush with the balsa. Gently countersink the 4mm tube a little for the M3 countersunk screw head.

Next up will be the tail feathers.

[cirrusRC]

Neil, is there enough access to replace motor?  Not obvious from the pics.

Enjoying the build thanks.

[CTR]

Hi Dave, yes, but it’s a fiddle!

two options: Cut away a circular area of the triangular strips each side of the motor access hatch (rear end of hatch where there’s enough width to leave some to support the hatch); or put the motor in via the servo access hatch and pass it through former F4 towards the nose. The motor access hatch is where you can get to the connector for the motor.

The motors tested with the Raven are 23mm dia. direct drive. Both in stock at Hyperflight. One at £17 and the other at £34 if I remember correctly. The £17 one has performed very well during the last season with an 8x5 prop and a 3s 450mA battery.

[CTR]

For those wanting to keep track of the weights during the stages of build, the fuselage after shaping, including pushrods, before covering, works out in the range 75 to 80g. Not too bad for a fully built up job. About 18g sanded away during the shaping process.

the stabiliser was interesting. The one I’ve been flying with is a flat strip balsa frame covered with film. This was okay with no flutter; so why change it? Well, aesthetically it’s not brilliant. Also aerodynamically it’s only okay.

I’ve experimented with different constructions, all using ribs, see the photo. One with a balsa spar, another with a 2mm carbon spar and the latest using a balsa spar and 1/32” LE sheeting. Interestingly, the weight spread is between 9.7g & 10.6g before covering; the sheeted one being the heaviest but also the stiffest! The sheeting is only on the underside which gives sufficient strength and clean aerodynamics. This is the design which will be included in the kit. A bit more building time but I think it’s worth it! The added 0.9g at the tail will mean about 4.5g added to the total flying weight which is less than 1%.

Note the sliver of balsa holding the assembly into the jig. The outer frame and centre pieces, laser cut from 3/16” balsa, are glued together on a flat building board first. That’s then put in the jig for the ribs and spars to be added, followed by the sheeting. The ribs have 3/16” ends to match the frame thickness, All very straightforward to assembly.

The stabiliser centre section is flat so it still sandwiches properly between the fuselage and fin. There are doublers top and bottom which hug the shape of the fin on top and the fus’ on the bottom. This should avoid accidentally putting the stab’ on upside down! These doublers also give enough thickness for sanding to the proper airfoil shape, guided by the ribs each side of centre.

Not sure if I’ll do the fin or wing next, but that will be revealed in the next couple of days!