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!