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Rusty Fergusson

Crop Duster Design

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Lindsay Todd28/05/2020 12:44:04
1774 forum posts
1833 photos

Thought i would just add this design which i have been working on for a while will post a build blog if sufficient interest

Model is 65" span for 60-70 size four stroke or electric equivalent, model now built and ready for paint and test flights. Free plan? maybe Linds

rf model.jpg

Lindsay Todd02/06/2020 08:39:45
1774 forum posts
1833 photos

Well I thought i would offer just a couple of pics up front of the model prior to paint but on completion of the construction. All very standard balsa ply traditional format, an older OS 70 four stroke up front should give more than adequate power and generous wing area should make for nice slow speed performance.


Its a big cockpit area so I need a character pilot for this one I think. I was going to g with flaps but decided not to bother in the end as the weight and wing area is so good but we'll see when it comes to flight performance if some retro work might be required.



Kevin Fairgrieve02/06/2020 08:42:29
1719 forum posts
2989 photos


Looks like a "Free Plan" to me.

Martian02/06/2020 09:36:33
2695 forum posts
1270 photos

I like it Lindsay it appeals to me

Lindsay Todd03/06/2020 10:44:20
1774 forum posts
1833 photos

Thanks for the interest guys as with all my own design these days followings drawings i get Slec to make a set of laser cut parts for me as this cuts down a lot of time and not too expensive to do.


First job was going through and identifying all the bits, i should probably get them engraved but never seem to get round to that and it is never a bad thing to go through all the bits and see if i have missed anything anyway.

Job started with the front fuselage sides. these are in two sections each due to the depth and then have ply doublers glued to the lower edge and wing seat. Also added the first of the 6mm square to the front lower curved edge.


I then marked the position of the formers on both fuselage sections and used them to set the positions of the other 6mm square strips down the fuselage sides. Everything was then glued in place on one side first and when dry fixed the second side in place

img_0003.jpg I also glued the first of the undercarriage fixing plates into position which helps keep things square



Geoff S03/06/2020 11:26:37
4032 forum posts
68 photos

Is there any consideration to battery access for swapping if opting for electric power or will it need to modified by the builder?

This could well go on my 'models to build' list. It certainly looks like my kind of model.


Lindsay Todd03/06/2020 12:43:56
1774 forum posts
1833 photos

Hi Geoff, there is plenty of space and good access via tank hatch for electric conversion, the only mod required would be a quick release hatch catch rather than the screws i used.

Moving straight onto the rear 6mm hard balsa lattice assembly using formers to ensure correct position then fixing uprights in place that the formers get glued onto and a 6mm insert at the tail end


The assemblies go this far only (no diagonal bracing) before assembly onto the front fuselage as the rear formers will glue onto the forward face of the vertical bracing


Rebates in the fuselage sides locate the lattice frames, the lower ply doublers also extend to add strength at the bottom.



Dwain Dibley.03/06/2020 22:37:27
1700 forum posts
1648 photos

Nice and Interesting Lindsay, watching at Dibley Towers. With Electric in mind of course.


Lindsay Todd05/06/2020 08:15:38
1774 forum posts
1833 photos

With the two latices frames attached now i started to add the rear formers one at a time working my way back down the fuselage. Each former being glued to the front face of the verticals of the lattice


The fuselage is clamped at the rear also fixing top and bottom 6mm balsa plates in place. Note the whole in the rear former for the elevator push rod control


More 6mm square bracing across the base of each former


The top stringers are again 6mm square and now added to the top rear fuselage back to former number 9 and also added the diagonal bracing down the sides between each former, again 6mm square balsa


Further 6mm sheet inset flush between former 9 and 10 as added support for the tail plane.



Lindsay Todd08/06/2020 10:41:37
1774 forum posts
1833 photos

At the front the cowl area is built in-situ and starts by tack gluing a couple of plates to set the angles for the front faces of the cowl, these will be removed later


The front is made up of two 6mm balsa faces and a 3mm ply ring. the balsa faces need a chamfer carving both side such that the assembly sits square a bit fiddly but the photo makes much more sense


I did not have my engine available at this point so had to measure carefully but you could fit the engine prior if you wished. Once the side plates are dry then add the nose ring


The position of the ring can be set by a block across the top and then measure side to side, just make sure its square to avoid messing about later.


The top of the cowl is then sheated with laminates of 6mm soft balsa, carved and sanded to shape.



Richard Harris08/06/2020 11:42:41
2234 forum posts
2057 photos

Looking forward to this one Linds thumbs up

Lindsay Todd08/06/2020 12:57:30
1774 forum posts
1833 photos

cheers Richard, feels like its been a while since i did a design although Terrysaur is still with the mag, anyway a bit more.

The underside is now sheeted, 6mm at the front so we can blend into the nose ring then 3mm behind back to the second uc plate which is glued onto the previous.


The second UC plate is 3mm shy each side, these are filled with scrap balsa simply to make rounding the edge of easier


Front end now sanded to shape on the underside. Note i have marked the position of the front face of F1 this is so i can cut on this line later to free the cowl from the fuz with a razor saw



Lindsay Todd09/06/2020 08:22:33
1774 forum posts
1833 photos

The tank hatch or battery hatch sits behind F1 and is made from two sub formers with 6mm balsa side rails and bridging longeron across the top. The sides are slightly built up and then chamfered to match the profile of the formers.


3mm sheet is then added in two sections with joint along the centre line


Before sanding i also started to prepare the top fuselage cockpit area by adding 6mm square struts and chamfer to the fuselage sides from F2 back to the rear of the cockpit ready for sheeting. This way when i come to trim and sand the hatch and fuselage will all blend together.



Lindsay Todd11/06/2020 12:59:06
1774 forum posts
1833 photos

3mm sheeting added using centre brace as the joint line and overlapping the chamfer cut to the sides of the fuselage


With both sides done trim and sand together with the front hatch so all is level and then trim the cockpit area to shape using a template and sharp scalpel


before sheeting the rear top section i glued a series of strips onto the top longeron aligned with the formers as extra gluing contact area


3mm sheeting was then glued in place in two sections each side using the longerons as a centre joint from cockpit F5 former back to tail plane insert F9




Lindsay Todd12/06/2020 00:04:22
1774 forum posts
1833 photos

Should have mention cockpit floor sheeted to top of upper logeron from 2mm balsa sheet and then the underside of the rear fuselage sheeted with 3mm balsa save for a rear ply plate to take a tailwheel assembly. all sanded and rounded off at the edges so time to start the canopy frames. These are made from 6mm square balsa , i used cocktail sticls glued through diagonally drilled holes to add a bit of strength to the joints, one for the front and one for the rear.


The rear frame is glued to the front face of the rear cockpit former F5 sitting on the fuselage sides


a 6mm semi circle is glued to the frame this having a cut out slot for the angled rear brace which is 6mm square spruce


A centre 6mm sheet is then added and then the front frame added to this to set the position, a further semi circle of 6mm to the front of this and a further 6mm square spruce brace at the front. The whole assembly then has a further 6mm sheet added to the top


further 6mm bracing is then added to the rear frame glued to the front face of F5 which really beefs up the structure. some further triangular infills around the bracing at the front and rear struts complete the basic frame work. the assembly is then sanded to match the angles so that the glazing will wrap around. will become clearer later.


Now for the brave bit, take a razor saw or hack saw and cut the cowl free from the fuselage flush to the front of F1, the lines made early are a huge help. When done i fitted the engine mount and my rather well used OF70 Surpass and set about making clearance holes and fixing ply plates to secure the cowl back in place.


lots of measuring and slowly carving and grinding produced a reasonable fit. i also added the lower air scoop simply from 3mm sheet



A couple of strips of 0.5 ply to the sides of the cowl stop screws from crushing the surface and as i had some laminating epoxy work to do i coated the cowl inside and out and the front face of former F1.


just to help things along i made up the u/c from 3mm aluminium sheet and a 4mm dia length of piano wire. the assemble screwed to the u/c ply plate with the aid of a saddle clamp for the wire. an old set of wide 21/2" wheels that have hung around for years complete the job.



Lindsay Todd12/06/2020 08:27:17
1774 forum posts
1833 photos

The tank / battery hatch had a couple of ply rails added just to get a nice secure fit, will be retained by two straps with screws into ply plates.


With the fuz put to one side time to make the fin and tail plane components. These are simply 6mm balsa strips cut to width and length and glued together


Once dry the rudder was sanded to a taper profile leaving a thickness of about 2mm at the trailing edge and a chamfer on the leading edge, a 0.5mm ply plate will be glued each side for the horn fixing


The tail plane is very similar construction, made from 3mm medium density balsa strips with a simple solid sheet section for the elevator, leading edges rounded and taper as per rudder to the elevator again with ply plates either side for the horn fixing


The fin needs to have a location slot formed on the fuselage between F9 and F10, use some scrap 6mm and glue strips either side making sure the slot is centred and sguare


The fin can then be slotted into face and further balsa added to make up thickness such that it can be carved and sanded to match the rear fuselage line. This can be done prior to gluing the fin in place.


Lindsay Todd14/06/2020 18:44:51
1774 forum posts
1833 photos

Once sanded time to look at fit of fin and tailplane to the fuselage so decided to get these covered using heat shrink nylon. I then also located the elevator outer snake down the fuselage exiting at the rear slot in former F11 and added the 6mm F11 which will be rounded off.0050.jpg

Once happy that this was set square i glued the tail in place having removed some of the covering from the underside. I also extended the top balsa fairing with some more soft block


A quick test fit of the elevator to make sure all working fine. Also note the ply plate for the rear commercial tailwheel i had lying around, a world models item i think


Before fitting the fin and rudders i installed the elevator and rudder servos onto ply plates. The rudder is closed loop so sited centrally and the elevator offset to one side. also note the throttle snake sited. Std servos used both futaba, 30001 for rudder and 148 for elevator as no great loads expected.


Fin and rudder also covered and glued in place and horns fitted


cables run down the fuselage and exit through two abs tubes for guides, perhaps brass would have been better but i did not have any two hand and as the cables are nylon coated any wear should be minimal. I also covered the rear fuselage open lattice structure again with heat shrink nylon.


Back to the front of the model and time to create the air exit for the cowl into the underside of the fuselage. I planned for this with a slot cut into F1 but for some reason added a radii to the corners so first job was to mark out and trim material away from the underside and remove the radii from the former


Trimming done and 3mm balsa sheet added, this combined with a small cut away at the rear of the cowl should ensure adequate cooling for the engine



Lindsay Todd20/06/2020 23:51:19
1774 forum posts
1833 photos

Time to show the wing construction, all ribs from 3mm balsa, constant chord so all basically the same except for some cut outs for servo trays and center spar braces which require the ribs to be cut in half.


Other items are spruce 6mm square for the main spars, 3mm marine ply spar braces and aileron servo fixing strips, 1.5mm servo plates. so nothing unusual


I built the wing with the first panel carrying the spar braces but we start with siting the lower main spruce 6mm square spar, the rear 6mm hard balsa lower spar and then setting out the ribs, including ply strips for the aileron servo fixings. Spars are aligned on the center line joint.


I used the servo plates to ensure correct spacing so i get a nice neat fit later, simple tip that can remove a real pain later getting the plates to sit flush with the underside of the wing


Top spars now added together with main spar brace, rear spar brace and wing bolt plate. I added the rear section ribs to the centre section together with a couple of small triangular blocks for support. Trailing edge stock was also added which is just a strip of 6mm medium density balsa cut to size.


My bench is long enough to now prop the first panel up such that the spar brace second side can sit flush on the bench to account for the dihedral and the second panel can be made as per the first in-situ and joined to the spar braces as you go. If necessary it could be made separately and added after.


With the basic wing complete check the position of the center line and then fit the front dowel plate to the front of the spar which is in effect a further wing spar brace and also set the position for fixing the front outer riblets that run either side of the fuselage side when the wing locates so worth a quick check against the fuz to check positions and fit are ok


Front riblet now added together with the 6mm balsa leading edge strip to both sides


When dry the wing was flipped upside down and the trailing and front 1.5mm sheeting was added, note i want to keep the structure flexible at this point so the cap strips and tip and centre mid sheeting were left till later as we need to incorporate some washout into the wing which i will do next



Stephen Jones21/06/2020 00:32:38
2924 forum posts
1660 photos

Hi, been watching this one quietly in the Background, it`s coming along nice.

I always look at your build and wounder to myself how deos he design his models so meticulously even down to using cocktail stick to strengthen a joint.

Well done for the work so far i am sure it will be a Great flyer. thumbs up


Dwain Dibley.21/06/2020 01:04:30
1700 forum posts
1648 photos

I've just caught up Linds....Wow...What a read, really enjoyed it, superb detail and super pics.....

Keep up the good work Matey..


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