Challenger RCME September 1984

[Nigel R]

Hello everyone.

 

I like classic pattern airframes. They are a good size for a sport model, usually around 60" or so - big enough to deal with the UK weather, small enough to use cheap standard radio. And of course, built straight, they fly smooth and accurate maneuvers. Lots of the converted on this forum, I think.

 

Thus the next project on my bench is a classic pattern airframe; Steve Burgess' 1984 design, Challenger. A lovely sleek bit of design work that nevertheless managed to keep to a simple structure for a reasonably easy build. It was produced just as the contest rules changed from ballistic to turnaround. The original article showed a nice affordable side exhaust motor with a pipe hung underneath, and a radio install consisting of just four standard servos plus a fifth to actuate the air retracts (although the plan also shows fixed gear)

 

There is a scan of the 1984 article at this website:

 

https://scottishf3a.tripod.com/id12.html

 

The design was also updated in 1992. This latter design seemed only to make some detail changes, such as showing two wing mounted servos, adding a hatch under the tank, and a different wing section. It did also show a rear exhaust motor - which at that point was probably standard kit for F3a. By and large, everything else remained as was.

 

https://outerzone.co.uk/plan_details.asp?ID=10185

 

As can be seen from the articles and plans, the fuselage is little more than a box with a thick top deck to allow for some shaping, with a razorback deck stuck on top, and a wood 'canopy' area. Lots of razor plane action will be needed to get a nice shape. Wings were shown as foam, however as usual I will be doing a built up construction.

 

This first post is really just here to put down some details and references for the design - I have achieved little yet, other than putting in an order at SLEC for the timber and some knick-knacks. I am still deciding on how to kit the thing out - I have several suitable motors to choose from - a side exhaust OS 61 SF and a rear exhaust OS 61 RF, both of which are pumped and will use a pipe. Both have also been recently renovated with fresh bearings and piston/liner combos made by Brian Gardner (which are excellent bits of kit). Radio will be straightforward - standard size standard servos all round. Retracts were de rigeur on classic pattern. I think I will probably use some. I have some standard mechanical units I can use, and a pukka retract servo to drive them.

 

I'll post more when I have some progress worth reporting on.

[Nigel R]

A start has been made.

 

First, the plan as per the outerzone link above.

 

 

Typical for the type and age, the wing is shown as veneered white foam. Whereas I will be doing a built up version instead - I am not set up for foam wing cutting and skinning. A built up is a little more effort but in the end, not actually that much more, the most difference in effort comes from shaping a number of individual ribs, all the other jobs in building a wing are much about the same for either structure. To that end, I first made a CAD version of the wing (DevWing) to plot the rib shapes.

[Nigel R]

 

Each panel is 32" of structure with a 1" block tip (not shown on the CAD model). 13-1/4" root chord and 8-1/4" tip chord for an area of around 700 sq in.

 

Wing section is a fraction under 15% thick. This was one thing that differed between the original Burgess Challenger and the later Aussie Challenger, the former had a standard looking section and the latter had something with a much blunter leading edge.

 

I am using one of the sections provided by the CAD programme DevWing (the RAF30) with just a minor adjustment to make the trailing edge 1/8" thick (I am not a fan of trying to feather the edge down to nothing). I have used the section on a few wings and it is well behaved. Developed in the 1930s I believe - nothing new under the sun.

Edited June 21, 2022 by Nigel R

[Nigel R]

 

First proper job thus begins - 24 ribs to cut out. A few hours work to get through all of those. As you can probably guess I am not a fan of the sandwich method. CAD templates are so much more accurate.

 

More when this job is complete.

Edited June 21, 2022 by Nigel R

[Outrunner]

Nice one Nigel, from the days when pattern ships were a sensible size and don't look like something from a Disney movie!

 

I have the same feeling with the "sandwich method" and I always cut my ribs from a templates.

 

Good luck with the project and I'm following with interest....

[Nigel R]

Several hours later...

 

 

Abracadabra - 24 individual rib shapes. Next job is to cut some strip wood - spars, leading edge, trailing edge, and then some slots in the ribs.

Edited June 28, 2022 by Nigel R

[Nigel R]

More kit cutting work. 

 

 

Spar slots in every rib. I may revisit these - probably going to run a double height spar out to mid span or thereabouts. 

 

Then got busy with this:

 

In order to straighten up the edges of all my 1/16 sheets. Which are then stuck together using masking tape and pva, like thus:

 

 

Ready to be cut to the right shape to make wing skins. 

 

I also cut and shaped false leading edge and false trailing edge strips.

 

Some time back I made a simple fixed balsa stripper type tool for cutting spars. Only does 1/4 wide. But, makes accurate strips, quickly. I used this to run off some 1/4 x 3/16 hard spars. As mentioned these will likely be double up at the root to 1/4 x 3/8.

 

I'm nearly ready to start assembling the wing structure now... I will need to do two things first - figure out some details on the retract mounting so I can make ply doublers, and cut a slot in the root rib to take the wing locating peg. 

[Torsten Spitzner]

I absolutely agree with you. These classic pattern planes make really good sport planes now. Not too big, not too small and fly very well.

I'm 3/4 through scratch building a 'Dirty Birdy' myself but progress is being slowed down by othe projects jumping the Q...

[Nigel R]

This week I've been working on the retract installation. The plan suggests a location - which I've ended up close to - but I'm quite constrained as to where the retract unit can fit, both by wing thickness near LE (i.e. can't go too far forward), and the spar location (can't go too far back). I'm trying to get the retract unit as far forward as I can. That position still meant that the wheel would bash the main spar. To get around this I planned on adding a kink to the leg, about an inch up from the axle position - pictures speak a thousand words here. Note that if I'd thought about it earlier, I could have put the spar a bit further back and saved the trouble...

 

Armed with a wire bender I made a pair of legs:

 

 

 

 

Have to love the wire bender, it does a great job if you're careful when setting up and you get the bends in the right place. Make good coils too.

 

Now I know where the wheel centres are, and where the retract units are, the next step is to make up the ply plate to mount the units on, and then the ply rib doublers.

Edited July 7, 2022 by Nigel R

[Nigel R]

Mounting plates, with a doohickey for the coil in the leg.

 

These are 1/4" thick. I'm thinking I may put a piece of 1/8 behind it to (a) give the screws more wood to bite into, or (b) give enough thickness for T nuts, and (c) to close up the area behind the coil cutout.

 

That said, M3 bolts & T nuts, or wood screws? I haven't done retracts in many moons and can't remember the normal method.

 

Edited July 8, 2022 by Nigel R

[Nigel R]

I've not been idle. Just busy actually flying!

 

The skeleton of the wing is together now. As you can see I use a very simple jig for tapered wings - one strip of carefully cut / tapered balsa, near the trailing edge. It's essential to keep centre line marks on the ribs when doing this - I find I can adjust the rib positions just ever-so-slightly to make sure both trailing and leading edges stay in a nice straight line. Those printed rib templates really help here.

 

 

I also added some internal structure to take the retract servo and aileron servo. I am only going to use one servo on ailerons. Torque rods are still my preferred solution for strip ailerons. Retract mounts are also in place at this stage.

 

 

 

Wing structure again from the underside. I started forming some of the wheel well (at least the bit for the top of the leg), but cannot go much further with that until the sheeting is in place.

 

 

I built up the retract mount plate with some scrap balsa, so the sheeting can be cut back neatly around the retract unit:

 

 

Servo boxes detail / some thick stuff to hold the wing dowel: 

 

 

I made ("precut", I guess) slot for dowel:

 

 

 

At that point, I discovered I do not have any contact adhesive (for wing sheeting) so I moved on to the other surface... tailplane structure internals, made from 1/4" thick wood:

 

 

I'm deviating from the plan with the tailplane - I'm not building up a "proper" airfoil section for it, just making a flat plate. In this case it is going to be 1/4" internal structure sheeted with 1/16", for a total of 3/8" thick.

 

Fin and rudder is shown as being carved from a lump of 1/2" (!) but I might well do the same style built / sheeted structure as the tailplane. I haven't decided yet. 1/2" is kind of a thick and expensive lump of wood though.

Edited August 3, 2022 by Nigel R

[Nigel R]

I built a structure for the fin in the same style as the tailplane. i.e. some scrap pieces of 1/4" in the area where the fin is glued to fuselage, and 'ribs' cut from leftover 3/32 and 1/8 scraps. Both tailplane and fin also received their sheet skin, PVA, weighted down and left overnight. After that they have a soft 1/4 sheet LE applied and a hard 1/4 rear edge (sort of a spar I guess).

 

I needed one fresh sheet of 1/16" and some clean strips of 1/4", but much of the rest of the tail components were made from the scrap box.

 

 

Light and stiff.

 

Anyway. The impact adhesive arrived in the post, so these will be put to one side for now, and I will return to working on the wing.

Edited August 5, 2022 by Nigel R

[Nigel R]

A small amount of progress this week. The lower wing skin is on, and some work has been done to open up the wing for the retracts. However, there is more work required yet on the retracts before I can close up the structure with the upper skin.

 

After adding the skin, I carefully sanded out a circular wheel well:

 

 

Tape wrapped around handy chewing gum container made the sanding 'drum'.

 

And a quick test fit of the retract unit:

 

 

All looks good so far.

 

I've also managed to mount the unit 1/16" too deep, i.e. below the wing surface. So a couple of birch ply shims will be needed to raise the unit up a touch, no big deal to fix.

 

There is also a little more sheeting to cut away around the retract unit / coil / leg, before I reach the full size of the well I created. If it looks too open I can build back up with some soft block.

 

Edited August 12, 2022 by Nigel R

[Nigel R]

Slow progress this last month - been on holiday and busy with work and the kids are back at school etc etc.

 

Anyway. The wing has moved on! It's all sheeted now, and leading and trailing edges are done. I've also done the usual tidy-up around the root (fixing up the dihedral angle on the root rib, in this case I needed a bit of scrap wood filler, see first pic).

 

Fully shaped LE...

 

 

Leading edge is formed from two strips of 3/16 med/soft wood. Gives plenty of meat to get the shape into.

 

 

And a fully shaped LE / aileron...

 

 

That used to be a lump of 1/2" wood. I admit carving and sanding that down to a nice shape takes a long time but does give a good result.

 

 

 

Hey presto...

 

 

Two panels! These are not quite ready to join yet. Some detail work to do first. Thus, I'm now in the process of forming the torque rods, getting the tip blocks on, and cutting free the ailerons from my TE before fitting hinges (including getting the torque rods sunk into the TE)... then I'll be about ready to join the wing halves, and think about trial fitting the servos and getting aileron/retract operation fineagled.

 

 

Edited September 20, 2022 by Nigel R

[Nigel R]

The wing has progressed a little.

 

First off, some careful bending and gouging has seen these installed:

 

 

Yes, I know, everyone does two servos, because it's obviously better and easier. But is it ? I actually find torque rods take less time than doing two outboard servos. Plus you get an easier time when it comes to covering, and it looks neater. Differential is not a big deal. Bend the torque rod appropriately or use an offset on the servo horn. A non issue for aerobatic models like this though.

 

Moving on... the ailerons had the hinges added, then the  panels were joined, with a very small amount of dihedral on the top surface. Then the retracts were ticked and installed:

 

 

So that the Retract servo could be installed and set up:

 

 

Down to getting the bandage on now, and shaping some wing tips.

 

Then I will make the rudder and elevator, to finish the tail surfaces, before moving to the fuselage. 

[GrumpyGnome]

22 minutes ago, Nigel R said:

Yes, I know, everyone does two servos, because it's obviously better and easier. But is it ? I actually find torque rods take less time than doing two outboard servos. Plus you get an easier time when it comes to covering, and it looks neater. Differential is not a big deal. Bend the torque rod appropriately or use an offset on the servo horn. A non issue for aerobatic models like this though.

 

My Wotty had the best/worst of those options..... 2 inboard servos, each attached to a torque rod. Think, on reflection, it was the best option.

[Rich Griff]

Nice, could you post the rib templates please ?

 

That would be very much appreciated by a lot of people on here I feel.

[GrumpyGnome]

Me? I have no rib templates.

 

I joined the wings, then drew around 2 servos side by side, then gouged a hole in the foam big enough for the servos, plus a couple of rails to screw them to, plus a little extra for some epoxy-soaked fibreglass. Then I fibreglassed the wing joint.... voila! Easy peasy.

[Rich Griff]

Nigel, the chap making the balsa ribbed wings, please could you post template images please.

 

Many would appreciate those template images I feel.

 

Thanks

[Nigel R]

I could, no problem (a bit later on, because I'm hard at work, etc etc) but, massive caveat, they are not the same section as is shown on the plan. I always dial up a standard section from the DevWing library, and then adjust the wing saddle / fuselage cutout to suit. That said, I'm quite happy to post what I used, if you would like it. Last point - I am not sure what format DevWing exports to, I have simply printed out from within the application.

[Nigel R]

Another month. Not much worthy of pictures, I finished the wing for now by getting the bandage on and then moved on to the fuselage

 

 

I've not taken many pictures as it is entirely conventional in construction. Balsa sides, 1/32" ply doublers, balsa longerons etc.

 

Cutting the formers took a bit of time, I tried to get as much done before glueing things together, so e.g. the engine mount is fixed to firewall, wing dowel hole is drilled in another former. Plus the sides needed the wing saddle cut and fit to the wing etc etc. 

 

Anyway. Nearly ready for glue up. 

[Nigel R]

Following on directly from the above, the fuselage is now glued together. During that process I added some meat to the tailpost, and some 1/32" ply doublers around the tailplane saddle.

 

At which point, it is about time to get the internal structure and fittings in. Radio, for instance - easiest if it is done while the inside is accessible. 

 

 

Fuselage is upside down; we're looking in through the wing saddle there, towards rear of fuselage, at the bare servo rails. 1/2" wide ply, laminated with some hard balsa.

 

The ply tongue thing behind the servo area, that's a sort of internal hatch, to keep the RX battery in place. I try to engineer a place to tuck it away, just behind the wing saddle, in case I need it there to get the CG correct. In this case there is enough space up in the decking.

 

 

 

 

Left to right, throttle, rudder, elevator. The rudder servo is raised, to reduce interference with the elevator and throttle servos.

 

 

Sullivan blue snake for elevator, my usual.

 

 

Exit holes are cut using a sharpened tube and a jig. The 'jig' is a piece of tea tray ply with holes drilled in the right place for the elevator snake and the rudder closed.  Makes things easy and repeatable and provides some guarantee I won't end up with the elevator horn getting in the way of the rudder horns.

 

 

Orange tubes are for closed loop guides. They're the insides of SLEC's cheapest snakes. I still have no idea what I could use the leftover outer tubes for!

 

I've taken to using foamboard internal formers quite a lot these days. Light, cheap, quick, and you can just punch holes in them dead easy to keep control runs in place. Much easier than fitting and drilling wood supports. They double up as reinforcement for the sides (to stop them flexing/squishing in when you grip it) and help keep things stiff during this phase of fuselage construction.

Edited November 23, 2022 by Nigel R

[Gary Clark 1]

Really like the idea of foam board supports inside Nigel. Never thought of that before but several benefits 

 

Gary

 

 

[Nigel R]

Well worth having a pack of foamboard on hand, I find.

[Nigel R]

Getting on with some more fuselage jobs.

 

Wing bolt blocks clamped in place:

 

 

These were made from layers of scrap ply, hard balsa, and liteply. Scrap box job.

 

 

Reinforcing the firewall with some corner gusseting:

 

 

More scrapbox material for that job.

 

Wing treblers - not fully cut to shape yet:

 

 

Soft balsa, leftover from cutting the wing's LE and TE strips, I think.

 

Decks, front, rear:

 

 

Challenger calls for a slab of soft 1/2" at the front and similar at the rear. The deck sides are soft 3/16". Plenty of meat to get some shape into when the razor plane comes out.

 

I have a little more fitting out to do inside before dosing the inside with fuelproofer and closing things up with the underside sheeting, but its coming along nicely at the minute. The first half of fuselage building never seems to look much like progress as it involves the inevitable "cutting a kit of parts" phase, and getting the radio in place, but at this point it starts to look like a proper airplane.

Edited December 5, 2022 by Nigel R