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Mama Mia!

Joe Bridi's Behemoth from 1982

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Martyn K24/07/2017 11:58:52
5074 forum posts
3678 photos

A bit of a lull in the build program. I managed to wreck my 61 powered Kwik Fli IV a couple of weeks ago when the tailplane failed in flight (again). Although the decent was quite gentle, the rock that final stopped the decent wasn't and the model (and engine - which took the impact) destroyed itself..

So I need a replacement. I have always fancied this, it was featured on the cover of Model Builder (American mag) back in 1982 and there were two things that immediately appealed to me. Sadly the young slim model was one of them.. <sigh>


The other model is huge compared with other models of the era. But compare the outline with a modern F3A (monoplane) and you have

- A highly swept back wing

- A long moment arm

The tailplane looks a little big but that will keep it stable, maybe a bit ponderous

A plan was procured and the specs off the plan look like this.


That 11lb AUW with a big wing and draggy fuselage - even powered by a hot 61 looks a little excessive, so my plan is to replace the Rossi with a Webra Speed 91 with pipe, which hopefully will make the performance more or less similar to other models of the era. I am also going to try and shed at least 2lbs off the weight with careful wood selection and removal of some of the excess. There is a lot of wood - especially in the fuselage in this model.

One other change is that the wing ribs will be cut from 6mm Depron. Simply because I cant afford the amount of balsa required for the wing ribs.. There will be other lightening efforts as well during the build, but hopefully I will end up with a nice flyable model that is in character.

So my first attempts with Depron and the tailplane

I have never used Depron before so it was a case of taking things very steadily., clamping all parts together while the glue dries and constantly testing the strength of the assembly .


Starting from the centre section, spars cut and pinned down and the first 3 ribs installed and allowed to dry. Using PVA for this which seems to be working well. The lower side of the tailplane is built flat so there will be a little effective anhedral


So far so good


One thing I did notice is that the Depron has springiness. So pushing the spars into a well fitting slot results in the Depron pushing the spar back out again...

Hey ho..


Elevators are assembled in the same way except built directly onto the sheet. All going together well but a little more slowly as I am letting the glue dry thoroughly before moving to the next step.


Tailplane and elevators just about completed after planing and carving and sanding the excess wood away. A little weight has been saved by using cap strips behind the spar instead of fully sheeting the tailplane. Hinge slots have been cut and its ready for covering.

I have also cut out one set of wing ribs for the wing. To give some idea of the size of this here is a shot of the paper templates laid out on the Depron.


That's a 1m rule at the back..

So far so good...

More to come


John Timmis24/07/2017 12:38:57
237 forum posts
298 photos

Hi Martyn

That wing looks like an ideal candidate for a 'laser' construction. (Hybrid foam & balsa).

Cheers John

Martyn K24/07/2017 13:03:39
5074 forum posts
3678 photos

Thanks John

Not sure what a laser construct looks like, but I am planning on building the wing the same way as the tailplane (foam ribs replacing balsa ribs). What is the difference?

John Timmis24/07/2017 14:05:44
237 forum posts
298 photos

Hi Martyn

It's a wing construction that combines traditional balsa construction with wire cut foam ribs. It sounds a bit complicated & it is a bit different, but it works very well. I tried it on a smaller wing & was well pleased.

Google " laser method of wing construction by Patrick Mullen" for a full explanation.

Cheers John

Martyn K24/07/2017 14:23:45
5074 forum posts
3678 photos

Hi John

Yes, I am aware of that method. The way I saw it written, it involves gluing strips of blue foam at the correct rib spacing then cutting as normal using a bow with templates at each end, build the top half then cut the lower section, flip the wing and complete (IIRC)

I did think of doing that with this model but I didn't have any blue foam.

I may try that on my next slope soarer though

Simon Chaddock24/07/2017 15:19:51
5686 forum posts
3024 photos

Martyn K

That all looks very sound.

The only thing to be aware of is the fact that Depron is much weaker than the same section of even the lightest balsa so using Depron ribs you have to be careful that the spars are adequately supported. I note the tail plane has balsa shear webs between the inner pair of ribs but beyond that there is only the soft Depron rib keeping the spars apart This could represent a 'weakness' that might not have occurred using balsa ribs. It would depend on the thickness of the balsa compared to 6 mm Depron.

Most models are built grossly over strength so doubt it will really make any difference but it is worth bearing this sort of structural issue if you use Depron where the forces do become significant.

Martyn K24/07/2017 15:22:10
5074 forum posts
3678 photos

Thanks Simon..

I am using 6mm Depron for the ribs. The plan shows no shear webs at all on the tailplane so these have been added to the 3 inner bays each side. No problem adding them across the full span - just trying to save those grams and the target kg will hopefully look after itself.

Martyn K22/11/2017 14:05:08
5074 forum posts
3678 photos

errrr. Apologies. I had forgotten I had started this blog. Spotted it yesterday while I was looking for another build blog.

So there is a bit of a catch up.

The fin. Its big - very big for a classic F3a. Quite simple construction and built directly off the plan except I have used Kevlar cloth as my hinge material


That's a 300mm rule to give some idea of bigness.

As expected, there were no real problems with this, a simple cut and glue exercise.

The Fus - despite my misgivings was almost built as per plan. It is still stupidly wide and boxy but I have increased the depth of the 1/6 ply doublers so that the whole of the inner fus to behind the wing has extra reinforcement. the plan only shows a 30mm or strip which seems to be asking for trouble.


All the formers except the firewall are built up. The upper side has a 1" Triangle which doe its length is in 2 pieces. I am using harder wood at the front - lighter at the rear which as you can imagine caused a few problems when I pulled it together at the rear.


Lines up in the jig and overhanging front and back.. Note the tailplane has been aligned level at this stage with the top of the fus. All very simple and typical build method of the boxy models of the era.


Now with the 12mm top deck added and the fin and tailplane fitted.


I added extra triangulation around the side of the fin for strength but also to improve the airflow over this area.


The elevator push-rod is a length of 6mm C/F tube with two M3 steel push-rods epxied then bound with Kevlar thread and more epoxy. It seems quite stiff strong and light.


These terminate with some decent horns on the elevators. The rudder will have a closed loop cable system.

More to come

Martyn K22/11/2017 14:19:16
5074 forum posts
3678 photos

The wing was fairly straightforward except it didnt fit either my building board or my bench.. Not the span, but the 20" root chord..


Apart from the use of Depron ribs and additional ply reinforcement around the retract bay its fairly straightforward. Again - its not complex, just big


To make sure that the TE was straight. I aligned it vertically. The plan shows two separate halves joined with FG bandage - as you would use for a foam cored wing. I have used a 3mm ply dihedral brace

The fus sits in the gap at the LE - you can see the 1/16 ply doublers for the UC blocks here


Sheeting the wing was very straightforward, surprisingly not that much was used as many of the cutoff pieces were reused


Wing fitting was entertaining. It was at this point I spotted another error on the plan. The LE gap  for the fus fitment was about 12mm too wide. So additional thick ribs were added to fill in the gap. There are lots of errors on this plan. None insurmountable but it looks like Joe passed the model to the draughtsman and said "draw that". Just fundamental errors all the way through where basic reinforcement is missing for example. I gather that this was not as the top of the Bridi 'Must Fly' list.


The ailerons were laminated from 1/2 and 3/16 sheet balsa (seriously), glued then planed to shape.Very satisfying when you can plane accurately like this




Aileron servos have been exercised and I have fitted them internally. One thing I did notice was that they tended to flex the ribs so the two adjacent ribs were linked with 1/8 balsa which stiffened them up nicely. The servos are screwed to a ply backing plate that is glued to the ribs.


The wing bellypan area was fitted with wing attached. It uses a M6 bolt at the front and 2 x M4 steel bolts at the rear. Needs quite a bit of interpretation regarding wing mounting plates etc. The front bolt is guided into place down a length of 12mm internal diameter brass tube. Otherwise it would likely get lost in that bellypan area

More to come



Edited By Martyn K on 22/11/2017 14:48:31

Martyn K22/11/2017 14:27:36
5074 forum posts
3678 photos

A bit out of step.

The wheel wells have been a bit of a pain. I used a 3" dia jar as a mold and wrapped damp 1/16 balsa around them and glued them directly onto a piece of 3mm balsa

That worked really well and I was certain I would get round wells.


I didnt get them . They are not too bad but still look decidedly angular in places


The canopy has been fun





The canopy is a glass fibre item that contains the tuned pipe.A foam core cut on the bandsaw then covered liberally with three dollops of plaster of paris. What a mess. Great fun. When dry, a surform was used to shape it then finished with sandpaper. About 5 coats of polyurethane (not water based) varnish to seal it. A very liberal coating of Beeswax and a plug was produced using Fibre Glass Tissue and Mat


This has turned out quite well but unfortunately the plug got damaged when I tried to pull it.I am also having a problem with blue PVA rel;ease agent not flowing very well - or rather condensing into clumps after application..I darn't try and make the final canopy unless I am sure it will release OK.


The cowl is again fairly straightforward and formed from thick balsa sheet triangle etc and then carved to shape.

My daughter came round at the weekend and this is what is currently looks like.


Coming along nicely I think. I need to get a manifold for the exhaust next.
More to come.

Edited By Martyn K on 22/11/2017 14:50:40

Geoff S22/11/2017 14:48:27
3586 forum posts
14 photos

An interesting build.

I was glad to see I'm not the only one to hoard apparently useless lead acid batteries to use either as weights or true 90 degree supports for fuselage construction. It must be quite a long fuselage to overhang both ends of a SLEC fuselage jig. As looked at your picture I had a glance at my own jig leaning up against the adjacent wall.

I'm never sure whether it's best to build the entire airframe and then cover it or to cover before assembly. My Mew Gull looks a bit odd at the moment as the fuselage is incomplete (just a few small jobs) but the tail plane, fin, rudder and elevators are all covered because I chose the latter option. I assume you prefer the former from you photographs.

I needed some carbon pushrods for my quarter scale Mew Gull and acquired some 5.5mm carbon tube at the recent LMA Gaydon static show. There were also some neat alloy ends that allow 3mm threaded rod to be epoxied into hole. It makes very neat and more than adequately stiff push rods. I can't remember which supplier it was unfortunately and, as I paid cash, I don't have card receipt.

btw I hope you aren't going to press your daughter to reproduce the pose on the magazine cover


Martyn K22/11/2017 14:59:23
5074 forum posts
3678 photos

Thanks Geoff

My daughter would probably shy away at that although I wouldn't dare make her mind up for her.

I have never been really happy terminating pushrods to the wire bits. Especially in this case they had to be done in situ or risk damaging the wood trying to fit them afterwards. I think (hope!) they will be strong enough though. Not especially neat, but secure.

I tend to leave the covering until last. The next big job will be to glass the fuselage then I'll get the sealing iron out.

The fus length now the cowl and rudder is attached is just less than 2m It dwarfs my Aurora which I thought until now was pretty big


Nigel R22/11/2017 15:01:55
3737 forum posts
585 photos

Very nice.

And very large!

What sort of prop/rpm are you seeing on the webra? I must admit I think you might want a bigger motor to make it go. Maybe a TT120Pro? They are routinely on ebay.

Martyn K22/11/2017 15:07:20
5074 forum posts
3678 photos

Hi Nigel

The Webra 91ABC is actually BNIB but about 30yo. I bought it last year for the princely sum of £70. I still haven't run it. When I get the manifold, I'll bench run it and experiment to get a good combination.

I am hoping to get 10k rpm using a 14x8 prop. I have a Graupner 90 sized tuned pipe for it, Anything in that area should be OK though


john stones 122/11/2017 15:12:09
11450 forum posts
1516 photos

Looking good Martyn, Webra, never owned a bad one. yes

Martyn K22/11/2017 15:16:00
5074 forum posts
3678 photos

Thanks John

Sorry Nigel - your other point. I don't want to make this model too much out of character. It would probably take a petrol engine and I did consider a 120FS and a bigger prop. I am limited though by the size of the undercarriage retracts. 14" is about the biggest diameter I can go to with the current setup. If it too revvy on a 14x8, then I'll try a 14x10 or a 13x10.


Nigel R22/11/2017 15:19:25
3737 forum posts
585 photos

That should get it going ok. Perhaps I let myself fall into the 'fit too much power' camp a bit too easily.

I'd go 13x10 given the choice, a bit of extra clearance, and would provide a bit more thrust at speed, for similar power/revs.

Trike gear I presume?

Edited By Nigel R on 22/11/2017 15:22:37

Martyn K22/11/2017 15:22:08
5074 forum posts
3678 photos

Too much power is never enough. SO I have been told. laugh

David Oxilia22/11/2017 17:51:56
27 forum posts
10 photos



great to see the fast "all at once" progress on your MM!

When your thread caught my attention some months back, I wanted to post my "interpretation" of the model as I also found that it was rather portly at 11 lbs and could easily shed 2 of them. In doing so, after sketching my ideas down on paper, I naturally gravitated to the plans and started to redraw the model shedding weight all around. I also found it way too wide so it became sleeker as well. Gone were the big blocks throughout on the fuse in favor of additional shaped formers and a 3/32" skinned top deck section. I also went with plugin wings as I couldn't see a model with a wing this large taking up space in my shop when being stored. You're a braver man than I am.

If I can figure out how to post pictures here again, I'll post the drawings so you can take a look. A little late I suppose as they might have given you some additional ideas for your own build.

The Webra 91 should make for a nice power plant. I originally set out with a Picco 80 RE in mind that I also picked up for a song but like some ST RE engines, sorting out the header (manifold for you lads across the pond) was a bit of a challenge due to the built in stub exhaust that these Italian engines have. It wasn't too hard to locate a vintage Rossi 90 RE though so that's what I will likely use when I get her cut and built. Current NovaRossi (Cesare Rossi's new company after separating in business from his brother who renamed the old company AXE Rossi) headers fit the vintage engines very well. Another very interesting option is the K&B 100 RE which is currently still in production. Maybe Father Xmas will bring me one this year as I was able to source some headers for it before Macs (our American header and pipe manufacturer) unfortunately closed its doors.

I look forward to your ongoing fast progress.


P.S. I think I figured out the system for posting pictures. I renamed my version of the model "Mamma Mia!" using correct Italian spelling. Some nuanced differences to the design other than a rather different construction is the blended airfoiled vertical fin which transitions up from the fuse into the fin. Much like the Aurora does.

mamma mia plans_fuse_org.jpg


mamma mia plans_wing_org.jpg


Edited By David Oxilia on 22/11/2017 18:02:42

Edited By David Oxilia on 22/11/2017 18:04:53

Edited By David Oxilia on 22/11/2017 18:05:10

David Oxilia22/11/2017 18:10:00
27 forum posts
10 photos

For those interested in this model, you may find the following two links of interest:

K&B 100 Aero:

K&B 100 Aero Review:

However..., I just noticed they are sold out. Too bad.

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