Scratch-building a 1.8m MDM-1 Fox - RC Groups
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Jan 29, 2004, 10:08 AM
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Scratch-building a 1.8m MDM-1 Fox

I've been fascinated with the MDM-1 Fox for quite a while and following the sucess of my Lenger 3m Duo Discus inspire me to design and build my own little Fox for our local slope. With thermalling capabilites in mind, I chose the long wingtip version for more wing area.

HQ wing section is chosen and wing is constructed from white foam core and balsa sheeting. Fuselage is 100% wooden except for canopy.

Fin and rudder uses foam core and balsa sheeting as per the wings. Horizontal tail surfaces are made from 1/4" solid balsa.
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Jan 29, 2004, 10:11 AM
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Fuselage constructed by balsa planking. Nose cone made of balsa lamination. Provision for functional wheel intended.
Jan 29, 2004, 10:14 AM
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Wings are joined by steel rod. Wing spans at 1.8 metre. Canopy construction still in progress.
Jan 29, 2004, 02:23 PM
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That looks really good. I am about to embark on a 1/4 scale scratch built Pilatus B4. Well I am at the drawing stage having gathered the data. I would be interested to see and hear more of your balsa planking the fuz. I was thinking of 1/64 ply sheeting.


Jan 30, 2004, 11:21 AM
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SoarNeck's Avatar
Looks very nice. I've had a fibreglass fuselage for a Fox 2m in my basement for a while, but I've been having trouble cutting wings for it. I've done at least a hundred good cores for other models, but the severe taper ratio of these wings is tricky. Any advice? How did you cut yours?
Jan 30, 2004, 10:37 PM
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Thanks. Some scale soarers designed by Cliff Charlesworth uses the method you mentioned, but I personally haven't try it yet as the prices for 1/64" ply is quite high, at least in this part of the world. It's more suitable for structures with a very narrow pitch between formers, or in other words, a lot of formers.

My Fox fuselage has only 5 formers and balsa strip planking should be more feasible. I use strips of 3/32" tapered balsa with the edges bevelled for better fit. For turtle deck, the length of strips runs from aft edge of the canopy to the forward of the fin. The bottom planks are split to 3 sections. If you are skillful enough, you can try to plank the bottom in a full lengthwise, depending of the size of the fuselage. In fact, this planking method is very popular in older WWII gas warplane kits.


Once again, thanks. I've done it by the hard way. I experimented with different wires and lengths and had ruined several wing cores before finally get the right one, although still not perfect ,still sufficient for my use. The trick is to use cutting wires of low heat, just enough to cut the foam,the reason being that the wire need to be moved slower at the narrow wing tip than the broader root chord, and this builds up more heat at the tip, causing the foam to shrink further,causing distortion. In the worst case, the heat built-up may even start burning into the template, depending of the material used. I always use plywood.
By using low heat, it prevents the foam at the wing tip from shrinking too much, but the downside is the cutting process is usually quite slow.
Jan 31, 2004, 11:14 AM
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There are two problems I have had when cutting severe tapers with a Feather Cut:

1. The panel gets ridges towards the tip because the tip pulling wire slackens and re-tautens.

2. The slow moving wire at the tip over burns the foam.

To solve the first problem I pivot the bow and drag the root chord end of my bow by hand with excellent results. See . I have even made an 8 foot bow to cut a 30" panel with 60% taper ratio (pivotin the end of the bow and supporting the wire at 33" from the root end of the bow - 8 feet of unsupported wire really sags).

To solve the second problem (apart from minimising the heat as suggested by Joe Yap) I cut multiple shorter panels. The consensus is that cutting panels longer than 24" with a bow (not CNC) allows wire sag which increases proportionally to the length of the bow. I know that 30" panels I have cut suffer from "thick LE syndrome" in the middle when I use my 48" Tekoa bow. If, for example, you were to cut a 40" panel with a 40% taper ratio, you'd probably end up with a thick LE at the centre of the panel due to wire sag and over burned tips because of the taper. Now, if you were to cut two 20" panels instead, the root panel would have a 70% taper ratio and the tip panel would have a 57% taper ratio - both better than 40% from a burning point of view and with no noticeable wire sag.

For sport models, if you are going to cut short panels, why not make a multi-taper wing for better aesthetics and more elliptical lift distribution? It is not that much more work and your models are more distinctive.

My 2 pennies' worth anyway.


Jan 31, 2004, 10:18 PM
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Good point Kenneth! As the matter of fact, my fox wing panels are cut out from 2 sections of 15" length panel instead of 30".
Feb 01, 2004, 02:35 PM
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I have got the Cliff Charlesworth book and was considering doing the ply legthwise. The rear part of the fuz behind the wing to the tail is a taperd tube with oval cross section. SO I reckon 2 sheets (one each side) should do the rear of the fuz. Mind you I think it wise to have some extra formers so if when I dry fit it does not work I can do it the CC way.

Feb 01, 2004, 11:00 PM
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John Walter's Avatar
I have used 1/64th" ply on the fuse of my 1/5th scale Blanik L-13. I used PS foam for the upper and lower curved areas then glued down the ply with yellow woodworkers' glue. I used a separate piece on each side of the fuse and cut darts forward of the wing to cover the compound curves at the nose. It worked very well. The foam added a great deal of torsion resistance without too much wieght (I hope!).

I intend to add 1 (or 2?) layer(s) of 1.9oz kevlar forward of the wing to provide abrasion protection at the slope.

I planned to use this mostly for thermal, but it may be a bit heavy so it may turn out to be a sloper after all. Anyone with suggestions about number of layers of kevlar?

Feb 02, 2004, 08:35 AM
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For bigger project like yours, I agree with you that it's wiser to have more formers in place in order to achieve accurate cross-sectional shape of the fuselage. The build-up fuselage section of my Fox is only about 24 inches and I reckoned that 5 formers is sufficient to achieve proper cross-section since the furthest distance between all the formers is only 6 inches. I'll probably end up with a much heavier fuselage with the CC moethod for plane this size with all the additional formers and glue joints.

For planking method, you usually do not need a lot of formers since the long strips of wood will confirm to the bends nicely when they are in contact with the formers. You can use slightly thicker wood to do so, which gives you more material to be sanded into a nice curve later on.If you are going to do the CC method, the fuselage MUST have a lot of formers in place. If not, you'll probably need a lot of filler in order to achieve proper shape.

IMHO, both methods has it's own pros and cons. It all depends on the size and shape of the models and personal competance to decide on which method to choose.
Feb 02, 2004, 08:43 AM
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I personally do not like the idea of adding anything permanent to the LE of the wing as it might change the airfoil profile and the performance of the wings, unless you are talking about doing so within the wing structure under the covering. If I were you, I'd rather stick a strip of glass-reinforcement tape to the LE, which can be easily replaced or omitted in the future. I personally think that carbon cloth may have better abrasive resistance than kevlar.
Feb 29, 2004, 08:45 AM
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It seems that the Foxes have stirred up quite a lot of interest in this forum. Well, I'm here to tell my story.

I haven't really complete my Fox yet, but the anxiety to know how it flies before the wind dies down, has prompted me to rig up the functional components and systems to make it airworthy, before finishing the details. At 1.8m and 20 oz AUW, it flies nicely in light lift and can do aerobatics very well when most flying wings at the local slope had a hard time keeping airborne. With the HQ airfoil, inverted compabilties is somehow limited but otherwise, a very pleasant slope machine and I suspect that it can even thermal well. With another 1 oz of ballast, it really moves and penetrates and really is a blast to fly! It's now my favourite sloper next to my 3m Duo Discus. I'll post some pictures when it's ready.
May 06, 2004, 09:41 AM
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Only recently, I finally obtained a digital camera. While I had flown it since 2 months ago, it's only now I'm able to take some pictures to post in this forum.
May 06, 2004, 12:10 PM
Piscine Promulgator
surfimp's Avatar
Joe, that's a beautiful little Fox! Great work. Any chance of more info regarding the all up weight and wingloading? How does it fly? How is the thermal performance with the larger wingtips?

Sweet plane.

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