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August Edition CAP 21 Plans

Problem understanding dihedral angle

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Howard Heffer19/07/2007 18:13:00
15 forum posts
HI Folks. Just got back into modelling after 10 years or so hence I am a bit rusty. After reading this months RCM&E mag I've decided to give the CAP 21 plans included in the mag a go.

I'm having trouble understanding how the dihedral of the wings allows a flat peice of ply (called the Undercarriage mount on these plans)to be joined from (left wing) Ribs R2-->R1<-to->R1-->R2 Ribs (right wing). Surely the dihedral of the wing is centered between the 2 R1 ribs meaning that when the wings are joined together and balanced (3/4" under each wing tip) there is no flat/straight section for this plywood UC mount to fit??

Also to allow for the dihedral, will the R1 ribs have to be angled away from vertical so they join flush with each other. This is how my old glider worked, but it is not mentioned in these plans.

Thanks for the help, I hope you guys understand my muddled question!
kdc19/07/2007 19:25:00
41 forum posts
It looks to me as though the notch for u/c plate in R1 should be slightly deeper than R2. I suggest sanding deeper as necessary at the final stages.
Yes it would appear that root ribs need angling slightly. Note that the dihedral is not as great as some models and therefore hardly any angle may be needed.

Have you checked the plan for errors in drawing? Peter Millers other plans are notorious for their errors, particularly in the fus formers not being correct and other fundamental errors which only become apparent at a later stage.
kdc20/07/2007 00:17:00
41 forum posts
Further study of the plan shows that the dihedral is mostly caused by it being a tapered wing. That is to say that the tip rib is thinner than the root rib, so if the root rib is at 90 degrees there will be about quarter inch of dihedral anyway.( remember the tip rib is half inch lower than the root- 1/4 above and 1/4 below! ) So only very slightly less than 90 degrees is required. Therefore the u/c notches may be almost correct.
The dihedral brace looks too wide at the outer ends - wider than R3.
Also all the ribs seem a millimetre or two shorter than shown on the wing plan - no real problem.

F7 appears 2 mm narrower than the plan - again a minimal problem.
Otherwise the plan looks reasonably accurate.
Howard Heffer20/07/2007 13:36:00
15 forum posts
thanks for the answers, just goes to show you have to have an eye for details. I understand what you have said and will check the plan out again tonight to put your word into images!

Thanks again!
Howard Heffer20/07/2007 18:34:00
15 forum posts
Ah yes now i have checkd the plans again and done some trig, it seems the the dihedral of each wing is only 1.62 degrees - not a great deal at all.

Another question if i may. The aileron hinges on the plans appear to be dotted lines - is he giving the builder discretion over what type of hinge to use? Would mylar hinges be suitable, or would a pin type be better?

And as for the elevator hinges?!? is this what is meant by the 14swg joiner?

Thanks for the input!
kdc21/07/2007 12:21:00
41 forum posts
The aileron joiner just ensure that both halves move together when the pushrod is connected to just one side.
Any type of hinge would be suitable for a small model like this. A hinge slotting device is handy - Sullivan is one make. Check with clubmates if you are unsure how to fit hinges - a crucial safety point- dont take chances. Pin all hinges for safety.

I have examined the drawings further and make the following comments:-

The material for the wing spars is not shown, so presumably balsa. I would suggest spruce is better unless really hard balsa is available. Note that the rib notches are drawn as 6mm not quarter inch ( the spar material might actually turn out to be 6mm.)

I think that R2 will need wider notches to accommodate the dihedral brace. Therefore three eighths wide notches top and bottom instead of quarter ( drawn as 6mm ) this comes very close to the hole for aileron pushrod.

R1 and R1A would appear to need a very slightly deeper recess for u/c mount than R2 and R2A. Perhaps half a millimetre deeper. If this recess is insufficient then R1 will push the top sheeting too high and cause the wing to fit incorrectly.

The dihedral brace seems to be 8mm too long and about 2mm too wide at the ends to fit against R3.

The aileron servo connection is not shown, however several other Miller designs show this in detail. Are you familiar with this type of double connector? Or do you intend to fit 2 small aileron servos in the wing in the modern style? 2 wing servos would enable the elevator, throttle and rudder servos to be moved further forward to help the balance if required.

The elevator and rudder servo bearer seems to be over the servo not under! A bigger hole in F4 would give more clearance for snakes or pushrods connections.

All these points are easy to fix if you do them at the initial rib and former cutting stage, but very tricky to do once the components are installed. No doubt you will have experienced this before!

All told this is a nice looking design and a more accurate plan than most of the Miller plans I have examined
kdc21/07/2007 17:04:00
41 forum posts
First a clarification. When I said pin all hinges for safety, I meant peg or pin all hinges with tiny dowels (cocktail sticks)

This is a model for experienced fliers only. Scale models with tapered wings are tricky especially in small sizes. No doubt Peter Miller expected that experienced fliers would also be experienced builders too, and therefore left hinging etc to the builders preference. Also the bevels to the elevator and rudder are not shown neither are servo mount details.

The elevator needs 3 hinges on each side (6 in all) , the rudder 4 hinges in my opinion. One hinge to be quite near the horn. The ailerons could do with 4 hinges each rather than the three shown in dotted lines in my opinion. One could be nearer the aileron horn and the extra one about 5 or 6 inches further out.

Fitting Hinges. My method.
The position of each hinge is marked on the control surface at the bare balsa stage and then marked on the fixed part. A hinge near the horn is desirable. Horn clevis hole should be exactly in line with hinge line. A gadget ( Sullivan, Dubro or SLEC etc) is used to mark the hinge slot accurately down the centre line of both parts and the slot dug out with hinge slotter ( a V shaped prong tool and a hook shaped tool to go in a craft knife holder ). Trying to dig a wide slot with a scalpel is difficult. Try to borrow a hinge slotter if you cannot buy one - an experienced modeller could cut all your slots in 5 minutes once you have marked the positions. I use the flat one piece nylon barbed type hinges made by Radio Active. Or if using the round type hinges a hole is drilled accurately at right angles in both planes. All hinge pivots ( crease lines ) must be concentric. Do all this at an early stage before tapering / shaping. If it is not absolutely right throw it in the scrap bin and start again. Then bevel the hinge line on the moving part, and complete tapering and shaping.
When the hinges work freely, glue the hinges in the moving part only with epoxy, making sure each hinge is sunk right up to the crease line exactly. Wipe off any surplus epoxy from crease line. When the epoxy is dry drill thro hinge and balsa with a drill of slightly smaller size than the diameter of cocktail stick. Push the point of cocktail stick right thro and out the other side until full diameter appears. Glue with thin cyano or PVA. When dry cut the cocktail stick off flush using electricians end nippers or a pair of nail clippers. Use eye protection, the points fly off towards your eyes! Paint the balsa part around the hinges with Clearcoat ( Solafilm product ) allow to dry before covering with Solarfilm etc. After covering the control surfaces hinges are inserted into the fixed ( and covered ) part. There should be almost no gap along the hinge line and hinges should work freely. Then the hinges are glued & pegged on the other (fixed ) side. This time the cut off cocktail stick shows, so a dab of paint can disguise it- this looks better than trying to cover with little bits of Solarfilm.
Hinges that are just cyanoed in are likely to pull out

kdc21/07/2007 17:07:00
41 forum posts
Making Elevator Joiners

An oversize length of piano wire is bent (cold )at right angles in a vice, using a hammer and block of hard wood. A second bend is bent parallel to the first. If necessary one end is held in the vice and ‘tweaked’ with a pair Mole grips/ pliers to get parallel. At this stage you will appreciate the extra inch or two oversize is useful for leverage. Then cut to length using a good quality 3 cornered file, not a hacksaw, just notch either side and snap off in a vice. Eye protection is vital! File the ends to flattened shape.
Then the tricky part of drilling into the elevator at right angles in both planes. Use an old fashioned hand drill turned slowly. Throw the part away if the drill penetrates the outside - it will be too weak as well as ugly. De-grease the piano wire before inserting into the holes. If necessary tweak the joiner again (out of the balsa!) to get the elevators parallel. Epoxy into holes at final stages when finally inserting hinges into tailplane ( after covering )
Howard Heffer25/07/2007 13:11:00
15 forum posts
Thanks for so much information. It just what the magazine article left out! I have just bought my own hinge cutter so this should make things much easier for me. I have also been looking for the hinges you mention you used for your ailerons, but I'm not sure which ones exactly you meant - "I use the flat one piece nylon barbed type hinges made by Radio Active". Can you tell me where to buy these online??? I'm also trying to find some Leading edge, trailing edge and Aileron Shaped balsa strips, but am having trouble locating an online retailer. My local model shop does not seem to supply these items. Have you any idea of where I can buy these items online?? Thanks!
Mike Rolls25/07/2007 15:41:00
500 forum posts
22 photos
Re drilling into the elevator (or aileron or rudder, for that matter). Unless the surface is big enough to make some sort of jig appropriate I use a drill bit in my fingers - I find it easier to keep it square in both planes that way.
kdc26/07/2007 19:50:00
41 forum posts
Check out for hinges.

For balsa I use the Balsa Cabin see
You could try
Best of all go to a show such as the BMFA Nationals at Barkston Heath which is this month I think. Usually a trader selling balsa at good prices. Go early for best choice!
I have planed LE strip from a ordinary sheet. Use a full 3 or 4 inch sheet stick it to a piece of melamine board with a few tiny bits of double sided Sellotape.Use a razor plane plane and gradually plane a bevel until it is wide enough. Then trim to size remembering to make the edge a right angle triangle if that is needed. Or a non right angle triangle ( is that an isococles Triangle?)for ailerons. You could also make a preliminary cut with a bandsaw and an extra fence, but always keep a full size sheet and watch your fingers!
kdc27/07/2007 14:27:00
41 forum posts
The LE on this model does not appear to be the shaped commercial type. See the drawing of R5. It is made from a piece of eighth sheet which is bevelled slightly so that the sixteenth sheeting covers it over. Then a further, narrower, piece of eighth sheet (LE capstrip ) is glued on front ( with balsa cement, cyano, or aliphatic not ordinary PVA ) and then sanded to the LE shape. This is a better way because the LE sheeting to LE joint has a greater glue area and the PVA glue joint is hidden & does not ‘pull out’ when sanding. That is why balsa cement. aliphatic or cyano is used on the LE capstrip.

Howard Heffer01/08/2007 17:07:00
15 forum posts
K, heres a stupid plan for you. All the plans I ever used have had both wings printed on them? Any tips for duplicating a mirror image of the right wing? A3 photo copier isnt big enough?!

Myron Beaumont01/08/2007 18:52:00
5797 forum posts
51 photos
Grease- proof paper (if you can't get hold of proper tracing paper) will do the job.Put it over your plan and trace the lines .Turn over & use as a plan Boom! Boom!

& it won't stick either!
Howard Heffer01/08/2007 20:20:00
15 forum posts
Ok then as simple as that huh!? A small snippet of genius me thinks! I was expecting a precision way of doing this but if you can get away with a hand copied drawing then I'm all for it!

Thanks for the idea; I will go away, drink something to steady the hands and give it a go!
Myron Beaumont01/08/2007 20:26:00
5797 forum posts
51 photos
Howard ! don't use the inverted copy upside down or you'll have to do two more the wrong way as well & end up with a bi-plane !
Howard Heffer01/08/2007 20:30:00
15 forum posts
Haha, yes I'll make sure I have the thing the right way up before I put it all together. Thanks for the heads up tho, thats just the sort of stupid thing I would go and do by mistake!
Myron Beaumont01/08/2007 21:09:00
5797 forum posts
51 photos
If you get it all wrong you can do this "B" thing (which I think involves being inverted ) with no problems & if you get it all wrong you'll still have a spare set of wings!
PAUL FOX01/08/2007 22:04:00
13 forum posts
KDC wrote "Peter Millers other plans are notorious for their errors..."

I haven't had any experience of Peter's plans, but I am presently building from one of another designer's plans and am finding significant errors.

I am fortunate in having drawing office skills, and, therefore, to iron out things that don't fit together. What I don't yet have is aircraft design experience that can tell me whether the alterations I make are aerodynamically significant. That must be the case for many competent builders, so I guess there must be a good few out there that give up on free plans before committing to a long building project that could easily be wasted.
Myron Beaumont01/08/2007 23:06:00
5797 forum posts
51 photos
I too have spent time in drawing offices & it was actually at RR Derby . I was designing model A/C long before I got the apprenticeship & I got it(I tnink 'cos I had learnt about aerodynamics a bit by designing my own models(I think)
I might ramble on for hours about designs etc etc
but at the end of the day I don't think that (in fact I know ) that these folks have never had to design parts of a gas turbine engine (for instance )that required that the drawings were "SPOT ON".

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