Levanter Posted January 1, 2018 Share Posted January 1, 2018 Unconcealed delight at another Peter Design for the Mass Build and I have been wanting to build a Grumpy Tiger Cub and my first twin for quite some time. I have a bit of a head start having a nearly finished Oodalally. Apparently the wing is basically the same so the aluminium rib templates will go to immediate use. My Oodalally has split flaps and I think I will try this again. I have never built a model with retracts and Peter warned that weight would be a problem as well as space. I think Peter had to add a little nose weight so one possibility might be to lengthen the nacelles a fraction to bring the engines forward and the trade-off might in weight and space might allow retracts to be fitted. The good thing is that I don't have to make that decision right now and being a Mass Build I will be able to see how others get on. Power is from a pair of OS FS26 Surpass engines. It would be really interesting if I could engineer a different cam shaft for one of them to have the propellers counter-rotating. If later on the power looks a bit marginal I have a pair of Saito 30s that would have a bit more grunt. I am a slow builder but I get great satisfaction from attention to detail and trying to build light. It does seem however that although one can easily shave a few grams off the structure, this quickly goes to pot if the aircraft is loaded with unnecessarily heavy equipment. So here we go and this is the start of the "kit" and Grumpy begins his journey. Levanter Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted January 1, 2018 Share Posted January 1, 2018 I don’t think power will be an issue if it’s the standard size GTC, I’m scaling mine up by 120% (65” ws) and reckon that a pair of 30FS will be plenty and that’s with trike retracts! Quote Link to comment Share on other sites More sharing options...
Greg Cavey Posted January 2, 2018 Share Posted January 2, 2018 Levanter, I’ve only used copied paper plans as my templates so I’m curious how you do that? What thickness aluminum do you use for your templates, how do you cut it, and how do you get it to match the plan. Can you cut it to the line using tin snips or is there s lot filing involved? How do you cut inside corners say like the slot of R1 where the wing dowel goes? Maybe it’s easier than I think? Thanks - Greg Quote Link to comment Share on other sites More sharing options...
Levanter Posted January 2, 2018 Author Share Posted January 2, 2018 Hi Greg I made these in aluminium for durability as they were used on the Oodalally wing as well being needed for Grumpy. Also because the wing is tapered I now have a set of useful templates for other aircraft of different sizes. Peter says the section has been very successful and I think other people have used it in their Skywriter (last years mass build) instead of the flat bottomed aerofoil on the drawing. I am a big fan of Peter so my theory is that if I was building a smaller aircraft I could choose any one of them and make a parallel wing if I wanted. The aluminium is 1mm thick so a bit of a handful for tin snips but I have a small Proxxon bandsaw which gets used a lot. I photocopy the ribs off the plan, cut them out accurately with scissors and then stick the paper ribs onto the aluminium using Pritt Stick. Because we are not worried about grain direction on the ally, the templates can be nested together closely to cut down wasted material. The bandsaw will cut very close but I always leave a tiny bit of meat to be sanded off to the line. The old saying is that it is easier to take material off than it is to put it back on and it is worth doing this as accurately as possible. For the main part I used a Permagrit tool and for the inside corners a small file. Aluminium picks up very easily and this can be avoided by using chalk on the sanding tool and file. Probably if you went down to 0.5mm you could use tin snips but the templates would not be quite so durable. To make the templates more versatile I just make a narrow saw cut on the forward side of any spar slots and use this as a reference by cutting the balsa through the slit. I am then not tied to any particular spar size and it also means if there is any slight variation to the spar dimensions I can adjust the width of the slot to fit closely. Once the template is finished, a light sprinkle of water and the paper floats off the ally. A quick smoothing of any sharp corners and stamping or scribing the rib type and number and the jobs done. Cutting the balsa ribs around the template is then a doddle. On more specialised aerofoils that I am unlikely to use again I use 3mm MDF which is easier to cut and sand to shape. I harden the edges with cyano and don't bother to take the paper off. I use MDF because it doesn't have a grain but this time it helps the scalpel not digging into opposing grain as it can do with plywood. And yes I do use just paper very often especially on formers. I am a great believer in templates, jigs and fixture for anything that will be repeatable but do remember to label them for what they do. I have a small collection of bits that were very useful at the time but I have no idea what I used them for!! Quote Link to comment Share on other sites More sharing options...
Greg Cavey Posted January 2, 2018 Share Posted January 2, 2018 Thanks for the additional info on your templates. Your regular bandsaw blade can handle 1mm aluminum no problem? I take it you cut the balsa parts with a razor knife against the template and avoid having to sand to a line like I have to with paper templates. Especially helpful for ribs that are the same or have minor differences such as a notch for the dihedral brace. Got it. Thanks again. Quote Link to comment Share on other sites More sharing options...
McG 6969 Posted January 2, 2018 Share Posted January 2, 2018 Hi 'Levanter', Glad to see you're in the starting blocks again. If it's not a Southern European state secret, what happened to your "Pas de Deux"? ... Is it still in the 'cardboard box'? Cheers Chris Quote Link to comment Share on other sites More sharing options...
Levanter Posted January 2, 2018 Author Share Posted January 2, 2018 Chris Say no more Quote Link to comment Share on other sites More sharing options...
McG 6969 Posted January 2, 2018 Share Posted January 2, 2018 Really 'no probs', sir. I had some long-ish hesitations as well regarding the choice of Bella's final deco scheme... Cheers Chris Quote Link to comment Share on other sites More sharing options...
Levanter Posted January 8, 2018 Author Share Posted January 8, 2018 Have not been totally idle in the last few days while I try to figure out what I can fit in realistically. Here is the wing view. And here is the wing view with the nacelle superimposed. The recess for the leading edge of the wing is about 20mm further back allowing for the taper of the wing. Lo and behold and Peter Miller has the formers of the nacelle and the fuselage lining up. How nice and easy! The centre of gravity has been re-marked on the copy of the nacelle. Next is the outline of an OS FS 26 Surpass as a sidewinder. Immediately one can see that the position of the carb behind the motor means it needs to go further forward. The OS FS 26 is about 100g heavier than the SC 15 two stroke that the model was designed for so the extra weight and being further forward might mean nose weight is not required. I am not yet decided about sidewinding due to the lack of symmetry on a twin so may well go for inverted. So here we are inverted and immediately I can see that the cylinder head is well clear of the ground in case of a nose-over. Actually I really like inverted because of cleaner lines and it worked out very well on the Oodalally. The outline of the engine was derived from the mounting template supplied with the engine in the photo above and from actual measurements. The outlines are now scanned and available to print any time. I don't have any 4oz Kavan tanks so I am showing SLEC 6oz which a do have. They fit - just but probably an overkill for the four strokes. I don't need the aircraft to stay up all day and I am not planning on fitting lights! Almost certainly I will get the Kavan tanks as suggested. The nacelle is filling up quickly. Still thinking of retracts I have shown Hobbyking size 60 electrics. They would fit but the 2" wheel sticks well outside the nacelle which would have to be considerable longer to accommodate it. The other problem is that I can't see a way of mounting the retracts forward of the cg in order to keep Grumpy a tail dragger. They won't fit in the wings because the nacelles get in the way and I have no pans to move the nacelles even if it was possible. This means I would have to fit a nose wheel and although thee is plenty of room in the fuselage I think it is getting too crowded in there and a bit OTT. Decision Time: Grumpy will have the OS FS 26s inverted. The SLEC tanks will go back on the shelf and 4oz Kavan's will take their place. We will do away with the idea of retracts and Grumpy will drag his tail. Exactly what type of gear can still be decided later but this is progress - phew! Quote Link to comment Share on other sites More sharing options...
McG 6969 Posted January 8, 2018 Share Posted January 8, 2018 I really do like your 'rational approach', young man. Of course retracts should make it look even better, but it gets indeed quite 'crowdy' in there. Prolonging the nacelles behind the TE and making them 'deeper' doesn't even seem to bring a solution if you keep it as a taildragger. Good progress anyhow. Cheers Chris Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 16, 2018 Share Posted January 16, 2018 Levanter, Interesting following your logic re the retracts/cg & tail dragger or trike. I had a similar train of thought with my build but it went along the lines. If I want a trike how do I get the gear far enough back and still in the wing. Had no sensible answer so stuck with the tail dragger. Retracts in front of the cg were no problem however as I was electric and did not need the tank space. Interesting world. Tim Quote Link to comment Share on other sites More sharing options...
Levanter Posted January 16, 2018 Author Share Posted January 16, 2018 Tim Yes it is funny where the mind takes us sometimes. I was even thinking about making some tinplate tanks to fit in the wings to free up space for the retracts. This would probably mean fitting the engines upright to get an approximate fix on the tank height. The engines have now gone full circle! Then I thought to myself this is a hobby - dammit! Why put myself through it? I still harbour thoughts of contra-rotating engines though. Hmmmm! Levanter Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted January 16, 2018 Share Posted January 16, 2018 Think laterally, why about mounting the retracts vertically so that in normal retract position they are down and when down they are up? Quote Link to comment Share on other sites More sharing options...
Tim Ballinger Posted January 16, 2018 Share Posted January 16, 2018 With you Ron but you would need some xtra long legs to make that work would you not ? Tim Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted January 16, 2018 Share Posted January 16, 2018 A bit unfair of me really as I'm designing mine to have nacelles that are extended back to behind the TE to mimic the FS Tigercat. I think that if I mount my retracts behind the F4 position I can have 4" legs and 2" diameter wheels which will retract into the nacelles which themselves are going to be about 5" diameter. I have also looked at retracts that give more than 90 degrees plus ones where the motor is on the opposite end to 'normal'. Quote Link to comment Share on other sites More sharing options...
Levanter Posted February 4, 2018 Author Share Posted February 4, 2018 After more musings and suggestions I am going to save my retracts for another model. I did take Ron's lateral suggestion literally and looked to see if the gear could go alongside the motors and tanks and let me achieve the position to stay with a tail dragger. The answer was yes (probably) but made the nacelle quite wide and to make it symmetrical about the centreline would be too much. The extra length used by Ron would certainly be helpful but would interfere with my idea for flaps. I want to keep the nacelle well inside the chord of the wing so the flap can extend to the outboard side of the nacelle so getting a decent area. My reckoning was that the space between the fuselage and nacelle was not enough but more of that later when I come onto the wing. The start of the build was put back slightly to get Toot Sweet completely finished ie not my usual 95% and that happened last week. I have a nice big box to collect the various parts as they come along although Grumpy is complaining that I treat him like a vampire in that he only gets to come out at night. Yes, really grumpy? I get a sense of embarrassment when making modifications to an established designer's plan so apologies in advance. I can't help it so I just try to keep it in check. I am raising the top line of the fuselage aft of the cockpit to get a bit more of a back-rest for the pilot and the new line is just visible on the photo above. This of course means adjusting the top sections of the formers. Using F6 as an example the new height is taken off the profile and marked above the former. The new curve does not affect the straight section of the fuselage. To do this perfectly accurately the sections should be "stacked" and projected from the end sections but as there are only a few formers I decided to do it by eye using French Curves. The chosen section of curve should end up tangential to the top and side lines and the required section has been marked with a small piece of masking tape so the curve can be mirrored. Like this. I find this better than folding down the centreline and cutting with scissors as you can see the finished result and adjust it if necessary before cutting and because I like to leave a little bit of meat on the template for sanding back to the line. The wide spacing of the main former means that small discrepancies in the curve will not be noticeable. Where the formers are decidedly incline over the wings I have left a bit more meat on the base line. The only adjusted formers that have to conform well are F5a and F5b and these will be pinned together as a pair and sanded to the correct bevel. The only other change planned for the fuselage is for the fin mounting. Here is Oodalally's fin on the left. The aft post of the fin extends right down through the fuselage and take the place of the spacer shown on the plan. I don't suppose it make a huge difference to Peter's method of fitting side block but it does help to make the fine self jigging. In Oodalally's case the leading edge strip was extended slightly to fit in slot in the tailplane. Like this and is completely self jigging on final assembly after covering. This is the modification for Grumpy. Firstly I can never resist built up tail feathers although it is quite possible I might skin them. You can see the aft post going to the bottom of the fuselage in one piece but this time the leading edge is slotted into F8 with the slot increased in depth accordingly. This will provide a very accurate jigged fit. The lower section of the fin is raised 8mm able the fuselage line to give the covering something to stick to although this will be redundant if I do decide to fit skins. The good thing now is that I am ready to start cutting wood and start filling Grumpy's box with "kit" bits. About time some will say. Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted February 4, 2018 Share Posted February 4, 2018 The full size has flaps on both the inside and outside of the nacelles so a torque rod between them would work. It’s something I’ve been thinking about as I am splitting my wings outside the nacelles so would make a logical break between flaps and ailerons Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted February 4, 2018 Share Posted February 4, 2018 Of course, the flap area can be increased by making them wider, yet another option! Quote Link to comment Share on other sites More sharing options...
McG 6969 Posted February 4, 2018 Share Posted February 4, 2018 Still following here, Levanta-er. I totally agree with your 'build up' sub-assemblies. I even laminated a fin/rudder for my Vicomte. Quite convinced with the 'self-jigging' tail bottom pieces as well. I did the same with 'Vicky' (my current build's new nickname). Keep it to yourself, of course. But if I was building a Grumpy, I would make the aft fuse even 'higher' and simply use a commercial Corsair cockpit, making your GTC totally 'unique'. Yep, I know about those naughty men in their 'white coats'... Cheers Chris Quote Link to comment Share on other sites More sharing options...
Levanter Posted March 2, 2018 Author Share Posted March 2, 2018 I have not been completely idle in the last few weeks doing more building than posting and there seems to be a bit of a shortage of Grumpy Tiger Cub posts (although not grumpy posts) so it is time to catch up a bit. Cracking on with the fuselage then. Here are the collection of formers in their raw state with the paper templates stuck on with a light smear of Pritt Stick. With the paper kept in place until the last minute it is easy to mark up and try ideas without affecting the wood. The red lines highlight where I am going to remove material which shaves off a few grams here and there. I know it is not much in the great scheme of things but I tend to put any saved weight into a "credit" box and trade it for other things which I will explain later. On F2 I have kept the cut-out fairly high because there will be a support glued on the aft face to hold a tray for the flight battery. I don't have an electric fret saw an having tried a hand fret saw with some difficulty I switched to chain drilling. In the little Proxxon pillar drill (one of my favorites) and this is very quick. The edges are then tidied up with a small sanding drum in a Dremel again using the paper as the guide. At this point if I remember I put a small centre-pop dot on the vertical centrelines. Later this will get scribed into the wood with a hardish pencil to leave a permanent line. These centrelines are extremely useful references throughout the build and are the only lines that stay on the finished model. On the firewall I always scribe the horizontal centreline which then defines the thrust line. I made two parts to F8. The main former has the deeper slot which forms the register for the base of the fin and the smaller section is glued to the fore side to support the end of the top spine of the fuselage. This F3 which takes the dowel at the leading edge of the wing. Here it is being fitted with a small Liteply doubler to increase the bearing area for a thin walled aluminium tube that will carry the dowel. Like this. Just above the tube you can see the centre-pop mark so I can always find the centrelines even if the pencil marks get rubbed off. The idea of the tube is not mine but I have seen Peter use it before. It ensures that any wear takes place on the dowel which is easily replaced. Another thought is that in the event of a crash it will promote the dowel to shear on the hard edge and perhaps save some structure elsewhere. So far an untested theory because only one of the planes I have built has flown to date and that was a small glider with the wings banded on. The disadvantage is that the tube has to be fitted with precision so care is needed. Any adjustment that is needed has to take place on the leading edge of the wing where the is not much to play with but enough if you are careful. I think that's enough about formers for now. Levanter Quote Link to comment Share on other sites More sharing options...
Levanter Posted March 2, 2018 Author Share Posted March 2, 2018 Fuselage sides and doublers net on the list. The copy of the plan I had made at the local print shop is fast reducing in size and big chunks came out when the sections of the fuselage were cut out and the short tail section stuck to the main body with drafting 3m drafting tape. Wherever possible components are made in pairs. Apart from the considerable tine saving it does ensure symmetry. It isn't generally the end of the world if some discrepancies creep in but at least any differences will be mirrored. In the usual way the paper template was stuck to one of two 2.5mm sheets (Grumpy is totally metric being of Spanish heritage) and the two sheet pinned together. Cutting closely to the line, it is an easy job to adjust to the finished size by light sanding. It was slightly different for the doublers because the plywood does not accept pins. Here, the plan section is stuck to the top doubler blank and the two blanks were stuck together with 3m Spray Mount. This keeps the two parts securely held together through a variety of operations but is easily separated when required by slipping a steel rule into the joint and working them free. Any residue wipes of easily with white spirit (ordinary thinners). Here is a finished doubler. The angled slot on the left will take the plate for the wing bolts. The register on the bottom will take a support for the Rx tray and the two recesses in the cut-out on the right are for the servo rails. These make very secure anchorages for the internal components and guarantee accurate orientation. The 5mm longerons to run along the lower edges of the fuselage could not be persuaded to follow the curve at the nose so a little prior help was needed. The balsa sections were dampened slightly and given a pre-bend using these wedges. As the started to relax the wedges were pinched up to give progressively more curve. It was then left to fully dry for a couple of hours after which the longerons conformed with no further argument. The two fuselage side are now ready and the positions marked with a soft (B or 2B) so that only very light pressure is necessary to make a good line. Normally I follow Peter's advice and use contact adhesive for doublers which works very well. The downside for me is that I don't like glue on the balsa surface where there is fresh air for the cuts outs so carefully applying the glue to a line or masking up takes a bit of time. In this case because there are no wide areas I used aliphatic glue followed by weighting down. The glue face of the doubler was scored and criss-crossed using the rough side of a Permagrit. Plenty of weight is needed and it helps to put a suitable flat board under the weights to ensure even distribution. Here are the finished formers for the front section. F2 has two rails near the bottom edge to take the front of the battery tray and this is the same height as the bottom edge of the lower cut-out in F2. The centrelines are clearly visible and the centre-pop marks as well. These are the rear formers slimmed down as much as possible. The doubler on F8 will be fitted later as it will need to cater for the bevel of the rolled top of the fuselage. We are now ready to start assembly. Levanter. Quote Link to comment Share on other sites More sharing options...
McG 6969 Posted March 2, 2018 Share Posted March 2, 2018 Very neat build, Levanter. And nice explanations as well. I like your provisioned "registers" to be able to position cross members or hardware more easily afterwards. Perfecto Cheers Chris Quote Link to comment Share on other sites More sharing options...
Levanter Posted March 2, 2018 Author Share Posted March 2, 2018 Time to get the glue out. My favourite above everything else is aliphatic. I use epoxy only if there are compelling reasons why I can't use aliphatic. In some areas I use aliphatic with cyano assist but we will come on to that. I decant the aliphatic into small jam jars. The type used in hotels where one is not quite enough but two is too much. I never miss an opportunity to collect. I keep two small brushes in a small jar of water. Why two - well when I forget to put a brush back in the water and it goes semi hard I have a second ready to get me out of trouble. This happens often. F5 being glued in place. F4 which is the same width is simply being used to support the weight parallel to the base so that it is stable while the glue sets. The notches were left to the last minute and were individually cut out to suit the longerons at each of the stations. Peter's plan is Imperial so I could not use the paper templates and anyway after fine tuning the fuselage edges after fitting the longerons some minor differences can easily be accounted for. F4 being glued in place and now you can see I am being braver with the weights. The strip of balsa is acting like a shore on the base of the square keeping it properly aligned. The three formers in the parallel section of the fuselage are now in place and this is to double check that they have all arrived at the same height. Any adjustments can be made by light sanding. It is good to check on accuracy at early stages and to keep doing so. I find errors have a nasty habit of accumulating so I try hard to correct things as I go along. Sometimes this takes much willpower (I am currently making my third dihedral brace!) The other side is now glued in place and before placing the weights the alignment at the tail is check using the square end of a Permagrit. This is very important because if the two sides are not exactly on the same plane, when they are brought together for joining at the tail a twist will be induced. That is a problem very much better to avoid. We are now ready to put the basic fuselage into the jig. Levanter Quote Link to comment Share on other sites More sharing options...
rgarde Posted March 2, 2018 Share Posted March 2, 2018 Nice to see that one of the GTC builds is making progress. You build very neatly Kind regards, Richard Quote Link to comment Share on other sites More sharing options...
Levanter Posted March 4, 2018 Author Share Posted March 4, 2018 Now for a little bit if background on my fuselage jig. Space is my apartment is always at a premium so I try to be organised with my storage. Here is my jig stored on the back of one of the built-in wardrobe doors. The doors are very sturdy and flat so ideal. The board sits on a rest with a shallow lip and the top is retained with a toggle like this: Having worked a lot with sheet materials of many types I know only too well that boards simply stacked against a wall will end up with a bend over time. This way the board is held perfectly flat and out of the way. My main building board is kept on the other door likewise. The jig is instantly recognisable as the SLEC version. To ensure good adhesion of the sticky-backed vinyl the board was primed first with water based acrylic primer and then lightly sanded to remove any fluffy bits and raised grain. Another important thing with boards and laminates is that they are "balanced". IE if you do something to one side you should also do something to the other in order to avoid warping. Both sides were painted, one immediately after the other. The printed grid is useful but if I was to criticise anything I would have preferred the lines to be much finer and as I think mentioned elsewhere the plastic clamps are a bit flexible. I order extra clamps and many extra washers and was glad I did so. The blind nuts are recessed on the underside of the board to retain a flat surface. Which covered with cork tiles to give me another building board. It is a bit big for my normal use but very handy for the longer items. Apart from the lateral clamping points I also put a number on the centreline and these are extremely useful as I will show later. Levanter Quote Link to comment Share on other sites More sharing options...
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