Tempus

This article first appeared in Radio Control Model World in February 1991.

Plans available via Traplet  at £10.00  (Feb 2006 price). There is a fairly torturous route to get there. Go to the Traplet Home page, then follow: UK >> Product Catalogue >>> Construction Plans >>> Aircraft Plans >>> Sports Designs. Sorry I can't give a direct link but the site uses frames.  Let me know how you get on if you buy some.

The young lady on the front cover is Vanessa.  The 'photo-shoot' for this was not quite as I imagined it might be. 

Far less glamorous! Click here for more information.

TEMPUS first started to take shape in the summer of '89. I wanted a good looking low wing model that could perform reasonably well (at least to the limit of my ability) and would suit the range of engines I had at that time.

Earlier I had bought a Firebird from Galaxy Models, as this had the looks, and the blurb with it promised no tip stalling. As it turned out it proved to be a slightly cranky model to fly, was rather too high on wing loading for my taste (25 oz/sq. ft.), and never really gave me any pleasure. It ended its rather short life performing a spectacular flick roll into the ground. (Not exactly what I intended). The result can be seen in the picture left.

At this stage I reverted to the good old WOT 4 while I considered what to do next. After much pondering I decided that I knew where the Firebird went wrong and set out to design my own.

Basing the fuselage profile on the pylon racer type configuration, I quickly sketched up the outline for Tempus, although at this stage it was "the plane with no name". Other areas of concern were the width, to accommodate three servo’s abreast in comfort, and the strength, always with a mind to lightness as I have a mortal fear of high wing loading The fin and tail areas were chosen to give reasonable stability, with relatively large rudder and elevator areas to give the desired manoeuvrability. The wing was originally going to be a conventional 15% symmetrical type section, with a parallel planform to give a reasonable area (That old wing loading again!). The plane would also be a tail dragger for ease of take off when the patch was bad - which is nearly all the time.

Having done all this I then proceeded to build the fuselage etc, ready for a Magnum .40 2-stroke, but got side tracked before the wings were started. (I obtained a Laser .60 at an irresistible price, and went off to build a Puppeteer to do it justice but that's another story).

Before I got back to building a wing, a work colleague and fellow modeller Graham Probut was telling me about a fantastic new wing section he had found in an American mag (Radio Control Modeller I believe). He was flying it in a high wing configuration, which gave a stable, but manoeuvrable model that was nearly impossible to stall.

Designed by Dan Santich, who used to be Chief Designer with TopFlite Models, it is a double reflex section; that is after the standard convex leading edge to approximately 1/3 the cord, the remainder of the wing is concave. Hopefully this is clear from the picture. According to Dan Santich this means the section can handle the constant changes in attack angle experienced by models, by holding the air in the reflex section laminar, for longer. It seemed fair enough to me and I was told by Graham that his model (and others built by members of his club) proved that it worked in practice.  

It seemed like the ideal section for the type of model I wanted, so I took down the complete fuselage, dusted it off (this was now some twelve months after I started), built an appropriate span wing, and modified the fuselage to accept it.

Also about this time I acquired a Saito .45 4-stroke at an irresistible price, (I'm actually rather weak willed when it comes to nice 4-stroke engines as you may have gathered) and decided that this would be an excellent power plant for the model. To fit it I hacked about half an inch off the nose, to accommodate the extra length and weight, re-made the cowling, and "hey presto" Tempus was born. The name, by the way, is a horrible Latin based pun; I will say no more!

Sticking It All Together

For all those who want to find out how it flies, skip to the end section, while I describe how to build it. Not that it takes much description, as it is quite straight forward, and quick. However I am assuming this is not the first model you have tackled, because despite its relatively easy handling this is not a trainer, but it would make an excellent first low winger.

In all cases build it light. There is no excuse for this model exceeding 51/2 lbs.; the original weighed only 5 lbs., with the heavier 4 stroke. This gives a wing loading of 20 oz/sq.ft. which is fine, although I was aiming for only 4 1/2 lbs., and 18 oz/sq.ft.

It is recommended that construction commence with the wing, as this is needed to check alignments, and the fixing has to be arranged before the fuselage is finished. (Note; this is exactly the opposite of the way I did it!) I used cyano for the most part, with PVA and epoxy where appropriate.

The Bits That Hold It Up In The Air

The wing construction is very simple, and conventional. Having cut the ribs out by your favourite method glue them onto the lower spar, using suitable packing piece at the trailing edge. I cut the centre ribs exactly as the others, and cut out for the dihedral braces and servo box when they are firmly in place. Build every thing onto this base before removing from the board. There is no washout at the tips. (At least none I intended, or know about!)  

The wing span chosen is critical, and is exactly 26.34in from the inner edge of one centre rib, to the outer edge of the last rib. By doing this the inter rib spacing is exactly 3in across the outer bays, and 2 in on the inner two. This saves cutting the vertical webbing to less than a standard sheet size (I never could understand why wings are designed so that each bit of webbing has to be trimmed. I've even had plans where every rib spacing was different from every other! Maybe that was to stop vibration resonance effects! ) If your sheeting is metric, space the ribs accordingly; a few millimetres either way won't make any difference. I have also positioned the spars so that the leading edge sheeting is 3in wide; I mean one doesn't’t want to wear ones knife out!

It is worth cutting the centre sheet to the outline shown (or similar) in the third bay. If the sheet is just stopped at the third rib it creates a discontinuity in the structure, and can lead to the trailing edge cracking at this point when subject to high stress. I normally adopt the outline shown, but I got lazy and didn't do it this time Guess what happened on the first heavy landing!

You may be able to find TE stock for the ailerons of the right size. I couldn't and laminated mine from what I could find and some 1/16" sheet as shown on the plan. Once I had these to approximately the correct section I tack glued them to the rear spar, and continued planing and sanding until the correct wing section was obtained. Then they can be freed, the ailerons cut out and finished etc. I always use Robart steel pin hinge points these days, as they are easy to fit, free acting and tough, but any good hinge system will do.

The Bit In The Middle

The fuselage is basically a standard square box, with 3/16in sheet sides, and 1/16in ply doublers over the nose and wing. With any model it is necessary to pay attention to alignment. Provided the wing seat is square to the tail plane, and the fin is at right angles and aligned down the centre there should be no problems.

I always start by cutting out all the formers, and constructing F4, so that its is well set when needed. Note that I have indicated the grain direction on all the gussets, bottom sheeting etc, it is important for strength that these directions are used.

You will need 4in wide sheet for the sides if you want to cut them from one piece, which I recommend. To glue the 1/16in ply doublers I use a combination of cyano and PVA. First spread a thin but complete layer of PVA over the centre, leaving about 1/8in around the edges clear. Then run lines of cyano around the edge in this area, don’t go mad with the glues, a little goes a long way. Then when the doubler is laid onto the 3/l6in sheet, it bonds immediately by the cyano around the edge, and eventually the PVA bonds the rest. This allows very rapid fixing without the problem of holding the parts together, or the edges of one sheet curling away from the other, or using a lot of expensive cyano!

Build the sides onto the formers in the conventional way, ensuring every thing is square as you go. I epoxied Fl, and eventually the l/4in ply under carriage plate. Do not fix this however until you have offered the wings up and drilled through for the wing retaining dowel. I used a short jobber drill to fit between Fl and F2, if you don't have this you may have to mark first and drill later.

The rear 3/32in sheeting is put on before the ½ in top cap and the tailplane slots are best cut after as well. I used a piece of scrap 1/4in for the tail post above the tailplane when fixing the side sheets, which was removed when the fin was fixed in place.

You may need to steam or soak the front deck sheeting, depending on the grade of wood you select; don't force it round, it may split later.

The canopy should be available in your local model shop; mine was left over from my Firebird, so presumably Galaxy of Norwich will be able to supply this.

The battery compartment behind the wing should not be needed if you are fitting a 2-stroke, but probably will if you fit a 4; it can be left until the radio gear is fitted and the battery position used to achieve the correct balance.

The End Bits

The tailplane, elevator, fin and rudder are all simply formed from 1/4" sheet as shown on the plan. Do note the grain direction where shown. The holes shown in the tailplane, fin and rudder will be required if you fit a light 2-stroke engine. It’s always easier to achieve the correct balance if you build the back end light. I actually have them on mine, even with the 4-stroke, but as I said I did originally intend to fit a lighter engine. Again I used Robart hinge points.

Noisy Bit Up The Front

The engine can of course be mounted any way you choose, but I prefer side mounting for the clean look it gives in profile. In this case inverted would be equally acceptable, and may give better airflow over the fuselage for knife edging. I used a good quality nylon engine mount, again to keep weight down, although I generally prefer a metal mount. The nylon works well, and does tend to break if crashed, thus saving the engine from damage, but I find they flex significantly, and I feel this results in a slight loss of power. If true, this will be more of a problem with 4 strokes, with their less frequent but slightly higher force ignition stroke. I have never put this theory to the test however, and, as I say, have had no trouble with the mount used.

Finally if you do fit a 2-stroke bring Fl forward to the position shown on the plan. You may need to carve some of the fuselage away to clear the silencer, alternatively fit an extension stack.

The Bits That Hold It Off The Ground

The undercarriage is formed from 8 SWG and 10 SWG wire on leading and trailing edges respectively, bound and soldered in the conventional fashion. The upper spreader need only be 12 SWG or 14 SWG, what ever comes to hand. Once made (and don't forget to thoroughly wash any flux residue off) epoxy or cyano balsa to fill in the gap. This can then be covered with your favourite finishing material to give a solid looking undercart. Alternatively you could use a commercial pre-formed aluminium alloy unit, but in my experience these end up heavier and are less forgiving when the model inexplicably throws itself at the ground. They all do at some stage!

My model has the U/C fixed with two nylon clamp straps, with a single fixing bolt, per strap, a' la WOT 4. Although this works well on the WOT 4, it did give some difficulties on Tempus, as described below. That is why the plan calls up saddle clamps.

Please make the spats, they finish the looks of the model and must help reduce the drag to some extent. As shown on the plan they are simply made from soft 1/2" and 1/4" sheet, with the inner width to suit your wheels, and it is worth shopping around for narrow profile wheels in this case. The spats are simply fixed to the wire U/C with one plastic saddle clamp, while collets or soldered washers etc retains the wheels as you choose. I have had no trouble flying off rough pasture with these on, and they seem to take all the knocks of landing as well.

The Bits That Move The Other Bits

Radio installation is simple. I always fit the gear immediately prior to finishing, and adjust the position of the servo's & batteries etc to achieve the balance point. I have not shown the position of the snake runs on the plan, as they are simple and a matter for personal preference. I used a secondary crank fixed to the front of the engine mount to transfer the servo linkage to the throttle. This avoids the throttle cable having to be connected close to the firewall, when the carburettor is at the back.

The Fancy Bits

 I finished mine in Solarspan, as it is my favourite material. Use what ever suits you, but at the risk of sounding paranoid, do watch the weight build up with traditional finishes. With the concave wing sections, covering with iron-on film is slightly different. I used Balsaloc on the capping strips and sheeting to ensure a good bond, and then ironed the film down along each rib, working from the spars backwards. The leading edge was in a contrast colour and top and bottom were applied with one piece. Having attached the material to all the capping strips etc, I then proceeded to shrink it. The holes shown in the wing ribs allow the heated air to escape, and reduce the tendency for it to balloon out in the bays, and lift the covering off. It’s not difficult, just common sense and a bit of care.

Flying, Or The Nerve Racking Bit

Having finished the model, and spent a reasonable amount of time checking the radio, engine, showing it to fellow aficionados, taking photos and other essential pre-flight checks, the moment of truth arrives. I went out to the field on a bright, fairly calm Saturday afternoon, ideal for a maiden flight. With the model rigged and ready to go I had a host of inquisitive eyes, following my every move. I then spend the next half hour trying to move the cows away, and persuade them that our take off patch was not the greenest grass they had ever seen, and could they please go a stand some where else.

With the cows eventually out of the way, I took the model out to the patch, pointed it into wind and let it go. It ran smoothly along the ground for a short distance (with my left thumb making small corrections to dodge the cowpats) and lifted easily into the air. My first reaction was how easy the model was to fly. It is extremely positive and goes precisely where you ask it, no more and no less. After a couple of circuits to check trims, which where OK as set, I was soon rolling and looping around the sky as if I had flown the model all my life. I even had the nerve to try spins, control line and knife edge flight. After 10 minutes or so I set up for landing and it grooved in as clean as you like, and rolled to a gentle stop just past the patch. Needless to say I was fairly elated at this point, and didn't take too much notice of the fact the U/C was loose. I just tightened the screws, wondered why I hadn't done it properly the first time, tanked up and shot off into the blue again.

All was going well until in the middle of a roll at the far end of the field a small white object was seen to part company with the model and fall groundward! I was tempted to watch the U/C to see where it landed, but very rapidly decided that without the U/C weight the model's C.G. would move back, and may make it more sensitive, so I concentrated on the flying. As it was the model flew just the same with the U/C off, and looked quite pretty, with even cleaner lines than before. Having landed without trouble, I fortunately found the U/C on the very edge of a large bramble & nettle patch, and retired home to devise a more robust fixing method.

Back home, I replaced the original large self tapping screws holding the clamp plates, with T-nuts and nylon bolts, exactly as per the WOT 4. Full of confidence I returned to the field Sunday afternoon, which is the usual club session, to another audience; human this time not bovine. With the air of somebody who knew what he was doing, I ran the model down the patch and it climbed gracefully into the air, leaving the U/C behind on terra firma! The flight complete I found the nylon bolts had sheared, and with hind sight I now believe I have the fixing bolts too far rearward, which amplifies the tensile forces on the bolts.

At the field I fitted brass bolts, and so far these have proved satisfactory. However the simple to replace weak link has gone, so that now if I suffer a really heavy prang the bottom of the model is likely to give.

With these minor problems a thing of the past the model is a pleasure to fly. It has a reasonable vertical performance with the Saito 45, but it isn't going to break any rate of climb records. It is in fact fairly close to the performance of a good full size aerobatic ship. For full-bloodied control response I use the following throws, measured at the TE of each surface, either side of neutral;

Ailerons ¾ in        Elevator 1 in       Rudder 1 ¾ in.

It is probably safer to start with half of this on ailerons and elevator, unless you are a real masochist.

The wings stall performance is all I was led to believe, and the model will fly at an angle of attack of about 25° without trouble. When it does stall it is very gentle and predictable. This makes slow landings a real treat. It also treats strong and gusty winds with much the same contempt as Clint Eastwood treats punks!

Tempus has had one or two knocks in its short life, (all my models do), including falling nose first out of a tree, that I had inadvertently landed in while trying a touch and go with no fuel left! Apart from a new engine mount and a bit of extra glue on the cowl following that jaunt, the model has proved quite robust. That's about it then. Build and enjoy; but I say again do keep it light and you should be able to fly the model to your hearts content.