home

WING BOXES AND TONGUES

by Geoff Graham

I am currently building a 1937 design model called Falcon. Though I am using a Ben Buckle kit, the a/c was originally designed by a guy called Ben Shereshaw and named "The Cloud Cruiser". The Ben Buckle version is a modified design first sold by Keil Kraft in 1946. Born in 1938 and an aeromodeller since the age of about 11, I am fascinated by these lovely old period designs. They are a "good build" for those who enjoy their building and because of that, splendid experience for someone who was born much more recently but who wishes to explore the way that models were built 50 or more years ago. A free flight design in the thirties, it is now a 3 channel R/C model and I am using a Laser 70 4 stroke which being quintessentially British, suits this aircraft absolutely.

The Falcon has a span of 96 inches but the wings are in two pieces to make carriage easier. As I have a Ford Puma (a fancy Fiesta but with less boot!), I NEED that kind of knock-down design. The two halves of the wing are joined by a very simple looking arrangement comprising of three spars secured to one wing and which then slide into tailored box section receptacles in the other wing root. The prospect of making and positioning three boxes of this type is at first a little daunting but it needn’t be if approached as a woodworking task. Woodworking, like any other fabricating hobby, requires accuracy and precision. Precision is required when the boxes are made as they just have to be a very good fit. Tight, but not too tight. If perfectly made, withdrawal of the tongue will be like that of a good fitting piston. You should be able to feel the drag as the partial vacuum left behind is filled with air slipping around the edges of the tongue. Accuracy is then needed to fit the boxes so that the spars all register and present themselves for easy insertion into the boxes in the other wing root.

The wings are built in two halves and so is the centre section (C/S). One half of the C/S has the wing boxes secured within it and the other has the spars. The spars are all different sizes. The forward spar is hardwood and a substantial 1 1/4 by 3/8 , the main spar is a flat tongue about 2 inches by 1/4 and in ply, the rear spar (hardwood again) is the smallest at 3/8 x 1/4 inch.

The only way to make certain that all three spars marry accurately when presented to their respective boxes is the make all the cuts in all C/S ribs simultaneously. Never try to do it one rib at a time or you will almost certainly have a disaster on your hands. First bind the ribs together. Position a scrap piece of the upper or lower wing spar to make certain that the ribs are properly aligned then use masking tape to bind them. There are six for the Falcon; two x 1/8 ply and four x 3/32 balsa. Make certain that you get them in the right position relative to where they will end up in the C/S.

Now, because my kit did not have the pieces of hardwood called for by the plan, I had first to make these which was a bit of nuisance but added interest. Picture 1. shows me honing my favourite chisel on an abrasive impregnated polishing wheel. This produces a superb, razor edge - essential as I may need to pare fine shavings around the edge of the holes I will soon be cutting.

First step is to mark on one of the outer ribs the position of the tongue. You can only do this with care, consulting the plan details. If you use the following technique you will not need to mark both sides. I next drill four small holes using a 1mm drill bit in my pillar bench drill. (This is why you need only mark out one side as the drill stand will ensure a true, vertical hole through the ribs). One hole is placed in each corner of the slot I am about to cut. This prevents tearing and splitting in the corners of the final cut. Next I drill another hole using a larger bit but one a little less wide than the finished hole will be. Make certain that you have a flat piece of mdf or similar held firmly behind your ribs as this will prevent the drill bit tearing out the underside in a messy fashion. Next I take the work to my electric scroll saw, threading the blade through the larger hole and after tensioning the blade, cutting the hole very carefully. Take your time with this! It is well worth it because with care you can make this the final action with little filing or paring needed to adjust the hole size.

Picture 2. shows the end of this stage with the tongue a very nice, tight but not binding fit in the hole I have made.

Picture 3. shows the wing box into which half of this tongue will fit eventually. To make the wing box I take a piece of balsa exactly the size of the tongue breadth and thickness. I coat it with candle wax simply rubbing it with a candle kept in the workshop for that very purpose. The top and bottom of the box are cut exactly to the width of false tongue but the sides over-size (waste removed later). One edge of each side is made true and straight (Permagrit 300mm block). These will be the bottom edges. I use cyano medium and place the "false" balsa tongue on the base of what will be the box. Then apply cyano to each edge and present the two sides with the true edges on the bottom. (Do this on a sheet of polythene). When the joint has gone off enough, lift this assembly and carefully clamp it until the cyano is well off. Now, leaving the false tongue in place, cyano the top piece of the box between the two sides making sure that the false tongue is gently but intimately held, then clamp.

When the cyano has gone off safely, remove the balsa tongue. It should come out OK but if it won’t simply drill it out. That is the beauty of using balsa. You would be hacked off if you had used the ply tongue! Now you need to mark the holes for the two remaining spars.

  Picture 4. shows these marked out. 

In picture 5. I am drilling the 1mm corner holes in each of the newly marked cuts.

Picture 6 shows the result.

Picture 7. has the C/S ribs on the scroll saw and picture 8 has the result showing all three holes pretty cleanly cut. All I then had to do was to enlarge the tongue hole in three of the six ribs so that it would take the prefabricated box. Before splitting my 6 pack of ribs, I inserted the tongue in to its housing in the ribs. Next the box was eased on to the tongue until it abutted the ply, outside rib. 

Picture 9 shows me marking round this box with my pencil. 

Picture 10, shows the extra material that has to be removed. Now the three ribs requiring this work are split away and carefully re-bound with masking tape using the three spars as a positive means of aligning them. It is back to the scroll saw now cutting very carefully on the waste side of the pencil line. It is better to have a hole too small than one too large!

  Picture 11. shows the invaluable Permagrit small, flat file at work taking out the last, small pieces of wood required to achieve a good fit. 

Pictures 12 and 13 show the finished set of six ribs threaded sweetly on to the spars. Now all that needs to be done is to build them into the centre section over the plan. I will use the spars to ensure alignment during this building progress. Finally, when the C/S is completed I will bind the tongue box with either nylon or glass fibre cloth and resin so as to give it that added bit of strength.

See more of Geoff's Falcon on work in progress

 - the answers!

Here are two interesting and helpful responses to the questions on the charging of sealed lead-acid batteries that were raised on #32.

ROGER RYTON of Newbury, UK, offers this: 

"The batteries are classed as non-spillable, having unique sealing techniques to guarantee that no electrolyte leakage can occur. The batteries are provided with a means of recombining gas internally during normal usage, and in the event that a build-up of gas at excessive pressure should occur, a low pressure venting system, operating at 7 to 10 psi, will dispel the excess and reseal itself.

The non-spillable feature of these batteries means that they can be operated in any position, unlike the usual form of vented lead-acid battery which is strictly 'one way up'. A service life of 5 years should be expected if the batteries are used most often in the 'floating' or 'standby' modes of operation, where top-up trickle charging is regular and discharge is infrequent. The batteries also feature a low 'self discharge' rate of only 3% of rated capacity per month, allowing the battery to be unused for some time without loss of efficiency or any appreciable deterioration of performance.

Because these lead-acid batteries are sealed, some care has to be exercised whilst charging, and it is recommended that a stabilised constant voltage source should be used, with current limiting proportional to the battery's Ah rating. Car battery type chargers must never be used. Battery performance and service life will be directly affected by the choice and efficiency of the charging circuit used. Constant voltage charging is the most suitable method, and the output must be within 2.25 to 2.30 volts per cell for trickle charging, or in the case of 'cyclic' use (regular discharging) 2.40 to 2.50 volts per cell. 

From the table of batteries that follows I have extracted the data for the NP 7-12.

Dimensions 151 x 98 x 65mm Weight 2.65kg Voltage 12v Capacity 7Ah
Short duration max. discharge 210A
Preferred constant V charging @ A max
For cycle use 14.4-15v @1.75A max.
For standby use 13.5-13.8v @ 14A max

Just two batteries in their range of 21 types are specifically designed for cyclic use. They comment "These batteries have been designed to withstand 500 charge/discharge cycles but, with care should handle many more." These are rated however at 17Ah and 24Ah respectively at nearly treble the price of the NP 7-12!

I'd look perhaps at Lead-Acid Leisure batteries. Although twice the price, the rating is 60Ah minimum and they have deep cycle capability. The drawback is the weight and the liquid electrolyte, but then glow fuel requires careful handling anyway.

To check the accuracy of the charger ratings, connect a digital multimeter directly across the terminals (under load) to check the voltage. For current, connect the multimeter in series (in-line) to read the amperes.

The old one probably did through a combination of old age and how it was treated.

The charger you use (Simprop Multicharger) is not a good choice for these cells, you can overcharge and damage the battery. It is intended for charging nicads and is constant current. (This means that the charger has circuitry which keeps the current the same during charging) This is correct for nicads but not for Gel cells. They need a constant voltage charger. This keeps the charger voltage constant and means that the charge current will go down as the cells reach their full charge. A lead acid cell increases in voltage as it charges (one way of checking how much charge is left) As it nears a full charge the charging current will drop and with the correct charger you can't over charge as it will only give a trickle charge at this point. If you go to Red's Battery Clinic he explains it much better than I can.

Get one of the chargers dedicated to charging Gel cells; you can get one from from your local model shop, or from Maplins. Also charge the cell more regularly, lead acid cells can be damaged by letting them get too low on charge. The new cell should be partly charged when bought but an overnight charge with the correct charger will bring it to full capacity.

Having read Roger's and Jim's responses, I discovered that Roger has just purchased a new 12v 7Ah battery together with this very nice JP dedicated 12v gel cell charger that does, indeed, do it all for you! Just plug it in to the mains, connect your battery and a red LED illuminates to show that you are successfully charging; when the light goes out, the charge is complete. It comes from your Perkins stockist for £11.99. 

The outcome? I've followed Roger's practice and James' advice and bought a gel-cell charger! Anybody want a Simprop Muliticharger?