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18 Dec 1913. Wednesday.

18/12/2013

Today, Frank was finishing off the tailplane. Having drawn up the metal bits yesterday afternoon, this morning he drew up the wooden ribs that were fitted to it.

511 Empennage Wood Ribs 18 XII 13

To our eyes, using steel to make the tail surfaces seems a bit odd, since there are all sorts of good reasons for trying to keep the tail as light as possible. Nevertheless, it seems to have been standard practice, and you can see it on many other aircraft of the period, made by B&CA and others. The Sopwith Camel, for instance uses this method. Generally, they use sheet steel ribs throughout, and Barnwell is unusual in using wood. They are also of very lightweight – frankly flimsy – construction, with 10mm x 5mm strips of wood top and bottom and a few – a very few – little plywood blocks to give them shape. He must have reckoned that the fabric would give them additional stiffness, but prior to covering, one has to be very careful about handling the tailplane in case you break them. 

You can see that the cross-section is symmetrical; the top and the bottom are the same. This is a non-lifting tailplane, and is almost universally used today, because the main purpose of the tailplane (or horizontal stabiliser) is to press downwards during flight, counteracting the natural tendency of the main wing to pitch forward. It means the aircraft is naturally safe in a stall. But in those days it was more common to have a lifting tailplane, because the power was so limited and the drag so high that one needed all the lift one could get and benign stall characteristics were rather lower down the list of priorities!

So even the famous Deperdussin Racer, which was far and away the highest performance machine of that time, had a lifting tailplane. Which makes it all the more interesting that for the production Scout ‘C’ they returned to a lifting tailplane by replacing that 1″ cross tube with a deep wooden spar that made the bottom surface flat and the top surface curved. I can’t find any immediate reason for this, and indeed later on they returned to a symetrical section.

One other beneficial design change in the production models was to silver solder light metal clips to the leading and trailing edges to which the wooden ribs were attached. But they never did get rid of the horrible flimsy ribs themselves! We hope this means that they didn’t get any problems with them in service…

Today was another productive day, which saw the drawings for the elevators (Air Elevator Flaps) completed.  (There is no date on drawing 513, so I’m assuming it was done on the 18th).

512 Air Elevator Flaps 18 XII 13 513 Elevator Flaps Wood Ribs

Construction is similar to the tailplane, but they may have had some problems with the wooden ribs, since they were changed to steel on the Scout C, and the same applies to the lever plates (called horns in today’s parlance), which were beefed up in production.

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