Harvard II (oz4300)
About this Plan
Harvard II - North American Texan. Rubber scale model trainer. Scale is 1/12. Plan shows geared rubber power, and retracting gear.
Quote: "This month's flying scale model is contributed by one of the country's leading authorities on the type. It has been thoroughly flight tested, giving an excellent performance, and has proved itself very robust. Harvard II by HJ Towner.
THE Harvard II is a revised version of the Harvard I designated NA-16-3 by its makers, the North American Aviation Incorporated, and is a two-seater advanced trainer used extensively in this country and Canada and is equipped to accustom the pupil to modern fighter practice. It is powered with a 600 hp Pratt & Witney Wasp motor and the top speed is in excess of 200 mph.
Bearing this in mind, the builder will expect his model to have a performance on these lines, and consequently the model is sturdily constructed and will stand a tremendous amount of rough handling. However, to suit all builders there are two versions of this model, the fully detailed type, ie with gearbox and retracting undercarriage which of course is heavier and therefore faster, and the simpler type with a straight drive motor and no undercart (this gives the effect of a retracted under-carriage in flight), each version giving a good account of themselves.
It must be remembered, however, that neither type model can be just thrown into the air, but must be properly flown and their respective per-formances will be in strict proportion to the ability of the pilot, but naturally the lighter job will give the longer duration.
The only controllable surfaces are the elevators and rudder and these are only controlled by trimming tabs. The tab on the rudder is to correct any turning tendency in the glide or to make the model circle if desired, while the tabs on the elevators are chiefly to allow for any warping which may occur, most tailplanes being very bad in this respect.
The direction and altitude under power are controlled by the front portion of the model which carries the propeller. It will be noticed in both the geared and ungeared versions that the crankcase is fitted to the distance piece and is in-dependent of the front, packing pieces being inserted so that this central unit can be set at any desired angle in relation to the centre line of the model. This arrangement remains 'put' and does away with the very bad practice of putting in bits of balsa. or stalks of grass to give the correct thrust setting.
It will be appreciated that a clearance of about 1/16 in must be left between the crankcase and the rest of the front to allow for this movement. To amplify the drawings on the plan, AA is the front, AC the crankcase, BP the backplate, DP the distance piece, and BB the bearing block. The gears are assembled on BB, allowing a slight clearance between the teeth and the centres marked on the block, which is drilled and bushed and the gears fitted on their shafts.
DP is now screwed to this block, having previously cut away portions to clear the cup washers on the ends of the 'lagshaffs.' When all is running free, remove DP and screw the crankcase on to this distance piece ensuring the line up is correct and reassemble. A slight amount of fitting may be necessary to allow for a true running shaft and you will be surprised how very easily the whole assembly will run.
On the original, thin brass gears of 1/16 in across the teeth and 34 teeth per wheel were used, which gave quite a scream when revolving under power, and in no small way reproduced the characteristic whine of the prototype.
The gears were obtained from the scrapbox of the local watchmaker, who kindly bored out the centres and riveted and sweated bushes about 1/4 in long by 1/8 in dia, which in turn were soldered to the shafts. The cost was 2s. 6d.
Undercarriage. As previously mentioned the simpler version of the Harvard has no retracting undercarriage but plug-in legs can be made for display purposes, and, of course, should be omitted for flying. All landings are on its belly, hence there are no such things as 'nose overs' and cracked up fins and rudders.
The original model was designed for automatic retracting undercarriage, but eventually it was decided to cut it out and use a manually operated one instead, as the flying speed is high, and in the event of a landing with the undercarriage down the said undercarriage would be promptly written off besides ripping out a lot of the centre section.
The system used is quite simple if carefully carried out and consists of a crank on each leg connected by a wire 18 g 'link rod' to a gear wheel, so that the rod is on dead centres with the gear in either the up or down position.
Care must be taken to see that the distance travelled by the crank equals the distance between the two dead centres on the gear. A screw forms the shaft of to gear, the head of which is used to rotate the gear and thus lower or raise the undercarriage..."
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Update 07/01/2019: Added article, thanks to Mary at https://rclibrary.co.uk/title_details.asp?ID=2378 with many thanks to Martin La Grange.
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User commentsHello, In a recent document on RCLibrary [Title=AeroModeller_1943_01.pdf. Filesize=16908KB], link : https://rclibrary.co.uk/title_details.asp?ID=2378] on pages 636-640 there's a lot of information about how to construct the model, and even a cutaway drawing giving a lot of good detail on construction as well as functions e.g. the working retractable gear. I'd like to suggest using the images as well as pages extracted as an additional PDF as well as illustrations for the plan. When I've finished mine (I've just started!) I'll send along some photos - I'm doing one as an SAAF Harvard, of the sort that used to produce a blatting roars over flying over my house when I was a child in Cape Town. With kind regards, and wishing you a blessed and happy Christmas (It's not yet 12th night !)
Martin La Grange - 07/01/2019
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