Curtiss Hawk 75 (oz14664)

About this Plan

Curtiss Hawk 75. Radio control stand-off scale model for electric power with 540 motor and belt drive gearbox.

Quote: "When looking for a new aircraft to model my main requirements are that I have sufficient information on the chosen subject and that it's of a lesser known aircraft. Okay so I've probably made more Spitfires than most (I'm on my 8th at present) and I've also done the usual Hurricane, Typhoon, ME 109, FW 190 and so on, (you'll gather I'm rather keen on scale subjects - preferably WW2) but there comes a time when you don't want to get stuck in the same old rut. Taking up electric power seemed to act as a spur somehow, so there I was hunting through my reference books. Have you noticed how hard it is to get books from your local book shop which illustrate anything more than the usual front line high profile jobs?

I eventually settled on the P36 because it was one I had never seen a model of and I had already drawn up and built the Kittyhawk for a .25 glow motor. Being roughly the right size for a 540 with gearbox I thought I might only have to modify the front end and all would be well. Wrong! On checking the 3-views I found that there were actually quite extensive alterations made during the development from Hawk to Tomahawk and then to Kittyhawk, although many concepts ran through all. It just goes to show that you can't judge the whole plane by things like wing planform or the fact that it looks similar in certain respects. For example, that characteristic rear to the cockpit. In the end, of course, I ended up redrawing the whole thing, but at least much of the basic structure was already there from the Kittyhawk drawing. Several days later the outlines were all down on paper and the structure sketched in.

The Reality: The Hawk first flew in April 1935 and went on over the next few years to equip an impressive number of air forces. It was considered something of a watershed as it entered service, but in Europe it found itself outclassed within two years of joining front line squadrons. Those undelivered to France at the time of its capitulation were taken over by the RAF under the name of Mohawk.

It was apparently a pleasant aircraft to fly, with excellent manoeuvrability and light and well harmonised controls, but lacked speed especially in climb and dive which made it no match for the BF109. It is of course a matter of British pride that the Americans didn't have a fighter worth a bean in the European theatre until the Merlin powered Mustang came along. Nevertheless, the Curtiss Hawk fought on in Burma, India and the middle east until early in 1944.

The Model Design: I decided to construct the prototype model using built-up wings with the outer rear sections unsheeted. The fuselage would be built onto the completed wing structure. I had used this system on my 52 in FW 190 A-8 (although with a foam wing) and it had resulted in a very light and strong airframe which performed beautifully right down to low speed using a .35 glow motor and three function radio (until I tried a reversal too low and found someone had jacked the field up a couple of feet!). I keep wondering whether to give it a new lease of life especially if I could gear up a couple of 540s to power it - it was only the cowling I damaged.

I made only one design decision on the Curtiss which erred from a full scale model, and that was to use full span ailerons. I had done this on my Jet Provost to great effect, producing a very impressive (if not frightening) rate of roll, and knew as a result that only small control movements would be required. I also designed for differential throw on the ailerons.

In the event, my decision for full span ailerons resulted in another minor departure from scale in that I was unable to find a trailing edge section of sufficient depth in one inch chord, and had to accept a one and a quarter inch section with the inevitable result that the wing chord grew by a quarter of an inch over the whole span.

Rib Sandwich: I tried the sandwich method for cutting the wing ribs but with such a highly tapered wing it turned out to be more trouble than it was worth and I ended up doing so much adjustment by eye (even after the wing panels were constructed) that it would have been quicker to visit the print shop in town and have the root template reduced by the required percentages. This would have given the remaining rib shapes (suitably modified for washout) which could then be cut individually. Next time!

I sheeted both top and bottom of the wings forward of the main spars before joining the two halves together and sheeting the rear part in one piece. The centre section sheeting went to one third span but could be cut back a bay if going for lightness.

The wings are joined using a plywood brace connecting all the spars, but the wing leading edges are not fitted at this stage. The plywood fuselage doublers were then cut to fit the wings and glued to the insides of the innermost ribs where the servo well had been left. A cut-out had been left in the underside of the wing immediately aft of the spars with the thought that it might serve as both a hand launch facility and a ventilation air exhaust. Now the model is complete I think the former impractical as I left insufficient space to ensure safe withdrawal of the fingers after the launch. The motor mount and bulkhead assembly was then constructed and glued to the front of the doublers and the fuselage formers cut and added, together with the rear underside of fuselage. The two main stringers were then attached to the top of the doublers / formers and to each other at the tail end.

The basic structure for the rear fuselage can now be added together with the sheeted sides, although before the sides went on I fitted some 3mm sheet over the section F4 to F5 so there would be plenty of thickness to allow shaping immediately behind the cowling. The 'medium hard' sheet sides need moderate care in fitting but will pinch in as required forward of the cockpit, especially if wetted on the outside surface..."

Curtiss Hawk 75 from RCM&E, June 1997.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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