HS 91 Clou (oz12065)

About this Plan

Graupner HS 91 Clou. Radio control sailplane model.

Note plan shows wing layout at wingspan of 97 in, and also an alternative short aerobatic wing at 75 in.

Note plan also shows optional installation of nose-mounted Cox Tee Dee .051 engine.

Direct submission to Outerzone.

Update 06/04/2020: Added kit review from Radio Modeller, March 1968, thanks to RFJ.

Quote: "Review. Graupner's HS 91 CLOU. An established model now making a new name for itself.

The Graupner Clou is not by any means a new kit, having been introduced to the British market some three years ago, but seems only recently to have realised anything like its full potential here. Last season's contests showed it to be an outstanding aerobatic flier, as well as a delightful fair-weather 'floater', so we asked BERNARD WILDMAN to give us his appraisal of this interesting sailplane.

ON opening the box containing the embryo Clou, it is immediately obvious that this is no 'quickbuild' model. Stacks of balsa, all of medium to soft grade, await the attention of the enthusiatic builder, who has not succumbed to the 'build it today - fly it tomorrow' cult. The kit is probably unique in that plans and materials are included for building two sets of wings; one pair giving a span of 97 in and the other 75 in, to enable one to cope with varying conditions or requirements. So there are lots of ribs - all die-cut in mainly very light grade wood. Sheet balsa for the larger sections, such as fuselage panels, is clearly printed with outline shapes, and the smaller parts are die-cut. You will need to dig out your fret-saw to cut out the parts which are printed on 1/8 in 5-ply, however.

All parts are numbered in the order suggested for assembly and can readily be checked against the very detailed, finely drawn, two-sheet plans, which are annotated in German and English. Instruction booklets are also provided in these languages, the English version being quite successful in clearly presenting a practical guide to assembly.

Constructional pointers: Great care must be taken, when seating the ply cabin platform, not to distort the planform of the partly assembled fuselage, and constant checking against the plan is required - in spite of optimistic suggestions to the contrary in the illustrations! The fuselage went together nicely, however, with everything fitting according to plan, until coming to the ply and balsa panels against which the wing root ribs are butted. The printed positions for the front wing tube holes were approximately 1/16 in too low. It might be worth your while checking this point; mine was an early kit, and it could have been corrected in subsequent batches. I fitted the moulded plastic nose-piece supplied, but as this fractured on the model's very first landing, I would suggest it be replaced with a carved block, from the start.

In building the fin, it seems I had taken insufficient care in positioning the all-flying tailplane (sorry, I should have said 'stabilator'!) fairings as, after flying trim was achieved, the neutral position of the stabilator roots did not align; a point worth watching.

Wing construction presented no snags, apart from the time consumed by building two pairs of wings (a chore which is well rewarded later), making all those splices in the spars and in the sheeting of the large span wings. My only departure from plan was in not fretting out the inside ply ribs. There didn't seem much to hold the brass tubes in place, so I took extra care to roughen the outsides of the tubes and coat them, and everything in their immediate vicinity, with Araldite.

The all-moving stabilator is of symmetrical section and is built upon a cradle jig. There doesn't seem much point in disturbing the jig and, turning the plan over, as suggested, to build the left-hand panel. Since the section is symmetrical, one has only to assemble one panel with the tip upside-down, to produce a pair.

Finishing: After the complete structure had been given two coats of sanding sealer and rubbed down, the fuse-lage was covered with lightweight silk, clear doped, followed by a further three coats of sealer, with appropriate rubbing down between. All flying surfaces were covered with the white tissue supplied in the kit. I gave two coats of normal strength clear dope, but this caused some pulling-in of wingtips, and slight distortion in the areas of extra soft sheeting. One coat of clear dope followed by some non-tightening proofer would probably be a safer proposition. Finally, fuselage, fin and leading edges of wings received one coat of polyurethane paint.

Installation: I can't remember when I last had a model turn out nose heavy on completion! With RCS Comp 10 receiver and 500 Deac in the
nose, a little weight in the tail was needed to get the balance correct. In order to fit two RCS servos, I made a hatch in the top of the fuselage, immediately aft of the cabin, for access to the elevator servo, and my Deac charging socket. The rudder servo, receiver and Deac were all comfortably contained in the cabin compartment. All-up weight was 39-1/2 oz with the short wings, and 42 oz with the large span pair.

Flying: A few hand launches on our local playing field quickly established a safe neutral setting for the stabilator. The long, floating glide with the big wings fitted seemed particularly promising. First flights from the ridge were made in a wind strength of about 10-12 mph, with the short wings fitted, when the glide angle proved good, and general stability quite adequate.

After adjusting stabilator movement to 5/16 in each way, from neutral (measured at LE of root rib) and rudder to 1-1/8 in each way (measured at rearmost point on lower curve) some very lively flying was enjoyed. Inside loops could be very tight if desired, and I could get about three-quarters around outside loops, neatly rolling out with a touch of rudder before the speed dropped off too much. For really hairy aerobatics, subsequent flying has shown that stabilator movement may be increased to 1/2 in each way - but mind you don't tear the wings off! With this amount of movement, bunts are well within the model's scope - as demonstrated at the last Ivinghoe rally. The Clou flies inverted surprisingly well and will maintain a quite respectable glide path.

A really magnificent fair weather floater with the large span wings fitted, the Clou will soar in the lightest summer zephyr. It really looks quite majestic in the air when flown gently and smoothly - but make sure, when flying close in to the ridge, to maintain flying speed in the turns or an incipient spin may catch you unawares. The fact that this model will spin quite well with the large wings fitted is very helpful in getting it down safely out of thermal lift, without building up too much flying speed. Recovery requires only neutralising of controls - it's as easy as that. Elegant loops are easily achieved from a very gentle dive. Gentleness is encouraged by the manner in which the wings flex after clumsy manoeuvres!

The all-moving tailplane works very well, if care is taken to reduce backlash to a minimum and I am seriously thinking of using a similar system on future designs of my own. It certainly makes for a clean tailplane surface and gets the control-horn and linkage out of the airstream.

A plea to 'Beef-it-up' types: I think one should accept the Clou as a lightweight for gentle breezes - for therein lie its charms and virtues - and not add weight in an attempt to turn it into a rugged type model. Under suitable flying conditions for the design weight, it is adequately strong and is a really delightful performer. A small amount of fibreglass, sparingly applied, however, could just give that useful reinforcement, against the pilot-error thump. First, inside about the first 3 in of the nose, to make a good strong Deac compartment. Secondly, before adding the fuselage top decking, very thin fibreblass inside parts 33, 34, lapping the rear cabin former, brass tube carrying formers and down to flow of the fuselage. This will really lock the formers in place and also prevent shock fractures along the grain of the fuselage sides. But please leave it at that.

Summary: A very cleverly designed model, interesting to build - for those who really enjoy building - and must not be rushed. In flight, a really fine performer which is a joy to fly. After some six months' flying, I still feel this is a really first class model which, apart from giving many hours of flying pleasure, can more than hold its own in the contest field. In addition, it should be ideal for sheer thermal searing, with the Hi-start techniques now beginning to create so much interest."

Update 11/11/2024: Added instructions translated into English, thanks to Grant Kerambrun.

Supplementary file notes

Instructions. 24 pages, in German.
Review.

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