Accent on Power

By P G F Chinn. Originally published in 'Model Aircraft', December 1950.


Clean lines of "Yulupa's" fuselage are evident in this photograph. Note small air intake on top of the close fitting cowl to ensure adequate air reaching the engine.

With the adoption, at the beginning of the 1950 season, of the S.M.A.E. C/L sub- committee's very sensible "appearance points" rule, the days of rough-looking box-like stunters for serious contest work definitely appeared to be numbered. This has been confirmed by the results of many of the more important stunt contests this season, and all being well, next season should see the virtual elimination of the shoddy contraptions which, in the past. have often been tolerated under the much-abused description of "functional models".

Undoubtedly, British stunt model design has come a long way in quite a short time. Our progress has been far quicker than the Continental countries, and it is probably no exaggeration to suggest that we are, if anything, now slightly in advance of the U.S., particularly in tile case of under 5 c.c. engined models. In general, American stunt models have remained unaltered in design over the past two or three years and tend to heavier and bulkier construction than the latest British designs. Of course, from the commencement of C/L flying, the Americans showed a good deal more interest in speed than stunt, hence their great prowess at the former, probably to the detriment of the latter. A similar situation has developed in Continental Europe, where average 5 and 10 c.c. class speeds are higher than those normally reached by British speed models.

Another feature of the 1950 stunt season, has been the wide increase in the popularity of 5 c.c. class models. This, undoubtedly, has been largely due to the good, low-priced 5 c.c. glow-plug engines now available, as well as kits to suit them. Medium and large size models have much to commend them, being smoother in flight and more spectacular and several newcomers to stunt flying have told the writer that they find the larger machines easier to fly than the small jobs. Views probably differ on this point, however, and, since larger models, in general, are bound to be more vulnerable than smaller types, the latter are still the obvious choice for the raw beginner.

For contest work, a powerful 5 c.c. engine, permitting, as it does, a model of around 300 sq. in. wing area, seems to be a suitable choice. Among kit models of this group conforming to the modern stunt formula of a fast, streamlined, semi-scale layout are Henry Nicholls's Monitor and Musketeer, Bill Dean's Skystreak-40 and Brian Hewitt's Stunt Queen now being kitted by Keil-Kraft.

The latter model is particularly interesting inasmuch as it won the most important British aerobatics competition of 1950, the Gold Trophy at the Nationals, and represents an interpretation of modern stunt requirements by a designer who, hitherto, has favoured the better type of functional model: Hewitt, of course, having previously built up an impressive stunt record, including last year's Gold Trophy, with his earlier Stunt King. The writer found the model especially interesting also in view of a general similarity to his own 5 c.c. stunt model Yulupa, designed late in 1949. Neither of us knew of the existence of each other's models until after the Gold event, but it appears that thinking was along similar lines in regard to what was required of the model and of its general layout, the precise manner in which this would be obtained, however, differing by detailed design. Some general design data for the two models, which shows this, is given in Table I. The aerodynamic layout of the Stunt Queen is shown in the accompanying 3-view drawing and a 3-view of Yulupa was given in the July issue of 'Model Aircraft'.

Stunt Queen drawing

Both models, it will he noted, were designed around the Yulon 5 c.c. engine, an early 1949 "30" type (actually one of the lirst five of these engines built) in the case of Yulupa and a 1950 "29" model in the case of the Stunt Queen.

Table I. General Data

Stunt QueenYulupa
Designed byB. G. HewittP. G. F. Chinn
Built byB. G. HewittJ. I. Chinn
TypeMid-wing monoplaneLow-wing Monoplane
Area276 sq in. (gross: 295 sq. in. approx.)275 sq. in. (gross)
Span40 5 in.40.00 in.
PlanformTapered L.E. & T.E.Tapered L.E. & T.E.
Chord, root8.375 in.8.00 in.
tip6.375 in.6.00 in.
Aspect ratio5.735.82
SectionNACA 0016 (16%)Symmetrical 12.5%
Dihedral2 degrees3 degrees
Total area60 sq. in.68 sq. in.
= % wing area20.324.7
Elevator area22 sq. in.30 sq. in.
= % tail area36.744.1
Moment arm (C.G. to elevator hinge)18.25 in.14 in.
= X mean chord2.472
Maximum (emergency) elevator movement25 deg. up and down40 deg. up and down.
UndercarriageFixed, faired, 2 in. wheelsFixed. I.5 in. wheels.
EngineYulon 29 (4.87 c.c.).Yulon 30 (4.95 c.c.).
InstallationBeam. Side mounting.Beam. Inverted.
Fuel tankBalloonVented balloon.
Airscrew9 x 6 Truflo or StantOwn 9 x 6 or Stant.
SpinnerKeil 1.75 in. plasticKeil 1.625 in. alum screw.on.
Glow.PIugChampion V.G.3K.L.G. Miniglow.
Rigging data
C.G. (relative to mean chord)15% chord25% chord.
Pivot point (to mean chord)40% chord40% chord.
Line rakeNil2 deg. back.
Engine offset2 deg.Nil.
Fin offset2 deg. plus 10 deg. rudder.5 deg.
Outer wing weight1/2 ounceI ounce.
Wing10 oz./sq. ft.10.7 oz./sq. ft.
Power (approx.)3.5 lb./h.p.3.5 lb./h.p.
Overall length28.5 in.25 in.
Total weight20.5 oz.20.5 oz.
Lines used60 -70 ft. (Russell 0.0096 in. 4 strand)60-70 ft. (P.D.Q. or Light Laystrate).
Normal level flight speed65 m.p.h.68-70 m.p.h.
Aerobatic capabilitiesFull stunt scheduleFull stunt schedule

The "30" had previously given a good account of itself in an earlier stunt model, and there was little doubt that Yulons were remarkably well suited to stunt work : their ability to function evenly through C/L manoeuvres being a quality shared by few other motors of similar capacity, and no reason, therefore, was apparent for adopting another type. The engine had, of course, the added attraction of easy starting and light weight which would allow the greatest possible scope in design. At the same time the available power output was quite adequate. No doubt similar arguments influenced Brian Hewitt's choice of a Yulon for the Stunt Queen.

With the capabilities of the " 30" already known, wing area and weight were quickly decided. A speed of 65 m.p.h. on 60 ft. lines was being aimed at, with a permissible fair weather increase to 70 ft. lines. Since a fixed undercarriage was desired, together with slight line rake and rudder oflset, both features which would increase drag and decrease speed, an area of about 275 sq. in. was decided upon. This was also dictated by the requirements of structural strength. Allowing 70 - 75 per cent, total weight for the finished airframe, an all-up weight of 20 - 22 oz. was indicated for a finished airframe weight of about 15 oz., a reasonable figure for adequate strength in a model of this size.

For a gross area of 275 sq. in. and an aspect ratio not exceeding 6 (a fair upper limit figure for stunt models) the span and root- and tip-chord conveniently worked out to round figures of 40 in., 8 in. and 6 in. The slight dihedral was incorporated mainly from the point of view of appearance. The tail moment it was decided to keep reasonably short without being excessively so.

A practice which has grown up with stunt models is that of measuring the tail moment from the trailing edge of the wing, at the root, to the leading-edge of the tailplane at the root, but with the great variation in planform shapes of wings and tails - some tapering sharply on wing trailing-edges and tailplane leading-edges, this method of measurement affords no accurate standard on which to base comparative dimensions. A better, yet equally simple, system is, in the writer's opinion, to measure the moment arm from the c.g. (from which all moments should in any case he calculated) to the hinge-line of the tailplane. For the moderate to short tail moments currently favoured for stunt models, the figure will then come out at between 1.5 and 2.5 times mean wing chord. For Yulupa, 2 X mean chord was chosen, i.e., 14 in.

A tailplane of planform shape to match the mainplane was laid out, area being 68 sq. in., or a fraction under 25 per cent. of the wing area. The elevator area is 44 per cent. of the total tailplane area, or 30 sq. in. No attempt was made to dispose the surfaces symmetrically, the wing being I in. below the thrustline, while the tailpiane is placed 3/4 in. above it.

Inevitably, some modifications to the original design were carried out during construction. Principal among these was the elimination of the manoeuvring flaps which, at one stage, it had been decided to try. Another alteration which the builder saw fit to make was the designer's fin shape, for which a nice " Bill-Deanish " outline (albeit somewhat spoilt by an offset rudder) was apparently deemed preferable.

No rush job, the paper work was started on at the end of last season and the model itself finally emerged in May this year.

Brian Hewitt, on the other hand, wasted no time over the design and construction of the Stunt Queen. Having been persuaded, a bare three weeks belore the event was due to take place, to have another crack at the Gold Trophy this year, Brian took three days to get his design on the drawing-board and scheduled his construction over a fortnight.. Thus it was that he had only one or two test hops before taking the model to York, while the engine had not been run-in at all.

Table II. Structural Data

Stunt QueenYulupa
FuselageSheet sides. Upper deeking planked 1/8 in. x 1/4 in. Two plywood nose formers. Whole cowl draws off forward when prop is removed. Retained by two bicycle spoke ends and nipples.Semi "crutch" basic structure. Plywood nose formers. 1/16 in. balsa planking. Sheet bottom aft of wing. Detachable upper cowling over engine and tank compartment.
Engine bearersAsh. 4.5 in. long, into both ply formers. Formers and bearers plastic wood filleted to each other and to fuselage sides.Ash. 8 in. long, extending well back above wing to strengthen fuselage.
MainplaneFixed. 1/16 in. sheet leading-edge covering ribs every 1.8 in. Bellcrank in centre section.Fixed. Two tapered spars. Ribs every 1.75 in. Bell-crank in centre section. Silk covering.
Tail unitSolid 1/8 in. hard sheet balsa. Split elevators with control horn enclosed within fuselage. Nylon hinges.Solid 1/8 in. hard sheet bales. One-piece elevator with external control horn on lower surface. Nylon hinges. Tissue covered.
Undercarriage12 s.w.g. steel wire. Bound to second ply bulkhead. Faired with 1/8 in. sheet balsa attached with cellulose tape. Celluloid wheels.12 s.w.g. steel wire. Bound to engine bearers. Rubber tyred aluminium hub wheels.
Control systemEnclosed. Flexible lead outs on original with paxolin bell-crank. 14 s.w.g. push rod in two parts. Piano wire elevator horn.Enclosed. 20 s.w.g. lead outs in wing. 14 s.w.g. push rod. Duralumin bellcrank and elevator horn.

On the morning of G-Day, while attempting to become better acquainted with the new machine, he had the unenviable experience of the "up" control lead-out tearing out of the paxolin bellcrank he was then using. It says a good deal both for Hewitt's flying skill and for the handling qualities of his model that he was able to bring it down, in a succession of outside loops, with no damage other than to the propeller, and then, despite such a discouraging start, to go on and win the trophy and by quite a useful margin - although he, himself, dismisses his performance as a " terrible flight"!

Brian told me a few years ago that this event actually occured with his lesser known Stunt Prince and not with the Stunt Queen. DD

The general similarity in appearance of the Stunt Queen and Yulupa are obvious from the photographs. The Stunt Queen, however, with its longer tail moment, smaller tail surfaces and bigger fuselage does suggest better scale proportions. It is, of course, a slightly bigger aeroplane all round, although Brian Hewitt succeeded in keeping the weight of the original model remarkably low - only 20.5 oz. - precisely the same as that of Yulupa. The layout of the two machines differ in that the Stunt Queen's horizontal surfaces are located on the centreline of the fuselage, for similar flight characteristics upright or inverted, whereas, on Yulupa, the wing and tail are located helow and above the centreline respectively. This latter arrangement has no noticeable effect on the inverted flight performance, however; the model handles equally well through manoeuvres from the inverted and will maintain an absolutely level course in either attitude.

The popular side-mounted engine installation is used for the Stunt Queen. The "sidewinder" has become almost universal in Britain, but although originating in the United States, this type of mounting is less popular both in America and Continental Europe, upright installations being more common.. Almost unheard of, however, are stunt models with inverted engines, and it was not without some protest from the pilot that the designer determined to use the Yulon "30" inverted in Yulupa. The original intention had been to entirely enclose the engine within a "beard" cowling but with such a short fuselage, this would have given a rather bulky appearance to the nose and so a compromise was eventually reached and the cylinder-head left exposed.

Apart from better appearance, the inverted layout did offer certain practical advantages with this particular design. Firstly, the engine bearers could be extended well back over the top of the wing to strengthen the fuselage at this point and thus permit the whole nose cowling above the centre-line to be made detachable. This completely solved the balloon tank location problem as the balloon would then lie in the ideal position, immediately behind the engine and properly lined up with the jet. Accessibility to both tank and needle-valve was then excellent, while the control stem of the latter was not in the usual vulnerable position on top of the fuselage. The objection to the vertical engine on grounds of vulnerability did not arise as, with the fixed u/c (conveniently anchored to the engine bearers) the cylinder is adequately protected.

Yulupa engine installation

Removable cowling on "Yulupa" gives easy access to engine needle-valve and tank. Balloon type stunt tank is housed in compartment behind engine. Glow-plug is wired to socket between U/C legs.

With regard to ihe functioning of the" 30" in the air, this has proved satisfactory, and only when one is a little too nonchalant in one's starting drill and drowns the glow-plug with an oversize prime, are the Yulon's quick starting qualities impaired.

Both models use balloon tanks. For Yulupa, the "vented" type is used to simplify re-filling. This was first tried out on an earlier model, and was later described in the June issue of 'Model Aircraft'.

Of the machines, the Stunt Queen appears to be the slightly more lobust. Actually, with the more complex structure, and higher flying speed, of the modern streamlined semi-scale stunt model, as opposed to earlier box-car and profile types, more serious damage in crashes is bound to result if wing loadings are kept at similar desirable levels. The modern contest Stunter, therefore, is not necessarily the model on which one learns to do vertical eights and then goes on to win a national contest. Better to knock up a fully acrobatic "trainer" and then to build the semi-scale job for the Gold... or, if you prefer to have decent appearance to start with, to resign yourself to building a duplicate for serious contest work. It is not entirely a matter of the semiscale job being the weaker, as of the scars acquired by any model used for training which may reduce its chances of high "appearance" points in any important stunt contest.

The Stunt Queen wing uses a thick section (NACA. 0016), with the leading-edge sheeted top and bottom, which must obviously provide a stiff structure. Yulupa has a section of moderate thickness (12.5 per cent.) with two spars running through the ribs and a certain amount of reliance is placed in the doped silk covering to provide adequate rigidity. Both models have lead-out wires running through the ribs and belleranks are mounted in the centre-section.

Both fuselages feature balsa planked and sheeted covering over ply and balsa formers. In the Stunt Queen, two ply formers are used in the nose section immediately in front of the wing leading-edge and these support the engine bearers and undercarriage. A short one-piece cowl carved from block balsa is fitted ahead of this around the engine crankcase. Plastic wood fillers are used to strengthen the various ply to balsa joints; 1/8 in. planking is used. Yulupa's fuselage features lengthened engine bearers extending well back over the wing, as already mentioned, with two plywood nose formers and the remaining formers assembled on two shaped balsa longitudinals extending to the tail which comprise the fuselage sides below the centreline. Above the centreline the fuselage is planked with 1/16 in. hard balsa.

Respective rigging is interesting. Both models have a 40 per cent pivot location, although c.g. locations are different, Yulupa balancing closer to the pivot at 25 per cent. Fin offsets are used and are approximately equal, but while the Stunt Queen has a 2 deg. engine offset, Yulupa uses 2 deg. line rake. Outer wing balance weights are, of course, used. Sum total of these features suggests that the Stunt Queen will pull outwards a little more under power, and, also, being the very slightly larger machine, will be slightly slower, assuming the power output of the two engines to be similar. This appears to be the case. Brian Hewitt quotes the speed of his model as 65 m.p.h., while Yulupa has returned figures ranging from 64 to 70 m.p.h. on various occasions using Truflo, Truflex and Stant 9 x 8, Stant 9 x 6, 9 x 5, and a homemade 9 x 8. The finer pitch propellers are definitely the best for stunting - the note of the engine hardly changes from that in level flight - and, with these, the Yulon must be approaching very near to its peak output in the air.

The Stunt Queen is an undoubted advance in the pursuit of more scalish stunt models in that it closely approaches scale proportions in tail moment length and tail surface areas, while remaining fully capable of all stunt manoeuvres.

In a later article, it is hoped to publish some impressions of precisely how the handling characteristics of the two models differ in the air. It has often been said that "anything" will fly on lines and that almost anything will stunt given enough power and elevator movement. There is a grain of truth here inasmuch as a good engine can "make" a stunt model, while the stability problems associated with free-flight models are happily non-existent.

But that is not to say that stunt model design presents no problems of its own, or that study of these problems cannot be repaid in superior models. The difference between a good and poor stunter can be likened to that between a thoroughbred sports car and a family saloon. Both model and car can be lively yet a pleasure to handle, willing to go where they are directed with precise accuracy, yet safe and manageable in an emergency. And while it is probable that no more than 5 per cent. of those who take up stunt flying can ever hope to achieve national contest standard, the pleasure to be had from handling a thoroughly good model is undoubtedly well worth the effort put into its design and construction.

A very interesting article, but I love the "I thought of it first" attitude. DD

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