Heading photograph shows Dennis Allen's Super Cyclone powered stunt model taking off at the recent Round Pond television show put on by members of the West Essex club.
Whilst the popularity of the control-line movement in America was initially built up on speed and sport flying, the past two years has seen a remarkable swing over to stunt and aerobatic flying. Now the emphasis is on the latter and models of this type have been developed to a high degree of perfection.
Jim Walker's "Fireball" - the model which really started the control-line movement - was actually the first model to fly inverted and loop, although this was essentially a "sport" design. Then followed numerous other designs and gradually the range of possible manoeuvres grew. Certain basic requirements soon became established - the need for a symmetrical aerofoil section for inverted flying, for example. Yet the famous"Fireball" still held the stage. Fitted with a special symmetrical section wing and lightened to a degree it was the first model to perform manoeuvres from inverted flight position and to demonstrate square loops.
The first stunt models were almost invariably large and fitted with the most powerful medium-speed motor available. Slagle's 1946 American Nationals winner, with its 415 square inches of wing area powered by the Super Cyclone 10 cc. motor is typical, 'But more recently smaller and lighter models have appeared with (American) Class B motors which perform equally well and have generally proved capable of taking hard knocks with less serious damage to the airframe.
Crashes with stunt models are still pretty frequent - even amongst the experts. Things happen quickly with the model flying at anything between 50 and 70 m.p.h. and it is all too easy to do the wrong thing at some crucial point in an advanced manoeuvre.
The basic requirement for success is, of course, a model capable of performing the necessary manoeuvres. There are no hard and fast rules as to the design layent for such a model, apart from one or two generalisations. By far the best method in designing a new model is to work on data relating to previously successful models.
In spite of the fact that there are a considerable number of highly successful stunt models in America these data are not always easy to find, especially by the average model builder with no American contacts. Table I has therefore been carefully prepared to meet this requirement and covers most of the well-known American - and a few British - designs which are fully aerobatic,
Of these models listed, "Hot Rock" - winner of the 1947 American Nationals Stunt Event - "Green Dragon", "Super Zilch" and "Fireball" (stunt version) have proved that they can do every manoeuvre possible with a control-line model. Probably a good many of the others could, as well, but data is lacking on this subject. Hence the figures in Table I should provide a very useful guide for new designs.
For stunt work, of course, a powerful and reliable motor is absolutely essential. The motor must run smoothly throughout the flight in whatever attitude the model assumes. This necessitates a stunt tank as a standard fitment, these tanks being specially designed to combat both centrifugal force (tending to pile the fuel against one side of the tank) and provide constant fuel flow in different flight attitudes.
Given the model, and the right motor and tank combination, the remainder is up to the flier. The only golden rule to success is practice - and the more practice an individual gets in, the better flier be will become. Dave Slagle - a consistent winner, although only in mid-teen age - practised for several months, flying for an hour or more every day to perfect his stunt routine for the 1947 Nationals. On the other hand, given the right model success may come more quickly. Paul Bender won the Columbus Ohio stunt event with a "Super Zilch" (built from a kit) on the model's tenth flight-and the Ohio State Championship on its seventeenth flight.
Certain leading data is omitted from Table I and covered in later tables. This is because many of the American designs listed are kit jobs and to avoid infringement of the Jim Walker patents on control-line, details of control assembly, control plate layout, etc., are omitted from the plan.
Table II attempts to remedy this omission by listing "standard" control plate sizes.
|Type||a in.||c in.||d in.||y in.||Application||Examples|
|BAT||2||1/4 - 7/16||1/4 - 3/8||0 - 1/4||Most small control-line models 16-24 in. span.||Phantom, Wizard, Goblin, Stunter, Phantom Mite.|
|MERCURY||2||11/32 - 17/32||1/4 - 3/8||0 - 1/4||Most small controI-Iine models 16-24 in. span.||Phantom, Wizard, Goblin, Stunter, Phantom Mite.|
|MERCURY||2-3/4||5/8 - 15/16||5/16 - 1/2||0 - 1/2||Medium size models up to 6 - 7 in. chord.|
|AMERICAN JUNIOR||3-7/8||5/8 - 7/8||1/2 - 3/4||0 - 1-1/4||Fireball Kit|
|STANDARD AMERICAN LARGE TYPE||3||5/8 - 7/8||3/8 - 5/8||0 - 1||Most American stunt models||Dronette, Hot Rock, Slagle, Green Dragon. etc., etc.|
Table III gives loading data, which is very useful as a general guide in preliminary layout of a new design.
|Model||Wing Area, sq. in.||Weight, ozs.||ozs per sq. ft.||ozs per 100 sq. in.||Power Loading, ozs per c.c.|
Table IV gives rigging data on nine designs which can be linked up with similar models in Table I.
|Model||n in.||y' in.||Remarks||d in.|
|DRONETTE||-||1/2||"n" dimension not specifIed on plan||-|
|HOT ROCK||2||1/2 - 1||-||1/2|
|GREEN DRAGON||3-1/2||2-1/2 - 3||-||-|
|PLAYBOY||4||2||C.G. in front of front line.||3/4|
|FIREBALL||3-3/4||1-1/8 - 1-3/4||Pivot point located well aft. C.G. in front of front line.||1/2 - 3/4|
There remains but the picture, the actual outline shape of these various models. Almost invariably the wings are parallel chord with rounded tips. Fuselages are short and mainly based on crutch construction with sheet balsa sides, top and bottom. Tail surfaces are almost invariably cut from sheet balsa, linen tape hinges being common, although metal hinges are now coming into wider use. Linen or tape hinges tend to fray and tear under continual vibration.
The four general arrangement drawings chosen are those of outstanding models with many contest places to their credit. Leading dimensions are given and the outlines are accurate enough to be scaled, if required.
The Green Dragon has an outstanding reputation for manoeuvres from the inverted flight position and is typical of the larger type of model preferred by West Coast fliers.
Slagle's 1946 Nationals winner is typical of the large general-purpose stunt modej, although the modern trend is towards a cleaner design:
Both the "Dronette" and "Hot Rock" are typical of Eastern practice, and both models have an outstanding contest record. A Drone diesel is the power unit in each case.
The "Dronette" has a fully aerobatic range, although the wing is a little small in area and a little too thin in section for best possible performance from the Drone diesel. Tucker's model is essentially based on the "Dronette" and is a particularly nice model to fly.
Most of the models listed in Table I are bigger than those used in this country and are flown on 70 ft. lines as standard. The medium size models, like the "Dronette", "Hot Rock" and "Rookie" give their best all-round performance on 55-60 ft. lines. Long lines on a model with a high power loading tends to make a true wing-over a breath-taking job - particularly to the pilot - but are an advantage for looping and similar manoeuvres.
The following generalisations may he applied to the various data given in the tables.
|(i)||Reliable, powerful motor fitted with stunt tank.|
|(ii)||Symmetrical section aerofoil.|
|(iii)||All rigging angles zero, i.e., wings, tail and thrust line. Offset thrust is sometimes employed, but is not generally advised. Correct C.G. position and rudder offset should be sufficient to maintain taut lines.|
|(iv)||Steel lines are invariably used - .010 - .012 for medium size models; .012 - .015 for larger jobs.|
|(v)||The rear line is invariably the "up" or "climb" line - i.e., the control horn is mounted below the elevators.|
Whilst most of the designs detailed call for motors more powerful than those generally available in this country, scaled-down designs have proved quite successful. In fact, it has been recently proved that stunt flying is possible with small diesels of around 2 cc. capacity, although the actual flying may not be as spectacular or as smooth as that of the larger jobs with 6-10 cc. motors.