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FLIGHT International, 23 July 1970

Aerobatic technology

THE WORLD AEROBATICS CHAMPIONSHIPS began on schedule last Wednesday. July 15. The first programme, the known compulsory sequence, was scheduled to be completed on the following day. Practice had been a little interrupted by a slowmoving front and earlier by high winds but all pilots had an opportunity to practise, once in the local area and once over the airfield. At the time of going to press, preliminary results were not available but a glimpse of some of the competing aircraft revealed some interesting developments.

An unknown quantity at the World Aerobatic Championships is the Akrostar, a brand-new European competitions aircraft. It was designed by Arnold Wagner, the Swiss aerobatic champion, who is a Swissair captain, and uses a unique system of control interconnection between elevator, ailerons and flaps. The idea is to boost the lift coefficient in various flight conditions. When the stick is fully back, ailerons and flaps droop the flaps half the amount of the ailerons. This increases the wing camber and provides more lift when pulling positive g. Similarly, when the stick is fully forward, ailerons and flaps "droop up." With a completely symmetrical-section wing, lift available in inverted flight is therefore the same as in erect flight.

For turns, the flaps also move half the amount of the ailerons so that the aircraft effectively has full-span ailerons with the inner section (the flaps) constantly in the engine slipstream. Stirring the stick seems to make just about everything move but Wagner claims that no complex control linkage is required. A system of interconnected flaps was fitted by Mr Wagner to the KZ8 which he recently sold to Bob Mitchell. The idea was borrowed from the model aircraft world.

The Akrostar is fitted with a 220 h.p. Franklin engine and Wagner claims that although the aircraft stalls power-on at 65 k.p.h., it will continue climbing at full power at this speed (or perhaps "mush" upwards) at 700ft/min, 3.5m/sec, and still with lateral control. The measured thrust from the Hartzell constant-speed propeller is 1.0341b. 4.6kN, for an all-up aerobatic weight of 1.3231b, 600kg, which compares with the Zlin's 6601b, 2.9kN of thrust and all-up aerobatic weight of 2,0721b, 940kg. Wagner has also designed the engine's unique inverted wet-sump lubrication system. It is fed by two pumps and an extra sump has been fitted to cope with inverted flight.

The Akrostar has epoxy spars and ribs and is covered with plywood skinning. Operating limits are + and —8g with ultimate limits of + and — 12g. The epoxy construction means that fatigue is not a problem. In a normal competition sequence, Wagner reckons to pull +7 g and — 5g.

Wagner sought advice on the aerofoil from Professor Eppler, who designed the wing section for the Phoebus and SHK gliders. The design parameters were processed by the computer at Bolkow and the outcome was virtually identical to Eppler's predictions. The result is a wing of symmetrical section with no dihedral and slight sweepback and mounted at zero angle of incidence.

The aircraft was built by Wolf-Hirth GmbH . It was paid for mainly by Jo Hoessl, the German champion who is flying it at the world championships in addition to Wagner. The total cost is estimated as £15,000, of which £9,000 was taken up with building and the remainder in research and development.

FLIGHT International, 6 August 1970

"9.6m/sec gusting 9.7"
Random notes on the 6th World Aerobatic Championships

by the contest director, JOHN BLAKE*

ARNOLD WAGNER'S slightly cynical comment on the morning .met reports forms a reasonably apt title to any article written in the immediate post-contest period concerning Hullavington. Throughout the contest, low cloud and high winds plagued the organisers and although the rules cater for both contingencies—after a fashion—one or other condition, and frequently both, put an effective stop to contest flying on many days. A comparatively large number of contest days exhibited marginal conditions; cloud along the 1,000m line, more or less, and winds at or around 10m/sec. Understandably', the organisers viewed met conditions more optimistically than pilots, and Wagner's phrase sets the problem neatly.

The rules say that when the cloud base is below 1,050m or the wind strength is greater than 10m/sec flying may not take place, with the escape clause that programmes may be split, with the agreement of team managers, and flown in two halves, height being regained in the interval without penalty, if the cloud base is below 1,050m a.g.l. but above 650m a.g.l. Against the 10m/sec rule there is no exception.

The application of the break clause becomes progressively more difficult as the competition develops (literally); in the free programmes, no team manager with a pilot still in the running for a top place is going to concede the use of this two-edged weapon.

As it happened, the competition was decided after the third programme, a bitter disappointment to the Americans, who were dominating the competition as a team and who—after some tense and exhausting hours following Bob Hefendeen's engine failure—were robbed of the opportunity of seeing if Egorov could have met the former's powerful challenge for world championship. (Herendeen flew his fourth programme, Egorov did not, and the lack of ftyable weather to complete this programme left them resting on the totals for Programmes 1, 2 and 3, with the American less than ten points behind.) As the conclusion of the last programme could have resulted in an American as easily as in a Russian victory, one sympathises with their disappointment and applauds their sportsmanlike acceptance of the situation.

Final Results—Men

Place Name Country Total Score Comp

(3 sequences) No.

1 Egorov, I. USSR 12,801.875 44

2 Herendeen, R. US A 12,678.375 39

3 Hillard, C. USA 12,418.875 40

4 Wagner, A . Switzerland 12,222.5 38

5 Williams, N. GB 12,205.625 1

FLIGHT International, 6 August 1970

Eastern victors
but only just


But the greatest single achievement of the meet was probably the fourth place finish of Swissair pilot Arnold Wagner in his still-new Acrostar. Despite a minimum of testing and the usual prototype bugs, the nimble and powerful little monoplane outshone all the others in such manoeuvres as the vertical eight and vertical S which it seemed to do as easily as the Zlin does a roll. Several tentative orders were placed for the Acrostar during the meet and its limited production is expected to begin soon.

FLIGHT International, 17 August 1972


The Akrostar was much in evidence in its Mark 2 version, with the Federal German and Swiss teams. Designed entirely to win World Aerobatic Championships on the Aresti system, the Akrostar does not appear to be an easy aeroplane to handle; with practically no neutral moment of stability it seems to require constant attention and the German team in particular seemed to be having problems with one or two manoeuvres that the aeroplane itself was quite capable of flying.

In the hands of the Swiss team, however, it showed itself to be a potential dominator of future Championships, with Swiss pilots coming into fourth and eighth place and winnig third Team Prize behind the Americans and the Russians. As the Akrostar was originally conceived in Switzerland by Arnold Wagner, now no longer competing in World Championships (at any rate for the moment), but very much in evidence with the Swiss team at Salon, this was perhaps not very surprising.

Fünf ACROSTAR MkII am Salon de Provence