Two Swedish Air Force aerobatic teams, ACRO Hunters and ACRO Deltas, delighted airshow spectators across the country during the 1950s and 1960s, as Jan Forsgren recounts
The Swedish Flygvapnet (air force) has a proud history of aerobatic display flying. Since 1976, Team 60 has been the air arm’s official demonstration team. Flying six SK 60 (SAAB 105) twin-engine jet trainers, Team 60 has appeared at numerous airshows across Sweden and Europe. However, prior to this outfit, perhaps the most acclaimed of all Flygvapnet teams were the ACRO Hunters (active between 1956-62) and ACRO Deltas (1964-66). But, before describing both, a condensed history of their predecessors is appropriate.
Non-regular airshow slot
Although formation aerobatics in Sweden had occasionally been featured during 1920s and 1930s airshows, particularly during the inauguration of Bromma airport in May 1936, the Flygvapnet had no dedicated display team. In 1939, a group of flight instructors from F 5 Ljungbyhed, where air force primary flight training took place, rehearsed a programme using Focke-Wulf Fw 44Js (designated Sk 12 in Sweden), which included inverted formation flying. The team was to make its public display debut at the Malmö airshow in September 1939, but the event was cancelled at the outbreak of war.
During the early to mid-1950s, many Flygvapnet air wings fielded formation aerobatic teams. Most were known by their leaders’ last name (usually the respective squadron leader), eg Fogde-Group and Haglind-Group. The usual mount of these teams was the SAAB J 29 ‘Tunnan’, but there was even a J 33 (DH Venom NF.51) group that performed a few night-time displays. A rare foreign excursion took place during the 1956 International Air Meeting at Dübendorf, Switzerland, when four Flygvapnet J 29s performed an acclaimed seven-minute display.
British style
With the Flygvapnet expanding rapidly, an order for 120 Hawker Hunter F.50s was placed in 1954. Designated J 34 (J for Jaktflygplan or fighter aircraft), the first airframe arrived in August 1955. The initial unit to convert to the British fighter was 3 Squadron of F 18 Wing.
In the spring of 1956, the F 18 Wing commander, Överste Erik Raab, ordered the formation of an aerobatic team to perform at the wing’s tenth anniversary airshow. It was initially known as the Lampell-Group, after its leader Sven Lampell. The year before, Lampell obtained first hand knowledge of the Hunter, having been seconded for six months to 98 Squadron RAF, then based at Jever, West Germany.
Two J 34s each from F 18’s 1 and 2 Squadrons were put at the disposal of the Lampell-Group, with flight training commencing in March-April 1956. The aircraft were fitted with smoke dispensers, with the diesel oil being fed from the modified ammunition compartment to the engine nozzle. Yellow and blue smoke – emulating the colours of the Swedish flag – could be produced by mixing various chemicals into the diesel oil; white smoke was pure diesel oil.
ACRO Deltas Flying Display
Formations
Square, Arrow, Column, Flank Immelmann turn Looping
1 Take-off in square formation
2 Passing right-hand turn
3 Immelmann right
4 Square roll
5 Immelmann left
6 Square looping
7 Immelmann right
8 Regroup from arrow using external roll
9 Immelmann left
10 Arrow looping
11 Immelmann right with regrouping to column and right flank
12 Flank looping
13 Roll with regrouping to square
14 Immelmann left
15 Split down
16 Landing
Roll
Roll with regrouping from flank to square Regrouping from square to arrow with external roll Immelmann with regrouping to column and flank Split-down
The Lampell-Group comprised Sven Lampell (1), Bruno Söderblom (2), Olle Linder (3) and Claes Croneborg (4). The first public display, at F 18 on June 1, 1956, was a resounding success, with the media commenting favourably on the team’s performance.
After flying several displays during the summer, Lampell decided a new name was required, to signify aerobatics as well as the name of the aircraft used. Thus ACRO Hunters was born. During its career, the ACRO Hunters would follow the British practice of performing the entire programme, including formation changes, in full view of the audience. Contemporary US display flying consisted mostly of high speeds, much noise and high G-loadings with square turns , with the aircraft not always in view of the spectators.
Having a Flygvapnet aerobatic team appearing at local airshows and other events across Sweden was a good way to ‘fly the flag’. Additionally, the recruitment potential, as well as popularising aviation to the public, was not lost on the air force high command. Since the J 34 Hunter was a foreign designed aircraft, contemporary regulations meant that ACRO Hunters were banned from appearing at airshows outside of neutral Sweden.
It was not uncommon for the team to perform up to three displays each day across the length of Sweden during the summer. According to Lampell, performing at F 10, located at Ängelholm in southern Sweden, and then flying to Luleå in the north, some 864 miles away, for a display later the same day was a practical demonstration of the high rate of mobility exhibited by the Flygvapnet. As a result of this hectic schedule, a semi-permanent group of three technicians was attached to the ACRO Hunters. Weather allowing, there were two alternative display routines:
• Cloud-free conditions, or with a cloud base not lower than 4,900ft
• In poor weather with a 2,300ft cloud base.
Each display began in the same way, with the quartet of J 34s in a line facing the crowd. The aircraft were fitted with cartridge starters, and a cloud of black smoke would burst from each aircraft simultaneously as the pilots pressed the starter buttons. Take-offs were always made in formation, which could be problematic on narrow and short runways. The display routine usually lasted between 10-12 minutes, with the J 34s carrying a fuel load of about 154 imp gal.
Finger four, one solo
A fifth J 34 was introduced in 1957, being flown by Björn Perback. During that year, Linder and Croneborg were replaced by Jan-Henrik Torselius and John Eriksson. The fifth aircraft used the same runway, but took off in the opposite direction to the group of four J 34s, before performing a roll. The quartet and solo performer then flew toward each other at an extremely low level and high speed in front of the crowd. Other manoeuvres included loops and Derry-turns (named after British test pilot John Derry; the turn involved making a 270° right-hand roll, followed by a sharp turn to the left) with the various formations including Box, Right Flank, Fork and Swan. The routine always ended with a Split Up in front of the audience, with the coloured smoke taking the shape of a flower.
In anticipation of the 1959 season, the display routine was tightened, making it more compact and with no ‘dead spots’. If the group of four J 34s was far off centre, the solo aircraft would grab the audience’s attention, and vice versa. One particular aspect of the solo was an eight-point roll.
Before the 1960 season, Claes Cronberg took over from Sven Lampell as leader and the team was reduced to four aircraft. By this time, the ACRO Hunters had become subordinated to 1 Squadron/F 18 Wing. For the team’s final displays in the summer of 1962, the J 34s were marked (by special permission from Flygvapnet HQ) with the distinctive ACRO Hunters logo, designed by J 34 pilot Bertil Skogsberg.
The final ACRO Hunters display took place at Malmslätt on September 1, 1962, to mark the 50th anniversary of Swedish military aviation. Several ACRO Hunters aircraft were subsequently transferred to F 9. Interestingly, the ACRO Hunters insignia remained on the J 34s for several months.
From Hunters to Drakens
With the J 34s replaced by the SAAB J 35B Draken, a new aerobatic display was formed in 1963. The team was called ACRO Deltas – a highly suitable name as it associated the team with both its predecessor as well as the double-delta J 35 Draken platform. The following year, after much discussion, the J 35Bs were adorned with highly distinctive markings, yet again the work of Bertil Skogsberg. With the supersonic J 35B having different speed characteristics than the J 34, it was possible to create a more powerful and noisy display. Led by Claes Jernow, the ACRO Deltas formation comprised four aircraft, with one solo spot usually being flown by Göran ‘Boris’ Bjuremalm. Smoke was not used, as this would have required an external tank, reducing performance as well as making the formation less tight.
Claes Jernow later wrote of his experiences in leading the ACRO Deltas: “To the right side on the ground, I can see our square shaped formation moving swiftly forward, with it immediately revealing if someone is dragging behind. ‘The shadow is good’, I say, also hoping that information will provide a release of tension with the others, what I am experiencing during every take-off – one of the most difficult moments during the display – has worked well.
“The right-hand turn shall be completed, so that from an altitude of 300m we can descend toward the spectators and pass in front of them at low level. During the descent, the speed increases to about 700km/h and, to avoid flying over the spectators and perhaps scare them, we must successively increase the G-loading during the turn. At 5g, in a sharp right turn, we present the square, with my aim being to fly as low and close to the crowd as I can.
“Peter and Inge are keeping a distance of 5m from me – the half wingspan of the Draken. Behind me is Hasse, flying close to number two and three. He cannot fly in the disturbed air right behind my aircraft, which would make his uncontrollable. He must keep lower, but not so low as to make the formation seem uneven. I start a banking movement as softly as possible in the disturbed air. Despite the knowledge that disturbances due to air pockets are not visible from the ground, it feels rather awkward to experience this shaking during the roll, considering the difficulties of the others. Things become significantly calmer when inverted – we have reached an altitude of 500m – and during the latter part of the roll, I reduce height to 100m. We leave this manoeuvre in a nice manner and I order: ‘Immelmann left – and then looping!’ I strive to make the pull-up as softly as possible. If I were to pull at the control column right now, it can very easily be seen that our places in the formation, particularly our respective height distance, is awkward. I concentrate to make a completely straight loop – a warped pull-up gives a tardy impression. We pull about 4g, and I feel it is just about right as I am pressed down in the seat. With our noses pointing straight up, I see only sky, with the feeling of disappearing upwards and away from the audience. If we had smoke generators, this is the moment where we would have used [them], to be more visible.
“When taxiing in, we hear ‘Boris’ [Bjuremalm] taking off in majestic solitude with a lit afterburner for his solo display. We catch glimpses of his aircraft flashing past in a rolling movement at extreme low level or in a vertical turn with white streaks after the wingtips, when he pulls around 7g. He keeps the afterburner lit during the whole display, ending by landing using only 500m of runway.”
During its brief existence, eight different J 35Bs were used by the ACRO Deltas: serial numbers 35223, ‘231, ‘232, ‘233, ‘251, ‘256, ‘263 and ‘266. Although the ACRO Hunters never performed outside of Sweden, the Deltas participated in the 1965 Paris Air Show, co-oordinated with the F 16 display team, flying J 35As. On this occasion, smoke was used to enhance the visual spectacle. In 1966, the ACRO Deltas disbanded, leaving behind thousands of impressed spectators. To keep the ACRO legend alive, a former Swiss Air Force Hunter F.58, now with the civil registration SE-DXM, is operated by the Swedish Air Force Historic Flight, painted in the ACRO Hunters colour scheme as 34033/G. One J 35B, 35221 (not one of the original ACRO aircraft) has been finished in the Deltas livery and is displayed at the Östra Vemmerlöv air museum in southern Sweden.
Thanks to Miro Patocka and Michael Sanz for their assistance in sourcing photographs.
