Aerobatic displays, be they solo, synchronised, or in formation, are usually the highlight of any airshow. The thick plumes of white smoke that trail behind the aircraft as they fly through their display routines does a huge amount to enhance the presentation.
Why display pilots trail smoke
Besides the visual aesthetic, the entire impact of the display is improved by the smoke that is being trailed. Spectators on the ground find it easier to spot the aircraft as they approach the display arena, and then to keep them in sight throughout the duration of their performances.
On many occasions there could be plenty of aircraft flying a complex routine in front of the crowd. The trails of smoke ensure that as the pilots fly their various manoeuvres, they are more readily able to see each other’s aircraft in the display “box”. Safety is therefore enhanced as the risk of a mid-air collision between the performers is minimised.
The typical smoke system
The smoke is produced by a system that starts with a smoke-oil tank. Pitts Specials, for example, carry about 20 litres of smoke-oil in an aluminium tank that is mounted close to the engine firewall on the floor of its front cockpit. This tank has to be very sturdy and well secured, as when it is full and high g aerobatic manoeuvres are performed, the forces on the tank could well exceed 100 kilograms.
The Harvards, with their 9 cylinder, 550 h.p. engines, need to carry between 40 and 50 litres of smoke oil. Its tank is mounted under the seat in the rear cockpit. When the tank is full and aerobatics are flown, the forces on the tank are enormous ….but then, the Harvard is a mighty strong aircraft!
On certain other aircraft types, the smoke tank might be built into the structure of the wing, or even the baggage compartment. Most tanks are fitted with a “flop tube” which is a length of flexible hosing that is weighted at its end and that can pivot up and down and left and right so as to reach every corner in the tank.
Under the effects of positive or negative g, and also knife edge flight, the tube will follow to where the smoke oil is being displaced. An uninterrupted and constant pick-up of the smoke oil will be continued.
Smoke oil or even diesel
The oil that is used is usually a paraffin-based mineral oil that should be both biodegradable and non-toxic. In South Africa we only have the words of assurance from the fuel representatives that their smoke-oil products are not harmful.
On odd and rare occasions, when aerobatic pilots have flown shows in remote areas, they have had to resort to using pure diesel in their smoke tanks. While this has worked very well and given huge plumes of smoke, it has turned out to cause a certain amount of breathing discomfort for the pilots and also to discolour the paint schemes on the aircraft.
An electric fuel pump is mounted close to the tank and moves the oil through pipes to nozzles that are connected to the engine’s exhaust pipes. As the oil makes contact with the hot pipes, it burns instantly and the high-visibility white smoke is created.
The volume of smoke oil pumped into the system needs to carefully metered to match the exhaust. Too little oil or a clogged nozzle will result in a weak or insipid plume. Too much of a flow will result in the tank running dry prematurely.
Activating the smoke system
The smoke system is activated by the pilot who has an On/Off switch in the cockpit. A soloist would manage the use of the smoke system on his or her own, switching it on for each manoeuvre and then switching it off on completion of the manoeuvre.
When formations are being flown, in order to have all the aircraft switching the smoke on and off at precisely the same time, it is the formation leader that will make the very famous call….”SMOKE ON…GO”!