Concorde Air In-Take System « Heritage Concorde

Concorde Air In-Take System

Concorde G-BOAC air in-take and engine nacelle

A suitable engine is only the beginning of a supersonic Mach 2 powerplant. Because no jet engine can accept air in its compressors at supersonic speeds. It is therefore necessary to slow it down from Mach 2 to Mach 0.5 ( from about 1,350 mph to about 500 mph)before allowing it to enter the engine. This is done over the eleven foot length of the air in-takes. The air in-take is best described as a self-starting in-take, which means that the engine does not lock into a surge necessitating engine shut down. The Air Intake Control System must satisfy the basic requirements to supply the correct amount of air at high efficiency in a form acceptable to the engines at all flight and engine operating conditions. Experience on the prototype Concorde’s brought about a bold change from analogue to digital control late in the development programme, this decision was initially, vehemently opposed by the French.

Air in-take and engine

One of Concorde’s most interesting technical features is its variable geometry engine air in-takes. Their function is to ensure that the Olympus engines get the right amount of air moving at the correct speed through a wide variety of airspeeds. The air in-take is rectangular in cross section and is of variable geometry in that it embodies two moving ramps in the top surface, the ramps forward and aft, which do not meet, but move up and down to control airflow. There are also two smaller doors which either let in more air or spill it when it is not required by the engine. The moving parts of the air in-takes are operated hydraulically under computer control

Although all the in-takes look the same, they are in fact toed-in along the incident lines of entry flows. At supersonic speeds this produces the optimum entry conditions but at take-off with all engines rotating in the same direction, the effect on the blade vibrator modes is markedly different between engines. So much so that N1 (Engine speed relationship) engine 4 is automatically held down up to 60k, when it is released.

Above Mach 1.3 the ramps and spill doors are positioned between scheduled maximum and minimum angles determined by the intake pressure ratio, Mach numbers, engine speed N1, angle of attack and actual ramp/spill door positions. Changes in ramp angle alter the in-take of air and create a shockwave system that originates at the ramp angle and which converges on the lower lip of the intake when optimum configuration of the ramp is reached. Consequently, this is an external compression system and it is this shockwave system that slows down the air entering the in-take to Mach 0.5 as already mentioned. Once the scheduled ramp angle is reached any excess air is dumped through the spill door.

A.- TAKE-OFF

(See diagram 1& 2)

Diagram 1

During this time the engines require maximum airflow

The ramps are fully up, the auxiliary inlet vane wide is open, and this is held open aerodynamically. Once at Mach 0.93 the auxiliary inlet will be closed, and above Mach 1.3 the ramps come into play, lowering to form a series of shock waves starting from the bottom lip of the intake. This would have the effect of slowing the air down. By the time that Concorde has reached mach 2, the ramps would have moved over half their possible travel.

Concorde’s engine at the end of its air in-take

The auxiliary intel vane is vide open

B. – Noise abatement

(See diagram 2)

Once Concorde has taken off it enters the noise abatement procedure, the re-heats are turned off and power is reduced. The secondary nozzles are opened further to allow more air to enter, this now as the effect of quietening down the exhaust system. The Secondary air doors would also be open at this stage; this would allow air to bypass the engine. When Concorde is travelling at slow speeds all the air the air which goes into her engines is primary airflow, so they would keep the secondary doors closed. This would also stop the engine from ingesting any of its own exhaust gases

Diagram 2

C. – Supersonic Cruise

(See diagram 2)

At Mach 2 the air is not only slowed down, it is compressed and considerably raised in temperature. The compression is helpful because it means that the engines own compressors have less work to do, but the rise in temperature of about 200C leads to the necessity for special metals in the engine.

As Concorde flies along, it meets all the changes with air temperature and pressure which causes disturbances to the wave pattern in the in-takes. The computers sense these changes and make final adjustments to ramp positions to maintain the airflow required by Concorde’s engines. Equally, any changes in engine power settings would require some changes in the airflow. Concorde’s computers deal with this in the same way.

Each in-take, then, presents high-pressure hot air at Mach 0.5 to the first stage of the Olympus engines, which is itself a conventional two-spool turbojet fitted with an afterburner.

ENGINE FAILURE

If this had happened to Concorde at supersonic speeds, it could theoretically have caused a catastrophic failure of the airframe

The most difficult case of change in demand would be if an engine was to fail, or shut-down at high speed. Suddenly, the engine would require little or no air. So the ramps would go down fully, diverting some air over the top of the engine, while the spill door opens wide to pour air out of the underside. The speed of this operation would obviously critical. Concorde’s ability to deal with sort of problem is most impressive slam-closure of a throttle at Mach 2 makes Concorde react, but the engine doesn’t even hiccup.

This need suddenly to dump air produces the one odd flying characteristic of the aircraft, if an engine fails at supersonic speed, Concorde banks the wrong way. Any aircraft, Concorde included, will yaw towards the dead engine; thrust on that side has suddenly been lost, and the engine has become a producer of drag. As a result, the wing on the side opposite to the failed engine will temporarily move faster, gain lift and rise.

The combined effect is both roll and yaw towards the dead engine. But at Mach 2 there is all the excess air to get rid of. This spill door opens and the in-take air is deflected downward. This causes the wing to rise and Concorde to bank away from the dead engine. The technique for dealing with this situation is first to level the wings, then to counter the yaw with the rudder. The auto-stabilizers will already have applied some rudder so it is not a difficult process for the pilot.