September 14, 2018, ©. Leeham News: In the last Corner we looked at the change of aerodynamic center when passing Mach 1 and the resulting trim change.
We also discussed the high pitch angle of a delta like the Concorde when landing, which brings visibility problems for the pilots. Now we look into the problem of skin heating at high Mach flight.
When airliners fly at normal cruise altitudes and speed, the temperature of the aircraft skin is around -25 to -30° Celsius.
When we increase the speed to Mach 2.0, the speed of the Concorde, the temperature of the outside skin varies between 90°C to 105°C, dependent on the position on the aircraft. At the tip of the nose, the skin temperature is 127°C.
Should the Concorde have flown at the originally projected speed of Mach 2.2, like the Boom Supersonic’s project, the highest temperature increases to 150°C.
The problem is not only the high temperatures but the temperature cycle of close to 200°C. When climbing to the altitude for passing the sound barrier, the aircraft will be chilled to around -30° to -40°C. Then the surface will heat to over 100°C when cruising at 50,000 to 60,000 feet.
At these varying temperatures, the long-term fatigue characteristics of materials become critical. The Concorde lowered the speed with Mach 0.2 to stay with Aluminum alloys in the aircraft skin.
Boom Supersonic is planning on using high-temperature Composites to master the temperature challenges.
While there has been research into high-temperature Composites, the proof of their suitability for commercial transport with a structural life of 30,000 hours, exposed to such temperatures, will be a long and hard process.
The problem is that testing for long-term hazards for aeronautical materials can only be accelerated to a degree. Long-term testing takes…..a long time.
The real hurdle will not be candidate materials. It will be getting FAA acceptance and certification of a Supersonic airframe flying at Mach 2.2 for hours, every day of the week, and lasting 30,000 hours.
Aerion Supersonic’s SST avoids the problem by flying at a modest Mach 1.4. At those speeds, the skin heating stays below the effect of the Sun on a parked aircraft. The skin will be heated to 20°C.
In the next Corner, we will start the discussion about the SST’s biggest challenge, the engines and nacelles.