October 19, 2018, ©. Leeham News: In the last Corner we discussed the temperature challenges an SST engine faces.
Now we address an even larger problem for SST engines, the takeoff and landing noise.
A jet or turbofan engine have two noise sources. The exhaust and the intake areas.
The intake noise comes from the fan’s fan blades. At takeoff thrust fan RPM the fan blades’ tips rotate with supersonic speed. This creates shock waves and it’s these shock waves we hear as a see-saw sound when an airliner start the takeoff.
The inlet sound is dampened by perforating the inlets and putting dampening materials in the cavity behind the perforations, like an exhaust silencer for a car. The inlet sound is not the main problem for an SST engine. It’s the exhaust sound.
One talk about high bypass ratio engines as being the way to a low noise. It’s not the bypass ratio in itself which gives the low noise, it’s the resulting lower overspeed of the air out the back to produce a certain thrust level. The engine speak name for the overspeed is specific thrust.
An engine’s thrust has two components, airflow and specific thrust (the overspeed of the airflow out the back relative to the aircraft). If we increase the airflow we decrease the specific thrust for the same thrust level.
The noise from the back of the engine is created when the jet from the engine hits surrounding air. This creates strong turbulence which creates pressure waves = sound. The faster the jet, the stronger the turbulence and by it the sound.
This sound has a lower frequency than the intake sound. It, therefore, travels longer and propagates around objects easier (dampening or sound walls are less effective).
The way to dampen this sound from an engine is to lower the specific thrust. Modern engines like the Pratt & Whitney GTF and CFM Leap have a specific thrust below 300m/s for their highest thrust variants at takeoff thrust.
Yesterday’s medium bypass turbofans had a specific thrust of 400m/s or more and Concorde’s specific thrust at takeoff was 900m/s.
If we lower the specific thrust to lower noise we must increase the engine’s airflow to keep the same thrust level. But we have seen increased airflow increases Ram drag (SST Corner 7).
A heavier chunk of air must be accelerated to within 0.5 Mach of the aircraft speed, then braked to zero and accelerated in the other direction to produced thrust (all relative to surrounding air. We observe an air molecule traveling through the engine).
The increase in airflow is done by increasing the bypass of air around the core. A higher bypass ratio increases the diameter of the engine for the same thrust. At high speed, a low specific thrust engine needs to increase the airflow to the level where the engine diameter creates high volume wave drag (SST Corner 2). The engine weight increases as well.
GE has made a clever move for the Affinity engine for Aerion’s AS2 (Figure 1). The engine has a static full throttle specific thrust of around 500m/s but this thrust level is not needed for takeoff. The engine’s maximum thrust is needed when the AS2 shall pass the sound barrier at 40,000ft.
At takeoff, the engine is throttled to a specific thrust of around 350m/s, as more thrust is not needed. By it, the AS2 can comply with the more rigorous Stage 5 takeoff and landing noise regulations which comes in effect by 2020.
In the next Corner, we will look at key data for different engines for SSTs. We will examine engines with different specific thrust levels and look at how Ram drag versus Net thrust varies as we vary speed and what happens with engine diameters and engine weight.