January 15, 2021, ©. Leeham News: In last week’s Corner, we looked at how hydrogen consumed in the rear fuselage tanks of Airbus’ ZEROe concept affect the airliner’s efficiency.
Now we look at other aspects of the rear placement of the tanks.
We look at what to consider when a hydrogen airliner has its tanks placed like Airbus’ ZEROe turbofan concept in Figure 1. It has two tanks placed in the rear (two for safety reasons), Figure 2.
We could see as the Center of Gravity (CG) shifts during a flight when the Liquid Hydrogen (LH2) is consumed, it causes an efficiency degradation by 1.4% on the maximum range flight of 2,000nm and 0.5% for the typical flight of 800nm.
Now we look at other effects. How large is the CG shift from a flight? Will it run the aircraft into CG range problems?
An airliner has an acceptable range for the Center of Gravity. It’s determined in the design process and verified in flight tests. It’s about the authority of the horizontal tail at low-speed maneuvers like rotation, lift-off, and landing. The A320 range is how in Figure 3.
As a test, let’s assume this diagram applies to our airliner as well. We have an initial CG where the red line starts to the right and now calculate how much of the Mean Aerodynamic Chord (MAC) Center of Gravity range is eaten up by our LH2 consumption during the flight.
For the maximum range flight of 2,000nm, the increase in the nose-down moment was 340,000nm. A moment is force time distance. To get the CG travel of the aircraft, we divide the moment with the aircraft’s gravitational force which is 61.7t times the gravity constant, g = 605kN/136klbf.
The CG travels 0.56m forward for the 2,000nm flight. For the 800nm flight, the moment is 154,500Nm, and the CG travel is 0.25m. Is this change of CG acceptable? To understand it, we must understand how many % of Mean Aerodynamic Chrod CG travel the displacements represent.
The Mean Aerodynamic Chord of an A320 is 4.2m. Thus, the maximum range flight moves the CG 13% and the typical flight 6%.
A 13% move of the CG forward during flight is not ideal (it’s about the length of the red line in Figure 3), but it can be handled. But a hydrogen airliner with the tanks in the rear will have a narrower passenger and cargo loading CG range than today’s A320/737.
As you prepare for a maximum range flight you have to include a forward movement of the CG with 13% during the flight. Your allowed CG moves back 13% compared to an airliner that doesn’t shift the CG during flight. As the rear limits remain your CG range shrinks 13%.
The CG shift of 6% is less problematic, but once again, it shall be included in the pre-flight analysis.
A hydrogen airliner with a tank arrangement like Figure 2 can include means to compensate for the CG shift. One possibility is to shift masses around as Airbus does with the tailplane tanks for the A330. You can also move the Aerodynamic Center of the aircraft.
We look at some of these methods in the next Corner.