August 17, 2018, ©. Leeham News: In the last Corner we outlined several challenges facing a supersonic airliner or business jet.
We will now go through these challenges one by one. We start with the aerodynamic challenge.
When an aircraft goes from subsonic to supersonic flight, the aerodynamic changes in a major way. We covered the changes in the Corner series on “Aircraft drag reduction”. The Transonic and Supersonic bits were discussed in Part 14 to 17. Here we are only recapping some main points.
In subsonic flight, the dominant drag types on an aircraft are Air friction drag, which is the drag due to the size of the aircraft, and Induced drag, which is the drag due to the weight of the aircraft.
For a Boeing Dreamliner, these drag types are typically around 10,000lbf each at cruise. The so-called drag coefficients are around 0.012 each, for a total drag of around 0.024.
Should we force the Dreamliner to fly supersonically, the fuselage alone would have an added drag of 0.1 or 80,000lbf. This is four times as much as the whole aircraft when flying at M0.85. To cruise the aircraft we then need two 50,000lbf engines instead of the two 10,000lbf engines of today (the developed thrust by the Dreamliner engines at a cruise altitude of 38,000ft).
Why is this?
Because we go from smooth subsonic aerodynamics, where the air molecules can move out-of-the-way of the approaching fuselage, warned by the pressure wave from the nose of the fuselage the aircraft is coming. This creates little air movement drag, called Form drag (Aircraft drag reduction Part 10).
When the nose of the fuselage is traveling at or over the speed of the air’s pressure wave, the air molecules can’t flow out-of-the-way anymore. We get a violent collision with the fuselage’s nose, much like when a curling stone hits another and both change their state and direction. We enter the region of “bounce aerodynamics”.
If our object hitting the air molecules has a blunt nose with a large diameter, there will be a lot of bouncing and much momentum loss for the object. The supersonic Volume Wave drag will be high.
The Volume wave drag describes how much added supersonic drag we create when we pass a body with a certain volume (like for a cabin) through the air at supersonic speed.
If we fly a fuselage the volume of the Dreamliner supersonically, but make it four meters in diameter instead of the Dreamliner’s six and compensate by doubling the fuselage length, the Volume Wave drag decreases nine times.
If we reduce the diameter to the three meters of the Concorde, we reduce the Volume Wave drag down to 3% of the original Dreamliner Wave drag, but now we need to double the length again to transport a 787 cabin volume.
We now understand why we shall make all volumes on a supersonic aircraft slim and long, with sharp and angled noses, just like the fuselage of the Concorde. Any extra diameter we want to use for cabin comfort will cost us dearly in Volume Wave or “bounce” drag.
Besides the Volume Wave drag we also add a Wave drag from the lift we create on the aircraft. We dive into this in the next Corner.