4 ms·

>> "The faster you go in an atmosphere the more the atmosphere resists." This is actually not even approximately true; like many aerodynamic phenomena, drag is

>> "The faster you go in an atmosphere the more the atmosphere resists." This is actually not even approximately true; like many aerodynamic phenomena, drag is super nonlinear in airspeed and not even monotonic, with a bunch of weird outcomes around mach 1. The first chapter of every aero textbook probably includes a variation of this famous chart [0], showing drag coefficient or sometimes drag force (estimated for some famous/classic airframe) as a function of mach number. This is why prolonged supercruise can exist for airplanes without giant comical fuel tanks etc. That said, your main point about CAD/physics definitely stands and is a good one; imho it would be unlikely to design and build e.g. a 10x better airplane than the Concorde today, even with all the fancy computers simulations etc. [0] https://www.researchgate.net/figure/Drag-coefficient-as-a-function-of-the-Mach-number_fig18_256292310 https://www.researchgate.net/figure/Drag-coefficient-as-a-fu...

deleted 4y ago
[deleted]
linkdink 4y ago
You're conflating drag with drag coefficient. The plot you linked doesn't say anything about drag. Drag is more strongly proportional to speed than drag coefficient. That type of proportionality analysis is what's usually in the first chapter of aero textbooks. Computers are great for marginal gains and saving labor costs, but they can't give a 10x improvement unless there's already a 9x improvement in materials and techniques. Small nit, but CAE is simulations, while CAD is just drafting.
Schroedingersat 4y ago
Drag is Cd * A * v^2 Even if your drag coeficcient goes down from 0.3 to 0.25, doubling your velocity will still more than triple your fuel consumption over constant distance. Then the tradeoffs in airframe to make it not fall out of the sky in the transonic regime, then the loss of ISP from much lower bypass engines, then the smaller cabins, then carrying the extra fuel... you can carry far fewer passengers. Sure your drag went down substantially from mk 0.98, but noone is proposing doing a transonic trip because that would be insane. You wind up using many times more energy per passenger, meaning you pretty much only have first class due to noone on a budget even considering paying 10x as much. Throw in the cost of SAF and you're looking at the price of a house per ticket in order to save 5 hours in the air from a total 15 hours travel/boarding/waiting time.
- credit_guy 4y ago
  > Drag is Cd * A * v^2 You are making two assumptions, one explicit and one implicit. That the cross-section area is constant (it’s not, it depends on the angle of attack, which is smaller at higher speeds) and that the air density is constant (it’s not, supersonic planes fly at much higher altitudes, where the air is thinner).
  - Schroedingersat 4y ago
    No. I was directly answering the assertion that the non-linearity of drag around the transonic regime somehow implied a supersonic craft would have lower drag than a subsonic one. The faster you move air to exert a force on yourself, the less energy efficient it is. KE=1/2 mv^2 and P=mv It is very simple physics. You're making a similarly dishonest equivocation by carefully trying to frame the problem as if it were a mystery and the problem hadn't been reduced to lift/drag ratios 70 years ago. Which have an easily calculable upper bound for supersonic flight, by the way. A supersonic craft has a much worse lift to drag ratio than a subsonic one. A supersonic craft even has a worse lift to drag ratio than the same supersonic craft at subsonic speed. You can't even optimize your craft for that high altitude high speed regime because it still has to fly at lower speed and altitude and make it through the sound barrier without getting destroyed. You will be using at least three times the fuel, so will need at least three times the ticket price. But that's not the only place a basic understanding of kinetic energy disqualifies the entire concept. Your engines/fans will also need at least double the effective exhaust velocity so their energy needs per unit of thrust will be doubled. With this we can predict that all other things being equal, a supersonic craft will use about 6-10x the fuel per passenger mile as a subsonic one. A 747 has a bit over double the range and 4-6 times the passenger count as a concorde, so the naive approach got us in the ballpark. Then for long haul flights (the only kind where supersonic will save time) existing airliners are almost half fuel by mass, which means you're in the regime of the rocket equation rather than having fuel scale linearly with distance. Then you can't offset business and first class with economy tickets and freight because noone is going to fly economy supersonic for 6-10x the cost or pay 6-10x the shipping for no perceptible time saving. With the previous in mind though, the supersonic craft is going to be limited to about 4 hours in the air before you start to lose passenger capacity to fuel. So it's not even great for long haul. In short. Unless fuel prices drop dramatically ticket prices for a new supersonic craft would be commensurable with a new apartment for time savings of around 4 hours or at best 30% of the total door to door time.

deleted 4y ago

[deleted]

linkdink 4y ago

You're conflating drag with drag coefficient. The plot you linked doesn't say anything about drag. Drag is more strongly proportional to speed than drag coefficient. That type of proportionality analysis is what's usually in the first chapter of aero textbooks. Computers are great for marginal gains and saving labor costs, but they can't give a 10x improvement unless there's already a 9x improvement in materials and techniques. Small nit, but CAE is simulations, while CAD is just drafting.

Schroedingersat 4y ago

Drag is Cd * A * v^2 Even if your drag coeficcient goes down from 0.3 to 0.25, doubling your velocity will still more than triple your fuel consumption over constant distance. Then the tradeoffs in airframe to make it not fall out of the sky in the transonic regime, then the loss of ISP from much lower bypass engines, then the smaller cabins, then carrying the extra fuel... you can carry far fewer passengers. Sure your drag went down substantially from mk 0.98, but noone is proposing doing a transonic trip because that would be insane. You wind up using many times more energy per passenger, meaning you pretty much only have first class due to noone on a budget even considering paying 10x as much. Throw in the cost of SAF and you're looking at the price of a house per ticket in order to save 5 hours in the air from a total 15 hours travel/boarding/waiting time.

credit_guy 4y ago
> Drag is Cd * A * v^2 You are making two assumptions, one explicit and one implicit. That the cross-section area is constant (it’s not, it depends on the angle of attack, which is smaller at higher speeds) and that the air density is constant (it’s not, supersonic planes fly at much higher altitudes, where the air is thinner).
- Schroedingersat 4y ago
  No. I was directly answering the assertion that the non-linearity of drag around the transonic regime somehow implied a supersonic craft would have lower drag than a subsonic one. The faster you move air to exert a force on yourself, the less energy efficient it is. KE=1/2 mv^2 and P=mv It is very simple physics. You're making a similarly dishonest equivocation by carefully trying to frame the problem as if it were a mystery and the problem hadn't been reduced to lift/drag ratios 70 years ago. Which have an easily calculable upper bound for supersonic flight, by the way. A supersonic craft has a much worse lift to drag ratio than a subsonic one. A supersonic craft even has a worse lift to drag ratio than the same supersonic craft at subsonic speed. You can't even optimize your craft for that high altitude high speed regime because it still has to fly at lower speed and altitude and make it through the sound barrier without getting destroyed. You will be using at least three times the fuel, so will need at least three times the ticket price. But that's not the only place a basic understanding of kinetic energy disqualifies the entire concept. Your engines/fans will also need at least double the effective exhaust velocity so their energy needs per unit of thrust will be doubled. With this we can predict that all other things being equal, a supersonic craft will use about 6-10x the fuel per passenger mile as a subsonic one. A 747 has a bit over double the range and 4-6 times the passenger count as a concorde, so the naive approach got us in the ballpark. Then for long haul flights (the only kind where supersonic will save time) existing airliners are almost half fuel by mass, which means you're in the regime of the rocket equation rather than having fuel scale linearly with distance. Then you can't offset business and first class with economy tickets and freight because noone is going to fly economy supersonic for 6-10x the cost or pay 6-10x the shipping for no perceptible time saving. With the previous in mind though, the supersonic craft is going to be limited to about 4 hours in the air before you start to lose passenger capacity to fuel. So it's not even great for long haul. In short. Unless fuel prices drop dramatically ticket prices for a new supersonic craft would be commensurable with a new apartment for time savings of around 4 hours or at best 30% of the total door to door time.