4 ms·

Air resistance goes up by the square of the velocity, so it takes more and more thrust to go faster. The most efficient thrust per unit of speed is therefore so

by ses1984 7y ago

Air resistance goes up by the square of the velocity, so it takes more and more thrust to go faster. The most efficient thrust per unit of speed is therefore somewhat less than the most efficient thrust per unit of fuel. I've only had some college physics and no aeronautical engineering so I could be way off. Of course there are other factors like the amount of lift per unit of velocity and so on...

CPLX 7y ago
> Air resistance goes up by the square of the velocity But you measure speed in this context relative to the air, not the ground. The fact that the plane is now moving across land much much quicker due to winds aloft is completely irrelevant from an aerodynamics perspective. The plane doesn’t even “know” that it’s getting there sooner.
- ses1984 7y ago
  I'm strictly arguing with the point about engines being efficient at 90% thrust which is independent of wind.
danaliv 7y ago
> Air resistance goes up by the square of the velocity This is true of form drag. Airplanes are also subject to a somewhat counterintuitive induced drag that is inversely proportional to airspeed. The minimum total drag is therefore somewhere between a slow speed and a fast speed. There are three optimal speeds depending on what you're trying to optimize. Maximum range (distance per unit of fuel) is best glide speed, which would be a painfully slow way to get somewhere. Maximum endurance (time per unit of fuel) is roughly max endurance divided by 1.316—even slower. "Optimum cruise," or Carson's speed (max speed per unit of fuel) is roughly max endurance times 1.316.
- ses1984 7y ago
  What's the relationship of these values to the efficiency curve of the engines? I'm pretty sure best glide speed times 1.3 can be achieved with less than 90% thrust from the engines of a typical commercial airliner.
  - danaliv 7y ago
    No idea. :)
glennpratt 7y ago
> the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph https://www.wired.com/2012/10/can-we-build-a-more-efficient-airplane-not-really-says-physics/ https://www.wired.com/2012/10/can-we-build-a-more-efficient-...
- danielmg 7y ago
  laden or unladen?
eximius 7y ago
If you have a tail wind, you face far less resistance.
- civil_engineer 7y ago
  Negative. The aircraft doesn’t know it’s flying with a tailwind. The onboard GPS and the human bladders know.
Scoundreller 7y ago
But more speed = more lift, so you can cruise through thinner air at higher altitudes. It’s still less efficient, but some of that squaring works to your advantage.

CPLX 7y ago

> Air resistance goes up by the square of the velocity But you measure speed in this context relative to the air, not the ground. The fact that the plane is now moving across land much much quicker due to winds aloft is completely irrelevant from an aerodynamics perspective. The plane doesn’t even “know” that it’s getting there sooner.

ses1984 7y ago
I'm strictly arguing with the point about engines being efficient at 90% thrust which is independent of wind.

danaliv 7y ago

> Air resistance goes up by the square of the velocity This is true of form drag. Airplanes are also subject to a somewhat counterintuitive induced drag that is inversely proportional to airspeed. The minimum total drag is therefore somewhere between a slow speed and a fast speed. There are three optimal speeds depending on what you're trying to optimize. Maximum range (distance per unit of fuel) is best glide speed, which would be a painfully slow way to get somewhere. Maximum endurance (time per unit of fuel) is roughly max endurance divided by 1.316—even slower. "Optimum cruise," or Carson's speed (max speed per unit of fuel) is roughly max endurance times 1.316.

ses1984 7y ago
What's the relationship of these values to the efficiency curve of the engines? I'm pretty sure best glide speed times 1.3 can be achieved with less than 90% thrust from the engines of a typical commercial airliner.
- danaliv 7y ago
  No idea. :)

glennpratt 7y ago

> the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph https://www.wired.com/2012/10/can-we-build-a-more-efficient-airplane-not-really-says-physics/ https://www.wired.com/2012/10/can-we-build-a-more-efficient-...

danielmg 7y ago
laden or unladen?

eximius 7y ago

If you have a tail wind, you face far less resistance.

civil_engineer 7y ago
Negative. The aircraft doesn’t know it’s flying with a tailwind. The onboard GPS and the human bladders know.

Scoundreller 7y ago

But more speed = more lift, so you can cruise through thinner air at higher altitudes. It’s still less efficient, but some of that squaring works to your advantage.