Theory of Flight

[Bubblehead]

During high altitude transit by a jetliner, how much power does the aircraft need to maintain altitude and speed considering that the air is thinner which means less lift but also less drag.

[chornedsnorkack]

During high altitude transit by a jetliner, how much power does the aircraft need to maintain altitude and speed considering that the air is thinner which means less lift but also less drag.

The lift is completely unchanged. For the simple reason that the lift necessarily equals weight (or else the plane could not sustain either level flight or steady climb).

What the plane does is increase true airspeed so that increase of true airspeed compensates for decrease of density. Indicated airspeed remains constant.

On propeller planes, drag remains constant on constant IAS climb, because the Mach numbers stay subsonic. When Mach number grows to nearsonic (which they do on jets), L/D degrades so drag increases (slightly on the subsonic jets).

But since power is the product of drag and true airspeed, the power increases both because of TAS increase and drag increase

[olderndirt]

On propeller planes, drag remains constant

Think I've flown a few of these

[beaky]

I think Bubblehead is asking about a throttle setting, or percentage of available engine power...?

If that's the question, the answer is: "It depends". It would vary from aircraft to aircraft.

[C]

I think Bubblehead is asking about a throttle setting, or percentage of available engine power...?

If that's the question, the answer is: "It depends". It would vary from aircraft to aircraft.

Indeed. It's made more complicated as the power settings aren't linear, they're exponential (so for example 75% N2/HP RPM doesn't produce 3/4 of the thrust produced at 100%, probably more like 20-30%, as it's not far off flight idle).

For the type I fly, we cruise, depending on weight and height at around 85-90% N2, but as Sean says, it varies from type to type.

[BSW727]

An average power setting for cruise at say, FL310 or FL320 at 155,000lbs in the 727 would be around 1.85 to 1.95 EPR. Or about 88% N1 +/-.

And it does depend...

The real advantage is the reduced fuel flow at cruise altitudes.