Simviation Forum - Theory of Flight

	Search the archive:

Simviation Main Site | Site Search | Upload Images

Simviation Forum › Real World › Real Aviation › Theory of Flight

(Moderators: Mitch., Fly2e, ozzy72, beaky, Clipper, JBaymore, Bob70, BigTruck)

‹ Previous Topic | Next Topic ›

Pages: 1

Theory of Flight (Read 351 times)

Nov 7^th, 2009 at 12:47pm

Bubblehead Offline
Colonel
Clear the bridge
San Diego, California USA

Gender:
Posts: 696

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.

IP Logged

Reply #1 - Nov 7^th, 2009 at 1:15pm

chornedsnorkack Offline
Colonel
I love YaBB 1G - SP1!

Posts: 363

Bubblehead wrote on Nov 7^th, 2009 at 12:47pm:

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

IP Logged

Reply #2 - Nov 7^th, 2009 at 8:42pm

olderndirt Offline
Colonel
Flying is PFM
Rochester, WA

Gender:
Posts: 3574

chornedsnorkack wrote on Nov 7^th, 2009 at 1:15pm:

On propeller planes, drag remains constant

Think I've flown a few of these

. Kidding aside, an interesting answer.

THIS IS NOT A PANAM CLIPPER

IP Logged

Reply #3 - Nov 8^th, 2009 at 7:27am

beaky Offline
Global Moderator
Uhhhh.... yup!
Newark, NJ USA

Gender:
Posts: 14187

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.

IP Logged

Reply #4 - Nov 8^th, 2009 at 2:13pm

C Offline
Colonel
Earth

Posts: 13144

beaky wrote on Nov 8^th, 2009 at 7:27am:

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.

IP Logged

Reply #5 - Nov 9^th, 2009 at 8:58am

BSW727 Offline
Colonel
Please upload all images
to SimV.
Inside a Boeing 727

Gender:
Posts: 202

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.

IP Logged

Pages: 1

‹ Previous Topic | Next Topic ›

« Home

‹ Board

Top of this page