artsyman
Posts: 4516
Joined: Wed Feb 28, 2001 12:35 pm

Minimum Required Power To Maintain Altitude?

Sat Dec 06, 2003 2:23 pm

After many conversations about twins and their overpowered engines, I was wondering if in the case of inflight shutdown, what % of engine thrust is required to keep the aircraft at altitude. I am not talking having to maintain speeds of 500 knots, just to maintain altitude. I am also not talking about failures on take off, just for example a mid flight shutdown. We often hear of the added strain on an engine when it is the only one active engine. But if the goal is just maintaining altitude as opposed to altitude and speed, then is this still the case. I am aware that you would want to land asap, but if landing was not an option.

Take the 777 / 767 / 757 / a330 for example, could one engine keep the plane in the air at 30 %, 50%, 80% thrust ?.

Jeremy
 
XFSUgimpLB41X
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Joined: Fri Aug 25, 2000 1:18 am

RE: Minimum Required Power To Maintain Altitude?

Sat Dec 06, 2003 4:19 pm

In a jet...50% of your power is found above 80% N1....most of the time in cruise you are in the 90's or so. This varies from airplane to airplane. In the CRJ our correction factor for approach is about 5% increase N1 in single engine ops. Not too much. It's got excellent single engine perf...now for a light twin..thats a different story....
Chicks dig winglets.
 
artsyman
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Joined: Wed Feb 28, 2001 12:35 pm

RE: Minimum Required Power To Maintain Altitude?

Sat Dec 06, 2003 4:24 pm

Cruise has you at 500+ miles per hour, what if you were going at 200 miles per hour or whatever is just above the stall threshold, what power do you think that would require. I am surprised to hear that standard cruise is using 90% + of the available thrust. You'd know better than I

j
 
XFSUgimpLB41X
Posts: 3961
Joined: Fri Aug 25, 2000 1:18 am

RE: Minimum Required Power To Maintain Altitude?

Sat Dec 06, 2003 4:36 pm

In cruise we only indicate around 290 knots usually at .74 mach..which is around 470-500 knots true airspeed... Single engine max altitude depends on weight..we'll use around 210-225 knots (the equivalenth of Vyse in the CRJ..called Vt-variable with altitude and weight) indicated airspeed and drift down to that altitude.

Remember that is 90% RPM....not power. Turbine engines get most of their power at high compression rates.
Chicks dig winglets.
 
vikkyvik
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Joined: Thu Jul 31, 2003 1:58 pm

RE: High Wing Vs. Low Wing Aircraft

Sat Dec 06, 2003 5:04 pm

The equation for thrust available is simply:
TA = TSL*(densityalt/densitySL)
where TSL = max thrust available at sea level, densityalt = density at altitude, and densitySL = density at sea level
If one engine is out, you have half the thrust available.
The thrust required on a given flight is:
TR = weight/(L/D)
where L = lift and D = drag or:
TR = weight/(CL/CD)
where CL = lift coefficient and CD = drag coefficient
CL = weight/(0.5*densityalt*velocity^2*wingarea)
CD = profiledrag + (CL^2)/(pi*e*AR)
where e = wing efficiency and AR = aspect ratio = (wingspan^2)/wingarea
With all that, I think you should be able to figure out some sort of answer for this, though I haven't completely thought it through yet. Hope this helps...
~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
artsyman
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Joined: Wed Feb 28, 2001 12:35 pm

RE: Minimum Required Power To Maintain Altitude?

Sat Dec 06, 2003 5:26 pm

Vickyvic, If I could understand what you wrote, I probably wouldn't have needed to ask the question in the first place  Smile

Jeremy
 
vikkyvik
Posts: 11852
Joined: Thu Jul 31, 2003 1:58 pm

RE: High Wing Vs. Low Wing Aircraft

Sat Dec 06, 2003 7:53 pm

Artsyman,
Haha, I think I got way too complicated up there. I'll try and figure out a way to explain it without just throwing a bunch of equations that I haven't even sorted through in 2 years. Or maybe someone else can be more concise. I will however refer you to a recent discussion on a related topic...
http://www.airliners.net/discussions/tech_ops/read.main/73545/
Basically, the altitude limits the speeds at which an airplane can fly, whether it be due to lack of thrust or the "coffin corner" described in the above discussion. At a given altitude with a given thrust available, there is a range of speeds at which the airplane can fly, limited by excessive drag at too low or too high speeds.
~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
prebennorholm
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RE: Minimum Required Power To Maintain Altitude?

Mon Dec 08, 2003 8:38 am

Dear Artsyman, as you probably know any plane has to be able to take off losing one engine after V1 speed on the runway. That stunt - with flaps and gear down and with immense drag due to high angle of attack, and the requirement to climb - that requires a lot more power than any straight and level flight.

But it all depends on drag and altitude (air density).

A twin usually have a power to MTOW ratio between 3.0 and 3.5. On one engine that becomes 6 - 7.

In clean configuration at optimal speed an airliner will have a lift to drag ratio around 18.

So you see, a twin on one engine will have almost three times the power needed for level flight in clean configuration at sea level at at maximum weight.

But at, say, 35,000 feet a turbine engine will have lost roughly 75% of its power due to the thin air. So you wouldn't be ably to maintain such an altitude unless you were way below maximum weight.

Drag is at optimal speed very close to proportional to weight.

That explains in very layman's terms exactly what Vikkyvik explains in a much more professional way.

But there are more complications. In most twins with an engine shut down you have run out of redundancy on cabin pressurization, so you will in any case descend to 10,000 feet in order not to harm your pax in case of troubles with the lone surviving aircon pack. Down there any plane will have plenty of power for level flight with one engine shut down.

Happy landing, Preben Norholm
Always keep your number of landings equal to your number of take-offs
 
411A
Posts: 1788
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RE: Minimum Required Power To Maintain Altitude?

Mon Dec 08, 2003 12:36 pm

Three engine designs are a horse of a different color, for example...

L1011-100 (RB.211-22B), enroute weight 370,000 pounds.

One engine shutdown, max altitude (ISA+10) FL190.
Two engines shutdown, max altitude 8000 feet.

Driftdown is rather important for sectors with high terrain.

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