iffy
Topic Author
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question regarding speed at altitude

Mon Nov 14, 2016 9:56 pm

Hello forum users,

This is my first post on this forum and hope to post more often, I'm not a Pilot, crew member or even involved in the Aviation industry at all what so ever - I'm merely a person whom is interested in the engineering, technology and operation of an aircraft. So please excuse any silly questions I ask as I have ZERO knowledge in this field.

The first question of many I want to ask is - are aircraft jets close to max jet rotation/propeller rotation/Jet speed at high altitude (30k feet plus) to achieve the cruising speed of 500 miles plus? I've always thought that the jets will only be at around 50% power as the air is thinner at higher altitude which equals less friction etc?
 
OzzyPirate
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Re: question regarding speed at altitude

Mon Nov 14, 2016 11:57 pm

What you say in your last sentence would be true if jet engines produced uniform thrust from sea level to cruise. In reality, altitude is a double-edged sword - the air is a lot thinner and thus higher True Airspeeds (and therefore groundspeeds) are attainable, but jet engines have significantly less available thrust at altitude.

So it's quite common for jets to be at the same engine RPM in the cruise as they are at takeoff, except they'll be producing significantly less thrust in significantly thinner air, thankfully at significantly lower fuel burn.

To give you an idea how little excess thrust is available up high, if we're near maximum altitude and a wind change drops our indicated airspeed by 10 knots, it may take over a minute at maximum continuous thrust to recover those 10 knots.
 
skyhawkmatthew
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Re: question regarding speed at altitude

Tue Nov 15, 2016 12:53 am

The short answer is yes.

In the cruise the engines are normally sitting around 85% of maximum RPM, but as OzzyPirate said, burning significantly less fuel and producing significantly less thrust than at sea level.

To build on Ozzy's example, and provide a comparison, look at the 777-300 and ER. The ER has a slightly bigger wing, but its engines are rated at over 30% more thrust output. At high weights and altitudes in the -300, if a significant airspeed drop occurs, the only option in the anaemic -300 may be to initiate a descent to recover the speed; the -ER can generally much more easily power out of such situations, as it is much more wing- rather than thrust-limited at high altitude.
Qantas - The Spirit of Australia.
 
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CARST
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Re: question regarding speed at altitude

Tue Nov 15, 2016 7:43 am

OzzyPirate wrote:
To give you an idea how little excess thrust is available up high, if we're near maximum altitude and a wind change drops our indicated airspeed by 10 knots, it may take over a minute at maximum continuous thrust to recover those 10 knots.


skyhawkmatthew wrote:
To build on Ozzy's example, and provide a comparison, look at the 777-300 and ER. The ER has a slightly bigger wing, but its engines are rated at over 30% more thrust output. At high weights and altitudes in the -300, if a significant airspeed drop occurs, the only option in the anaemic -300 may be to initiate a descent to recover the speed; the -ER can generally much more easily power out of such situations, as it is much more wing- rather than thrust-limited at high altitude.


I am no expert, and you two can probably add something to what I have to say, but what you describe here should probably be put into perspective. AFAIK most passenger airplanes at cruise level are very close to stall speed and maximum speed at the same time, especially when heavy. E.g. a heavy 773 might cruise at 290 knots indicated air speed at FL 340, the maximum speed could be at 305 kt IAD and the stall speed at 270 kt IAS. So that 10 knot loss you speak about can be very important. My numbers are made up, but I am sure someone can provide real world numbers, that should be in the ballpark of what I mentioned.

iffy wrote:
The first question of many I want to ask is - are aircraft jets close to max jet rotation/propeller rotation/Jet speed at high altitude (30k feet plus) to achieve the cruising speed of 500 miles plus? I've always thought that the jets will only be at around 50% power as the air is thinner at higher altitude which equals less friction etc?


These 500 mph are ground speed (GS) anyway. The true airspeed (TAS) will be different (adjusted to the wind speed and direction) and the indicated airspeed (IAS) will be way, way lower. The IAS is the one that really matters in this case of the engines and the topic I talked about above. It factors in the atmospheric pressure. And because (as you mentioned) the air is thinner in higehr altitudes, you have less air going through the engines and pitot tubes, which measure the speed of the aircraft. So while at FL350 you make a 500 knot ground speed, you might fly only with (just an example) 285 knots indicated airspeed. But to produce the same thrust for that "given" groundspeed the engines have to work harder (faster) than at a little lower altitude (e.g. FL280). But they also burn much less fuel...
 
OzzyPirate
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Re: question regarding speed at altitude

Tue Nov 15, 2016 8:56 pm

CARST wrote:
I am no expert, and you two can probably add something to what I have to say, but what you describe here should probably be put into perspective. AFAIK most passenger airplanes at cruise level are very close to stall speed and maximum speed at the same time, especially when heavy. E.g. a heavy 773 might cruise at 290 knots indicated air speed at FL 340, the maximum speed could be at 305 kt IAD and the stall speed at 270 kt IAS. So that 10 knot loss you speak about can be very important. My numbers are made up, but I am sure someone can provide real world numbers, that should be in the ballpark of what I mentioned.


Aircraft *can* be close to stall speed if they're near maximum altitude, but there's no requirement to be up that high, and I'd only ever go there if conditions were suitable. In turbulent or unpredictable conditions, it's a very brave crew that flies around with minimal margin to buffet and/or stall. Any conditions likely to involve wind changes as I mentioned above, and there's no way I'd be going that close to maximum altitude - and therefore we'd have a much better margin to buffet and stall.

Near maximum altitude, there's no such thing as recovering from a low speed (buffet) condition without altitude loss, either (well maybe in the 300ER as Matthew said above, but certainly not in the 737) - there simply isn't enough thrust to get the speed back on target without trading some altitude. So yes, we do cruise with these margins from time to time, but certainly not always.
 
RetiredWeasel
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Re: question regarding speed at altitude

Tue Nov 15, 2016 9:10 pm

CARST wrote:
The true airspeed (TAS) will be different (adjusted to the wind speed and direction) and the indicated airspeed (IAS) will be way, way lower.
..

Your right..you are no expert. TAS has nothing to do with wind speed. It is IAS or EAS (equivalent airspeed) corrected for pressure and temperature. GS is the only speed that
uses wind factors.
 
timz
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Re: speed at altitude

Sat Nov 19, 2016 6:52 pm

Maximum continuous thrust at FL350 at Mach 0.8 is perhaps 20% of maximum thrust at standstill at sea level.
 
atcpeter
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Re: question regarding speed at altitude

Sat Nov 19, 2016 9:19 pm

Look up "coffin corner" on Google. Most modern jets at cruise altitude (could be several thousand feet below max altitude) have less than 20kts of *indicated* airspeed difference between aerodynamic stall and never-exceed speed. Of course the closer you fly to the plane's service ceiling, the narrower that range, since the aircraft must fly at a greater angle of attack to maintain level flight at a given speed.

Sorry if there is too much aeronautical terminology there, but the differences are significant when it comes to flying.
 
OzzyPirate
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Re: question regarding speed at altitude

Sat Nov 19, 2016 11:25 pm

atcpeter wrote:
Most modern jets at cruise altitude (could be several thousand feet below max altitude) have less than 20kts of *indicated* airspeed difference between aerodynamic stall and never-exceed speed.


Sorry but I can't agree with that, unless you're defining max altitude as certified maximum only. If you're several thousand feet below max altitude as defined in the FMC (could be certified limit, thrust limited or buffet limited), you're going to have a lot more margin than 20 KIAS between the red bricks.

Just checked a recent photo I took up front in the cruise, we were around 3000' below max altitude, with 80 KIAS between stick shaker and overspeed.
 
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Starlionblue
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Re: question regarding speed at altitude

Sun Nov 20, 2016 4:08 am

Agreed with OzzyPirate. The optimum altitude is lower and will give a larger margin.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
flyboy80
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Re: question regarding speed at altitude

Mon Nov 21, 2016 7:52 pm

This post makes me very curious about the flow of fuel to the engine. Some have stated the fuel burn decreases significantly at altitude, yet the engine speed is similar to lower altitudes-although delivering much less power obviously. Is this simply because the engine can run faster with the less dense air at cruise altitude? If so that would mean if fuel flow to the engine was the same between takeoff and climb the engine would technically overspeed at cruise? I probably have it all wrong.
 
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Matt6461
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Re: question regarding speed at altitude

Mon Nov 21, 2016 11:06 pm

flyboy80 wrote:
If so that would mean if fuel flow to the engine was the same between takeoff and climb the engine would technically overspeed at cruise?


What do you mean by overspeed?
Engines reach their maximum rotational speed and max OPR at the top of climb typically. That max OPR/speed seems to be a hard limit - exceed the OPR and your temperature exceeds the design limits for the engine, which would be catastrophic. If that's what you mean by overspeed, I think that's right but certainly defer to others here.

Your question about fuel burn decreasing with altitude...
If you mean raw fuel flow, then yes and this connects to the earlier point about maximum speed. An engine produces thrust by changing the momentum of the air. In less-dense air, an engine with a given maximum rotational speed produces maximum thrust that is decreasing with air density. You can't spin the fan any faster than X, which accelerates Y particles of air, producing Z thrust. No point in feeding more fuel than is required to produce that maximum thrust (or the desired cruise thrust somewhat short of maximum).

If you mean that engine SFC decreases (efficiency increases) with altitude, this is generally true too. Colder air increases the thermal efficiency of the engines by increasing the difference between inlet air temperature and combustion chamber temperature. That temperature difference dictates how much useful work can be extracted from the energy of combustion (if one were flying on the surface of the sun, for example, the combustion chamber's ignitions wouldn't be as useful in forcing hotter gases out of the engine). Temperature decreases until you hit the stratosphere at ~FL 37 under standard conditions (stratosphere level varies across the globe and by day).

I think that's right but others may want to check my "work."

Related question - It seems like engine efficiency continually improves beyond FL 37. I seem to recall a Ferpe post showing better SFC at FL 39 than at 37, for example. I may be wrong about this. Is there a reason for jet engines to be more efficient in less-dense air of equal temperature?
 
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Starlionblue
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Re: question regarding speed at altitude

Tue Nov 22, 2016 12:09 am

flyboy80 wrote:
This post makes me very curious about the flow of fuel to the engine. Some have stated the fuel burn decreases significantly at altitude, yet the engine speed is similar to lower altitudes-although delivering much less power obviously. Is this simply because the engine can run faster with the less dense air at cruise altitude? If so that would mean if fuel flow to the engine was the same between takeoff and climb the engine would technically overspeed at cruise? I probably have it all wrong.


As Matt6461 explains, the amount of air going through the engine decreases. The engine will run in the same rotational speed range at altitude, but given less dense air there is much less mass of air going through it, and consequently much less fuel being used. Compare the take-off and cruise fuel flows for one 330 engine and you can see that despite N1 not being so different, the fuel flow is very different.
- Take off: 9-10 tons/hour at 95% N1
- Cruise: 2.5-3.5 tons/hour at 80% N1
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Bellerophon
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Re: question regarding speed at altitude

Wed Nov 23, 2016 3:24 pm

OzzyPirate

At max (certified) altitude with 140 kts between MMO and VLA.

Not a modern jet though! :D

Image

Best Regards

Bellerophon
 
GalaxyFlyer
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Re: question regarding speed at altitude

Thu Nov 24, 2016 4:34 pm

Barbados-bound, Bellerophon?
 
Bellerophon
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Re: question regarding speed at altitude

Thu Nov 24, 2016 9:59 pm

GalaxyFlyer

...Barbados-bound...?...

Yes, well worked out! :bigthumbsup:

The (true) heading and max altitude are a big clue!

Best Regards

Bellerophon
 
OzzyPirate
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Re: question regarding speed at altitude

Mon Nov 28, 2016 1:59 am

Bellerophon wrote:
OzzyPirate

At max (certified) altitude with 140 kts between MMO and VLA.

Not a modern jet though! :D

Best Regards

Bellerophon


Awesome!

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