Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
Trimeresurus
Topic Author
Posts: 119
Joined: Fri Jun 02, 2017 6:06 pm

How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sun Nov 08, 2020 9:34 pm

Due to air being thinner at such altitudes, RPMs being equal, less air will be pushed away to generate thrust. Another factor to decrease thrust might be that there's less oxygen available for combustion, hence taking away a bit of vitality from the engine, though I'd think that'd be an issue mostly for turbojets or engines with lower bypass ratios.

How much less thrust does an average airliner produce during cruise that it'd otherwise do with the same N1 at sea level?
 
User avatar
SheikhDjibouti
Posts: 2348
Joined: Sat Sep 30, 2017 4:59 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sun Nov 08, 2020 11:35 pm

Trimeresurus wrote:
Due to air being thinner at such altitudes, RPMs being equal, less air will be pushed away to generate thrust. Another factor to decrease thrust might be that there's less oxygen available for combustion, hence taking away a bit of vitality from the engine, though I'd think that'd be an issue mostly for turbojets or engines with lower bypass ratios.

How much less thrust does an average airliner produce during cruise that it'd otherwise do with the same N1 at sea level?

LOL; many many posts ago I asked a similar question based on fuel consumption. As the answers came in, I suddenly had a "eureka" moment when suddenly it all made sense.

So, whilst you are waiting for one of the grown-ups to give you a proper technical answer, I'll just say that it is a BIG difference. And AFAIK it isn't really dependent on turbojet or low vs high bypass ratio. I suggest you start thinking in terms of air density at altitude (& don't forget the temperature!), and then consider Newton's 3rd Law.

Ball park figure from memory.... a CF6 rated at 50,000 lb (static) take-off thrust might only produce 11,000 lb thrust at 40,000 feet at temperature X, and at speed Y. The good news is that in the thin air at altitude the aircraft only requires a very much reduced thrust, and it only consumes a correspondingly lower fuel figure. You could try feeding more fuel in, but without sufficient oxygen, it is just going to waste.

As for taking away the vitality of the engine; I'm going to suggest the engine is operating more efficiently, albeit at a lower net thrust. For one thing, the compressor section is operating under much less "stress".
 
426Shadow
Posts: 249
Joined: Tue Sep 11, 2007 8:13 am

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 09, 2020 12:29 am

It is very dependent on bypass ratio. A GE90-115B will have a bigger thrust lapse than a JT8D. The main reason for this is the fact that the closer you get to your exhaust speed in relation to your forward speed (in the atmosphere) the less thrust you create. Since turbojets and low bypass turbofans had much higher exhaust gas and bypass fan velocity they aren't as affected by this effect. In addition when it comes to the bypass air at least, the air is thinner so to get the same thrust you would have to expel it faster which is impossible because of the reason I mentioned before. A high bypass fan may see a decrease of 70 or more percent at cruise where a low bypass or especially a turbojet will typically be less. The performance section in airliners flight manuals typically have a graph that shows the thrust lapse vs speed/altitude.

Here are some examples I found online.


Image
Image
Image
 
User avatar
Horstroad
Posts: 613
Joined: Thu Apr 08, 2010 8:19 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 09, 2020 4:02 am

426Shadow wrote:
[...]

Is there something wrong in the data of the JT-9?

All of the engines come in between 18.00% and 25.93% of their respective takeoff thrust at cruising altitude, while the JT-9 appears to still have 87.15% of its takeoff thrust available during cruise
 
Trimeresurus
Topic Author
Posts: 119
Joined: Fri Jun 02, 2017 6:06 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 09, 2020 8:12 am

426Shadow wrote:
It is very dependent on bypass ratio. A GE90-115B will have a bigger thrust lapse than a JT8D. The main reason for this is the fact that the closer you get to your exhaust speed in relation to your forward speed (in the atmosphere) the less thrust you create. Since turbojets and low bypass turbofans had much higher exhaust gas and bypass fan velocity they aren't as affected by this effect. In addition when it comes to the bypass air at least, the air is thinner so to get the same thrust you would have to expel it faster which is impossible because of the reason I mentioned before. A high bypass fan may see a decrease of 70 or more percent at cruise where a low bypass or especially a turbojet will typically be less. The performance section in airliners flight manuals typically have a graph that shows the thrust lapse vs speed/altitude.

Here are some examples I found online.


Image
Image
Image


It's interesting that RB211 didn't have FADEC despite coming out at 1990.
 
Trimeresurus
Topic Author
Posts: 119
Joined: Fri Jun 02, 2017 6:06 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 09, 2020 8:22 am

SheikhDjibouti wrote:
I suggest you start thinking in terms of air density at altitude (& don't forget the temperature!), and then consider Newton's 3rd Law.

Ball park figure from memory.... a CF6 rated at 50,000 lb (static) take-off thrust might only produce 11,000 lb thrust at 40,000 feet at temperature X, and at speed Y. The good news is that in the thin air at altitude the aircraft only requires a very much reduced thrust, and it only consumes a correspondingly lower fuel figure. You could try feeding more fuel in, but without sufficient oxygen, it is just going to waste.

As for taking away the vitality of the engine; I'm going to suggest the engine is operating more efficiently, albeit at a lower net thrust. For one thing, the compressor section is operating under much less "stress".


But Newton's third law doesn't dictate that the force must be in relation to the ambient environment. Eg.The force exerted upon a baseball pitcher by the ball that he's thrown doesn't depend on if the ball is hit by the fielder or not, as soon as it leaves his body, it's set. Same with an engine, otherwise rockets wouldn't work in space because there is zero ambient air. So to produce enough thrust to speed up the aircraft to a velocity that'd produce enough lift(by that thin 40k feet air) by the wings to defeat the gravity(which isn't much lower at 40.000 feet than 0 feet) is still a challenge. If it wasn't, aircraft wouldn't have a ceiling at all.
 
User avatar
SheikhDjibouti
Posts: 2348
Joined: Sat Sep 30, 2017 4:59 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 09, 2020 11:29 am

426shadow wrote:
It is very dependent on bypass ratio. A GE90-115B will have a bigger thrust lapse than a JT8D
My bad. However in the big scheme of things the small difference between 78% thrust reduction (CFM56 / bypass ratio 6.6) and 74% (JT8D / bypass ratio 1.0) is somewhat less significant than the fact that the vast majority of thrust available at sea level is simply not available at altitude. I was attempting to cover the basics, not the detail between specific engines.
Your point about the GE90-115B is entirely valid though. :bigthumbsup:

The table is great, and many thanks for that, but I wonder about the selection of engines provided.
For instance, why JT8D-17R, introduced in "1970", but basically the same engine as the original from 1963. :scratchchin: (unlike the JT8D-200 series, which was indeed a significant step forward)
And also this gem...
Trimeresurus wrote:
It's interesting that RB211 didn't have FADEC despite coming out at 1990.
The RB211-524 first ran on 1 October 1973, long before FADEC was a thing. As with the JT8D, the date is only applicable to the introduction into service of a sub-variant.
I was going to add that it might have been a problem applying FADEC to such an existing design, except... it turns out the -524G/H specified in the table does indeed have FADEC. :lol:

As I said above, the specific choice of engines in that table is curious, and maybe should be viewed against a bigger picture. Meh, what do I know? :old:
 
User avatar
SheikhDjibouti
Posts: 2348
Joined: Sat Sep 30, 2017 4:59 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 09, 2020 11:55 am

Trimeresurus wrote:
SheikhDjibouti wrote:
I suggest you start thinking in terms of air density at altitude (& don't forget the temperature!), and then consider Newton's 3rd Law.

But Newton's third law doesn't dictate that the force must be in relation to the ambient environment. Eg.The force exerted upon a baseball pitcher by the ball that he's thrown doesn't depend on if the ball is hit by the fielder or not, as soon as it leaves his body, it's set. Same with an engine, otherwise rockets wouldn't work in space because there is zero ambient air. So to produce enough thrust to speed up the aircraft to a velocity that'd produce enough lift(by that thin 40k feet air) by the wings to defeat the gravity(which isn't much lower at 40.000 feet than 0 feet) is still a challenge. If it wasn't, aircraft wouldn't have a ceiling at all.

You're not wrong.
But in this case, Newton's 3rd Law comes into play because it is the "ball" thrown by the pitcher that is 80% lighter/less dense at 40,000 ft. :o

Basically, the pitcher is throwing a tennis ball instead of a baseball. That's going to have an effect. Indeed, if a baseball pitcher ever "threw" a handful of fresh air, he would probably over-balance and fall flat on his dignity. Try it yourself!

And whilst his arm swing might be even faster (because the tennis ball is lighter) there will also be physical limitations on his muscle memory, and operation of shoulder and elbow joints. Not that any of this is relevant to our discussion.

As for rockets working in space; I'm guessing here, but I suspect the fuel mixture includes it's own oxygen supply. If you fit a big enough oxygen tank to an aircraft, those engines could continue producing thrust all the way to the moon. (or maybe not...)

If it wasn't, aircraft wouldn't have a ceiling at all.
Again, not part of this discussion, but commercial aircraft may also limited by how quickly they can descend in an emergency after loss of pressurization. Someone else can probably find the actual regs.
wikipedia wrote:
Compared to service ceiling, the absolute ceiling of commercial aircraft is much higher than for standard operational purposes. In the Concorde's case, it was tested to be 68,000 ft
 
Sokes
Posts: 2773
Joined: Sat Mar 09, 2019 4:48 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Tue Nov 10, 2020 3:01 am

426Shadow wrote:
It is very dependent on bypass ratio. A GE90-115B will have a bigger thrust lapse than a JT8D. The main reason for this is the fact that the closer you get to your exhaust speed in relation to your forward speed (in the atmosphere) the less thrust you create.

That's why I wonder why planes fly 830-900 km/ h instead of let's say 750-800 km/h.
Anybody knows the exhaust speeds of turbofan engines?

I believe lift increases square to speed?
A longer wing would also do, but then wing material required increases by cube while lift increases only square to wing length.

At least for lower MTOWs one should assume that a long wing for low speeds is better.

To come back to the question:
If speed is same I assume 20% of air density means 20% of thrust.
But how does thrust fall with increasing speed?
 
User avatar
SheikhDjibouti
Posts: 2348
Joined: Sat Sep 30, 2017 4:59 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Tue Nov 10, 2020 11:58 am

Sokes wrote:
But how does thrust fall with increasing speed?
Who says it falls? :scratchchin:
This might well depend on numerous factors, including engine type.
OK, so a typical modern high bypass turbofan engine (the actual subject of this thread) will obviously fail at supersonic speeds, but I don't know that the thrust graph is a linear line. It might something more like a bell curve for some engines, peaking at some value other than when stationary on a runway.

Concorde's engines were famously more efficient than anything else available for decades, mainly due to supersonic speeds. Of course "efficient" doesn't automatically translate to "more thrust", but it tells us the equation is not necessarily a simple one.

And for instance thrust definitely increases if the engine is a ram-jet.

Unfortunately I am seriously under-qualified in terms of giving you the complete answer, but I would be wary of simple answers. :white:
 
Sokes
Posts: 2773
Joined: Sat Mar 09, 2019 4:48 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Tue Nov 10, 2020 3:08 pm

SheikhDjibouti wrote:
Sokes wrote:
But how does thrust fall with increasing speed?
Who says it falls? :scratchchin:

Him:
426Shadow wrote:
It is very dependent on bypass ratio. A GE90-115B will have a bigger thrust lapse than a JT8D. The main reason for this is the fact that the closer you get to your exhaust speed in relation to your forward speed (in the atmosphere) the less thrust you create.
 
426Shadow
Posts: 249
Joined: Tue Sep 11, 2007 8:13 am

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Tue Nov 10, 2020 6:20 pm

Sokes wrote:
SheikhDjibouti wrote:
Sokes wrote:
But how does thrust fall with increasing speed?
Who says it falls? :scratchchin:

Him:
426Shadow wrote:
It is very dependent on bypass ratio. A GE90-115B will have a bigger thrust lapse than a JT8D. The main reason for this is the fact that the closer you get to your exhaust speed in relation to your forward speed (in the atmosphere) the less thrust you create.


This is very specific to engine types as well. As mentioned by Sheikh there are engine types where this is not the case. Yes the thrust will fall with any engine that isn't a straight turbojet or ramjet. And even if the thrust increases like in an after-burning turbojet or LBP turbofan the drag will also increase and when that equilibrium is reached, forward acceleration will stop. So you could still have Mach 2.5 exhaust for example yet be stuck at Mach 2.1 because if you are creating 80,000lb of thrust but also 80,000lb of drag you cannot go faster.

For high bypass turbofans there is a severe difference in bypass velocity and core exhaust velocity and the core exhaust is not a limiting factor in cruise speed because it is such a small portion of the thrust.
You may for example (made up numbers BTW) have a CF6 with cruise fan exhaust velocity of 500 mph and a core exhaust velocity of 620 mph. Obviously airliners typically don't cruise at 620 mph but they do typically cruise faster than 500mph. So the core exhaust will help increase the higher overall cruise speed but at the same time where there is shearing there is inefficiency. This is why the Concorde is more "efficient" at altitude where it is traveling closer to it's exhaust velocity. On takeoff high bypass turbofan's are very efficient relative to how much power they make and that decreases with altitude while the actual fuel burn decreases. Turbojets and low bypass turbofan's are odd in that there is a certain speed range where they start to become more and more efficient even while using more and more fuel. I could go on for days but i think you get the picture.
 
Sokes
Posts: 2773
Joined: Sat Mar 09, 2019 4:48 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Wed Nov 11, 2020 6:12 am

426Shadow wrote:
...
You may for example (made up numbers BTW) have a CF6 with cruise fan exhaust velocity of 500 mph and a core exhaust velocity of 620 mph. Obviously airliners typically don't cruise at 620 mph but they do typically cruise faster than 500mph.

I don't understand.
The fan exhaust velocity is in relation to surrounding (still) air or in relation to the plane?
 
tommy1808
Posts: 14691
Joined: Thu Nov 21, 2013 3:24 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Wed Nov 11, 2020 6:15 am

SheikhDjibouti wrote:
And also this gem...
Trimeresurus wrote:
It's interesting that RB211 didn't have FADEC despite coming out at 1990.
The RB211-524 first ran on 1 October 1973, long before FADEC was a thing. As with the JT8D, the date is only applicable to the introduction into service of a sub-variant.
I was going to add that it might have been a problem applying FADEC to such an existing design, except... it turns out the -524G/H specified in the table does indeed have FADEC. :lol:

As I said above, the specific choice of engines in that table is curious, and maybe should be viewed against a bigger picture. Meh, what do I know? :old:


also: FADEC had fairly strict export restrictions.

best regards
Thomas
 
426Shadow
Posts: 249
Joined: Tue Sep 11, 2007 8:13 am

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Thu Nov 12, 2020 6:47 pm

Sokes wrote:
426Shadow wrote:
...
You may for example (made up numbers BTW) have a CF6 with cruise fan exhaust velocity of 500 mph and a core exhaust velocity of 620 mph. Obviously airliners typically don't cruise at 620 mph but they do typically cruise faster than 500mph.

I don't understand.
The fan exhaust velocity is in relation to surrounding (still) air or in relation to the plane?


The absolute velocity. That is to say the velocity as it leaves the nozzle.
 
AvgWhiteGuy
Posts: 35
Joined: Tue Jun 25, 2019 2:44 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Thu Nov 12, 2020 11:33 pm

Going by hard numbers, in the 737NG, which at FL400 on our flight an hour ago, was burning 3,550 pounds an hour per engine at top of climb and settled at 2150 pounds per hour per engine at cruise, would have approximately 4666 total pounds of excess thrust available from both engines. If it took approximately 7,166 pounds of thrust to maintain M.79 at FL400 for us today at 123,000 pounds, and having another 4666 available, it would be a percentage difference of 53.6%. This is assuming the, "cruise flat," design of the engine, where the SFC is roughly constant at higher rpm's and an SFC of .6 at the .80M FL400 benchmark, holds true. If the engine is slightly less efficient at 101% N1, which it most likely is, then the math would be linear with the efficiency difference. It should also be noted that our optimum altitude was greater than the NG's certified ceiling, FL410, and hence the margin of excess thrust will obviously decrease when heavier and climbing to optimum altitude.
 
AvgWhiteGuy
Posts: 35
Joined: Tue Jun 25, 2019 2:44 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Fri Nov 13, 2020 2:27 am

I just realized that I didn't answer the question directly, so here is an addendum to my previous post. Our engines make a total of 54,400 pounds of thrust at sea level and approximately 11,800 pounds at top of climb. That is 21.7% of sea level thrust, assuming the cruise flat is just that - flat. More realistically, the flat is probably off by a percent or two at 101% N1, so roughly .61 lb/lb/hr and so I would round that down to 21%.
 
phugoid1982
Posts: 145
Joined: Sat Sep 10, 2016 4:02 am

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Fri Nov 13, 2020 7:41 pm

Since the induced drag in Steady level flight is proportional to the Weight^2/ dynamic pressure squared, at the cruising speed of typical airliners at altitude this produces a (1/4) reduction in thrust required due to q infinity due to velocity as well as a reduction to Thrust required due to density changes.
 
WIederling
Posts: 10041
Joined: Sun Sep 13, 2015 2:15 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sat Nov 14, 2020 10:18 am

Sokes wrote:
426Shadow wrote:
...
You may for example (made up numbers BTW) have a CF6 with cruise fan exhaust velocity of 500 mph and a core exhaust velocity of 620 mph. Obviously airliners typically don't cruise at 620 mph but they do typically cruise faster than 500mph.

I don't understand.
The fan exhaust velocity is in relation to surrounding (still) air or in relation to the plane?


engine must increase impuls of air mass going through.
intake impuls is -m. * Vin
output impuls is m. * Vout
Vout >Vin ( the Vdelta * m. massflow is effective thrust.)

low bypass does small m. * large Vdelta
high bypass does large(r) m. * small(er) Vdelta.

energy requirement for increasing impuls is m/2 * Vdelta²
reason why high bypass is more efficient : massflow is linear towards impuls and energy
while Vdelta is linear towards impuls but square relation towards energy.
 
Sokes
Posts: 2773
Joined: Sat Mar 09, 2019 4:48 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sat Nov 14, 2020 11:22 am

WIederling wrote:
engine must increase impuls of air mass going through.
intake impuls is -m. * Vin
output impuls is m. * Vout
Vout >Vin ( the Vdelta * m. massflow is effective thrust.)

low bypass does small m. * large Vdelta
high bypass does large(r) m. * small(er) Vdelta.

energy requirement for increasing impuls is m/2 * Vdelta²
reason why high bypass is more efficient : massflow is linear towards impuls and energy
while Vdelta is linear towards impuls but square relation towards energy.

Thanks.

Questions to all:
If air is accelerated by 400 km/h it has 400 km/h compared to the surrounding air, but above the speed of sound compared to the engine.
Any boom?

I assume to preserve energy the air is accelerated only 100 km/ h or so? Somebody knows more?
 
WIederling
Posts: 10041
Joined: Sun Sep 13, 2015 2:15 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sat Nov 14, 2020 1:30 pm

Sokes wrote:
I assume to preserve energy the air is accelerated only 100 km/ h or so? Somebody knows more?


simplistic you can divide thrust ( N ) by massflow ( kg/s ) and get delta V ( m/s )
( N = kg * m / s )
For the JT8 on the 727 you get
72900N / 148kg/s = 492m/s
high exhaust speed is broken down fast via turbulence.
explains the exorbitant noise they made ( on ground, starting..)
 
User avatar
Lifes
Posts: 4
Joined: Fri Jun 19, 2020 7:37 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sat Nov 14, 2020 9:43 pm

WIederling wrote:
Sokes wrote:
I assume to preserve energy the air is accelerated only 100 km/ h or so? Somebody knows more?


simplistic you can divide thrust ( N ) by massflow ( kg/s ) and get delta V ( m/s )
( N = kg * m / s )
For the JT8 on the 727 you get
72900N / 148kg/s = 492m/s
high exhaust speed is broken down fast via turbulence.
explains the exorbitant noise they made ( on ground, starting..)


Also, the speed of sound increases with the square root of air temperature (in Kelvin, please), meaning that exhaust velocity can be higher without being supersonic.

In fact, even the core exhaust flow will never be supersonic as it is mach locked to 1 in the turbine nozzle throat. For supersonic exhaust flows you need a convergent-divergent nozzle design, like turbojet and rocket engines have.

Most of the noise you hear is created by turbulent mixing of core and bypass flows with surrounding airflow. You can see how this has been mitigated with CFM LEAP-1A and -1B. Even though they are on the same-ish thrust region the -1B has to do with a lot less mass intake area as B737 MAX nacelles are a lot smaller for ground clearance. They had to add chevrons to comply with noise regs.

Granted, fan blade tip speeds are supersonic but great care is put on the intake diffuser and fan housing/nacelle to mitigate this.
 
Sokes
Posts: 2773
Joined: Sat Mar 09, 2019 4:48 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Sun Nov 15, 2020 3:51 am

Learnt something.
Thanks to both of you.
Looking forward to more posts of you.
 
744SPX
Posts: 617
Joined: Mon Jan 27, 2020 6:20 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 16, 2020 9:47 am

I'm thinking this is all about bypass ratio and overall exhaust velocity. I'd hazard a guess that the CF6, PW4000, and RB211-524 with their sub 5-to-1 BPR's have superior high altitude/high speed performance vs the current generation engines like the GEnx, GE9X, and Trent XWB with their ~10-to-1 BPR's . Cruise thrust at FL40+ should theoretically be higher for lower BPR engines.

There is a reason the Gulfstream G650 and Bombardier Global Express have engines with a BPR below 5...
 
User avatar
SheikhDjibouti
Posts: 2348
Joined: Sat Sep 30, 2017 4:59 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 16, 2020 2:52 pm

744SPX wrote:
I'm thinking this is all about bypass ratio and overall exhaust velocity. I'd hazard a guess that the CF6, PW4000, and RB211-524 with their sub 5-to-1 BPR's have superior high altitude/high speed performance vs the current generation engines like the GEnx, GE9X, and Trent XWB with their ~10-to-1 BPR's . Cruise thrust at FL40+ should theoretically be higher for lower BPR engines.

There is a reason the Gulfstream G650 and Bombardier Global Express have engines with a BPR below 5...

Like the Curate's egg, your post is good in parts.

"Cruise thrust at FL40+ should theoretically be higher for lower BPR engines".
That is to say, there is less reduction vis-a-vis take-off thrust.
As is often discussed here on a.net, different aircraft have different reserves of power at take-off, allowing superior climb-outs or better single-engine performance. The 757 springs to mind. Such an aircraft will continue to have a useful reserve of power at FL400+, regardless of whether it has high or low BPR engines. And what of it anyway, if you are already pushing the airframe mach limits?

There is a reason the Gulfstream G650 and Bombardier Global Express have engines with a BPR below 5...

Because there isn't yet an engine with higher BPR in their thrust category?

Whilst it may be true that some clients will be swayed by crossing the Atlantic 20 minutes faster by virtue of a slightly higher cruising speed, they can achieve the same result by picking an aircraft with more engine than it really needs. Or simply instructing the pilot to push the levers to the wall. After all, people who can afford such exec jets are not going to cry so much over slightly higher fuel consumption. And they might appreciate that extra power when getting in & out of smaller airports.

But as it happens, I am not seeing these "higher" cruising speeds anyway. Perhaps you could elaborate?
 
User avatar
Lifes
Posts: 4
Joined: Fri Jun 19, 2020 7:37 pm

Re: How much less thrust does a modern high bypass turbofan engine generates at cruise altitude as opposed to sea level?

Mon Nov 16, 2020 3:55 pm

As our colleague writes above a big part of engine thrust selection is second segment one engine out requirements per CS 25.121(b).

At cruise you will reach the airframe drag-divergence mach number a lot sooner than you run out of delta V for the bypass flow, meaning that all your 'excess of thrust' will only really introduce aerodynamic and potentially aerolastic issues. The airframe limits max speed.

Regulations aside, such as emergency descent oxygen requirements, an aircraft could use its excess of power to climb to its maximum absolute ceiling which is when drag equals max thrust for the lift coefficient needed to keep level flight. This will change based on lots of things such as weight, aerodynamics... I would imagine an empty A359 could reach in excess of 45-46k ft based on XWB output only.

To conclude, no, high bypass turbofans do not perform worse at altitude and speed expected for large transport category aircraft flying at transonic speeds. For other applications such as supersonic flight then yes, they would be less suited than low bypass turbofans.

Who is online

Users browsing this forum: No registered users and 25 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos