please explain "residual rate of climb"

[Bungfai]

please explain "residual rate of climb". It is some thing involves the maximum cruise altitude.Thanks.

[Pugilistic Animus]

Well rate of climb is dependent on the amount of power available, jet or thrust available piston, over that which is required to over come the drag---

as the engines of all planes lose power with altitude---

meaning, when the amount of power or thrust available no longer equals that which is available you end up with a situations whereby you reach certain points on the thrust altitude curve where---

you'd reach a service ceiling ---the exact definition of which depends on the certification standard depends of the service ceiling depends on the certification whether far 25 or 23 [military airplane have combat ceilings also],


but, if you really wanted to---

you could keep climbing with a 'residual rate of climb' until you reach a point where the power available is equal to the power required and climbing is no longer possible However,

you would have to maintain the best rate of climb to reach this point...


with high altitude and high speed you run into other troubles due to shock waves building up shaking the plane because the air can't escape the path of the airfoil and gets compressed, but that's a complex topic do a search for 'coffin corner' and you may find interesting stuff.

[Bungfai]

Thank you so much.
According to my manual,in maximum altitude table indicates that at certain weight and temp and load factor(1.2G, bank angle=33), the aircraft maximun altitude is XXX with 300 FPM residual rate of climb in level flight.Does it mean that at max altitude of XXX,with wing level, the airplane still could climb at rate of climb 300 FPM until an altitude that cannot climb any more? And what speed do we use ?

[SuperRanger]

bungfai,

there are 2 factors that affects your Max Alt. One will be your aerodynamic limit i.e. 1.2G, etc. the other factor will be your engine thrust!

for 4-engine airplane usually the Max Alt is govern by the aerodynamic limit. this is where you hear about the 'coffin-corner'.

for 2-engine airplane usually the Max Alt is govern by engine thrust. this means aerodynamically your airplane can actually go higher to reach the 1.2G limit but unfortunately, your 2 engines can only produce that much thrust at that altitude i.e. 300fpm.

what speed? well, it depends what you enter in your FMC. if you change the CRZ speed, the Max Alt will change. so whatever speed you entered in your FMC, at that Max Alt and CRZ Speed, you will only get 300fpm at your max. CLB thrust (this is for the B777). if you do not have your FMC, then the QRH will give you the Max Alt at LRC at different ISA.

as i've said above, for most instances, for a 2-engine airplane (B777) you will most likely be residual climb rate limited.

hope this clears it up for you?

SR

[dudduddud]

That is very interesting.

Why the difference between 2 and 4 engines?

[SuperRanger]

i guess a 4-eng airplane have more thrust compared to a 2-eng airplane at altitude.

[Bungfai]

SuperRanger,
Thank you so much.Aerodynamic is sometime hard to understand. But now you give me a clear picture, THANKS!

[Old Smokey]

Probably a bit late for a topic which may have run it's course, but an interesting topic, particularly for Airbus pilots.

Residual climb is pretty much as the name implies, it's the climb performance remaining after the aircraft has reached it's optimum level.

There are two optimums to be considered in ascertaining optimum operation, (1) Optimum speed for the airframe, and (2) Operation of the engines at optimum speed for Thrust Specific Fuel Consumption (TSFC).

For each and every weight, at each and every level, there is one speed which provides for the optimum performance of the Airframe. For the engines, there is one engine speed (with some variation with altitude for N1/N2/N3 relationship) which provides optimum TSFC, i.e. best thrust output for minimum fuel expenditure for each unit of thrust. A "ball park" RPM for best TSFC is around about 88%-90% N1 (at least for the aircraft that I fly).

At low levels, optimum engine speed produces thrust well in excess of that required for optimum airframe speed, and climb is possible. If this "perfect state" climb were to continue during climb at the optimum TSFC engine speed, thrust would steadily decrease, and at an altitude where optimum TSFC engine speed maintained optimum airframe speed, we are at Optimum Altitude. If this were to continue, a "creep" or "cruise" climb would follow, as further climb would only be available as aircraft weight decreases, typically at about 70 ft/min.

The problem with such a climb is that the last several thousand feet of climb to optimum altitude would be VERY protracted, taking hundreds of miles. The practical solution is to "bump up" the climb thrust to a little above optimum TSFC speed, tolerate a small fuel penalty, but enable a more positive climb to optimum level. Residual climb is usually optimised (by Airbus) at about 400 ft/min, causes much bitching amongst A340 pilots in particular, but provides very good economics by enabling an acceptable climb to optimum level.

Typically, MCR is a little above optimum TSFC engine speed, and MCL somewhat more.

There are often instances where an advantageous wind gradient will warrant a climb above the still air optimum level, and this residual climb capability is useful in climbing those extra couple of thousand feet to a more advantageous level.

An interesting interchange of thoughts within this thread regarding 2 engine Vs 4 engine climb capability. All other factors being equal, the 2 engined aircraft more often than not is capable of much higher initial levels, due to a considerably greater thrust excess. If it is remembered that a 2 engined aircraft approximately must achieve on 1 engine what a 4 engined aircraft does on 3 engines, in normal all engines flight the 2 engined aircraft has a 100% excess of thrust over minimum required, whilst the 4 engined aircraft has a mere 33% excess above minimum, and..... climb performance depends upon excess thrust available.

Regards,

Old Smokey

[Bungfai]

Old smokey,
I am with boeing.Your great knowledge really enhances my interest of learning. Thank you so much.

[mutt]

In the Boeing Manuals "residual rate of climb" means the climb rate achievable when you reach that altitude. If for example you can still achieve 300 fpm at FL350 but only 290 fpm at FL360, then the maximum altitude would be FL350.

Generally there can be differences between the QRH altitudes and the FMC due to a varience in the inputs.

Old_smokey, welcome back.... :):) We are just moving back into the Airbus world, so your explanation is interesting and appreciated.

Mutt