Speeds

[TAAMGuy]

What speeds change with the mass of the aircraft, other than V1, V2, Vr, and Vref? Climb IAS, or cruise IAS, or what?

[ZimmerFly]

Generally, ALL speeds increase with increasing weight(mass)...EXCEPT Limiting Speeds.

[CabinMaster]

Climb and Cruise speeds are the same, at least for a jet that cruises above cross-over altitude. The less well known "green dot speed" changes with weight.

[411A]

Climb and Cruise speeds are the same, at least for a jet that cruises above cross-over altitude.

Hardly.
Even early FMS-equipped aircraft (the L1011 was the first) have different climb and cruise speeds dependant on weight (mass, for our Euroland friends) when the 'economy' mode is selected.

[CabinMaster]

Depends, only if you chose "minimum fuel climb".

From what I know (and that may not be much) aircraft usually climb "best profile". For an A320 that is 250kts below 10000ft (due to aTC restrictions), and then either 290 or 300 knots up to cross-over altitude (Mach Cruise = Climb Airspeed). A weight-optimized climb would maybe see different speeds. But with a jet, the sooner you gain altitude, the better for overall performance.

[CatPilot]

as for 737cl, climb and cruise speeds DEPEND on weight and also wind(head/tail).
737-3/5 usually have climb speed approx. 280kts above FL100 until changeover alt.
while 737-4 (which is more heavy) has 300kts.

[411A]

(which is more heavy) has 300kts.

If we get heavier still, with other types, one can certainly see the difference.
For example.
Economy climb with an L1011-500 type:
Light TOW, 250/290/M.80.
Whereas, at max TOW...
250/345/M.825.
All FMS derived.

Only been this way for...thirty years.

[CabinMaster]

Haven't seen anything similar in A320 or A330 manual. Maybe its not there. Maybe someone knows. From the physical perspective, higher weights will have higher green dot speed (best glide ratio). But from the overall performance perspective, even at lower weights, quick climb is more desirable.

345 KIAS as climb speed is quite a lot. What was the VMO of the L1011?

[HazelNuts39]

Originally Posted by CabinMaster

But with a jet, the sooner you gain altitude, the better for overall performance.

Exactly. And the best rate of climb is obtained at a certain lift-coefficient (1). That means that the theoretical optimum speed for climb varies approximately as the square root of weight. However, for practical reasons many aircraft use a schedule of constant speeds.

regards,
HN39

(1) See the parallel thread "Max Endurance, Max Range ..."

[411A]

What was the VMO of the L1011?

375 knots
Mmo M.90

Pilots that fly lighter (smaller) jet transport aircraft normally have a somewhat difficult time understanding the concept of very heavy weight models (especially those designed for higher cruise speeds, in the past) and the necessity of faster than what might otherwise seem prudent climb speeds...compared with the lighter/smaller/slower cruisers that they normally fly.

In other words, B737 and A320 aircraft, they are not.

The L1011 was designed for a normal enroute cruise speed of M.86, and a LRC speed of M.84, and if at heavier weights, a slower than optimum climb speed was attempted, the time to climb would be longer.
Guaranteed.

[CatPilot]

slower than optimum climb speed was attempted, the time to climb would be longer.

Exactly! But not so obvious at 737.

[CatPilot]

However, for practical reasons many aircraft use a schedule of constant speeds.

I guess FMC equipped planes uses data from computer, which calculates max rate climb speed which depends on weight and some other factors.

[Another St Ivian]

Originally Posted by 411A

if at heavier weights, a slower than optimum climb speed was attempted, the time to climb would be longer.
Guaranteed.

It's a pretty small effect however - For average higher end weights of the -500, a spread of 100 knots sees a delta of ~7 mins (from memory) on time to T/C.

(NB Still flying around with the ARMA, 411A?)

[CabinMaster]

Rate of Climb = (Thrust - Drag)/Weight * True Airspeed

Assuming that thrust is always the same (not depending on weight), we have to minimize the drag.

Drag can be described as:
Drag = [CD0 + k * CL^2] * Dynamic Pressure * Wing Area
CD0 and k are pretty much constant up Mach cruise.

CL = Weight/(Wing Area * Dynamic Pressure) [short: W/(Qdyn*S)]
So:
Drag = [CD0 + k * [W/(Qdyn*S)]^2] * Qdyn * S
Qdyn = True Airspeed ^2 * Air Density * 0.5

So, more true airspeed means less lift coefficient, but it increases dynamic pressure and so the total drag (not as coefficient, but as force).

When you put all into a spreadsheet (yes, sometimes I use Excel ) , the higher speeds will always show better rate of climb. I increases almost linearly with the true airspeed, with some exceptions at high weights and low speed (here the quadratic influence from the drag polar kicks in).

My conclusion: the fastest true airspeed (which is also the fastest indicated airspeed) is always quickest climb. It may not be minimum fuel. You might not have best climb gradient.

To sum all up:
Google: "getting to grips performance"
Open the PDF you'll hopefully find.
Go on PDF-page 158ff.
Read.

It says: most economic is always highest rate of climb. The best indicated airspeed depends on the aircraft design, higher wing loadings will see higher speeds.

And yes, please be critical with this post, not sure I got everything right.

[411A]

(from memory)

That's the problem
Ten minutes is more correct...a significant amount, IMO.

[HazelNuts39]

CabinMaster;

Thanks for your patience and the very useful link to the Airbus Performance 'Tutorial'.

I must retreat. It is not correct to say that the speed for max rate of climb is simply a certain cL independent of weight. On the other hand, I'm not quite sure that you 'got everything right'. The Airbus document doesn't quite say that:

2.2.3. Climb at Maximum Rate
Climbing at the maximum rate of climb speed enables a given altitude to be
reached in the shortest time. (...) (refer to “Climb at minimum cost”).

2.2.4. Climb at Minimum Cost
(...) for a given cost index, an optimum climb speed (IASECON) and an optimum climb mach number (MachECON) are calculated by the FMGS as a function of the aircraft’s weight. (...)

regards,
HN39

[CabinMaster]

But the document says:

To minimize overall fuel consumption during flight, a low cost index must be
used. As the climb phase is fuel consuming, it is advantageous to minimize climb
duration. This is achieved at the maximum rate of climb speed.
CI = 0 ⇒ IASECON = Maximum rate of climb speed

For Cost Index = 0, IAS Econ is maximum rate of climb.

For higher cost index the airspeed is increased up to VMO-10.

[rudderrudderrat]

Hi CabinMaster,

Where do you stand with your first statement

Climb and Cruise speeds are the same, at least for a jet that cruises above cross-over altitude. The less well known "green dot speed" changes with weight.

Since "green dot speed" or "minimum clean speed" change with weight, and ECON climb varies as a function of (min clean speed), have you convinced yourself now that ECON Climb & Cruise speeds do change with weight?

[HazelNuts39]

Originally Posted by CabinMaster

For Cost Index = 0, IAS Econ is maximum rate of climb.

Correct, and according to Airbus Perf 2.2.4, that speed is:

calculated by the FMGS as a function of the aircraft’s weight

I believe, but stand corrected, that the max rate of climb speed for constant thrust increases with increasing thrust-to-weight ratio. The FMGS in addition takes account of the change of thrust with airspeed. Shall we leave it to the FMGS to resolve our dispute?

regards,
HN39

P.S.(1) The question is discussed in more detail in 'Boeing Jet Transport Performance Methods', Chapter 30, page 30-6:

How about the speed for the best rate of climb?
(...)
For increasing weight the speed increases, as it did for the best angle of climb. However, the difference in time to altitude changes only slightly with speed, so it will be acceptable to use a single speed schedule for all weights.

P.S.(2) Just to satisfy my curiosity (and perhaps yours), I made this graph. You will note that for T/W = 0 (a glider) the speed for max roc corresponds to Vmp (as defined by Keith on the other thread), and for level flight (T/W = minimum D/L) to Vmd. The speed for max roc increases with weight, but the rate of increase becomes very small when T/W is greater than 0.15, i.e. speed for max roc greater than 1.5*Vmd. (See here).