Climb gradient vs flap setting

[LGB]

What flap setting gives the best climb gradient (not rate of climb, but purely gradient), say on a 747? Since obstacle/noise critical departures are made with flap 10, it could be assumed flap ten gives the best gradient?

When accelerated and cleaned up, shouldn't the gradient be better than at flap 10, even though a higher speed must be maintained? (disregarding loss of climb when accelerating and cleaning up)

What I am specifically thinking of is when the SID says "Cross xyz at or above ...", say 8000 feet (from MSL), will flaps 10 get me to 8000 feet in a shorter distance (forget the wind for now), than if I clean up?

[Old Smokey]

LGB,

I think that you answered your own question within your question when you stated -
(disregarding loss of climb when accelerating and cleaning up)
That is the key item to be considered for any OVERALL gradient up to ther point where gradient no longer matters. The loss of gradient during acceleration to flap reduction speed is very significant - I always monitor Flight Path Angle on every departure, and (on the B777) the gradient routinely is as low as 1° (1.6%) or less. For a 4 engined aircraft, this will be even more limiting due to the considerably lower excess thrust with all engines operating.

I know of no aircraft where even low flap extensions produced a gradient better than in the clean configuration, and the clean configuration would undoubtedly yield an improved instantaneous gradient than would 10° Flap, the problem is the severe overall gradient loss during the acceleration to the better configuration and speed.

Regards,

Old Smokey

[LGB]

So, if it is not for close in obstacles, but a quite high altitude restraint in the distance, then cleaning up and keeping Vx (clean)is the better solution?

[Old Smokey]

LGB,

The simple answer is YES, but a definition of when a close-in obstacle becomes a far-out constraint is required.

Consider a constant climb at Flaps 10°. Gradient is less than optimum, but for a near obstacle we dare not reduce the gradient to accelerate to the higher speed and clean up until passing the obstacle. Now consider a 'composite' climb, initially at Flaps 10°, then a much reduced climb gradient whilst we accelerate to clean speed, and a final improved climb gradient in the clean configuration. At one point, the two vertical paths will cross each other. Prior to their intersection, Flaps 10° is the answer, beyond the point, the clean configuration will provide the optimum climb gradient locus. Therein lies the definition of "near-in" and "far-out".

Regards,

Old Smokey

[LGB]

How clever! Of course.

Any idea of what order of magnitude this is, realizing that many factors are involved here?

Yeah, I know, I can get the books out and do the math, or I can make a practical test (NOT :rolleyes: )

If there were no obstacles, no noise abatement procedures, but I wanted to be as high as possible when reaching a certain point (let's just say 50 track miles after take-off), I would want to clean up as early as possible, then go Vx (clean) I guess.

PANS OPS III to IV, the acceleration segment (introduced in III) was cancelled again, is there any link between the above and said acceleration segment?

[Old Smokey]

There are far far too many variables involved, in particular the acceleration distance required for clean-up, and the loss of gradient involved.

(1) If it is a near-in obstacle that is of concern, maintain the flap setting and the appropriate lower speed until the obstacle is passed.

(2) If there are no 'near-in' problems, accelerate to Vx as quickly as possible when prudently safe after takeoff, as you have suggested.

As stated, far too many variables to categorically state an 'all purpose' best gradient profile, but a 'finger in the wind' guesstimate puts the crossover point somewhere between 20 and 30 miles for a typical heavy jet aircraft.

Regards,

Old Smokey

[LGB]

OK, thank you for the answers, I feel enlightened now :p

[mutt]

20 and 30 miles

All engine or engine out?

Mutt.

[Old Smokey]

Hi mutt,

All engines, bearing in mind that the acceleration to clean speed and clean up will be accomplished somewhat before 20 to 30 miles, but the crossover point occurring later, about 20 to 30 miles out.

All 'finger in the wind, a guesstimate for day to day observations - no rocket science. With OEI, 20 to 30 miles would only represent the point where 4th segment began.

Regards,

Old Smokey

[Intruder]

When accelerated and cleaned up, shouldn't the gradient be better than at flap 10, even though a higher speed must be maintained? (disregarding loss of climb when accelerating and cleaning up)

Yes, and therein lies the "great Quaker Life debate" over Flaps 10 vs Flaps 20 for takeoff...

Flaps 20 gives a shorter ground roll and lower takeoff speed, but lesser climb gradient. Flaps 10 takeoffs often require higher thrust settings and may contribute to faster tire wear and more close-in noise.

SOPs among 747 operators vary as to which is preferred or mandated. It will likely be the case for some time...

[411A]

For close in/medium distance obsticles, Boeing (on the B707) and Lockheed (for the TriStar) produced specific runway charts for overspeed/improved climb performance (for several takeoff flap settings) which, if properly applied, works quite good.

This is only applicable however, where excess runway length is available, for these slightly higher V speeds.

ZRH, runway 16, is a perfect example, where this has worked for me in the past.

Several ways to skin the cat.:}

[Old Smokey]

Intruder,

Just like the Flaps 10° to Clean transition, there is also a 'crossover' point for the obstacle clearance advantages when considering Flaps 10° Vs Flaps 20°. Near in, Flaps 20° gives the best runway and obstacle performance, after the 'crossover' point, Flaps 10°, at the expense of poorer runway performance, offers the best obstacle clearance.

Regards,

Old Smokey

[Intruder]

But where is the crossover point, with reference to the standard climb segments?

[mutt]

often require higher thrust settings

Runway dependant, Flaps 10 normally gives a higher takeoff weight that then results in a greater thrust reduction, so i dont understand your comment!

Mutt

[Intruder]

Given the same runway and gross weight, Flaps 10 will require a higher takeoff speed than Flaps 20. Therefore, if runway limited, more thrust will be required to accelerate the airplane to T/O speed.

[john_tullamarine]

Are we missing a point in that the operator has a number of ways to achieve the goal of getting this much stuff off this runway on this particular day ?

The operator pays the bills and has the right to nominate how the aircraft should be flown, in terms of setting the priorities for the set of takeoff configurations published - subject to the captain's veto on the day due operational assessment causing a less preferred option in the nominated set to be flown.

[Ignition Override]

If the fix is within 8 miles or so, how about after flap/slat retraction pushing the throttles to climb instead of quiet EPR? At Chicago O'Hare (ORD), there is no way to always be sure about 3000' at 5 miles or 4000' at 8, so I tell the other pilot that quiet Epr is OUR choice-we are the pilots and are still expected to make these oft-forgotten restrictions and with reduced (5*) flaps or a tailwind, we will either forget the quiet EPR to 3,000' or tell Dep. Control "Dep, FrozenTundra 292 is ------------- 4,000" . "-Blocked----Say again...".

To heck with reduced climb power at times like that. We would probably have our transmission blocked anyway if we tried to warn ATC; US ATC operations are based, historically, on many corners being cut, in terms of readbacks and extra warnings or concerns at larger airports or enroute from the Great Lakes to the East Coast. The Captain makes whatever decisions he or she considers necessary-the book is a good guide, but no replacement for pilots. And we don't have "magic" nor autothrottles, just two hot, very busy pilots (even with operative APU air), two VORs :uhoh: and an altitude hold .:)