747 Less thrust = Loss of Lift???

[atpcliff]

FCTM says when u lose thrust in a engine, u will also lose lift...I think it says stall speed increases 4 kts?

I don't understand...

Any and all answers and commentary welcome!!!

Thanx!

[peekay4]

I think this is actually loss of maneuvering margin when an engine fails during a flap retraction?

The way I understand it: after takeoff when you retract flaps, some lift is lost. But because the airplane is also accelerating (higher speed == more lift) you also gain lift and can maintain maneuvering margin.

However, if while retracting the flaps you lose an engine, now you're not accelerating as fast (or at all). You're not generating as much lift and therefore you've effectively decreased your maneuvering margin compared to a normal climb.

So the 4 kts is the equivalent loss of the maneuvering margin, not an increase of stall speed.

(I'm sure someone will correct me if I'm off base here).

[john_tullamarine]

Suggest we are looking at the vertical component of thrust's providing some defacto lift. Stall speed with lots of thrust will be less than nil thrust. Hence stall speeds for certification relate to low thrust.

[tom775257]

Assuming a seized N1 (or perhaps even just windmilling), will the turbulent air spilling off the engine affect lift, especially at high AoA?

[wiggy]

atpcliff

It's not "my" equipment any more but I don't remember being told/warned about a 4 knots difference if engine out and I can't find any mention of it in the current version of the 747 FCTM I have access to. Can you provide a reference?

[oggers]

What JT said plus loss of thrust>pitch down moment>more elevator back pressure required= net loss of lift. Maybe.

[peekay4]

I found the FCTM wording in Chapter 1 General:


Conditions Affecting Maneuver Margins

For a fixed weight and altitude, maneuver margin to stick shaker increases when airspeed increases. Other factors may or may not affect maneuver margin:

Gross weight: generally maneuver margin decreases as gross weight increases
Altitude: generally maneuver margin decreases with increasing altitude for a fixed airspeed
Temperature: the effect of a temperature change on maneuver margin is negligible
Landing gear: a small decrease in maneuver margin may occur when the landing gear is extended. This loss is equivalent to 2 knots of airspeed or less
Speedbrakes: generally maneuver margin decreases when speedbrakes are extended
Engine failure during flap rectraction: a small decrease in maneuver margin occurs due to the reduced lift experienced with the loss of thrust. The loss is equivalent to 4 knots of airspeed or less



Notice the 4 knot loss of maneuver margin is only for engine failure during flap retraction.

[JammedStab]

I found the FCTM wording in Chapter 1 General:

Conditions Affecting Maneuver Margins

For a fixed weight and altitude, maneuver margin to stick shaker increases when airspeed increases. Other factors may or may not affect maneuver margin:
•Gross weight: generally maneuver margin decreases as gross weight increases
•Altitude: generally maneuver margin decreases with increasing altitude for a fixed airspeed
•Temperature: the effect of a temperature change on maneuver margin is negligible
•Landing gear: a small decrease in maneuver margin may occur when the landing gear is extended. This loss is equivalent to 2 knots of airspeed or less
•Speedbrakes: generally maneuver margin decreases when speedbrakes are extended
•Engine failure during flap rectraction: a small decrease in maneuver margin occurs due to the reduced lift experienced with the loss of thrust. The loss is equivalent to 4 knots of airspeed or less


Notice the 4 knot loss of maneuver margin is only for engine failure during flap retraction.

These items are listed for the 747-8. For some reason, only the first one is listed for the 747-400.

[peekay4]

This is not type specific I think... you will find the same or similar wording even for the 777, 787, etc.

[tdracer]

Assuming a seized N1 (or perhaps even just windmilling), will the turbulent air spilling off the engine affect lift, especially at high AoA?
That particular aspect is designed for. If you look at the inboard side of most high-bypass engine inlets, you'll see a large strake (we sometimes call that the 'engine ear' :E) I don't recall off hand if said strake is used on the 747-400, but its on the same engines on the 767 (and is definitely on the 747-8). It's purpose is to energize the spillage airflow from a shutdown engine during low speed/high angle of attack conditions so that there isn't airflow separation over that area of the wing during approach/landing conditions.

[wiggy]

peekay

Found it thanks, (must read the book again....).

As you say..

This is not type specific I think... you will find the same or similar wording even for the 777, 787, etc.

(actually it's not on our 777 sub-para of the joint FCTM, but given it's seems to almost apply across the board that might be an editorial issue).

As for a reason: If pushed my gut feeling is that JT has it (thrust against weight) - at/just after take-off you've got a combination of highish AOA/body angle, and changing configuration towards clean all with high power. Maybe once you're clean and the AOA's and/or body angle has reduced or at times of lower power (e.g on approach) the effect is less pronounced and the margin is less critical so Boeing don't think the effect is worthy of further mention....perhaps.........

[peekay4]

Hmm, possibly, but the FCTM wording specifically calls out "during flap retraction".

If the effect is just about the thrust vector component then it should apply at any time during high-power, high-angle climb, not only during flap retraction?

[wiggy]

Don't disagree with you, there must be some logic somewhere.......

[Derfred]

Perhaps the logic is that the only time you intentionally fly below min manouever speed with an engine out is during flap retraction.

[JammedStab]

Perhaps the logic is that the only time you intentionally fly below min manouever speed with an engine out is during flap retraction.

Not on the 747-400.

[Gysbreght]

Of course JT and wiggy have it right. The thrust contribution to lift is greatest at high AoA. Flap retraction has nothing to do with the lost of thrust component, but does of course affect the lift. Even Boeing can be wrong sometimes.

[Derfred]

Not on the 747-400.

Ahh, ok sorry. It's a while since I flew that beast. It certainly is true on the 737.

[JammedStab]

Ahh, ok sorry. It's a while since I flew that beast. It certainly is true on the 737.

Might be considered if two engines out and lots of drag and trouble accelerating.

[john_tullamarine]

Couple of comments, if I may ...


If you look at the inboard side of most high-bypass engine inlets ..

I had thought that these were for a more general high alpha stall margin concern with larger engines ? Chines - another example of VGs in action - are associated with reduced nacelle-wing clearance and resultant shed vortex flow over the wing (rather than under as with the smaller diameter low bypass and straight jet engines of a bygone era).

See, for instance, -

(a) http://www.smartcockpit.com/aircraft-ressources/Vortex_Generators.html

(b) http://www.icas.org/ICAS_ARCHIVE/ICAS2012/PAPERS/542.PDF

(c) (having trouble linking to this paper - google Thierry Sibilli PhD thesis and Cranfield ) ... pp 155-177.

The theory can get a tad heavy but the pictures are great ... even slower chaps such as I can get the gist of what's going on ...

I recall, years ago during the early 733 days in Australia, a CASA airworthiness engineer asking me over coffee whether I would be concerned taking a 733 minus a chine without any engineering consideration. I think my ashen face gave him my answer ... it transpired that a local operator (the other one - not mine) had done just this a few days prior ... CASA had, quite appropriately, taken a dim view of the matter ...


the FCTM wording specifically calls out "during flap retraction"

Perhaps the OEM emphasises flap retraction as that is when the margins routinely are likely to be reduced. I recall, during a period working for an operator with 744s, that the problem was a routine crew concern for high weight departures.

[atpcliff]

Thanx for all the replies!

namaste

[tdracer]

John T, I suspect it depends on the wing/engine interface. Back during the original development of the 767, I was a fresh faced young engineering working nacelle aero - and at the time it was my understanding that we only needed the strake for the engine out condition.
Since then, engines have gotten progressively larger in diameter and more closely coupled with the wing (the 737 being an extreme example of that). So you may well be correct that it now goes beyond the engine out scenario.
On the 747-8, the need for the strake was identified very early on, and there was a pretty good fight between the aero types and where they wanted it to go, and the mechanical types that actually had to make the location workable :rolleyes:

[No Fly Zone]

Engineer, not transport driver, here...
Moderator John commented:"
"I recall, years ago during the early 733 days in Australia, a CASA airworthiness engineer asking me over coffee whether I would be concerned taking a 733 minus a chine without any engineering consideration. I think my ashen face gave him my answer ... (emphasis added) it transpired that a local operator (the other one - not mine) had done just this a few days prior ... CASA had, quite appropriately, taken a dim view of the matter ..."

I would take more than a dim view of this. I would not start the SOB in that condition, let alone fly with it. I'd like to think John is making fun or yanking our chains a bit here, but he does not do that. CASA's 'dim view,' must be the understatement of the year; they should have grounded the entire fleet where located, until complete inspections were done. This falls into the 'you've got to be kidding me,' class, except that again, Moderator John does not do that. CASA nailed it; a seriously stupid practice. I have to wonder if there was an investigation and/or report covering the circumstances. John?? If so, can you dredge up a link? Thanks...:eek:

[FE Hoppy]

looking at the list it appears that things that cause nose up pitch help and things that cause nose down don't!

Not really a surprise then that when you have to increase tail downforce you lose manoeuvre margin.

[john_tullamarine]

I suspect it depends on the wing/engine interface.

Indeed.

Presuming you work where I presume you do, I shall give due weight to your comments.

A great value of PPRuNe is the wealth of serious knowledge which is available from folks such as your goodself.


they should have grounded the entire fleet where located

As I understood the tale, it was an inappropriate one-off during the early post-introduction days of the model and not at all typical of the operation .. which was every bit as tight as that at the other mob for which I flew.

Interestingly, I don't recall any discussion of the chines during my endorsement on the Type around the same time. I guess the importance to the performance numbers hadn't filtered down to appropriate training levels within the system ?

[Linktrained]

Aircraft with their engines "underslung " have the advantage, normally, of more power tends to give a climb ( which may be compensated by the A/P). Looking at the traces of AF447, when power was reduced to Flight Idle briefly, the nose came down by a few degrees. (A/P was OFF) The aircraft may have been in an unrecognised stall at that time.

Even simple gliders tended to be wiinch launched from under the Pilot's seat . Or aerotowed from the nose. In each case this was to get the better performance.

Perhaps something like the 4 knots mentioned may apply to other aircraft with underslung engines, depending on where the engines are located in relation to the " Vertical C of G". (Usually one only considers the Fore and Aft C of G.)
LT