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]

Originally Posted by 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.

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' ) 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]

Originally Posted by Derfred

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]

Originally Posted by Derfred

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...enerators.html

(b) http://www.icas.org/ICAS_ARCHIVE/ICA...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