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Ruddman
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Why does a 737-700 use flap 1 and not zero flap for take off?

Sun May 15, 2022 11:54 am

Since it’s such a small amount of extra lift generated?
I’ve been told by a real pilot, using 26k ‘bump’ thrust, at MTOW, a 737-700 only needs around 2500-3000 ft maximum runway for take off using this flap setting. So why not just use zero flap?
 
JustAnFO
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Sun May 15, 2022 12:42 pm

No model of the 737 is certified for a flaps-up takeoff. The only approved takeoff flap settings are 1, 5, 10, 15, and 25.

I suppose an operator could apply to have the type certificate amended to permit flaps-up takeoffs, but that would likely be a fool’s errand in terms of cost/benefit.


Sent from my iPhone using Tapatalk
 
DH106
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Sun May 15, 2022 4:00 pm

Since Flap 1 extends the LE devices I'd say that there'd be considerably more than just 'a small amount of extra lift generated'.
 
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zeke
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Sun May 15, 2022 4:29 pm

Ruddman wrote:
Since it’s such a small amount of extra lift generated?
I’ve been told by a real pilot, using 26k ‘bump’ thrust, at MTOW, a 737-700 only needs around 2500-3000 ft maximum runway for take off using this flap setting. So why not just use zero flap?


More for the leading edge flaps and slats
 
GalaxyFlyer
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Sun May 15, 2022 10:58 pm

I’d like to see take-off data showing a TORR of 3,000’, too.
 
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Starlionblue
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Sun May 15, 2022 11:50 pm

DH106 wrote:
Since Flap 1 extends the LE devices I'd say that there'd be considerably more than just 'a small amount of extra lift generated'.


The leading edge devices don't so much increase lift as increase the AoA margin, though of course as you "use" the higher AoA, you do get more lift.

Either way, a higher AoA margin does come in handy for takeoff. :)

Image
 
kalvado
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 12:01 am

Starlionblue wrote:
DH106 wrote:
Since Flap 1 extends the LE devices I'd say that there'd be considerably more than just 'a small amount of extra lift generated'.


The leading edge devices don't so much increase lift as increase the AoA margin, though of course as you "use" the higher AoA, you do get more lift.

Either way, a higher AoA margin does come in handy for takeoff. :)

Image

AoA as measured by body to airflow angle or AoA as wing chord to airflow angle? I believe B uses body definition . And leading edge devices would allow higher body angle at a cost of lower chord AoA.
And I would think that tail strike is more limiting than stall at takeoff
 
Okcflyer
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 2:22 am

While it's an interesting academic question to ask, the first question is what benefits may be realized from the configuration IF it were possible. What were you thinking the benefits are/were? I'm just not getting what those might be. The 737 reaches flat retraction speeds very quickly even at reduced thrust. I just can't imagine a benefit that makes it worth the (many) cons.
 
kalvado
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 2:38 am

Okcflyer wrote:
While it's an interesting academic question to ask, the first question is what benefits may be realized from the configuration IF it were possible. What were you thinking the benefits are/were? I'm just not getting what those might be. The 737 reaches flat retraction speeds very quickly even at reduced thrust. I just can't imagine a benefit that makes it worth the (many) cons.

And what are the cons you think of?
Thinking about it, it may be about lift and drag control by leading edge devices during takeoff roll, and corresponding tyre traction. Seems that floor angle in cruise and at gate also comes into it....
 
GalaxyFlyer
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 3:24 am

kalvado wrote:
Starlionblue wrote:
DH106 wrote:
Since Flap 1 extends the LE devices I'd say that there'd be considerably more than just 'a small amount of extra lift generated'.


The leading edge devices don't so much increase lift as increase the AoA margin, though of course as you "use" the higher AoA, you do get more lift.

Either way, a higher AoA margin does come in handy for takeoff. :)

Image

AoA as measured by body to airflow angle or AoA as wing chord to airflow angle? I believe B uses body definition . And leading edge devices would allow higher body angle at a cost of lower chord AoA.
And I would think that tail strike is more limiting than stall at takeoff


Tail strike, Vmu, must be limiting to prevent a ground stall. Part 25 requires the plane can take-off at a geometrically limited body angle.
 
DH106
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 5:49 am

Starlionblue wrote:
DH106 wrote:
Since Flap 1 extends the LE devices I'd say that there'd be considerably more than just 'a small amount of extra lift generated'.

The leading edge devices don't so much increase lift as increase the AoA margin, though of course as you "use" the higher AoA, you do get more lift.
Either way, a higher AoA margin does come in handy for takeoff. :)


My understanding is that slats in the takeoff position usually have the slot sealed, and so in effect act in a similar manner to the inboard kruger flaps in that they increase the chamber of the wing in a similar manner to the trailing edge flaps. To increase the AoA margin, doesn't the slat have to be fully deployed to open the slot and thus provide increased AoA margin by accelerating the air through the slot, reenergizing the boundary layer?
 
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jetmech
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 11:38 am

DH106 wrote:
To increase the AoA margin, doesn't the slat have to be fully deployed to open the slot and thus provide increased AoA margin by accelerating the air through the slot, reenergizing the boundary layer?

Slats increase the useable AOA in the sealed position.

This is due to the increase in leading edge radius they provide, which reduces the suction peak in the region of the leading edge.

Reducing the suction peak in turn, reduces the following adverse pressure gradient. As such, the airfoil can achieve a higher AOA before flow separation occurs.

Regards, JetMech
 
kalvado
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 1:29 pm

jetmech wrote:
DH106 wrote:
To increase the AoA margin, doesn't the slat have to be fully deployed to open the slot and thus provide increased AoA margin by accelerating the air through the slot, reenergizing the boundary layer?

Slats increase the useable AOA in the sealed position.

This is due to the increase in leading edge radius they provide, which reduces the suction peak in the region of the leading edge.

Reducing the suction peak in turn, reduces the following adverse pressure gradient. As such, the airfoil can achieve a higher AOA before flow separation occurs.

Regards, JetMech

How much of effect can be achieved with trailing edge devices only?
I suspect leading edge role, at least partially, is to kill lift (hence drag - while also improving traction) during takeoff roll. If we want cabin floor to be more or less level both in cruise - when positive AoA is a must; and at the gate - hence during takeoff roll - moving leading edge for takeoff can be beneficial.
 
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zeke
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 4:24 pm

jetmech wrote:
Slats increase the useable AOA in the sealed position.


On the 737 inboard of the engines it has leading edge flaps, it has slats outboard of the engine.
 
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jetmech
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 5:20 pm

kalvado wrote:
How much of effect can be achieved with trailing edge devices only?

Not sure to be honest.

What I can say is the main-plane and trailing edge flaps operate much more effectively when interacting with attached flow. The role of leading edge devices is to promote attached flow by delaying or reducing the likelihood of flow separation.

kalvado wrote:
I suspect leading edge role, at least partially, is to kill lift (hence drag - while also improving traction) during takeoff roll.

I'm really not sure about this. I can understand killing lift upon landing to get as much weight on the wheels and increase traction for braking, but takeoff?

Regards, JetMech
 
kalvado
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 6:52 pm

jetmech wrote:
kalvado wrote:
I suspect leading edge role, at least partially, is to kill lift (hence drag - while also improving traction) during takeoff roll.

I'm really not sure about this. I can understand killing lift upon landing to get as much weight on the wheels and increase traction for braking, but takeoff?

Regards, JetMech

1. Lift means drag, longer takeoff roll etc.
2. I don't know how much lift you need to depart surface and go into ground proximity effect, but I am pretty sure one of the points of rotation technic is to have a positive "turn on lift and start climbing" moment, as opposed to soft field takeoff. Maybe some old taildragger videos can be helpful.
 
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Ruddman
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Mon May 16, 2022 6:52 pm

GalaxyFlyer wrote:
I’d like to see take-off data showing a TORR of 3,000’, too.



Same as I've only seen charts that show around 5200 ft at MTOW (obviously I'm not a pilot nor claim to have much knowledge in aeronautical subjects.) 26k Engines. Not sure what flap setting.

I was just curious though because I've seen it written where the -700 often uses Flap 1 for take-off. So at lighter weights and long runways (10-12000 ft), what really is the risk that Boeing saw where a zero flap take off isn't allowed?

Allowed as in different to possible.
 
GalaxyFlyer
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Tue May 17, 2022 12:09 am

Remember, at light weights, de-rates will result in shorter TORR due to lower Vmcg at the de-rated thrust. I believe that TORR at MGTOW, standard day, but likely not Flaps 1, probably F10 or F15. The reason for reducing flaps is second segment climb, not take-off run
 
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jetmech
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Tue May 17, 2022 5:45 am

kalvado wrote:
1. Lift means drag, longer takeoff roll etc.

Sure, but you're going to have huge amounts of drag on takeoff in any case due to the extended landing gear and in many cases, partially extended trailing edge flaps.

If anything, extended leading edge devices reduce the suction peak and thus may slightly reduce overall lift, but any drag reduction benefits from this are likely to be relatively small.

Furthermore, I suspect the amount of lift - and hence lift induced drag - produced by the wing is not too significant prior to rotation.

Regards, JetMech
 
kalvado
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Tue May 17, 2022 2:43 pm

jetmech wrote:
Furthermore, I suspect the amount of lift - and hence lift induced drag - produced by the wing is not too significant prior to rotation.
Regards, JetMech

And I would think we want it that way. Another datapoint for the lift generated is that a clean wing in cruise produces 1g at about zero pitch. A takeoff roll with clean wing would have same pitch, lower speed, higher air density, and a ground effect in addition. Not sure where it would end up, but probably a fairly significant number.
 
744SPX
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Wed May 18, 2022 3:38 am

Ruddman wrote:
GalaxyFlyer wrote:
I’d like to see take-off data showing a TORR of 3,000’, too.



Same as I've only seen charts that show around 5200 ft at MTOW (obviously I'm not a pilot nor claim to have much knowledge in aeronautical subjects.) 26k Engines. Not sure what flap setting.

I was just curious though because I've seen it written where the -700 often uses Flap 1 for take-off. So at lighter weights and long runways (10-12000 ft), what really is the risk that Boeing saw where a zero flap take off isn't allowed?

Allowed as in different to possible.


I'd bet a 737-600 with the max thrust option could pull that TO distance off. Certainly with the APB winglets which were sadly not certified for the -600 because it wasn't deemed to be worth it for so few aircraft.
 
GalaxyFlyer
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Wed May 18, 2022 4:10 pm

You don’t understand take-off performance—the 737 will do better with a de-rate simply because of Vmcg will be lower and the plane won’t have to accelerate to the higher Vmcg. Also, don’t confuse AEO run with take-off distance required which plans for the OEI case.
 
kalvado
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Wed May 18, 2022 5:46 pm

GalaxyFlyer wrote:
You don’t understand take-off performance—the 737 will do better with a de-rate simply because of Vmcg will be lower and the plane won’t have to accelerate to the higher Vmcg. Also, don’t confuse AEO run with take-off distance required which plans for the OEI case.

Do I understand it correctly that such Vmcg difference means that v1 is lower for low thrust configuration? Does that cover actual set thrust, or maximum possible thrust for the airframe?
How does that affect actual takeoff roll - higher thrust means more runway required for engine failure, but less of an actual roll?
 
GalaxyFlyer
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Re: Why does a 737-700 use flap 1 and not zero flap for take off?

Wed May 18, 2022 9:49 pm

kalvado wrote:
GalaxyFlyer wrote:
You don’t understand take-off performance—the 737 will do better with a de-rate simply because of Vmcg will be lower and the plane won’t have to accelerate to the higher Vmcg. Also, don’t confuse AEO run with take-off distance required which plans for the OEI case.

Do I understand it correctly that such Vmcg difference means that v1 is lower for low thrust configuration? Does that cover actual set thrust, or maximum possible thrust for the airframe?
How does that affect actual takeoff roll - higher thrust means more runway required for engine failure, but less of an actual roll?


Yes, lower thrust rating will produce a lower Vmcg and Vmca. Those speeds are “red lines” to protect controllability and must be considered for all take-off planning. Vmcg dictates V1 and minimum Vr. A lower Vmcg will normally be more advantageous than increasing thrust and accepting the higher Vmcg and related V1.

All take-offs are planned using the RATED power selected, set thrust is not a factor. Changing the thrust rating in the FMS is, for planning, exactly like installing a different engine. This is very different than “reduced thrust” using assumed temperature, note, the thrust rating is the same when using reduced thrust and rated thrust Vmcg and Vmca are used.

Every take-off plans on engine loss, so using a incorrect, but greater, thrust rating would shorten the ground run, but compromises controllability in the event engine loss.

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