ANITIX87
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Full Flaps At Takeoff?

Fri Oct 22, 2010 5:54 pm

Hi, everyone.

Forgive the seemingly silly question, but a thought occurred to me while reading a thread on "hot and high" airports. I know that a standard takeoff uses the first or second flap setting available (and that on the 767-200 and A300, a flap-less takeoff is considered SOP as well).

If an aircraft is at a "hot and high" airport, where max tire speed could be an issue, why wouldn't the aircraft be able to use more flaps and reduce V2? Is this simply a limitation of flap retraction speed? If so, why not set full flaps just to be able to lift off and then just retract them at shorter intervals as you climb out and accelerate?

Or is this an issue with ground effect, or something else? I can't imagine it being an aerodynamic restriction, but that's why I'm asking, just in case there's something I'm overlooking.

I'd love some input. Thanks.

TIS
www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
 
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akiss20
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RE: Full Flaps At Takeoff?

Fri Oct 22, 2010 6:32 pm

The induced lift versus drag ratio from flaps is very non-linear. You get most of your extra lift with the first 15 or so degrees of flaps, after that you are creating mostly drag. Thus full flaps at takeoff is useless as you are getting barely anymore lift but a heck of a lot more drag.
Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
 
PapaChuck
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RE: Full Flaps At Takeoff?

Fri Oct 22, 2010 8:28 pm

If you're attempting to takeoff in a "hot and high" situation, engine-out climb performance will be a major consideration. Using full flaps just might get you off the ground a little quicker if everything is working, but it would be like dragging an anchor behind you once you get airborne. Having all that extra drag when you suddenly have to clear that apartment complex at the end of the runway with half of your original thrust could really ruin your day.
In-trail spacing is a team effort.
 
kalvado
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RE: Full Flaps At Takeoff?

Fri Oct 22, 2010 10:11 pm

Quoting AKiss20 (Reply 1):
Thus full flaps at takeoff is useless as you are getting barely anymore lift but a heck of a lot more drag.


Looks like I miss something here, would be great if someone can enlighten me:
What is the difference between takeoff and landing, making flaps useless on takeoff, but needed for landing?
Thanks!
 
b78710
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RE: Full Flaps At Takeoff?

Fri Oct 22, 2010 10:54 pm

Quoting kalvado (Reply 3):
What is the difference between takeoff and landing, making flaps useless on takeoff, but needed for landing?

landing you want to be going as slow as possible for the maximum amount of lift. ie. lots of drag to slow you down

taking off you want maximum lift for minimum drag, to get up to takeoff speed asap.

as said above. after a certain flap setting, your just adding drag rather than increasing lift
 
ANITIX87
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RE: Full Flaps At Takeoff?

Fri Oct 22, 2010 10:57 pm

Quoting kalvado (Reply 3):

Looks like I miss something here, would be great if someone can enlighten me:

If I'm understanding AKiss20 correctly, this is what I failed to grasp (and maybe what you're confused about).

At takeoff, you're depending on thrust to create airspeed and lift so you can accelerate and climb. Therefore, additional drag is really bad, especially at MTOW or at a "hot and high" airport. When you're landing, since the goal is to slow down and the plane is lighter after having burned off its fuel during the flight, you can afford the additional drag since you have additional engine thrust available to maintain a lower speed, which is the crucial point when landing to improve landing performance. Once you land, the drag also helps you slow down and dump lift faster.

Thanks for the input, everyone!!

TIS
www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
 
prebennorholm
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RE: Full Flaps At Takeoff?

Fri Oct 22, 2010 11:19 pm

Quoting kalvado (Reply 3):
Looks like I miss something here, would be great if someone can enlighten me:

Yup. First of all, both your assumptions are incorrect.

Quoting kalvado (Reply 3):
What is the difference between takeoff and landing, making flaps useless on takeoff...

Flaps are not useless for takeoff, they are used all the time for takeoff.

Quoting kalvado (Reply 3):
...but needed for landing?

They are not needed for landing, but very practical.

This thread is about optimal flap setting at takeoff from H&H airports. PapaChuck explains that perfectly in reply #2.

I might add that reduced engine power at H&H conditions makes PapaChuck's statement even more relevant.

It is not like it would be physically impossible to take off with landing flaps at 30-40 degrees. It is only so inefficient that I doubt that the plane manufaturers ever bothered to publish the performance data.

At H&H conditions most planes will be weight restricted in order to maintain the minimum climb gradient with an engine out. With the enormous drag from landing flaps that weight restriction would be much more severe.

Quoting ANITIX87 (Thread starter):
If an aircraft is at a "hot and high" airport, where max tire speed could be an issue

Tire speed is seldom an issue. Reduced TOW due to weight restrictions means that V2 speed is seldom significantly higher than at a sea level takeoff at MTOW.
Always keep your number of landings equal to your number of take-offs
 
PapaChuck
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RE: Full Flaps At Takeoff?

Sat Oct 23, 2010 2:46 pm

I'm not trying to say that full flap takeoffs have never been done, it's just that they aren't practical. Someone please correct me if I'm wrong, but I'm pretty sure the B-25s used on the Doolittle raid departed the carrier using full flaps because that resulted in the shortest takeoff roll. How does that relate to the original topic? It doesn't. Just some stupid trivia.
In-trail spacing is a team effort.
 
SchorschNG
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RE: Full Flaps At Takeoff?

Sat Oct 23, 2010 8:14 pm

One issue you have is pitch authority.
Flaps add significant nose-down pitch moment.
At full weight the pitch up authority might be an issue.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
aviopic
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RE: Full Flaps At Takeoff?

Sat Oct 23, 2010 9:36 pm

Quoting prebennorholm (Reply 6):
they are used all the time for takeoff.

Not on all aircraft.
Guess it all comes down to the chosen wing profile apart from other circumstances like temp, t/o weight, runwaylenght etc.

Quoting PapaChuck (Reply 7):
but I'm pretty sure the B-25s used on the Doolittle raid departed the carrier using full flaps because that resulted in the shortest takeoff roll. How does that relate to the original topic? It doesn't.

Maybe because there are many different type of flaps or maybe the "full" setting in terms of degrees is not so full compared to other aircraft.
The truth lives in one’s mind, it doesn’t really exist
 
SchorschNG
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RE: Full Flaps At Takeoff?

Sat Oct 23, 2010 10:07 pm

Quoting PapaChuck (Reply 7):
I'm not trying to say that full flap takeoffs have never been done, it's just that they aren't practical. Someone please correct me if I'm wrong, but I'm pretty sure the B-25s used on the Doolittle raid departed the carrier using full flaps because that resulted in the shortest takeoff roll. How does that relate to the original topic? It doesn't. Just some stupid trivia.

Those B-25 had ~50kts of headwind.
As the previous poster said, the B-25 had simple drop-down flaps.

Each flap setting has an optimum lift coefficient. Theoretically, one could take off with full flaps. It just doesn't solve any problem. The huge drag will reduce acceleration, and in case of one-engine-out the thrust of the remaining engine will be insufficient to cancel the drag. I can't see how a twin will have positive second segment climb rate with full flaps.

A modern airliner has a best lift-over-drag at full flaps of 8 to 10. More complicated flaps (like three-slotted) will see values like 7.A failed engine significantly reduces lift-over-drag, maybe pushing it to 5 to 7. Assuming a thrust-to-weight of .3 (twin), after an engine failure the minimum lift-over-drag is ~7. Anything below will see no positive climb rate.

Look at: http://www.youtube.com/watch?v=fI5xTmmPbsY
Especially last 1-2 minutes.
Too much flaps, to little thrust. As soon as you have lost your airspeed, you are in a world of pain.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
PapaChuck
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RE: Full Flaps At Takeoff?

Mon Oct 25, 2010 1:46 am

Check this out:

http://www.boeing.com/commercial/airports/acaps/767sec3.pdf

Take a look at page 13 where the takeoff performance tables start. For the 767, flaps 20 always results in the shortest takeoff run, but that setting can only be used at less than maximum takeoff weight. As weight and altitude increase, flap settings have to be reduced, and I assume that is because of engine-out climb performance as noted before. Also, at the top of the chart, max tire speed is factored in as well. Take a look at the takeoff performance tables for the other Boeing aircraft and you will find the same trends.

What does all this mean? More flaps mean less runway needed for takeoff, but other factors come in to play that prevent full flaps from being an option. Also, max tire speed can also be a limiting factor in a "hot and high" situation. Hope this helps!
In-trail spacing is a team effort.
 
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SEPilot
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RE: Full Flaps At Takeoff?

Mon Oct 25, 2010 2:13 pm

One factor not mentioned previously is that flaps are important in landing to help slow down. Going from cruising speed to landing speed while descending with jet engines (which provide considerable thrust even at idle) is not a trivial exercise. The landing gear adds considerable drag, but not enough (and I think flap deployment speed is usually higher than landing gear extension speed). My first flight instructor told me that up to 20 degrees flaps added more lift then drag, but over 20 added more drag than lift. That seems to hold true for all types that I have examined. Older Cessnas have 40 degrees of flaps; most of them will not climb at all with anything close to max weight at full flaps, even with full power.
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readytotaxi
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RE: Full Flaps At Takeoff?

Mon Oct 25, 2010 7:09 pm

http://www.flightlevel350.com/Aircra...rline_KLM_Aviation_Video-4845.html

Interesting landing, with full flaps at a high location. MD11
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DocLightning
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RE: Full Flaps At Takeoff?

Mon Oct 25, 2010 11:37 pm

Quoting PapaChuck (Reply 7):
Someone please correct me if I'm wrong, but I'm pretty sure the B-25s used on the Doolittle raid departed the carrier using full flaps because that resulted in the shortest takeoff roll.

I don't know if that's true, but supposing it was, a B-52 doesn't have the engine-out problems that a 767 does. If a B-52 loses one on takeoff, they "only" have seven remaining engines to maintain their planned gradient.
-Doc Lightning-

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XT6Wagon
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RE: Full Flaps At Takeoff?

Mon Oct 25, 2010 11:39 pm

one thing to note is early jet engines took a week to spool. ok, it only feels like a week when you needed more power. So landing in a high drag configuration allowed them to keep the engines power up, reducing the time to full power by a massive margin. Trying to land something like a comet or 707 with the engines at idle is more or less the same thing as trying to land one with the engines off. 12secs to generate meaningful thrust is a literal lifetime when things go wrong.
 
vikkyvik
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RE: Full Flaps At Takeoff?

Tue Oct 26, 2010 1:30 am

Quoting DocLightning (Reply 14):
I don't know if that's true, but supposing it was, a B-52 doesn't have the engine-out problems that a 767 does. If a B-52 loses one on takeoff, they "only" have seven remaining engines to maintain their planned gradient.

Read again. B-25, not B-52.

If there were B-52s at the Doolittle Raid, then someone managed to keep a pretty big secret!  
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
PapaChuck
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RE: Full Flaps At Takeoff?

Tue Oct 26, 2010 3:04 am

Actually, Doc may have inadvertently brought up a good point. Anyone familiar with B-52 engine-out procedures? It's not exactly a fair comparison to airline ops since a Buff doesn't carry passengers and the crew can punch out if things turn ugly. Still, eight vs. two engines has to make a difference. I wouldn't mind hearing a little more on the subject.
In-trail spacing is a team effort.
 
SchorschNG
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RE: Full Flaps At Takeoff?

Tue Oct 26, 2010 8:25 am

Quoting PapaChuck (Reply 17):
Actually, Doc may have inadvertently brought up a good point. Anyone familiar with B-52 engine-out procedures? It's not exactly a fair comparison to airline ops since a Buff doesn't carry passengers and the crew can punch out if things turn ugly. Still, eight vs. two engines has to make a difference. I wouldn't mind hearing a little more on the subject.

First, the crew cannot punch out, at least not all. 2 of the crew members have downward-facing ejection seats and those are not recommended to be used below 1000ft.
Second, the B52 cannot rotate at take-off. So it virtually needs to wait until the lift is sufficient to take off. It will usually lift off with slight nose down attitude.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
Woof
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RE: Full Flaps At Takeoff?

Tue Oct 26, 2010 12:07 pm

Quoting DocLightning (Reply 14):
If a B-52 loses one on takeoff, they "only" have seven remaining engines to maintain their planned gradient.

I never miss an opportunity to quote one of the many aviation related funnies:

[A military pilot called for a priority landing because his single-engine jet fighter was running "a bit peaked." Air Traffic Control told the fighter pilot that he was number two, behind a B-52 that had one engine shut down. "Ah," the fighter pilot remarked, "The dreaded seven-engine approach."]
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