Isn't it supposed to be wicked dangerous to take off a jetliner with no slats extended?

 

fwiw.

 

http://en.wikipedia.org/wiki/Delta_Air_Lines_Flight_1141

Isn't it supposed to be wicked dangerous to take off a jetliner with no slats extended?

Whats wicked is you spent half a million on a RAAS system and never listened..;-)

Isn't it supposed to be wicked dangerous to take off a jetliner with no slats extended?

In that case every Fokker 70/100, F.28, ERJ, CRJ100/200 and BAe146/RJ take off is wicked dangerous.

Every thing that differs from the house operation instruction can result in something dangerous.

 

I'm splitting hairs a little here, but that's not quite right.

 

Slats themselves don't add any lift at all. They permit the wing to fly at a higher angle of attack before stalling. As lift is proportional to angle of attack, slats delay the onset of stall, allowing the wing to generate more lift.

 

And flaps may generate some drag, but not many departures are power limited, so I'm not sure how that's relevant.

Ok, that's mostly correct, but, the take off is accomplished by rotating the aircraft, in that case the lift must be sufficient and there must be a sufficient speed to let the AOA within limits. As previously told by vagabondo, the tyres, and other systems are not made to resist at a too high speed, so, slats are required in order to achieve a good AOA that let the aircraft lift from ground with no risk of a stall.

The flaps drag cannot be not relevant, it is relevant if flaps are in a landing configuration, where the drag is pretty high. It is for that reason that the plane cannot TO with flaps 25 to 40.

It is less relevant with TO configurations where basically the added lift is better than the produced drag.

Every thing that differs from the house operation instruction can result in something dangerous.

 

Ok, that's mostly correct, but, the take off is accomplished by rotating the aircraft, in that case the lift must be sufficient and there must be a sufficient speed to let the AOA within limits. As previously told by vagabondo, the tyres, and other systems are not made to resist at a too high speed, so, slats are required in order to achieve a good AOA that let the aircraft lift from ground with no risk of a stall.

The flaps drag cannot be not relevant, it is relevant if flaps are in a landing configuration, where the drag is pretty high. It is for that reason that the plane cannot TO with flaps 25 to 40.

It is less relevant with TO configurations where basically the added lift is better than the produced drag.

Of course flaps in landing config create lots of drag, they're supposed to. I've no idea what that has to do with slats on takeoff though.

In that case every Fokker 70/100, F.28, ERJ, CRJ100/200 and BAe146/RJ take off is wicked dangerous.

Alright, so not EVERY single jet liner.

Of course flaps in landing config create lots of drag, they're supposed to. I've no idea what that has to do with slats on takeoff though.

Nothing

Slats point down therefore don't they remove lift?

Slats point down therefore don't they remove lift?

No, extending slats increases the angle of attack that the wing stalls at so the stall speed is reduced.  That means the takeoff speed is reduced too.

Disclaimer, I'm not an Aero Engineer, and I'm trying to relate this in some basic terms, so go easy if are, and you feel like picking it apart:

 

Perhaps it would be good to jump down to some aerodynamics 101 here, because a lot of people seem to not fully understand how wings work.

 

Wings generate lift through forward movement through the air, which causes a difference in pressure above and below the wing (in the case of asymmetric wings).  You may be thinking "well duh," but I have to make sure we're on a common ground of knowledge here.

 

As forward movement creates lift - ignoring all of the tire limit speeds and so on - it is perfectly possible to take off without flaps on any aircraft.  You cruise without flaps and slats, so clearly their lift and AoA alterations are not needed for flight.

 

For the purposes of this discussion, you can consider flaps/slats as "flight envelope and margin of error increasing devices."  What I mean by that is that the flaps and slats allow the aircraft to operate at slower speeds without compromising safety.  They offer a margin of error between the AoA at which you'd like to operate, and the AoA at which the wing will stall.

 

Stalls are based off of AoA.  While you can make inferences into the speed at which that AoA is hit - this is how they come up with stall speeds - in the end, it's the AoA that determines where you're going to stall.

 

Isolating each surface:

Slats

Allow the wing to operate at a higher AoA.  Slats do not generate lift directly.  Deploying slats delays the onset of boundary separation, which means that the AoA at which the wing stalls is increased.  Allowing the wing to operate at higher AoA allows the wing to generate more lift, but at the cost of induced drag.

 

Slattish, but not: Krueger Flaps

Allow the wing to operate at a higher AoA.  They generate lift through the alteration of the wing shape.  Deploying Krueger flaps increases wing camber and the coefficient of lift.  As they deploy downward from the leading edge of the wing, this moves the chord line in the negative (nose down) direction decreasing the AoA, while also increasing camber.  This increased camber produces an increase in lift.

 

Flaps:

In general - because there are so many different types of flaps - wing flaps for airliners increase both the planform area (can be approximated as "surface area") and camber of the wing.  While increasing the coefficient of lift (as in the cases above) increases lift, so does increasing wing area.  This increase in area allows the aircraft to generate the same amount of lift at lower speeds.  As the flaps increase in their angle, the lift generated also increases, but the wing camber also increases generating more drag.  Deploying flaps actually increases AoA (remember, AoA is the chord line versus the relative motion).  Because of this increase of AoA, the wing will stall earlier than in a clean configuration (again, angle- not necessarily speed-wise).  In order to combat this increase in AoA, many large aircraft also employ leading edge devices, above, that decrease AoA.

 

 

 

 

TL;DR:

Excepting tire limit speeds and so on, you can make a perfectly safe takeoff without flaps.  Concorde did it for 27 years, but it also had some pretty special tires rated for 220 knots.  Within the limits of a normal operational environment, you'd be pushing your luck with your tire limit speeds by taking off without flaps and slats, but it could be possible depending on the aircraft, weight, and weather.

I got this quote from a pilots BB, and I quote:-I have flown out of Fallon NAS in Nevada a number of times inDC9's. Fallon is at 3934 ft and when the temp is above 110f (verycommon in the summer, sometimes as high as 125F) Normal procedureis a very long roll with the flaps always up. I have seen airforce737 navigation trainers take off there too, also with flaps up.Ken Ehlers, PA-C<103441.3241>

That's called an improved performance takeoff. The idea is if you have a long runway to build a lot of speed before rotation then you use the extra momentum to gain altitude more quickly. It helps with the critical stages of the climb especially if you lose an engine.

 

Unrelated to that I used to fly the Fokker 100. It was normal to do no flaps takeoffs in that.

I thought I'd do some flight tests as I certainly don't recall accelerating to the high speeds the earlier posters guessed at when inadvertently taking off with no flap.  I set up a max weight takeoff condition (174.7 tonnes, 20% CG) with no derates.  Initially I input flap 5 for takeoff and the FMC gave a V2 of 159 knots.  I rotated at the computed Vr and followed the FD.  AOA got to over 12 degrees but the aircraft flew.  In the air the stall bricks appeared showing Vstall at about 165, but by that time IAS was around 174.  Redoing this with flap 5 actually selected meant the stall speed was reduced considerably.

 

I then tried a flap 1 takeoff, which produced a V2 of 165, just about stall speed.  Naturally this flapless take off was less problematic.  After takeoff, selecting flap 1 resulted in about 1 deg less AOA on climbout and a reduced Vstall.

 

So as I said earlier in the thread the flapless takeoff is possible at programmed V speeds, but margins to stall are reduced to nothing and for Flap 5 if you try and maintain V2 you will be in the stall region.  My NGX didn't actually stall but it was giving a buffet alert, so in reality it would be very uncomfortable in the aircraft.  When thrust reduces to CLB the situation gets worse, but if you are at V2+20 the sim flies with no problem.

 

Unrelated to that I used to fly the Fokker 100. It was normal to do no flaps takeoffs in that.

 

I flew in the back of a couple of those from IAD-ORD and back.  By the time I'd gotten to them, though, they were pretty well beaten up.