# How do slats reduce stall speed?

I've read in PHAK that slats reduce stall speed, but in what way? Is it because of the increase in surface area?

• Could you include a link (or better description) to what PHAK exactly is?
– Bram
Aug 19, 2019 at 10:17
• I'm sorry, but I actually read it somewhere else (I forgot where, I read too many stuff), but PHAK is Pilot's Handbook of Aeronautical Knowledge by FAA ( faa.gov/regulations_policies/handbooks_manuals/aviation/phak/… ). Aug 19, 2019 at 13:08
• In some very rare cases they don't just reduce stall speed but eliminate it altogether.
– vsz
Aug 20, 2019 at 4:02
• Nice info! But it says there that it only has a very low stall speed (50kph), not necessarily eliminated? Aug 25, 2019 at 1:49

It's because leading edge devices allow a higher Angle of Attack.

The four types of leading edge devices work by pointing the nose of the wing downwards so that at higher AoA there is no flow separation at the upper surface near the nose. Deflecting the slats does not increase $$C_L$$, the aeroplane must increase AoA to do that. Lower stall speed comes with the higher $$C_{Lmax}$$

From this answer, which also contains a graph showing that flaps increase $$C_L$$ at constant AoA.

• The leading edge flaps are the ones which deflects downward, and the slats technically extend forward to create slots (which prevents immediate airflow separation) don't they? I'm a bit confused with the main difference of these two, because they function the same Aug 19, 2019 at 14:31
• Yeah their function is the same, of all four: re-align the nose to be more in line with the free stream at high AoA. Note that if increased wing area would be a factor, the $C_L$ would rise at constant AoA, which it doesn't. Aug 19, 2019 at 14:59

The two main leading edge devices are leading edge slots and leading edge flaps:

Slots energize the boundary layer to delay stall by allowing higher pressure air from below to "leak" in a controlled way to the upper surface. The slot is effectively a convergent duct, large at the bottom and small at the top, so the air is slightly accelerated as it flows through the outlet at the top, more or less parallel to the skin surface. Because the air is slightly accelerated relative to the freestream just above it, being squeezed through a convergent duct in effect, Coanda effect enhances its ability to follow the upper surface contour and help the freestream stay attached.

Leading edge flaps, basically drooping leading edges, increase wing camber the same way that a trailing edge flap does, increasing Clmax somewhat at a given angle of attack.

If I combine the leading edge SLoT and flAp together, I get a SLAT. The slat extends its nose down and forward from the normal LE position, increasing camber somewhat, like a LE flap. The slot created when the slat extends energizes the boundary layer and delays separation, allowing the wing to generate lift into the mid 20 degree range. So you get a little bit higher CLmax for a given AOA and it can operate to a much higher AOA, giving you the highest possible CLmax and lowest stall speed.

Fixed slots were all the rage in the 30s and 40s to control flow separation forward of the ailerons on airplanes like the Globe Swift, negating the need for wing washout, but these didn't really reduce stall speed, just kept the ailerons working in the stall. They fell out of favour because the drag penalty of the always open slot was worse then the efficiency loss of washout.

• I see what you did there. Very nice way to remember it! Aug 25, 2019 at 1:53

Slats increase the camber of the wing, which increases the coefficient of lift.

When you deploy slats, the AOA is actually lowered for that part of the wing. This is the same washout principle Dunne ingeniously applied to the D.8 swept wing bi-plane in 1912!

You now have a more heavily cambered wing at a lower angle of attack, so more lift is to be had by increasing pitch more. Since the Clift is higher, one can generate the same amount of lift at a lower speed.

Airflow underneath the wing and its contribution to lift may be worth further study. The action of the air curling around the wing leading edge and impacting the "underside" in an upwards and forward manner (Eye of Jupiter?) or a bit of compression may help explain why they work so well (in addition to the benefits of lift creation by increasing camber on the upper part of the wing).

Slats are greatly helpful in reducing the AOA and increasing the Coefficient of Lift of part, or all, of the wing. You can now pitch up a little more, but be careful.

• Slats are greatly helpful in reducing the AOA and increasing the Coefficient of Lift is not what they do. Leading edge devices don't raise $C_L$ at constant AoA, see graph. They are not helpful in reducing AoA, they allow for greater AoA without stalling. Aug 25, 2019 at 6:17
• Mate - please look it up. AoA as defined by the angle relative to the free stream. Aug 25, 2019 at 10:38
• But it’s OK, I can delete the comments if you like. Aug 25, 2019 at 10:39
• Right, look at the chord line angle to the free stream after the slat is dropped. What is there is a more cambered wing at a lower AOA. So that PART of the wing now has different properties. Lower AOA AND more camber. So you've washed out the wing tip (relative to the rest of the wing). The reason why this is so important (slats really helped the Sabre Jet (the first of the too small horizontal stabilizer (guess what happens with a swept wing))aircraft) to understand their role in lowering AOA relative to the rest of the wing and assisting horizontal stabilizer in preventing stall. Aug 25, 2019 at 13:40
• Because when an aircraft goes to excessive AOA, the lift of the horizontal stabilizer should be going positive to help lower AOA, even without neat tricks like pushing the stick forward (ha, ha, ha). Good slats will also help as will generate a pitch down as they (on a swept wing) are behind the CG, and by virtue of their increased lift at higher alpha (from your graph). Aug 25, 2019 at 13:53