# What theoretically would happen if the flaps were extending even further?

The angle of flaps is limited to a specific value. It's around 30° to 40° on most aircraft what I've seen.

But what happens when they get extended even further? 50°, 80°, 90°, ... Let's also say they don't break due to high load on them. I just want to know it theoretically. What will happen to the plane?

• Will drag be increased?
• At which point will the aircraft loose it's lift due to the extreme deformation of the wing?
• What about the slats?
• Are there any aircraft that use a higher than usual angle of flaps?
• Are you asking if the flap mechanism broke? Or if designers actually allowed the pilot to extend them to those additional detents? What kind of flaps are we talking about here? Fowler flaps? Plain? Split? Slotted? – Ron Beyer Apr 19 '17 at 19:11
• @RonBeyer I edited the question. I know, there are differences between the kind of flaps, but let's just talk about flaps in general, devices that create lift/drag. – Noah Krasser Apr 19 '17 at 19:18
• Further extension makes the flap act like spoilers as there is no airflow attached anymore so they are basically acting as an air brake. The flaps in the picture are so called slotted flaps, here the slot ensures that the flow is attached to the flap, but even those flaps will eventually stall as the angle of attack to that section of the flap system becomes to large and the flow will be fully separated. The now fully disturbed air aft of the main wing will affect the controllability of the aircraft as "dirty" air is hitting the aft wings. – Brilsmurfffje Apr 19 '17 at 23:59
• Note that 30°, 40°, 60° etc. are just for reference purposes. Look at the Boeing 777 inner trailing flaps, their deflection is quite pronounced (maybe 60 or 70°) Flaps are designed according to manufacturer's computations. There's no point in building flaps that requires excessive and heavy structural devices. However, some modifications can be done to allow a model to perform better on high and hot/short runways (eg: B737) The aircraft looses lift when airflow above the wing is disrupted : it stalls; extend the flaps even further and at some point (speed/AOA), the airflow won't cope anymore. – Karl Stephen Apr 22 '17 at 12:31

## 3 Answers

Depends on the kinda of flaps installed on the aircraft; in the example above with fowler flaps, the design would begin to just create excessive drag without gainful lift and would require a pretty bulky structure to accommodate for the aerodynamic loads imposed upon them.

Now some other types of flaps like the split flaps or dive brakes like those found on the Spitfire fighter were designed to extend 90°.

• Those flaps must have an interesting effect on the exhaust from the radiators. – FreeMan Apr 20 '17 at 15:19

The type of flaps shown in your question (Boeing 727, right?) work because of the small gaps between the single elements. Those help to re-energize the flow, and if more are added, a steeper angle will be possible. Deflecting the flaps more without adding more elements and gaps, however, will lead to flow separation when the wing is kept at the same angle of attack.

This separation reduces the pressure recovery towards the trailing edge and adds more suction on the rearward-facing parts of the wing, so separation increases drag. But the deflected flaps will also block the flow on the lower side of the wing more, which increases local pressure and, consequently, lift. However, this lift increase is small and is bought with a massive drag increase, so it is not economical to increase the flap deflection angle.

Note that increasing the angle of attack when flaps are deflected will cause a gradual increase in flow separation, beginning at the trailing edge and moving forward as angle of attack is increased. While the flow over the flaps is attached at the approach speed and angle of attack, during the flare it will start to separate. The drag increase from this separation is actually desired because it helps to slow down the aircraft.

Less complex flaps do use higher deflection angles and the flow separation that comes with it on purpose. Take the SB-10, for example: Here the inner camber flaps can be set to 70° for landing. This setting will produce the highest lift coefficient, but the drag is excessive. With an L/D of over 50, the SB-10 is not easy to land, and the high drag helps!

A typical landing works like this: First, the approach is flown at 90 km/h with normal flap setting for low speed (+10°) in a glide pointing at a location far beyond the desired landing spot. In about 30 to 50 m altitude the pilot pulls the nose up and slows the airplane to 70 km/h so the dynamic pressure on the flaps is low enough to deflect them down to 70°. Then he has to push the nose down quickly to avoid stalling the glider, and now he can point the aircraft at a point on the ground maybe 50 m ahead of the desired landing spot. Pushing more will increase speed only moderately because of the high drag of the flaps (and the landing gear; it also adds maybe 50% of the drag of the glider with normal flap settings). Now the speed will be between 90 and 100 km/h and the flaps will create a loud whistling sound - great to alert everyone on the ground that the SB-10 is coming in for landing! In 5 m altitude he has to start a well-timed flare because speed will now decrease quickly and the aircraft will settle down immediately without any floating. Great for landing out in a restricted space.

Below you see the combined flap and speed brake which was used in the Glasflügel Mosquito and the Schempp-Hirth Mini Nimbus. Figure 1 shows the regular deflection range, while Figure 3 shows the approach configuration. Note that the segment ahead of the flap starts to open in order to act like a spoiler. While this increases drag, lift will be unaffected. Figure 2 shows the landing configuration which will produce the highest lift, but also the highest drag.

Flap and speed brake of the Glasflügel Mosquito.

• I've always wondered, the FAA's Pilots Handbook of Aeronautical Knowledge describes the split flap configuration to be the most effective design, but I always intuitively though that the Fowler flaps would be a more efficient and effective design. Since they would seem to produce the most drag and a lot of flow separation, why are they considered to be more effective? – Carlo Felicione Apr 21 '17 at 2:19
• @CarloFelicione: I agree, Fowler flaps create much more lift. I think the FAA handbook only compares flaps which don't increase wing area, and in this category the split flap (more precisely, the Zap flap) has the highest effectivity. In terms of lift and drag with the least mechanical effort, split flaps are top. – Peter Kämpf Apr 21 '17 at 6:59

The aircraft will stall.

At some point, the drag created by these "panels" (now hanging more and more vertically) will exceed the maximum thrust of the engines. The airspeed will decrease significantly. You can either increase angle of attack to maintain lift, which will decrease speed further and you will stall; or you can pitch down aggressively to maintain speed, but it does not matter - sooner or later you will hit the ground.

If you have engines capable of producing enough thrust to overcome drag, you have created a wing with such a poor lift-to-drag ratio that it would be more efficient to simply point the engine down than to use wings.

That is assuming the wing does not break, of course.

• Further extension will have the flaps act as air brakes as there is no flow attached to it (no flow over the airfoil) – Brilsmurfffje Apr 20 '17 at 0:00
• No, Kevin, the aircraft will not stall. Maybe it cannot maintain level flight anymore, but it will fly just fine. – Peter Kämpf Apr 20 '17 at 20:17
• @PeterKämpf if an attempt is made to maintain level flight (assuming sufficient elevator authority), it will eventually stall due to ever decreasing airspeed (the engine is underpowered to overcome drag), am I correct? – kevin Apr 20 '17 at 22:53
• @kevin: Yes, you are correct. But the flaps are meant for descent, approach and landing. Then the drag becomes quite helpful. – Peter Kämpf Apr 21 '17 at 7:08