How does flying with flaps differ from flying with airbrakes?

Question

I've got a private pilot license and learned on a Scheibe Falke SF25. For the last year however I've only been flying a C42 Ultralight.

I find flying the UL way easier for one particular reason: it has flaps where the Falke has just airbrakes.

I don't know how many hundred landings on the Falke I have done, still I have never felt comfortable. The reason is the airbrake. For some reason it scares me. With the UL I set the speed, lower the flaps and if I'm too far away I can add a little power. The flaps lower the stall speed and make the plane slower.

The airbrakes also make the plane slower, however they increase the stall speed. I see how (especially flight instructors) pull the air brake to the maximum and then do their approach. If I'm too far away with the Falke, I have to retract the air brakes. This affects the plane very directly, especially compared to adding a little power on the UL.

My questions are:

• How to correctly land a plane with airbrakes? Full out or just a bit? Do you leave them in one position or do you vary during approach?
• How much do they really affect stall speed?

Answer 1

The SF-25 has such a low wing loading that it easily tolerates the small reduction in maximum lift which the airbrakes cause. I have done very few flights in it (I'm too tall to fit comfortably in it's cramped cockpit), but from what I remember the airbrakes are not very big, having little effect on minimum speed. My experience, however, is with the SF-25 B, and you linked to the SF-25 C, so I have to assume that my experience is not fully relevant.

I have more experience with real gliders and composite motorgliders, and here my experience is: Start your approach, stabilize the right approach speed, set the airbrakes such that your glide path looks right and do not play around with them too much. Set your approach such that you need medium airbrake extension, so you have margin for corrections in both directions. Normally, I come in a little high so I am not caught by windshear, and when I am sure I will reach the intended touchdown point, I extend the airbrakes fully to land with the shortest distance possible. However, when touching down, make sure you do not fully extend the airbrakes: This gives you more time for stopping the descent. Especially in gliders with very effective flaps and airbrakes (ASW-20 C, ASH-25) I never made a good landing with fully extended airbrakes. Reduce them to half extension at the most when touching down!

In the ASW gliders the airbrakes are linked to the very effective wheel brake, and touching down with fully deployed airbrakes means that the wheel is braked at touchdown. This is another reason to reduce airbrake extension when touching down.

In high-performance gliders it is easy to float too long, so it is very important to control your approach speed and to continue the descent all the way down to the touchdown point. Don't pull too early! The SF-25 will not float as much, but with its low wing it shows a pronounced ground effect, and exercising strict discipline when landing helps a lot to make the landing better.

I could not find a number for the stall speed change due to airbrakes in the SF-25 manual, but I would guess it is at most a few km/h.

Answer 2

I would not recommend flying an approach to land with maximum airbrakes. Instead, what I was taught to do in a glider (or a motor-glider without the engine running) is to plan and fly the approach with about 1/2 airbrake extension.

This allows you to use the airbrake handle much like you would use the throttle in an aircraft with a running engine:

• Getting a little low or slow - Retract the airbrakes as far as needed to return to your normal approach angle.
• Getting a little high or fast - Extend the airbrakes as far as needed to return to your normal approach angle.

You then touch down with whatever airbrake extension was required, and then fully extend them afterwards.

Flaps are unrelated to airbrakes, and you can have gliders with or without either of them. Some gliders have both! The flaps allow you to slow further, which reduces your ground roll and also increases drag. This is important in high performance gliders because it can be very hard to actually make them go down without gaining speed when you want to land.

As far as how much they affect the stall speed, this is very much dependant upon the specific aircraft and the design of the airbrakes/wing. The POH should describe any special procedures (such as flying your approach speed + 10 with full airbrake extension) if they are needed.

Answer 3

It's bad to compare flaps and speed brakes and use flaps as brakes. They are meant for different things. To slow down and to make stall speed slower. Of course it's possible to use flaps as speed brakes but it's a bad practice. Boeing 737 flight manual warn's pilots not to use flaps to slow down. High performance aircraft usually has both. Another important think is that speed brakes doesn't have speed limit. Flaps have speed schedule and are meant only for slow speed. For example you want to descent fast you maintain maximum speed and extend brakes.

Answer 4

Flaps and spoilers(airbrakes) have a similar affect by creating more drag. However, they have the opposite affect when it comes to creating lift. A great way to think of this is the car analogy. There are two ways to slow down a car: the hydraulic brakes and the engine brakes. Hydraulic brakes, analogous to spoilers/airbrakes, decreases revs as it slows down the vehicle. On a standard transmission, applying the brakes without hitting the clutch will eventually cause revs to go to low and stall the car. Similarly on an aircraft, using spoilers will increase the angle of attack(decrease lift) and of course increase stall speeds.

Conversely, flaps work like engine brakes. If I'm cruising at 50 mph(80 km/h) and my revs are at 1500 rpm in 6th, and I put my car in 5th while maintaining the same speed, I will have to increase the power I put and my revs will jump to about 2000. SAME CONCEPT IN AN AIRCRAFT! If I'm cruising at 180 knots IAS and my angle of attack is 10 degrees, and I apply flaps from up to 10 degrees, the engine will have to use more power to maintain 180 knots IAS, but my angle of attack will decrease, and I'll get more lift. Just like a car has speed limits for its gears, aircrafts have maximum speeds to use every level of flaps. Using engine brakes can help you slow down just like flaps can help you fly at a lower speed and eventually land at an optimal speed.