How do airbrakes help to lose height quickly without increasing speed? And do they affect the stall behaviour?
These are good questions – time for an answer, before we attempt Approach Control. But a caveat first: different models of glider will respond in their own ways, so get to know their behaviour at a safe height before you need to use airbrakes close to the ground.
Primary Effect and Use
The primary effect of airbrakes is to increase drag by a variable and controllable amount. This causes the glider to lose height more rapidly, and controllably, as a result of the degraded glide angle. Airbrakes are mostly used on the Approach, to steepen the glide angle without increasing speed. Modern gliders have such good glide angles that without them you would have to approach from a ridiculously low height a long way out – and if you hit sink in that situation, you have no way out. So, they are used to provide sink as and when we want it, and by so doing, allow us to touchdown where we choose with a good margin of safety
Airbrakes also reduce lift over their section of the wing, which further contributes to a decrease in the glide angle. The outer section of the wing is still producing all of its lift and so can now act as a pivoting force around the airbrake box area. Hence,…
They can limit speed, to a point. If you allow the nose to stay below 45 degrees down, there’s every chance that speed will build to Vne. Read the Pilot’s Handbook for the glider before you try it. Within these limits, airbrakes can be used to prevent excess speed if it threatens to run away with you.
The airbrakes are operated by the blue handled control on the left hand side of the cockpit. Push forward to close them, pull back to unlock (usually over an over-centre lock) and then pull back to progressively open the airbrakes. Control forces vary between models, and dependent on airspeed. Beware – some are attached to a wheel brake operated a little beyond the fully open position (take care when touching down).
In Condor, airbrake operation may be compromised. If they are assigned to buttons, you can probably only operate them in fixed increments of 20% per press. This lack of finesse does not seem to be much of a problem, and if you have only one slider control, it is best assigned to the trimmer.
Airbrakes can affect pitch (and therefore speed) significantly. Some gliders are unaffected. Again, get to know the glider before you need them.
It stands to reason that if you reduce the lift of the wing by opening the airbrakes, then the stall speed will rise. The more you open them, the higher the stall speed. Be careful close to the ground.
Scenario and Demonstration
The demonstration shows how the Standard Cirrus behaves in Condor. It is just an example of how pitch, sink and speed can vary as a result of using airbrakes. The glider is trimmed for 50Kts, a typical (minimum) approach speed. The undercarriage is put down. In this steady state, the vario shows sink of about 1.5 knots down.
The airbrakes are then opened fully, without adjusting the elevator to control pitch. The glider initially slows down briefly, and pitches down before then building speed. Left to its own devices it would carry on at around 60 knots in a nose down attitude. The mechanical vario is off the scale, and the electric vario shows the glider is losing height at up to 10m/s (20kts). Whilst losing height effectively, the attitude change has caused a significant increase in speed. If we were landing, this is a lot of extra energy to be disposed of.
The airbrakes are then closed fully, and the elevator continues to be held steady. The glider pitches up, slows down as a result, and oscillates a while. It isn’t quick to settle, so we assert control and settle again at 50 Kts. Sink returns to around 1.5kts.
We then open the airbrakes fully, coordinating with elevator to maintain speed. This leaves the glider in a slightly nose down attitude, with speed held at around 50kts – sink is now 9kts down.
Closing the airbrakes with coordinated use of the elevator pitches the nose up a little, resulting in some loss of speed, which would be easily maintained with better coordination!
We then go on to explore the stall characteristics. Airbrakes are fully opened, and the stick eased back, wings level. Pre stall buffet occurs at around 42Kts. We then close the airbrakes and compare it with the stall speed with just the wheel down – it is about 38Kts. So we see that stall speed increases with the airbrakes open.
We repeat the stall exercise with the airbrakes open, and allow it to stall – we are immediately rewarded with a rapid wing drop. No fuss or drama above 2,000′, but unwelcome on approach for instance. Recover by easing the stick forward, closing the airbrakes (especially if low), allowing speed to build and then ease back on the stick to regain the normal attitude and recover some height, wings level. So not only is the stall speed higher, but a stall is likely to be more vicious and deeper with the airbrakes open.
The video is best viewed in YouTube in Full Screen mode, to easily see the on-screen messages during the demonstration. Use view, pause and rewind as needed to grasp the content and timing of the messages displayed, then focus on the action.
Performing the Exercise
Repeating the exercise is straightforward. Settle at 50kts, then open the airbrakes fully without adjusting the elevator. Note the change in speed, attitude, and sink. Close the airbrakes and observe the recovery. I find that in Condor, the glider (Standard Cirrus) oscillates a lot, and requires some active control to settle. Then explore the stall speed, with and without the airbrakes open. Practise recovering while closing the airbrakes. Doubtless you’ll try recovering with the airbrakes open – note that you need to fly faster and will lose more height. You may even stall again post-recovery.
Further Reading and References
Gliding From Passenger to Pilot, 2nd Edition: Page 109-110
BGA Instructors’ Manual, 4th Edition: Section 2, Chapter 11