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Airbrake controls the glide angle. Elevator controls speed. At least that's what I was taught.
You have never been gliding, have you.
The airbrakes place the glider on an artificially steeper glideslope. So if you lose airspeed due to windshear, you can close the airbrakes and maintain the same speed (because the artificially steep glideslope will now allow the glider to accellerate).
If you had to lower the nose every time you lost airspeed in a glider, you would soon find yourself surrounded by sheep, and separated from the airfield by a barbed wire fence.
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