Answer: We recommend using no more than 50% of the maximim upward deflection that the flap is capable of. Any more than that may make your model unstable and very difficult to control safely. 

Better Answer: If you overdo the speed brake deflection beyond 50% of the maximum upward deflection that the flap is capable of, you will get additional braking effect but will substantially increase the drag and possibly make the model unstable. There is also a reduction in the effectiveness of the vertical stabilizer and the rudder when as speed brake deflection is increased. This loss of effectiveness can reduce the ability to counteract and control Yaw making it particuarly important to ensure that both speed brakes are deployed to the same degree at all times. Differential (non-equal) speed brake deflection can result in a crash due to roll and pitch changes that can be very difficult to overcome and control.

We suggest phasing in the amount of speed brake deflection, starting with very little and working up to the 50% max noted above. At each step, test your control over Yaw (using the rudder) and Roll (using the ailerons). If you can't maintain control over Yaw and/or Roll or the model is Pitching suddenly up or down, back off on the amount of speed brake deflection.

 

A more sophisticated setup can allow for speed brake deflection beyond 50% but only when the model is on the ground. This helps to reduce runout length upon landing. This can be done in a model with a high end computer radio that allows for speed brakes to depoly in stages. The flyer then ensures that the max speed brake deflection is limited to 50% until such time as the model is on the ground.