The following article is taken from the AMA National Newsletter.
OPTIMIZING CONTROL SURFACES
Control surfaces enable us to maneuver our aircraft and maintain a controlled flight path. Too often these important devices are attached without proper consideration for their function. They can be misfitted, tight and binding, or without sufficient movement. Worse still, they can be laden with surface finishing and attached with loose or sloppy connections that make them flutter candidates. Some ARF aircraft have even shown up with no adhesive on the hinges.
Here are some considerations related to aerodynamics, control effectiveness, and aerodynamic flutter:
Ailerons consume some of the wing area and must be fitted to minimize the hinge gap to preserve effective wing area. The best option is to gap seal the joint, but it is not always worth the complexity on the average model. However, if you are working with a high wing loading (greater than 30 oz. per square foot) model, gap sealing is very desirable to stabilize the low speed environment. Gap sealing will reduce the aerodynamic drag and increase the control effectiveness, thus requiring less deflection for the same outcome. A simple gap seal can be achieved by laying a strip of adhesive tape over the joint while holding the control surface at full deflection to preserve its movement. Such a seal will require replacement from time to time as it will degrade during service.
Stabilizer control surfaces are not required to be snug-fitting unless you are dealing with a fast and slippery model. In some circumstances it is desirable to open the gaps to reduce control sensitivity. This may be the case on a training model. Some trainer-type models have a huge control surface gap on the rudder and elevator. The purpose of this gap is to provide a soft feel around the neutral position and a strong control response toward full deflection which results in the gap closing. This control response is analogous to an exponential-type movement available on most computer radio transmitters.
Control surface flutter is the curse that will destroy your model quicker than you can say "What's that noise?" Flutter is caused by a lack of balance of the control surface about its hinge point. In smaller models, it is hardly ever evident due to the low mass of the surfaces. However, the larger the model and the more surface finishing materials used (covering, fillers, primers, and paint) the more susceptible the surface is to flutter. If the surface is susceptible to flutter, then it is only a matter of speed before the flutter happens and structural failure becomes imminent. It is commonly believed that removing control system slop and stiffening the control linkage will eliminate flutter. This is not true. It will only defer it to a higher speed. The only solution to control surface flutter is to mass balance the surface (add mass to the control surface ahead of the hinge line to achieve a balanced condition). If you identify flutter and survive to rectify the problem, then you can consider yourself very fortunate. Many have never been able to identify the cause of their model's demise or been able to recover from the situation.
Please see bottom of page for contact information.
This article is by Geoff Burdon from Vapor Tales.
from Vapor Tales by Geoff Burdon
Fred Harvey, editor
307 N Brook Forest Rd
Derby KS 67037
