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Control Surface Flutter, Balance, and VNE

Posted by Martin Bedding on |

  • Category: Tips and Tricks
  • First, let’s define what flutter is why it’s important what it can do and how we can prevent it from happening. Flutter is instability due to an interaction between aerodynamic, inertial, and elastic forces it’s known as aeroelasticity.

    Incidentally, it’s not restricted to aircraft. The original Tacoma Bridge spanning the Puget Sound in Washington collapsed on 7th November 1940 as a result of aeroelastic flutter. All it took was 42MPH wind to make it happen.

    Control surface flutter on aircraft was discovered in the early days of aviation and became a particular problem during the first World War as aircraft performance began to improve and the top speed increased. It’s a situation that can lead to the destruction of the aircraft if left unchecked and many pilots have been killed as a result. It can happen with model aircraft and not just full size. Many models have crashed due to control surface flutter and you may not even realize it’s happened. Even after you pick up the pieces you may never know what happened. The higher the performance or airspeed the more likely it is to happen. Generally, you will hear a buzzing noise and this is the start of it.

    All aircraft, models as well as the full size will have a VNE (Velocity Never Exceed). In the full size, the VNE figure will be listed in the cockpit on the dashboard. So where is with a model I hear you all say? Yeah, well, that’s the problem it isn’t listed anywhere. In most cases a kit-built model or a ready to fly model you are unlikely to encounter a problem. However, if you increase the performance of a given model by fitting a more powerful motor or engine in an effort to “go faster” it is possible to encounter problems.

    All aircraft have parts that flex and bend this includes the wings tails and the control surfaces attached to them. Flutter usually begins with a slight vibration that increases to an oscillation as the force increase with airspeed the oscillation increases until a fracture occurs and the parts fail. You can find many videos on YouTube showing the effects of flutter. If you encounter it and recognize it as flutter immediately reduce the airspeed this will stop if. Reduce power and land.

    Mass Balance

    Statically balancing the control surfaces will help reduce the effects of flutter. It will certainly prevent control surface flutter. It will also reduce the load on the servo gear train.

    Using an elevator as an example disconnected from the servo and hanging freely on its hinges the natural portion will be for the elevator to hang in the full down position. This is considered a tail heavy position and the same thing happens with the ailerons. At some point, a control surface in this condition will flutter. The problem is we will never know what speed this happens. The good news is we can prevent it and to do this we have to add weight forward of the hinge point. In most cases, this is not easy to do but it’s not impossible. You will have to fit a hard point as close as possible to the leading edge of the control surface in question and then attach an “L” shaped piece of wire to the hardpoint with the part of the wire facing forward to which a small weight is attached. If you are building a model from scratch use hard balsa for the leading edge of all of the control surfaces and a soft lighter balsa for the trailing edge. If the model has an all-flying stabilizer simply add lead shot to the leading edge at the most forward point from the pivot point. In most cases, it doesn't have to be perfectly balanced just adding a little weight forward of the leading edge will work.

    Full-size aircraft have overbalanced controls basically what this means is the elevator would hang above neutral in the up position rather than hang down. Weight added to the trailing edge of a control surface will create a problem. Mud from a grass runway for example collecting on the trailing edge of the elevator will cause flutter if it is not removed before flight. This would be considered part of a walk around preflight check.

    Flutter is less likely to occur in slower models such as trainers and slow flying models. High-performance EDFs can have an issue especially if you install a more powerful motor and fan set up.

    It’s often thought removing any backlash in the pushrod/clevis will prevent flutter, this is not the case at all. The only sure solution is to mass balance to be sure.

    All control surfaces should be close fitting and preferably with a sealed hinge gap. This will reduce drag, and make the control more responsive especially as lower airspeeds. The hinges should be secure and checked on a regular basis. The pushrods, ball joints clevises, servo horns, and control horns should also be checked for wear and security as well. All these parts should be checked before the first flight of the day in case anything has damaged after the last flight and during travel home and back to the field.

    Servos

    If a control surface is balanced the servo will have less load on the gears than one that isn’t balanced. If the control is not balanced the servo is having to hold the control in the neutral position before it even moves it and this is without flight loads. A balanced control will require less force to move than one that isn’t. Most ready to fly models using standard servos will be adequate for general flying. The only time this may become an issue is if you increase the motor size in an effort to increase the top speed. If you require a stronger servo usually you get a feel because the elevator control feels soft and this is because the control surface is pushing against the servo because of the air pressure is pushing the control surface towards the neutral position. If this proves to be the case there is a formula for calculating the servo power required for a given surface area. However, you do need to know the likely top speed to calculate it accurately and this is something you are unlikely to know. If you are in any doubt about the required servo torque user a higher torque one.

    The servo torque is measured 10mm from the center servo arm retaining screw. Generally, all servo manufacturers use this point to provide torque figures. Remember the further away from the center screw you mount the servo pushrod the less torque you have at the control horn and also the closer the control surface you mount the pushrod the less torque you will have on the control surface. Personally, I always mount the pushrod on most of my servos at the 10mm point I know then I am going to get maximum torque from the servo.

    I hope this helps and gives you a little insight into control surface flutter and how to prevent it.



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