Noisy Plane ?

Oct 19, 2011   //   by admin   //   Tech Info  //  No Comments
  • Noisy Plane ?

Not all the noise emitted from an aeroplane is exhaust noise. Although properly designed and fitted silencers work very well, the final choice of propeller can make a considerable difference. Scimitar type blades with curved ends will reduce most of the noise created at the tips, especially if the blade tips go supersonic. Wooden propeller blades tend to self damp with regards to natural resonances. Zinger and Master propellers with square ended tips are noisier than most others. As with any  propeller, they must ALL be carefully balanced before use, with no visible damage, and have a smooth finish.

New propellers are seldom in perfect balance. In my experience, less than 10 % I have purchased over many years have NOT required any re-balancing. An unbalanced propeller will cause vibration that will be transmitted to the airframe of the model..  Excessive vibration will reduce engine and airframe life, by loosening the engine and silencer mountings and also damaging the servos and control mechanismst. Most model aircraft engines have just a single cylinder. Only a percentage of the reciprocating piston and conrod mass can be balanced by the rotating crankshaft mass. Thus, some vibration will be present at all times. Although this will vary in frequency with engine rpm and amplitude according to the crankshaft “ balance factor “ and any damping effect of the engine mountings.

 Electric motors are easily damaged by out of balance propellers. The motor bearings will have a much shorter life and the internal magnets have been known to come loose. A badly out of balance propeller can even damage the output shaft of the motor.  

Rubber mounting any engine will reduce the vibration transmitted to the airframe. Thus, less noise will be radiated from the model. Foam cored wings tend to damp airframe noise quite well. I have “heard” many modern models with strongly constructed lightweight airframes and solidly mounted engines . Although the design and construction has been of excellent quality, the airframes tend to be very noisy and resonate like a drum at certain engine speeds. These really are a case for rubber mounting the engine, as airframe vibration at these levels will cause a significant reduction in servo life and control fittings. This is assuming the receiver is properly secured and isolated from vibration. Velcro mountings on their own do not properly isolate receivers from vibration.

Ocasionally, a plane that seems reasonably quiet when on the ground, can become noisier when being flown. This is often due to increases in engine speed the airframe resonance. Two of my own planes have exhibited this phenomenon. On both occasions it has been due to undercarriage vibration. One plane, a “Limbo Dancer” once the exhaust and propeller noise had been reduced to acceptable levels, I could then hear the wheels vibrating on the axles in flight. So I bushed the wheels with plastic sleeving from some electrical wire and lightly oiled the axles. Job Done!…..Vibration cured……………. On a 1970’s vintage aerobatic design with a fixed tricycle undecarriage, the torsion bars of the main undercarriage vibrated in sympathy with the exhaust note to produce a loud vibration noise. So again, I fitted plastic sleeving from electrical wire to prevent the torsion rods vibrating in the wing mountings.

 So, having a quiet plane is not just about keeping the neighbours happy. It is also about good engineering practice and reducing electronic and airframe failures. And, saving a lot of your hard earned money in the long run! I clearly remember the radio controlled model aircraft of the 1960’s. Nobody ever used any silencers at all !!  And boy!!!………….did those McCoy 60’s, Merco 49’s and similar engines …………



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