General Overview

• The subject of noise is a very emotive one for all model aircraft operators. Like it or not, each and every model owner must adopt a sensible approach to noise, since excessive noise can and does lose flying sites.

• However all is not lost – read and digest the following tips!

Some Very Simple Guidelines

• Use APC, Bolly Clubman or G-Sonic propellors ONLY. Their scimitar shape does a lot for noise reduction. Good as they are, avoid Graupner and Master Airscrew propellors – the square tips generate a LOT more noise.

• Increase the pitch of the propellors you normally fit. This will maintain the thrust and reduce the revs to reduce the propellor tip speed. The only noticeable difference is a slightly slower acceleration at take-off. You really will NOT notice any difference in the air.

• Some recommended propeller sizes for the most popular engines in the Club!

• Fit your engine with a Super Quiet Silencer and preferably one which has an internal baffle! Most modern engines are designed to run with these silencers having two chambers. I am afraid you will have to pension off that 25 year old .40 that you have had sitting in it’s original box all those years! In those days, the silencer mainly served to shoot the oil residue away from the airframe rather than try to quieten the engine……….. Go to our links page to view the Just Engines web site who supply a large range of silencers including their own design Super Quiet Silencer.

• Fit your silencer with a rubber extension piece. They are cheap, help to keep the oil residue off the model and also provide additional silencing!

• Fit your model with a slightly larger engine! Instead of a .40, fit a .46, or instead of a .46, fit a .52 or .53! You can then fly round nice and quietly at half throttle and have ample power in reserve to do those advanced aerobatics!

• Your flying style can also go a long way to reducing noise. You have gone to the trouble and expense of fitting a throttle servo so use it! A model flying round at full throttle throughout the flight is much more noticeable to the general public. Throttle back after takeoff and only use full throttle for those aerobatic manoeuvres requiring the extra power. After all, full size aircraft (even fully aerobatic aeroplanes) never fly at full throttle all the time, so why do we need to?

• Fly using straight fuel or with 5% nitro at most.

• Some more detailed stuff follows for those of you with a more technical interest!

Propellors

• Selection of the correct propeller is important for two reasons. Firstly, the faster the engine runs, the more exhaust, engine and airframe noise is produced. Secondly, high propeller speeds produce more noise. Although it is difficult to lay down strict criteria for calculating ideal propeller sizes, as a rule of thumb, the following target RPMs need to be approached if noise emissions are not to exceed 82dBA at 7 metres:

• • .40 cu.in 2 strokes: 10000-11000 RPM
  • .50 cu.in 2 strokes: 9500-10500 RPM
  • .60 cu.in 2 strokes: 9000-10000 RPM
  • .90 cu.in 2 strokes: 7000-8000 RPM

• Some commentators have suggested that the propeller tip speed should be kept below 350 mph to achieve the same effect. It is true that if the propeller tip speed becomes too high, the tips will emit a very significant amount of noise. This is not usually a problem on model aircraft unless large propellers running at very high RPMs are used. Tip speeds approaching the speed of sound are required for this effect to become significant, and a 12 inch propeller will not approach this speed even when running at 15000 RPM.

• Whichever rule is followed, the correct way to reduce RPM is to run coarse pitch propellers rather than large diameter propellers.

• This prevents RPM from rising dramatically in a dive or at high speed (this would negate any benefit of reducing RPM on the ground, leading to increased noise output). Incidentally, a coarse pitch propeller will often result in higher achievable airspeeds – think of it as an aerodynamic gearbox; coarse pitch equals a higher gear ratio. Note that the same principles can be applied to 4 stroke engines. Some useful prop sizes are given below:

• Multi-bladed propellers have a place, especially for very large, fast running engines. Where unacceptably high tip speeds would occur with the diameter of a two bladed prop necessary to absorb the power, an extra blade can be used to absorb the power at a more modest tip speed. For most engines however, a multibladed propeller offers no advantages over a coarse pitch two bladed prop.

• The other point to note is that the torque peak for most model aircraft engines lies quite near the above recommended RPM ranges. This means that a comparatively large propeller can be turned, and benefits result by virtue of moving a larger mass of air over the wings and control surfaces. Thus although the power output of the engine at these lower RPMs is lower than the BHP peak, more of the available power is converted into thrust.

Exhaust and Silencers

• There are now several effective exhaust silencers available on the market for a range of engine sizes. These come in two basic types: an integrated primary silencer, or an add-on secondary silencer. All good silencers incorporate internal structures to break up the sharp pressure wave that is produced by rapidly expanding hot exhaust gases. It is this pressure wave that is responsible for the vast majority of exhaust noise. The flow of exhaust gas is usually very quiet; this can be illustrated by exhaling very quickly. The noise produced by this simple act is far less than that produced by any model aircraft engine exhaust, despite the volume of gas moved per second by your lungs being more than by a 0.90 cu.in 2 stroke running at 10000 RPM!

• An add-on silencer should not cause any power loss when attached to a primary silencer. Power loss is caused by back pressure – a result of exhaust gas not being able to exit the silencer quickly enough. If all the holes in a silencer are large enough, back pressure does not occur to an appreciable extent and power is not lost.

• The best way to measure power is by using maximum RPM as a surrogate measure. The maximum RPM should be measured both before and after fitting the silencer, with no other changes, except that a minor degree of re-tuning may be needed after fitting the silencer. If the RPM has remained within 200 RPM (allowance for experimental error), no power loss or gain has occurred.

• Tuned pipes can be used very effectively as silencers, provided that they have a suitable internal structure (‘quiet’ tuned pipes), and that any increase in power provided by using such a device is not used to increase the RPM. Instead, a propeller with increased pitch should be used to absorb the increased power whilst maintaining a constant RPM.

• A couple of other points are worth making on the subject of silencers. Firstly, silencers should not be hard mounted to the fuselage or wing of the aircraft. This increases the amount of vibration transmitted to the fuselage and hence airframe noise. Secondly, the noise meter can be a useful tool in determining the comparative effectiveness of silencers. A series of tests should be carried out, all at the same RPM with the same propeller, changing nothing but the silencer. In this way, meaningful comparisons of silencer effectiveness can be achieved.

• You will often notice that the reading most affected by adding a silencer is not that taken with the exhaust facing the meter, but that taken with the starboard side of the aircraft facing the meter (on an aircraft with an upright or side-mounted engine with side exhaust). This fact can provide a useful pointer as to whether the exhaust is the dominant source of noise. If the starboard reading is higher than the others, the exhaust is usually the dominant source.