[dmichael]

Educating decision makers in a club, modeling organization or - worse yet- an outside group like a homeowners organization about what is an effectively muffled power system can be a tricky task - especially when it concerns large gasoline engines. For those of us who have been around planes with the "proper" prop, engine and exhaust setups designed for the lowest possible in air noise levels like the one you have, we have become aware that measurements taken on the ground can provide little or no prediction about if a plane is quiet in the air. Perceived noise levels when flying is where it actually counts, not on the ground.

I have witnessed large gasoline powered engines equipped with three bladed props and cannisters that, when held to the standard "db test at X feet on the ground" were measured to be noisier than much smaller glow powered planes. However, when in the air, the glow powered plane was easily the noisier of the two planes.....

This dynamic is why IMAC instituted an in air score. The IMAC sound committee did exhaustive work to try to find the best way to predict how to properly equip the large gas powered planes so that they put out acceptable noise levels in the air. In the end, their conclusion was that ground testing really didn't help much. The in air score was a way to judge and manage how loud a plane really was in the air where it counts. You might ask why the ground testing wasn't scrapped? IMAC cites a few different reasons- the biggest was to meet AMA expectations on their effort to reduce noise.

I believe we saw a clear reduction in noise levels at IMAC contests as a result so the IMAC noise reduction is having an effect. Even so, the work is not done. The bottom line for your particular situation, however, is that ground measurements are not going to provide an accurate reflection of how quiet your plane is in the air and you may need to help your club understand this.

I apologize for the length- for those who want to see it, here is a statement from Wally Pitts, Sound Taskforce Chairman, on this subject:






"I have often been asked why we have a ground test. The short answer is: The AMA has (to my knowledge) always measured aircraft sound on the ground.


In the past it has worked out perfectly well for the AMA (F3A and Pattern). Unfortunately for IMAC (and the STF) the ground test has become increasingly troublesome. Most problems being related to aircraft size, prop size, and of course the fact that the test is taken on the ground, which is in itself a great reflector of sound.


Looking back it seems to me that the purpose of the ground test was to predict (in a fashion) what an airplane would do in the air. The theory being that the quieter the plane was on the ground, the quieter the plane would be in the air. To date, we have found this relationship to be non-predictive. As the STF started to study Decibel theory we decided to base our enquiries on the theory that we should be able to predict (by measuring a model on the ground) what it will dB in the air. To date we have found that no relationship exists between the two values (at reasonable ground measured distances). The ground test therefore has no relationship to the sound levels a model will generate in the air. Why is this? In many ways it has to do with the inherent reflective nature of the earth’s surface itself. When a plane flies through the air, it is surrounded by an "atmosphere" if you will, of air. Put another way, there is a blanket of air surrounding the plane for 360 degrees in every direction from the plane that not only carries sound but allows it to dissipate as well. On the ground the model looses half of that atmosphere to a highly reflective hard surface.


Complicating the issue further are the measuring devices themselves. It would be unreasonable to expect IMAC (or any other SIG for that matter) to invest in expensive sound spectrum analyzers, and distribute them across the country. So what we are left with are commercially available consumer retail units from Radio Shack (RS). Initially we tested various RS units in the lab (at General Dynamics) and found them to be within one dB of each other. So we quite happily employed them in the field as Pattern has done in years past. Unfortunately, as is the case with many systems, once employed in the field the inaccuracies started cropping up. It was not uncommon to find the RS meters consistently 2dBA apart on readings, and in many cases we have found units to be as much as 3dBA apart! When one considers the fact that the intensity of sound doubles for every 3.01 dBA, the use of the RS meters can (and has) given widely varying readings during the ground test. Another persistent problem, is that we have 40% 150cc powered planes on canisters (turning 32 and 33 inch two blade props) barely making ground test levels, yet recording readings in the air less than that of planes with engines half their size. These 40% planes are recording Very Quiet Sound Scores, yet are barely making the ground test levels! Having said all this the ground test will not soon be going away and as the RS meters are the only reasonable game in town and I still recommend every IMAC member have access to one.


Naturally then, one should ask, if the ground test has no predictive value, why do we have it? The answer is multi faceted. First we have it because the competition rules say we have to. The rules are dictated by a much larger organization of which we are but a small part. To be frank, there were those in this larger organization who were (at the outset) very skeptical of our sincerity, drive and desire to reduce the sound of our planes. Fortunately, we put solid work and established theory into our Sound Plan, and as a result we have a great working relationship with the AMA. A negative was truly turned into a positive, and we have a renewed credibility with those who make decisions that affect our SIG. This relationship will (in my view) bear fruit in other areas of IMAC’s relationship with the AMA as well. So in the end we have a ground test because it is of value. While not a predictor of airborne sound, the ground test does give one a general view of what levels might be expected in the air. We know for example that if one dB’s at 98 over a hard surface, one should not be too loud in the air. Conversely, if one dB’s over that same surface at 103, then in all likelihood a noisy flight awaits. Going forward it is my goal to have everyone think of the ground test as a Gateway, not an end all test.


So you ask, knowing all of this what do I as a CD do with the Ground Test at a contest? First keep in mind that we do not intend it to be the "end all test." Folks come to fly at our events, and we want to give them every reasonable opportunity to do so. After all, if someone is Too Loud, the In-Flight Judging Criteria will "catch" them. So in answering this question, we should again consider the inherent inaccuracy of the RS sound meters. Since there can be as much as a 3dBA difference in readings from meter to meter, you as a CD don’t know if your unit is on the high side or the low side. In my view, no one should be turned down from flying if they test within 3dBA of the limit. When this problem first surfaced IMAC began recommending that the use of ATV was appropriate for the ground test. Therefore use the Ground Test for it’s true purpose, giving competitors an expectation of what the plane will do in the air. Our recommendation: Never ground a competitor unless they are substantially (3dBA) over the limit."