6 October 2002

The effect of decreasing venturi diameter and/or silencer rearmost end diameter.

 

Below, first my letter that I have send to the CIAM control line subcommittee the 28 th of July and second the measurements I made.

 

Subject: 
Fly-aways in F2D
Date: Sun, 28 Jul 2002 14:13:14 +0200
From: Loet Wakkerman <wakkerman.f2d@wxs.nl>
Organization: WAK-Engines
To: "ciam-f2@fai.org" <ciam-f2@fai.org>


Hallo,

During and after the recent WC in Sebnitz I have been thinking about
the big amount of fly-aways in F2D combat.

Solutions to solve this problems were told to me by several persons.
Thicker lines, smaller engines, shut off systems are in my opinion not
the right solutions.
Thicker lines won't solve the problem, I foresee more models crossing
through the circle. 1.5 cc Engines will destroy the fun 1.5 cc classes
in several countries. Shut-offs make the construction more difficult
and more expensive and is not working always.

The way to reduce the amount off fly-aways is lowering the power of
the engines. A combat bout with slower models is less destructive in
any aspect.

My idea is:
We have 2 simple and adequate devices which are named venturi and
silencer.
We could make the venturi smaller, lets say 3 mm diameter and reduce
at the same moment the rearmost end of the silencer to 6 mm diameter.
This will reduce power (speed), noise.

I have the possibilities to make some venturis and silencer in the
next 2 weeks and test them at he competition in Grave Holland. Rob
Metkemeyer will do the dB measurements. I will focus on a 3 and 3.5 mm
diameter venturi and silencer rearmost end with 6 and 7 mm diameter

As fanatic combat pilot I also like to fly as fast as possible but I
foresee problems in the near future if we let it go. Up till now,
happily, nothing serious has happened but we were very close this
time.

Please give me your opinion about this subject.

Loet Wakkerman

 

 

 

 

And finally the results of my rpm/db measurements. 

The aim of the test was to see what the effect of a smaller venturi diameter and /or silencer rearmost end diameter is. As side effect the noise level was measured. The intention of these measurements was NOT to lower the noise level but to lower the rpm of the engine. A lower noise level is a pleasant side effect.

The engine was mounted on a test bench and the propeller was shortened so much that the engine made the same RPM as in level flying conditions.
The noise  level was measured at 3 m from the center line of the test bench/engine. The engine was running at full power and the measurement was taken at 90 degrees to the 'flight path.' dB measurements were done by Rob Metkemeijer.

I have been using my own engines (twin exhausted) and therefore the rearmost end is different from what is shown below.
I have used 5.65,   4,94   and  4.24 mm diameter which equals 8,  7,  6 mm diameter respectively for a single exhausted engine. Here I show what it would be for a single exhaust engine.

All the dB measurements were rounded to halve values, e.g. 91.1 being 91 or 91.6 being 91.5.

 

Combination Venturi diameter in mm. Silencer rearmost end in mm. RPM dB
No. 1 4 8 30600 97
No. 2 4 7 29200 93
No. 3 4 6 27900 92
No. 4 3.5 8 29600 96
No. 5 3.5 7 28100 93
No. 6 3.5 6 27800 92
No. 7 3 8 28100 94.5
No. 8 3 7 27600 92
No. 9 3 6 27500 91

 

As expected the rpm and noise level would decrease when venturi inlet and/or silencer rearmost end is smaller.

From experience I know that when an engine is making 1000 rpm's less the model slows down by appr. 1 second per 10 laps.

In the table above some fluctuations can be seen which might be interpreted as reflections from interested people passing by. Some measurements were done more than once and the average was calculated.

If we want to make the model appr. 1 second per 10 laps slower we could choose for combination No. 2 or No. 4. 
I would choose for combination No. 2. As I told before a lower noise level would be a pleasant side effect (4 dB is a lot). Secondly, there seems to be engines with non replaceable venturis.
Make up your own conclusions and remember that the values in the table above will be different for other engines.

I don't want to say that we should do this or that, these measurements were made because I strongly believe that we need to do something more about safety.  I believe that a slower model is a contribution in this direction. From lots I have heard that they believe that shut off's are the solution. Happily nobody thinks that thicker lines will help but increasing the line pull test from 15 to 20 kilo's or more seems to be a good idea. Some lines that are sold now are inferior and will not withstand a heavier pull test. I am also sure that we will need and find better lines.

I hope these or other ideas will lead to a safer combat flying. It is hard to say what the best solution is but for sure: something must be done!