Modutech Helmets wind tunnel test

1) Abstracts
Tests contents on this paper has been made to compare performances of motorcycle and car helmets distributed on world-wide market since 1990 to 2011.
Modutech started to measure any kind of aerodynamical behaviour weights on comfort and active security since 1986, to acquire more informations about how the helmets “works” during use with a motorcycle or a car.
Particularly Modutech has worked in order to increase repeatability of measurements, assured by a medium 0,29% (2010 data) value spreading during test.
Parameters taken in use are the following:
• Aerodynamical non-dimensional coefficient Cx;
• Front surface in use Sf;
• Noise pressure level Vs air speed;
• Noise frequency spreading Vs air speed;
• Temperature decreasing Vs speed;
• Ventilation system volume proceeded;
• Intake system efficiency (flux non-dimensional coefficient);
• Buffeting frequencies;
• Internal comfort ratio;
• Heat comfort index
Technical description of tests are available under request.


2) Test description
2.1) Measurement of aerodynamical drag forces and non-dimensional coefficient Cx.
To evaluate helmets shape efficiency is useful measure drag forces in order to verify how the geometry used behaves in the air.
This means calculate an interesting aerodynamical parameter, called “aerodynamical non-dimensional drag coefficient” or, shortly, “Cx”; smaller is the Cx value, better the helmets behaves in terms of drag forces made during use in speed.
To evaluate Cx during wind tunnel tests it is used the following experimental formula:

Fd = 1/2 r Sf Cx V^2

Where:
Fr represents measured drag force,

r represents air density,

V represents axintotic air speed,

Sf represents frontal area surface in use,

Cx represents non-dimensional drag coefficient we are looking for.


Symbols are the same used on previous equation, system control parameter is drag force, measured by a double strain gauge system placed on an aluminium staff deformed by the drag force: measurements has been executed at various speed (from 20 to 80 m/s, step 10 m/s), and obtained value has been mediate to obtain better results (dispersion of data never exceeded 0,29 percent during all tests - 2011).
To measure drag forces it has been used the following hardware:
1. Dummy in driving position;
2. Fiberglass head dummy;
At the beginning the helmet is photographed in use condition (centre of visor tangent in use perpendicular to the ground), designed and measured with a CAD software in order to give frontal surface, truncated at third decimal.
Helmet is placed on dummy medium head with visor centre tangent perpendicular to the ground, air speed perpendicular to the visor centre: perpendicular axe of helmets is about 7° to the perpendicular of ground.

2.2) Measurement of noise level and frequency spreading.
About Phonometric tests, analysis in wind gallery has meaning only to evaluate noise made by air moving across helmet during use, according with aim posed at the beginning of this research: noise level has not to be intended to be absolute, but only to be compared between various kind of products: the right way to intend these data is comparative analysis to understand how is possible increase performances of products, particularly with frequency spreading.
For example, basically high frequencies peaks means a non conformity on visors geometry or mounting (not appropriate closing on the gasket) or bad shape of visors movements: these are really very noisy when inappropriate.
Higher importance has been given to repeatability of tests: for this reason tests has been made in wind tunnel to have always the same condition for every helmet in test: maximum deviation measured in last test was 0,78 % in noise level and 0,87 % in frequency.
To measure noise and spreading level it has been used the following hardware:
1. Sennheiser MKE X tie-microphone placed in left ear zone of dummy.
2. Dummy head in driving position (about 5° frontally placed );
3. Windows win7 PC with data acquiring system and Spectra Pro IV software;
4. Hewlett-Packard oscilloscope model HP4415S;
Helmet is placed on dummy (medium head) with visor centre tangent perpendicular to the ground, air speed perpendicular to the visor centre: perpendicular axe of helmets is about 7° to the perpendicular of ground: balancing test is made at 33 m/s to evaluate buffeting or instability effects.
Dummy dressing operations:
1. Medium size helmet (58);
2. Helmet tightly linked to the false head;

2.3) Measurement of ventilation system performances.
To measure temperature and mass air processed it has been used the following hardware:
1. Laser Doppler Anemometer system Dantec mod. LDA2300;
2. Dummy head in driving position (about 5° frontally placed );
3. Portable Pc with AER10 rel.9 software.
4. Thermometric probe system Thermometrics data mod. TH2341;
5. Thermocouple systems AIM Datapro;
6. Standard helmet positioning (see previous paragraph).

About temperature: helmet must be placed in a conditioned area for a minimum of 4 (four) hours with a constant temperature of - 40 °C, placing time must be less than 30 seconds and wind tunnel internal temperature at 21 °C.
About air mass proceeded: points (linked to formula #3) must be twenty placed in regular distances and far from last zone one millimetre, with flow ideally linearly decreasing in this distance form measured speed to zero; Air mass is measured linking flow speed to cross sectional area.

Total mass processed Q is calculated by summing n differential contributes on A area divided in elementary surfaces dA, following this formula:


2.4) Measurement of comfort index.
Condensation temperature is an excellent parameter in order to have a good indicator of heat stress during use of motorcycle helmet.
Comfort index IBcB is the difference between the internal temperature and Dew point one (TBdB).
Hr is the relative moisture of air.

As developing this equation we elide the contribution of temperature, we decided to acuire a brand new index (Thom index, It) which gives us an excellent indicator of thermal stress in function of measured internal temperature:

Thom index is quite similar to the medium temperature, so a good way of proceed can be the following one, inserting the dynamic comfort index Dci:


 
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