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  1. #1

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    Temperature change and resulting RPM

    What would the RPM change be from 60* air to 70* air, roughly? Does that number change from 70 to 80 and 80 to 90 and so on??

    I understand there are other factors, but can has anyone noticed any relative consistency?


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    Quote Originally Posted by pdxrealtor View Post
    What would the RPM change be from 60* air to 70* air, roughly? Does that number change from 70 to 80 and 80 to 90 and so on??

    I understand there are other factors, but can has anyone noticed any relative consistency?
    There is so many factors involved it would be impossible to say.. It's not always the temp that get's you, most of the time it's the humidity. You hear guys around here say "RAD" alot, that's the best way to track your performance or lack of

  3. #3

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    Quote Originally Posted by Alex13 View Post
    There is so many factors involved it would be impossible to say.. It's not always the temp that get's you, most of the time it's the humidity. You hear guys around here say "RAD" alot, that's the best way to track your performance or lack of

    I have seen RAD a lot and always just assumed it was RADAR like RADAR gun... no?

    The reason I ask is I have a couple or RPM numbers with different temps and am trying to give my prop guy the number of R's to tune.

    You ask why I need this info if I have two RPM numbers with different temps?? LOL.... one RPM number was banging the limiter......

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    Quote Originally Posted by pdxrealtor View Post
    I have seen RAD a lot and always just assumed it was RADAR like RADAR gun... no?

    The reason I ask is I have a couple or RPM numbers with different temps and am trying to give my prop guy the number of R's to tune.

    You ask why I need this info if I have two RPM numbers with different temps?? LOL.... one RPM number was banging the limiter......
    The best thing is to have the riva nozzle kit with the 81,82 and 83mm and have to props one for summer and 1 for the winter then you fine tune the RPM with the Riva ring nozzle kit.

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    Quote Originally Posted by pdxrealtor View Post
    I have seen RAD a lot and always just assumed it was RADAR like RADAR gun... no?

    The reason I ask is I have a couple or RPM numbers with different temps and am trying to give my prop guy the number of R's to tune.

    You ask why I need this info if I have two RPM numbers with different temps?? LOL.... one RPM number was banging the limiter......

    It's something like?? Reliative Atmosphere Density..

    The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity. At sea level and 20 C, air has a density of approximately 1.2 kg/m3.
    Contents

    [hide]

    [edit] Relationships


    [edit] Temperature and pressure

    The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:
    where ρ is the air density, p is absolute pressure, R is the specific gas constant for dry air, and T is absolute temperature.
    The specific gas constant for dry air is 287.05 J/(kgK) in SI units, and 53.35 (ftlbf)/(lbmR) in United States customary and Imperial units.
    Therefore:
    The following table includes the air density - temperature relationship:
    Effect of temperatureTemperatureSpeed of soundDensity of airAcoustic impedance in Cc in ms−1ρ in kgm−3Z in Nsm−3−25315.81.423449.4−20318.91.395444.9−15322.1 1.368440.6−10325.21.342436.1 −5328.31.317432.0 0331.31.292428.4 +5334.31.269424.3+10337.31.247420.6+15340.31.22541 6.8+20343.21.204413.2+25346.11.184409.8+30349.01.1 64406.2+35351.91.146403.3

    [edit] Water vapor

    The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic.
    This occurs because the molecular mass of water (1 is less than the molecular mass of air (around 29). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume. So when water molecules (vapor) are introduced to the air, the number of air molecules must reduce by the same number in a given volume, without the pressure or temperature increasing. Hence the mass per unit volume of the gas (its density) decreases.
    The density of humid air may be calculated as a mixture of ideal gases. In this case, the partial pressure of water vapor is known as the vapor pressure. Using this method, error in the density calculation is less than 0.2% in the range of −10 C to 50 C. The density of humid air is found by:
    [1] where:
    Density of the humid air (kg/m) pd = Partial pressure of dry air (Pa) Rd = Specific gas constant for dry air, 287.05 J/(kgK) T = Temperature (K) pv = Pressure of water vapor (Pa) Rv = Specific gas constant for water vapor, 461.495 J/(kgK) The vapor pressure of water may be calculated from the saturation vapor pressure and relative humidity. It is found by:
    Where:
    pv = Vapor pressure of water φ = Relative humidity psat = Saturation vapor pressure The saturation vapor pressure of water at any given temperature is the vapor pressure when relative humidity is 100%. A simplification of the regression [1] used to find this, can be formulated as:
    IMPORTANT:
    • This will give a result in mbar (millibar), 1 mbar = 0.001 bar = 0.1 kPa = 100 Pa
    • pd is found considering partial pressure, resulting in:
    Where p simply notes the absolute pressure in the observed system.

    [edit] Altitude

    To calculate the density of air as a function of altitude, one requires additional parameters. They are listed below, along with their values according to the International Standard Atmosphere, using the universal gas constant instead of the specific one:
    • sea level standard atmospheric pressure p0 = 101325 Pa
    • sea level standard temperature T0 = 288.15 K
    • Earth-surface gravitational acceleration g = 9.80665 m/s2.
    • temperature lapse rate L = 0.0065 K/m
    • universal gas constant R = 8.31447 J/(molK)
    • molar mass of dry air M = 0.0289644 kg/mol
    Temperature at altitude h meters above sea level is given by the following formula (only valid inside the troposphere):
    The pressure at altitude h is given by:
    Density can then be calculated according to a molar form of the original formula:

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    Someone had an easier format to work with than what you see above but I can't find it..

  7. #7
    Moderator Franko's Avatar
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    Here is a handy RAD calculator:

    http://wahiduddin.net/calc/calc_hp_dp.htm

  8. #8
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    Quote Originally Posted by Franko View Post
    Here is a handy RAD calculator:

    http://wahiduddin.net/calc/calc_hp_dp.htm

    THat is the one i use and it works very good for calculating the air quality.
    With the same barametric pressure, and altitude the RAD will go up with temperature increasing.
    The barametric pressure has alot to do with it.
    For example if your barametric pressure is low say 29.0 the rad will be much higher at the same temp compared to pressure at 30.0+. If you ever see say 30.0 to 30.4 your air quality will be awesome and make peak power.

    Just play arround with the above calculator changing only the temp and then changing on the pressure and you will see quickly what i am talking about.

  9. #9
    West Texas RXP skoepp's Avatar
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    I just played arround with the calculator.
    Example:
    70 degrees
    29 pressure
    50 dew
    500 ft
    equals RAD of 1,350 ft.

    with all the same above with 80 degree temps RAD is 2,000 ft.

    Now if you keep the temp at 80 degrees and change the pressure from 29 to 30 the difference is double on the RAD.

  10. #10
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    I just tagged this tread so I can find that damn worksheet.. Thanks Franko

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