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Thread: Speed vs Pitch Charts and mis-marked props

  1. #1

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    Default Speed vs Pitch Charts and mis-marked props

    In the thread about climb vs cruise prop airspeed, a formula was shown. I then made a couple of charts. Neither of those matched the speeds I'm getting from my A65 powered Vagabond and a wood Sensenich W72CK42 prop.
    I'm using a TruTach optical/digital tachometer and averaging my gps speeds over 4 directions. I'm consistently showing TAS/average GS of 84-86 mph @ 2150rpm. (I wish that I was getting the speeds that the airspeed indicator shows! )

    Static RPM = 2055 @ 850ft MSL @ 80F.
    Climb at max gross @ 83F, about 400fpm.

    I'm thinking that my prop was stamped with the incorrect pitch by Sensenich (42"). 46" seems more like it. Has anyone seen this before?

    I have attached 4 different spreadsheets using two different formulas.
    Also, are these formulas correct? (Please forgive my basic algebra.)
    Attached Files Attached Files
    Last edited by DaleLindstrom; 08-10-2019 at 05:27 PM.

  2. #2
    Vagabondblues's Avatar
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    Default Re: Speed vs Pitch Charts and mis-marked props

    Most cell phones have a bubble protractor function or app. You can use it like a propeller protractor and find the blade angle and then calculate pitch using the attached Sensenich letter.

    If if you have an IPhone open the level app and put your phone on the flat part of the hub, or on the crush plate. The bottom of the phone should perpendicular to the centerline of the crankshaft. As your aircraft is a tail dragger you need to zero out the bubble level. To do this press on the level image and hold until it zeros out on the new level position. While maintaining the I phone direction of orientation place the phone on the face (the flat side that faces the pilot) of a level (blades are parallel to the ground) ascending side of the blade at the 75% chord. 75% chord is D (72”) divided by 2, which is a radius of 36” so your phone should by placed 9” inboard from the tip. Blade stations are measured from the center of the hub, so 27” is 75% of cord but 9” (36-27=9)from the tip is easier to find if you have a spinner or skull cap installed. You should see something in the teens like around 14 to 17 degrees. If it is higher than that, flip the phone around to the other side. You can then plug the angle into the attached Sensenich formula to find pitch.

    Wood propellers have a thicker cord than metal. This is to provide the strength needed at the shank. The thicker cord creates drag and is not efficient as a thinner more aerodynamic metal prop. In other words they are slower. Also wood changes dimensionally with each season change, which requires the prop bolts to be retourqed every 25 hours. If the friction on the crush plate is lost you also loose prop efficiency as the prop slides back and forth in the crush plates.
    Last edited by Vagabondblues; 08-11-2019 at 07:30 AM.

  3. #3

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    Default Re: Speed vs Pitch Charts and mis-marked props

    Using Sensenich's formula I came up with a 42" pitch on my prop. That makes the formulas in my spreadsheets suspect.
    But, they were only to predict how a change in pitch might effect airspeed.
    Last edited by DaleLindstrom; 08-11-2019 at 02:21 PM.

  4. #4
    tnowak's Avatar
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    Default Re: Speed vs Pitch Charts and mis-marked props

    Hi Dale,
    My Vagabond is A65 powered and has a locally (British) made Hercules wooden prop. Diameter / pitch is 72" x 47.32".
    Your performance figures for TAS and rate of climb are near identical to what I get for my Vagabond.
    My home airfield is 550 Ft AMSL, so very close to where you are based.

    The only thing that seems a bit low is your static max RPM. I get a static of around 2180-2190 RPM.
    Is it possible that your tacho is under-reading slightly? 2055 RPM does seem a bit low.
    TonyN

  5. #5

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    Default Re: Speed vs Pitch Charts and mis-marked props

    My TACHOMETER shows about the same range as yours, static. The TruTach optical/digital tachometer shows 2055, +/-5rpm, static. Also, my indicated airspeed is in the 85-90mph range. (I wish that were true!)
    I measured the prop again today, making sure that it was level horizontally, and came up with a 14.5 degree pitch. This calculates out to 43.9" according to Sensenich's formula. This makes the other spreadsheets more reasonable.

    Pitch = 3.141159 x (.075) x (diameter) x (TAN((degrees) x 3.14159/180))

    See attached spreadsheet.
    Attached Files Attached Files

  6. #6
    Vagabondblues's Avatar
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    Default Re: Speed vs Pitch Charts and mis-marked props

    Here is another pitch angle and blade angle calculator
    Attached Files Attached Files

  7. #7
    tnowak's Avatar
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    Default Re: Speed vs Pitch Charts and mis-marked props

    My Indicated airspeed is around 92-93 MPH with the side window closed at 2150-2175 RPM straight and level cruise.
    However I know the TAS is closer to 85 MPH. Have checked that by the GPS method and flying 3 headings 120 deg apart.
    If I open the side window the IAS drops a few MPH.
    TonyN

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