Hi Bernie,

 

Certainly not an expert here, but my understanding is that at the atmospheric altitudes we fly at that air temperature is the dominant factor governing the speed of sound, not density, not humidity, not pressure – at least not significantly.  But, then my interpretation could be wrong.

 

Forward speed of the aircraft is a factor to consider as others have pointed out.  I know that compressibility effects start long before a shock wave is created at the speed of sound, the latter  which you certainly want to avoid.  I imagine that the shape of the object probably has a lot to do with it not to mention that you have the radial speed component and the horizontal speed component all interacting.  Airflow around the prop is probably being affected long before the speed of sound – but, how long I simply don’t know.  I would suggest research on the internet on the topic.

 

Ed

 

 

In a message dated 7/8/2009 8:32:41 AM Central Daylight Time, eanderson@carolina.rr.com writes:

Subj:[FlyRotary] Spreadsheet for Prop Tip Speed PropTipSpeedCal.xls
Date:7/8/2009 8:32:41 AM Central Daylight Time
From:    eanderson@carolina.rr.com (Ed Anderson)
Sender:    flyrotary@lancaironline.net (Rotary motors in aircraft)
Reply-to:    flyrotary@lancaironline.net (Rotary motors in aircraft)
To:    flyrotary@lancaironline.net (Rotary motors in aircraft)

File:PropTipSpeedCal.xls (28672 bytes) DL Time (49333 bps): < 1 minute



Hi folks,

Attached is a spreadsheet for calculation of propeller tip speed both static
and moving through the air.  Thanks to Bernie and Bill for the equations - I
have not attempt to verify the equations, but they appear to be accurate and
further more if you put zero airspeed in the moving prop equation, you get
the same calculated values as the static prop - so appears they are in
agreement and accurate.

You may change any value shown in color Blue - the rest of the spreadsheet
is protected so folks don't accidentally erase an equation in a cell by
putting a number in it.  But, you can view the equations in each cell, so if
someone wants to duplicate it unprotected - have at it.

Enjoy

Ed



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Subject: [FlyRotary] Spreadsheet for Prop Tip Speed  PropTipSpeedCal.xls
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That spreadsheet is really cool!  I did not know about what affects speed of sound (humidity, etc.), I thought it would be air density (altitude). Because of such arbitrary influences that can't be accounted for in a formula, I would probably "red-line" the RPM at no more than 100 MPH below speed of sound. Those influences could, at times, change the formula speed of sound while you are flying, so that your prop RPM could enter the speed of sound range just by chance of changing air properties.

In my sample formula, you can see what the difference in RPM can do. Add 500 RPM, and it puts your prop tips a hair away from speed of sound.

Do you think a 100 MPH below speed of sound limit is reasonably safe?

Thanks, Bernie