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From: "Bob Darrah" <RDarrah@austin.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-1350586@logan.com>
Subject: Re: [FlyRotary] Re: prop speed
Date: Sun, 27 Aug 2006 16:19:11 -0500
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Well, guys, I think you should look at this a little differently.  The 
rotational tip speed pf the prop is at right angles to the forward speed and 
should be added  vectorially.  As a result, it adds very little.  For 
example, a 72 inch prop turning 2400 RPM going 180 MPH results in a 
rotational speed of 6' times pi or 18.85 feet per REV.   Now, the forward 
speed  in that length of time (1/30 of a second at 2400 RPM) is only 8.8 
feet.  Add those vectorially (the two sides of a right triangle) and the 
result is 20.8 feet per rev,  almost within 10% of using RPM only.


FWIW

Bob Darrah


----- Original Message ----- 
From: "Ernest Christley" <echristley@nc.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Saturday, August 26, 2006 11:53 PM
Subject: [FlyRotary] Re: prop speed


> Barry Gardner wrote:
>> That would mean that the rpm they calculate would be too high for an 
>> aircraft (except for the first five seconds of takeoff) and the prop 
>> length recommended would be longer than an aircraft could use because the 
>> longer tip could go supersonic when the forward velocity is added in.
>>
>> Anyone want to tackle this or straighten me out?
>>
>>
> No straightening needed.  Your course line is perfectly straight.  Your 
> conclusions are solid.
>
> --
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