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From: "George Lendich" <lendich@aanet.com.au>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-4498839@logan.com>
Subject: Re: [FlyRotary] Re: Prop Chord vs RPM
Date: Sat, 9 Oct 2010 08:21:39 +1000
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Ernest,
Sounds logical!
George (down under)

>A prop is a wing that flies in a circle.  Right?  The most efficient wings 
>(the ones that put on gliders), are very long and thin to get a high aspect 
>ratio.  Right?  Given this premise, it only stands to reason that the best 
>prop would be the longest one that would leave some ground clearance, and 
>then absorbs any remaining Hp by increasing the chord.
>
> That's my story, and I'm sticking to it (unless I'm wrong).
>
>
> Ed Anderson wrote:
>> Lynn, I've looked into trying to calculate such several times  and after 
>> getting a headache each time I gave up.  Prop calculations makes duct 
>> calculations appear simple {:>).  Having said that here is one equation 
>> (below) that relates Thrust "T" and propeller diameter "D".  There are 
>> only three other significant factor in the equation.  There is the speed 
>> of the air stream into the prop disc "v" and the acceleration imparted to 
>> the air stream by the prop "Dv" - air density is what it will be and you 
>> can't affect that (except by where you fly). so if you look at the 
>> factors one could conclude that Prop diameter appears to be the most 
>> signficant factor (at least in this momentum equation).  The Thrust is 
>> directly proportional to the square of the diameter.  So if all else is 
>> equal then a 74" dia prop would produce 74^2 = 5476, whereas 76 " dia 
>> prop would produce 76^2 = 5776
>> or approx 5467/5776 = 0.9467 giving approx 5% more thrust for the 
>> slightly longer prop.
>>  I went from a 68" prop to a 76" prop when I switched from my 2.17 to the 
>> 2.85 gear box.  I was very impressed with the change in take off 
>> performance.  So comparing the two (Yes, RPM went down a bit on the prop 
>> with the 2.85 so in real life the comparison is skewed), but in any case 
>> at the same PROP rpm.  The 66^2 = 4356
>>  going to the 76 gave me 5776, comparing thrust for the two 4356/5776 = 
>> 0.75 or a gain of 25% in thrust for the same prop rpm - now that will get 
>> your attention. However, this equation clearly does not take prop chord 
>> into effect and based on the simple chart of three data points - it would 
>> appear to play a major role in static rpm and thrust.
>>
>>
>> *From:* Lehanover@aol.com <mailto:Lehanover@aol.com>
>> *Sent:* Friday, October 08, 2010 1:54 PM
>> *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
>> *Subject:* [FlyRotary] Re: Prop Chord vs RPM
>>
>> In a message dated 10/8/2010 11:48:42 A.M. Eastern Daylight Time, 
>> eanderson@carolina.rr.com <mailto:eanderson@carolina.rr.com> writes:
>>
>>     I did a plot of chord of the three props mentioned recently.  The
>>     props were of 74-76 inches in dia and made by Catto (2) and
>>     Performance Prop (1).  Note my Performance Prop was shorter only
>>     74 compared to Sams and Dennis Catto Props.  When mine was 76" in
>>     dia my static was lower than 6000 which would have all three props
>>     pretty much on a straight line.  I don't know if one could make
>>     much of a linear relationship between prop chord and rpm based on
>>     just three data points, but thought some might find it interesting.
>>
>> And now how about the thrust for each diameter and blade chord and pitch. 
>> What is the most effective prop for the most common diameters?
>>  Lynn E. Hanover
>
>
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