Props for the Lucky Lindy
     
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I've been asked about propellors for the Lucky Lindy model many times.  Recently flyer and friend Bob Sowder asked them, and below is how I answered.  Of course there are many other engines and props that will work well for the Lucky Lindy.  What's here summarizes my own experiences over the years.

TheAeroSmith 


Bob:
 
As far as my own experience, the prop that has flown the FAI sized LL the very best has been an APC C-2 8X4.  There areseveral different props carrying the 'C-2' tag.  I've asked them about that, and only gotten an answer that's incorrect; was told that the C-2 (or other) is for the mold for that prop.  I have at the very least, two different APC 8X4's that say they are C-2's.  Obviously, they didn't come out of the same mold.
 
The very earliest of the 8X4 APC (C-1) flew the model very well.  A later version, a C-2, looks similar, but has a beefier hub and has less Scimitar shape.  That one I've used most.  That's also the prop that I blew apart at about 9 seconds out, which made the model look and sound as though it had been hit by a rocket.  I now have epoxy/glass/carbon props of this version.  I thinned the hub area some, and did a very thorough job matching the blades for thickness, width, balance, pitch, and tracking.  This one won't come apart without some serious provocation.  If there's an interest, I may make up a dozen or so to sell.  I think they will have to sell for about $20 each + mail to be worth spending the time on the project.  That's a lot for a prop, but if the model doesn't explode, it's really cheap.
 
There is a newer 'C-2' that is much more conventional looking...pretty straight blades.  I think it may be the best of the bunch, but I have to make a 'glass one before I'll fly it to see how it performs.  As soon as I can get some altimiter testing done with the new prop, I'll let you know the results.
 
Bob

  Several props used on the FAI Lucky Lindy.

Difference between APC props seen in the side view.
 

Some information on prop tip speed and how to calculate it might be of interest to some.  This data courtesy of P.Ponk Aviation (www.pponk.com).

To determine propeller tip speed:

Prop Diameter X Pi / 12 X RPM X 60 / 5280 = Tip Speed in MPH

 

Example: 86" prop turning at 2800 RPM
86 X 3.1416 / 12 X 2800 X 60 / 5280 =
716.4 MPH

To determine the speed of sound:

Square Root (absolute temp + ambient temp) X 33.4 = Speed of Sound

Example: 59 degree F. day
Square root (460+59) X 33.4 =
760.9 MPH (speed of sound)

To determine propeller tip mach speed:

Tip Speed / Speed of Sound = Tip Mach Speed
716.4 / 760.9 =
.942 MACH (too fast)