Re: Re: Grain Direction in Moulded Prop Blades

From: Bill Gowen <b.gowen_at_comcast.net>
Date: Sun, 11 May 2008 10:43:27 -0400

John
My own take on your questions is that the method of blade construction is pretty much class dependant - and this is mostly due to weight considerations. Maybe a simple answer is that making a blade a single piece of wood (as in Limited Pennyplane) would either be too heavy or too weak for such classes as F1L or EZB. Conversely a blade formed from edge gluing oblique sheets of wood is more complexity than is needed for heavier classes like LPP and PP.

I always strive to make my airframes as stiff as possible. I'm not as good a builder as many of the people I compete with. The combination of these 2 things means that when I get a finished airframe I usually don't have a lot of weight left for the prop. Also, I like BIG props, which further complicates the issue. An example would be that for F1L going in this direction only leaves about .26g for the prop. To get the size prop that I want and have it weigh .26g requires .009" 4# balsa for the blades. I think you can see intuitively why a single piece of .009" wood could not do the job with the grain parallel to the spar.

A cloudier example would be F1M. Because F1M has a 3 gram min. wt. you have a wider variety of choices for prop blades. I've used PP style blades, EZB style blades and built-up blades. Again, part of the choice is going to be determined by the weight of the rest of the airframe. In my case I have about .7g available for the prop. I use a robust VP hub that I think weighs about .3g, leaving .4g for the blades and spars. At this point a LPP style blade leaves the picture (too heavy) and I'm left with a choice of EZB style blades or built-up blades like an F1D. Early on in my F1M flying I tried built-up blades for a year or two. I came to 2 conclusions: (1) shearing the blades off of the prop a high percentage of the times the model landed.(in the case of built-up blades) was not a good way to be successful in competition and (2) a sheet wood prop is PROBABLY more efficient than a built up prop (this is just a guess - Aki has been very successful with built-up props).

I agree with every thing Mark said and am just adding that many of the things we do as "standard practice" got that way for pretty mundane reasons instead of from aerodynamic theory.
Bill Gowen
(hoping to become an indoorist again soon!)

  ----- Original Message -----
  From: John Barker
  To: Indoor_Construction_at_yahoogroups.com
  Sent: Sunday, May 11, 2008 4:48 AM
  Subject: RE: [Indoor_Construction] Re: Grain Direction in Moulded Prop Blades


  Mark
  Thank you for an answer in you usual competent style. It constantly
  surprises me that although most of us have pursued this hobby for a long,
  long time there are still questions to be asked on what, at first sight, may
  seem quite simple matters.

  I think a further advantage of oblique grain, that I did not think to
  mention before, is that it resists the tendency of the blade to acquire
  additional, unwanted, undercamber. Following from this it does look as if
  the classes that use thicker material for the blades do not usually run the
  grain obliquely.

  John

  [Non-text portions of this message have been removed]



   


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Received on Sun May 11 2008 - 07:43:30 CEST