Re: Re: Hints for Cat IV LPP flying

From: John Barker <john.barker783_at_ntlworld.com>
Date: Fri, 5 Mar 2010 17:48:00 -0000

 
Quote< First approx for optimum rubber wt is double airframe wt (6g on LPP). Twice the energy of 1:1 and only 50% higher wing loading.

Adding in the reduced drag coefficient of the faster flying (higher Reynolds Number) the ideal ratio is closer to 2.5:1. > end quote




I am puzzled by this claim. Long ago aeromodellers applied a little calculus to a simple duration equation and concluded that the optimum rubber weight was twice the airframe weight. (I first saw this in the 'Aeromodeller' about 70 years ago and it was probably old even then!)_Now the first point is that the rate of increase of duration with rubber weight is very slow as the rubber weight gets higher such that if you use a rubber weight which is the same as the airframe weight (1:1 ratio) the duration will still be over 90% of the duration at the theoretically best 2:1 ratio.




However it is necessary to go further than this. The old calculations assumed that the amount of rubber could be increased without an increase in airframe weight. Any top indoor flyer can tell you that this is nonsense. A heavier motor demands a bigger, stronger and heavier motor stick and probably a stiffening in other components as well. Bob Meuser did one of his excellent papers in the NFFS Sympo a few years back which dealt at length with the reduction in the theoretical 2:1 figure as structure weight increased.




John Barker - England
Received on Fri Mar 05 2010 - 10:33:04 CET