Gary
I'm afraid I'm the guilty party in the use of the "weight factor". It's
not a scientifically derived number. It comes from my use of the Hunt
design program and the changes I've made to it for my own use.
Basically I changed the program by removing references to the program's
use of a 1.4x factor for rubber weight compared to model weight. I think
when the program was developed it was for 65cm F1D's and a rubber weight
of 1.4g was considered either optimum or maximum (I'm not sure which)
for the 1 gram model weight. I didn't think this factor had any bearing
on the models I fly so I removed it from the program.
The result of that and some other fiddling I did was to remove the
rubber weight from having any effect on the program's outcomes except
for its effect on the all up weight of the model and the CG location. In
other words adding rubber weight had the same effect on the performance
numbers as adding ballast at the midpoint of the space between the
hooks. The performance number that I was interested in was "Sink speed
as glider". The more rubber weight you use the worse the sink rate of
the model becomes.
Obviously if you have more rubber you also have more energy available so
the degradation of the sink speed is not the whole story. When I test
motors a heavier motor will almost always show a better "score" than a
lighter motor. But as seen in the modified design program adding rubber
weight makes the sink rate worse. Since I'm not smart enough to
calculate how all these effects work together I took a simple route to
get an idea of the total effect, I used the design program to calculate
the sink rate based on the lightest motor I was likely to use and then
did it again with the heaviest motor I was likely to use. Then I
calculated the slope of the change in sink rate between these 2
extremes. This slope is used to calculate the "weight factor" in my test
routine. I calculated the "weight factor" for each different type of
model that I fly.
I think some of my test routines have been messed up by copying from one
model type to another. The original weight factor for A6 was calculated
with this formula:
=0.69/(((0.13333*(D2-0.9))+0.69))
where D2 is the motor weight, .69 is the sink rate in ft/sec with a .9
gram motor and everything after that is the calculation for the sink
rate with a motor that weighs more or less than .9 grams.
So if you're testing a motor that's heavier than .9 grams you'll have a
"weight factor" that is less than 1. Multiplying the weight factor by
the motor score gives an approximate reduced score for that motor.
If your test motor weighs less than .9 grams then you'll get a weight
factor that is more than 1 and your corrected score will be higher than
the raw score.
A really good .9 gram motor may have a worse raw score than a normal 1.1
gram motor but when the weight factor is applied the lighter motor may
have a higher score and MAY produce higher flight times.
I'm sure there are better/more accurate ways to figure out these effects
but this is the way I'm able to look at the problem based on my very
limited knowledge of the physics involved, and this method seems to
produce usable results.
On 10/24/2013 10:53 AM, Warthodson_at_aol.com wrote:
>
> I have adopted the method of testing individual motors that Bill Gowen
> uses which involves winding to some arbitrary % of Max. torque & then
> backing off in increments & recording the torque. The individual
> torque readings are added together to get a total raw score. The raw
> score is modified by a "weight factor" to arrive at an "adjusted
> score". I presume some of you are also using similar procedures.
> I assume the purpose of the "weight factor" is to adjust the "raw
> score" such that you can compare motors of different weights to
> determine which motors are the "best", but I do not understand the
> origin (or derivation) of the formula. Weight Factor =(29.6-(3*Motor
> Wt.))/26
> Can someone enlighten me?
> Gary H
>
Received on Thu Oct 24 2013 - 09:04:50 CEST
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