Gary
As I've probably told you and others I'm not saying that my methods are
correct or that someone else should adopt them. In this case what I've
done is found an approximate result based on modifications I've made to
someone else's hard work. There are a lot of ways to go wrong in that
process but in general it makes sense to me. What I (and most everyone
else) would really like to have is a program that would spit out
everything you need to know to maximize a model and a motor for a given
flying situation. We're not at that point and probably never will be.
That being said there's not really any correlation between my motor
testing routine and picking the proper motor length and weight. To get
length and weight you have to input (into your brain's store of data)
everything you've learned about flying. In the case of events without
rubber weight restrictions you also have to decide what your flying
style is going to be. In simple terms I fly longer and heavier motors
than most people use so I consider that my style.
When I'm preparing for a contest I usually will decide on motor length
and weight and then make a variety of motors that generally meet those
criteria. I will make motors from different sections of rubber and often
from different batches as well (usually 3/02 and 5/99). What I HOPE to
learn from testing this group of motors is which ones will give me the
best chance of success. Since there are usually variations in the length
and weight of individual motors and since heavier motors will usually
score higher, I use the weight factor to normalize the scores.
My version of the Hunt program still gives flight time estimates but
because of the changes I've made I don't think they mean much. I use the
"sink speed as glider" and the "static stability margin" to determine
whether changes are good or bad. And even those numbers give very
suspect results in some cases.
On 10/25/2013 5:48 PM, Warthodson_at_aol.com wrote:
>
> Bill (others are encouraged to jump in and comment, too),
> Thanks for the through explanation. It will take me some time to
> absorb all the subtleties of the changes you made to the Hunt program,
> but you have given me food for thought. I do understand the basic
> concept & reason for what you did.
> When I began testing motors using your method I tested all the motors
> that I had that were already tied up. As you mentioned, I quickly
> realized that a heavier motor would almost always have a higher raw
> score that a lighter motor. It occurred to me that if I divided the
> raw score by the loop length & total weight of the motor the result
> would be a dimensionless number (score) that might indicate the energy
> potential of each motor. I.E. the higher the "score" the more energy
> potential, regardless of motor weight &/or length. I would still have
> to decide which motor weight/length combination would be the best
> choice for any given situation. Do you see any merit or flaw in this
> approach? Also, I assume your approach does not determine for you
> which motor length & weight will be optimum of any given situation
> (ceiling height, etc.). Is that a correct assumption?
> I have used the Hunt program to test the theoretical performance of
> different A6 designs, but I have always looked at "Approx fixed pitch
> time corrected for low rubber ratio" as my indicator of a good design.
> A quick review of a few different designs seems to indicate a direct
> relationship between "Approx fixed pitch time corrected for low rubber
> ratio", sink speed & Power needed for level flight. I wonder if there
> is always a direct relationship between these. In other words is the
> best time also always the best sink speed & the best power for level
> flight? I did notice that there is not always a direct relationship
> between best time & best L/D ratio.
> Gary H
> **
>
> -----Original Message-----
> From: William Gowen wdgowen_at_gmail.compower
> <mailto:wdgowen_at_gmail.compower> needed for level flight
>
> 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 Fri Oct 25 2013 - 15:33:17 CEST
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