Hi Gary,
You are correct that the Hunt program is mainly concerned with the models sink rate as a glider. Hunt figured out a factor, 14.63 divided by the sink rate times prop diameter efficiency factor. This equation accurately predicts duration of old rules F1Ds with a 1.4 gram motor.
The duration prediction corrected for the low rubber ratio is the rubber weight divided by the 1.4 times the model weight. Based on this equation, I believe that Hunt thought the rubber weight should be 1.4 times the model weight for all classes. However, as the rubber weigh decreases, the sink rate improves. So, there may be a good compromise between sink rate and rubber weight some where in between.
For A-6, the prop efficiency factor is considerably outside the expected inputs of the program. The program compares everything to an 18 inch F1D prop. A prop less than 18 inches is considered less efficient with a factor less than one, while a prop greater than 18 inches has an efficiency factor greater than one. Because A6 uses small flat bladed props I would be wary of any duration predictions the program makes.
Regards,
Nick
Sent from my iPad
> On Oct 25, 2013, at 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 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 Sat Oct 26 2013 - 06:12:44 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:48 CET