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Author Topic: Rule of thumb: Duration penalty when overweight?  (Read 283 times)
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ChrisH
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« on: August 10, 2019, 12:17:09 PM »

Ignoring any penalties through a model becoming floppy if too flimsy, is there a rule of thumb guide for the likely indoor duration duration benefit if a model is built meeting the class weight limit, rather than 5%, 10%, 15% overweight?

I am thinking particularly about a Sainte Class cabin model built for small (1 badminton court) hall flying.   My old repaired model is now 2.21gm without rubber so 0.21gm overweight, and a lighter model should benefit from flying on a lighter rubber motor too.

I assume such a 'rule' would apply to EZB and F1D too, but a small limited span cabin class like Sainte or Peanut Duration, is probably less likely to suffer a growing lack of stiffness as weight is reduced.
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piecost
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« Reply #1 on: August 10, 2019, 12:53:36 PM »

This is a good question and one i have thought about myself. I don't have an.answer but suggest factoring the duration by the ratio of airframe weight. Having said that; a 12% reduction in my legal eagle from about 3.4g to just under 3g resulted in a 53% increase in duration! I had changed the motor and atrribute this huge improvement as due to a much better optimised motor/propeller combination than on the heavier model.
I will be interested if anyone has a more concrete answer
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ChrisH
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« Reply #2 on: August 10, 2019, 05:06:30 PM »

Thanks Piecost,

If Duration Gain = (Airframe and rubber) % weight reduction x 4 (or more)......that would be a hell of an improvement with no other variables!

As you said, some of your huge 53% improvement may have come from a better prop/cruise torque fit....or maybe you swapped to a better rubber batch too?

Hopefully the Bob Baileys or Bernie Hunts of this world, and their peers, may have data showing benefit from changing one variable at a time?
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piecost
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« Reply #3 on: August 10, 2019, 08:08:37 PM »

Hi Chris,

Your reply got me thinking. I can only attribute the massive increase in performance with my lighter model as being due to the propeller suddenly working efficiently. Both models employed their rubber motors similarly. The rubber batch performance was similar too.

I found a Bernie Hunt spreadsheet for F1D model design which i had plugged in my legal eagle models a couple of years ago (but not for duration comparisons). It gives a estimated duration for the cardinton hanger. This may be the biggest problem in that it is applicable a 150' ceiling and not a typical leisure centre of around 25'.

Now, i have not unpicked how this works so take the following with a large pinch of salt.

The ratio of indurance seems to vary with the ratio of total model weight (airframe + rubber) raised to the power of -1.42. The thing is that the rubber weight will change with airframe weight. Perhaps you can assume that the rubber mass will vary on proporsion to airframe mass.



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ChrisH
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« Reply #4 on: August 11, 2019, 07:24:44 AM »

Piecost,

Thanks for the reminder.   I remember Bernie produced a program that he ran on a Psion pocket computer, published in the 1986 Free Flight Forum.   I did get it to run in a (Lotus 123?) spreadsheet at work, but have never seen a spreadsheet version published by Bernie - is it still available anywhere online?

The program had many variables, and calibration factors gleaned from observation, but it might be interesting to see if a simplified version will emulate flight of a simple model in a small hall with no ceiling bashing.   I guess a given model could be flown repeatedly with a standardised motor, size/turns/backoff/launch torque, but adding ballast each flight.    Hopefully results and predictions might be in the same ballpark!

I know Bernie Hunt dabbled briefly with Peanut Duration, but don't  know whether he ever used his program to predict results.   He was certainly successful in its application for F1D and the American Intermediate Stick.
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piecost
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« Reply #5 on: August 11, 2019, 09:30:37 AM »

Hi Chris,

I'll send you the spreadsheet if you pm me your email addreess

Cheers

Tim
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frash
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« Reply #6 on: August 11, 2019, 10:48:32 AM »

Somewhere I have some old programs from Bernie Hunt. He came to Johnson City, Tennessee (USA) near me and flew for a few years at the USIC, AMA Nats, and/or F1D World Championships whatever was held that year. This was 1987-1992 approximately.

Since he already published most of this, I speculate that he would not object to a re-posting if you credit him.

I'll look and see if I have anything promising. Bill Gowen is likely to have some of Bernie's calculations also.

Fred Rash

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Olbill
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« Reply #7 on: August 11, 2019, 11:20:17 PM »

I have an unmodified version that I got in 2000. I also have a version with some changes made that are specific to what I want to know. All of the rubber models I fly are based on spreadsheet info.
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