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Author Topic: Rubber Energy  (Read 543 times)
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rivers
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« on: February 15, 2017, 03:05:06 PM »

Hi, I'm looking for some good numbers regarding the energy delivered to the prop in F1B models. The purpose is to compare the climb efficiency of modern F1Bs to that of current F1Q electric models.

As a starting point I've put together the following estimates:
F1B model weight = 230 grams
Height gain of the better F1Bs  = 100 to 120 meters, say 110 meters from reports I've read
Therefore, calculated potential energy gain = 25.3 kg-force meters which equates to 248.1 joules

Now the input energy from rubber is very roughly estimated from reports as follows:
Unwinding turns = 450 which equates to 2827.4 radians
Average unwinding torque = 50 oz. inches which equates to 35.9 gm-force meters
Energy to prop = 2827.4 x 35.9/1000 = 101.7 kg-force meters which equates to 997.3 joules

Therefore, climb efficiency = (248.1/997.3) x 100 = 24.9 %

Does anyone have some good or better data for rubber energy?
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« Reply #1 on: February 15, 2017, 05:26:34 PM »

Dick,

Good rubber should yield about 4200 foot-pounds per pound. With a 29.3g motor, that works out to about 271.3 foot -pounds per motor. Assuming that all that energy was used, it would lift the model about 535 feet. Taking 320 feet as the actual altitude gain, that works out to about 59.8% efficiency.

Comparing an F1B to your F1B-based F1Q, the bigger F1B prop is probably more efficient, but your climb rate is relatively constant, so the F1B has higher drag during the first part of the climb (the burst). Your motor run is maybe 5 to 10 seconds shorter than an F1B motor run; could the F1Q climb speed be slightly higher than F1B cruise?

It is important to note that a high percentage of the stored energy in a rubber motor comes from the high-torque burst, which only lasts a few seconds. Where did the 50 in-oz average unwinding torque come from? It seems high. A typical motor might start at 120 in-oz (or higher) but drops very rapidly to cruise level, which might be in the 30 in-oz range for much of the flight.

You might take a look at some of Peter King's Sympo papers. In one of them he graphs torque against unwinding time. But note that the data may be for the older 35g motors.

Sorry about the mixed use of English and metric numbers.

Louis
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rivers
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« Reply #2 on: February 15, 2017, 06:26:01 PM »

Louis,

Yep, I probably overestimated the average unwinding torque. My 50 oz-in number was based on a quick glance at some curves found online.

When you say good rubber yields 4200 ft-lbs per pound are you using the word yield to represent unwinding energy vs wound energy? If so, that would say the energy delivery by a 29.3 gram motor would be 37.5 kg meters ... a lot less than my 101.7 kg meters.

That in turn would lead to a climb efficiency of 67.5 % which to me is an astonishing result. That would say F1B models are remarkably efficient machines.

Dick
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ykleetx
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« Reply #3 on: February 15, 2017, 06:54:06 PM »

I would use 5500 ft lb per lb as the energy density. That brings down the efficiency a little.
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rivers
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« Reply #4 on: February 15, 2017, 08:17:19 PM »

OK on the alternate energy number for rubber.... Thx.

Using 5500 ft lb/lb the climb efficiency for the F1B drops to 51.5 %. Still a remarkable value.

My own F1B-based electric model can climb to the same height as the better F1Bs. But it takes more energy input to do that. First there is the energy input from the battery which is limited by the rules. Second, motor efficiency of about 70% reduces the energy delivered to the prop. Overall, I've calculated that the climb efficiency is 38 % at best based on energy to the prop.

The good news is there is considerable room for improvement. However, I doubt that electric  will ever be as good as rubber.

Dick 
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« Reply #5 on: February 15, 2017, 08:51:33 PM »

Dick,

I found the Peter King torque vs time graph. It was in the 1995 British Free Flight Forum.  It was for 28 strands of (1/8?) Tan II but year and month were not noted. Torque went from 115 in-oz to 30 in 6 seconds and hit 20 in-oz at 15 seconds. Total time was 51 seconds, which is longer than current practice (35-42 seconds). In 1995 rubber weight for F1B was 35g. The curve looks like a very steep sky slope.

Kang,

Where can I get some of that 5500 ft/lbs per pound stuff? Please don't tell me it was May 99. I sold off my stock (about 7 pounds) in 2005, unfortunately before prices got really crazy. Frankly, though, I didn't like it for F1B--too fragile.

Louis
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ykleetx
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« Reply #6 on: February 15, 2017, 10:09:18 PM »

Louis,

I was not talking about May 99.

I've heard of mid 5,000's for Tan SS for some time now (a few years). There's considerably more energy at the peak.
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« Reply #7 on: February 16, 2017, 06:38:55 AM »

Kang,

Thanks for the info.  I used to rely on Fred Pierce's rubber test data, as did a lot of other people. Usually Fred had foot-pounds per pound info out for every new batch. Has anyone continued his work? I have not seen any data on recent batches of Super Sport, only rumors about a certain batch being good, bad, etc. 

You are right about the energy build-up with SS. Those last few turns really do matter.

Is May 99 Tan II still the rubber of choice indoors? 

Louis

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ykleetx
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« Reply #8 on: February 16, 2017, 10:43:01 AM »

Louis,

Yes, May 99 is what almost all of us use. The F1D motor is only 0.4 g, so we use this valuable rubber sparingly Smiley

-Kang
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« Reply #9 on: February 16, 2017, 12:46:00 PM »

Kang,

The last price (about 5 years ago) that I heard for May 99 was $350 per pound.  That works out to about 1133 F1D motors, so about 33 cents a motor. At current prices a Super Sport F1B motor works out to about a buck and a half.  Glad we don't have to fly F1B with May 99 at about $24 each.

Louis
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ykleetx
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« Reply #10 on: February 16, 2017, 01:45:28 PM »

And each F1D motor lasts 25 to 30 minutes. Even more "bang" for the buck!
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ricardo
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« Reply #11 on: February 16, 2017, 08:21:56 PM »

Good rubber should yield about 4200 foot-pounds per pound. With a 29.3g motor, that works out to about 271.3 foot -pounds per motor. Assuming that all that energy was used, it would lift the model about 535 feet. Taking 320 feet as the actual altitude gain, that works out to about 59.8% efficiency.

I've got a little article on P30 props at http://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=142&mode=search under Miscellaneous Articles.

Applicable to other rubber models too.  It's about prop efficiency but it suggests that your 59.8% TOTAL efficiency is probably very optimistic.  The prop itself is likely that efficient only during a small part of the flight.

The energy return from the rubber is good only during the 'Linear' part of the motor run.  During the initial burst, ie above the torque knee, the rubber will only return a small part of what's put in ... and the prop will be inefficient too.

To make the prop more efficient during the burst requires LESS pitch which is the opposite of 'common' practice for variable pitch rubber.
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« Reply #12 on: February 17, 2017, 07:38:16 AM »

Price update:

Just saw a post on Free Flight 4 Sale by Mike Woodhouse (Free Flight Supply) that he had some May 99 for sale. Mike said that he had heard that the going price is a dollar a gram. That works out to about $450 per pound, so up a hundred dollars in five or six years. Should this stuff be traded on the New York Stock Exchange?

Louis
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