Hi,
Normally I would agree unless another goal supersedes:
Quote:
Originally Posted by P-hack
Hi Bob. I'm a big fan of keeping it simple, so I'm scratching my head why a $20, 8lb 12v 12ah battery . . .
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I will have to measure the weight of the assembled NiMH pack but the back-of-the-envelope:
- 8.38 lbs - SLA/AGM, 12 AH
- 3.98 lbs - 11, D cell, NiMH, 10 AH
Saving 4.39 lbs helps mitigate the addition 9.2 lbs from the inflight, adjustable prop. In my rebuilding effort, I am concentrating on losing weight and reducing drag while addressing
risks:
- 172.6 Lbs HAPI VW, 60 hp -> 112 lbs Hirth 3502, 60 hp
- 3,200 rpm prop -> 2315 rpm geared prop, lower drag
- wet oil sump -> oil injection, no oil changes or filters
- carburetor -> fuel injection, electric, no mixture, no carb heat
- flywheel alternator @3200 rpm -> flywheel alternator @5000 rpm, 20A
- 3.5 gal/hr @60 hp -> 3.9 gal/hr @60 hp
- 60-140 mph, fixed air cooling -> 60-140 mph, adjustable liquid cooling, lower drag
- 52" fixed pitch prop, 5 Lbs -> 52" adjustable pitch, 14.2 Lbs
- no electrical load -> 30A peak to 4A sustained, risks fuel injection ECU
- 0-60 mph lugs engine -> 0-60 mph full power, takeoff
- 60-100 mph full power -> 60-100 mph full power, climb
- 100-140 mph over-revs engine -> 100-140 mph full power, cruise
- previous VW battery -> Odyssey PC625, 13.2 lbs, 18Ahr
- single 12V electrical puts fuel injection ECU at risk
- 'soft-start' may allow use of lighter battery
- independent 12V for fuel injection ECU, fuel pumps to complete flight
- all other loads on the primary 12V including starter
The desire for 'simple' has been one of the chief complaints about our Prius because it has an independent, electrical, motive power system. The Prius transmission is very heavy due to the electric motors, traction battery, and power inverter. But the Prius complexity gives us nearly a doubling of fuel fuel efficiency, 25 MPG -> 50 MPG. Fuel efficiency trumps simpler but inefficient cars.
Today we have options that were not possible decades ago. So we have choices IF we pay attention to the requirements, design, and throughly test what we about to do. In this case, I'm looking at NiMH batteries which offer a significant weight savings without the complex battery management needed for LiON.
Thanks to our Prius, I feel comfortable with NiMH batteries and confident in my ability to manage the charge/discharge. In contrast, I don't share that confidence with LiON because of the flatter SOC vs voltage curve and more abrupt transitions at the charge limits. As for lead-acid, weight is the problem, and though somewhat more tolerant of abuse, they still suffer the same problems as NiMH and LiON.
With luck, I'll bring the empty weight of N19WT from 695 lbs to 650 Lbs. This means faster acceleration on takeoff (i.e., shorter runways,) faster climb (i.e., avoiding trees, power lines, and reaching cruise altitude sooner,) and more payload (i.e., ~8 gal or 2 hours.)
Other Dragonfly owners have put larger engines, O-200, rotary, Corvair, 2100cc VWs, and rediscovered the problems of higher weight and fuel consumption. Instead of landing at 60 mph, they are at 70 mph. Instead of flying nearly 600 miles, they plan 400 mile legs. They fly solo or two small people with no overnight bags (my second passenger is my wife.)
Bob Lutz calls losing vehicle weight 'the virtuous cycle.' It leads to less drag, lower fuel consumption, and better performance. So that is why I'm using the technologies that are available since 1980, the year the first Dragonfly flew. These are technologies not available 35 years ago.
Bob Wilson
ps. I use "lbs" for specs or paper numbers but "Lbs" for those I have measured. I fly 'in the real world.'