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Brom Nader
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*** streamlines -> streamlined Any reason why comments cannot be edited, and that there is no "Reply" button?
@Insider - Yes I agree. The Beaver is not streamlines, and the pontoons are a huge drag as well. It is a large and heavy plane. In my last comment I was referring to a streamlined 4 or 6 seater. The problem is that if a beaver needs at least 100 kW to cruise as is claimed, and that is the plan for this electrification by Harbour Air, then today's battery technology at 250 Wh/kg will not work for them. You might as well have added to the article that this project is infeasible.
Thanks for the valuable information @gryf, @engineer-poet, @insider One reason power cut only to 75% may be due to the inefficiency of a ground adjustable pitch propeller at lower RPM speeds. Thus in order to maximize full power thrust, lower power thrust is sacrificed due to lower pitch efficiency at reduced RPM. One way to heat the batteries would be to use motor and inverter waste heat. If the motor is 94% efficient and inverter 97%, then 9% of the power is thermal waste. If all this thermal energy can be captured, at 30 kW cruise, that is 2.7 kW -- quite a bit of heat that can easily warm the cabin, and batteries. Probably liquid cooling of the motor and inverter would be needed to capture significant waste energy.
@insider 30 kW I claim is equivalent to 60 hp of ICE (not 40 hp) - the 'Brom Rule'. A streamlined aircraft (and Beaver is not one of them) consumes not more than 20% more than an EV at "economy cruise". You can easily do 80 to 100 kph with 10 kW on a Leaf. That is why the Pipistrel can cruise at 10 kW. The ratio between climb and cruise is far more than 1:75% or 1.33:1. My own experience with an EV shows that every meter of hill climb is equivalent to an extra 40 meters of flat surface roll. That is the cost of the potential energy. If we apply this to an aircraft climbing at 1000 FPM or 300 m/min or a climb of 18 kph, then you get 1000 FPM climb is equivalent to 18 * 40 = 720 kph of extra travel. So the climb at 100 kph horizontal and 720 kph vertical and its ratio to 100 kph economy cruise would be 8.2:1. This is a far cry from 1.33:1. (Note that the aircraft is not flying at 820 kph but at 100 kph when climbing, so the drag is the same). If the Beaver needs 300 hp to takeoff, then it should only need (300 * 0.74)/8.2 = 27 kW to cruise (or 18 kW according to the 'Brom Rule' of 1 hp = 0.5 kW). These are less than the 30 kW that I have assumed to arrive at battery size. Note that the top speed on the Beaver may not be limited to engine power but more likely to airframe structural integrity and controls. I think electric propulsion would be very attractive to tours. Foremost is cut in the noise and fumes. In a Beaver you need noise cancelling headphones to have a decent conversation. This requirement will probably go away. Then there is the anxiety of an engine that is 40 years old from going kaput. Also the novelty of electric flight and it being green. Less disturbance to wildlife. Less chance of fire. Ability to take off in tight spots improves because there is less chance of an engine failure. Also performance at high elevations is superior. Finally, an electric motor can provide 3x more power than its continuous rating (if the inverter and batteries comply), to get out of tight situations like a pre-stall or tailwind takeoff.
@sd - Very good points. When engine power is reduced to 55%, is that the measure of the fuel flow or is that RPM? If it is RPM, then clearly the power drops to far less than 55% due to the nonlinear torque curve on ICE. For some reason, this does not agree with what pilots are reporting on the Pipestrel Alpha electric plane. The difference between takeoff and cruise can be 50 kW to 10 kW or down to 20% reduction or 5:1. While on ICE, the reduction is being reported at 55% or 1.8:1. Possibly the Alpha has real-time variable pitch props so it can adjust the RPM and pitch to optimize cruise power. But still there is a huge difference between reduction to 20% and to 55%. Compare that to an EV. An EV starting with high acceleration and climbing a hill will easily consume 80 kW. But while cruising on a flat surface at 80 kph, 10 kW is plenty. That is a 8:1 ratio. What gives?
@sd Have you considered a 5 blade prop instead of 2 blades, so RPM can be reduced?
@Herman Interesting comment - thanks. I would guess that for a beaver with a limited max takeoff weight, most efficient cruise speed would be not more than 30 kW of power. So storage required would be 30kWh, and if you add an extra contingency of 50%, that would be 45 kWh for 30 minutes of flight or 70 kWh for 1 hour of flight. 150 kWh that you propose will give you 3hrs with the contingency and 4.5 hours of flight time without contingency. This is way too rich and not required. At 5 kg/kWh, 30kW would translate to 150 kg or 225 kg with extra contingency (not 700 kg). The batteries can be put in a compartmentalized modified pontoons, and thus the issue of spontaneous combustion, which is very rare, would be addressed. 200 kW power that you propose is way too rich. Note that due to the flat torque curve of an electric motor and the persistent over-specification of engines by the manufacturers, one hp on an ICE translates to 0.5 kW (and not 0.74 kW) on an aircraft. There are no need for battery swaps. 45 kWh can be fast charged in 30 minutes. That is a one hour flight, because reserve is not normally recharged. With a 1hr fly + 0.5 reserve battery of 70 kWh (which includes 50% contingency), the DoD will not dip below 40% (and not 20%). This greatly improves the cycle time to a few thousand cycles. Thermal management is strictly a charging matter, and that can easily be done with land-based blowers (air cooled), or pumping seawater (liquid cooled). During a few minutes of takeoff where 150 kW may be needed, the rush of ambient air can cool down the pack which is discharging at 2C - a very modest rate. I believe I have addressed all your objections?
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Mar 27, 2019