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Electric Propulsion and Density Altitude question
I am doing a new performance course updating the current performance ground school module https://elearning.sportaviationcenter.com/ and trying to include something for electric propulsion and density altitude.
These are my thoughts below. Does anyone have any actual data, testing or engineering basis on this subject that can be specifically referenced?
Electric propulsion and density altitude
With the upcoming electric propulsion for LSA, the density altitude effects are completely different.
As we looked at in the previous discussion of engine power/thrust verses density altitude, we determined the internal combustion engine loses about three percent power per 1000 foot density altitude. As we discussed in the previous examples, that is 100 HP to 70hp at 10,000 feet density altitude. With a fixed pitch propeller, and the density ratio .74, there is less drag allowing the propeller to maintain most of its RPM thus keeping the optimum angle of attack on the prop. This is not the case for electric propulsion. The power (HP/KW) does not drop with altitude. However, thrust is a different story.
From a simplistic viewpoint, if you are putting the same power to the propeller at high density altitudes, the propeller will rotate faster than its optimum design at sea level standard conditions (or what ever the prop is designed for). Generally, the faster the prop, the more drag and the greater prop loss in efficiency. This is similar to the turbo charged reciprocating engine without a variable speed/pitch propeller.
With this electric constant power and higher RPM of the propeller, it can be expected to lose X% thrust (starting point) per 1000 foot density altitude for the electric motor. 10,000 foot density altitude is a good reference for comparison.
Future variations would be a variable speed/pitch propeller or a speed limiting governor for the electric propulsion.
Appreciate your thoughts on this subject…
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3 Replies
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Based on my original comments, the feedback I am getting for high altitude electric performance, is a reduction in power from the batteries with lower temperatures. NO specifics but good enough for now. Moving ahead at finalizing this project which will be a great help for Trike ground school.
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With a naturally aspirated engine, the horsepower reduces, very similarly to the reduction of resistance on the propeller. Meaning if you’re getting 5500 RPM at sea level you can expect to get very close, although it will be slightly lower, RPM at high altitude at WOT.
With an electric motor, the RPM is limited by the KV, or RPM per volt. So the reduction in drag on the prop at high altitude will have no bearing on the RPM. And sure, the amp draw will reduce significantly at high altitude, but it will simulate the same effect as a naturally aspirated motor at high altitude.
Much like a turbo charged engine you MUST either repitch the prop for high altitude or have a variable pitch prop to be able to even take advantage of the motor keeping its HP at high altitude.
So to sum in all up, an electric motor with a fixed pitch prop will give you a very similar reduction of thrust, the higher you go, to a regular combustion engine without a turbo.
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Thanks Larry this correlates with my other findings.
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