4 ms·
L/D ratio is not enough, you also have to take into account speed. A L/D of 6 at 120 mph has an equivalent per mile efficiency of an L/D of 12 at 60 mph (and a
by deepnotderp 5y ago
L/D ratio is not enough, you also have to take into account speed. A L/D of 6 at 120 mph has an equivalent per mile efficiency of an L/D of 12 at 60 mph (and a better von Karman efficiency).
The figure of merit is maximum (speed * L/D), not L/D alone, and this is where the efficiency of airships fall apart. Due to their greater parasite drag compared to aircraft (necessitated by the lifting gas volume), airships generally can’t match the same per mile efficiency, and even if they can, not at the same von Karman efficiency.
- joefigura 5y agoThat's not the case. The energy per mile depends only on the L/D ratio and the mass of the aircraft. L/D is a ratio of forces, and work = force times distance. Going faster at the same L/D decreases the travel time but increases the power used, for the same energy usage. Generally L/D depends on speed, but you can get an apples-to-apples comparison of the energy usage of two different aircraft by comparing only L/D. I think we might be discussing two different figures of merit. The efficiency that I care about is "energy per lb of payload per mile." You haven't defined what you mean by "efficiency per mile," but from the reference to von Karman efficiency, I think you have a figure-of-merit in mind that includes speed - specific resistance or productivity or something like that. But the economics of our vehicle and use case don't really change with speed. The middle mile legs that we're flying are not fully-utilized - the most important thing is to get the cost per flight down, so that we can dispatch the aircraft whenever there's a delivery ready.