Solarsail said:
Yes, and I did not claim that the L/D ratio is sufficient to determine power.
In fact I did not even talk about L/D ratio. I talked about absolute drag (and lift efficiency). Larger planes have higher drag values, and thus need more power. And the more weight, the more lift is needed, and thus higher speeds, which results in more drag. That should be obvious.
I don't mean to argue, but in fact you are talking about L/D. That's because L/D and glide ratio are identical, as in they are mathematically and theoretically one and the same. I'm happy to explain this if you'd like, but it's a little outside the scope of the original discussion.
Solarsail said:
Can you please provide the formula to arrive at the sustaining power from the L/D ratio and weight?
Check out the second paragraph of https://endless-sphere.com/forums/viewtopic.php?f=38&t=106128#p1555722. It's a trigonometry calculation, where you know how much weight you need to support (plane weight) and then because of L/D ratio you know how much drag (force) which results from that much weight. So if your plane weighs 1500N, and has a 50:1 glide ratio at a certain speed, then you have 30N of drag. If you know the speed, then you can determine the work, which is speed * force.
In short, (mechanical) cruise power = weight(force) / glide_ratio * speed(@glide_ratio)
Please note that these calculations do not take into account any inefficiencies in the system.
Solarsail said:
kubark42 said:
Furthermore, the important part of power is more about climbing than sustaining. The sustaining power can be determined by the L/D ratio and the plane's weight.
Power (kW) for climbing has never been a problem for electric flight. Just like acceleration has never been an issue for EVs. The important issue is range or energy with the unit kWh - and thus cruise power. A small 15 kWh battery pack can easily give you 45 kW of continuous power. And a small PMAC electric motor can easily give you 30 kW of power continuous. Given that "Flight hp" (i.e. the hp advertised by Rotax and others) is only about 0.5 kW (not to be confused with electrical or mechanical hp which is 0.74 kW), even a small battery pack and a small motor of 30 kW can give you 60 hp of sustainable flight, and 100 hp of peak climbing power. 100 hp for an LSA plane is far more than necessary for good climbing. Electrical aircraft will always have faster climb rates than general ICE powered aircraft.
I understand where you're coming from, but strongly disagree that the important value is range. As a glider and powered plane pilot, I have some small experience with watching the ground retreat on takeoff. More to the point, I have experience with watching the ground approach on takeoff, which is *very* scary. If you sufficiently powerful hit sink right after takeoff, and I did just about did yesterday, you can wind up in a situation where you have no options whatsoever other than to hope that the sink goes away before the tow plane hits the ground, with you about 2 seconds behind. Sometimes the sink is stronger than the tow plane and the result is tragedy.
The only protection is climb rate. The higher your climb rate the more you can combat sink and the sooner you can get to a safe altitude. Once I get to 1000' in a glider I can catch some lift and have a five hour flight without issue.
I also disagree that climbing is not a problem for electric flight. Climbing is *the* problem for electric flight, as the heat capacity of the components is very low. The motors I'm using to convert a glider to an eGlider will fail much beyond 100C. Since the motor only weighs 1.2kg, it doesn't take much internal heating before it reaches peak temperature and can no longer run at full power.
Batteries and speed controllers have similar problems. None of them like to go much beyond 80C. Some can handle as much as 100C, but that's really getting close to system failure and you're overstressing components, esp. electrolytic caps.