Flaps are part of it, but I suspect a larger part is bled off in approach turns, which are in part required to do just that.mgb_phys said:It still depends on the amount of power lost by aerodynamic drag.
If when you turn off the engine you lose speed fast enough due to drag then using a regenerative propeller to slow even more isn't possible. It would only be worth it if with no engine you still had to use flaps to dump speed...
If you have to use active means to lose speed then regenerative braking is in theory worth it - wether it would be worth it in terms of added weight, complexity, reliability etc .mheslep said:Flaps are part of it, but I suspect a larger part is bled off in approach turns, which are in part required to do just that.
They increase lift and drag.russ_watters said:Flaps aren't used to bleed speed, they are used to allow the plane to fly at a lower speed.
There have been a bunch of Nasa and secret projects to make a long endurance UAV as a sort of mini-satelite.mheslep said:not to mention that an essentially indefinitely orbiting high altitude aircraft has practical value,
Yes, thanks for the NASA reminder: Pathfinder-'95, PF+-'98, Helios (solar+ fuel cells)-'01.mgb_phys said:There have been a bunch of Nasa and secret projects to make a long endurance UAV as a sort of mini-satelite.
Apparently the Solar Impulse crew believes the batteries are there if they are to fly across at 70km/h. Regards energy requirements, I expect a better metric is solar efficiency divided by battery energy density greater than a constant for indefinite flight, so progress in either one gets you there.If you want to fly in cloud or at night you also need batteries (or fuel cells) and these aren't quite there yet in power/weight. The sunsailor team estimate that you need about 40% efficency panels to fly indefinitely in good conditions (they are in sunny Israel) which is still a little way off.
.Q: Commercial Photovoltaic cells still have fairly low efficiency (typically >20%). At what level of efficiency do you believe they will start proving to be more useful in commercial aviation (both manned and unmanned)?
A: Truthfully, Solar cells low efficiency is only half of the problem. The means of energy storage such as rechargeable batteries or fuel cells is the other half. But solar cells for large scale commercial aviation will have to go a long way before they can supply the power that is needed. Even a 40% solar cell covered wing on a sunny day will barely be enough for the cruise power requirements of a small manned aircraft
The solarimpulse is intending to fly to very high altitudes during the day and slowly descend during the night. So the battery just has to generate enough power to reduce the descent rate.mheslep said:If 20% panels can now carry a person across the Atlantic w/ good batteries, then I expect 20% panels can keep up a UAV indefinately.
Yes I saw the day/night altitude plan. I don't see that flying at night by itself is risky. The battery energy is definite, the sunrise time is definite. Weather is another issue, though they can fly above most cloud cover ascending during the day, and they don't care about cloud cover when flying lower at night. Otherwise they have the option to simply avoid the more intense weather. I did notice they were concerned about head winds at night. This must mean they intend to chase the sun rise and gain an ~hour of sunlight. If things are that tight then they are certainly at the margin.FredGarvin said:One of the big things the Solar Impulse is relying on is that they gain altitude during the day when they don't have to draw off the batteries. At sunset, they initiate a slight decent and try to extend that for as long as possible to some low altitude point. From then until sunrise, they are on batteries. Kind of risky especially when you have to also think about having to possibly avoid weather issues.
Why? Based on the energy - time profile claimed for this manned vehicle, then a UAV should weigh ~200kg less without a cabin, have less drag, require no cabin environmental power. Thus I would think a fuel-less loitering UAV immediately possible w/ the same technology. It might have other undesirables, such being slow to move on-off station, or forced off station by weather.mgb_phys said:I suppose if the aim is just to stay 'up there' then you could hunt for thermals and soar - only using the engine to get to the next thermal.
But if you want to stay over a city to relay TV signals or loiter over a battlefield you are going to need a lot more power.
They don't have much leeway to avoid weather at night, they are going to descend to a certain altitude by a certain time however bad weather is down there. They also don't have the energy reserves at night to fly a few hundred miles around a large storm system.mheslep said:and they don't care about cloud cover when flying lower at night. Otherwise they have the option to simply avoid the more intense weather.
? They have essentially infinite reserves. Unlike a fueled aircraft, they can afford to fly in any direction they care to. They only need to avoid weather for the night and can choose any compass point w/ out regard for the nearest airport. Granted some 1000 mile wide severe front will cause problems.mgb_phys said:They don't have much leeway to avoid weather at night, they are going to descend to a certain altitude by a certain time however bad weather is down there. They also don't have the energy reserves at night to fly a few hundred miles around a large storm system.
Agreed, though I wouldn't underestimate. Given the budget one can build very strongly w/ carbon fiber. Edit: yes its carbon fiber honeycomb. Expensive and very strong. And they claim 220Wh/kg in the batteries. That's almost double what's cited for the new EV car batteries. Twice as expensive too I believe.Most of these solar powered aircraft also look pretty flimsy to be flying into turbulence.