On Thu, Jul 10, 2014 at 06:00:14PM -0700, meekerdb wrote: > On 7/10/2014 4:42 PM, Russell Standish wrote: > >On Thu, Jul 10, 2014 at 01:23:35PM -0400, John Clark wrote: > >>On Thu, Jul 10, 2014 at 3:28 AM, Russell Standish <[email protected]> > >>wrote: > >> > >>>IIRC, the average insolation is something like 1kW/m^2, so that would > >>>make John's solar cells around 3% efficient > >> > >>At noon under ideal conditions a square meter of solar cells can produce > >>about 150 watts, but midnight is not ideal conditions and I was averaging > >>over 24 hours. 30 watts sounds about right to me. If you rigged the solar > >>panels to follow the sun you could do a bit better but it would be > >>dramatically more expensive. > >> > >>I assume that the quoted 333MW engines for a 767 was actually the maximum > >>>power, which is only required for take off, > >> > >>No. The number I started with was 140 megawatts, and that's the AVERAGE > >>amount of power a 747 uses during a flight, during take off it would be > >>considerably more than that. But after I sent my post I did realized I made > >>a mistake, I was assuming that the jet engines on the 747 were 100% > >>efficient which is nonsense, 50% would be more like it. So the factory > >>needed to provide the fuel to keep just one 747 in the air would cover > >>closer to 6 square miles of the Earth’s surface than 3 as I originally > >>said. And that my friends just isn't practical. > >> > >Of course, you're free to clarify where your figures came from, but usually > >consumption is quoted in litres per hour, and power generated is the > >maximum power output of the engine. Think about how you car is rated, > >which is usually done using the antiquated units of miles per gallon > >for consumption and brake horsepower for peak power. > > > >So you would need to have multiplied the aircraft's consumption by the > >calorific value of avgas to arrive at your starting figures. Since you > >didn't say that, I assumed you were using the maximum power figures, > >which seemed more likely to be at hand. > > > >Actually, doing a quick Google, I see you figure of 140MW is splashed > >around by Wikipedia. But it still sounds suspicious. > > > >Looking at http://en.wikipedia.org/wiki/Boeing_747#Model_summary gives > >a maximum thrust of 223kN for each of the 4 Rolls Royce engines. Given > >that maximum thrust and power would be developed at takeoff, we can > >multiply this value by the takeoff speed to determine the maximum > >power of the aircraft, which I estimate to be somewhere around 30 m/s > >(100km/hour approx). > > > >So 223kN x 4 engines x 30 m/s = 26MW > > Although that's maximum thrust (46,000lbf) it's not maximum power. > The thrust doesn't fall off with speed. They probably produce > virtually the same thrust at 250m/s.
They possibly can, but unlikely to do so, as maximum thrust is only required on takeoff. Once the plane reaches cruising altitude and speed, the thrust is backed right off. More to the point is when maximum power is reached - I would still guess its at takeoff. -- ---------------------------------------------------------------------------- Prof Russell Standish Phone 0425 253119 (mobile) Principal, High Performance Coders Visiting Professor of Mathematics [email protected] University of New South Wales http://www.hpcoders.com.au Latest project: The Amoeba's Secret (http://www.hpcoders.com.au/AmoebasSecret.html) ---------------------------------------------------------------------------- -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
