Last edited:

- Thread starter autoreply
- Start date

Last edited:

Great sentenceAdvertising is much more efficient at generating propulsive power.

It is recommended for fertilising the antigravimetric field. The yeld of fools believing in this stuff will be higher.I have some Unicorn poop I am willing to sell you.

Let's take our hypothetical 'world beater' ecraft and see how it fares in reality

Cruise speed 50m/s (about 100mph)

Flat plate area 0.15m2

Takeoff weight 200kg (oh come on, you can convert that!)

Altitude 46,000 (probable safe limit for pure oxygen breathing)

Air density 0.226kg/m3

Span 39' (11.9m)

Flat plate drag area 0.15m2 (about 1.5sqft)

Flat plate drag at altitude:

0.5 x 0.226 x 50 x 50 x 0.15 = 42.4N

Flat plate drag power

Drag x 50 = 2120W

With 75% propulsive efficiency

2120/0.75 = 2830 W

Induced drag, assumed span efficiency 0.8

200 x 200 x 9.8 x 9.8 / (0.5 x 0.226 x 50 x 50 x pi x 11.86 x 11.86 x 0.8) = 38.5N

Induced drag power

38.5 x 50 = 1920W

With 75% propulsive efficiency = 2560W

Total cruise power required at 46,000' and ~100mph = 5.4kW

3 hours of cruise = 16.2kW of battery

Zero batteries are around 180Wh/kg. So we'd need 90kg of batteries just for cruise. Better increase the T/O weight and recalculate.

400kg T/O weight, 70m/s. We now have 5,120W worth of induced drag and 7750W of parasitic drag. Total power for 100mph cruise is now 12.9kW. Uh, oh. Now I need 38.7kW of battery for 3 hours of cruise. 215kg of zero batteries. Good job I allowed so much weight!

Wait, I forgot climb!

14,000m x 400 x 9.8 = 55MJ

x propulsive efficiency = 73.3MJ

In kWh, 73300/3600 = 20.3kWh

Oh dear, better add on another 113kg of zero batteries.

I must be getting close now?

80m/s, 550kg

Parasitic power: 11600W

Induced power: 12100W

Climb energy: 27.9kWh

Ah, poop. Now I need 99kWh of batteries. That's 550kg of zero batteries.

Weight a minute. That's 100% of the aircraft weight. Nothing left for luxuries like wings and fuselage or motor. I suspect that I may be in a losing battle here? I have no clue how to build a 160mph aircraft that light.

Topspeed, could you please show me where my maths is wrong? I seem to have reached a cognitive impass.

You forgot the solar cells.

Let's take our hypothetical 'world beater' ecraft and see how it fares in reality

Cruise speed 50m/s (about 100mph)

Flat plate area 0.15m2

Takeoff weight 200kg (oh come on, you can convert that!)

Altitude 46,000 (probable safe limit for pure oxygen breathing)

Air density 0.226kg/m3

Span 39' (11.9m)

Flat plate drag area 0.15m2 (about 1.5sqft)

Flat plate drag at altitude:

0.5 x 0.226 x 50 x 50 x 0.15 = 42.4N

Flat plate drag power

Drag x 50 = 2120W

With 75% propulsive efficiency

2120/0.75 = 2830 W

Induced drag, assumed span efficiency 0.8

200 x 200 x 9.8 x 9.8 / (0.5 x 0.226 x 50 x 50 x pi x 11.86 x 11.86 x 0.8) = 38.5N

Induced drag power

38.5 x 50 = 1920W

With 75% propulsive efficiency = 2560W

Total cruise power required at 46,000' and ~100mph = 5.4kW

3 hours of cruise = 16.2kW of battery

Zero batteries are around 180Wh/kg. So we'd need 90kg of batteries just for cruise. Better increase the T/O weight and recalculate.

400kg T/O weight, 70m/s. We now have 5,120W worth of induced drag and 7750W of parasitic drag. Total power for 100mph cruise is now 12.9kW. Uh, oh. Now I need 38.7kW of battery for 3 hours of cruise. 215kg of zero batteries. Good job I allowed so much weight!

Wait, I forgot climb!

14,000m x 400 x 9.8 = 55MJ

x propulsive efficiency = 73.3MJ

In kWh, 73300/3600 = 20.3kWh

Oh dear, better add on another 113kg of zero batteries.

I must be getting close now?

80m/s, 550kg

Parasitic power: 11600W

Induced power: 12100W

Climb energy: 27.9kWh

Ah, poop. Now I need 99kWh of batteries. That's 550kg of zero batteries.

Weight a minute. That's 100% of the aircraft weight. Nothing left for luxuries like wings and fuselage or motor. I suspect that I may be in a losing battle here? I have no clue how to build a 160mph aircraft that light.

Topspeed, could you please show me where my maths is wrong? I seem to have reached a cognitive impass.

- Joined
- Sep 17, 2008

- Messages
- 5,942

Reality is hard for everyone. Math is hard for some. They consider it optional.

Let's take our hypothetical 'world beater' ecraft and see how it fares in reality

Cruise speed 50m/s (about 100mph)

Flat plate area 0.15m2

Takeoff weight 200kg (oh come on, you can convert that!)

Altitude 46,000 (probable safe limit for pure oxygen breathing)

Air density 0.226kg/m3

Span 39' (11.9m)

Flat plate drag area 0.15m2 (about 1.5sqft)

Flat plate drag at altitude:

0.5 x 0.226 x 50 x 50 x 0.15 = 42.4N

Flat plate drag power

Drag x 50 = 2120W

With 75% propulsive efficiency

2120/0.75 = 2830 W

Induced drag, assumed span efficiency 0.8

200 x 200 x 9.8 x 9.8 / (0.5 x 0.226 x 50 x 50 x pi x 11.86 x 11.86 x 0.8) = 38.5N

Induced drag power

38.5 x 50 = 1920W

With 75% propulsive efficiency = 2560W

Total cruise power required at 46,000' and ~100mph = 5.4kW

3 hours of cruise = 16.2kW of battery

Zero batteries are around 180Wh/kg. So we'd need 90kg of batteries just for cruise. Better increase the T/O weight and recalculate.

400kg T/O weight, 70m/s. We now have 5,120W worth of induced drag and 7750W of parasitic drag. Total power for 100mph cruise is now 12.9kW. Uh, oh. Now I need 38.7kW of battery for 3 hours of cruise. 215kg of zero batteries. Good job I allowed so much weight!

Wait, I forgot climb!

14,000m x 400 x 9.8 = 55MJ

x propulsive efficiency = 73.3MJ

In kWh, 73300/3600 = 20.3kWh

Oh dear, better add on another 113kg of zero batteries.

I must be getting close now?

80m/s, 550kg

Parasitic power: 11600W

Induced power: 12100W

Climb energy: 27.9kWh

Ah, poop. Now I need 99kWh of batteries. That's 550kg of zero batteries.

Weight a minute. That's 100% of the aircraft weight. Nothing left for luxuries like wings and fuselage or motor. I suspect that I may be in a losing battle here? I have no clue how to build a 160mph aircraft that light.

Topspeed, could you please show me where my maths is wrong? I seem to have reached a cognitive impass.

Ah poop, that would surely change everything!You forgot the solar cells.

Yeah. I'm eager to know how we get that very low (provided) drag number with a wing top surface covered with solar cells.Ah poop, that would surely change everything!

And if the math indicates thing don't work with zero structural weight, it'll be interesting to see how the induced drag from the weight of the solar cells "improves" things.

Time to think farther outside the box. "Pilot Bill, have you prepared for today's flight? Weather briefing? Mandatory meal of 2 kg of cabbage and beans? Okay, plug into the methane collector and feed that fuel cell. The low pressure at altitude will help get every last cc of energy. Good luck!"

Bare solar cells could be built flush into a composite laminar wing. Curvature is limited.Yeah. I'm eager to know how we get that very low (provided) drag number with a wing top surface covered with solar cells.

And if the math indicates thing don't work with zero structural weight, it'll be interesting to see how the induced drag from the weight of the solar cells "improves" things.

Time to think farther outside the box. "Pilot Bill, have you prepared for today's flight? Weather briefing? Mandatory meal of 2 kg of cabbage and beans? Okay, plug into the methane collector and feed that fuel cell. The low pressure at altitude will help get every last cc of energy. Good luck!"

More span and less speed would be the way forward. I have NFC how to build something with the necessary weight and speed at 70kg.

Blimp, not the structure of a dirigible and like a balloon it may continue to expand with altitude.

Let's take our hypothetical 'world beater' ecraft and see how it fares in reality

Cruise speed 50m/s (about 100mph)

Flat plate area 0.15m2

Takeoff weight 200kg (oh come on, you can convert that!)

Altitude 46,000 (probable safe limit for pure oxygen breathing)

Air density 0.226kg/m3

Span 39' (11.9m)

Flat plate drag area 0.15m2 (about 1.5sqft)

Flat plate drag at altitude:

0.5 x 0.226 x 50 x 50 x 0.15 = 42.4N

Flat plate drag power

Drag x 50 = 2120W

With 75% propulsive efficiency

2120/0.75 = 2830 W

Induced drag, assumed span efficiency 0.8

200 x 200 x 9.8 x 9.8 / (0.5 x 0.226 x 50 x 50 x pi x 11.86 x 11.86 x 0.8) = 38.5N

Induced drag power

38.5 x 50 = 1920W

With 75% propulsive efficiency = 2560W

Total cruise power required at 46,000' and ~100mph = 5.4kW

3 hours of cruise = 16.2kW of battery

Zero batteries are around 180Wh/kg. So we'd need 90kg of batteries just for cruise. Better increase the T/O weight and recalculate.

400kg T/O weight, 70m/s. We now have 5,120W worth of induced drag and 7750W of parasitic drag. Total power for 100mph cruise is now 12.9kW. Uh, oh. Now I need 38.7kW of battery for 3 hours of cruise. 215kg of zero batteries. Good job I allowed so much weight!

Wait, I forgot climb!

14,000m x 400 x 9.8 = 55MJ

x propulsive efficiency = 73.3MJ

In kWh, 73300/3600 = 20.3kWh

Oh dear, better add on another 113kg of zero batteries.

I must be getting close now?

80m/s, 550kg

Parasitic power: 11600W

Induced power: 12100W

Climb energy: 27.9kWh

Ah, poop. Now I need 99kWh of batteries. That's 550kg of zero batteries.

Weight a minute. That's 100% of the aircraft weight. Nothing left for luxuries like wings and fuselage or motor. I suspect that I may be in a losing battle here? I have no clue how to build a 160mph aircraft that light.

Topspeed, could you please show me where my maths is wrong? I seem to have reached a cognitive impass.

I don't aim that high with a 68 kg empty weight motorglider. 30 000 ft and 110 kg of batteries...900-1200 watts minimum cruise power....climb possibly 2-3 kw.

Above is an image of an efficient electric aeroplane.

Last edited:

There's your problem. Well, one of them. About 1.2 hp. Nope.900-1200 watts minimum cruise power.

I used the numbers you gave in response to a request for its specs. I guess you've now moved the goalposts to a lower altitude?I don't aim that high with a 68 kg empty weight motorglider. 30 000 ft and 110 kg of batteries...900-1200 watts minimum cruise power....climb possibly 2-3 kw.

View attachment 104430

Above is an image of an efficient electric aeroplane.

Show us the maths for any of your designs. I gave you a template to follow. I even resisted the temptation to derive equations for necessary span/weight/battery fraction, or check your flat plate area.

So the cruise power numbers you keep giving out are plucked out of thin air?

Why am I not surprised?

We have a wide selection of aircraft for sale. Search our database to find the best new and used aircraft for sale such as business jets, helicopters, UAVs, Drones, and more now.

www.trade-a-plane.com

So the cruise power numbers you keep giving out are plucked out of thin air?

Why am I not surprised?

Well the neighbourhood is right.

Your neighbourhood is cloud cuckoo land?Well the neighbourhood is right.