[Professor Mega-Mind (OP)]

What , you don't like my giant butt ?
Let's make it a giant head instead !
P.

[Kryptid]

So much for a productive discussion...

[Professor Mega-Mind (OP)]

Skuza , it's my Animal House days come back to haunt me !

[Bored chemist]

Stop dreaming and provide some sort of estimate of the cross sectional area your idea would have to push through the air.

So much for a productive discussion...

We can't have a discussion if you don't give us something to discuss.

[alancalverd]

So far we have discussed a machine for carrying passengers backwards in an airway, or exposing its crew to enemy fire whilst attempting to remove scrap metal from a battlefield. There being little market interest for either, I don't see much point in  discussing the details until someone finds a bit of the atmosphere around 12,000 ft altitude, that doesn't move.

This afternoon I'll be passing over the Flying Bum en route from Cambridge to Oxford. It's a great landmark, but that isn't the primary function of an aircraft.

[Professor Mega-Mind (OP)]

...................Tank Yank
There is always ebb and flow on a battlefield .  Ebb is the time to go get your $7 million tank , and run it back to the depot for a quick and safe turnaround .  Meanwhile , you can be shuffling thousands of troops and their equipment around the battlefield fast enough to get pinch-offs galore on the enemy !  Drop a couple of  MOABs on that box and it's "enemy no more !" .
 Remember now , when I talk about the military stuff , I'm talking about the all-motor version .  The Hyper-Guppy version could work in certain civilian applications . 
Alrighty then !
P.M.

[alancalverd]

Ledt's have a recap on the size, cruise speed and landing speed of your machine.

[Professor Mega-Mind (OP)]

Okay , I'm big on the Flying Bum today .  How's about we cut it in half horizontally , so that it's much flatter . We give it powerful engines with VTOL propulsion . 
Top Speed : 200 mph.
MTO. & MLD. : 0 feet .
Alright , really high-speed ferry !
........P.M.

[Bored chemist]

OK, so we are talking about this
https://en.wikipedia.org/wiki/Hybrid_Air_Vehicles_HAV_304/Airlander_10

with these specs
Capacity: 10,000 kg (22,050 lb)
Length: 92 m (301 ft 10 in)
Wingspan: 43.5 m (142 ft 9 in)
Height: 26 m (85 ft 4 in)
Volume: 38,000 m3 (1,300,000 cu ft)
Gross weight: 20,000 kg (44,092 lb)
Max takeoff weight: 33,285[52] kg (73,381 lb)
Powerplant: 4 × 4 litre V8 turbocharged diesel engines, 242 kW (325 hp) each


So, it can carry about 10 tonnes in load, and you are suggesting we halve the thickness- so the capacity falls to) a bit less than) 5 tonnes.

According to this page
https://www.army.mod.uk/equipment/combat-vehicles/
"Challenger 2 (CR2) is the British Army's main battle tank. ".

And it weighs in at 62.5 tonnes.
So you need 13 "half arses" to lift one.

Unlucky for some...

[Professor Mega-Mind (OP)]

Massive Contra-fan units , VTOL  and putting out 2to300k lbs. of thrust each double-disk propulsor .
Add that million pounds to your 5k!
Nyuh-nyuh-nyuh !
P.

[Bored chemist]

Massive Contra-fan units , VTOL  and putting out 2to300k lbs. of thrust each double-disk propulsor .
Add that million pounds to your 5k!
Nyuh-nyuh-nyuh !
P.

At which point you have a badly made helicopter

[Professor Mega-Mind (OP)]

That can lift half a forest , or a tank division , or a drone wing , or a blue whale , etc. , etc....
"Keptin , there be whales here !" .
P.M.

[alancalverd]

There is a faint air of feasibility here. The largest available contraprop turbofans can generate around 12 tonnes of static thrust and weigh about 3 tonnes, so if the airframe and fuel weigh nothing, seven Kuznetsov NK12s could just lift a tank vertically.

That is why we don't recover battle-damaged tanks by airship.

[Bored chemist]

300k lbs. of thrust

That's 5 times the thrust of the jet engine on a jumbo jet.
https://sciencebasedlife.wordpress.com/2011/07/25/extreme-engineering-the-boeing-747/

Please stop inviting "designs" that need magical ingredients which don't exist.

[Professor Mega-Mind (OP)]

Gentlemanses ,
I just gave you time to research monster turbofans .  Let’s take my fave. , the GE-90 .  Manufactured over a quarter-century ago , it's maximum sustained ( 30 hrs.) static thrust at sea-level is documented as 130k.lbs .  We could do much better nowadays .  150k.lbs is conservative .  A free-turbine double-disk version would thus produce 300klb of static turbo fan thrust per propulsor .  Yes , that is almost five 747 engines , ain't progress great ?!
Go ahead and fetch yer mega-tank, Wardog !
P.M.

[alancalverd]

True, a GE 90 could lift 6 times its own weight compared with the contraprop 4 times, giving a payload of 40 tons per engine if we ignore the weight of the rest of the aircraft. You can eliminate the need for a tank altogether by just flying your machine over the enemy and literally blowing them away.

[Professor Mega-Mind (OP)]

Yaaaas , M.O.A.B.s !!

[Bored chemist]

Let’s take my fave. , the GE-90 .  Manufactured over a quarter-century ago , it's maximum sustained ( 30 hrs.) static thrust at sea-level is documented as 130k.lbs . 

And its rated thrust was a lot less
" with thrust ratings from 81,000 to 115,000 lbf"
 from
https://en.wikipedia.org/wiki/General_Electric_GE90

So, what you are saying is we could magic 100,000 and pretend that it is 300,000 them magic it some more to make it able to sustain that peak performance indefinitely.
Meanwhile, you magically lift the 8 tons or so of engine into the air with a blimp that has a capacity of 5 tons on a good day.

At which point, as I said, you made a shockingly bad helicopter.
If you can vector 1500 tons of thrust you can lift what ever you damned well please.

What's the point of the bag of helium at this point?
It's just an anchor, mired in the air.

[Bored chemist]

i may have found a use for the gas bag- it can hold up the fuel.
If this is right
https://www.airliners.net/forum/viewtopic.php?t=771027
The engine burns about 16 tons per hour.
So 5 tons would run it for about 18 minutes.
As far as I recall, you were thinking of using 4 of them.
So that means the balloon bit could carry enough fuel for nearly 5 minutes.
With a top speed of 200 MPH that's long enough to cover about 15 miles
In most urban areas there are fuel stations that close together so we should be OK, as long as you can use diesel- that's probably OK in a jet engine.
So, your 15 mile hop takes 5 tons of fuel- that's roughly 5000 litres at £1 per litre or so.
Only £300 per mile.
To be fair, that's only 10 times the cost of my local taxi company.

[Professor Mega-Mind (OP)]

...........Oh , ye of little faith !
Let us examine the GE-90's normal performance .  A 777 cruising for 15 hours consumes under 200klbs of jet fuel , utilising 2 engines at 1/2 throttle .  A "flying diaper" with 4 pairs of these might consume 4 times as much while heavily loaded and going flat out , but it only transports a short distance .  Mostly , it would be loading , unloading , or standing by .  The all-engine version would consume more while hovering , but would have a jet-like top-speed .  Either version would do mostly short trips  , with cargo .  The gas-bag version would work like a powered para-glider ; it would save gas .  The all-engine version would function as a giant VTOL jet .  1.2mlbs. of thrust gives you a lot to work with !
 See how much better simple and clear math makes everything ?
Alright , save nation , peace .....P.M.