Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Supervolant on 08/10/2016 01:18:21

Title: Would a vehicle as light as air have problems flying?
Post by: Supervolant on 08/10/2016 01:18:21
Hello there,

let me give you a few facts before starting right into this little thought experiment I am really not sure of what to think.

- We have a flying vehicle
- The vehicle is highly aerodynamic
- The vehicle has a static structure
- The vehicle is as light as air & maybe even lighter

So, here is the scenario we have:

The highly aerodynamic flying vehicle is passing trough the air aaaaand... at this point my physical imagination stops working in terms of what would actually happen to the vehicle?

My difficulty is in fact to imagine how a vehicle would move trough the air when it is in fact as light as air or even lighter then air.

Would the vehicle float around like a balloon or is it possible to accelerate up to fast speeds?

In the aviation the goal was always to make planes the lightest possible... but is there a too light?


I recently signed up into this forum and look really forward to the knowledge that could be gained of this of mastermind group.
Thanks really much for messing with my request, I am looking forward to your answers.

Title: Re: Would a vehicle as light as air have problems flying?
Post by: Colin2B on 08/10/2016 09:03:13
Do you mean an airship, but more aerodynamic to reduce air resistance?
I suspect that airship speed is not just limited by drag but by the limit on weight/power of the engines the structure can support.
A light structure would also be more affected by side winds than a heavier aircraft.
Title: Re: Would a vehicle as light as air have problems flying?
Post by: alancalverd on 08/10/2016 09:16:34
The lighter the ship, the better it flies in the ambient air.

Problem is that the ambient air isn't always going where you want to go. Steering and landing an airship or even a glider is a hell of a lot more difficult than a 747. Wing loading (mass divided by wing area) determines stability and penetration of turbulence. Even in the light plane zone, you can feel the difference between a Cessna 172 (13.8 lb/sq ft) and a Piper Archer (15 lb/sq ft), both of which are used as trainers and family runabouts:  the Cessna floats and dithers over the runway but the Piper goes exactly where you put it. Landing an airship is like berthing a tanker in three dimensions.     
Title: Re: Would a vehicle as light as air have problems flying?
Post by: Supervolant on 09/10/2016 15:13:43
I thank you both very much for your answers. They helped me to understand the topic way more and see it out of a new perspective.

alancalverd & Colin2B

Yes in fact, I mean an airship with a more static / strong shell + more aerodynamic in order to reduce the resistance. Of course the Airship is limited in the end to it's engine. But how much more would they be able to put out in terms of thrust when the vehicle they have to move trough the air is as light as air itself. Of course this is highly hypothetical but still alone the though bugs me a lot.

Side winds are a very good point to hang on guys. But what if the monocoque of the vehicle itself is shaped so that from all directions the wind would just slide on top and bottom away? Maybe imagine a round vehicle formed like a frisbee but using a standard wing profile. Would sidewinds maybe even help that way to lift the vehicle?

I would love to go deeper into this thought experiment with you guys. Thanks.
Title: Re: Would a vehicle as light as air have problems flying?
Post by: alancalverd on 09/10/2016 16:42:06
I'll encourage you to keep thinking!

Fact is that sidewinds are less of a problem if you have a high power/weight ratio, because you can always direct an airship into the local wind when approaching your mooring, unlike a plane where you have to crab or slip through the wind to align the wheels with the runway. A bow thruster would also help, as would steering jets to assist the rudder. So the trick would seem to be to use a high bypass turbofan with vectored thrust like a Harrier.

Worth looking at modern rigid materials. I did some back-of-envelope calculations which suggest that a 100m x 10 m diameter aluminium beer can (0.1 mm thick) would fly very well if filled with hydrogen. And now comes the geometry! If you increase the radius r of the tube, the mass increases by r but the volume increases with r^2, so a fat tube will carry more payload than a thin one of the same volume. It turns out that the traditional dirigible airship is pretty close to the ideal shape for travel at up to 100 mph.If you flatten the envelope a bit, it begins to look like a wing with significant dynamic lift.

Worth considering a sliding payload. Suppose the cabin can move backwards. Bring the ship to a mooring using a combination of dead lift, vectored thrust, gas displacement, and aerodynamic lift, because the "dead" ship is slightly heavier than air, and lower the whole contraption to the ground. Disembark and reload at ground level, then slide the cabin to the back of the ship. The nose rises because the front of the ship is nsw lighter than the back, and when it reaches the takeoff angle of attack, apply full engine power, gradually rotating the thrust vector as we gain height and speed, to get airborne again.