Is it possible to generate lift without thrust?

chizwoz

I'm having a bit of a disagreement with someone and I hope some of you can enlighten us.

It sounds rather silly but the debate is around the physics used in a video game (Batman Arkham City). In this game, the character can jump off buildings and shape his body into a downwards dive (much like a skydiver trying to gather speed). He can then open his cape and this not only slows his descent but he can also supposedly use the resistance of his cape to then fly back upwards.

My understanding of physics tells me this is in principle impossible, unless there is something like a huge upward gust of wind which provides more upward force than that provided by his (downward) weight and since the character can do this anywhere at will, it's safe to assume no such gust is present.

I hope a few of you can weigh in (giggle) on the issue.

russ_watters

Welcome to PF!

In this case, thrust is provided by gravity during the descent, but then it is lost. The glider has to build up speed, which it later exchanges for altitude via conservation of energy. So he can only glide upward until he's lost so much kinetic energy/speed that his wings can no longer provide lift.

AJ Bentley

Well, in theory he turns himself into a glider.

Given that, he can use forward speed to generate lift, up to the limit of the kinetic energy available. That is, he will be able to regain height up to a significant fraction of the drop he just took - but no higher.

However, in a cityscape he could quite possibly use the wind turbulence around buildings to pick up lift.
If you've ever watched a seagull in flight, they are absolute masters of this trick and make it look like they can fly wherever they please without so much as a flutter of their wings.
So maybe Batman is just concealing his secret.

sophiecentaur

I have a problem with threads about something that just came out of a fiction writer's head. I frequently dream about flying and this is always achieved (in my dreams) by holding my body in a particular way and sort of launching off. . . . .
Would there really be any percentage in discussing the possible Physics of this?

I realise I am being a grumpy old man about this but my issue is that these days, Science, Magic and Science Fiction are just one blur for most young people (and a number of old ones too). I am sure that Matrix -type and other computer generated activities are taken as real and the Star Trek 'science' is considered very very seriously by those who should know better.

The rules of PF actually forbid speculative and unfounded topics yet we keep getting posts about how computer game simulations can be made 'realistic'. It worries me.

I guess I should point out that this particular thread is far less questionable than many you can find here.

russ_watters

I think the thread is ok here because it isn't speculating about non-mainstream physics, it is merely asking if the physics demonstrated in a game is accurate. It isn't a science fiction question or a crackpottery question, it is a question about if something presented in fiction is actually real.

My first thought on seeing this question was this:

rcgldr

Assuming that updrafts could be found within the city, the main issue would be the lift to drag ratio. Perhaps if the cape was replaced by a hang gldier setup. Wiki article:

wiki_glide_ratios.htm

chizwoz

Well with all do respect, what I was trying to do with this question was actually clarify the difference between fact and fiction, not blur the line.

And thanks for the answers people.

russ_watters

I don't think this has anything to do with updrafts since the OP is talking about a no-thrust situation and an updraft enables gravity to provide thrust.

AJ Bentley

True - but then the answer is simply a boring 'no'.

rcgldr

For a glider in a steady descent with respect to the air, gravity always provides some thrust = force in the direction of flight (assuming lift to drag and corresponding glide ratio is not ∞). An updraft only changes the situation with respect to the ground, so I don't get your point here.

Getting back to the op

Yes in the case where the caped person could temporarily gain altitude in exchange for reduction in speed. If the question was "is it possible to generate lift without drag", then the answer would be no, but this is not the same thing.

russ_watters

Um...no, it isn't. The answer is yes.

russ_watters

The OP is not talking about a glider in a steady descent, the OP is talking about a glider swooping upward. Right: so the steady state is not what the OP was asking about. The OP was asking if what he saw in the video game was accurate. From his description, it sounds like the answer is yes.

rcgldr

My fault, my last edit to my previous post didn't take (sometimes physics forums locks up on me). I corrected it to read: Yes in the case where the caped person could temporarily gain altitude in exchange for reduction in speed. If the question was "is it possible to generate lift without drag", then the answer would be no, but this is not the same thing.

A.T.

Sure. Lift has nothing to do with "thrust". It's just a function of your airspeed and shape. If you have enough speed (kinetic energy) you can always convert it into height (potential energy). See this video at 2:30:



Here he gets the kinetic energy from the plane and has no initial downward speed to reverse, so it is "simple". But theoretically the KE could also come from a long fall with arms and legs retracted. I'm not sure if such a wingsuit (or a cape) can be made efficient enough to then cancel downward speed, and go upwards. But it wouldn't violate any law of physics. A more rigid wingsuit would definitely allow this.

Assuming no wind: If he started the fall with zero speed, he could never get higher than he started.

Interestingly you don't need updrafts to gain energy. An efficient glider can use a downdraft to gain extra kinetic energy during the fall, and then convert it back into potential energy outside of the downdraft in still air, ending up higher(but same speed) than he started:
http://www.icarusengineering.com/Dyn...ing-and-SE.htm