Expert: Mark Janus - 2/4/2010

Question
QUESTION: I am trying to found out if one were to create a low pressure(L.P.) above a surface (as in the way a wing does when moving) but by a different method, would this still create lift. I have an idea to 'move' a L.P in side a rotating cylinder that suddenly 'exposes' the L.P. to the atmosphere via a slit (quickly simulating a wing??) The cylinder continues to rotate and exposes another L.P. area, etc,etc. A few people
I have chatted to say it is a CFD problem. Can you help?
Neal  

ANSWER: Hi Neal,
No matter what method is used, the pressures on the surface (summed up over the entire surface(s)) will result in a force...  the component of that force perpendicular to the direction of travel is the lift....   

You can also analyze your device via the bigger picture...   a lifting body exerts an equal and opposite force to the air surrounding it and thus is pushing that air down....

You may find that whatever you are using to lower the pressure (inside) is costing you more energy than what we have right now (wings)...   you have to consider those costs (and the human cost if the device were to fail and "lift" ceases)...to determine the efficacy of the design....

Thanx,
mj

---------- FOLLOW-UP ----------

QUESTION: MJ! Thank you very much for responding to my inquiry! Also, sorry I took so long to reply and think I sent a this reply to a wrong address.
Indeed, as you said ‘no matter what method is used a result will occur’, but
what I want to know is, will the idea work?
Please refer to the p.d.f in the attachment.
A’s=air pressure on object.
B=area where less pressure is available.
C=force upward?
D=The cylinder is a tube with a slot cut in to it with one ends sealed, the
other where air is exhumed from. Good seals are required.
E=The vane fits closely into the cylinder and can rotate.

  Around an object at ground level a force of 14.7lb/in2 (A’s) is pushed upon it.
The idea is to expose, an area of less air pressure to the upper side of the
cylinder (d). Done by evacuating the chamber that the star like vane (E)
occupies, then to expose that low pressure through the slit at the top and as
the vain rotates. As it continues rotation, another area is exposed (B) to
create an imbalance of the overall pressure around the object hopefully inducing
a force in the up ward direction (C).

  You mention energy and efficacy; this dosen’t concern me at the moment, it’s
just seeing if the idea works! Along with if it were to fail and any human cost,
these too are not concerns at this time and are not the resultant requirements
of the end product.
  Also as many people have also mentioned, that if it works, it will have a
ceiling operation height, due to air pressure being less at altitude, this is
also of no importance at the moment. The only interest is to create a stable and
uncomplicated lifting mechanism.
   
Thanx
Neal.

Answer
I would say, based on what you've shown the idea would produce a resultant force in the vertical direction. How much force would depend on a lot of factors...  size of the hole, level of low pressure in the star region, speed that it rotates...etc....  That said, I would not consider it an "uncomplicated lifting mechanism"...  a wing is a passive "device" that is a uncomplicated lifting mechanism...that only becomes complicated at the limits of its operation.  that is why I pointed to the other issues of human cost and energy cost...

Hope this helps..

mj