Jump to content

Bernoulli Vs. Newton


Recommended Posts

I was talking with one of my friends about helicopter aerodynamics. He is an airplane CFI with an ME Degree. He asked what percentage of lift comes from bernoulli's principle and how much comes from newton's third law.

Unsure, I told him I would look into it and get back to him. I have had a hard time finding much in the way of any kind of answer in my text books. I also have had little luck online. I realize its a dynamic situation based on the speed of the relative wind and the angle of attack. So Im assuming were talking about a range. He told me that in his CFI checkride the examiner told him that in cruise flight its about 80% from newtons third law and 20% bernoulli. that sounded really off to me. if for no other reason than my RFH says newtons third law provides aditional lift. Anyone have an answer?

Link to comment
Share on other sites

Take a perfectly round object, such as a ball. How much lift will Bernouilli's aerodynamics generate? None. Airflow divides evenly each side, with a net zero lift.

 

Throw the ball, without spin. It doesn't fly, it just follows a ballistic path.

 

Now throw it (or kick it) with a backward spin. It floats level or climbs for a bit, then falls. This shows that it is generating lift. The Coanda effect causes air to travel around the top of the ball and depart with a downward vector. Newton says that if the air gets pushed down, the ball goes up.

 

For an extreme demo of this, use a balloon, even though it isn't perfectly round.

 

Giving a downwards velocity to the air flowing around the wing is the prime generator of lift, Newton style. A flat plate does the same, though with a lot of drag. The aerofoil shape reduces the drag so you don't need an afterburner to make the flat plate fly. :blink:

Link to comment
Share on other sites

Mathematically, a calculation of Bernoulli's Pricipal will produce the same result as Newton's III concerning lift on an airfoil. Some schools say Bernoulli's is 2/3 and Newtons is 1/3. The FAA uses Bernoull's exclusively because it is more widely accepted by engineers and is more scientifically verifiable. Bernoulli's Principal of Massflow is a simpler formula to apply than calculating Newton's deflection of air. This was told to me by an aerodynamicist so dont be too hard if you disagree.

I've always had problems with Newton's III because it doesnt deal with pressure differentials, which obviously exist if we try to explain induced flow. If a parcel of air 10 feet above the disk is going to pass through it eventually, how is that an equal and opposite reaction to the air at the disk? Nor does it demand a curved shape on the top of the airfoil, if Newton's III was it, we'd all be flying around with cheap, flat pieces of metal over our heads.

 

Also, Coanda Effect is generating lift by blowing air across a surface and has nothing to do with airfoils. And what you're describing with a spinning baseball is Magnus Effect, not Coanda or Newton.

 

I realize this is a touchy subject, much like discussing politics or religion: believe what works for you.

Link to comment
Share on other sites

Ah! C of G,

 

I love your ability to throw a cat amongst the pigeons! Hmmm, our old friend, 'The Magnus Effect' a.k.a Robins Effect. That little known corner of physics that touches so many of our lives.

 

Do we have an 'anti-Bernoullite' on our hands?!

 

 

Let me explain it simply for the benefit of anyone who hasn't seen it before.

 

If you take a cylinder and flow air over like this:

 

image032.gif

 

You get this which represents the net tangential velocity .

 

image033.gif

 

Using integration you can can get this:

 

image034.gif

 

Thus the net lift on the cylinder can be represented thus:

 

image035.gif

 

Hey presto, the Magnus Effect.

 

Simple eh?

 

 

Here's some very good bedtime reading for anyone, one of my childhood favourites actually: Bedtime Reading!

 

Joker

Link to comment
Share on other sites

Here's some very good bedtime reading for anyone, one of my childhood favourites actually: Bedtime Reading!

 

Joker

 

 

HOLY COW ! :o Ya make me feel like a "deprived child"---alls i had on my shelf was : the 3 bears, clifford the B I G red dawg,,,,,,,,,, :(

 

 

The Warp Engine :D :D :D

 

 

i had one of those in a 68 MG midget ! :P son of a ! if i didnt haftah yank that warped head off like TWICE one weekent too ! :lol:

 

did a warped clutch on it a few times also :angry:

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...