HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Nov 16, 2012, 10:45 AM
Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
451 Posts
Quote:
Originally Posted by Crossplot View Post
Fluid static pressure is changed as the fluid mass inertia resists (reacts) to that acceleration.
When Bernoulli's equation holds, we can say that the static pressure is proportional to the velocity squared. We could instead say speed squared because we only need the magnitude of the velocity vector.

Therefore, an acceleration that only changes the direct of the velocity vector (i.e. centripetal acceleration) does not change the static pressure.
DPATE is offline Find More Posts by DPATE
Reply With Quote
Sign up now
to remove ads between posts
Old Nov 16, 2012, 10:46 AM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
6,917 Posts
Quote:
Originally Posted by eflightray View Post
Can the threads title be reversed ?, 'You can't have downwash without lift'.

Or, 'You can't have lift without turbulence'.

Sorry, I know, I shouldn't really be here
Nah - you make good sense -
How anyone can actually confuse lift with the turbulance which may occur also seems odd to me .
But then it appears to be entertaining to some to try to ferret out the obvious . .
go figure
richard hanson is offline Find More Posts by richard hanson
Reply With Quote
Old Nov 16, 2012, 10:54 AM
Registered User
ShoeDLG's Avatar
Germany, BW, Stuttgart
Joined Mar 2012
805 Posts
Quote:
Originally Posted by ciurpita View Post
is the angular momentum of the air changing?
When you talk about an object's linear momentum, you're talking about something that's unambiguous (as long as you pick one reference frame and stick to it). When you talk about an object's angular momentum, there's some ambiguity introduced because you have to specify the point you're measuring the angular momentum about. A logical point in many instances is the object's center of mass.

In the case of a lifting wing moving through the air, you could measure the air's angular momentum in the wing's reference frame using a point on the wing's centerline. If the wing is left/right symmetric, the air's angular momentum around both the axis that points in the wing's direction of motion, and the vertical axis (assuming wings-level flight) won't change. The left-right symmetry says that it can't.

The air's angular momentum about the "wing line" axis can change (the flow around a 3D lifting wing cannot be front-back symmetric). In fact, you can show that if the linear downward momentum in the wing's wake is increasing, the air's angular momentum about the wing line axis has to be changing (which raises the question of where the torque is coming from to cause this change in angular momentum).
ShoeDLG is offline Find More Posts by ShoeDLG
Reply With Quote
Old Nov 16, 2012, 01:02 PM
greg
ciurpita's Avatar
somerset, nj
Joined Feb 2005
371 Posts
Quote:
Originally Posted by ShoeDLG View Post
The air's angular momentum about the "wing line" axis can change (the flow around a 3D lifting wing cannot be front-back symmetric). In fact, you can show that if the linear downward momentum in the wing's wake is increasing, the air's angular momentum about the wing line axis has to be changing (which raises the question of where the torque is coming from to cause this change in angular momentum).
As i said, i wasn't sure if angular momentum is the proper term, you seem to be using that term by its definition, which is what i was hoping to avoid.

at least when hurricane sandy past through where i lived, there was a great deal of wind. We all know is was flowing around the center of the storm 40 miles away, but from my perspective it was relatively straight.

when i push my hand through a basin of water in a linear direction, a circular flow is created from the water being pushed in front of my hand, around my hand to fill in behind my hand. It seems to me that at least an angular flow of water was the result of me moving my hand in a straight line.

in post #721, you included a colorful picture of the wing tip vortex. Was there a torque applied that created it, or was it simply the vertical forces above and below the wing tip due to a vertical lift force that resulted in a displacement of air in a radial path because of air blocking the flow a wing chord distance or more below the wing and the vacuum formed above it.



this is becoming frustrating ... do you need to consider the state of the air behind or to the side of the wing when trying to understand the mechanism of lift at the surface of the wing?

isn't the force of lift roughly perpendicular to the surface of the wing? doesn't it affect the air near the surface (< 1/4 chord) of the wing? don't the pressure regions causing lift also apply a force to the mass of air near the surface of the wing potentially accelerating it downward? is the air flow also affected by it's ability to move (is it's flow impeded because of the surrounding air)?

perhaps i'm not seeing the forest for the trees, or the trees for the forest.

greg
ciurpita is offline Find More Posts by ciurpita
Reply With Quote
Old Nov 16, 2012, 02:13 PM
Registered User
Joined Dec 2011
62 Posts
Quote:
Originally Posted by DPATE View Post
When Bernoulli's equation holds, we can say that the static pressure is proportional to the velocity squared. We could instead say speed squared because we only need the magnitude of the velocity vector.

Therefore, an acceleration that only changes the direct of the velocity vector (i.e. centripetal acceleration) does not change the static pressure.
Take the given examples, train on the curved track, the merry-go-round or water in a swinging pail there is pressure against the curving object. as long as air has mass it will experience outward pressure from centipetal acceleration.

Tangential velocity required for Bernoullis principle cannot be calculated from its relationship to the wing surface. That is not its inertial velocity. What is true is that the pressure calculation from centripetal accelerarion reduces down to being the same as Bernoullis equation. (hence all of the Bernoulli confusion.)

For some previous questions: The acceleration in centipetal acceleration is to all intents and perposes linier.

I cannot find again a free download copy of "Understanding wing lift" by J.Silva and A.A.Soares. If you can find it it is a must read. Also look for Dr klous Weltner and Gail M. Craig
Crossplot is offline Find More Posts by Crossplot
Reply With Quote
Old Nov 16, 2012, 02:15 PM
Registered User
ShoeDLG's Avatar
Germany, BW, Stuttgart
Joined Mar 2012
805 Posts
Quote:
Originally Posted by ciurpita View Post
this is becoming frustrating ... do you need to consider the state of the air behind or to the side of the wing when trying to understand the mechanism of lift at the surface of the wing?
You dont need to consider the state of the air anywhere but right at the surface of the wing to calculate the lift. I think you do need to consider the air elsewhere if you want to understand what gives rise to the state of the air at the surface. I can't pretend that I really understand the mechanism behind lift. Nmasters said it well in the "Understanding Flaps" thread...

"Fluid dynamics is the hardest problem in classical physics."

I've written a lot in this thread, but I haven't been trying to peddle an explanation for lift. I've been trying to show that a simple explanation based on pure vertical momentum exchange is flawed.

I first encountered this problem as a graduate student in the following context... For a jet engine to produce a given amount of thrust, it needs to impart momentum to the air at a given rate. In imparting momentum to the air, the jet inevitably also imparts energy. If you can reduce the rate at which a jet engine imparts energy to the air, you can improve its propulsive efficiency. A jet that imparts a small change in velocity to a large mass of air can achieve the same momentum exchange with less energy expenditure than a jet that imparts a large change in velocity to a small mass of air (one of the reasons you see big engine diameters where fuel economy is important).

In a conversation with my thesis advisor, I tried extend this concept to a lifting wing. I suggested that a higher aspect ratio wing imparts less velocity to a larger mass of air and therefore can impart the same downward momentum with less energy. His reply was basically: "Interesting observation, but at what rate does a wing impart downward momentum to the air?". I answered: "At a rate equal equal to the lift... Obviously". He challenged me to actually calculate the air's rate of vertical momentum change. After going through the exercise, I was very surprised to find that my intuition (based on incorrect application of Newton's 2nd Law) was completely incorrect. It turns out that the rate at which the air's vertical momentum changes depends on how the air is bounded (even when the boundaries are very far from the wing).

I'm glad I had that conversation because it has helped me grasp some subtle features of a lifting wing that I otherwise would have missed.
ShoeDLG is offline Find More Posts by ShoeDLG
Reply With Quote
Old Nov 16, 2012, 03:22 PM
t=1007606
HX3D014's Avatar
Joined Dec 2006
423 Posts
Quote:
Originally Posted by ShoeDLG View Post
I don't understand what you're saying. If you have a string of boxcars that form a closed loop, there is no way for the translational momentum of that string to change (as long as it stays on the track).
I was talking about the fact that the train track is stuck to the medium from which you are applying your force from (the ground).

It should be noted that in your example (due to the ground) there were two forces applied. One by you and one at the centre of the axis by the ground.
If it were a free body where the track was on a steel plate that could slide on the ground , frictionless) then when you apply a push, the trains will move in an angular and Translational way.

The air and aircraft are both free.

The Video is an example of a feature evident in helicopters, (there is a flat glass plate on the scales) induced flow. I read your last post
Quote:
In a conversation with my thesis advisor
I'm glad I had that conversation because it has helped me grasp some subtle features of a lifting wing that I otherwise would have missed.
Heli Hover Scales3.mpg (0 min 13 sec)
HX3D014 is offline Find More Posts by HX3D014
Reply With Quote
Old Nov 16, 2012, 03:38 PM
Registered User
ShoeDLG's Avatar
Germany, BW, Stuttgart
Joined Mar 2012
805 Posts
Quote:
Originally Posted by HX3D014 View Post
The air and aircraft are both free.
In the same post you say the air is free and then show a video where the glass plate exactly balances the force that the helicopter exerts on the air.

The glass plate is preventing linear momentum addition to the air in the same way that the tracks prevent momentum addition to a closed string of boxcars.

If the glass plate is pushing up on the air with the same force that the helicopter pushes down, the unbalanced force on the air is?
ShoeDLG is offline Find More Posts by ShoeDLG
Reply With Quote
Old Nov 16, 2012, 03:43 PM
t=1007606
HX3D014's Avatar
Joined Dec 2006
423 Posts
The helicopter, weighs 30g
HX3D014 is offline Find More Posts by HX3D014
Reply With Quote
Old Nov 16, 2012, 03:50 PM
t=1007606
HX3D014's Avatar
Joined Dec 2006
423 Posts
Quote:
Quote:
Originally Posted by ShoeDLG View Post
I don't understand what you're saying. If you have a string of boxcars that form a closed loop, there is no way for the translational momentum of that string to change (as long as it stays on the track).
I was talking about the fact that the train track is stuck to the medium from which you are applying your force from (the ground).

It should be noted that in your example (due to the ground) there were two forces applied. One by you and one at the centre of the axis by the ground.
If it were a free body where the track was on a steel plate that could slide on the ground , frictionless) then when you apply a push, the trains will move in an angular and Translational way.

The air and aircraft are both free.
Getting back to the train tracks. You created an example where the Track could not move in a translational way due to the way you set it up, and suggest that it is Possible to add Angular momentum to a body with out there being any Translational momentum. But it was not evident to most that this example is not similar to the Aircraft and the Air it is flying in due to the fact the train tracks are stuck to the ground.

You can not add angular momentum to a free body by way of one single application of force without there being a translational component. However you can (Theoretically) do the reverse.
HX3D014 is offline Find More Posts by HX3D014
Reply With Quote
Old Nov 16, 2012, 04:00 PM
Registered User
ShoeDLG's Avatar
Germany, BW, Stuttgart
Joined Mar 2012
805 Posts
Just as the tracks can prevent change to the train's linear momentum, the ground can prevent any change to the air's linear momentum. The helicopter video shows this well.
ShoeDLG is offline Find More Posts by ShoeDLG
Reply With Quote
Old Nov 16, 2012, 04:06 PM
t=1007606
HX3D014's Avatar
Joined Dec 2006
423 Posts
Please review.
back in a few hours (its 9:06am here in Au)
HX3D014 is offline Find More Posts by HX3D014
Reply With Quote
Old Nov 16, 2012, 04:54 PM
Registered User
Joined Dec 2011
62 Posts
Quote:
Originally Posted by ShoeDLG View Post
"I've written a lot in this thread, but I haven't been trying to peddle an explanation for lift. I've been trying to show that a simple explanation based on pure vertical momentum exchange is flawed..
Now that could leave a reader a little bit afloat!

My quest is to establish for certain as to whether the downwash aft of the wing is an actual reaction to lift or is it a byproduct of the system created to generate lift.
Crossplot is offline Find More Posts by Crossplot
Reply With Quote
Old Nov 17, 2012, 12:46 AM
Registered User
Joined Dec 2011
62 Posts
[QUOTE=ShoeDLG; I've been trying to show that a simple explanation based on pure vertical momentum exchange is flawed.QUOTE]

Shoe, you have obviously put a lot of effort into this. Would it be possible for you to summarize what you feel is the impact into our understanding of the creation of lift?
Crossplot is offline Find More Posts by Crossplot
Reply With Quote
Old Nov 17, 2012, 01:11 AM
Registered User
ShoeDLG's Avatar
Germany, BW, Stuttgart
Joined Mar 2012
805 Posts
Quote:
Originally Posted by Crossplot View Post
Would it be possible for you to summarize what you feel is the impact into our understanding of the creation of lift?
Sure.

http://www.rcgroups.com/forums/showp...&postcount=420
ShoeDLG is offline Find More Posts by ShoeDLG
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Dymond D47 servo, Which version do you have on hand, how can you tell??? TDboy Hand Launch 5 Apr 21, 2012 02:26 AM
Discussion Is there anything listed that you can't live without? P-51C Life, The Universe, and Politics 44 Aug 05, 2011 02:55 PM
Discussion Fast food that you just can't do without. Bilbobaker Life, The Universe, and Politics 101 Aug 17, 2010 11:30 AM
Joke What Would You do to get that Aircraft You Can't Have ? Chophop Humor 10 Feb 09, 2010 07:00 AM
Discussion HELPIf you have a problem that can't be solved here ccowboyearl Beginner Training Area (Heli-Electric) 2 May 14, 2009 12:31 PM