Nov 14, 2013, 05:51 PM
Grumpy old git.. Who me?
Aberdeen
Joined Mar 2006
13,822 Posts
Quote:
 Originally Posted by Rockin Robbins NASA seems to use a "not recognized aerodynamic term" in their instructional materials. Holy crap! They use the same CL abbreviation that I used and have seen on plenty of model airplane and some full sized plans!
The abbreviation CL (or sometimes Cl) is as I'm sure you know Coefficient of Lift, not Centre of Lift.

As for the site you linked. The site is specifically aimed at junior to high school children and so uses very simplified language. It is hardly an authoritative reference for proper 'grown up' aerodynamic terminology or theory, and in some cases is plain wrong..

As an example of the quality of the information on the site here is it's explanation of how wings make lift :

Quote:
 Lift Lift is produced by a lower pressure created on the upper surface of an airplane's wing compared to the pressure on the wing's lower surface, causing the wing to be "lifted" upward. The special shape of the airplane wing (airfoil) is designed so that air flowing over it will have to travel a greater distance faster, resulting in a lower pressure area (see illustration) thus lifting the wing upward. Lift is that force which opposes the force of gravity (or weight).
http://www.allstar.fiu.edu/aero/fltmidfly.htm

So forgive me if I don't take that site too seriously, even if it is part funded (not written) by NASA.

If you want to use the term that's fine, lets just agree that it means the same thing as Neutral Point then at least we know what each is talking about?
Last edited by JetPlaneFlyer; Nov 14, 2013 at 06:13 PM.
Nov 15, 2013, 03:27 PM
United States, FL, DeLand
Joined Mar 2009
5,989 Posts
Quote:
 Originally Posted by JetPlaneFlyer The abbreviation CL (or sometimes Cl) is as I'm sure you know Coefficient of Lift, not Centre of Lift. As for the site you linked. The site is specifically aimed at junior to high school children and so uses very simplified language. It is hardly an authoritative reference for proper 'grown up' aerodynamic terminology or theory, and in some cases is plain wrong.. As an example of the quality of the information on the site here is it's explanation of how wings make lift : http://www.allstar.fiu.edu/aero/fltmidfly.htm So forgive me if I don't take that site too seriously, even if it is part funded (not written) by NASA. If you want to use the term that's fine, lets just agree that it means the same thing as Neutral Point then at least we know what each is talking about?
There's a thousand more. I just picked that one because it had the NASA name on it. Obviously their use belies the contention that there is no such thing. In any event the diagram supplied by XFLR5 was ridiculous. The plane as shown wouldn't even be able to fly. At least the NASA/FIU diagram, even if labeled wrong, which I don't think it is, shows a flyable plane!

This situation smells like a transition between long used and useful terms to a new nomenclature that is less user friendly so is meeting with some resistance.
 Nov 15, 2013, 03:50 PM buyer of the farm United States, FL, DeLand Joined Mar 2009 5,989 Posts Cool stuff here! http://www.americanflyers.net/aviationlibrary/pilots_handbook/chapter_2.htm. It agrees with Jetplaneflyer. It appears we're in the middle of a transition on how aerodynamicists think about flight. It also looks like the new paradigm is useful. It appears there was a collision between fluid dynamics and aerodynamics and the fluid guys won. Happens all the time in science. Two groups have separate languages, discover they're both working on the same phenomenon, in order to talk to each other they merge their nomenclature. Everybody else gets upset.
 Nov 15, 2013, 08:11 PM Red Merle ALES United States, Mt, Helena Joined Apr 2002 6,448 Posts Lift Coefficient Cl (small l) is for the surface you are referencing CL (Capital L) is for the whole plane Curtis
Nov 16, 2013, 02:10 AM
Grumpy old git.. Who me?
Aberdeen
Joined Mar 2006
13,822 Posts
Quote:
 Originally Posted by Curtis Suter Lift Coefficient Cl (small l) is for the surface you are referencing CL (Capital L) is for the whole plane Curtis

Thanks Curtis for the clarification, but it is of course still 'Coefficient of Lift'... not 'Centre of Lift'.
Nov 16, 2013, 02:14 AM
Grumpy old git.. Who me?
Aberdeen
Joined Mar 2006
13,822 Posts
Quote:
 Originally Posted by Rockin Robbins It agrees with Jetplaneflyer. It appears we're in the middle of a transition on how aerodynamicists think about flight.
To be fair, aerodynamisuists would never have used 'Centre of Lift' as recognised terminology. It seems to be something that in the main modellers only use, but it's exact definition is unclear, hence the confusion and why I dont think it's a good idea to use the term.
 Nov 16, 2013, 10:08 AM Registered User United States, CA, Lancaster Joined Jan 2009 1,521 Posts The wing produces lift and has vectors protruding from this depending on Cl and Cm. So since pressure changes according to AoA this is the center of pressure that resides in the lift envelope. So the accumulative effect is the area that produces the most lift and is referred as center of lift. I still refer to Cl and Cm charts for reference. But AC should be the refrence point and not the center of lift or pressure. AC is refernced at 25% MAC and is the greates area for lift at the planes normal operation. Pressure does changes across this area as AoA changes. So other than NP, AC should be refenced as well. But on flying wings these two pionts are close and sometimes on top of each other. So CG placvement is in front of this for a longitudal stability will make AoA negative. So to pitch up a reflex airfoil or more twist is needed in the wing. I always look at the plots to see CG poisition and what the Cl and Cm plots are telling me. I can also see were the center of pressure is in relation to the wing. Last edited by p901P901; Nov 17, 2013 at 04:21 PM.
Nov 16, 2013, 10:39 AM
United States, FL, DeLand
Joined Mar 2009
5,989 Posts
Quote:
 Originally Posted by JetPlaneFlyer To be fair, aerodynamisuists would never have used 'Centre of Lift' as recognized terminology. It seems to be something that in the main modellers only use, but it's exact definition is unclear, hence the confusion and why I don't think it's a good idea to use the term.
The center of lift is clearly understood as the normalized lift vector of either a plane as a whole (the more useful usage), or the normalized lift vector of the wing. It is the sum of all lift vectors as if all the lift were coming from a single spanwise line on the wing. On a side view it's shown as a single arrow, just as shown on the CP diagrams that are presently in style.

No its definition is not unclear. It is well understood and useful for a hundred years. Pull out a Frank Zaic yearbook from the 1930s and you'll find extensive and still very useful applications of what you think is an unclear concept.

When aerodynamicists were trying to find empirical means of designing airfoils they ran up against the barrier of trying to explain in more detail what happens to every point of an airfoil and how to anticipate air pressure at every point. That's when the fluid dynamics guys said "Hell, we've been doing that stuff for years! You'll have to change your nomenclature and then we can talk about it."

Now there's a mathematical relationship between shape and air pressure about an airfoil. They've even quantified the balance between the Newtonian wedge model of lift and the Bernouli model. showing the the increased pressure on the bottom of a flat bottomed airfoil is lower than the lift generated by the low pressure generated on top of the airfoil.

Even so, their mathematical models don't conform to reality. Checking out the documentation to Ilan Kroo's LinAir4 software, he clearly shows plots by his program are different from experimental results. The computed results are similar enough to be useful but not useful enough not to require testing and modification of the airfoil and planform designed.

There has been progress but designing an airplane is still an art. It's telling that even the best-flying electric 2-meter RTF sailplane, the Radian, still uses the Gottingen 398 airfoil, designed in 1919! Why? Because it workes darned well, that's why!

# Images

Nov 16, 2013, 10:44 AM
United States, FL, DeLand
Joined Mar 2009
5,989 Posts
Quote:
 Originally Posted by JetPlaneFlyer Thanks Curtis for the clarification, but it is of course still 'Coefficient of Lift'... not 'Centre of Lift'.
There is both a coefficient of lift and a center of lift. They are not and have never been synonymous terms. One is a quantity (a relationship really), one is a location on a wing profile. The center of lift is the normalized sum of all lift in the airfoil as if it were a single vector, a force with location and direction. It changes constantly in location with airspeed and angle of attack. The lift vector changes in direction and magnitude as well. The point from which the normalized lift vector works has been called the center of lift for a hundred years.

Here's a great explanation of how wing airfoils are designed with some examples and why the modern aeordynamicists use the terms they do. It's a pdf and well worth printing out to read. It's both technical and a good read.

By the way, from the McGraw-Hill Science & Technology Dictionary yet another clearly defined definition of a supposed imaginary term: center of lift. The fact is that the center of lift is a clearly understood, clearly defined quality which has been in use for a hundred years. Modern aerodynamics, used in theory, not by model airplane builders, has replaced that term using surface pressure analysis. It has done that for reasons that have no relevance to us as builders of model aircraft, although we do benefit from it with better modern airfoils as those designed by Eppler, Selig and Drela. It's important to realize that when Parkzone was designing a standout electric sailplane it chose a seat of the pants airfoil designed by Gottinger in 1919, before any of this newfangled language and methodology was dreamed of.
Last edited by Rockin Robbins; Nov 16, 2013 at 12:38 PM.