Define some terms for me
 

I was wondering what the exact technical definition is for the terms "Moment," "Moment coefficient," and "washout." is there a website or another forum that I can go to that has the meaning of other airplane related words like these?

RE: Define some terms for me
 

There's a whole jargon connected with aerodynamics.
Many of them are best understood when explained in context.
I suggest getting a copy of Andy Lennon's "Basics of R/C Model Aircraft Design", from MAN, available from Amazon.com.
The terms commonly encountered in design and performance are illustrated nicely..
But for now,
"Moment" refers to a force applied to an object at a distance from some point on the object, or it relates an area's location to a point.
"moment coefficient" refers to the value of a number which describes how an airfoil responds to airflow around it,
and "washout" refers to a twist in a wing, from root to tip. This can be mechanical; the tip built with a twist in it, or aerodynamic, the twist being a result of choosing specific airfoils using their zero-lift lines to achieve "washout" without building it in.

Start here.....
 

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html is a good start. It is in the basic aerodynamics link at the top of the forum.

RE: Define some terms for me
 

Futaba, has some aeronautical terms on there site. Might help with some stuff.
Maybe we need a section devoted to these terms. I have often wondered about some terms myself.
I consider " moment" to mean amount of leverage. Kind of like a pipe wrench handle.
I am sure someone with more knowledge than I will set us straight. ( It wouldn't take much )
Aloha , Les

RE: Define some terms for me
 

The Boeing Flight Test Manual, a standard in the industry (due to engineers following the work) has 11 pages of definitions. :)
Some of which have little connection to model airplanes but sound really technical when used wrong! :)
Some NACA reports also compile definitions.

RE: Define some terms for me
 

There is also "Stick and Rudder" by Wolfgang Langewiesche. While it is orientated to the full sized fixed wing pilot, it is a very easy read.


http://www.amazon.com/exec/obidos/tg...glance&s=books

Warning: I got so enraptured by this book I finally went back to school, got my degree, and became a professional pilot. (I land a lot better now than when I was learning to fly Patches, my old Gentle Lady.)

RE: Define some terms for me
 

Regarding the coefficient part of "moment Coefficient"... you'll see tons of co-efficients in aerodynamics. This is cause engineers hate to have to use units when they talk to each other. ;) Making something (like Lift, Moment, Drag, etc) into a coefficient means that you have divided it by something that has the same units. (i.e. To make lift, in lbs, into a coefficient, you divide it by "SOMETHING" that has the units of lbs.). This is called non-dimensionalizing. You can choose what you use to divide it by arbitrarily, but there are conventions for the standard coefficients. For instance, to get coefficient of lift from lift force, you divide the lift by dynamic pressure and wing area.

Often the coefficients are more useful to work with because the aircraft geometry, and flight condition are taken out of the picture. If an airfoil has a maximum Cl of 2.1, then that's it's maximum at any speed, or wing size. This way one set of wind tunnel data can apply to many different wings (assuming you match the Reynolds and Mach No. but that's another thread :P).

I hope I haven't confused you, but the main thing to remember is that when you see "coeffient of XXX", it means that they've somehow removed the units from XXX so that it can apply to more than one situation.

Regards,
Tom

RE: Define some terms for me
 

As Tommy mentions, there's quite a few "non-dimensionalized" coefficients and numbers related to aeodynamics, and indeed most scientfic areas of interest where things can be quantified.
Many of them have little application to practical model flight. About the only one which gets attention is the Reynolds number, which is in essence a "scale factor" which can be used to predict wing performance.
It's based on airspeed and the wing chord, and experience with wind tunnel experiments and models place lower limits on what is a "good" value for it on a model, and what might be poor.
Model aerodynamics deal with very small numbers relative to full-scale, and good reliable research in the field is rare. But for most of our model stuff it's been found that for an RN (also seen as Re) below 100,000, flight performance may be poor unless pains have been taken to design the wing for the low-speed, small chords which this value represents.
For example in the Canard thread, it's noted that small chords work poorly... extending the chord is the same as increasing the RN for the surface.
Dr. Mark Drela has done extensive research into the low-speed area of model flight and has developed a series of airfoils more suitable than the typical fat curved surface seen most often on slow flying or small models.
Aerodynamics is a complex science with many factors influencing all the other factors, and it's difficult to pick it up by merely listening to what often times is ignorant speculation.
A good library is a must if you want to get more into design than merely TLAR, just to seperate the baloney from reality.

RE: Define some terms for me
 

I know about what a coefficient is, but it is the moment part that I am having trouble understanding. At one point, someone told me that the moment coefficient was a measure of how stable the aircraft is and where the center of lift is located. Is this correct. Also, do you want to have a low moment coefficient or a high one? Damn aerodynamics is confusing, BUT I LOVE IT!!!

RE: Define some terms for me
 

"someone told me that the moment coefficient was a measure of how stable the aircraft is and where the center of lift is located. Is this correct."
NO!
This is precisely WHY it becomes necessary to refer to real world technical data!
The vast lore of urban legends about aviation is always available from the self-appointed expert. Something on the order of Cliffie in "CHEERS"... but there everyone knows he's a blowhard.
Running into the same situation where fact are requested and not knowing the difference between a snow job and a fact creates a lot of confusion.
.
THE pitching moment cofficient is a characteristic of the shape of the airfoil. When an airfoil is mounted in a wind tunnel supported at the 25% mac point, the airfoil may or not "pitch" around that axis.
Symmetrical airfoils do not pitch.
Cambered airfoils do. Generally it is, relative to the 25% point, a nose-down pitch.
The camber can be tailored to eliminate the pitch completely or have a nose-up pitch. The Clark-Y has a nose-down pitch.
Airplanes using it require a horizontal tail. Its derivative the Clark YH has a slightly smaller nose down pitch, the difference being in the aft portion of the profile, and make the shape suitable for flying wings (tailless) but more pronounced treatments of the aft part of the airfoil are now available for those interested in tailless flying.
Before the NACA release of their 4-digit series the C-YH was used for a few fighters in the late 30's.
The amount of the coefficient is a measure of the amount of camber the airfoil has. Some shapes have a small coefficient, others have large values. The coefficient's value is one of the parameters investigated when designing the airplane, as it drives what measures might be required to achieve the performance desired.
The mathematical sign of the coefficient signifies the direction of the pitch, with a negative sign indicating nose-down.
As the camber can be tailored to suit the operational enviroment of the airplane, in the real world it frequently is, with some parts of the wing optimized for low-speed flight, other parts for high speed, with the camber adjusted in the initial design phase to not affect the performance outside the region it is intended for.
High values, which indicate a strong nose-down moment for instance can be used with a blend into a shape with low values, for ease of flight control between landing speed, and cruise speed.
.
"center of lift" and its buddy "center of pressure" have been discarded as useful terms by NACA since 1939, with the more accurate identification of the pitching moment, static margin and neutral point now the preferred terminology.
For park-bench aero theorizing, they're OK but they are not scientific.

RE: Define some terms for me
 

Some of these definitions are my interpretation of them and for the most part I went with practical usage when it made the concept easier to understand.

http://www.airfieldmodels.com/inform...sary/index.htm

RE: Define some terms for me
 

Another excellent online source for aerodynamic stuff is here:

http://www.av8n.com/how/

it's basically an online text book. I've found that it's one of the most readable texts I've ever seen (including all the one's I've bought over the years).

The section on pitch stability is especially good.
http://www.av8n.com/how/htm/aoastab.html#sec-teeter

I especially like his comment on how the empennage of the aircraft DOES NOT work like the feathers of an arrow.

The amount of pitch stability you'll have is more closely related to the location of the CG than the amount of pitching moment your wing is making. On all aircraft there will be a CG location where there iS NO restoring moment caused by a change in AOA. This point can be thought of as the aerodynamic center of the entire aircraft (generally this point is aft of the wing aerodynamic center)... Putting your CG at this location is great for doing 3D aerobatics, but not good if you want a pleasant sunday flier.

FYI: Aerodynamic center is the point where the pitching moment is constant with AOA.

Tom

RE: Define some terms for me
 

Oh I have looked through this site many times. I have never had the time to sit down and try to absorb it all, however. But the parts I have read are so informative. But, at this moment, I am going to hold out on any other questions I have until I can find time to read the section you told me to. Thanks for the input everyone.