in reguard to my earlier post concerning incidence in the post," new guy with a question", i have a question or two to confirm my thinking about your excellent responce.
i think i have a fairly good understanding of "incidence" and of "angle of attack " and how they relate to each other,but i'm not sure how AOA is defined given different airfoil sections.
i understand that with a fully symetrical wing, AOA is a line prjectedthrough the centers of both, leading edge and trailing edge, making the wing neutral, thus needing pos. incidence to creat lift bymounting the horizontal stabilizer with some negative incidence. the wing on my plane is semi symetrical- flat bottomed. so, given the definition of "AOA", i should mountmy wingwhere the bottom flat plane of the airfoilis parrallel to the fuse's reference line, which is intended to be horizontaly level in level flight. my incidence would thenbe the angle projected by a line throught the thecenters of the leading edge and the trailing edge of my wing, resulting in a positive incidence ofabout1-1/2degrees.......is this correct?or do i have to physicly mount the wing so that the bottom flat plane of the airfoil is angled a degree or soup in relation to the reference line?. by the print, the hor.stab is also mounted 0, so i should bring it's leading edge down a degree or so, correct?
thanks in advance, you've been very helpful!
Ron.

Ron, the AOA is defined as the angle between the wing chord line and the airflow at that point. So you have to be actually flying to measure it. What you are referring to is the old 'Riggers angle of
Incidence' or 'Wing to body angle'. This is simply the rigged angle of the wing to the fuselage and will dictate what angle the fuselage will ride at during straight and level flight. All these angles use the chord line of whatever section you use as the reference line. You can forget the bottom of the airfoil for any meaningful measurements. As you know the Chord line joins the centre of the leading edge radius to the centre of the trailing edge, and will be the longest straight line you can fit through the section.
Evan, WB#12.

Having go that, then the next bit. You seem to want to see the fuselage flying at some attitude when the airplane is cruising along 'S&L'. So draw the fuselage at the attitude you want, then stick a straight line through it, horizontal to the horizon. This will be the datum line for all our measurements. Now you have to make some guesses, like how heavy will the model be? How fast is this cruising speed to be? You have already stated that you are using a cambered section, and from 'a few years model design experience' I can tell you that setting the wing somewhere between one and two degrees positive (assuming you can build that accurately) will allow the wing to fly at an AOA providing enough lift 'S&L' at a speed somewhat less than the max available, assuming two things. One, the wing loading is moderate (16 to 25 oz.in) and installed power is adequate. So now you can draw the wing chord line at (say) 2 deg positive to the datum line. Now you can ask yourself how much speed stability you want. This is the 'built in trimmed speed' of your model and is directly related to the angular difference between the wing and tailplane chord lines. If you leave the tail chord line parallel to the datum you will have two degrees positive angular difference between the two, and this is a pretty normal setup that will provide just a bit of 'self stabilising' speed control, as during flight the tail will try to maintain the main wing at two degrees AOA which if you maintain height will always be the same airspeed. Less power and the model will descend, at this speed, more power and it will climb, at this same speed. So, all wing incidence measurements are taken from the chord line. Tail to wing difference determines cruise (S&L) speed AOA. Weight determines cruise speed. You cannot measure AOA unless you can put the finished model into a wind tunnel, and even then you might not get the right answer, as in the real world it is constantly changing as you fly the model.
Evan, WB #12.

Ron, pimmnz has covered much of it but I'd like to add a couple of thoughts.

With full size aircraft the incidence or rigging angle is very important. Real planes normally fly for long distances and they want the fuel economy to be as good as practical. So the designers set a cruise speed and "normal" weight. Knowing the speed and weight and wing area and airfoil test parametersthey can easily figure out what the angle of attack in flight will be. Contrary to pimmnz we can actually do that as well if we want. We can make an educated guess at the weight and pick a flying speed. From that we can figure out what the angle of attack will be to generate the lift needed. we would then set that as the incidence angle so that the fuselage is flying as directly into the oncoming air as possible to generate the least drag. Pimmnz, the tools are all avaiable online to do this. Normally I'd have agreed that it's not practical but other folks have wrapped up the equations in various online applications such as Xfoil and Foilsim to allow us to actually do such a thing.

Now that's all fine and dandy but it's not how we fly our models other than some very special cases such as thermal sailplanes used for contest work where they want to achieve the minmum drag during some speed runs or for best L/D at the best cruise speed. The rest of the time we only fly at a single speed for a very short time and distance. If we hit the speed where the fuselage is dead in line with the airflow then it's good luck more than good planning. Normally we're constantly climbing, diving or turning.

The reality of this becomes the fact that we modelers only need to use the incidence angles of the wing and stab to establish the basic decalage angle, or longitudinal dihedral if you wish, to set the wing to stab angle that will be used to achieve pitch stability. We don't really need to even do that if we don't mind having the elevator angled up so it's not flat to the stabilizer itself. But most of us don't like that look so we try to minimize such things by incorporating some angular difference between the wing and stabilizer to "get in the ballpark.

Thanks for the added info BMatthews, but I was always told that 'pitch stability' was a function of balance, not wing/tail angular difference.
Other than that, and not actually having used any of the 'on-line calculators' (I do mostly scale stuff, so most of the designs I draw are already fixed so far as layout, areas etc are concerned) I have to agree with you. My biggest beef is the interchangeability of 'incidence' and 'AOA' as used by modellers, even though the two things are totally different. Which was what I was trying to illuminate in the discussion, without much success.
Evan.

oh well-

Pimmnz, you're right of course. Sorry if I made it sound differently. I passed by the CG issue.

The CG location ahead of the neutral point sets the stability margin. BUT.... the wing to stab angle isthenselected so the model trims to level flight at the desired airspeed for that CG location if one wants to avoid a healthy case of upelevatortrim angleto stabilize to level flightat the trimmed airspeed.

This last bit is the part I left out. I sort of assumed that folks would fill it in. Does that make it more complete?

hi fella's,
   wow!, i've been answered!!.....
               please excuse my loose use of terminology, i am quickly  learning that much of what i've read/learned has been, as pimmnz puts it, "interchanged terms"!.
              i do have a fairly good grasp on the dynamics of pitch stability, the relation of neutral point to cg and how the stab's incidence/elevator's trim works  against the neutral point/ cg moment.... much the same as the relationship of the centers of effort and resistance and how a lee or weather helm of a sailboat 's trim will change as the boat heels and the underwater form changes.
     this " basic decolage angle" is what i am asking about. 
         as i see it,...per the print, if the datum line is shown with the fuse drawn as in level flight and the datum line  level during level flight as drawn, my semi sym.-flat bottom wing, with the flat bottom plane of the airfoil section mounted on the datum line, will have 1.5 degrees positive incidence and "by the print" the hor. stab is mounted parrallel to the datum,... the decolage angle is 1.5 degrees positive. is this decent starting point or should i add a degree of negative incidence to the hor.stab. cconsidering the the c/g is moved forward a bit.
        my thoughts;.... the incidence angles are easily adjust with shims at the mounting points of both wing and stab. i say mount them as is 1.5 degr. pos. decolage and see how much trim i need at the recomended 1/2 throttle airspeed......does this sound reasonable??
            

Ron - where you say the incidence angles are easily adjusted with shims for the wing and stab - well, Lol - they maybe on your plane mate!

If it was easy to re-angle the flying surfaces we'd not need to get so worried during the designing/building phase of our hobby - just flight test and correct as required. Unfortunately, re-jigging the incidence on a finished project can be a nightmare close to a full re-build, depending on what has to be taken apart!  A further fly in the proverbial ointment is that if you change the wing seat angle it will upset the vertical thrustline of the motor. I fell foul of this last year in a small vintage cabin model that had too much incidence - only solution in the end was to cut the tail off and re-jig it to four degrees (or so) more incidence at the tailplane.  On my flat bottomed wings i now set the tail parallel to a datum drawn from the wing TE through to the middle of the LE. This can make the wing appear to be angled down - but in fact its still producing +ve lift and on a cabin model some down thrust is also still required ( to counter slight ballooning on opening the throttle, not helped of course by the Centre of Drag line being well above the thrust line).

If that's how they set up the model it implies that it's not a trainer.  Or they just expect the builder and pilot to shim the wing as Rick mentioned. 

If you're building a model to train other new flyers then I'd likely build in another degree of positive in the wing.  Putting it in the wing does two things.  It increases the decalage and it sneaks in a degree of downthrust since you're altering the wing to thrust line angle at the same time as the wing to stabilizer angle.

reality check:
a degree (one degree) -is aprox 1/4" in one foot!
this kind of angle in model airplanes is HUGE-
From experience - - make adjustments in increments measured from a flat referrence
and typically on a outdoor model - in 1/16" increments
or less.

1 degree over a 12 inch length is.209 (41 thousands of an inch less then 1/4)
I think its easier to say just less then 1/4 LOL

po-tay-toe  =  0.209 is approx 1/4"

po-tah-toe  =  0.209 is less than 1/4"

Yup!  Looks about right to me.</p>

Sorry about using "aprox".
It is a shorter way of saying "about"
Using degrees instead of accurate referrence measurements on model planes is not a great idea
My findings are that 1/16" vertical difference in le to te is BIG difference in many models
but then I have built only a thousand or so flying models

rick,
       for this particular plane, it is truly that easy to change inidence/decolage, not only for the wing but for the hor. stab as well. the flat plane of the wing's underside sits on top of the fuse sides secured typicaly with four nylon machine screws . the top of the sides are also the datum line. so, with no shims the line through the center of the the leading edge and trailing edge of the wing gives 1.5 degrees pos. incidence (in relation to the datum) which is supposed to be level in straight and level flight. similarily, the tail wing sits atop the vert stab, who's top is parallel to the datum, so getting a degree negative means simply trigging the offset and shimming the appropriate thickness under one of the two small machine screws that secure it to it's saddle.... and yes, the vert stab is built strong enough. built hollow foil  from 1/32" ply.  the foil is 3/8" at it's thickest point with a 1/8"thick by 3/4" wide ply platent epoxied on top for the saddle. the elevator linkage is by belcrank, up through the sta and a custom made control horn allows the entire assembly to be inside the vert. stab. all a change in incidence requires is the proper shim and a small linkage adjustment to return the elevator to neutral after shimming. so both surfaces are very easily adjustable.
      my concerns are whether i am correct in assuming that the incidence angle of the wing is defined by the line through the leading and trailing edge, in relation to the datum. i fairly well understand the whole theory of incidence angles and how decolage, the cg,  the stability margine and airspeed  work together (or against each other)  to stabilize the plane....very simar to the same elements, with different names of course on a sailboat. i just want to know if the line, as described, is the referance for incidence for my particular airfoil shape, namely, a semi-symetrical-flat bottom wing, just a line on a full symetrical is used when setting decolage.
     being sure of this one particular element  (the purpose of my question, initially) and all of this board's excellent advice, will allow me to continue, knowing that i have at least, got an initial decolage set up that should work.......you gotta start somewhere!

Ron, you will be fine as it is. If you shim the tailplane leading edge down you will increase the 'longitudinal dihedral', decrease the 'S&L' cruise speed, (cause the tailplane will try to maintain the main wing at a higher AOA) and slightly change the fuselages flight attitude. The old speed trim thing again. Leave as is, balance at 25% MAC for the first flights, and see what it is like. Then try playing around with the rig. Yes, the various rigging angles are DEFINED by the relation of the CHORD LINES to each other and whatever DATUM the designer has chosen. (And that datum can be any line, anywhere, it is only a reference). The CHORD line is defined as that line that joins the centre of the Leading edge radius and the centre of the trailing edge. And we try not to have 'semi-symmetric ' wing sections, that's a bit like being just a bit pregnant, something is either symmetric, or it is not. Better to define your sections by describing the CAMBER, that line half way between the top and bottom of the wing section, ie the line that divides the wing section into equal areas, top and bottom. In your case you have a wing section with a bit of positive camber, which is usually expressed as a percentage of the wing chord (there's that term again). A symmetric wing has, of course zero camber, or you could say that the chord line and the camber line are the same.
Evan, WB #12.

pimmnz

pimmn,
thanks much......chordline....that's wat in needed to know and now it falls into place. these sites and the net is fantastic, but sometimes answers tothe simplest of questions get get expanded into whole dissertations with way too much info and way too many opinions. no offense intended to anyone who helped me, as i know it was all done in being as helpful as possible! andi did learn quite a bit in this thread, as well! you guys are great. i'm not much of a reader any more and it's good to know a guy can get one small elementthat he's just not sure of sorted out without haning to read a whole book on the subject. asmentioned, this is a first build after a 40 yr.absence, the building part is no problem atall andi'veretained most of what i learned years ago, there's just a spot here and there that i,m not sure about.
hopefully, at some later date i will learn how to post pics and put this "flying contraption" up for all to see.....beforei turn it intoa pile of splinters, anyways!! the airframe is just about ready for final sanding, sealing and covering, it like i'll be right on track for good weather at the feild in spring.
one thing i've discovered is that years ago, i'd pound through a build as fast as possible to get in the air, now, it's more enjoyable to take time and make every joint fit good and have the satisfaction of knowing it's built as good as possible and finished perfectly as possible....with age comes logic!.
thanks for all your help,
ron.