Augustinev,
What about the vertical stabilizer's size? How does it effect the stability?

Directional Stability, (Most of the times it is the limiting factor manoeuvring at high angles of attack) and as an offshoot lateral stability (If the fin is dorsal then it contributes positively to lateral stability and if the fin in ventral then negatively

Two questions.
1) The stabs and the fins have a ratio to the wing area. for example, in a high winger tractor, the stab should be about 15-20% of the wing area. While if it is less, then it may not provide necessary stability, but what happens if it is more?

2)While scratch building I have noticed that it is nearly impossible to make the wings/stabs parallel to each other, nor for that matter the fin would be exactly perpendicular to the stab. There exists a difference even if it is less than a degree in angle or 1mm in distance. The materials that we use for models is soft. Sometimes due to storage or due to faulty constructions or due to repeated hard landings, these angles/measurements changes. How do these variations effect an RC model (not the real plane).

Not answering the questions just suggesting that while you are thinking about stab sizes you should also consider the movement arm length. Refer image posted by Augustinev on reply #174.

ps: this is my favorite thread on RCIndia

Arm length part is almost similar to a basic lever ( effort - fulcrum - load). So that part I can relate to

Well,
These questions are pushing me to cover issues which will scare away guys  

There are two terms called Decalage and Longitudinal Dihedral

Decalage When two airfoils have different angles of incidence, (from the French word for “shift” or “offset”)  The more wing to tail decalage you have, the more vigorously the airplane will oppose any attempted deviation from its preferred angle of attack. +ve decalage is + ve responce by the model, -ve decalage is -ve response from the model (If you watch carefully the model while it is flying is slight wind condition , without your input when hit by a small gust of air see what it does (See Image), if updraft of air and the nose comes down it is +Ve and nose goes up it is -ve, there are theories of where the centre of mass is etc which i am not going into presently)

The Angular between the angles of incidence of the wing and tailplane is called Longitudinal Dihedral. Positive if the wing incidence is greater. it is like the Dihedral you give for lateral stability, while scratch building give slightly more positive incidence for the main wing and have a different and less incidence in the tailplane, aeroplane will be longitudinally stable as well

See image for Balance of forces using Rainfall Analogy to make things clear (As brought out by SLS)

Like I said, in simplistic terms, I am able to understand the wt x distance from the fulcrum needs to be same on both sides of the fulcrum ( CG /NP ). So what this means is that if the stab area is more, then not only the effect of rainfall is more pronounced, but as the weight of the tail also increases, it would mean additional weight on the nose. In theory as far a stab and wings are concerned, this is understood (did I understand correctly?).

What about the vertical fin? how does this get represented on the rainfall/level theory? The simple stuff on the above diagram is that the lift produced by the stab and the wing are the two loads. While this is understood for pitch movement, on the Yaw front, the tail fin appears to be the only load so what is it balancing against?

Are my questions making sense?

bigger tail will have more tail lift , nose will pitch down

Fin maintains direction and contributes slightly to lateral stability as well

not always because some planes have inverted airfoil for tail so that nose pitching is reduced. So if the nose is heavy it can be compensated by an inverted airfoil for the tail

in the design consideration inverted aerofoil (Like the Dornieir 228) is for completely different purpose. We will only complicate the matter. at RC Level let us keep it simple

ya i agree its not used for this purpose.
i just wanted to put up the point.
i had a problem with a heavy nose once and tried out and it worked out well. the only thing is it decreases the speed of the plane.
and sure it complicates things a bit.

The simple moment figures are clear.

Now, If the CG resides just below the wing lift point ( one arm length = 0), so there is no need of stabilizer,

and

if the CG goes left (As per Pic right arm length = -ve), one moment should be in opposite to other theriotically to stabilize and stabilizer force should be downward insted of upward.

A simple MOMENT Calculation only (Recall Strength of materials).

Am I Right? Is this related or useless?

? about centre of mass? how will aeroplane behave? that's why i said let's leave it at that

Augustinev,

Please look at the problem in post #24 of this thread http://www.rcindia.org/electric-planes/scratch-build-powered-glider/new/#new

Look at this HK model... http://www.hobbyking.com/hobbyking/store/__8472__Hobby_King_Piaget_EPP_CF_3D.html

Seems to have a tail fins type of assembly on the wing as well. Does this serve any purpose?

Chato Read this
http://en.wikipedia.org/wiki/Wingtip_device

Read this thread and the reply for some insight into 'V' tail config

http://www.rcindia.org/self-designed-diy-and-college-projects/coro-whistling-racer/msg84898/#msg84898

Basic points that could be considered while designing scratch built glider (Some more reading on the subject will be required)
http://www.rcindia.org/electric-planes/scratch-build-powered-glider/msg83934/#msg83934

Flaps, Spoileron & Crow/Butterfly

I will try and keep it as simple as i can,

Flaps

1. what lifts the wing is the angle that leading edge  of the wing and the trailing edge of the wing makes with the path that the aeroplane follows,
2. when you lower flaps what happens is the trailing edge of the wing is lowered and the aeroplane without changing the position of the nose and the speed gets a boost in the lift. also the camber increases.
3. what happens in take off is that it is able to lift off at a lesser speed and at a lesser runway length, but the drag also increases
4. that is why after take off the flap is raised and the wing now becomes clean and produces less of drag ,
5. same thing happens in landing with increase in lift the aeroplane is able to, and at a lesser speed make landing easy and reducing the landing roll.


1. Flaperon in RC parlance is when you you use a full length aileron, also as flaps


2. for reducing the landing roll you can use it like a flap and aileron, increased lift can be used for not only landing you can use it to fly the aeroplane at very less speed with stalling (Wing dropping) some use it even in a rolling harrier. in real life aeroplane too you use it, in fact hawker hunter had flaps at 15°, 23°,30°, 38°, 45° & 60°, initial couple of positions of the flaps were called Combat Flaps used during the low speed regimes of the combat so that aeroplane had adequate lift and does not spin or stall

Spoileron rolls and yaws the ailerons, not by differential lift ,  by dumping lift on one side, in RC, it is used to negate cross coupling, however in real life it is used in a short span wing where this method is more effective than aileron , in a sweeping wing (Like the F-14, MiG 23 & MiG 27 ) at the fully swept back position spoileron is used, it is also used in long wing where aileron can cause twisting moment and break the wing or negate the differential lift due to twist caused by the aileron.

crow/butterfly flaps is when ailerons go up and flaps go down. works quite like an airbrakes, to reduce speed

wash out/ Wash-in Due to Flexure

Disclaimer (This discussion pertains to RC aeroplanes and esp foamies, therefore aeroelastic tailoring has been left out of the discussion)

When a swept back wing flexes due to less of torsional rigidity the incidence at the wing tip reduces , effective lift production is concentrated inboard  of the wing (See image) wing tip therefore looses lift, the centre of pressure moves forward and aeroplanes pitches up, if the tip lift loss is high, she will pitch up and yet loose altitude  

How does this happen (You need to have some imagination  , easier way is to fold a paper and see)

When a swept back wing flexes under load, all chordwise points at right angles to the main spar are raised to the same degree,  see image two, the points A and B rises through the same distance and the points C and D rise through the same distance but through a greater distance than A and B.Thus C rises further than A and there is a consequent loss in incidence at this section. This effect is termed, ‘washoutdue to flexure’, and is obviously greatest at the wing tips. It is most noticeable during high g manoeuvres when the loss of lift at the tips and the consequent forward movement of the centre of pressure causes the aircraft to tighten up in the manoeuvre. (very applicable on foamies).

Now washout is structurally not dangerous, because wing tip is unloaded of high lift

Wash-in Due Flexure on a swept forward wings (Like su-47 berkut)

In a swept forward wings if you see the image above due to flexure the wing tips will produce more lift, this will overload wing tips and it will break away clean . The image i have put here is from the rcgrops (http://www.rcgroups.com/forums/showpost.php?p=15309060&postcount=9), you can see how cleanly it has broken, if you see the thread he has attributed it to flutter, it is not, it is due to wash in due to flexure

read a little bit more about berkut here

http://www.rcindia.org/electric-planes/su-47-berkut-tomhe-plans-build-and-fly-log/msg91398/#msg91398

Why are Flaps required ? here
http://www.rcindia.org/electric-planes/using-ailerons-as-flaps/msg96159/#msg96159

Adverse Aileron drag (I have talked about it earlier) and why should you setup aileron differential



For years,  modelers often used 'Y' cables for ailerons and offset servo output arms Very rarely in India though) to achieve differential aileron movement. Today, however, using separate aileron servos and the aileron differential program menu in your radio has greatly simplified the task. Before we take a closer look, let’s first check out the Aerodynamics  > of our model during a turn or a roll to understand why aileron differential is important.

Adverse Aileron Drag

Typically, most models are set up with equal amounts of elevator (pitch up and down) and rudder (yaw left and right) control surface movements. But when it comes to ailerons, equal amounts of up and down (roll left and right movement), can cause the model to yaw in the wrong direction. Reason: When the ailerons are at their neutral positions, the lift and drag produced by each wing panel is equal and the model tracks straight ahead. But when a model has ailerons that move in equal amounts both up and down, the amount of drag (and lift) created by the wing panel with the down aileron becomes greater than the one with the up aileron. The panel with the aileron pointing downward moves up because it creates more lift. The opposite panel goes down (less lift) and causes the model to back toward the up aileron. Because of the increased drag caused by the upward motion, that down aileron wing panel also slows down; this causes the model’s nose to yaw in the opposite direction of the roll. The model yaws nose right in a left-hand bank/turn  . This condition is known as adverse yaw. Without aileron differential, most airplanes require a certain amount of coordinated rudder to prevent, or at least minimize, adverse yaw while the model is banking through a turn. For sport and scale planes, this can be done manually or with a program mix-however, it won’t work in all types of flight conditions. in some high performance ARF like the CARF Yak 55 Sp and in real life aeroplanes we have something called the frise aileron (More about it later). This adverse yaw thing is also an important consideration while flying aerobatic planes. Aerobatic pilots need to set up their models to react in pure yaw, roll and pitch motions. During a roll (whether it’s executed on a horizontal or vertical line), the model must roll axially without its nose yawing or wandering off the straight line of flight. Aileron differential helps keep the model’s tracking straight.

Symptoms of a an Adverse Yaw
 
1. The model skids through turns.
2. The tail drops during a turn.
3. The nose swings out of the turn.
4. It’s very difficult to roll your model in a straight line.

Using your radio’s programming is the easiest way to get the job done.

Radio Programming

Assuming you have dual aileron servos (One or more servo / aileron). One connected to the aileron receiver port and the other in the Aux.1 port (Typically Ch 6). Make sure the aileron servo moves in the proper direction.

Activate the flaperon wing type or, depending on your radio system, the dual aileron function.

Start with 30% to 40% differential (down going aileron 30 or 40% less than up going aileron).

If differential mix is backwards (more down than up), reverse the servo connections by switching the aileron and Aux. 1 servo leads.

Adjust the differential percentage after flying the model (You need to watch it carefully). Land the model before making adjustments and test fly again.

Happy Differentialling