Prop has something called the Helical Twist. It is CW or CCW. tractor pusher is a wrong terminology , why? Can a tractor prop be used in a pusher configuration? Of course you can , whether in front or rear , a CCW prop will rotate CCW to throw the air back , a CW prop will do the opposite,
Pankaj, you are thanking sukurt for simplifying matters? Actually he did not, there is nothing called tractor prop and pusher prop, it is very colloquially, loosely and incorrectly used terminology,

To simplify and put matters to rest
(a) viewed from front if a prop is rotating Counter Clockwise ( CCW) then it is CCW prop, it could be a tractor or it could be a pusher.
(b) viewed from front if a prop is rotating clockwise(CW) then it is a CW prop,
CW prop is used as a port inboard and starboard outboard prop ( in a two/four engine configuration, like the super Connie, this too is not a rule though, beaufighter is an example where both prop rotated to the same side, advantage was you needn't look for a spare port or starboard engine, one spare engine worked for both, ground looping landing accidents outweighed this advantage though) , also used as a pusher prop

I hope I have made myself clear. If there is still some doubt, feel free to shoot

To be more specific, the front of the motor, not the airplane.

OK I am confused.

Hobbyking does sell "pusher prop"  so not sure why they misrepresent on a 7x4 prop these are two

Pusher   http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8044
Tractor http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8011

so now as I understand, when I mount the motor in the front and facing the front, it is called a tractor and when I mount it near the tail and facing away from the nose, it it called a pusher. In the pusher arrangement, even if I mount a normal prop, the marking is towards the nose and I just change the direction of the wind by interchanging two wires connecting the motor to the esc. Aparantly,interchanging the wires makes the motor rotate anti clock wise.  In both these cases the wind travels from the nose to the tail hence the plane moves forward


So to understand the twin motor setup, both the motors have to push the wind from head to tail but one needs to move in clockwise direction while the other in anti clock wise. So how am I supposed to do this because given what I have described, one would throw wind in opposite direction to the other? Since  this is not what is supposed to be, I am sure I do not understand how the setup is to work

Thanks Sushil sir, there was a possibility of confusion there,

Now pankaj how do you do a contra rotating prop is a complicated, let me try to be simple.
If using same motor or engine you need a gearing mechanism which rotates outer shaft CW and the inner CCW. you can also use two engine/ motor through gearing doing the same thing.
Complication in a contra rotating prop is pitch of the rear prop, matching that without a CSU( constant speed unit, which keeps prop pitch optimum depending on flight conditions) is counter productive, in an Rc setup contra prop is optimum only for a particular speed , because the inlet velocity of the rear prop is the exit velocity of the front prop and it is a different direction. The interaction between both has something called Cascade Effect,
Bottom line contra prop in  just cosmetic in Rc, advantage accrued is far far less.

so what happens if the motor I use is not geared? direct drive brushless types?
secondly, I have not seen the EDF units being sold as pairs (as in left EDF unit being different than the right one) so aparantly they operate in the same cw or ccw direction (thats what I think) So how do they balance out?

Closer to drag line this effect is less in a edf unit.

I guess I am getting confused in two aspects.
1) the direction of rotation of the motor
2) the direction of the prop.

so if I understand correctly, a prop is like a screw you turn in one direction it goes in and turn in the opposite direction it comes out. Now if a standard screw would go in while turning clockwise and suppose there was a screw with threads such that it goes in anti-clock wise then this might be similar to the twin motor setup -right?

Bingo, that's why da vinchi called it and even now it is called airscrew

read this post for insight into Flaps

http://www.rcindia.org/rc-general-topics/flaps-up-or-down/msg77245/#msg77245

Back to the drawing board.

Made a pusher 50" span the wing is polyhyderal, shapped like this \______/. The flat part is 30". The elevated portions are 10" each and have ailerons. Chord is 7".
The horizontal stab is about 20% of the wing area (7x50) and the vertical fin is about half the stab.
CG checked at less than 1/3 the chord length.
Motor is 1400Kv with a 7x4 pusher prop.


The moment the bird takes to air, it kind of has a very erratic behavior. It banks sharply on left even with a full aileron trim. It goes up and down like a tail heavy bird, but I suspect CG is not the issue here.

So the question is what should be checked? I know it may not be possible but - any suggestions?

The symptoms indicate a warped wing

polyhedral if not made well will always give you trouble, few things i need to check please post some pics

hi,

your specs match mine. i have built a pusher with 50" wing span, 7" chord.
the wing is tip dihedral the same which you showed.
i have set it up for 3 channels, with a 1400kv motor, 18A esc and a 6x4 prop.

flys very stable. infact tested with a 2200mah lipo and the model was nose heavy would not glide
when motor was turned off. surprisingly flies with a 1300 mah bat and glides also very well. the CG seems to be at 50% wing chord. i wonder how 

pls see pics below.

There seems to be a lot of Doubt of Descend that's why this post, i urge all of you who want to know about descend to please read carefully

Knowing this is important, i have seen many people on engine cut pull back (They call"Saar i gave max up elevator yet i crashed ") and crash, GLIDING , SLOPE SOARING, LANDING BACK AFTER A CUT, for all of these this topic will be useful, please read carefully.

Angle / Rate of Descent in Glide

17.   As the aircraft descends on a steady glide path, it forms a finite angle with respect to the horizontal. This angle is called the Gliding Angle.  While gliding at a steady speed, the aircraft would also have a finite rate of descent, which will be proportional to its gliding angle and the speed of gliding.

18.   An aircraft in glide may be flown at any speed by choosing the angle of descent of the aircraft, i.e. by steepening the descent. The aircraft may be flown at a higher speed with a correspondingly higher rate of descent or it may be possible to fly the aircraft at speeds as low as its stalling speed. From the pilot's point of view, lowering the nose of the glider may not necessarily mean that the rate of descent would increase and the range of glide reduce. Similarly, slowest speed during glide neither means the shallowest angle of descent nor does it mean the slowest rate of descent. To understand the mechanics of a gliding descent let us understand the forces that act in a glide as indicated in Fig

Forces in a Glide

19.   The four forces in flight are Lift, Weight, Drag and Thrust. Since there is obviously no thrust available in a glide, the balance of forces of the remaining three forces is as follows:

(a)   Weight is balancing the lift as well as drag, however the addition is not 'algebric' but 'vectoral'. From the Fig 16-6, it can be seen that the total (aerodynamic) reaction obviously is balanced by the only available force, that is 'Weight'.
(b)   The drag accordingly, is being balanced by a component of weight  i.e.   W sin θ.
(c)   The lift is being balanced by another component of weight i.e.  W cos θ.

The Lift continues to be at right angles to the flight path and Drag continues to be along the flight path in the reverse direction of flight.  The only source of energy to do the work, which is being done against the drag, is the potential energy of the aircraft, which is being slowly traded off as the height of the aircraft is reducing.

Gliding for Endurance (For Slope Soaring)

20.   Gliding for 'endurance' means gliding for maximum time in air.  Obviously, from a specified commencement height, an aircraft with the least rate of descent would stay afloat in air longest.  

21.   In Fig we can see that for minimum rate of descent, V sin θ must be minimum.  From balance of forces in a glide:

D   =    W sin θ

Multiplying the equation with velocity V

DV    =    W sin θ.V   OR
Vsin θ    =    D.V                  
       W

From Fig  we know that

ROD    =    V sin θ

Therefore, from equation , for the ROD to be minimum, the value of D.V/W should be minimum.  Assuming the weight to be constant, the minimum ROD and hence maximum endurance of the aircraft would be achieved at the speed requiring minimum power.

22.   when flying at the endurance speed least ROD is required above and below this speed (Min power speed) the ROD would increase.

Gliding for Range When engine Cuts  

23.   Gliding for range means gliding in a fashion so as to cover maximum distance by the time the aircraft touches down on ground. From Fig , for a given commencement height, the maximum range would mean a point farthest on the ground from the point vertically below the commencement point.  This in turn would mean the smallest gliding angle or the 'flattest' glide.  For example, a TB 20 aircraft has approximately an angle of glide of 4.7 degrees and the CAP 232 aircraft has an angle of glide of about 12.95 degrees when flying at respective best L/D angles of attack.

24.   From Fig , the triangle formed by Lift, Drag and Total reaction is geometrically similar to that formed by Distance, Height and Glide-path.  Now, if distance is to be maximum, gliding angle must be minimum.  θ is minimum when Cot θ is maximum or  Tan θ is minimum.  From the balance of forces, since L  =  W . Cos θ  and D = W . Sin θ,   Cot θ  = L / W.  Therefore θ is maximum when     L / D is maximum.  Further, in most aircraft the gliding angle is quite shallow, being less than 15 degrees hence the lift can be safely assumed to be equal to weight. For the more mathematically inclined reader, truly speaking the lift will be equal to Cos function of the gliding angle, which in case of a glide angle of 15 deg works out to be L  X  Cos 15 = L X  0.9659 i.e. less than 2% error, which will be even lesser at lesser glide angles.

25.   Therefore best (smallest) angle of glide depends on maintaining an angle of attack that gives the best lift/drag ratio. Since lift is assumed to be equal to the weight and hence constant, best L/D ratio would correspond to the 'minimum drag' condition. For the mathematician, like mentioned earlier, in case of steeper gliding angles, the lift would be lesser than the weight by a factor of cos θ. In case the aircraft is equipped with an angle of attack indicator, the pilot needs to just maintain the same angle of attack as earlier; however if the aircraft does not have the AOA indicator and the pilot has to go by the EAS, he would have to reduce gliding speed by a factor of √cos θ to continue to maintain the optimum angle of attack for best range.

PS
For some of you initial reading might sound confusing, i do urge you to read and understand, shoot questions after you have done so

Quote from: SunLikeStar on November 16, 2011, 03:38:24 PM
The symptoms indicate a warped wing

What is this? How to check?

Another thing is that this is my third pusher. First one was 3 channel setup - flew very well, the next two I tried with ailerons ( one dihedral and one ppolyhydral). Same uneven flight characteristic noted on both wings. So was wondering if something special needs to be done for putting in ailerons on pushers - specially for scratch builds

hi,

ever wondered why the easystar does not have ailerons. it flies best with rudder only.

even the axn floater requires rudder input alongwith ailerons. i am mixing rudder with the ailerons on the axn.

as per my experience. slow flying planes fly best with rudder only.

for speed models ailerons are the best.

my current pusher is 3ch and it flys beautifully. hence no intention of putting ailerons. infact all my students like to fly it.

regds
sandeep

AFAIK this has nothing to do with speed of the plane, i have a super slow easyglider which almost refuses to turn without any aileron input. On the other hand Amar has a super fast easystar which he flies like a pylon racer just on rudder.

Quote from: PankajC on November 17, 2011, 06:51:05 PM
this is my third pusher. First one was 3 channel setup - flew very well, the next two I tried with ailerons ( one dihedral and one ppolyhydral). Same uneven flight characteristic

I doubt that this is the reason of uneven flight characteristics. Only thing to remember is with sufficient dihedral/polyhydral you wont need ailerons. Ailerons are not very effective for turning if the wing has dihedral/polyhydral and rudder is not very effective for turning if the wing does not have enough dihedral/polyhydral. Augustinev is the best person to explain dynamics of this but before that you should post pictures of your wing.

About warped wing; its when one wing "twists" out of symmetry from the other wing. PFA a picture worth many words
image source

i didn't want to take the path that about to take now, the reason for me to go a deep into it is that, there appears to be a great deal of doubt amongst even senior aeromodellors on the theory aspect, especially  of turn performance (Since theory  is lacking they are exhibiting the same in teaching and building models).well here it for you,


Axes of Movement of an Aircraft

1.   Lateral Axis.    The lateral axis is a straight line through the CG, normal to the plane of symmetry, rotation about which is termed pitching.  This axis may also be known as the pitching or looping axis.  If any component of the forward flight velocity acts parallel to this axis the subsequent motion is called sideslip or skid.

2.   Longitudinal Axis.    The longitudinal axis is a straight line through the CG, fore and aft in the plane of symmetry, movement about which is known as rolling.  This axis is sometimes called the roll axis.

3.   Normal Axis.    The normal axis is a straight line through the CG at right angles to the longitudinal axis, in the plane of symmetry, movement about which is called yawing.  This axis can be referred to as the yawing axis.

4.   Fixed Relationship.     The three axes are fixed relative to the aircraft irrespective of its attitude.  Fig below shows the major axes and the possible movements about them.

Now imagine a yaw (Caused by applying only rudder) in an aircraft the outer wing will travel faster than the inner wing this will cause the aeroplane to roll, this roll will tilt the lift vector which now supports the weight and also gives necessary centripetal force for the aeroplane to turn

If If, lateral stability of a model is good and directional stability poor (High winger, High Dihedral, small fin) , this yaw induced roll will corrected by the lateral stability and the aeroplane will right itself back, to turn with rudder if you apply more rudder she will do a kind of an unbalanced skidding turn (Evident by the way where if you have to push down on elevator to avoid the aeroplane to climb)

At last it is all the interaction of movement of aeroplane on one axis with an another (Including aerodynamic and inertial forces, do not discount inertial forces especially in a model aeroplane inertial forces play a large role)

Do not ever jump to conclusion without reading about it, it can be dangerous to the model and you

For an aircraft to turn, centripetal force is required to deflect it towards the centre of the turn.  By banking the aircraft and using the horizontal component of the now inclined lift force, the necessary force is obtained to move the aircraft along a curved path.

1. If the aircraft is banked, keeping the angle of attack constant, then the vertical component of the lift force will be too small to balance the weight and the aircraft will start to descend.  Therefore, as the angle of bank increases, the angle of attack must be increased progressively by a backward movement of the control column to bring about a greater total lift.  The vertical component is then large enough to maintain level flight, while the horizontal component is large enough to produce the required centripetal force.

Effect of Weight

2.   In a steady level turn, if thrust is ignored, then lift is providing a force to balance weight and a centripetal force to turn the aircraft.  If the same speed and angle of bank can be obtained, the radius of turn is basically independent of weight or aircraft type.  However not all aircraft can reach the same angle of bank at the same speed.

Effect of Thrust (See my MiG 29 Video you will realise what i am talking about)

3. as far the effect of thrust on level turns has been ignored.  However, the thrust, or lack of thrust, may be the determining factor as to whether the optimum speeds for turn i achieved.  Even in level flight it can be seen clearly with some aircraft that a component of thrust is acting in the same direction as lift due to the inclination of the thrust line from the horizontal.  This effect becomes more pronounced as the critical angle of attack is approached (see image).  The thrust component assists lift so that either less lift is required from the wing  or the turn can be improved beyond that indicated by simple theory.  Just as lift was split into two components in turn fig, one to oppose weight and one to provide centripetal force, so the component of thrust that acts in the same direction as lift can also be split into two similar components.  This is the reason for the remarkably  mall radius of turn of which some high performance aircraft like the Mig-29 and SU-30 (using thrust vectoring) are capable of. 

4.   It should be realized that many trainer glider aircraft do not have sufficient thrust to reach and sustain the optimum speed thrust assisted turn.

Quote from: sbajare on November 17, 2011, 10:14:24 PM
ever wondered why the easystar does not have ailerons. it flies best with rudder only.

Sandeep
See easy star two's flying chrac, same aerofoil see the difference

Quote from: sbajare on November 17, 2011, 10:14:24 PM
even the axn floater requires rudder input alongwith ailerons. i am mixing rudder with the ailerons on the axn.

That is because of excessive lateral stability, even high performance fighter jet requires rudder (Because high sweep increases lateral stability, but high sweep is required evil for supersonic flight, so it is all about lateral stability and directional stability and how harmonious the airplane is )

Quote from: sbajare on November 17, 2011, 10:14:24 PM

as per my experience. slow flying planes fly best with rudder only.

for speed models ailerons are the best.

Not a true statement at all, read the reps above, it is all the interaction of lateral stability to directional stability both static (the initial response) and dynamic (What happens after a while, to put it in a layman's term)

Does the longitudinal stability has anything to do with the size of the stabilizers (horizontal and/or vertical)?
Does the distance between the GC point on the wing and the hinge of rudder/elevator  has anything to do with stability?

Tailplane and Elevator.    The function of a tailplane is to supply any force necessary to counter residual pitching moment arising from inequalities in the two main couples, i.e., it has a stabilizing function.  This is achieved most commonly by fixing a hinged flap behind the tailplane by which the direction of force being generated, as well as its magnitude can be varied.  The force on the tailplane is positioned some distance from the CG means that it can apply a large moment to the aircraft (see fig). For this reason the area and lift of the tailplane is small compared with the mainplane.

Additional Information on the Tailplane

(a)    Design.    The tailplane may be required to produce either an upload or a download. Therefore the tailplane is usually (Not always) symmetrical in design.

(b)   Tailplane Flight Longitudinal Dihedral.    In most conventional aircraft, the tailplane operates in the downwash of the mainplane. The downwash reduces the effective angle of attack of the tailplane (usually the tailplane operates at half the angle of attack of the main plane). The reduced angle of attack of the tailplane is termed as the Tailplane Flight Longitudinal Dihedral.

(c)    Tailplane Riggers Dihedral.    In order to offset the reduction in angle of attack due to mainplane downwash, the angle of incidence of the tailplane is usually rigged to some positive incidence relative to the main plane. This is referred to as Tailplane Riggers Dihedral.

(d)    Trim Drag.   (Very Important fro RC Flying, if your aeroplanes nose is too heavy you will add to apparent weight to the aeroplane in terms of trim drag) In order to maintain straight and level flight, the tailplane is required to produce an up/down force to counter any imbalance between the thrust - drag and lift - weight couples. If the tailplane has to provide a downward force, this additional force adds to the effective weight of the aircraft. The increase in weight has to be countered by increasing lift (by increasing speed or angle of attack). The additional drag generated in this process is called trim drag.

(e)    Case of Reducing Speed and Stabilized Low speed.    When speed is being reduced, the stick is being brought back, i.e., the elevator is deflected upwards, implying that the tailplane is generating downward force (lift). However, this is a transient phase. In a stabilized low speed condition of level flight, the center of pressure has moved up (as the angle of attack is higher) and the nose down lift-weight couple is reduced. This would cause the nose down pitch to reduce. Now, to maintain straight and level flight, the force being produced by the tailplane is less downward. The converse occurs in a case of increasing speed and stabilized High Speed.

f.    Variation of Speed in Level Flight.    For level flight the lift must equal the weight. From the lift formula (L= CL ½ ρ V2 S) it can be seen that, for a given aircraft flying at a stated weight, if the speed factor is decreased, then the lift coefficient (angle of attack) must be increased to keep the lift at the same value as the weight.

g.    Aircraft Attitude in Level Flight.     At low speed the angle of attack must be high, while at high speed only a small angle of attack is needed to obtain the necessary amount of lift. Since level flight is being considered, these angles become evident to the pilot as an attitude, which will be nose-up at low speeds and nose lower at high speeds. The difference between low and high-speed attitudes is most marked on aircraft having sweptback or unswept wings of low aspect ratio,