when i flew my rc plane, it goes up but with in 2 second it goes down with nose down and hits the ground. can any buddy suggest me possible defect in my rc plane and how to rectify them.

First guess is stall.  Next guess is signal loss but even with signal loss, if your model's CG is correct, it should be a nice glide down.

Post some details of your model, and where the CG lies

In this case your airplane is very tail heavy and also your elevator throws are extremely high, I would suggest that you please re check the CG and for initial few flight keep the nose slightly heavy. Also if you are using a Futaba radio then please put minus 40 exponential on elevator and ailerons, in case of JR please add plus 40.
Regards 

here is some pics of rc plane balancing with help of pencil on thermacol.
is CG is correct or not?

If the CG is correct then please check the elevator deflection

Have you tried the glide test?  Install the lipo but don't connect it.  Find some tall soft grass.  Hand launch it to see if it glides.

It seems to me the wings are too close to the elevator. Tail moment might be an issue here and taking away from stability.

Next time you go for scratch build try to use the proportions from the J3 Cub.  For the plane as pictured, I would mount the wings a couple of inches forward and mount the battery directly the motor to get the CG.

Double check the travel to make sure the servo isn't reversed.

In all cases it is a stall, not enough lift to keep the craft afloat...

As a rule of thumb the cg should be about 1/3rd of chord, or just a little aft the thickest section of airfoil... Do post a side view

What is the flying weight of the plane and what is the power plant and prop? Can't spot a landing gear so most likely you are hand launching ... How far does it travel after the launch before dropping... and the launch is level into incoming wind?.. A video is a 1000 pictures (literally)

A glide test would be perfecto

motor power 1000kv
chord = 16cm, wing span = 1m , horizontal stabliser's area= wing area/ 4, vertical  stabliser area = 50% of horizontal, length = 75 cm

side view

There are many flyers in Lucknow. Take help from some experienced flyer

That motor looks like a 2830 1000Kv, should produce roughly about 175 useable watts on a 3s and a 9x6 propeller..have you metered using a wattmeter. What would be the weight of the plane? What prop?

Is it just me, or does the incidence of the wing look too high?

you are right, the wing seems to be too much inclined towards the top. this could cause stall.

make some glide test , power in off position throw it straight to your eye level not too high not in the down position ,

if it glide to some distance then

give full throtle and throw it to the eye level i mean straight little bit upward but not too much

if you trow it too much high angle then it will stall that could be your case i am not sure
if you trow it down side then it will go straight to the ground.

if c.g , thrust and balancing is proper then chance of sucess is more.

Main wing and elevator angle of attack should be in same line. you can put some packing in back side of main wing.
Black line is close the CG.

From the pictures, the wing angle of attack looks high (inviting a stall). However, a stall is characterized by a momentary loss of speed or a nose-up situation prior to the nose-dive crash. (Does this happen in your case)?
As observed earlier, the CG should be ascertained along with the full delivery of the motor (should be at least three-quarters of full throttle at launch). Also, wind direction (launch against the wind) should be ascertained.
Last, but not the least, the elevator movement is to be checked. A 3 to 5 degrees up-elevator during launch is typical.

I feel the plane is just underpowered given the other factors the attack angle, the large wing span, elevator neutral, aft cg, almost symmetrical airfoil, up/down thrust ... anything as little as a slight bank and it'll descend... and we still don't know what the flying weight is sire... It seems just about a Kg and with that wing area, the wing seems lightly loaded... so while stall speed shouldn't be too high... maybe the power system is flying it only notches higher than the stall speed...

here is an underpowered plane flying notches above its stall speed, see the landing and its tendency to descend...



Any common symptoms?

It will be most useful if you fly with someone at the local field, diagnosis will be easier... The medic almost always needs to see the patient than the papers... 

Quote from: ayub on April 30, 2014, 12:29:54 AM
Main wing and elevator angle of attack should be in same line. you can put some packing in back side of main wing.
Black line is close the CG.

+1 Incidence misalignment can cause this. Try to make sure the wing and stab are in the same angle with the thrust line.

Thanks everyone for suggestions. Main problem was wing angle of attack. Today it flew good. Thanks every one once again.

Great to know that. Post videos of flying.