Even if stiffners like carbon rods are used, I have noticed that when the model flies (the 48" span scratch builds that I make), the dihederal appear more than when on ground. Is this normal or am I doing something wrong?

Pankaj

not having much knowledge but i think it is normal at occurs due to variation in temp. & pressure by variation of height[just a guess-seniors correct me if i m wrong]

It is quite normal for a wing which is well reinforced to flex in flight, expecially when it is 48" wing span. As long as it is maintaining its original shape and not breaking due to forces of flight it is OK.

You will not be able to eliminate wing flexing inspite of the best reinforcement. Longer the wing more will be the flex.

Airbus wing also flexes and more than a fighter jet.

If you happen to take a window seat next to the wing on an airbus you will notice that the wing of the airbus also flexes when it hits a bad pocket of air and has rough flight.

Theoretically, you should be able to make a wing with pretty much zero flex, if you reinforce enough. 

What if the wing was made entirely out of CF rods laid out in parallel and tied/glued together ? Now that is an interesting build idea (Of course, deep pockets are assumed!)

Theoritically yes agreed but practically next to imposible to make one even with any amount of reinforcement even at the cost of loosing its aerodymnamics value.

Hmm... I have overhead wings, so with the CG below, I guess, the wings would tend to flex a bit. I guess that is the reason why some models have spars running from wing tips to the base of the fuse...

Pankaj

If you make a bridge, half and fix at one side, free on other, the bridge free end will be slight down due to gravity. again if you poor equal sand throughout the bridge, the bending is more and free end will come more down.

In case of Wing of an aeroplane, the thing is just reverse. Insted of vertical load due to gravity, the wing lift due to air velocity.
If the plane speed increase or dencity (Pressure) increase, lifting power per square area increases just similar to sand loading. and bending moment increases and bending will be more.
This is the bending moment and you can get it any book of Strength of materials.

It solely depends upon weight of fuselage where you need to calculate wing loading !
do this hold  wingtip to wingtip on ground if it bends too much than fuselage weight is more ,if not ,only in flights thats due to 'G' force acting on plane when your climbing ,pulling up from diving etc

even heavy aircraft like B747,b777 n A330 wingtip will raise during takeoff
especially B747-400 series with increased wingspan raises almost 1-3mtrs depending upon weight