Can scale model airplanes fly like the full size?, scale power, wing incidence, wing loading, CG, etc. A friend says they must be modified. Is he correct?.

Bob he is correct. The problem with any scale besides 1:1 scale is that the air molecules can't be resized. If we could use 1/4 size air the we could mimic the full scale in every way.. but we can't so we have to make up for it in other ways.. usually speed is where we make things up.
Our propellers are not as efficient as the full scale due to their smaller sizes.. so we typically run much faster RPM"s to make up for that. I think the CG can be the same and the incidences can be the same or close.
A P47 weights about 13,500 lbs for take off.. a 1/6 plane would need to weight 2250lbs!.. 1/6 the weight of the full scale. So you see.. things do change on the models.. The larger the models the closer you can get to the full scale in many areas..

Thanks Bob, very good explanation.

Weight is the cube of the scale. A 1/6 scale object with the same density would weigh 1/216th the weight of the full scale. In the case of a plane weighing 13,500 lbs, the 1/6 scale version would weigh 62.5 lbs. A 1/6 scale P-47 would have a wingspan of 81.5", a wing area of 600 sq in, and a wingloading of 240 oz/sqft. For a 1/6 scale plane to fly, you need to be closer to 60z/sqft, so consessions need to be made with the materials used to build the model.
Another problem is that the engines we use are lower torque, higher RPM than full size engines. As a result, we need to turn smaller blades in order to achieve reasonable performance. Our engines are not capable of turning scale props. If you try, the performance would not be enough to get airborne, if you could even get the engine to run. The only way to get close to a scale prop would be to gear down the speed of the prop with either a geared engine such as the RCV SP series, or with a gearbox like the Byron. Electric motors may also aproach the torque needed to turn a scale prop. In order to get reasonable performance, you would still need to turn at higher RPMs and fly at higher than scale speed.

Scott

Bob:

Many competition scale models are very true to the original in areas such as airfoil, incidences, etc. I wouldn't make a blanket statement that they "must" be modified.

That does not mean that they fly well...

I have found that the incidence of the fixed horizontal stabalizer must be modified to make the model fly correctly, I always set up my models with 3 degrees of wing incidence and zero degrees stabalizer incidence regardless of what the full size aircraft had (example: Grumman Wildcat).

You can also calculate the scale weight, wing area, horsepower and other factors. But like mentioned above our propellers turn at much higher RPM's and much smaller diameter. If you took a P-51 and scaled down the weight, horsepower and overall size it would be underpowered, overweight and would barely taxi. The model P-51 must be much lighter than scale weight and have more horsepower to fly correctly. This affect becomes more noticable as the model gets smaller and smaller. This is why Giant scale models fly better than smaller scale models.

Hey Scott, call me when you test fly your 1/6 warbird at 60oz/ft2 - that baby's gonna scoot

Seriously, go to an F4C comp and you'll see that scale sections, incidences etc CAN fly, but not with scale power, props, loadings or airspeeds. Bob's right - can't scale air, gravity, inertia etc. Still, if it was easy, where would the fun be in that!

Cheers,

Cam

Hi!
All flight surfaces and incidences could be scale! But you can't scale down weight.
What's important in a scale model is to build it light! Then all airplanes will fly at near scale speeds and be very easy to fly. Build them heavy and you will have to fly them fast.