Dynamically Similar Values
The identified parameters listed in
the table below are followed by the corresponding Dynamically
Scaled Values on the right which are provided in a decimal or fractional
measurement of a full scale aircrafts
corresponding parameter. These should be used to calculate a
scale models
dynamically similar value. The table calculates scale
factors for "K" which are decreased from 1/2 to 1/10 of full size
aircraft.
This chart was born from a truck
load of research gathered to develop an efficient airfoil
for a 1/4 scale P47 project as well as other common
disagreements that seem to loom over various scale modeling
meets that I've had the privilege of attending.
Example Conversation
Considerations:
What is Scale Speed?
How many times have you heard at the field "well the
full size P47D30 (for example) had a top speed of 428 mph so a
1/5 scale should fly at 85.6 mph (428 mph times 20%), that's
what scale would be!" Not True! If you
use this example and the factors in the chart below, a
1/5 scale model should have a top speed of 174.62 mph to be dynamically
similar to its full scale equivalent. Since a P47D30 had a cruising speed of 350 mph, than scale cruising speed for the judges
of a 1/5 scale version should be
142.8 mph. Also the P47D30 had a landing threshold speed
of 105 mph, or at 1/5 scale 42.84 mph  that's why you might
feel that this particular 1/5 scale model flying around at 50
mph looks a little like it's going to fall out of the sky!
Enlarging/Reducing Plans
& Wing Efficiency? In modeling and UAV development, compromises in weight
from those determined below may be necessary due to
inadequate runway facilities, engine size (power), air
density (elevation), desired performance imbalance (3D
flying) and/or piloting skills. Such deviations are
unavoidable particularly as model sizes (Scale Factors) are
decreased significantly. Also, Reynolds Numbers are
distorted when enlarging/reducing scale plans, therefore lift
efficiencies are greatly decreased depending on the final
Models or UAV's scale size selected. While the plane may fly
many of the efficiencies or total airframe balanced may be lost.
(Thanks Pat!  Advanced Aero Technologies, St Paul, MN)
