Laminar and turbulent.  Which flow type occurs on the wing's surface depends on the wing's 

	form
	surface's roughness
	chord length
	airspeed
	and the density to viscosity of the air.

Re (air density/air = viscosity) x air speed x wing chord

Air viscosity  (is measured in kilograms per meter per second)
     The standard value is: 0.0000179=20 kg/m/sec.

     For instance:

A wing with a chord of 1 meter at an airspeed of 1 m/sec and with the standard air density and viscosity will have the following Re:

(1.225/0.0000179) x 1 x 1  = 68459

 Thereby, a simplified formula may be obtained as follows:
         
 Re = 68459  x V x L

	V = airspeed in m/sec
	L = wing chord in meters.

Re increases as the airspeed, air density and wing chord increases. 

Since the wing chords of model aircraft are often much less than 1 meter one may get a Re value close enough for modeling purposes by one of the following simplified formula:

	Kilometers   Re = speed in  kilometers per hour x chord in
	centimeters x 189 (Metric units)
	MPH Re =3D = speed in miles per hour x chord in inches x 770
	(Imperial units)

    With a small wing chord (such as a model aircraft) the air viscosity is a dominant factor, whereas with the full-sized aircraft the viscosity effects of the air are insignificant while the aircraft's mass inertia becomes more dominant.
     That's why  one should not expect a scaled model aircraft to have the same flight characteristics as its larger counterpart.
     A  large wing that is flying fast has a higher Re and thinner boundary layer than
a small wing that is flying slow. The boundary layer is thinnest when its flow is laminar and thickens when it is turbulent.
     The turbulent flow may separate from the wing's surface producing  more drag and decreased lift, which may lead to stall.
     Thus, a  Re wing is more likely to suffer from laminar separation and to stall sooner than a wing with high Re.

     The area of  the flying surfaces (wings, fin and stabilizer) as well as the control surfaces (elevator, rudder and ailerons) should be proportionally larger in the model aircraft in order to obtain more controllable flights and landings.

     Wing loading is also more critical with smaller models. That means, a bigger model may get away with a greater wing loading than  a smaller one..

Some basic rules of thumb may be followed  when designing an easy flying trainer model according to the  pictures below:

Start by choosing the desired wing chord or wingspan
(Related dimensions may be calculated from the WS or chord).

High wings 

The dihedral angle is should stay between: 

	Without Ailerons 3 to 6 degrees
	With Ailerons up to 3 deg

A washout angle between 3 to 5 degrees is advisable in order to improve stall characteristics.

Motor thrust angles  

are usually found by trial and error.

	Flat bottom wings may need more down thrust than
	symmetrical and/or semi-symmetrical ones.
	Initially one may start with 2 to 3 degrees down and
	right thrust.
	The wing's and stabilizer's incidence may preliminary be  set at zero, and may be changed during test flights.

Landing gear placement 

	Tail dragger  axle coincident with the leading edge of the wing
	tricycle the main gear should be slightly aft of the CG balance point in order to get easier take-offs.

A tail-heavy aircraft will be 

	more unstable
	more susceptible to stall at low speed e. g. during the landing approach.

A nose-heavy aircraft 

	More difficult to takeoff from the ground
	More difficult to gain altitude
	Will tend to drop its nose when the throttle is reduced
	Requires higher speed to land safely.

	Gives high lift flying upright
	Poor lift at inverted flight
	Used in slow and relatively light powered models.

	Gives good lift at upright
	Gives slightly less lift inverted.

Gives equal lift upright and inverted

	Too much throw causes the aircraft to respond too quickly making the aircraft difficult to control
	Too little throw will result in to little control especially at  low landing speed.

6mm (1/4") up and down.

	5/16" (8mm) up
	5/32"  (4mm) down.

3/8" (10mm) left and right.

Moving the push rod connection to close to the control surface will increase the travel of the control however it will allow the control to flutter at airspeed.

Some transmitters have dual rate facility, which allows the pilot to change the max throws to suit the flying speed. 


Material and construction methods

	Strive to build as light and as strong as possible.
	Wing loading is the aircraft's weight divided by the wing area.
	In order to avoid stall a plane with high wing loading requires higher take-off and landing speed
	Two different sized planes with the same wing loading will have about the same stall speed but the smaller one will be more difficult to control especially during landing approach.

Typical wing loading with a 60 in  (150cm)   wingspan model is 
about 19-oz/sq. ft
(60g/sq. dm). 
(This value may be slightly higher with bigger models) 

Example 

The wing loading of a full-scale Cessna 152 is about 167-oz/sq. ft
(510g/sq. dm) 

A model aircraft with such a wing loading would hardly be able 
to fly.

	For instance a full-scale Piper J-3 Cub weighs 1000lb and has 36ft wingspan. A 1/6 scale Piper J-3 Cub model should weigh 1000/63  4.6lb
	The wing loading of the scale model should be reduced by the  scale factor's ratio.
	The 1/6 scale model should have 13.3 oz/sq. ft wing loading  instead of 80 oz/sq. ft as the full-size Piper J-3 Cub.

	1ft = 0.3048m
	1in = 2.54cm
	1lb = 16oz = 0.4536kg
	1oz =  28.35g
	1sq. ft = 144 sq. in