How to use:
- Select the units of measurements.
- Take your wing or entire airplan and align it in a right angle to a wall.
- Approximate your wing with max. 5 trapezoidal panel including
the panel within the fuselage - see examples:
Grumman X-29 Sukhoi Su-29
- accurately measure chord (R & T), sweep (S) and panel span (W) of each trapezoid according
sketch on top.
Remark¹: for extreme wing dihedral (V-shape) or for all
V-Tail use the planform dimensions projected onto the horizontal
- select the type of your tail (standard stabilizer, T- or
V-tail, canard, flying wing or delta) and repeat 2. to 4. for your
- measure the distance (D) from the leading edge (LE) of the main wing to
the leading edge of the stabilizer (see abouve).
- define the static margin.
- Verify the wing drawing does correspond to your airplane
- Verify the wing and tail span do match the span of your plane
- Verify the wing area corresponds to manufacturers information
- The Center of Gravity is measured in the middle of the
fuselage from the leading edge (LE) of the
main wing. Positive value are
towards the back, negative towards the front of the aircraft.
- Use a rather conservative CG value for inflight evaluation and
approch a lower static margin (decreased stability) in small
Other Examples: (Click on the examples for
It has been found both experimentally and theoretically that, if the
aerodynamic force is applied at a location of 25% of the Mean
Aerodynamic Cord (MAC), the magnitude of the aerodynamic moment
remains nearly constant even when the angle of attack changes. This
location is called the wing's Aerodynamic Centre (AC).
The AC value is always measured from the Leading Edge (LE) in the
center of the corresponding wing.
The Neutral Point (NP)
of an aircraft is the point where the
aerodynamic forces are balanced. Having two or more wings interacting on your
aircraft (e.g. main wing and tail) they influence the aerodynamic forces to
your aircraft. The NP value is always measured from the leading edge
(LE) in the center of the main wing.
The «tail effectivness» influences
the NP position and does not only depend on it's size, but also it's
location relative to the main wing.
V-Tail¹: project the V-Tail
horizontal plane and use the projected dimensions.
Flying Wings & Delta: Do not have a tail (second wing). Therefore Aerodynamic Center (AC)
and Neutral Point (NP) are identical.
Although the stabilizer is in front of the main wing, the stabilizer
has to be defined as «tail» wing. However, make sure the Distance
between main wing and tail (stabilizer) is defined as a
The Center of Gravity (CG) is the
point where the aircraft's weight is balanced. The CG value is always
measured from the leading edge (LE) in the center of the main wing.
stability the CG is placed 5% to 15% of MAC in front of the NP. This
margin for stability is called Static Margin. A lower
static marging will result in less stability, a grater elevator
authority (agility) and a more «tail heavy» aircraft. But any CG byond
NP will lead to uncontrolable flight conditions and aircraft upset.
higher static margin creates more stability, less elevator authoritiy
(slugish pitch) and a more «nose heavy» aircraft. Too much static
margin may lead to an elevator stall unable to pitch the aircraft for
take-off or landing
typical conventional aircraft design the CG is between 25% to 38% of
The Stabilizer Volume (Vbar) is a value for
maneuverability. The lower the more agile the aircraft gets. Typical
0.5...1.1 High-lift Jet
0.0 for Delta & Flying Wing (due missing
optimal Center of Gravity must be evaluated in flight. For safety
CG evaluation always in a conservative manner with a static margin of 15...5% for a good
longitudinal stability. Optimize CG in small steps only! Never
ever exceed CG on maiden flight!
Limitations - what does cgCalc
does not provide aerodynamic performance analysis.
- Propulsion and aeroelasic effects on incidence and dynamic
stability are not covered.
is not able to calulate NP of bi-planes.
- Canard: For canard configuration the stabilizer is significant
smaller than the main wing. For tandem wings use the «std
- Fuselage: cgCalc
does not take into account the lift effect of «fat» fuselage. Having
a fat fuselage in front the main wing, use an additional 5% static
margin (see Sukhoi example uses rather 15% than 10% static margin).
- Jets with intake below or ahead of the wing and twin
aircraft with wide nacelles do have a significant destabilizing
effect and is not taken into account by cgCalc.