MAC And Neutral Point
Enter the following data:	Wing	Tail
Root Chord (A, AA):
Tip Chord (B, BB):
Sweep Distance (S, SS):
Half Span (Y, YY):
Distance between wing & tail root LE's (D):
Desired Static Margin (5% = twitchy 15% = mushy 7-10% recommended) :
When making a change in this section, be sure to also click the Weight & Balance calculation button, too (but only if you've filled in that section).
Weight And Balance
	Weight	Arm
Left Main Wheel:
Right Main Wheel:
Nose or Tail Wheel:
Pilot:
Passenger/Cargo/Accessory:
Fuel Tank (gallons of gas):
Datum Point from Wing Root LE (neg is aft):

Calculated Results
Mean Aerodynamic Chord (MAC) =
MAC distance from root chord (d) =
Wing Aerodynamic Center aft of root LE =
Ideal Center of Gravity %MAC =
Ideal CG aft of root LE =
Actual Center of Gravity %MAC =
Actual CG aft of root LE =
Actual CG from main axle (neg. is aft) =
Neutral Point %MAC (NP) =
Neutral Point aft of Wing's root LE (NP) =
Distance between Ideal CG and NP (E) =
Distance between Actual CG and NP =
Desired Static Margin % =
Actual Static Margin % =
Gross Takeoff Weight of Aircraft =
Empty Weight of Aircraft =
Total Moment of Aircraft =
Other useful results
Wing Area =	Tail Area =
Wing Aspect =	Tail Aspect =
Wing Taper =	Tail Taper =
W sweep C =	T sweep CC =
W sweep Angle =	T sweep Angle =
Tail MAC =	T MAC dist =
Tail Arm =	Vbar =
Lt wheel mnt =	Rt wheel mnt =
Ctr wheel mnt =	Pilot mnt =
Passngr mnt =	Tank mnt =

The Equations

Wing Sweep, C =	(S * ((2 * B) + A)) / (3 * (A + B))
MAC (length) =	A - (2 * (A - B) * (0.5 * A + B) / (3 * (A + B)))
MAC location, d =	Y * ((A - MAC) / (A - B))
wing area, WA =	2 * Y * ((A + B) / 2)
tail area, TA =	2 * YY * ((AA + BB) / 2)
wing aspect ratio, ARw =	(2 * Y) / ((A + B) / 2)
tail aspect ratio, ARt =	(2 * YY) / ((AA + BB) / 2)
Tail Arm =	(D - wing AC) + tail AC
tail volume, Vbar =	(TA / WA ) * (Tarm / MAC)
NP (%MAC) =	0.25 + (.7 * Vbar * (As / Aw) * ( 1 - de / da));
ideal CG (%MAC) =	NP - Desired Static Margin
actual CG (%MAC) =	((Total Moment / Total Wt) - (Datum to LE + C)) / MAC
actual static margin % =	(NP - actual CG) / MAC * 100

Center of Gravity (CG) is the point where the weight of the aircraft is balanced.

Neutral Point (NP) is the point where the aerodynamic forces of the wing and tail are balanced.

25% - 35% MAC is the typical range for the CG of a conventional aircraft .

Static Margin (SM) is the percentage of MAC that NP is located aft of CG. A lower margin around 5% means you have a tail heavy condition that produces less stability and greater elevator authority, while a higher margin around 15% means your have a nose heavy condition that produces more stability and less elevator authority. A static margin greater than 15% (too much stability) may result in elevator stall at take off and landing.

Vbar is a unitless volume coefficient, based on the ratio of the wing surface area to the horizontal tail group (stabilizer and elevator) surface area and the distance between the wing's AC and the tail's AC. It quantifies the tail's aerodynamic effectiveness and is used to calculate NP. Typical values range from 0.35 (smaller or closer to the wing, thus less effective) to 0.6 (larger or farther from the wing, thus more effective).

CG & MAC equations adapted from the Public Domain.
NP equations courtesy Alasdair Sutherland (see C.G. Position article on Bloobird Radio Flyers)
Neutral Point and Weight & Balance Calculators developed by Dean A. Scott and are (c) 2005 - , all rights reserved. Unauthorized use of the codebehind, html, or content is strictly prohibitied.
Additional thanks to Geoff Haynes, ex-president, HMAC, for refinements to the code.
Email the author at dascott.mfa / at / gmail.com to obtain copyright clearances to redistribute, repurpose, and/or reuse all or parts of the code and content herein.