This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
First, build a few kits or plans and get some experience with
gliders. Designing a successful glider is a lot more
complicated than designing a successful rocket. Once you've
mastered building, trimming, and flying some existing designs
you are ready to try your own.
Glider stability is similar to a rocket stability, but a bit
more complicated. The equivalent to a rocket Center of
Pressure (CP) is called the Neutral Point (NP) of a
glider. There is an article on how to calculate this in the
1980 MIT Journal available from NARTS. I've used a program I
wrote (FORTRAN-IV for RT-11 and VMS) in the early 80s to
calculate the NP. Versions of this program for DOS and LINUX
are finally available on the net at
Just as a rocket CG needs to be ahead of the CP, a glider CG
must be ahead of its NP for it to be stable. 10-20% of the
wing chord (the distance from the leading edge to trailing
edge of the wing) is a good margin for free flight models. RC
models can get by with much smaller margins.
Here is a sample output of my neutral point program. The program itself is
based on a paper presented by Guppy at MITCON-11, and a later summary in the
The Flat Cat data is from the Quest model, emailed to me by
Andy Eng. I make no claim to its accuracy. I.E. if it's wrong,
it's Andy's fault. Andy specified all the surfaces as being
1/8" thick! I "corrected" this, making the stab and rudder
1/16", which sounds more reasonable to me. Besides, the
thickness is just used to calculate the frontal area; it
doesn't affect the NP calculation. I was surprised to see the
NP at 85% of wing chord. I'd really like someone to try
trimming this model with the CG at 70% back and let me know
how it glides.
Description: Flat Cat | | Wing: Span= 15.500 Root= 2.7500 Tip = 1.3000 PSpn= 14.671 Swep= 1.4500 Thck= 0.12500 Dihd= 2.5000 Angl= 18.819 Frnt= 1.9375 > MAC = 2.0250 Area= 29.710 AR = 7.2451 Xac = 0.42413 L =0.81490E-01> Stab: Span= 6.5000 Root= 2.0000 Tip = 1.0000 PSpn= 6.5000 %Wng= 0.32818 Swep= 1.0000 Thck= 0.62000E-01 Dihd= 0.0000 Angl= 0.0000 Frnt=0.40300 > MAC = 1.5000 Area= 9.7500 AR = 4.3333 Xac = 0.41667 L =0.65391E-01> Fin: Span= 1.5000 Root= 2.0000 Tip = 1.0000 PSpn= 1.5000 %Wng= 0.75733E-01 Swep= 1.0000 Thck= 0.62000E-01 Dihd= 0.0000 Angl= 0.0000 Frnt=0.93000E-01> MAC = 1.5000 Area= 2.2500 AR = 1.0000 Xac = 0.41667 L =0.30000E-01> Front: Dia = 0.75000 Hgth= 0.50000 Thck= 0.25000 BstA= 3.0003 GldA= 2.4960 Bdia= 1.9545 Gdia= 1.7827 Semi= 1.1474 > Tail: Momt= 6.0000 QCM = 8.4170 TVC = 1.3641 FVC = 0.43448E-01 Wash=0.70190 > The Neutral Point is located at 85.38% ( 2.348 units ) from wing L.E. NPg = 1.1540 Stability factor 0.20 Put CG at 70.65% ( 1.943 units ) from wing L.E. NPb =0.99614