# 8.2.9 I'd like to design my own glider. How do I know if it will work? How do I compute the CP for a glider?

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

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

ftp://eagle.he.net/pub/cssinc/Private/NP.zip.

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
MIT Journal.

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
```

There are several good articles on Boost Glider Stability in old Model
Rocketry Magazine and Model Rocketeers. Reprints of many of these are
available from NARTS and/or NARTREK.

When scratch building, selecting good balsa wood is
important. SIG has a great reference on balsa grain and
density in their catalog. Look for pieces of wood with
straight grain, and no knots or swirls. For wings and stabs
choose as uniform a piece as possible so you don't have
density variations in the surface. Also avoid splits and

There's an article in the MARS Pathfinder newsletter with a
V-tail BG design and calculations at:
http://www.marsclub.org/path/MP15-3.pdf

Continue to: