Mark Drela's Allegro 2m
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Home > Articles & Tips Index > Mark Drela's Allegro 2m Composite

[Courtesy of Mark Drela, drela "at", May 2000 - web presentation by Joel Foner, joel.foner "at"]

Construction notes are included in the drawings, which you can download as Acrobat v6 PDF documents in the links below. If you need a copy of the Adobe Acrobat Reader, you can download it free from


For some of you, Mark Drela needs no introduction...  For the others, Mark has held indoor HLG records and innovated in many aspects of HLG design for the indoor crowd. He is a well known aerodynamicist at the Massachusetts Institute of Technology, and as some of you have read on the Soaring Exchange, Mark is also working to see if he can get his XFoil airfoil design code released to the public domain by MIT.

Understanding the Allegro 2m

The Allegro 2m is the outcome of his RC 2m design analysis work this year. The Allegro is a surprising sailplane, from a couple of perspectives. One is the Allegro's "outrageously thin wing", which has proven so far to be winch-proof.  This thin, low drag section, combined with custom designed airfoils (Mark seems to never be satisfied with using a "standard airfoil" for his designs!) leads to a very high performance sailplane.

Mark with the prototype Allegro 2m ship

Drawings and Construction Notes

The Allegro is designed for 2m RES (rudder elevator spoiler) competition tasks. The Allegro uses a set of custom-designed airfoils, tuned for the airframe design parameters.  Detailed construction notes and target weights are provided in the plans - please pay close attention to these if you wish your Allegro to have the strength it needs, and the performance it should have!

Allegro Construction

Allegro 2m plan (Acrobat PDF file    CAD DXF file)

Allegro 2m fuselage details (Acrobat PDF file    CAD DXF file)

Allegro 2m tail shaping data and diagram (also see "Accurate shaping of solid balsa surfaces without templates" below - 20 kB Adobe Acrobat PDF file)

Building the Allegro "winch-proof" spar (10 kB Acrobat PDF file)

Building the Allegro "winch-proof" joiner (15 kB Acrobat PDF file)

Accurate Shear Web Cutting (13 kB Acrobat PDF file)

Spar and Joiner Construction Notes:

The Allegro "winch-proof" spar and joiner system is the result of both laboratory stress to failure testing and real-life testing on our club winches in contest conditions. On the plans, you'll notice that the spar joiner tapers in width from the center towards the tip. This taper is intentional, and allows the joiner to bend in a controlled curve, presenting a uniform load along the joiner box, rather than a point load at the ends.

The panel spar segments are first individually assembled. Before the spar skin is put on, the core is hogged out at the ends and the joiners and joiner boxes are glued in its place (this procedure is outlined in 'Building the Allegro "winch-proof" spar' document above.

The joiner areas get additional spar skin layers since the shear is vastly greater there. Over most of the spar the shear is some fraction of the max wing lift (less than 100 lb on one wing). Over the joiner the shear is equal to the spar cap load -- over 1000 lb.

Each spar segment gets built completely separately. There are five of these on the Allegro. The two outer breaks also get a little joiner and its box. This one is so tiny that just splooge flox/epoxy around it rather than use the basswood.

After the completed spar segments (with joiners and boxes) get installed in the cores and the panels are Mylar/bagged, the small outer joiner gets glued into its box making this joint permanent. The joint also gets a glass strip all around. I don't have any detail for this outer joiner since almost any reasonable joining method would work here. The outer joiner could be plywood for example.

Airfoil Coordinates and Polars

Airfoil Coordinates  
Compufoil COR files
ag16 ag16.dat ag16.cor ag16_polars.pdf (32 kB Acrobat PDF)
ag17 ag17.dat ag17.cor ag17_polars.pdf (26 kB Acrobat PDF)
ag18 ag18.dat ag18.cor ag18_polars.pdf (31 kB Acrobat PDF)
ag19 ag19.dat ag19.cor ag19_polars.pdf (26 kB Acrobat PDF)
ht08 ht08.dat ht08.cor tail section 1
ht12 ht12.dat ht12.cor tail section 2

Polar Comparison with Other Typical Airfoils

Related Construction Notes

Ultra-light tail construction (5 kB Acrobat PDF file)

Here is a view of the Allegro tail

allegro2m_tail_closeup.jpg (66787 bytes)

Integrated cloth hinge design (8 kB Acrobat PDF file)

Accurate shaping of solid balsa surfaces without templates (12 kB Acrobat PDF file)

Best of luck, and please let us know how you're doing with your Allegro 2m!

Document Version History

January 27, 2004 - jaf
- added CAD DXF files from Tomer

October 30, 2000 - jaf
- added tail shaping tangents drawing

October 11, 2000 - jaf
- Added "Accurate Shear Web Cutting" to the construction section

October 8, 2000 - jaf
- Added CompuFoil COR files for each of the airfoils, to make construction easier for those without a simple DAT file converter process

July 5, 2000 - jaf
- Added "Building the Allegro "winch-proof" spar
- Added "Building the Allegro "winch-proof" joiner
- Added spar and joiner construction notes
- Added pictures

June 27, 2000 - jaf
- "all new"


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