Mark Drela's Apogee HLG
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Home > Articles & Tips Index > Mark Drela's Apogee HLG

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

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 http://www.adobe.com/products/acrobat/readstep.html.


Table of Contents


Is the Apogee the Right HLG for me? [top]

The Terminator, which was our first web-published design (designed by Bill Grenoble and Denny Maize), was an effort to bring composite construction and HLG's to first-time builders. As such, the Terminator HLG site includes numerous construction details and a design that is fairly insensitive to design changes and construction "oopses".

Unlike the Terminator, which was designed for the first-time HLG builder, the Apogee is a (very) high performance design. Obtaining the Apogee's potential requires careful construction and a constant (obsessive?) attention to weight and construction details.  In return for this effort, if built from properly chosen materials, constructed carefully and accurately, the Apogee will surprise you with its capabilities.

If you have experience building light wood or composite airframes, have a gram scale and are used to using it while building, like building from scratch and want to build a world class HLG, the Apogee is for you!

Construction notes are included in the drawings, which you can download as Acrobat v4 PDF documents in the links below. If you need a copy of the Adobe Acrobat Reader, you can download it free from http://www.adobe.com/products/acrobat/readstep.html.


Introduction [top]

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 Apogee HLG Series [top]

The Apogee HLG series is the outcome of his RC HLG design analysis starting in 2000, and continuing with some recent updates. Comments on the Apogee, from those who have seen it fly, range from "wow" to "very surprised" to "how did he do that?".  There are several flying and in construction in the club, and we expect that once you build one, the number will multiply at a "pretty fast rate".

The Apogee HLG is the result of some careful thinking about the nature of the flight requirements of a high performance hand launched sailplane. 

One of Mark's early observations was that in hand launch, launch altitude is one of the most important parameters of a successful design. You can do some things to influence sink rate (the "hang time") of a design, but if you can get 20% or more launch height advantage, this will swamp any minor hang time advantages.  

A perhaps even more important consideration is maneuverability. This is where the small span of the Apogee really has a considerable advantage over "full-size" HLG's - even aileron "full-house" designs. The Apogee's fast roll rate, tight circling radius, and docile stall behavior allow very small or spotty thermals to be worked very close to the ground with confidence. The fast control also allows safe flying in high turbulence which grounds most 2-channel 1.5m gliders.

The Apogee hand launch glider obtains thermaling performance primarily via high launch and tight circling capability. Its key design features are:

  • small size -- 30-40 in. span, 125in^2 (30") -190 in^2 (40") wing area
  • light weight -- 2.75 oz (30") - 4.2 oz. (40") weight, 3.1 oz/ft^2 loading
  • very light extremities and fast rudder roll response
  • thin airfoils with 100% attached laminar flow at higher speeds

The all-laminar airfoils give good launches and surprising penetration. In a typical contest it is quite competitive with full size 1.5 meter HLGs. In spotty or choppy lift, the Apogee's maneuverability is strikingly good. An unexpected bonus is that the maneuverability makes the Apogee always extremely fun to fly!

What's better: 30", 36" or 40" span? The main tradeoff is between maneuverability and L/D,  as expected. If you want a "fun" glider, or fly in turbulence near buildings, then the 36" span is probably a better choice. For contest work the 40" span is suggested. One side advantage of the 40" span is that it's easier to build to a sufficiently light wing loading, and allows use of 120mAh batteries for longer flying time.  

What are the differences between the wood and composite Apogees? The 36" wood Apogee was the prototype to check everything out before the CNC metal molds were cut. 

The main difference is in the airfoils. The AG03 on the wood Apogee seems to favor the glide at some expense of launch height. The AG04 on the composite Apogee is a bit more of a "high-speed" section, and its flight behavior bears this out. These differences are quite minor however. The computed AG04 polars are a bit better, but the AG03 can be constructed and maintained more accurately in balsa so it may be better in reality. The AG03 is recommended for the wood Apogee versions.


Here are two 36" Apogee HLG's with a 40" wing.


Apogee Airfoils [top]

Mark designed a new set of airfoils for the Apogee, specifically oriented to the RC HLG flight tasks.  A basic goal of the airfoil design was to  minimize the launch-mode drag of the system, while not trading off cruise performance to the extent that "big glider" airfoils do in this application. 

Many current HLGs use "big glider" airfoils like the SA7035, MH32, etc. On small gliders these have large draggy separation bubbles. Thinning such airfoils, or using "fast" slope glider airfoils like the S6063, only partially alleviates this problem.

The AGxx airfoils were designed for very low Reynolds numbers from the outset. At thermalling speeds, they have quite small separation bubbles for good minimum sink and a docile stall. At higher speeds they exhibit 100% attached laminar flow, resulting in very low launch drag and exceptional penetration. Computed polars indicate very significant overall improvements over the S6063 and other adapted airfoils. The Apogee's performance and behavior matches these expectations.

There are two sets of airfoils, one set for use with molded construction, and another that are designed to be "buildable" with wood wing construction.


Drawings and Construction Notes [top]

The Apogee has been designed in two sizes, 36" and 40".  In addition, each design can be built either with a shaped wood (balsa) wing or a composite wing.

After building an prototype, Mark built fiberglass molded Apogees using a CNC-machined mold.  Not all of us have access to this technology, and there is good news here. The performance of the wood version is almost identical to the CNC-molded version IF you are accurate in your rendering of the airfoil and choose the right density balsa (light!).  If you choose to build the wood version please also refer to Mark's tech note on airfoil shaping in the wood sections below. 

NOTE: Please pay particular attention to the completion weights and construction notes on these plans. Each gram counts...


Molded Wing Composite Apogee Construction (see below for Wood Wing Plans) [top]

30" Molded Apogee Plan (Acrobat PDF file    CAD DXF file)

36" Molded Apogee Plan (Acrobat PDF file    CAD DXF file)

40" Molded Apogee Plan (Acrobat PDF file    CAD DXF file)

40" Molded Apogee Plan - "Tip Launch Rated" (Acrobat PDF file    CAD DXF file)

Molded Wing Composite Construction Notes:

  • The composite Apogee does not use an internal wing spar - the wing skin provides the strength.  
    - An extra layer of cloth was used for the inner 1/3 of the span to increase the local strength near the root.
    - The 40" composite version should be built with carbon fiber "spar" strips in the layup for some extra skin strength, especially if you are a "hard launcher" :-)
    - The 36" composite version does not need carbon fiber spar strips.
  • If you wanted to build a multi-taper (triple, quad) foam core composite wing to approximate the planform shown, rather than go the CNC mold approach, the performance degradation will likely be minimal.  At the same time, the effort would be pretty significant, and compared to the speed with which the balsa wing can be created, perhaps it is not a worthwhile strategy.  If you do give it a try, please let us know how it works out!
Airfoil Coordinates CompuFoil COR files Polars
ag04 ag04.dat ag04.cor ag04_polars.pdf (34 kB Acrobat PDF file)
ag08 ag08.dat ag08.cor ag08_polars.pdf
ag09 ag09.dat ag09.cor ...polars not provided here - flow not really 2d this close to the tip, so the analysis wouldn't be particularly meaningful... 
ag10 ag10.dat ag10.cor ...polars not provided here - flow not really 2d this close to the tip, so the analysis wouldn't be particularly meaningful...
ht05 ht05.dat ht05.cor tail section
Airfoil ag04 ag04 ag08 ag09 ag10
Location
(% of span)
0% 20% 80% 92% 99%

A quick note on the airfoil transitions for the composite version...
- If you recall, Mark's composite design was generated using a computerized numerical control (CNC) system, and thus the airfoils were actually continuously blended between the stations shown above.  The AG08, AG09, AG10 are small modifications to the AG04 to compensate for the lower Reynolds numbers. This is a bit overkill, but overkill is easy with CNC! Blending just the AG04 and AG08 should suffice for more conventional construction.


Wood Wing Apogee Construction [top]

36" Wood Apogee Plan (Acrobat PDF file    CAD DXF file)

40" Wood Apogee Plan (Acrobat PDF file    CAD DXF file)

Wood Wing Construction Notes:

  1. See the "Shaping Data" links below in the Related Construction Notes section for printable tangents for the ag airfoils used in the wood version of the Apogee wing.
  2. The wood wing Apogee has no separate spar. The wing is shaped from solid balsa, and the balsa provides enough strength to support the air loads.
  3. The basswood leading edge on the prototypes was laminated from two layers of Midwest 1/32" basswood.  (The LE adds no spanwise strength - it is really there to provide some crush resistance and help the leading edge maintain its proper shape after more "contacts" with things other than air!)
  4. The 40" wood Apogee can be built with a solid wing IF 4.0 lb balsa is available (weighed 4.0 lb balsa... not "this is pretty light" 4.0 lb balsa :-). This is more durable and will give better airfoil accuracy. Use diagonal splices near tip if using 36" wood sheets. Otherwise it is probably a better deal to stick with the built-up aft section of the wing as shown in the plans.
  5. The AG03 airfoil is used over most of the span, and 
    gradually blends into the AG11 in the last 2 inches at the tip.
  6. The shape of the upswept leading edge (the smooth curve on the front bottom of the airfoil) is especially critical for launch 
    and penetration performance. Please shape it very carefully. "Just sorta rounding it off on the bottom" like someone might do to a Gentle Lady leading edge strip won't cut it on the Apogee. It has to be accurately shaped for this airfoil to work correctly.
  7. If 4.0 lb balsa is not available, one can thin the airfoil 10% or even 15% to compensate for slightly heavier wood. Airfoil computations indicate that the thinner airfoil will penetrate even better, but the float and handling may degrade a bit. Difficult to say for sure without trying it.
Airfoil Coordinates Polars
ag03 ag03.dat ag03_polars.pdf (33 kB Acrobat PDF file)
ag11 ag11.dat ...polars not provided here - flow not really 2d this close to the tip, so the analysis wouldn't be particularly meaningful...

A quick note on the airfoil transitions for the wood version...
The AG03 airfoil is used over most of the span, and gradually blends into the AG11 in the last 2 inches at the tip.


Airfoil Polar Comparisons [top]


Fuselage and Tail Plan Options [top]

Here are a few pictures of the Apogee fuselage and tail...

apogee36nose.jpg (54292 bytes)
Apogee nose

apogee_compositevswood_fuse.jpg (115301 bytes)
A picture of the wood and kevlar versions of the Apogee fuselage

apogee36_compositetail.jpg (115197 bytes) apogee36_compositetail_closeup.jpg (81585 bytes)
And here are two shots of the Apogee tail construction

  • If you really want to build a "standard" tail instead of a v-tail...  Well, first you have to read the two reasons why you should build a v-tail! ;)  
    1) lighter weight - with a standard tail you will almost certainly need to add noseweight  
    2) less likely to get hit on landing.  
    If you still want to build a standard tail, you might want to try the following sizing:
    - horizontal area: 17 square inches
    - vertical area: 9 square inches
    Note:  Treat these as pretty educated guesses, since almost every Apogee is built with a v-tail, but this is better than starting with nothing, right?

Molded Fuselage Plan for 50 mAH receiver pack (Acrobat PDF file    CAD DXF file)

Molded Fuselage Plan for 120 mAH receiver pack (Acrobat PDF file    CAD DXF file)

Wood Fuselage Plan for 120 mAH receiver pack (Acrobat PDF file    CAD DXF file)

Wood Fuselage Plan for Apogee 30" (Acrobat PDF file    CAD DXF file)

Molded "squat fuselage" for Apogee 30" (Acrobat PDF file    CAD DXF file)
NOTE: Why a "squat fuselage"? There were basically two reasons for developing the squat fuselage design.  The first is that the finger hole can be much more spacious, which is an advantage if you have "significant fingertips" :)  The second is that it takes the side loads of tip-launch better.  Now you know!

New Molded "squat fuselage" for Apogee 40" (Acrobat PDF file    CAD DXF file)


Related Construction Notes [top]

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

Integrated cloth hinge design (8 kB Acrobat PDF file)

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

Improving Sanding Accuracy (some tips on how to sand accurately - especially useful for sanding things like the Apogee wing!)


Apogee Questions and Answers [top]

Q1: Why the curved outlines? 

A1: Mainly for looks. I like flying good looking airplanes. Considering the countless hours of flying enjoyment I've gotten out of my Apogees, the 1 or 2 extra hours needed to build the curved wings was a very worthwhile investment.

Q2: Why the straight V-dihedral? 

A2: See A1. There is little penalty of V-dihedral anyway.

Q3: Why the flat bottom airfoil? 

A3: There is little aerodynamic penalty with the mostly-flat bottom airfoil over a more general optimized shape. Having just the bottom front 15% chord curved is sufficient to gain a large speed range at these low Reynolds numbers. The mostly-flat bottom certainly allows a more accurate wing to be shaped and maintained, so in actual practice it may be better than a fancier section.

Q4: Why is the radio gear "trapped" in the fuselage? 

A4: A fully accessible radio gear would require a larger, draggier, more complex, heavier, and weaker fuselage. When it is necessary to replace any of the radio gear components, simply slice away the fuselage side and then glue it back, reinforcing the seam with narrow glass strips. This will need to be done at most a few times over the life of the glider, so surgery is easier than building a more complex fuselage to begin with.

Radio Gear Recommendations [top]

As shown on the fuselage plan, the Apogee is specifically designed for the Hitec 555 RX, HS-50 servos, and a 3-cell 120 mAh AAAA battery. This will give bulletproof reception and a safe flying time of 70-75 minutes. 

Smaller receivers such as the Berg and smaller 50 mAh 1/3AAA cells can of course be used for a significant weight reduction, provided the tail is kept light so that noseweight is not required with the lighter gear. However, the single-conversion RX will compromise reliability and the smaller batteries will limit flying time to 35-40 minutes. There is little reason to reduce the wing loading below 3 oz/ft^2, so that the lighter gear may be unnecessary if the airframe weight is kept down, especially on the 40" version. 

The weight savings of the lighter gear may be more important on the 36" version.

Any radio capable of elevon mixing can be used. 


Best of luck, and please let us know how you're doing with your Apogee HLG!


Document Version History [top]

Apr 25, 2004 (jaf)
- Fixed links to wood version of Apogee fuselage

Feb 4, 2004 (jaf)
- Added CAD DXF files generated by Tomer Jackman

July 12, 2001 (jaf)
- Added an explanation of why the squat fuselage design was developed.

July 10, 2001 (jaf)
- Added "40" Molded Apogee Plan - Tip Launch enhanced" to the Molded Wing plan links section.
- Added new molded "squat fuselage" plans for the 30" and 40" Apogee to the Fuselage and Tail plans section.

July 1, 2001 (jaf)
- Moved Apogee 30" plans to the Molded section (oops - they landed in the wood section at first!)
- Updated some of the wording in the Understanding the Apogee HLG series to accommodate the new 30" design.

June 29, 2001 (jaf)
- Added Apogee 30" plans
- Added Table of Contents with "top" jumps at section headers

March 28, 2001 (jaf)
- Replaced "Shaping data for AG03" in the Related Construction Notes section with updated airfoil tangents

October 17, 2000 (jaf)
- added info on basswood dimensions for the leading edge cap (wood wing construction note #3)
- added note on Apogee 40 conventional tail sizing (fuselage and tail plan options section)

October 8, 2000 (jaf) 
- added CompuFoil COR files for each airfoil, to assist users who do not have an easy DAT file converter.

Sept 1, 2000 (jaf) 
- added link to "Improving Sanding Accuracy"

July 6, 2000 (jaf) 
- updated description of Answer 3 (A3) in Q&A
- posted new version of pictures with adjusted contrast/brightness.

July 5, 2000 (jaf) 
- added "Wing planform tangent locations" PDF's
- added "Shaping data for tail airfoil"
- added Tail planform, hinge and chord shapes
- added pictures
- added updated wood fuselage design drawing
- expanded "Understanding the Apogee HLG series"
- added Q&A section
- added "Radio Gear Recommendations" section

June 28, 2000 (jaf) 
- updated comparative description of Apogee airfoils vs "big glider" airfoils. 
- added "Shaping data for AG03" pdf files.

June 27, 2000 (jaf) 
- added missing ag09 coordinates
- clarified design strategy
- expanded construction notes for wood wing version
- added comparative description of behavior and performance with the 36" and 40" versions, as well as the rationale for creating a "sub-standard" size HLG.

June 26, 2000 - jaf 
- "all new"

 


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