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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]
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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:
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.
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)
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)
Airfoil Polar Comparisons [top]
Fuselage and Tail Plan Options [top]Here are a few pictures of the Apogee fuselage and tail...
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)
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) Feb 4, 2004 (jaf) July 12, 2001 (jaf) July 10, 2001 (jaf) July 1, 2001 (jaf) June 29, 2001 (jaf) March 28, 2001 (jaf) October 17, 2000 (jaf) October 8, 2000 (jaf) Sept 1, 2000 (jaf) July 6, 2000 (jaf) July 5, 2000 (jaf) June 28, 2000 (jaf) June 27, 2000 (jaf) June 26, 2000 - jaf
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Construction notes are included in the drawings, which
you can download as Acrobat v
Molded
"squat fuselage" for Apogee 30" (