Is the Apogee the Right HLG for me?

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!

Introduction

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

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.

Apogee Airfoils

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

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.

Molded Wing Composite Apogee Construction

• 30" Molded Apogee Plan - PDF - 30 KB CAD DXF - 657 KB
• 36" Molded Apogee Plan - PDF - 41 KB CAD DXF - 601 KB
• 40" Molded Apogee Plan - PDF - 42 KB CAD DXF - 589 KB
• 40" Molded Apogee Plan - “Tip Launch Rated” - PDF - 34 KB CAD DXF - 518 KB

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

• 36" Wood Apogee Plan - PDF - 37 KB CAD DXF - 499 KB
• 40" Wood Apogee Plan - PDF - 48 KB CAD DXF - 686 KB

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	CompuFoil COR files	Polars
ag03	ag03.dat - 4 KB	AG03.COR - 4 KB	ag03_polars.pdf - 32 KB
ag11	ag11.dat - 4 KB	AG11.COR - 4 KB	…polars not provided here 1

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

• Comparison: ag03, ag04, ag08, s6063 (re 30k) - 26 KB
• Comparison: ag03, ag04, ag08, s6063 (re 40k) - 28 KB

Fuselage and Tail Plan Options

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

❮ ❯

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 - PDF - 67 KB CAD DXF - 954 KB

• Molded Fuselage Plan for 120 mAH receiver pack - PDF - 71 KB CAD DXF - 996 KB

• Wood Fuselage Plan for 120 mAH receiver pack - PDF - 39 KB CAD DXF - 551 KB

• Wood Fuselage Plan for Apogee 30" - PDF - 39 KB CAD DXF - 551 KB

• Molded “squat fuselage” for Apogee 30" - PDF - 38 KB CAD DXF - 599 KB

• New Molded “squat fuselage” for Apogee 40" - PDF - 37 KB CAD DXF - 827 KB

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!

Related Construction Notes

• Ultra-light tail construction
• Integrated cloth hinge design
• Accurate shaping of solid balsa surfaces without templates
• Improving Sanding Accuracy

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• Shaping data for AG03 PDF - 12 KB
• Shaping data for AG03 (leading edge blowup PDF - 7 KB
• Wing planform tangent locations page 1 - PDF - 11 KB CAD DXF - 172 KB
• Wing planform tangent locations page 2 - PDF - 24 KB CAD DXF - 388 KB
• Wing planform - full CAD DXF - 325 KB
• Shaping data for tail airfoil PDF - 8 KB
• Tail planform, hinge and chord shapes PDF - 15 KB

Apogee Questions and Answers

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

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.

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1. flow not really 2d this close to the tip, so the analysis wouldn’t be particularly meaningful… ↩︎ ↩︎ ↩︎