Acro Sport I / II

The Acro Sport family is a line of single- and two-seat aerobatic biplanes designed by EAA founder Paul H. Poberezny in the early 1970s specifically for the amateur builder. Poberezny created the Acro Sport I in response to surging interest in sport aerobatics and the formation of the International Aerobatic Club (IAC, a division of EAA). It first flew on January 11, 1972, and plans were released through Acro Sport Inc. of Hales Corners, Wisconsin. Construction is classic and accessible: welded 4130 steel-tube fuselage and tail, wood wing structure, all fabric-covered - techniques any patient homebuilder can master in a garage without specialty tooling.

The Acro Sport I is a single-seat, short-span biplane with open cockpit and spatted tailwheel undercarriage, powered by Lycoming engines from 85 to 200 hp (O-200 through IO-360), though 150-180 hp is most common. The Acro Sport II is an enlarged two-seat version (tandem, dual-control) intended as a trainer and for shared sport flying, almost universally built with the 180 hp Lycoming O-360. The Super Acro Sport is a hopped-up single-seater with 200 hp Lycoming and symmetrical airfoils for more serious competition work.

The design is beloved in the EAA community as a "builder's airplane" - simple enough to finish, pretty enough to show, and honest enough to teach aerobatics without biting. As of the most recent FAA registry data, 83+ Acro Sport IIs remain on the US registry and dozens of Acro Sport Is continue to fly. Paul Poberezny also designed the pober-pixie (a low-wing single-seat taildragger derived from the Corbin Baby Ace) and contributed to the Pober Junior Ace two-seater - both distinct from the Acro Sport line but part of the same "simple honest homebuilt" lineage.

Frontmatter reflects the Acro Sport II with 180 hp O-360, the most commonly built and traded variant.

Book numbers for an Acro Sport II with 180 hp: ~132 kt max, 107 kt cruise (123 mph), 46 kt stall clean, 1,100 fpm climb. Real-world cruise is usually 100-105 kt at 8.5-9.5 gph depending on prop and how clean the airplane is - open-cockpit biplanes all carry a speed penalty versus book. Climb with two adults aboard is still spirited but the Acro Sport II is not a 200 hp Pitts; vertical aerobatics are limited and energy management matters. The Acro Sport I, when built with an O-320 or O-360 and kept light, is a crisper-handling airplane than the II and will hold altitude through basic aerobatic sequences better.

Range is practically limited by bladder more than fuel - 24 gallons at 9 gph yields ~2.5 hours usable, or 250-300 nm with reserves. Density altitude softens performance noticeably; wood wings and a relatively low aspect ratio mean a dense summer day at 5,000 ft MSL will feel sluggish. Service ceiling of 20,000 ft is theoretical - these airplanes live below 8,000 ft MSL in practice.

With a 645 lb useful load on the Acro Sport II and 24 gallons (144 lb) of fuel at full tanks, you have 501 lb for people and gear. Two 180 lb adults with parachutes (25 lb each) consumes 410 lb, leaving ~90 lb margin - comfortable. Two 200 lb adults in parachutes leaves ~50 lb margin - still workable but watch the forward CG with heavy occupants.

Full fuel + 1 pilot (180 lb + 25 lb chute): 296 lb cargo capacity (but you have nowhere to put it - no baggage compartment).

Full seats + reduced fuel: Two 220 lb adults with chutes = 490 lb, leaving 155 lb (~26 gal) payload budget. Since tanks hold 24 gal, you can fill up and fly - the airplane is not volume-limited at reasonable occupant weights.

Aerobatic weight: Most builders limit aerobatics to solo or to a reduced gross weight (1,350-1,400 lb vs 1,520 lb normal category). Check the specific airplane's operating limitations issued by the DAR - these vary by build. A common pattern: 1,520 lb normal category, 1,350 lb aerobatic category.

Range/payload tradeoff: Essentially irrelevant - fuel capacity is small enough that you're bladder-limited before you're weight-limited. Plan 200-250 nm legs and don't try to stretch the airplane.

• Aerobatics (8/10): Purpose-built for sportsman and intermediate aerobatics. Positive/negative capability depends on whether the builder installed an inverted oil and fuel system - many Acro Sport Is and IIs are "upright-only" aerobatic. The Super Acro Sport with symmetrical airfoils is the best aerobat of the family. Not an unlimited competition mount - see pitts-s-2 and extra-300 for that. Well-suited to the aerobatics mission within its G envelope (+6/-5 typical).
• Local Fun Flying (9/10): This is the sweet spot. Open-cockpit biplane, honest handling, low operating cost, and the pride of a plans-built airplane. Ideal for the local-fun-flying mission.
• Flight Training (2/10): Two-seat dual-control makes the Acro Sport II usable for aerobatic and tailwheel transition training under Letter of Deviation Authority, but experimental-amateur-built airplanes cannot be used for commercial primary instruction. Type-specific transition only.
• Cross-country (2/10): Open cockpit, small fuel, no IFR, no baggage to speak of. Weekend fly-ins within 150 nm are fine; real travel is not.
• Business travel / cargo / backcountry / surveying: Not its mission.

Experimental aerobatic biplanes live at the minimalist end of the panel spectrum - see avionics-pricing.md for reference pricing.

• Day-VFR basic ($500-1,500): Airspeed, altimeter, slip-skid ball, tach, oil P/T, CHT, fuel. Handheld comm (Yaesu FTA-550) velcroed to a knee strap. Most Acro Sport Is are built this way.
• Modern experimental basic ($3,000-6,000): Single Dynon D3 or Garmin G5 portable EFIS, panel-mount uAvionix tailBeaconX (ADS-B Out), single Trig TY91 comm, handheld GPS. Typical for newer Acro Sport II builds.
• Fuller experimental ($8,000-14,000): Dynon SkyView HDX 7" or Garmin G3X Touch 7", single comm, transponder, engine monitor. Rare on Acro Sports - at this panel cost you're approaching the value of the whole airframe.

Glass beyond a single small EFIS is uncommon and arguably wrong for an open-cockpit biplane - glare, washout, and water all fight you.

Experimental-amateur-built aircraft do not use STCs - builders can modify freely as long as the airplane remains within Phase I test parameters or goes back into Phase I for a "major change." Common builder modifications:

• Engine upgrade (O-320 to O-360, or O-360 to IO-360): Common on Acro Sport I. Adds 20-40 hp, requires W&B rework and often a heavier prop. Cost: $25k-40k used runout engine + install.
• Inverted oil and fuel system (Christen-style): Essential for sustained inverted flight. Andair fuel valves, Christen inverted oil. Cost: $2,500-5,000 installed during build.
• Smoke system: Small belly tank + pump + nozzle in exhaust. $800-1,500. Popular for airshow and fly-in passes.
• Symmetric airfoils (Super Acro Sport mod): Builder re-ribs the wings using the Super plans. Substantial work; done at build time, not retrofit.
• Constant-speed prop: Hartzell or MT composite prop in place of fixed-pitch wood. $8,000-12,000. Improves aerobatic energy management noticeably.
• Canopy / windscreen variations: Some builders enclose the rear cockpit of the II with a sliding canopy for cooler-weather flying.

See maintenance-ecosystem for the general experimental support landscape.

• Plans and build support: Acro Sport Inc., P.O. Box 462, Hales Corners, WI 53130. Plans are still sold (roughly $125-$200 depending on set), along with the "How to Build the Acro Sport" construction manual. Aircraft Spruce stocks most raw materials (4130 tube, Sitka spruce, AN hardware, fabric).
• Builder/owner community: Active on the Biplane Forum, EAA forums, and the University of Florida-hosted Acro Sport completions page (dhinten). EAA chapters everywhere have at least one member who has built or owned one.
• Annual inspection: As experimental-amateur-built, the airplane needs a condition inspection each 12 months, which can be performed by the original builder (if they hold the Repairman Certificate) or any A&P. Typical condition inspection costs $500-1,500 at a shop, vs. free if the builder/Repairman does it.
• Typical annual operating cost: $5,000-7,500 assuming 50-75 hours/year, including fuel, oil, hangar, insurance, and reserves. Engine reserve at $8-10/hr is the largest single line item.
• Expensive items: Engine overhaul ($28k for O-360), recover ($10k-15k), prop overhaul or replacement ($2k-8k), and any wood wing repair (can be $$$$ if a spar needs splicing).
• A&P familiarity: Any tube-and-fabric-competent A&P can work on an Acro Sport - the design uses no exotic systems. Avionics and engine work are generic Lycoming, so nothing specialist is required.

See valuation-methodology for the general framework applied to experimentals.

Acro Sport II market spans roughly $18k-$45k, with typical flying examples at $25k-$32k. Valuation drivers, in rough order:

• Engine time SMOH: A 180 hp O-360 at 400 SMOH is worth $8k-12k more than a runout core. At TBO, budget $25k-30k for overhaul before flying - buyers discount accordingly.
• Cover age: Fresh fabric (last 5 years) adds $5k-8k over 20-year-old cover. Fresh paint adds less - cosmetics, not structure.
• Builder reputation: An airplane built by a known EAA chapter craftsman with complete build logs, photos, and FAA Form 8130-12 commands a premium. An orphan airplane with sparse logs sells at a steep discount.
• Inverted systems: Oil + fuel inverted = +$3k-5k. Without them the airplane is a "one-way" aerobat.
• Prop: Constant-speed adds $4k-6k over fixed-pitch wood.
• Avionics: Modern ADS-B + basic EFIS adds $2k-3k; beyond that avionics rarely move the needle on a $30k biplane.
• Acro Sport I vs II: Is trade roughly 10-20% below equivalent IIs in the US market - the second seat is highly valued because it lets you share the airplane and give rides.

Super Acro Sports rarely change hands and command a premium of 20-30% over a standard Acro Sport I when they do.

Experimental-amateur-built airplanes are not subject to FAA Airworthiness Directives on the airframe - but engine and propeller ADs absolutely do apply. Lycoming O-360 ADs (crankshafts, oil pumps, magnetos) must be tracked and complied with. Common pre-buy and in-service issues on Acro Sports:

• Wood wing spar condition: Sitka spruce spars, glue joints, and internal wing corrosion are the number-one structural concern. Moisture intrusion through fabric punctures or leaky fuel tanks can rot a spar in a few years. Pre-buy should include removing at least one inspection cover per wing panel and inspecting ribs, spar, and glue lines.
• Fabric condition: Polyfiber/Ceconite fabric lasts 15-25 years depending on hangar and UV exposure. Punch-test is mandatory at annual. A recover is $8k-15k in materials and shop time if contracted out.
• Welded tube fuselage corrosion: Particularly at the tailpost, lower longerons behind the seat, and wherever moisture collects. Borescope inspection through drilled access holes.
• Control system slop: Aileron pushrods, bellcranks, and rod ends wear - these airplanes get rolled a lot.
• Builder quality variance: The single biggest risk in any plans-built biplane is "who built it." A meticulous builder produces a 50-year airplane; a sloppy one produces a hazard. A pre-buy by an A&P experienced with fabric-and-tube biplanes (ideally an EAA Technical Counselor) is non-negotiable.
• Engine logs: Many Acro Sport engines are mid-time or better but have aerobatic hours - check for prop strikes, oil analysis history, and compliance with Lycoming Service Bulletin 388C (crankshaft AD).

See insurance-underwriting. Acro Sports fall into the experimental aerobatic biplane category, one of the more underwriting-averse niches in GA:

• Hull: 3.5-5% of agreed value annually. A $30k airplane hulls for $1,100-1,500.
• Liability: $1M/$100k smooth is generally unavailable; most owners carry $1M/$100k sublimited or $100k/$100k. Liability premiums $1,500-2,500.
• Typical all-in annual premium: $2,500-3,500 for a 500+ hour pilot with tailwheel and aerobatic experience.
• Open-pilot warranty: Usually requires 500 TT, 100 tailwheel, 25 in type (or 10 dual in type), often with a formal checkout from an aerobatic CFI.
• Aerobatics endorsement: Some carriers exclude aerobatics entirely on experimental biplanes; others cover aerobatics only with named-pilot endorsements. Read the policy carefully.
• Low-time pilots (under 250 TT or no tailwheel): Often uninsurable for hull on this type regardless of premium offered. Liability-only is the common fallback.

Carriers actively writing this class include Avemco (limited), Starr/BWI, and Falcon - see insurance-underwriting for current market.