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AC-21 Powered Sailplane

AC-21 is not a racer – this aircraft is all about maximum enjoyment of free flight. Sweet handling, slow flight capabilities and low sink rate allows exploring “microlift” conditions, when most other gliders are unable to stay in the air. AC-21 can turn really tight. You will be able to find and use a kind of lift that only hang gliders and paragliders could enjoy until now.

AC-21 powered sailplane is an evolution of AL-12 design that was developed in Ukraine during 2000 – 2005 as a joint project by two firms, Aeros and Aerola. In 2006 it was decided to make a next step in the development of the sailplane, splitting the project and leaving production of AL-12 (now Alatus) in Aerola. AC-21 joins unique features of AL-12 incorporated in the new design. More about sailplane concept .

We had three objectives in the development of the design:

  1. Meet the requirements with the most strict DULV and DAeC standard for ultra light aircraft LTF -UL 2003 with LSG  B 0105 amendment for UL powered sailplanes;
  2. Improve the power plant in both power and reliability;
  3. Improve aerodynamics of the fuselage and fully enclose the propeller.

So, let's see what we have done in all three aspects.

1. Certification requirements.

AC-21 structural capacity has been increased keeping an aim to meet actual sailplane standards (+5.3  -2.65 G load factors). It gives us a chance of wider margin over current DULV and DAeC requirements for ultra light aircraft (+4  -2 G).

Specifically

  • Wing leading edge D-spar had to get stronger by 25%;
  • Ribs structural capacity and rigidity was greatly increased and the ribs are now made of carbon;
  • The rest of the structural components either already met our new requirements or, in some cases, have been redesigned and tested.

 2. New Power unit

We have now moved up to the Hirth F33 BS.  This engine not only delivers 28 hp (a 33% improvement over the AL-12M's Corsair) but it does so with a factory recommended TBO (Time Between Overhauls) of 1000 hours at 75 % power.
Considering the type of use it will have most of the time (motoring up to the first thermal and switching off) we could say that this engine should last you for life.  It also allows the use of the AC-21 as a "power plane", great for those stable winter days. 
Four doors on fuselage complicate the construction, but greatly improve its aerodynamics with an engine out, improving both climb and cruise performances.
It also greatly increases safety when taking off in unfavorable conditions like high temperature, high altitude airfields, strong thermal activity or larger pilots.

3. New Fuselage design

Carefuly designed new fuselage insures laminar flow over its front part.  New design makes room for the engine and allows us to fully enclose  the propeller when in glider mode (it was partially exposed on the AL-12). We have also increased the transparent part of the canopy, which greatly improved front visibility.

So, where are we now? 

First impressions are good: the behavior of the sailplane is similar to its predecessors, and at the first glance its flight characteristics look close to the calculated values.
Next stages of the test program will allow us to collect complete information on AC-21 behavior with and without engine installation.


Happy landings,
Aeros Carbon Works 

 

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