LET L-13

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L-13 Blaník

A Blaník over the Alentejo hinterland in Portugal.
Type designation	L-13
Competition class	Two Seater
Number built	3000+ incl. variants
Crew	2
Length	8.40 m
Height	2.08 m
Cockpit width
Cockpit height
Wingspan	16.2 m
Wing area	19.15 m²
Aspect ratio	13.7
Wing profile	NACA 63²A-615 root NACA 63²A-612 tip
Empty mass	ca. 292 kg (645 lb)
Maximum mass	500 kg (1105 lb)
Wing loading	ca. 19 - 26 kg/m²
Maximum speed	253 km/h (136 kt)
Rough air speed	145 km/h (78 kt)
Stall speed	55 km/h (30 kt) (about 2 knots higher without flaps)
Minimum sink rate	ca. 0.82 m/s at 77 km/h (41 kt)
Best glide ratio	28:1 at 90 km/h (48 kt)
Roll rate	4 s at 90 km/h (L-23) -45º to +45º bank

The L-13 Blaník is a two seater trainer glider produced by Let Kunovice since 1956. It is the most numerous and widely used glider in the World.

Contents 1 History 2 Characteristics 2.1 Construction 2.2 Variants 3 Sources

[edit] History

The L-13 Blaník was designed by Karel Dlouhý of VZLÚ Letňany ca. 1956, building upon the experience gained with the Letov XLF-207 Laminar, the first Czech glider to employ laminar flow wing profiles. The L-13 was developed as a practical glider suitable for basic flight instruction, aerobatic instruction and cross-country training. This design concept was combined with true and tested technology: metal construction, NACA laminar profiles and many standard-issue components of the soviet aerospace industry.

The Blaník entered production in 1958 and quickly gained popularity as an inexpensive, rugged and durable type, which was easy to fly and operate. It was widely adopted in the Soviet block countries and was exported in large numbers to Western Europe and North America. Total production was in excess of 2650, or more than 3000 if variants are included. Nearly half a century after its first flight it is still the most common glider in the World.

In the cross-country role the Blaník achieved many two-seater World distance records during the 1960's in spite of having only fair performance.

The Blaník inspired other designs, notably the Démant and L-21 Spartak single-seaters developed to equip the Czech team in the 1956 and 1958 World Championships.

[edit] Characteristics

The effectiveness of the Blaník as a primary trainer is due to a blend of characteristics that facilitate progress of ab initio students towards solo flight, namely: slow landing speed, ample control deflections and an effective rudder. These are in effect typical of wood-and-fabric primary trainers such as the ASK 13, which the Blaník resembles in handling, though not in materials, construction and aerodynamics.

For this reason, pilots trained in the Blaník require differences training in a modern two-seater before transitioning to high performance plastic single seaters.

The Blaník is stressed for simple aerobatics. It cannot perform advanced maneuvers such as snap rolls (flick rolls in British English) and inverted flight is strictly single-occupant. Therefore, intermediate level aerobatic training in the Blaník must be done in solo flight with the instructor on the ground or in another aircraft.

There are two piloting issues that pilots should be aware of. The wheel retracts counter-intuitively, with the gear handle moving forward for wheel up and back for down (this is reversed on the L-23). More importantly, the spoiler and flap handles have the same shape and are close to each other. This has led to a number of incidents where pilots have mistakenly operated the wrong handle, particularly in the landing pattern. Instructors recommend that a positive visual check of the spoiler operation be made during the prelanding checklist, and that the pilot's hand stay on the handle right through full stop.

The Blaník empennages are vulnerable. The horizontal stabiliser is low enough to be damaged when landing in brush, and one must not push on the vertical fin when ground handling as it is not stressed to carry loads fore-and-aft.

[edit] Construction

• Fuselage of semi-monocoque construction employing longerons and bulkheads, with an ovoid cross-section. The cockpit is covered with a two-part acrylic glass canopy.

• Trapezoidal single-taper wings with forward (negative) sweep, single-spar, all-metal construction. Metal ‘torpedo’ tips. Flaps and ailerons have a metal frame and are covered in fabric. Metal DFS type spoilers on the upper and lower wing surfaces.

• The horizontal tail surfaces fold up parallel to the fin for transportation and storage.

• The elevator and rudder are metal frames covered in fabric.

• The landing gear is semi-retractable and sprung with an effective oleo-pneumatic shock absorber, excellent features which assure landings with little or no damage even if the wheel is left (forgotten) in the raised position.

[edit] Variants

• The L-23 Super Blaník is a modernised version of the L-13. The most obvious change is the swept-back fin and T-tail which are allegedly less prone to ground damage. The wings are shorter, with glass-fibre wingtips in place of the original metal “torpedo” tips. The Fowler flaps have been deleted. A larger canopy complemented with a new rear-opening transparent turtledeck improves the view and facilitates and the ingress and egress of the rear seat occupant.

• The L-13 AC Blaník is primarily intended for aerobatic training with a wider flight envelope enabling dual training up to intermediate-level. It combines the wings and cockpit of the L-23 Super Blaník with the single-piece canopy and conventional empennage of the L-13.

• The Vivat is a touring motorglider derivative. The wings, fuselage and tail surfaces of the L-13 are mated to a cockpit featuring side-by-side seats and a conventional firewall-forward engine installation with either a Mikron M III AE four-cylinder inverted inline engine or a Limbach L 2000.

• An auxiliary-powered Blaník was also developed, with an external engine permanently mounted on a pylon above the rear fuselage.

• The SL-2P twin fuselage Blaník was developed by Sportinë Aviacija in Lithuania as a flying laboratory for testing of laminar airfoils. The specimen profiles are fixed to a supporting frame erected between the fuselages. This variant is similar in concept to the modified Janus once operated by the DFVLR (today the DLR, or German Aerospace Center) for the same purpose.

[edit] Sources

• LET website
• Website of Blanik America
• Photo of the SL-2P flying laboratory
• Sailplane directory
• Aguiar J, Blanik L-13: um Planador de Sucesso, Revista do Ar, Jan 1994