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Lynch Air tankers operated from 1977 – 1992, where the aircraft were fitted with a STOL
kit, designed and developed by Kenny Lynch, they were designated the Lynch STOL 26
The Lynch high lift/STOL system was designed
and developed by Denny Lynch to perfect the operations of his fleet of A-26 Invaders, helping to improve take-off
and landings at "hot and high" airfields that were used each smmer. The density altitude was critical for these aircraft during
the season.
The STOL 26 used a modified wing section which was
also designed for efficient high-speed flight as well as for take-off and landing, as greater angles of attack were achievable
during these phases of the flight.
The new modifications consisted of a leading edge cuff that ran from the fuselage out to the
wing tip and four wing fences across the wing and mainplane.
Take-off and landing distances are strongly influenced by
aircraft stalling speed, especialy on the A-26 Invader and with lower stall speeds requiring lower acceleration or deceleration
and correspondingly shorter field lengths. It is always possible to reduce stall speed by increasing wing area, but it is
not desirable to cruise with hundreds of square feet of extra wing area (and the associated weight and drag).
As the Lynch air tankers would often fly at quite slow speeds
and coupled with low altitude, a means of guaranteeing stability at these conditions was important.
The Laminar flow wing ( NACA 65-215 ) used on the A-26, although ideal at high speed which
was its intended purpose, suffered from maneuvering difficulties at low speeds.
Nose extensions or droop slots
fitted to the STOL 26, are auxiliary airfoils fitted to the leading edge of the wing. The angle of attack of the slat
being less than that of the mainplane, produces a smooth airflow over the slat which tends to smooth out the eddies forming
over the wing.
Slats are usually fitted to the
leading edge near the wing tips to improve lateral control.
Later wing design used retractable leading edge slots.
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Supplemental Type Certificate
STC Number:
SA43RM
This certificate issued
to:
Lynch Flying Service Inc
STC Holder's Address:
Billings Logan Int'l Arpt
Billings
MT 59101
United States
Description of the Type Design Change:
Installation of stall
fence and wing leading edge cuffs.
Application Date:
Status:
Issued, 02/05/1975
Responsible Office:
ANM-100S Seattle Aircraft Certification Office Tel: (425) 917-6400
TC
Number -- Make -- Model:
L-3-4 -- Douglas Aircraft Co., Inc. -- Army A-26C
L-3-4 -- Douglas Aircraft Co., Inc. --
Army A-26B
AL-33 -- Martin-Marietta Corporation -- Army B-26C
Supplemental Type Certificate
STC
Number:
SA44RM
This certificate issued to:
Lynch Flying Service Inc
STC Holder's
Address:
Billings Logan Int'l Arpt
Billings MT 59101
United States
Description
of the Type Design Change:
Increase in gross weight.
Application Date:
Status:
Amended,
01/24/1979
Responsible Office:
ANM-100S Seattle Aircraft Certification Office Tel: (425) 917-6400
TC Number -- Make -- Model:
L-3-4 -- Douglas Aircraft Co., Inc. -- Army A-26C
L-3-4 -- Douglas Aircraft
Co., Inc. -- Army A-26B
AL-33 -- Martin-Marietta Corporation -- Army B-26C
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Above, airflow illustration showing air movement characteristics on modified
wing surfaces.
Section through wing showing STOL mod and directional fillets or Stall fences
( Hatched )
It can be quite clearly seen in the shots below, the droop nose
slot used on the STOL 26, as opposed to a conventional wing used on aircraft without the mod.
Also visable are the stall fences which maintains
longer aileron effectiveness through higher angles of attack.
As a result, the low speed handling
qualities of the aircraft are greatly enhanced, and then lower portions of the wing, depending on the aircraft manufacturer.
Above, the modified leading edge coupled with the double slotted
flaps gives excelent low speed maneuverability.
The two shots above clearly show the stall fences.
Below. the wing leading edge on an unmodified A-26
The Lynch Leading edge
cuff used to enhance the STOL 26 wing.
Leading edge devices such as nose flaps, Kruger flaps, and slats
reduce the pressure peak near the nose by changing the nose camber. Slots and slats permit a new boundary layer to start on
the main wing portion, eliminating the detrimental effect of the initial adverse gradient.
The nose extention (or droop nose slot) used on the STOL 26
leading edge coupled with the double slotted
flaps, produce lift in excess of drag.
The leading edge cuffs fixed to the STOL 26 is
an aerodynamic device employed on fixed-wing aircraft to modify the airfoil used. They may be either factory-installed
or, more commonly, and as in the case of the Lynch modification, an after-market modification.
In most cases a leading edge cuff will “droop” the
leading edge of the airfoil. This has the effect of causing the airflow to attach better to the upper surface of the wing
at higher angles of attack, thus lowering stall speed. This allows lower approach speeds and shorter landing distances. They
may also, depending on cuff location, improve aileron control at low speed. A further benefit is that a cuff may produce a
more gradual and gentler stall onset, particularly where the original airfoil had a sharp leading edge shape.
Where a factory modifies an existing design by the addition of
a leading edge cuff it is often due to an identified problem or deficiency in the original airfoil used. The A-26 Invader
had a tendancy to stall at low speeds if the angle of attack was to high for the speed and on a number of occations when an
A-26 would pull up out of an aerial attack on a wild fire target there was a tendancy for the wing to want to stall.
The method of aerial attack and the fact that it demanded a lower
approach speed, thus risking a possible stall, all went into consideration when designing the STOL 26 wing.
Several after-market suppliers of STOL kits make use of leading
edge cuffs, in some cases in conjunction with such other aerodynamic devices as wing fences and drooping ailerons.
Leading edge cuffs can exert an aerodynamic penalty for the lower
stall speed obtained. The amount of the penalty depends on the airfoil design they are installed on and the amount of “droop”
that they incorporate. The installation of a leading edge cuff often results in some loss of cruise airspeed.
The Lynch STOL kit was ultimately
developed to reduce or eliminate "MOOSE HUNTER STALLS", a spin initiated by an extremely uncoordinated tight turn which causes
a snap roll entry into a spin which commonly happens without enough altitude to recover. The increased stability and reduced
stall speed is a positive by-product of the leading edge cuff. The kit ultimately consisted of
Douglas A-26 Invaders converted to the Lynch
STOL 26
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Serial #: 44-34121
Construction #: 27400
Civ.
Registration:
N4805E
C-GHZM
Model(s):
A-26B
Consort 26
STOL
26
Name: None
Status: Stored
Last info: 2007
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History:
Delivered to USAAF as 44-34121, 19??.
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Stored at Davis Monthan AFB, AZ, Mar. 6, 1958.
- Surplused, June 11, 1958.
Rock Island Oil & Refining Co, Wichita,
KS, 1960-1969.
- Registered as N4085E.
- Converted to Rock Island Consort 26 configuration, Hutchinson, KS.
Koch
Industries Inc, Wichita, KS, 1972
Reeder Flying Service, Twin Falls, ID, 19??.
- Converted to tanker but not used.
Lynch
Air Tankers, Billings, MT, 1975-1992.
- Flown as tanker #58.
- Converted to Lynch STOL 26 configuration by Lynch.
Jerry
Slater, Seattle, WA, Sept. 25, 1992.
Tony N. Grout, Spanaway, WA, ?Aug. 10, 1993-2000.
Air Spray Ltd, Red Deer,
Alberta, Jan. 18, 2001-2007.
- Registered as C-GHZM.
- Stored Red Deer, 2001-2002.
- Engines and control services
removed.
- Marked as tanker #58.
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Serial #: 44-35371
Construction #: 28650
Civil
Registration:
N4818E
Model(s):
A-26C
TB-26C
STOL 26
Name:
None
Status: Airworthy
Last info: 2002
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History:
Rock Island Oil & Refining Co, Wichita,
KS, 1960-1966.
- Registered as N4818E.
- Planned conversion to Monarch 26 not completed.
- Stored, unrestored, Hutchinson,
KS.
Consolidated Air Parts Corp, Los Angeles, CA, 1967.
Denny Lynch/Lynch Air Tankers, Billings, MT, 1967-2002.
-
Converted to Lynch STOL 26 tanker.
- Flew as tanker #A28 (later #59).
- Damaged when nose gear collapsed on landing,
Billings, MT, June 28, 1975.
-- repaired.
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Serial #: 44-35497
Construction #:
28776
Civil Registration:
N3426G
C-FOVC
Model(s):
A-26C
RB-26C
STOL 26
Name: Safe Passage
Status: Airworthy
Last info: 2003
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History:
Trade school, Instructional Airframe.
Johnson
Flying Service, Missoula, MT, 1961-1970.
- Registered as N3426G.
- Arrived in black USAF scheme, converted to fire tanker.
-
Flew as tanker #A17.
Evergreen Air, Missoula, MT, 1975-1977.
- Flew as tanker #A17.
Lynch Air Tankers, Billings,
MT, 1977-1992.
- Converted to Lynch STOL 26.
- Flown as tanker #56.
Air Spray Ltd, Red Deer, Alberta, circa
1995-2003.
- Registered as C-FOVC.
- Flown as tanker #56/Safe Passage.
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Serial #: 44-34102
Construction #: 27381
Civil
Registration:
N4060A
Model(s):
A-26B
B-26C
STOL 26
Name: None
Status:
Destroyed
Last info: 1983
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History:
John M. Sliker, Wadley, GA, 1966.
- Registered
N4060A.
Kern Air Inc, Worland, WY, 1969-1970.
Lynch Air Tankers, Billings, MT, 1972-1983.
- Flew as tanker
#01.
- Modified to Lynch STOL 26 configuration.
- Crashed while fire bombing, Hubbards Fork, KY, Mar. 5, 1983.
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Serial #: 44-35721
Construction #: 29000
Civil
Registration:
N9425Z
Model(s):
A-26C
B-26C
Lynch STOL 26
Name:
None
Status: Displayed
Last info: 2002
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History:
Central Oregon Aerial Co., Inc, Bend Or, 1963-1964
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Registered as N9425Z
Lynch Air Tankers, Billings, MT, 1966-1992
- Flown as tanker #A24 (later #57).
- Converted to
Lynch STOL 26 configuration.
Robert J. Pond/Planes Of Fame East, Minneapolis-Fling Cloud, MN, Sept. 1992-1997.
- Flew
as 435721/Fire Eaters-Always.
- Restored as USN/435721/BP Invader, Chino, CA, 1993.
Palm Springs Air Museum,
Palm Springs, CA, 1997-2002.
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The benefits of STOL
STOL is an acronym for short take-off and landing, a term
used to describe aircraft with very short runway requirements.
The formal NATO definition (since 1964) is:
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Short Take-Off and Landing (décollage et atterrissage courts)
is the ability of an aircraft to clear a 15 m (50 ft) obstacle within 450 m (1,500 ft) of commencing take-off
or, in landing, to stop within 450 m (1,500 ft) after passing over a 15 m obstacle.
Many fixed-wing STOL aircraft are bush planes, though some, like
the de Havilland Dash-7, are designed for use on prepared airstrips; likewise, many STOL aircraft are taildraggers, though
there are exceptions like the de Havilland Twin Otter, the Cessna 208, and the Peterson 260SE. Autogyros also have STOL capability,
needing a short ground roll to get airborne, but capable of a near-zero ground roll when landing.
Runway length requirement is a function of the square of the minimum
flying speed (stall speed), and most design effort is spent on reducing this number. For takeoff, large power/weight ratios
and low drag help the plane to accelerate for flight. The landing run is minimized by strong brakes, low
landing speed, thrust reversers or spoilers (less common). Overall STOL performance is set by the length of runway needed
to land or take off, whichever is longer.
Of equal importance to short ground run is the ability to clear
obstacles, such as trees, on both take off and landing. For takeoff, large power/weight ratios and low drag result in a high
rate of climb required to clear obstacles. For landing, high drag allows the aeroplane to descend steeply to the runway without
building excess speed resulting in a longer ground run. Drag is increased by use of flaps (devices on the wings) and by a
forward slip (causing the aeroplane to fly somewhat sideways though the air to increase drag).
Normally, a STOL aircraft will have a large wing for its weight.
These wings often use aerodynamic devices like flaps, slots, slats, and vortex generators. Typically, designing
an aircraft for excellent STOL performance reduces maximum speed, but does not reduce payload lifting ability. The payload
is critical, because many small, isolated communities rely on STOL aircraft as their only transportation link to the outside
world for passengers or cargo; examples include many communities in the Canadian north and Alaska.
Most STOL aircraft can land either on- or off-airport. Typical
off-airport landing areas include snow or ice (using skis), fields or gravel riverbanks (often using special fat, low-pressure
tundra tires), and water (using floats): these areas are often extremely short and obstructed by tall trees or hills. Wheel
skis and amphibious floats combine wheels with skis or floats, allowing the choice of landing on snow/water
or a prepared runway. A STOLport is an airport designed with STOL operations in mind, normally having a short single runway.
These are not common but can be found, for example, at London City Airport in England.
A later development of the Lynch STOL kit was the Horton STOL
kit, which for all intended purposes bore an incredible similarity to the Lynch system.
Description of the Horton STOL Kit
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The Horton STOL Conversion will give the advantage of taking-off
and landing at a slower speed, the ability to get out of grass strips easier, obtain better performance at high altitudes,
with better aileron control at low speeds, reduce the tendency to spin and reduce the stall speed.
The slow landing
performance is not your main concern but safety is. The Horton STOL conversion will provide that extra margin of safety. In
the event of an engine failure and you are forced to land in an unfamiliar area, the STOL conversion gives you more time and
control of your airplane by allowing you to reduce forward airspeed, while reducing the sink rate.
The combination
of these results will give you the possibility of touching down at a fraction of the normal speed, the chances of personal
injury and aircraft damage is greatly reduced.
Your aircraft will be modified with only the necessary and approved
items to improve take-off, slow flight and landing characteristics.
Only STC's and certified materials and quality
parts are used in the Horton STOL conversions. The FAA has issued a Parts Manufacturing Approval (PMA) to Horton STOL-Craft
for the production of our STOL conversions.
Wing Foil Modification
Very important is the addition of
a special leading edge which increases the leading edge profile. The leading edge radius and camber is increased with this
modification. Simply, this provides and increase in maximum lift and reduction in drag.
For the owners of new Cessna
aircraft with the Camber Lift Cuff, HORTON STOL-CRAFT has designed and engineered a new leading edge, for increased performance,
to replace the Cessna Camber Lift Leading Edge.
Conical Cambered Wing Tips
Low speed aileron effectiveness
is improved by special wing tips. Induced drag is further reduced since the HORTON STOL-CRAFT tips reduce wing tip vortex
strength. They also increase the wing area and decrease the stall speed.
Control Surface Seals
Not readily
visible in a complete conversion, but very important are the control surface seals, also known as gap seals.
The control
surfaces of most conventional aircraft permit air flow from the underside of the wing where the high pressure area is present
to the upper portion where the low pressure area is developed. The air "leaks" through the space between the ailerons and
the fixed position of the wing. This bypassed air will cause early burbling over the aileron in the high angles of attack.
We
install a strip of aluminum in this gap between the wing and aileron to prevent or greatly reduce this unwanted movement of
air.
Stall Fences
As you recall from your flight training days, modern general aviation aircraft are
designed in such a manner that the stall starts at the inboard portion of the wing and then moves outwards as the angle of
attack is increased.
The HORTON STOL-CRAFT stall fence controls the stall span wise progression and therefore maintains
longer aileron effectiveness through higher angles of attack.
As a result, the low speed handling qualities of the
aircraft are greatly enhanced, and then lower portions of the wing, depending on the aircraft manufacturer.
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Below, the Horton STOL kit
As can be seen below, other methods of stability and
air flow control were also used on the A-26.
Wingtip devices are usually intended to improve the efficiency
of fixed-wing aircraft. There are several types of wingtip devices, and though they function in different manners, the intended
effect is always to reduce the aircraft's drag by altering the airflow near the wingtips. Wingtip devices can also improve
aircraft handling characteristics and enhance safety for aircraft. Such devices increase the effective aspect ratio of a wing
without materially increasing the wingspan. An extension of span would lower lift-induced drag, but would increase parasitic
drag and would require boosting the strength and weight of the wing. At some point, there is no net benefit from further increased
span. There may also be operational considerations that limit the allowable wingspan.
When air tankers were flying through a maize of vertical and lateral hot air
currents, these devixes did help with stabilty.
Wingtip devices increase the lift generated at the wingtip (by
smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving
lift-to-drag ratio. This increases fuel efficiency, in powered aircraft, and it increases cross-country speed in gliders,
in both cases increasing range. U.S. Air Force studies indicate that a given improvement in fuel efficiency correlates directly
with the causal increase in L/D ratio.
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