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Original Year
 1996
# of pieces
 465
Categories
 Aircraft:  Airplanes
Dimensions
 48(L)x50(W)x19(H)
Features
 Propellers,  ailerons,
retractable gear, flaps
Other Views

8425 Black Hawk
iso revolve
Click the image to download the LDraw file of this model.
Model by Benjamin Wendl
The 8425 Black Hawk was the third Technic airplane and was very similar in size to the previous 8836 but somewhat more full featured.  It features twin rotating turbo-prop engines, working ailerons, and a retractable tricycle landing gear system with slaved trailing edge flaps.

Like 8855, the wing is constructed using a number of ribs in an airfoil shape like a real semi-monocoque airplane.  The empennage is a T-tail configuration but has no active control surfaces.  The wing is a simple rectangular planform with no sweep, taper, or dihedral.  The cockpit has some stickers representing flight deck electronic detail, though the circuit breakers are difficult to reset.

Features
Engine
The turbo-props are driven from above by an input 24 tooth gear, then geared up 3:1 for a fast speed before passing through a set of bevel gears.  There are a number of other gears on the engine shaft itself, but they are just there to represent the compressor stage stylistically and have no mechanical function.

A pair of 2 blade propeller parts arranged at right angles are used here to simulate 4 blade props.  It is a pretty realistic prop which actually has a pitch angle, unlike the previous parts.

 engine    engine
Click for an animation of the engines in motion.
Ailerons
The primary control surfaces of aircraft which control roll are ailerons.  These panels on the trailing edge of wings move in opposite directions (one moves up while the other moves down).  Since this model has both flaps and ailerons, only the outboard surfaces are ailerons.

The ailerons are operated by a knob on the top of the aircraft that works through a couple of bevel gears to produce opposite rotation.  Nothing fancier than that going on here, though it is interesting that the aileron is not mounted on the gear axle but on another axle 1 stud further aft.
 ailerons    ailerons
Click for an animation of the ailerons in motion.
Landing Gear
The model's best feature is the landing gear.  This is not just because it is a spring locked retractable tricycle landing gear, but mostly because the trailing edge flaps are mechanically slaved to the gear.  When the gear is down, the flaps are deployed.  When the gear is stowed, the flaps are retracted.  While this isn't how a real plane works, you do use the flaps for take-off and landing so it makes perfect sense.

I've color coded the axles which are anchored to structure in red to make the system easier to understand, and even so it is quite complicated.  A knob on the bottom actuates the system.  It turns a longitudinal axle through a set of 12 tooth bevel gears.  ANother bevel gear set on either end operates the nose and main gears in synchronization.  The 3L black axle you can see on the nose gear acts as a down stop and carries the weight of the nose of the model.  The rear gear post goes over center and locks when the engine crankshafts you see above them stop against the fuselage.  These crank elements also serve as the connection to the flaps.

Make sure to watch the animation to appreciate the complex kinematics of this system.  The motion alone is worth the price of this model.
 gear    gear
Click for an animation of the landing gear in motion.
 

Other Views
Side View
side view
 Top View
top view
Front View
front view 		Back View
>back view
Computer Render
Render

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