What is a “compound helicopter”? It’s a HELICOPTER. It’s a JET. It is BOTH! But also, a compound helicopter is NOT a tilt-rotor (see my Bell 301 (XV-15) Tiltrotor or a tilt-wing (see Henrik Jensen’s excellent model of the LTV XC-142). Looking at the pictures at those links will show you that the names ‘tilt-rotor’ and ‘tilt-wing’ are apt labels.
A compound helicopter has two independent propulsion and lift systems: a main rotor for vertical takeoffs, and propellers or jet thrusters with wings for high-speed forward flight. Compound helicopters are pretty rare!
Only two S-72 aircraft were built in the 1970’s/80’s, serving as a real-life test-bed that exceeded what could be learned from technical analysis and wind-tunnel testing. The NASA photos below show this aircraft in its three most famous variants:
Two exceptional features of the S-72 deserve further comment:
The S-72 had an emergency crew extraction system, for which the designers needed to solve the problem of getting the crew away from the spinning main rotor during flight. Sikorsky created a design for ejector seats for this helicopter, which required first blasting all the rotors away from the mast, blowing away a canopy hatch, then using rockets to eject the crew in their seats so their parachutes could deploy away from the aircraft. Working at 1:39 scale as I did here, this function would have been impossible to mimic, but at a larger scale Gabor Pauler designed a Lego solution. You can see his work at this link for his brilliant Light Attack Compound Helicopter; scroll down to section 3.5.1 of his design drawings and explanations for a working ejector seat in Lego – fantastic work, Dr. Pauler!
The S-72 had an X-wing variant designed to stop the main rotor in mid-flight, and convert the rotor blades into supplemental lift systems in addition to the main wing extending from the bottom of the fuselage. The technique involved using compressed air generated by the engine to be expelled over the surface of the rotor blades. The NASA photo shows the real X-wing variant, followed by my image of it as a Lego model:
Here below is my Lego version with a standard 5-blade main rotor:
Here below is the fixed wing version:
About my design work:
This is my first design to use the ‘beta’ software from Bricklink called PartDesigner which complements their Studio software. I added realistic markings for the tail rotor warning and for the fuselage (‘NASA Sikorsky’). I began the design in Lego Digital Designer, thereafter I imported it into Stud.io to create the digital renderings. I decided to make the pictures of the model more interesting by adding bright red accents in various spots.
This is my second compound helicopter; earlier I designed the Fairey Rotodyne whose listing I just updated with more and better digital pictures.
This is my fourth model to have retractable landing gear. My Lockheed P-38 Lightning, Focke-Wulf FW-189 Uhu, and Sikorsky VH-92A ‘Marine One’ have wheels that fold up in line with forward motion (you can see pictures of that function in those listings). By contrast this S-72 has wheels that swing sideways into the belly of the fuselage. Plus, this model is my first to have doors on those wheel wells. The fuselage is only 6 studs wide, thus requiring each wheel to fully retract within a 3-stud distance -- that was a new problem in Lego design for me to figure out. The following three renderings show my solution. With the aircraft belly-up, the first image shows wheels deployed:
The next image shows the wheels retracted while the wheel well doors (part# 18910, hinge panel, curved) remain open (upper right and lower left in the image).
Lastly, the doors are closed. The remaining open area (dimensions are 2 studs by 4 studs) could be filled with plates and tiles attached to the doors, to effect 100 percent enclosure of the wheels.
The efficiency of this design is enabled by skipping the use of the typical Lego wheel holders – I am really pleased with my solution, even though the front wheelbase on the actual aircraft is wider than what my model depicts. Feel free to comment on the solution here! I learn from helpful critiques.
The rear wheel (under the tail) is non-retractable. Not using a standard Lego wheel holder, I improvised with another Lego part shaped just the way I needed. If you can identify what part I used, put your guess into a comment below.
On the S-72 the jet engines are suspended beneath an arch-shaped arm extending from the fuselage. For added stability, in the model I added axles extending into the sides of the engines.
The aircraft nose of my Guardian jet used a ‘meta’ or pre-cast piece, drawing advice in comments not to do that. My nose solution here for the S-72 goes so far as to include the pitot tube off the tip of the nose, and I like the fact that this design captures the large cockpit windows of this model. By contrast, jet plane cockpit windows have much smaller surface areas because the limited range of aircraft maneuverability plus its forward speed reduces the need for a wide field of view; but with helicopters, the ideal is a Plexiglas bubble so the pilot can see all around.
If you are a fan of Sikorsky aircraft, I’m your guy. This is my 9th Sikorsky model; the others are:
Quoting Gabor Pauler
Rare topic of MOCs, well constructed. I especially enjoyed the landing gear legs.
Thanks! I was happy to recognize your 'LACH' with its ejector seat - I wonder if the designer of the official Lego set 'James Bond Aston Martin DB5' borrowed your technique to create that function. It sure looks like a fun kit, I'm thinking of buying it.