The C-17 Globemaster III is the newest, most flexible cargo aircraft to enter the airlift force. It is capable of rapid strategic delivery of troops and all types of cargo to main operating bases or directly to forward bases in the deployment area. It has a payload capacity of 170900lb (77519 kg) and can drop 102 paratroopers. The aircraft is operated by a crew of three: pilot, copilot and loadmaster.
Technologically, the heart of the C17 is its propulsive lift system, which uses engine exhaust to augment lift generation. By directing engine exhaust onto large flaps extended into the exhaust stream, the C17 is capable of flying steep approaches at remarkably slow landing speeds. The aircraft has the ability to land 160000lb pay loads on 3000ft long runways. Slow, steep approaches help pilots make precision landings on limited runway surfaces. This was accomplished by diverting engine exhaust downward, giving the wing more lift. The engine exhaust can impinge directly on slotted flaps and is deflected downward to increase the wing lift.
Davie Matthews decided to create a model of this monster transporter the C-17 Globemaster III, “The Big Grey Cloud”. He also has his own website with a section dedicated to the modelling of the C-17. This page uses pictures and information from the website which you can access here.
Height: 5.5 ft
Engines: Four RAM 750F
Making a Kit of parts
The design and construction of the model took place after Davie visited RAF Brize Norton to see the full-size C-17 Globemasters during a short visit to England February 2002. The serious design of the model started in March 2002 using AutoCad and took about 600 hours to complete. The result was a set of detailed drawings from which a kit of parts could be constructed.
Construction was started on 28th September 2002. The process began with 5 hours being spent at the laser-cutters, getting five 8ft x 4ft 3mm lite-ply sheets, three 5ft x 5ft 6mm ply sheets, and seven 4ft x 1ft 3mm ply sheets cut from the 274 AutoCAD drawings. This resulted in a kit of 965 parts. The fuselage components were mainly cut from 6mm material whereas the wing ribs were cut from the 3mm sheets.
The initial components of the ‘kit’ were assembled quickly over a period of only a few days. After only four days work the basic fuselage with fin had been assembled. Pictures below show the result of this work and the use of six 25mm diameter aluminium tubes to join the two fuselage sections.
After another six days work the basic wing structure was completed. The wings are made from lite-ply ribs. These were cynoed to glass fibre sheets with the drawings underneath to ensure a totally flat, straight and true wing was made. All the flying surfaces were also done this way. After only 11 days work the outline structure was complete.
After the formers were fixed in place, 1/4″ balsa stringers were added along the full length of the structure at 3.5″ spacing. The framework was then covered using 1/8″ balsa sheets All sheeted sections of the model were glass clothed with Deluxe materials Aeropoxy.
Sanding and finishing of the woodwork was then completed ready for painting.
Painting the model.
All paint used was on the model was Lesonal 2K paint. Once glass clothed and undercoated the weight of the wooden parts was 98lb. The acetate sheeting used in the cockpit area was cut to size and glued in place using Deluxe Materials Speed Bond. Then the windows masked and car body filler used to blend in the panels. The framework was then covered using 1/8″ balsa sheet. With the nose section finished, aluminium tape was applied to create raised surface detail. The model then had a final spray coat of 2K paint to complete the basic colour.
The many masks for the signs and lettering were all drawing in CAD and then cut from, aircraft grade, paint masking material. To make the panel lines, 1.2mm wide tapes are placed on the surfaces and sprayed over. Once fully dry the tapes were removed. Rivets were burned in using a 1/16″ brass tube on a soldering iron.
All the flight surfaces are driven by Futaba 5301 servos. The standard servo arm discs were 1/2″ too short and so extension arms were made from 3mm epoxy/glass plate and held with 3mm HT bolts and nylocs. The horns were also made from epoxy/glass and held with four self-tapping screws into 1/4″ ply. Push rods were made from solid 3mm threaded rods with 5mm carbon tube outer sections to prevent any flexing. Each end has Dubro 4-40 ball links. Each surface was hinged with Dubro 1/4″ diameter hinge points. Six were used for each aileron, 10 on each elevator and seven on the outer rudder section. The inner rudder to fin hinge was made from 3mm epoxy/glass plate cut 1″ wide and hinged, with 1/8″ piano wire. Flap hinges were made from 3mm epoxy/glass plate and 6mm bolts with nylocs giving 40 degrees of movement. The top surface wing spoilers are surface hinged using Kavan flat hinges and controlled by Futaba 9303 servos, giving 60 degrees of movement. Each one of the twelve gear doors is operated by a Futaba 3001 servo. Other servos are used to retract the gear, operate lights, and to steer the model on the ground.
The turbine installation.
The four turbines were fitted with lots of tubing and a bucket full of Festo fittings. The turbines and associated fittings weighed 19lb. The turbines are mounted in the nacelles using large clamps along with the fuel pumps. The ECUs and 2200mAh battery packs are located under hatches in the nacelle arms. The turbines are started with compressed air with the air and gas valves situated under the nacelles. Mick Reeves Models made the turbine exhaust tubes inner and outer sections.
The 4.5li fuel tanks were made using epoxy/glass. The tank has two parts. The main tank is 4li and has a smaller hopper tank attached on top. These were made as one unit and weighed 10oz per tank. They allow a 14 minute safe run time. Fuel is fed to the engines via a felt clunk filter along 6mm tubing through a stop-cock ball valve into the UAT header tank and bubble trap. Then it passes to the electric fuel pump and via 4mm Festo filter to the turbine. For start-up, a two gallon reserve tank plugs externally into the main tanks. This allows plenty of time to complete the power checks etc before a flight is commenced.
The retract mechanism is based on the electric screw jack principle. These came courtesy of Black & Decker screwdrivers. The nose leg unit is 15″ long with a 12″ leg and uses braked 4″ diameter wheels. It was made from solid aluminium turned up on Davies’ lathe. The nose leg is sprung with HD springs as an oleo unit.
The two main units use 5″ wheels (12 of them) and a cam arrangement to achieve the twist and turn necessary. The two 6 wheel bogies and rubber shock mounted cam follower plates are again driven by screw jacks. Power to the motors is via a fused 6v source regulated from one of the three 12v 7Ah batteries in the model.
To do this needed a good set of servos, wiring and electronics:
4 x 309DPS PCM synthesized receivers. 9 x Futaba 5301 servos. 2 x Futaba 9206 servos. 4 x Futaba 9303 servos. 15 x Futaba 3001 servos 4 x 4,400mAh 6 Volt packs. 2x 2,200mAh 6 Volt packs. 2 x 1,800mAh 4.8 Volt packs 2 x 7,000mAh 12 Volt lighting packs 2 x opto-isolator units. 2 x battery backer units. 8x led battery checker units. 10 x heavy duty switch harnesses.
Having chatted with Steve Holland about the installation in his Comet, Davie fitted four receivers rather than the mandatory two. The reason for doing this was to reduce the wiring necessary and reduce the number of plugs that had to be connected when assembling the model. Less wiring also reduces the possibility of interference and voltage drops. Even so, 80ft of heavy-duty twisted wire was used. Another advantage of having four receivers is to further reduce the possibilities of a fault making control of the model impossible.
The fuselage system comprises of a Futaba 309DPS receiver powered by a 4.8v 1500mAh battery which is connected to a SM Services 8 channel opto-isolated board. A 6V 4000mAh main pack and a 6V 1800mAh secondary pack via a SM Services battery backer powers the servos. An SM Services led voltage monitor is used to indicate receiver pack charge. The two outer wing panels use the same receiver, but are powered directly by 6V 2200mAh packs. This is a simpler system because the wing panel loads are a lot less than those needed for the fuselage.
There are various lights on the model which also had to be wired up. Lighting is provided by 12V Halogen bulbs of various shapes and sizes. Twenty bulbs from 5W to 50W were installed. Also four 12V strobe units from Farnell were fitted to the wingtips, fin top and underside of the fuselage.
The servos, wiring, receivers, fuel tanks and battery packs contributed 40lb to the model’s weight.
Well it hasn’t yet! Big models need a lot of preparation and it is best not to rush such things. For example, It takes 1 hour from when the van doors open to get the C-17 assembled, fueled and ready to be started. Taxi trials were carried out on the 26th June 2004. Somehow Davie has resisted the temptation to rush the test flights. Due to him moving house and other commitments, test flying to scheduled to start late June 2005. Pictures and videos of the test flights will be placed on Davie’s website and hopefully some pictures will be added to this page.
Until then we can enjoy examining the model. It has great presence on the ground. It is bound to be spectacular in the air.
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