WARNING! The mechanism shown and explained on this page has NOT been flight tested and is my own creation! If you build this system for your plane you do so at your own risk! Do not attempt to apply this mechanism to an aircraft without consulting a professional. The original W.A.R. F4U Corsair gearbox design is different and has little similarity with what you see here.
To date I have only heard of one or two Corsairs fitted with a landing gear actuation assembly other than the stock gearbox. Of course some have added electric motors while retaining the handcrank but the system has basically stayed the same. With the stock (plans) system there have been some complaints and failures. It is my hope that the pneumatic system will alleviate those problems.
First let me explain the stock system. A bicycle chain and sprocket system on the side of the fuselage connect the pilot's handcrank to all three landing gear systems (mains, doors and tailwheel). A torque tube attached to the lower sprocket runs perpindicular to the pilot's legs. This torque tube via U-joints (costly U-joints) is attached to each main gear's gearbox. A toggle assembly which comes out of the gearbox has a cam mounted on top of it that operates a mechanism which supposedly opens and closes the gear doors. Nobody that I've talked to has had this "cam" system work properly. Instead most people have resorted to using a separate pulley system to close the doors. Note that I didn't say open - some Corsairs have lost their gear doors due to flopping with this setup. Finally, the tailwheel assembly is synchronized to the mains via a worm gear drive from the main torque tube. The worm gear drive is attached to cables which run to the back of the plane into a chain/sproket setup which actuates the tailwheel and it's door assembly.
Failures of the stock system include...
Drive chain breakage or falling off
Gerabox seizure
Tailwheel not remaining locked
Landing gear doors crushed and/or falling off
Underside view of pneumatic gear assembly (minus spur gear and lower leg) Toggle assembly is the "Y" shaped thing in the middle. Overcenter links are built into the toggle assembly which make it lock by hyperextension. System is not yet bolted but has been tested.
Your first questions are probably, "Are you crazy?" and "Why pneumatics?" Here are my reasons...
Look at a Yak! Everything with power is pneumatically actuated! Engine starting,brakes, flaps and landing gear are air powered.
Pneumatics handle shock better than non-dampened hydraulic systems.
Instead of tearing up delicate airframe parts with excessive unrelenting force that hydraulics may inflict, pneumatics offer a "cushion".
Plumbing is simpler with no limit switches and other specialized hardware which hydraulics require.
Pressures are lower than a hydraulic system allowing lighter tubing to be used. Although leakage is a concern, it's not as critical.
Gear doors can be actuated by separate small bore double-acting cylinders which forcefully hold the doors open or closed (as opposed to a handcrank system).
Pilot workload is decreased and gear cycling time is rapid (compare throwing two levers to 30 turns of the handcrank).
There are more reasons than these but I think you get the general idea.
The basic idea. A three inch aluminum double acting air cylinder pushes a steel rack across a spur gear. The spur gear is attached to the toggle assembly. Gearbox side plates are similar to the plans version but are made of steel.
Rack assembly in it's housing. I took a steel rack and reduced it's height so that it could mate with a 7075 aluminum spacer. The aluminum spacer serves two purposes. It raises the center height of the pushrod and lightens the assembly. The raised center height was required to allow the air cylinder to fit exactly in the middle of the truss.
Aluminum cylinders thread onto round 2024 aluminum adaptors. Of course I had to make these adaptors. On the left are two adaptors awaiting boring and threading. The middle adaptor has had threads chased and is ready to be screwed onto the air cylinder like the adaptor furthest right.
Partially installed system held in with c-clamps. With the lower leg installed, it was pushing with 50lbs of force at the axle! This was 10 -15 lbs more than necessary. Maximum air pressure is 150 lbs psi. Everything except the inboard truss, toggle and gear box are the same as the stock setup. The toggle assembly is different only in that it is offset to accomodate the inboard truss changes.
For those of you thinking of retrofitting a stock system with this one there are small changes. The inboard truss and gearbox need to be replaced. You simply can't get the three inch air cylinder to fit in the stock truss dimensions. Yes, the enlarged truss will protrude on the bottom side of the wing. On the "stock" system the gearbox protrudes anyway. This looks like an excellent place for a bomb/fuel pylon to be glassed in! It seems that the 3" air cylinder size is the smallest that will work. Oh yes, the toggle assembly is offset a little so that would need to be changed as well. Otherwise everything bolts onto the mainspar fittings per plans dimensions. Rear spar mounting holes are different for the inboard truss only.
