Rubber torsion landing gear?

[cluttonfred]

Has anyone ever seen this type of gear used on an aircraft? I could see it working well for fuselage-mounted main gear, taildragger or tricycle, with a wide fuselage. It doesn't seem like it would be too hard to fabricate from rounded square tubing, plate of various thicknesses, and a weld- or bolt-on axle tube.

 

[Vigilant1]

It would work, but the only damping that would occur is internal to the rubber, (maybe a small amount from rubber/metal friction). Because the wheels wouldn't splay out and scrub the ground when load is applied, you wouldn't get the type of damping a Wittman gear would provide.
Also, at least with trailers, the rubber does 'give" over time, and the axle sags if there's weight on it, even if the trailer doesn't get used.

 

[cluttonfred]

All true, though I suspect even a lightly-used trailer gets more ground miles put on it than even a 10,000 hour aircraft, a heck of a lot more. I haven't heard any complaints that trailers with such suspensions were more "bouncy" than traditional ones, in fact, just the opposite, they are said to ride better over small bumps than steel springs.

I was thinking of a square-section fuselage with left and right torsion axles bolted to the bottom with simple fairings over them, kind of a single-engine Short Skyvan look. That would imply a fairly tall fuselage to get the necessary clearance for the prop, but that's fine in this case as it follows on the recent "flying camper" thread.

It would work, but the only damping that would occur is internal to the rubber, (maybe a small amount from rubber/metal friction). Because the wheels wouldn't splay out and scrub the ground when load is applied, you wouldn't get the type of damping a Wittman gear would provide.
Also, at least with trailers, the rubber does 'give" over time, and the axle sags.

 

[wsimpso1]

Has anyone ever seen this type of gear used on an aircraft? I could see it working well for fuselage-mounted main gear, taildragger or tricycle, with a wide fuselage. It doesn't seem like it would be too hard to fabricate from rounded square tubing, plate of various thicknesses, and a weld- or bolt-on axle tube.

View attachment 118497

Not seen it. When you work through the travel we usually need in airplanes to soak up sink , you will need at minimum about a 20" arm, the moments get big with 1.5-1.75 time total airplane weight on each main gear times that 20" arm. Run through the numbers, but the weight seems to get big. Wittman gear legs, both plate and round, make a lot of sense in out size range.

 

[cluttonfred]

20" arm? Is that based on, say -45 degrees to 0 degree travel? I get about 14" vertical travel that way, which seems like quite a lot for a typical two-seat light aircraft. Something like this factory-built Jodel DR1050 clearly has nowhere near that much landing gear travel.

Not seen it. When you work through the travel we usually need in airplanes to soak up sink , you will need at minimum about a 20" arm, the moments get big with 1.5-1.75 time total airplane weight on each main gear times that 20" arm. Run through the numbers, but the weight seems to get big. Wittman gear legs, both plate and round, make a lot of sense in out size range.

 

[Turd Ferguson]

My camper has an axle like that and it's the torsion bar inside the axle housing that provides the "spring" the rubber is only there to keep it from rattling around inside the axle tube. My Dodge truck has torsion bars for the front suspension so I think I understand Billski's point: the torsion bar part has to be long enough for the twisting to provide even small amounts of suspension travel. The torsion bars on my truck are quite long, from the front suspension to the output end of the transmission so ~4 -5 feet ???

It might work on something like a FlyBaby where there is already a long axle.

 

[cluttonfred]

TF, to be clear, I was talking about independent half axles. Here’s an example:

FLEXIRIDE® Torsion Half Axles (935 Lb.) #FR-935

Stock# 7106013The Flexiride® Half Axles are perfect for light duty applications where full beam axles cannot be used. The half axles operate independently and are bolted to each side of the trailer, can fit any trailer width and are easily aligned. Sold as a pair. Hubs are sold separately. All...

www.easternmarine.com

 

[Dan Thomas]

My camper has an axle like that and it's the torsion bar inside the axle housing that provides the "spring" the rubber is only there to keep it from rattling around inside the axle tube.

Nope. I sold those things in the 1970s and the rubber is the spring, not the square bar. And they are HEAVY. Good for trailers, no good for an airplane.



Note that the inner end of the square bar is not anchored to the outer tube. Some are retained just by the friction of the rubber, and others have a washer welded on the inside end just inside a bulkhead with a round hole in it to keep the thing from sliding out.

 

[Turd Ferguson]

TF, to be clear, I was talking about independent half axles. Here’s an example:

Copy, the illustration in #1 shows a different type

 

[wsimpso1]

20" arm? Is that based on, say -45 degrees to 0 degree travel?

I went with it being down at 25 degrees with zero load, on the stops at zero degrees, then rounded up to 20” for arm length. I do not know what wing loading might be used so I went a little high. The short arm shown in the OP is just unrealistic. The higher the wing loading, the more vertical sink rate, and the more travel you need.

 

[Tiger Tim]

What would this assembly look like on an airplane, Matt?

 

[Pilot-34]

I’ve always thought they would make a good trailing link suspension.

 

[cluttonfred]

Thanks, all, for the comments. I had in mind something similar to the construction in post #1 but individual units like the link in post #7 and below.



Construction would kept simple, just a rounded square tube welded to a mounting plate for the housing and a lever arm welded to a thick-walled square tube for the moving part. The axle could be a bolt-on type or a welded tube. The housing tube could have a welded outboard end plate with a large round hole to keep the rubber in place and a removable inboard one with the inner tube poking through enough for a washer and cross bolt to keep everything attached.

What would this assembly look like on an airplane, Matt?

 

[Map]

Two rubber torsion spring elements are used on the LS series gliders landing gear. Works well. Sorry I don't have a picture.

 

[karmarepair]

I checked my copy of Pazmany, "Landing Gear Design For Light Aircraft, Volume I" (there is no Volume II) and see no examples of this type of gear. "Torsilastic" springs were used on the bow doors of LCM-6s Back In The Day, but when I needed to renew them after decades of wear and weathering, Firestone didn't make them anymore. My recollection is that they are non-linear (rishing) in spring rate, which could be a good thing.

 

[Vigilant1]

I'm still wondering where the damping would occur. In aircraft landing gear that use rubber for shock absorption there's usually some rubbing or friction element to reduce rebound. For example, bungees are utilized in multiple wraps that rub against each other when expanded and contracted, this creates friction/heat and dissipates energy. If rubber doughnuts are used, they often are contained in a metal sleeve and rub against that sleeve, or the centering rod, as the rubber expands when squished. And/or the doughnuts/metal washers rub against each other as the doughnuts change shape.

With these torsion designs there would be a lot of compression of the rubber (and some internal friction inside the rubber--demonstrated when a rubber band warms up when rapidly and repeatedly stretched), but there's not a lot of rubbing elsewhere.
To prevent bouncing, it might be important to use an especially "dead" rubber element that doesn't return a lot of energy on the rebound. More like a cold squash ball than a superball.

 

[Dan Thomas]

Stuff like this concentrates the forces in a small package that therefore has to be stout, and stout often means heavy. A simple trailing-link gear made of light 4130 tubing, with rubber pucks as used in Mooneys and Ercoupes would be lighter.

The axles I sold were made of steel tubing that was sawn lengthwise into two halves. The rubbers were placed in the halves, the square bar with its welded-on arm and spindle, was laid into one, and the other laid on top and clamped down hard, then the sawn edges welded together, probably with immediate cooling to protect the rubber. The problem with such a thing is that you can't replace the rubber when it gets old and tired. You might be able to pull the bar out, but you'll never get it back in with new rubbers.

 

[Blackhawk]

Hi Mathew,

There's nothing new about using this type of suspension design; you've seen this type used in aviation before.

You commented on my designed tailwheel back in May 2017

I've also designed the same princlple for main and nose wheel landing gear.

..............................................................................................................................................................................................................................................................................................................
May 11, 2017

My new designed tail-wheel could be of interest to the group for any type of tail-wheel homebuilt aircraft.


Specifications are:

CNC machined 6061-T6 billet Torsion block and axle with polyurethane compression bars

CNC machined 6061-T6 billet wheel cheek plates

316 stainless steel mounting shaft

2 x sealed 316 stainless steel case bearings with ceramic balls for pivot in torsion block

All bolts are 316 stainless steel

6" x 1-1/4" pneumatic wheel with 2 x sealed 316 stainless steel bearings and black alloy rim

Total weight as shown in photo is 1.68kg

Graeme

 

[Vigilant1]

It is a very nice thing if the landing gear can also play a big role in protecting occupants in the event of a flat crash. Wittman style gear can do this very well, absorbing energy as the legs plastically deform/bend and smoothing the deceleration until the airplane belly hits. And they can perform this function over a range of impact angles.
Would the rubber torsion gear help out in a flat crash? I suspect beyond its "normal" design stroke (rubber very compressed) that subsequent deceleration of the fuselage would spike ("ouch, my back!") until the mounting broke, the leg bent, etc. Maybe there's a way to get smooth (one time) deceleration over a long distance through:
1) Careful design of the leg (titanium rod?)
2) A bendable mounting (cantilevered thick ductile mounting plate?)
3) Choose inner or outer metal components of the torsion assembly that will yield (go from square cross section to rounded) and in this way keep absorbing energy and providing stroke distance after the rubber is fully sqiished.f
4) If the gear is mounted to a wing spar, the bending/deformation of the spar itself might be useful in this scenario.

 

[Geraldc]

The rubbers were placed in the halves, the square bar with its welded-on arm and spindle, was laid into one, and the other laid on top and clamped down hard, then the sawn edges welded together, probably with immediate cooling to protect the rubber

The ones in my trailer are inserted without cutting the tube. I think they are frozen then pressed in.