Trailing Link Landing Gear

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GESchwarz

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I've always liked the trailing link landing gear. It's found on the nose gear of the F-8, the main gear of the FA-18, and the tail wheel of the C-46, among others. The motocross dirt bike with the monostrut arrangement is another good example. It's been said that it's hard to make a bad landing on such a main gear.

I'm considering this style for all three positions of my tricycle gear. I'm planning on using an appropriately sized motorcycle shock absorber, which is a combination compression spring and piston damper. The trailing link would have multiple positions for adjusting travel and moment arm of the shock strut.

My big goal is to have the plane stick to the runway upon touchdown; zero bounce, even if I bring it down fast and hard, or in a crosswind.

Does all of this seem too heavy and complicated for a simple 2 seater?

Is there a simpler way to get the kind of “sticky” suspension I’m looking for?
 

RacerCFIIDave

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I do have some reservations about weight...

however...the use of motorcycle shocks is a good idea...they usually have significant compression damping as well as rebound... (former AMA Pro crewchief here)

A vertical leg with a short swingarm and appropriately sized shock and spring could be made reasonably light... as a main gear assy

I would not reccommend such a system for the nosegear (overkill) unless you plan on making carrier landings...(or at least carrier style landings...ie controlled crashes)

Do you have a good idea of the total weight you are looking at as well as the ground clearance required for prop clearance on your design...as well as the wheel sizes?

If so I may be able to help...I do know a thing or 2 about suspension...:gig:


Dave
 

orion

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Trailing link gear are wonderful and most pilots that I've known who have flown on them do indicate a strong preference for that particular design. But whether motorcycle shocks will work or not will depend on a number of variables, the chief among which are the loads and the required (or preferred) motion. And in case of retracts, the other issue is of course the wing's internal volume.

I think the loads may be your limiting factor - the landing gear needs to be designed for just a bit over 3 Gs. Assuming the average two place may have a gross weight of about 1,500 pounds, that means that the gear must be able to absorb something over 2,250 pounds, well in excess of the design loads seen in most motorcycle systems.

Another issue is deflection. If the trailing link gear has too much motion it becomes uncomfortable to handle on the ground. This will be one of the first items to address in your design since the amount of motion and the trailing link geometry will be principal in determining your energy absorption requirements. Below is a gear I did some design work on some years back - it's at least one example of a general aviation trailing link layout.

Another gear you might want to look at for an idea of simplicity is that of a Mooney. It's very durable and extremely simple in that it does not use an Oleo system, instead it uses a series of Neoprene pucks.
 

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RacerCFIIDave

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I think the loads may be your limiting factor - the landing gear needs to be designed for just a bit over 3 Gs. Assuming the average two place may have a gross weight of about 1,500 pounds, that means that the gear must be able to absorb something over 2,250 pounds, well in excess of the design loads seen in most motorcycle systems.


True enough...maybe shocks sourced from racecar mfgs might be better... Carrera Shocks out of Metro Atlanta is a good place to start... GREAT quality...sensible prices... and they will build custom shocks to meet your needs...loads...travel...etc


Another gear you might want to look at for an idea of simplicity is that of a Mooney. It's very durable and extremely simple in that it does not use an Oleo system, instead it uses a series of Neoprene pucks.

That has great potential too...I had forgotten about those Mooney systems...


Dave
 

Jeremy

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I haven't done the sums yet (too early in the morning here for math.......) but I would guess that some of the motorcycle spring/dampers used for off-road stuff might be up to it.

A typical 3g undercarriage free-fall certification drop test is from a quite modest height, often only a foot or so. Off-road bikes, although much lighter in weight, seem to regularly "land" from 4 to 6 feet up, maybe more, at similar forward speeds to a light aircraft.

I've no knowledge of this type of bike system, but have spotted that they often use mono-shock rear ends, with one unit taking all the rear wheel loads.

Weight might well be the major problem though, as "lightweight" in a bike context is most likely pretty heavy in an aeroplane context.

Jeremy
 

Topaz

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A possible issue with off-road bike struts could be the long stroke. You might be able to gang more than one per landing gear to take the load, but the stroke on those things is mighty long for aircraft use. Sportbike rear struts (especially for the heavier 1200cc bikes) might be a more viable option on this score.

Still, I'd be more inclined to go with the rubber-puck idea mentioned above. Not as finicky, can be used in a trailing-link gear, and can be tailored to exactly your design. Landing Gear Design for Light Aircraft by Pazmany has a good description of these.
 

RacerCFIIDave

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A possible issue with off-road bike struts could be the long stroke. You might be able to gang more than one per landing gear to take the load, but the stroke on those things is mighty long for aircraft use.

Absolutely! Lots ot those MX shocks have 15" of travel...you could drag a wingtip and not get a wheel off the ground:cry:

Sportbike rear struts (especially for the heavier 1200cc bikes) might be a more viable option on this score.

Even the sportbike shocks likely have excessive travel and light load cap...racecar shocks...can be found/ordered in most any combination of load cap, travel and damping options...

Still, I'd be more inclined to go with the rubber-puck idea mentioned above. Not as finicky, can be used in a trailing-link gear, and can be tailored to exactly your design. Landing Gear Design for Light Aircraft by Pazmany has a good description of these.
And...as much as I like hydraulic dampers...the weight advantage of this concept alone pulls heavily in its favor...

there are other advantages I thought of while writing this... a blown shock can dump hydraulic oil on hot brakes = fire!

and...well...a pile of pucks of differing materials cannot blow...:grin:

and...if you pick the right puck materials in the right order...you can tailor the spring rate to be a rising rate...


Dave
 

orion

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As an example, the gear pictured in my earlier post was designed for a airplane with about a 4,500# gross weight. The stroke on the Oleo in the system was somewhere around 3". The wheel travel was about seven inches.

To get the proper energy dissipation in that short a stroke, the system must have the internal metering components designed specifically for those conditions. Since most aircraft applications need at least a similar geometry, it is unlikely that any type of automotive or motorcycle based system could be easily adapted. At least not without internal modifications.

However, if you're going for something like a bush plane, where landing gear deflections might be sizable, an automotive system might stand a better chance of incorporation. But in all this, the design does really depend on the motion (especially the length of stroke), the amount of energy you need to dissipate and of course, physical size. I've seen some nice aluminum units on some of the bikes - with a bit of redesign, who knows?
 
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Gary,

I used an off road racing shock that has worked very well. Sway-a-way, Fox racing, King shocks...etc make an emulsion shock that is charged with nitrogen (no heavy external spring)...don't tell them its for an airplane! It is very much like a motorcross shock, but designed for heavier loads. Mine is modified internally to allow pressure variation on the fly. This setup may be overkill for you, but your idea of "not bouncing" can work. The downside with a long travel suspension, as was mentioned earlier, is that your plane will sway very easily in a x-wind. On strong wind days, I drain the pressure from the shocks so that the plane sits flat. Pressure can be added back via an on-board nitrogen bottle.

The plane has 25 inches of wheel travel. The empty weight is 1510#, and the shocks will just bottom out at 1000fpm - no flare. It's the most fun I've had landing a plane (probably because it takes no skill!). Unless you are bush flying, this would be serious overkill - but for the record, what you want to do can be done. The link below is a you tube video, about 600fpm.

Geoff

 
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Tom Nalevanko

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Geoff,
That is really a cool video. Reminds me a bit of the gyro that does a similar feat with a scissors type gear. Can you tell us more about your plane?
Blue skies,
Tom
 

PTAirco

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Do you have a G-meter in that plane? It would be interesting to see what loads a landing like that generates. Or lack thereof.
 
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arizonarotors

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Hi Geoff,

Where could a person get a few drawings (or even good pictures) of your main gear set up?

It looks really good.

How does it work on pavement? Is the tire scrub a problem on a paved runway?

Regards,

Jim Mayfield
 

JMillar

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On most sportbikes, there is quite a lever arm (the swingarm) multiplying the force on the shock/spring.. if you can decrease the lever length or have it direct acting that's a lot more weight with a much shorter stroke. Remember that such shocks are also (for modern examples) triple-adjustable, plus you can have the springs altered / replaced.
 
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Thanks for the nice compliments.

The landing gear geometry is similar to a pilatus porter, but with a much newer shock. The fuselage is a highly modified Maule -2' longer & a cabane "V" to optomize the swingarm geometry which included a lot of new tubing & welding to the original structure. Wings are extended to 34.5 & have leading edge slats. The engine is a hopped up IO-540 with a 90" borer prop. There are lots of other smaller modifications as well.

I don't have a g-meter, but from a seat of the pants evaluation, a 600fpm landing feels like a normal 3 point landing on stock oleo gear. A 1000fmp landing is very firm, but not harsh. I wouldn't want to do it every landing.

There is a considerable amount of scrub, and on pavement it is noticable in that the tire & gear combination tends to grab a bit. It's nothing dangerous or uncontrollable, but I avoid pavement because of the cost of the bushwheels!

I have quite a few photos that would help show the contruction of the gear, but I'm not sure how to post them - I'm new at this site. any suggestions?

Geoff
 
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Ok - figured it out. The first photo is one shock deflated, one extended -testing purposes only! The second is a shot of the additional structrure added to the Maule fuselage for the swingarm attachment. the third is normal ride height with both shocks equally inflated.

Geoff
 

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Dana

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Kinda resembles the gear on a Fairchild 24.

-Dana

And they shall beat their swords into plowshares, for if you hit a man with a plowshare, he'll know he's been hit!
 

GVanmeter

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Gary: I think I talked to you in the Camarillo EAA hangar where I'm building an all wood Stork. What did you end up with for gear? I think you were going to use some Foxx shocks? I'm Gary also.
 

GESchwarz

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I think I did look into the Foxx shocks. It would be the equalivalant to what might go onto a dune buggy. But of course to get it optimized you need to do the math on weight, decent rate, max g, suspension travel and angles.
 
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