At Airventure, I expressed interest in the new Zenith "Donut" nose gear suspension system and Roger recently provided me with a kit to retrofit my STOL 750 so I could provide an early evaluation and testing (Zenith has been testing this in-house for a year).  This system will also be available for the Cruzer and the 601/650.  The 701 system will follow, but apparently it will need some modifications from this design.

The original bungee system works well, but it does have a single-point failure potential (the bungee!) and apparently the last few years the production process has changed and bungee failures are occurring more frequently.  In addition, the bungee is non-adjustable for pre-load and induces some torsional resistance when the nose gear rotates.  

My kit arrived Friday and was very complete - the only additional material needed was some white lithium grease to lubricate the area where the donuts are located.  A detailed drawing and step-by-step instructions were included (note correction on Page 2 about spacer above last puck).  The total weight of the installed parts was 3.5 lbs (this is with one steel collar - the second collar is removed after pre-load adjustment).  The bungee and bungee pin removed were 0.5 lbs for a net weight of 3 lbs. Here's what's in the box:

I removed the nose gear by cutting the bungee and detaching the steering rods and lower bearing.  I had the stubs that hold the bungee on the upper end of the nose gear cut off and the resultant holes welded shut. (You can modify your own nose gear, send it to Zenith for modification, or purchase a new nose gear.)  I powder coated the lower, exposed portion of the nose gear (not required, but something I had wanted to do the next time the nose gear was off!), painted the area from the steering arms up to 10" from the end of the upper gear leg, and ground and profiled the welds to provide a smooth surface. 

I polished the upper exposed 10" with a #80 aluminum oxide abrasive disc backed by a foam pad (so as to conform better to the curvature of the tube).  It is important to polish the tube and profile the welds so the donuts can slide smoothly.

The kit includes 10 spacers and 10 rubber "donuts" or pucks that are stacked above the 2 steel shaft collars.  I found the spacers and donuts to be a tight fit, so I opened them up slightly with an oscillating spindle sander.  The sander removed very little material from the donuts, but easily opened up the spacers so they could slide on the tube without binding. The spacers and donuts are then stacked on the nose gear (start with a spacer, then a donut, and alternate, finishing with a donut).  The rubber donuts fit snugly, but will slide with a little lithium white grease for lubrication (recommended by Roger). I then drilled and riveted the front and rear angles to the upper stop.

These 8 rivets were drilled out in the forward firewall gusset on each side and opened up to #12 holes with the upper stop cleco'd in place.

After deburring and Cortec application, the upper stop is bolted in place with 16 AN-3 bolts - heads inboard and nuts outboard. The nose gear is then reinstalled and the stack is pre-loaded by tightening the lower shaft collar, prying up the upper collar with screwdrivers on each side, and then tightening the upper collar.  I then loosened the lower collar, moved it up, and repeated the process for a total compression of between 3/8"-1/2".  Some pre-load is necessary to permit the self-centering of the nose gear in the lower bearing.

Apparently I got the pre-load about right - when the aircraft sat back down on the nose gear, the steering arms rode approximately 5-6 mm above the bearing block, allowing for easy ground steering.  As I mentioned earlier, once the pre-load is adjusted, the second steel collar can be removed. (One is sufficient and they weigh 1/2 lb each!)  Zenith will eventually have an adjustable tool to adjust the pre-load and the second collar will no longer be necessary at all. With one shaft collar, Roger recommends Loctite on the securing machine screws.

My original bungee system worked great.  It was smooth and I couldn't even detect the self-centering "notch" as I swung the rudder from one side to the other. (IMHO, most rudder smoothness problems are due to over-tensioning the cables.)  However, I was amazed at the difference after installing the new "donut" system!  The suspension feels more compliant and is quieter. Steering effort on the ground was reduced and in the air, the rudder pedals were extremely light. However, when the rudder was centered, it seemed to hold it's position well.  The best way I can explain the difference in "feel" is it is similar to the difference between manual steering and power steering - it feels like the nose gear is turning on ball bearings - there is absolutely no torsional resistance!  I always felt my finger-tip dual stick forces were much lighter than my rudder, and now they are equally light.  After flight testing and bumping along on a turf strip, I checked the bearing marks on the grease on the strut below the bearing and it appears the gear was deflecting about an inch during landing and taxi, which is fairly similar to what I saw with the bungee.

About the only negative is the additional weight over the bungee, but that's a small penalty to pay for eliminating the potential single-point failure of the bungee and eliminating regular bungee replacements.  It was a fairly easy retrofit since the Jabiru is a light engine and there is plenty of working room between the engine and firewall.  I understand Zenith is going to make this system standard with new kits.

John

N750A

(Disclaimer: No business or financial affiliation with Zenith Aircraft.)

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I give the pedal bearing block a shot of LPS lubricant at annual and that seems to work fine.  LPS1 would probably be best as it leaves a dry film (to minimize attracting grit and debris), but it is supposedly effective for a short time.  I usually use LPS2, which supposedly lasts a year but leaves a thin, oily film - I wipe the excess away once it has wicked-in to the bearing.

I used to always see dramatic improvement from using white lithium grease on the nose gear lower bearing block, but even when thoroughly lubricated, there is no comparison to lack of friction and smoothness with the new donut system!  I think a good deal of the resistance in the original system is due to the resistance exerted by the rubber bungee as the nose gear strut rotates.

John

The part about bungy is certainly true but I have the issue as well and don't have a bungee. Have the spring. I keep trying to remember to lube the block before next flight and keep forgetting.

In case anyone missed it, Zenith has a blog post about the new suspension system here.

John

Thanks John. Apparently the 701 must be more of a challenge as I’ve not seen anything about the new system for the 701.

Thanks for your write-up. I will be testing the donut nose gear in Alaska on my Cruzer too.

Recently, I fabricated a Tie-down Ring Camera Mount and shot a Camera Mount Test Video to assess the stability of the mount, etc.  I deliberately included the front landing gear in the frame so I could also assess the new Zenith "donut" suspension system as the nose wheel bumped along over my turf runway at TN66 .

In the video, the suspension moves smoothly as one would expect.  A few years ago, I made a similar video with another action camera.  What struck me as I listened to the audio was the marked difference in the landing gear noise during take-off and landing!  The older video was with the original bungee system and you hear lots of thumps and clunks emanating from the nose gear.  In the recent video with the new system, it is completely silent!

I also noticed that with the bungee, the gear seems to very abruptly jump up and down but with the donuts, it is much smoother.  I think this is because as the donuts are progressively compressed, they expand horizontally and the internal diameter of the donut decreases, progressively gripping the landing gear strut tighter and thus providing a dampening effect both as the gear compresses and extends.  Also, since the donuts' pre-load is adjustable, it allows the suspension to be adjusted to be more compliant. The bungee, however, simply stretches as far as it can go and then snaps back.  

I had already reported that I thought the new system was much quieter from the cockpit perspective, but listening to these videos objectively demonstrates this!  I certainly don't think there's any harm in having "noisy bungee" landing gear, but perhaps the new system will spare the airframe some wear and tear.  Moreover, I do like mechanisms that operate smoothly and quietly! ;>)

John

Neat mount John.  I couldn't help but notice the airplane position moved around in the video. With fixed mount, not sure I understand that.  Is the mount flexible to some extent?

Sure think I would go for the donut system if I hadn't already changed to the spring.

But to cameras, do you know anything about the Ion Air Pro 3?  Also, I have the Dynon pitot and was thinking about clamping to it well behind front.  Guess I would need to do some test flights to see if it affected IAS any.

Keep the good info coming.

 I couldn't help but notice the airplane position moved around in the video. With fixed mount, not sure I understand that.  Is the mount flexible to some extent?

Funny you mention that, Joe - I can't quite understand why the airplane appears to move and the long-distance scenery doesn't!  The mount is not flexible (I think!).  I wonder if it has something to do with the digital image stabilization? Also, since the mount is out near the end of the wing, maybe the excursions of the fuselage up-and-down as turbulence is encountered are exaggerated relative to the background scenery?

It will be interesting to see what difference, if any, it makes now that I've shortened the mount nearly 2".  I hope I get a chance to fly it this week, but it's awfully cold here in East Tennessee at the moment!  If I don't get it done this week, it'll be nearly a month before I can fly again as I'll be out of the country for a few weeks.

The fuselage undulations in the present video do, however, reinforce that the plane is flying and not just stuck at a point in the sky!

Maybe some camera whiz can jump in here and explain what is happening!  ;>)

John

Hi John,  You are correct, the image stabilization is causing the movement.  It's my understanding the camera sensor samples more data than is needed for the frame resulting in an oversized frame, the gyro's in the camera detect the motion of the camera and a video processor will move the viewable frame within the oversized frame with some dampening to make the movement appear smoother.  Your video looked great!  Keep them coming!

a video processor will move the viewable frame within the oversized frame with some dampening to make the movement appear smoother.  Your video looked great!  Keep them coming!

Gee ... and I didn't even know what I was doing! Haha!  However, like I said, I kind of like the effect as the movement of the airframe within the frame makes the airplane look more "dynamic" or like it really is in-flight rather than being rigidly fixed in the frame.  Now that I think about it, I've noticed this same effect when looking at video's of close-formation flying, etc.

Thanks for the explanation!

John

Wow, you have a beautiful spot there.  

Rusty;
The plane was a total write off: on its back with a bent empennage, folded prop, bent wings, the works. I touched down firmly in a suddenly shifting 15 knot direct crosswind, bounced a bit, and as I added power to stabilize to either go around or touch down found that I had no rudder control at all. Rather than risking a spin or hitting the trees I elected to set it back down. When I lowered the nose, it kept right on dropping. The Front gear was completely retracted against that wire ring stop that is on the strut. That caused 2 things to happen: 1 - it jammed the nose gear steering and rudder control, 2 - when I placed the nose wheel the second time it wasn't "where it should have been" and hit with more and totally undampened impact.

With no rudder and no idea of why no rudder I decided to take my chances on the ground. That is what insurance is for; for bent airplanes not dead pilot's relatives benefits. But it happens fast and you don't have much time to make a decision.


By then I was off to the side of the grass field where the sod was soft from recent rains and because of no rudder and stiff cross wind I was not aligned in direction of travel. Impact plus soft earth plus side load folded the front gear. I used full backpressure to keep the nose as light as possible and flaperon to keep the wing out of the dirt. It slide on the nose about 100 feet. With loss of elevator effectiveness the plate mounted on the gear fork to accept a Piper tow bar, acted like a plow share and started digging a furrow. after about 25-30 feet of slide it had dug deep enough to catch and flip the plan on its back. It slid barely a foot after flipping, but with the rudder stuck in the soft dirt it caught and bent up everything.

The robust structure prevented any injuries. The forensics on the failed bungee showed localized embrittlement of about 75% of the fibers directly under the manufacturer's glued on tag, with no damage adjacent to the tag. The remaining fibers let go on the first bouncy landing. I highly suspect the use of an incompatible glue on the tag. The bungee was barely 2 years old and was inspected only 30 days prior.


Of course the FAA totally ignored the failure analysis, called it pilot error and lifted my license until I did remedial crosswind training and retook the my commercial check ride. It was an expensive, hostile, and humiliating experience. I didn't learn anything new about crosswind landings, but I did learn that the FAA has a quota of pilot scalps to fill and they are remorseless in trying to fill it. You don't want to be on their gotcha' list.


I am using a higher rated bungee which makes the steering and spring action very stiff. You can understand why I want to get rid of the bungee. I got a note from Zenith that they would look into donut conversion of an 801, but despite several emails and a call to them I have had zero response.


All I want to know is who makes the donuts? I can engineer the installation and make the rest of the parts myself, but can't find a source for the donuts. Are there any manufacturer identification markings on the donuts that come with the kit?

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