Fixed Gear Replacement Bumper
UPDATE: 10-9-2009. Late last month I was in Florida and visited the Velocity factory. S. Swing told me that the rubber shock has performed without any issues on the factory trainer, since it was installed. I had the chance to make some take offs and landings on the trainer and felt really smooth taxiing and upon dropping the nose. I found them working on a Rocket Racer, at a fairly advanced construction stage, which had the new rubber shock installed. Scott expressed his concern about lack of availability, which may force them to look for an alternative or go back to the original setup. There are still some units available from the manufacturer below but they continue to post a note about discontinuing the part. [J. Bujanda]
UPDATE: The factory has finally installed this new bumper on the Velocity factory trainer and it is currently being tested. S. Swing says that the retrofit was very easy to do, that he liked the quality and density of the rubber, and thought that we probably will not have problems with the rubber eroding over time. He also said that he would keep us informed of their testing and results. [J. Bujanda]
NOSE GEAR BUMPER MODIFICATION
by Jorge Bujanda
While still building, I was told by Kevin Steiner that there was a problem with the fixed nose gear design. He had gone through three nose gear rubber shocks in four years. After several landings, the steel face on the shock in contact with the nose gear strut would bend inward compressing the rubber and diminishing its shock absorbing properties (see pictures at bottom). This would create a gap between the rubber shock and the strut, while on the air (unloaded). Upon lowering the nose on landing, this gap would close and the strut would hit hard against the deformed metal face of the bumper with very little, if any, shock absorbing action. This would result in a somewhat rough nose gear drop and taxiing. In addition, he had experienced a significant shimmy a few times with properly-torqued nose gear washers that he had attributed, at least in part, to this condition. The fix from the factory was to add aluminum plates between the rubber shock and the strut to eliminate the gap.
To me, this kept the rubber of the shock compressed and did not bring back its load absorption capacity. Consequently, this would not make the nose gear drop or taxiing any smoother either. In my opinion, even if the metal plate would not bend, the shock absorption capacity of the original part was minimal due to the load being spread over such a wide area. Craig Woolston and John Schoorl started flying and developed the same problem. At some point, they experienced a strong shimmy episode that they also attributed to the above condition. Based on all this, I decided to formally look for a possible solution before flying myself. I thought about different possibilities and discussed them with my son Daniel (aerospace engineer). At the end, I felt that the easiest fix was to replace the original with a strong rubber part that would not have a metal plate on the shaft side. This would provide the shock absorption action without the deformation of the plate and the resulting problems. After spending a significant amount of time researching on the web, I finally found a part manufactured by Advanced Antivibration Components that seemed to give me what I wanted. It was a rubber bumper that was slightly longer than the original. It would not fit horizontally inside the keel however; I saw this as a benefit. I felt that installing it vertically against the canard bulkhead would further improve the design because of the larger contact surface between the rubber bumper and the nose gear strut. The depth of the part was very close to the original and the rubber specifications seemed appropriate for the requirements with a recommended maximum static load of 1200 lbs and occasional dynamic load of 2150 lbs.
I ran the part specifications and set up by Daniel and he believed it to be appropriate, with an estimated safety factor of at least 2.5 to 3 on a very rough landing. He also confirmed that it should work better if installed vertically. Kevin was waiting for Daniel’s opinion, as he was doing his annual and wanted to try an alternative to installing another original part. He would be the first one to test the modification. By the time we ordered the parts, a couple of other builder-owners believed in the set up and had joined. Kevin made a seat for it against the canard bulkhead with microglass and installed it under compression as planned. His comments follow.
“The airplane feels perfect taxiing and landing. The rubber shock shows no signs of deformity after several landings. It is still tight against the captivator.” Kevin Steiner “Two landings this weekend - both in x-wind conditions - no shimmy, no adverse issues.” Kevin Steiner
Bruce Topp had the same problem and was the second to install and test the part. He did not make a microglass pad for the bumper and just used a metal spacer. Initially it felt “bouncy” for him while taxiing, so he fixed it by adding a thicker metal spacer between the bulkhead and the strut to increase the base compression.
“I did taxi with the thicker spacer and the shock mount performed flawlessly. The real test will be how if feels during touchdown/landing.” Bruce Topp
“I finally flew today with the new shock mount. The shock mount worked perfectly. All crosswind landings and touching down with a slight crab on one of the landings. It’s great to know that if you don’t make a perfect landing, you won’t bend a part on the airplane. For my installation, the 5/16” spacer seems to be correct. This provided the shock mount to be installed with good compression. Nothing more to report because everything worked great..” Bruce Topp
The detailed description and pictures of my installation can be seen at: http://www.jbujanda.com/ConGear.html#NOSE
Many fixed gear Velocity owners have now made the modification and believe it to be a significant improvement over the original set up. Contact information for the part manufacturer is given by Andy Millin below.
For those of us that fly with the gear “down and cured” … If you have worked on the nose gear of the FG, you probably know there is a rubber bumper used as a shock absorber on the nose gear. Like several other parts, this was a “found part.” It is an automotive engine mount that seemed to fit the bill.
I have not flown yet, so I can only relate what I have been told. The rubber portion of the mount is sandwiched between plates of steel. When compressed, the steel will deform and does not relax to the original shape. The bent steel introduces slop. Over time, possibly a lot of slop. I am told that Velocity has recommended replacing the shock, or shimming from behind. Shimming will remove the slop, but not the original problem. The steel may deform even more.
A small group of builders/owners has taken up the task of finding a new option. The original research was done by Jorge Bujanda. http://www.jbujanda.com He located an industrial bumper. It is not a sandwich. It is a rubber pad mounted to a steel plate. Jorge’s son is an engineer and ran the numbers for suitability. His analysis showed it would be an excellent replacement.
Since it was not a proven solution, we didn’t think it wise to recommend it to anyone. We wanted to get our hands on it and try it first. The new part was ordered and installed by Kevin Steiner (Steiner_N111VX SELW/FG) and Bruce Topp (Topp_N125TP XL/FG). The new part mounts vertically on the canard bulkhead. The retaining strap on the current bumper is discarded.
The pad is hard but flexible. Taxi tests revealed the new shock must be installed under compression. Both Kevin and Bruce fabricated shims and adjusted the force so the gear leg would stay firmly held with a two passenger load in the front. Kevin did this by standing on the canard bulkhead and checking for any gap. I couldn’t to this. I’m pretty sure I would shoot a knee cap across the shop. I’ll have to find another way. :)
Once installed, the new shock has taxied, flown, and landed beautifully. There has been no sign of deformation or slop. It appears to be just the right thing. The manufacturer of the bumper is an industrial supplier. Unfortunately, the smallest run they make of the product is in the thousands. At the time the part was located, they had just 17 left on the shelf with no plans to make more.
Gary Paxton (SE/FG) ordered the last of the bumpers this weekend. He reported back that the manufacturer said this has become a “hot item” and they were going to tool up and make more. :) There should be enough for anyone that wants one.Jorge spoke with Scott Swing and they will be getting one. If they see them same results, this might become the standard part.
If you are interested in making this modification, here is the info:
ADVANCED ANTIVIBRATION COMPONENTS
2101 Jericho Turnpike
New Hyde Park, NY 11042-5416
Tel: 516 328-3662
Desc: Rectangular Bumper
Part Number V10Z 7-1011
Their catalog can be found here: [1] (6 MB download)
The part is listed on page 126. There is a picture. Dimensions are also given.
You can call and order it directly. The cost is around $50 (including shipping).
Andy Millin
Instructions
Jorge
- I opened 3/8” holes on the canard bulkhead
- I used a 3/16” aluminum spacer between the canard bulkhead and the rubber shock’s steel plate.
- I inserted a couple of wide area washers between the bulkhead and the spacer, on the top bolt, to ensure a flat support for the spacer against the uneven surface of the canard bulkhead
- I used 3/8 x 16 bolts and nuts
- I inserted a couple of wide area washers between the aluminum spacer and the rubber shock on the bottom to increase compression and ensure that the strut be tight in place against the captivator.
- I had to place 140 lbs on the nose compartment to compress the rubber shock enough to allow installation of the captivator back in place
- Once unloaded, the strut sits in place tight against the captivator.
- I may have to adjust the level of compression before flying.
Andy
- Remove the captivator plate.
- Remove the upper cross bolt.
- Remove the old bumper.
- Place the new bumper vertically. Center approximately over the old bumper bolt hole.
- Drill the top hole. 3/8"
- Remove the shock from inside the keel.
- Bolt it to the forward face of the canard bulkhead through the new top hole. Make it as close to plumb as you can.
- Drill the bottom bolt hole.
- Remove the shock.
- You will need to shim new shock. It is looking like 5/16" is the right number. You will need to verify what is right for your airplane.
- I followed Kevin Steiner and made a microglass pad for a shim. Bruce Topp and Jorge Bujanda fabricated a shim out of aluminum. Builder's choice.
- I used Grade 8 bolts 3/8x16, 1-1/2" Also purchased Grade 8 Nylock nuts. Can be purchased at the local hardware store. If you want to use AN hardware, you'll probably want AN6-14A or AN6-15A.
- Bolt the shock in place with the shim.
- Put the gear leg in and put the cross bolt in.
- Install the nose gear fork and tire.
- Turn the fork backward (point to the nose).
- Remove any sawhorse or other support; placing the weight of the aircraft on the nose gear forcing it to compress against the new bumper.
- Put enough weight in the front end to compress the shock. (on the canard bulkhead, on the floor close to the canard bulkhead, you in the nose, etc.)
- When you have enough weight, you will be able to bolt the captivator plate back in.
You're in business.
Old versus new part
Replacement Part
Installed Shock
Installed with Gear Leg
Shock Shown Pre-compressed
Damaged by a few landings N44VF
This bumper had only 15 flight hours N44Vf
