Roee Kalinsky

Electrical Engineer (Independent Contractor)
Specializing in FPGA, ASIC, and Embedded Systems Design

Nose Gear

Running Total Hours: 0.0

 

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2008.12.09: (0.0) Failures of the nose gear on the RV-xA models has been a highly controversial topic in the RV community for a few years now.  Anecdotally, there seems to be an alarming rate of occurrences of the nose landing gear leg buckling, often followed by the aircraft flipping over.  Some of the accidents in question occurred while operating on unpaved or otherwise rough surfaces, but others occurred on paved surfaces under what most would consider normal operating conditions.  Even under ideal conditions, the nose gear is known to have a tendency toward shimmy problems and large magnitude flexing oscillations in the gear leg.  There are those, including Van's, who insist that there is no problem other than perhaps poor pilot technique.  And then there are those, including myself and the NTSB, who believe that there are also one or more design deficiencies that are major contributing factors to these accidents.  Primarily, there are the ground clearance issue and the wheel rotational resistance issue.  The phyiscal properties of the gear leg itself have also come into questions, but the limited data that is available on that to date has not revealed a problem.

DESIGN OVERVIEW

The nose gear design on the RV A models is very simple.  The gear leg is made of solid 6150 spring steel rod and is bolted to the engine mount.  It has a circular cross section, and tapers down in diameter toward the bottom.  The main span of the gear leg has a forward sweep angle, and makes a bend to vertical at the bottom.  There, a free-castoring aluminum fork holds the wheel/axle assembly.  A fiberglass fairing mounts to the fork and covers the wheel assembly.  Another fiberglass fairing covers the gear leg.  The fairings provide aerodynamic drag reduction, but are not otherwise structurally significant.  The gear leg is fairly elastic, and there is no provision for dampening.

ISSUE: GROUND CLEARANCE

It has been suggested that these nose gear failures occurred because the front of the fork or the bottom of the gear leg made contact with the runway surface and dug in.  This idea is supported by damage that has been observed on those parts of the aircraft, as well as grooves on the runway surface at some of the accident scenes.  It makes sense that if that occurs, there may be enough force exerted on the gear leg to cause it to buckle.  However, that in itself doesn't explain what causes the fork or gear leg to conact the runway surface in the first place.

The pattern of RV A-model nose gear failure accidents eventually caught the attention of the NTSB, and a study was commissioned.  The study, NTSB Case No. ANC05LA123 released on June 21, 2007, focused on the ground clearance issue as the key to this pattern of accidents.  The study found that a number of factors including the fork design, low tire pressure, heavy firewall-forward configurations, and uneven runway surface can reduce the ground clearance to nothing.

REMEDY: NEW GEAR LEG AND FORK DESIGN

Although Van's has always maintained that there was no problem with the gear design (liability, I understand...), in February 2005, before the NTSB study was even commissioned, Van's quietly started shipping a redesigned gear leg (p/n U-603-3) and fork (p/n WD-630-1).  The design change was supposedly made for manufacturabiliy reasons, but in any case, the new design provided improved ground clearance.  On November 9, 2007, following the release of the NTSB study, Van's finally issued Service Bulletin 07-11-09 for the upgrade of older RV's to the new gear leg and fork.  My own finish kit was shipped in October of 2007, and already included the redesigned gear leg and fork, so I'm in compliance.

REMEDY: WEIGHT & BALANCE

On November 9, 2007, along with the mandatory Service Bulletin 07-11-09, Van's also issued a service letter containing further discussion as well as operational recommendations related to the nose gear.  Most notably, the service letter recommends a new limitation on weight & balance.  The recommendation is that the maximum static weight on the nose wheel for an RV-7A should not exceed 375 lb.  I plan to adopt this recommendation as an operating limitation on my airplane.

The service letter contains graphs for determining weight on nosewheel as a function of gross weight and CG for a typical RV-7A aircraft.  Alternatively, I could use the actual measured stations of the nose wheel and main landing gear wheels on my aircraft to calculate it numerically.

REMEDY: TIRE PRESSURE

Van's service letter of Nov 9, 2007 recommends that the nose gear tire should be kept between 25 and 35 psi.  I'll aim for the upper end of that range, and check it dilligently on a frequent basis.

REMEDY: RUNWAY SURFACE

Although many tricycle gear RV's, including Van's demonstrator aircraft, do regularly operate on grass or dirt airstrips, I would have to say that the gear design is not well suited for those types of surfaces.  And the accident history does point to unpaved surfaces as a major factor.  Let's face it: the RV is a wonderfully verstaile aircraft, but it is not a bush plane.  Most of the flying I do anyway is to destinations with paved runways in decent condition, so for me that's not a major limitation.

REMEDY: PILOT TECHNIQUE

This should go without saying, in any aircraft.  The nosewheel is for supporting the weight of the nose sitting on the ramp or rolling at taxi speeds.  It is not designed to sustain landing loads; that is what the mains are for.  For the RV, with its spindely little gear legs, this is even more critical than other aircraft.  On takeoff, lift the nosewheel off the ground as soon as practical.  On landing, hold the nosewheel off the ground as long as practical, then let it down gently.  Taxiing, or any time the nosewheel is on the ground, keep the weight off it as much as practical with up-elevator.

ISSUE: NOSE WHEEL ROTATIONAL RESISTANCE

The stock nose wheel and axle assembly design from Van's (using Matco components) inherently has a significant amount of friction even under ideal conditions, and is susceptible to creating extreme amounts of friction or even to binding completely with only moderate stresses or slight variations in geometry or axle bolt torque.  When the aircraft is rolling forward, resistance to rotation of the nose wheel clearly will impose a force on the gear leg trying to bend it aft.  The greater the rotational resistance, the greater the magnitude of the force will be on the gear leg.  Whether this is structurally significant as a static load is not known, but perhaps more important is its effect on the dynamics of the entire nose gear as a system.  It has been observed that the amount of rotational friction in the wheel has a significant effect on the oscillatory tendencies of the nose gear.  Those RV pilots that have replaced or altered the wheel and axle assembly to eliminate this friction in various ways report that the nose gear no longer shimmies and oscillates.  Some even report that the difference in overall rolling resistance in noticeable to the pilot in taxi operations.

Several design attributes seem to be the major contributors to this rotational resistance.  Firstly, the axle assembly design is such that the wheel bearings are directly squeezed by the axle bolt.  There is no spacer to set the axle geometry and bearing pre-load, so slight variations in axle bolt torque or flexing under load can have a significant effect on bearing pre-load.  Secondly, the bearings themselves have rubber seals that rub and create significant friction.  This also increases dramatically with pre-load.  Thirdly, there is nothing keeping the "mushroom" bushings from rotating themselves, which is what tends to happen once the bearing drag becomes excessive.

REMEDY: GROVE WHEEL AND AXLE

Grove Aircraft Landing Gear Systems, Inc. sells a far superior wheel and axle assembly that corrects the deficiencies in the Van's/Matco stock design.  It uses an axle spacer to precisely set the bearing pre-load, it includes an anti-rotation pin for the "mushroom" bushings, and uses bearings with felt seals for lower inherent drag.  The wheels themselves are also of very high quality, and are available in aluminum or magnesium.  I opted for magnesium to save weight ($30 more, 1 lb. less).  Those who have switched to the Grove product report a tremendous improvement in the ground handling characteristics of the nose gear, and that the oscillation problems practically disappear.  The part number is 59-2M-RV, which includes Grove's standard 59-2M 500x5 nose wheel and bearing, plus the axle components specific to the RV-6/7/8/9A (a different product for the RV-10 is also available).

On a historical note, Van's originally used a nose wheel and axle assembly made by Cleveland on the RV-6A.  Around 1998 they switched to the Matco product, no doubt because it was cheaper.  It is hard to find solid data, but anecdotally it appears that around the year 2000 (presumably when the 1998 finish kits started turning into airplanes) the troubles began.  Incidentally, the Grove design is practically identical to the Cleveland design used on the early RV-6A.

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2008.12.10: (0.0) Today I went over to Grove, which conveniently for me is located 30 minutes away at Gillespie Field, and bought their 59-2M-RV nose wheel and axle package.  Over there I met Gail, and company founder Robbie Grove, both of whom were very knowledgeable and friendly and just great all around.  These folks are true aviation enthusiasts, and are excited about innovating and putting out a high quality product.  This is the kind of company I like to do business with.

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2010.08.09: (0.0) Another plot twist...  There is an issue with the offset of the valve stem relative to the centerline of the wheel.  In summary, the RV -A models use a tire size (11x4.00-5) that is smaller than anything used on certified aircraft (note however that the same tire size is used on some Lancair, Glasair, and EZ models).  While "real" aircraft tires are available in this size (Aero Classic, Desser, Lamb / Cheng-Shin...), the inner tubes available in this size are made and marketed for go-carts, lawn tractors, etc., not aircraft in particular.  As such, these inner tubes don't conform to the aviation standard of placing the valve stem along the center line.  More commonly (and nowhere specified that I could find), the valve stem on these tubes is offset about 3/4" away from the centerline.

Now, the Matco wheel provided in the kit from Van's indeed has the valve stem exit hole cut at an offset from the centerline of the wheel, roughly matching that offset on these inner tubes.  The Grove 59-2M-RV nose wheel however, is actually just their standard 5.00x5 aircraft wheel, and as such it has the valve stem exit hole cut right on the centerline.  This means that with the combination of this Grove wheel and go-cart inner tube, the tube will be twisted and the valve stem will be under shear load due to being forced closer to the centerline of the wheel.

The ideal solution I was hoping to find is a standard aircraft tire and tube of the same or even similar size, that had the valve stem on the centerline, and therefore would be compatible with the standard Grove wheel.  But after much research I've come up empty handed.  So it'll have to be the stock tire and tube.

Through my online research I did find though that this issue has already been noted by others, and remedied by Grove by virtue of another wheel model (59-4M-RV) with an offset valve stem exit hole (the valve exit to the outside is also in a slightly different location that better fits this tube, and does not use a grommet).  I called Grove to ask about it, and they agreed to let me swap my 2M (which was still unused, in the box) for a 4M.  I drove down there in the afternoon and did the swap.

Note that when I was there, I spoke with Gail, who told me that many of the 2M wheels have been used with these offset inner tubes for a long time with no actual problems reported.  She said that the 4M wheel was more just a response to pressure from folks who expressed the same concern as me, without having actually experienced any tube failures.  I do believe her.  But I also do still prefer to "do it right" than to get away with something that's not quite right.  And since they do have a wheel now that solves the issue, I'm happier to have it.  Again Grove proves to be a stand-up company who cares about customer satisfaction.

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2010.09.05: (0.0) Mounted the tire (see main gear page for general technique).  First attempt was a failure.  Pinched the tube between the two wheel halves.  Many others have had this happen, and I can see why.  This is so common that Van's even ships you two nose wheel tubes by default...  Second attempt was successful.  Partially inflated the tube to give it its round shape (which I also did the first time), but also used my fingers through center of the bearings to push the tube out from between the wheel halves as I progressively tightened the wheel tie bolts to bring the wheel halves together.

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2010.11.03: (0.0) Started putting together the fork assembly.  Firstly, I just took the fork weldment and cleaned it up (it came very dirty with all kinds of tenacious residues).  I then did a little bit of filing to remove some surface defects and round over hard corners.  Finally, masked off the brass bushings and threaded holes, prepped the surface, and shot it with epoxy primer.  That should give it a good long corrosion-free life.  I then installed the two cap screws that act as swivel stops.  Used a little Loctite 242 on these guys.  I also installed the MS15001-1 grease fitting ("zerk" fitting) for the swivel joint.  Note: Van's doesn't specify a particular grease to use for the swivel joint, and different builders seem to be using anything and everything under the sun without any problems reported.  I plan to use Aeroshell 22 simply because that is the grease specified for the nose wheel bearings, so using the same grease for both there will be less chance of mishaps.

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2010.11.04: (0.0) Fitted the fork to the leg, set the breakout force, and drilled the cotter pin hole.

 

 


Send mail to roee@kalinskyconsulting.com with questions or comments about this web site.
Copyright 2003 Roee Kalinsky
Last modified: June 23, 2009