GLOSTER
Nosewheel
Thoughts
Contents
What are the significant factors?
New Nosewheel Forks Group Buy to
save shipping costs
Standards of fork and nose wheel
spacers
Update Feb 2010 – Nosewheel
spacer mod number is 12265, standard mod for any RV-6A, 7A, 8A or 9A
Latest news 01 Feb
09 – A few months ago a nosewheel RV that had been modified with a new
nosewheel fork came to grief in a landing tip-over accident in
Bill Knott is now shipping nosewheel forks that allow
a 5” wheel to be fitted, details at the foot of this article and a link to
contact information. Bill was injured in a flying accident last summer but is
well on the road to recovery, best wishes to Bill.
A builder in the
As a consequence Gloster Air Parts will not be
coordinating any further group buys of nose wheel forks.
23 Jan 08 – It
appears that LAA Engineering are about to recommend to the
Briggs Brothers Engineers in
Introduction. Many people may have seen the shocking
video on the internet of an RV-7A flipping over at a fly-in this summer. Some
of you may also have heard that this is just the latest in a series of such
accidents that call into question the RV nosewheel design. This article is an
attempt to present all of the data about nosewheel RVs and RV nosewheels that I
have been able to find over the last few months. I do have a vested interest in
the subject, I have owned an RV-6A for 5 years and now have about 300 hours
experience in “
Accidents are always traumatic affairs. Even if there
are no physical injuries, repairs are often costly and egos get a severe
bruising. In this article I have tried to avoid pointing any fingers at any
particular individual, and would ask that anyone who has been involved to date
takes this in the spirit of increased awareness for all and not in any way of
passing judgment on the actions of any particular person.
In early November Van’s Aircraft issued a Service
Bulletin, which was labeled as “Mandatory”, calling for all nosewheel aircraft
to be fitted with an updated design of nosewheel attachment yoke, with an
attendant modification to the nose undercarriage leg, at or before the next
annual inspection. See SB
There is a weight of opinion that says further
operation with the original design nosewheel set up is dangerous. One the other
hand, some owners have operated for hundreds of hours without incident using
the original design. For the average owner it seems difficult to determine what
the real situation is, I have tried to present the available information to
allow individual owners to decide for themselves what is the
best way forward.
There are four sources of reliable data on which to
draw,
Figure 1 Old and new nose
wheel forks
As of August this year the AAIB’s
database listed 8 reportable accidents involving nosewheel RVs in this country
that occurred during landing or take-off. Add the accident in the video to make
9. There have also been a few more incidents that have not required to be
reported. Of these accidents 3 happened on a hard runway, in 6 the pilot either
have less than 4 hours on type or had less than 6 hours in the last 90 days. In
some the ground was soft, in others it was rough. There is little data on the
nosewheel configuration (which mod standard was fitted, was a spat fitted), or
the pilot’s technique, the cg position (and hence the load on the nosewheel),
wheel fairing to tyre clearance or the nose wheel tyre pressure. There may be
other relevant factors that have yet to come to light.
The NTSB report concludes that as soon as the large
nut (that holds the nose wheel fork to the nose gear leg) contacts the ground
the pilot is almost certain to lose control as the nose down moment from the
nut acting as a plough is greater than the nose up pitching moment from the
elevator – as that is low because of the slow speed (and usually reducing on
landing). The
The new Van’s design is clearly an improvement on the
old as it provides another inch of static clearance (an increase of 25%)
between the nut and the ground. Van’s data shows that there have been no accidents
with the new design of leg & fork, but the number of hours flown with the
new design may be small compared to the original set up. But perhaps the basic
unanswered question remains, is the old design an
accident waiting to happen?
What are
the significant factors?
Reviewing all of available data, it appears that there are several factors that
influence the outcome of each landing in an RV-xA. It
is certainly not true to say that the old nosewheel fork is “an accident
waiting to happen”, the situation is far more complex
than that.
Pilot
proficiency and technique. AAIB data shows that in the majority
of accidents the pilot was inexperienced on type, had not flown much in the
previous 90 days or was relatively inexperienced (in half the reported accidents
less than 400 hours total). Van’s letter emphasizes that the nose gear does not
react well to techniques that might be acceptable in the Piper or Cessna that
you learnt in. In particular nose wheel first arrivals are unlikely to have a
happy ending. Hard braking on soft or bumpy surfaces, especially with a forward
c of g, can significantly increase the likelihood of nose undercarriage
failure. Typical crosswind techniques of allowing the nosewheel down onto the
runway at speeds approaching touch down speed may also not be helpful (two
Nosewheel spat. Flight
without the nosewheel spat fitted is specifically discouraged by Van’s and the
Nosewheel
bearing configuration. Van’s have
offered two different nosewheel axle and bearing support arrangements. The
first was offered with the original RV-6A in 1990, and the second version is
now supplied with any RV-6A, 7A, 8A or 9A. Figures 2 & 3 below, copies of
Van’s drawings, show the differences. Some owners
strongly recommend installing a spacer in the later configuration (see figure
4) to control the tension in the bearings and to prevent seizure.
Nose leg
damper. In the early RV-6A drawings Van’s shows a wooden
damper glassed to the back of the nose leg, that also
doubled as a fairing. In the late 1990s a glassfiber
fairing was introduced (see Van’s drawing C1 issued in 1999), and the damper
was deleted. It has been reported on some on-line newsgroups that Van’s is now
advising against the installation of a damper as it changes the bending and
resonance characteristics of the nose leg.
The
distance of the large nut securing the fork to the nose leg from the ground. As
pointed out by the NTSB, when the metal work contacts the ground the aeroplane
is very likely to stop abruptly. That may result in damage to the nose leg and
the aeroplane tipping over. Any thing that can increase the distance between
the ground and the large nut will decrease the likelihood of contact.
Compl
Load on the nose leg. As the aircraft becomes heavier, and the cg moves forward (not that
those two things happen together), the load on the nose wheel increases.
Van’s have published a series of graphs (Reference 1)
that give a maximum acceptable load on the nose wheel.
Level of
braking. Heavy braking will increase the load
on the nosewheel and could reduce the ground clearance. If making a landing
where several of the other factors mentioned here are present, such as a
forward cg and soft runway, pilots should avoid the use of heavy braking if at
all possible. It may be preferable to go-around rather than continue with a
landing that requires heavy braking to stop in the runway available.
Nosewheel
tyre pressure. Low nosewheel tyre pressure
significantly decreases the ground clearance, however high tyre pressures
increase the likelihood of nosewheel shimmy. Van’s
recommend a pressure of between 25 and 35 psi, RV-6A pilot
Runway
surface. Most accidents have occurred on soft
and/or bumpy surfaces, but that should not be news to most people! Clearly it
is up to the pilot to satisfy himself that the surface he is about to land on
is suitable. Long period larger bumps that change the load on the nosewheel
appreciably can be as troublesome as smaller ridges. No matter how far the
large nut is off the ground, if you drive into a large enough hole or ridge
there will be contact between the two.
RV-6A,
7A, 8A or 9A? Are nose wheel versions of each of the Van’s models
equally affected? Probably not, its difficult to be certain as the number of A versions in a fleet is not known, but from the NTSB data
RV-6s seem least effected while 7s & 9s are more at risk. It may be that
RV-8As are most at risk, but as there are no 8As flying in the UK yet that
might not be very relevant (the new design of nose wheel fork has been included
in all finish kits shipped for the last 2 ½ years so any UK -8As should be
fitted with the new set up).
So what
should you do? A
Mandatory Permit Directive to make compl
You might like to also consider the following:
- Make
yourself a reminder to check the nosewheel tyre pressure every month.
- Ensure
that the nosewheel has a spacer between the two bearings.
- Make
sure you have sufficient clearance between the nosewheel spat and the
tyre.
- Don’t
go flying without the nosewheel spat fitted.
- Take
some refresher training on your landings from an experienced RV A model instructor.
- Try
to avoid heavy braking when at forward c of gs,
when the ground is soft or very bumpy.
- Read
all the reference documents at the foot of this article.
Below are some questions that might be asked
along with the best answer that can be given at the moment.
Q. Van’s have made this a mandatory service bulletin, shouldn’t I do this right away?
A. As the LAA/
‘
Q. As this is a Van's SB do I need to get my’inspector to sign it off?
A. Yes, all work that is not on the list of approved
owner maintenance items must be signed off by your inspector.
Q. As this SB calls for the gear legs to be modified
at Langair in
A. No, as long as the nose geaI
leg is modified as shown on the appropriate drawing (the thread is cut rather
than rolled) the work may be carried out anywhere that is acceptable to the
inspector signing off the installation.
Q. Do I need to apply for a mod to install the new
components called out by the SB?
A. Definitely not! This is a modification specified by
the designer (Van’s Aircraft) and as such may be embodied by the owner, and
signed off by his inspector, without reference to LAA Engineering.
Q. When I built my aeroplane I modified the nose gear
leg/fork/spat slightly and applied for a mod, can I modify the new gear
leg/fork/spat to the same mod?
A. Probably no, but talk to your inspector. As the new
gear leg & fork have different part numbers to the old items, your original
mod submission may have to be updated to call up the new parts. There is no
guarantee that the updated mod will be passed by LAA Engineering.
Q. Can I convert my nose wheel RV to a tailwheel?
A. Yes you can, but is it really necessary? The conversion
is not cheap as a new engine mount and undercarriage legs are required.
The A model fleet have flown hundreds of thousands of hours with only
a very few problems. Heed the advice in Van's letter and you should have many
more hours of enjoyable flying.
Q. One of our group members consistently lands all our
RV-9A with all 3 wheels together, he says that is the way he was taught to fly
it. I feel the aircraft should be landed on the main wheels only with the
nosewheel held off. Who is right?
A. It is always difficult to arbitrate on a matter of
flying style from a written description. Van's letter specifically says that
the nose gear is not designed to withstand landing loads and implies that the
nose wheel should only be lowered on to the ground after touchdown and initial
deceleration. It appears that your group member is not heeding this advice and
that the way he was taught was incorrect. I would suspect he would not find
many people to endorse his technique on any aircraft type.
New
Nosewheel Forks. If you
decide to comply with the SB you will need the following parts as a minimum:
1 x Nosewheel Fork,
WD-630-1
2 x Brackets (L
& R), U-713C L/R
1 x Nosewheel
bearing spacer (not available from Van’s)
Gloster Air Parts
is no longer coordinating group buys of nosewheel parts.
The spacer is to be
placed in the centre of the nosewheel to maintain proper torque in the
nosewheel bearings. Those “in the know”, such as
To answer some of
the further questions that have been asked,
Re-threading and shortening of nose-gear
legs.
Thanks to Pete Greenslade, DV Godden Engineering in
RV-9A builder Les Clark, who runs Briggs Brothers
Engineers, has built a fixture to machine the additional thread using the same
thread milling techniques as Harmon Lange in the
I suggest owners use the RV Sqn to coordinate batches.
Supply of new
nose-gear legs (U-603-3X).
Please deal direct with Van’s for new gear legs. The
cost is $194.00, plus shipping, etc. If you would like to ship your old leg to
them Van’s will match drill your new leg for a further $58. I’m sure there are
engineering companies in this country that could carry out the match drilling,
but I have not yet heard of them. Harmon Lange’s website has a useful set of
instructions for match drilling legs (http://www.langair.com/matchdrilling1.html).
Because the legs are that much heavier than the forks, making
the shipping that much more costly.
Standards
of fork and nose wheel spacers.
From the information I have to hand this is the
situation as I can determine it.
Early RV-6As were supplied with a thin nose gear leg
(1” at the top as it exits the socket on the engine mount),
all
Until the mid/late 90s a “thin” nose wheel spat was
supplied, after that the “pressure-recovery” spats became standard. I suspect
the thin spat will not be able to use the new U-713C attachment brackets.
RV-6As, until the late 90s were supplied with a thick
axle, see drawing below, RV-6/A drawing number 62 dated
These aeroplanes will not be a candidate for using the
spacer I described above. Note also that the drawing shows a wooden damper
glassed to be back of the gear leg – in lieu of a fairing that was supplied
with later kits.
Nose wheel finish kits supplied after February 1999
used a different nose wheel bearing set-up as shown in the drawing below,
RV-6A, 7A, 8A, 9A dwg C1 R2 dated 10/10/01 (initial
issue 2/16/99).
This drawing shows the axle adapters, U-623-1, or
‘mushrooms’ that support the wheel bearing. The spacer
bears on the inner face of the bearing, not on the inner face of the mushroom.
Note also that no damper is shown on the nose gear leg. The other thing to note
from this drawing is the position of the wheel spat attachment brackets.
The drawing below is taken from the FAQ sheet made
available with the SB.
The costs of embodying the change (at the time of
writing) are $154 for the new fork, $15x 2 for new brackets and $75 for
shortening the nose leg and cutting a new thread (at Langair
in OR). The two British companies above can modify nose legs also. All prices
are exclusive of shipping and VAT, Langair requires noselegs to be packed in a sturdy wooden box.
Some have asked if they need to submit a mod to make
the change. The answer is definitely no! This mod has been designed by the
factory and as such can be signed off by your inspector after you comply with
the provisions of the SB.
1. Van’s Aircraft Service Letter dated
http://www.vansaircraft.com/pdf/Nose_gear_service_letter.pdf
2.
Van’s Aircraft Service Letter dated
http://
www.vansaircraft.com /pdf/letters/nosegear.pdf
3.
Van’s Aircraft Service Bulletin
http://www.vansaircraft.com/pdf/sb07-11-9.pdf
4. Nose gear service bulletin FAQs
http://www.vansaircraft.com/pdf/Nosegear_sb_faq.pdf
5. NTSB Structures Study, Case No.: ANC05LA123
http://www.ntsb.gov/publictn/2006/RV_Study.pdf
6. NTSB Structures Study, Case No.: ANC05LA123 – photos and data table
http://www.ntsb.gov/publictn/2006/RV_Photos.pdf
7. Further NTSB comment
http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20051006X01588&ntsbno=ANC05LA123&akey=1
RV-6A builder Bill Knott from
RV-10 nose fork & wheel on 2-seat A models
In the
“I have had several requests that I should post the
details of my nose gear modification to use a RV-10 nose gear, so here goes.
The reason I did the conversion was due to the lack of clearance between the
ground and the nose fork nut as well as the wheel pants. This is not a factor
when flying off of hard surfaces. I got about 1 1/4 inches more clearance after
the modification, I was hoping for more, but it's
better than it was. The gear leg is unchanged. I purchased a new RV 10 fork. It
is exactly one inch taller than the 9a fork (at the main part of the fork where
the gear leg goes through). I took this new fork to a local machine shop and
had them remove one inch from the bottom portion of the fork. Removing one inch
from the bottom rather than the top gives the most ground clearance. The hole
in the fork where the gear leg goes through is larger in diameter so I had the
guys at the machine shop cut me some new oilite
bushings to make things fit. You will also need the part that mounts to the
gear leg just above the fork, the part that the bearing rides against that has
the little ears on it for travel stops, this needs to be bushed to fit the gear
leg as well (you can use one of the original oilite
bushings that came with your -9A fork, it fits perfectly, just trim off the shoulder).
The nut and cup washers are the stock -9a parts. It uses the same size tire as
the main tires used on all of Van's two seat models, 500x5. No change to the
wheels. The stock -9a wheel pant will not fit.
“The mains really weren't necessary. I found a tire that was one inch larger in
diameter that would still fit on the stock wheels. In the end I only got 1/2
inch more clearance at the mains. My stock main wheel pants still fit nicely
with these larger tires, I only had to trim the opening a little where the tire
sticks out of the bottom of the pants.
“The whole reason I did this was that on my very first taxi test with standard
size tires and beautiful wheel pants, I hit a soft spot in the grass.
Fortunately I was going very slowly, but it damaged my front wheel pant and
upon closer inspection I could see that the nut on the bottom of the gear leg
had contacted the ground. This could be catastrophic if done at take off or
landing speeds. Since I have done the conversion I have not had any clearance problems.
And, to be honest, this is the reason I am selling the plane. I have decided to
find a tail dragger, something more suited for rough fields. I just hate the
thought of hitting a really large soft spot, even with the larger gear, at
higher speeds and what it will do to my brand new airplane. I hope this helps
answer your questions. Again, in the end, I did not get as much extra clearance
as I had hoped for but if it saves me from a collapsed nose gear and all of the
damage an incident like that would cause, then it was well worth it.
“I want to make it clear that this modification is untested by Van's aircraft,
so, any builder wanting to make this modification is on his own. Any body
concerned with tricycle geared airplanes on soft airstrips should consider a taildragger instead. I talked to Van's Aircraft about this
today. They made it clear that they do not want builders trying to contact
their venders to build a part for them that has not been designed and tested by
Van's Aircraft.”
http://lloydsplane.50megs.com/index.html
Bill Knott’s nosewheel fork
