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
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
