Nose Fork Modification Considerations

Disclaimer:
I am not an engineer.  Do not consider my calculations or processes
necessarily accurate or applicable to your project.  I have included
this information to simply demonstrate that comprehensive study is
needed when making these modifications.



The Increase in the Bending Moment with My Nose Fork Modification
 
The additional length of the new fork arm is 1.25" which amounts to a 17% increase and that is transferred to the leg.  My
7A nose weight
is 290#.  17% of 290 is about 49# which is the amount I subtract from Van's max. suggested weight on
nose wheel
of 375#.  That leaves me with a maximum permissible nose wheel load of 326#.  Using Van's chart, "Weight
on
Nose Wheel vs Gross Weight & CG Location", ( http://www.vansaircraft.com/pdf/Nose_gear_service_letter.pdf )
it is obvious that that limit can be exceeded under certain loading conditions.  I have to adjust my W&B to accommodate
that limit.

 
I have contacted Harmon Lang concerning building a custom leg for me that would be stronger, but got no response to my
email.  The ideal would be for Van's to offer a properly engineered leg/fork combo that would be available as an option
for
missions such as mine.  In the mean time I plan to strengthen the leg with a stiffener system that is similar to the one Van's
suggests
for the main gear using wood and glass or carbon fiber.  The goal is to return the load capacity for the nose gear
to the
original.


Added Mass of the Larger Fork, Wheel, and Tire

The weight of my new nose wheel assembly is 3.1# more than the new Van's fork setup which makes it about the same

weight as the original Van's fork assembly so I am not concerned about added mass there being a factor.


Below is an email sent to me by Scott McDaniels who is only representing himself.

His email has prompted me to post the additional information about my nose wheel modification on my website.
Those doing the larger nose wheel mod should consider this.


Hi Bob,
    While surfing around the web I ran acrossed a link to the landing gear
mods that you have done on your RV-7A.
From the looks of your work it is obvious you are a very capable craftsman,
but I felt inclined to jot you a quick note in regards to one possible issue.
The mod for installing the different main gear tire probably has value when regularly
operating off of a runway such as yours.  The thing that concerns me is the larger nose wheel tire.
Actually, not the tire itself, but the larger fork required to install it.
    I didn't see any mention of you installing a different nose gear leg.  With the installation of a
longer fork on the standard nose gear leg to accommodate the bigger tire, you have increased the
bending moment that the nose tire induces on the gear leg as a result of vertical load being applied to the
tire and through the longer fork.  No calculations have ever been done as far as I know, but with the longer
fork you could have effectively negated any clearance benefit you gained from the larger diameter tire
because forces applied to the tire can more easily bend the nose gear leg (which reduces the ground
clearance of the fork).  You also now have a higher mass on the end of the gear leg which could be
a factor also.  There has been a flip over accident in the U.K. involving an RV-7A with a
nose wheel mod very similar to yours.  I do not know if a determination has been made as to
whether the longer fork contributed to the accident or not.
 
Just hoping to prevent you from becoming complacent thinking you are now have a more robust nose gear.
In reality it may not be (though you may have already done an engineering evaluation yourself, if so then please ignore my E-mail)
 
Sincerely
Scott McDaniels


This was my response to Scott's email.


Scott,
 
Thanks for your input concerning my nose gear mod.  I would have responded sooner but I needed to
get to my remote hangar to acquire some figures.
 
<<With the installation of a longer fork on the standard nose gear leg to accommodate the bigger tire, you
have increased the bending moment that the nose tire induces on the gear leg as a result of vertical load
being applied to the tire and through the longer fork.>>
 
That is of course is true.  The difference in the arm is 1.25" which amounts to a 17% increase.  My 7A nose weight
is 290#.  17% of 290 is about 49# which is the amount I subtract from Van's max. suggested weight on nose wheel
of 375#.  That leaves me with a maximum permissible nose wheel load of 326#.  Using Van's chart, "Weight on
Nose Wheel vs Gross Weight & CG Location", it is obvious that that limit can be exceeded under certain loading
conditions.  I have to adjust my W&B to accommodate that limit.
 
 <<I didn't see any mention of you installing a different nose gear leg.>>
I have contacted Harmon Lang concerning building a custom leg for me that would be stronger, but got no response to
my email.  The ideal would be for Van's to offer a properly engineered leg/fork combo that would be available as an option for
missions such as mine.  In the mean time I plan to stiffen the leg with a setup similar the the stiffeners Van's suggests
for the main gear using wood and glass or carbon fiber.  The goal is to return the load capacity for the nose gear to the
original.
 
<<You also now have a higher mass on the end of the gear leg which could be a factor also.>>
The weight of my new nose wheel setup is 3.1# more than the new Van's fork setup which makes it about the same
weight as the original Van's fork assembly.  
 
<<There has been a flip over accident in the U.K. involving an RV-7A with a nose wheel mod very similar to yours.>>
I am aware of only one UK flip over which occurred at Croft Farm and involved the original Van's gear fork.  If you have
some information concerning a different accident I would appreciate hearing from you about that.
 
<<Just hoping to prevent you from becoming complacent thinking you are now have a more robust nose gear.>>
I'll be careful and thanks again, I appreciate you taking the time to email me.
 
---- lovin' my RV,
 
Bob Trumpfheller
http://mesawood.info

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