REFLECTOR: towbar

[Bob Jackson]

You might be!  The taildragger is only at risk of ground looping when you're
trying to stop it on landing, when it's decelerating, or when the force is
from the front.  And now when I think about it, the nose wheel is only at
risk of shimmy when you are landing, decelerating, and the force is from the
front.  I guess no one worries about shimmy (or ground looping) on take off!

 

But the bend in the nose wheel fork is there by design to put the CG (and
weight of the nose wheel tire and rim) behind the pivot point of the NG leg.
So I guess that's done to help keep the nose wheel aligned during take-off
and taxi.  And this is actually a de-stabilizing factor during landing and
braking, the root cause of shimmy, but hopefully minimized by the Belleville
washer tension.  

 

So, when we add 'tangs' for the towbar attachment, it sounds like we have a
choice of whether we want the weight of the tangs to de-stabilize take-off
NG alignment (if we put the tangs in the front), or de-stablize landing
alignment (contribute to shimmy) by putting the tangs in the back.  

 

It seems to me that shimmy de-stabilization is the more likely and
significant risk, so we really should have put our 'tangs' on the front, not
the back!  That would also make it a lot easier for the linemen to get the
towbar attached than in our arrangement with the tangs in back!  I guess
we'll have to consider changing them around to the front.

 

Can anyone else who's more professionally qualified (at least than I am)
verify all this...

 

Thanks for the insight Brian!

Bob

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Brian Michalk
Sent: Monday, November 08, 2010 10:05 AM
To: bobj at jaxtechllc.com; Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: towbar

 

I think I have it right.
A taildragger airplane is at risk of a ground loop because the CG of the
airplane is behind the main wheels.
A control surface is at risk of flutter if the CG is too far behind the
pivot point.

On 11/8/2010 12:38 AM, Bob Jackson (Jax Tech) wrote: 

I think having the CG forward of the pivot point will tend to make the nose
wheel unstable and want to flip around 180 -- didn't you say that backwards,
Brian?  Having it aft of the CG would make it more stable and less likely to
shimmy, correct?  What am I missing?

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Brian Michalk
Sent: Saturday, November 06, 2010 11:51 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: towbar

 

Nice design, but normally, you want CG as far forward of the pivot as
possible, not aft.

Also, there was an analysis here ... I think by Ulmann, who concluded that
our shimmy problems are not caused by the CG/pivot arrangement.

On 11/05/2010 11:46 PM, Bob Jackson wrote: 

Dave,

 

Our gear is retractable, so I'm not sure exactly how you might apply what we
did.  However, I can tell you that although many Velocity's don't make any
provision for it, we have found that adding a standard tow bar male 'tang'
arrangement to our nose gear leg has been an invaluable feature.  

 

Moving the plane around in a crowded hangar at home, or when you're
traveling, it has been 'priceless' for us to be able to use a standard small
Cessna type 'manually' controlled tow bar, or one of the big orange ones
(common at most FBOs) to move our plane around.

 

It might be more essential because our Velocity is so heavy, but when we
have a full load of fuel (93 gals), it is very difficult to move our plane
around -- either trying to push up small inclines, or for tight maneuvering
because with full fuel it is essentially impossible to pick up our nose to
make tight turns (unless you enjoy hernias!).

 

My aircraft partner designed (with some ME help) and had fabricated the tow
bar adapter shown below, as it is attached to the top of our nose gear fork.
This is the front view.  It is also important (as I'm sure you would
appreciate) that the extra weight of the 'tangs' is on the back side of the
fork -- for anti-shimmy reasons.