REFLECTOR: tow bar

[Laurence Coen]

Bob,

I learned to fly in J-3 Cubs and Aronica 7AC's and while it's more common to ground loop during a landing, folks manage to accomplish this maneuver during taxi and take off.  I watched a guy turn onto a runway in a 7AC for take off, lose it, take out a runway light and come to a halt inverted  Not everybody has that skill set.

Larry Coen
N136LC


From: Bob Jackson 
Sent: Monday, November 08, 2010 9:40 AM
To: 'Velocity Aircraft Owners and Builders list' 
Subject: Re: REFLECTOR: towbar


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

 


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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?

 


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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.