Re: "Exponential" differential via mechanics

David J. Gall


Allow me to rebut:

Part of the reason it skids is that the plans ratio moves it too much. At
higher speeds, the tailwheel is quickly and easily pushed past the limiting
slip angle and it begins to skid. Like an all-flying tailplane that has too
much throw, the pilot can push it right past "stall" and it becomes less
effective than it could be were it not stalled. Likewise, the plans ratio
allows the rudder control system to push the tailwheel right on past the
skid limit of the tire (between 3 and 7 degrees, depending on tire type)
when the pilot applies "a little" rudder pressure.

At lower speeds, when the tail can respond and a turning radius can be
accomodated, the amount of deflection can be more without exceeding the
limiting slip angle. So it makes sense to have a differential system. At
higher speeds the smaller throw near center helps to keep from skidding the
tailwheel, actually increasing effectiveness, and at lower speeds the large
throw needed for ramp maneuvering is still available.

If you think it is a lot of complication, consider that your airplane
actually has the opposite, a DEcreasing differential, due to the installed
angle of the rudder pedals and the absence of any thought given to the
design. The cables simply attach to the sides of the rudder pedals, so as
the rudder pedal is pressed forward the amount of linear pull on the cable
actually diminishes for increasing angular displacement of the rudder pedal.

Lack of thought does not imply simplicity; more thoughtful design does not
imply increased complication. I gave a perfectly valid suggestion that
increases the "fail point" count by exactly ZERO while reversing the
bass-ackwards differential that the fine folks at QAC gave you. Simply
change the shape of the one-piece rudder pedal so that the cable attach
point is aft of the hinge axis. This one change will give INcreasing
differential without all the monkey-motion of a belcrank, if you wish, while
still addressing the fundamental issue of an ill-executed design.

I'd wager that there aren't many airplane designers who actually take the
time to think about the linkages they create in their control systems, and
we consumers/pilots pay the price every day. Burt Rutan is not immune from
mess-ups in his designs, and having Tom Jewett do the detail design work on
the Quickie was no guarantee of error detection and correction. Gary LeGare
(the plumber) scaling up the design to two-place certainly didn't add any
particular expertise in the realm of control system design and the ensuing
redesign for mass production was not about refinement, either. So you have a
minimalist system that had no real thought given to it adopted in toto and
you now wish to defend it on the basis of its "simplicity" and limited
number of "fail points"? Hello? It doesn't work right, what about that?

The design itself IS a fail point, witness its failure to prevent the
tailwheel from skidding during the fast part of the ground run. Were it not
for the many, more serious design issues missed by the QAC, we would have
been on about this one, say, twenty years ago, but it has taken us this long
to synthesize a consensus on the JB6Pack to where we can actually start to
talk about fine-tuning. If that warrants poo-pooing, then so be it. Some
things truly aren't worth persuing; this one is, and I knew it in 1997 when
I first published my thoughts on the matter.

Carry on, O gravel-runway reflexor-less single-data-point friend....

David J. Gall
P.S. And your Norton Rotary wasn't a lot of complication for dubious
P.P.S. When are Jabiru going to finally hire an internal-flow consultant to
design proper cooling for their engines instead of telling owners to burn
'em in for 50-100 hrs until they "loosen up"? Some of the nicest engines,
some of the most pitiful cowls.... :(

-----Original Message-----
From: Q-LIST@... [mailto:Q-LIST@...]
On Behalf Of Peter Harris
Sent: Tuesday, October 24, 2006 12:07 AM
To: Q-LIST@...
Subject: RE: [Q-LIST] "Exponential" differential via mechanics

Thanks Larry.

To me it seems like an awful lot of complication for a
dubious result. The sensitivity of the tailwheel steering
increases as the speed reduces. At first touchdown the
aircraft momentum and tail makes it stay straight and any
attempt to deflect it is limited by the flex of the
tailspring and the grip of the tyre, so at first it will skid
rather than deflect the tail.
Later in the ground roll the plans ratio seems right for the
job to me. I think it is just something we learn to do and
get the feel with familiarity.

But I am in favour of most ideas as long as they are not compulsory.



Join to automatically receive all group messages.