Re: Taxiing before first flight. (long editorial)


David J. Gall
 

Mike,

You are SOOOOO right! It bears repeating:

I would like to point out something I think people loose
sight of regarding the Jim-Bob 6-Pack: The bellcrank mod
allows the builder to reduce the pivot arc (travel) of the
tail wheel relative to the rudder travel. That makes the
plane much less susceptible to sudden swerves at high speed,
and thus easier to control during takeoff and landing. The
tail wheel springs also desensitize the tail wheel.
Consider that a rudder can have twenty-five or more degrees of deflection
before it stalls whereas a tailwheel will start to skid (analogous to
airfoil stalling) at between three and seven degrees deflection depending on
the type of tire. So you really NEED to have some ratio between the rudder
deflection and the tailwheel deflection at any given rudder pedal
deflection.

A belcrank giving a 3:1 ratio of rudder to tailwheel deflection combined
with some springs on the tailwheel to let it trail against load somewhat
(giving an even higher effective ratio) might be about right to really
desinsitize ("tame") the Q2, but you'd have to put up with a large turning
radius at low speeds. Then, too, the ratios described above are mechanical
ratios; there is a complicating aerodynamic component that I'm ignoring
right now, due to the varying effectiveness of the rudder at varying
airspeeds. What ratios are people using, Jim, Bob?

Consider also that the main objective of the takeoff or landing run is to go
STRAIGHT and you soon realize that the ideal place for the tailwheel is
locked, dead straight. Many (most??) WWII fighters had locking tailwheels.
The "reduced arc" you refer to, Mike, was essentially nil with directional
control provided by rudder and differential brakes only. Of course, some
steering capability must be available for low speed taxiing with the
tailwheel unlocked....


David J. Gall

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Mike Perry
Sent: Thursday, October 19, 2006 8:30 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Taxiing before first flight. (long editorial)

I would like to point out something I think people loose
sight of regarding the Jim-Bob 6-Pack: The bellcrank mod
allows the builder to reduce the pivot arc (travel) of the
tail wheel relative to the rudder travel. That makes the
plane much less susceptible to sudden swerves at high speed,
and thus easier to control during takeoff and landing. The
tail wheel springs also desensitize the tail wheel.

(You could achieve the same thing using split cables and
changing the lengths of the control horns on the rudder and
tail wheel, but the bell crank is an easy place to change the
relative travel.)

I understand the tail wheels of some WW II fighters had very
little travel, but I can't seem to document that right now.
My opinion is that tail draggers that land at high speeds
need tail wheels that are "desensitized"
with a reduced pivot arc and tail wheel springs.

I understand the Q-2 plans from QAC used a tail wheel with a
wide pivot arc in order to negotiate tighter corners on
airports. This design worked OK on the Quickie, which lands
at a lower speed, but has been problematic on the Q-2 and
Q-200. The 6-Pack has full pivoting tail wheel and
differential brakes to allow you to negotiate tight corners,
and a reduced pivot arc and springs to desensitize the tail
wheel at high speeds.

Jim, I think this is what you meant when you said: "With just
splitting the cables, Wes has no way to differentiate
(detune)the angle of the tail wheel from the rudder." I just
wanted to say it more clearly.

Mike Perry

At 06:33 PM 10/19/2006 +0000, Jim Patillo wrote:


Fellas,

I had the opportunity to high speed taxi Wes Isbergs' Q200 Saturday
before his first flight and would like to share some info.
Even though
he did a fair job of controlling the plane down the runway,
I found it
very difficult to handle. I could not keep it straight either with
rudder or his braking system or the combination. Wind was
about 12K -
10-15 degrees off the nose. (Note: he has no bellcrank but has split
the cables internally to rudder and tail wheel and has a
modified dual
differential finger brake system).
Guess he didn't buy the Jim/Bob Six Pack Mod concept. My immediate
response to him was to park the plane before he got hurt and fix the
problem!

Now for the details. With just splitting the cables, Wes has
no way to
differentiate (detune)the angle of the tail wheel from the
rudder. He's
installed a modified dual finger brake that has a single stick that
slides sideways to apply pressure to one master cylinder or
the other
or both. That is not good because its way to sensitive and
hard to get
positive results from known inputs.
Further exacerabating the situation is his hand is on the
brake and not
the throttle when taking off or landing. Not a good thing! How many
serious tail draggers have anything other than toe brakes or heel
brakes? Its just not natural to have finger brakes on a tail dragger
and is to dangerous in my opinion. There are way to many
things going
on to be doing this additional "dance".

This leads me to a conclusion. The Q's with sixpack mods
installed are
really quite tame, ask anyone who has them. I cannot for the
life of me
understand why anyone would want to do anything different.
Yet we see
it all the time. Most new Q pilots have a very difficult
time handling
their planes initially and often crack them up or have an
incident even
before the first flight. Because you didn't invent it,
doesn't mean you
can't copy it! The situation with Wes; here's a very
intellignet fella
sitting on a field with sucessuful Q's all around, yet he
chooses to do
his own thing. Why?
To much additional cost? To much additional labor? Doesn't
have a clear
undrstanding of the problem? Doesn't know how to fix it - NOT!
What? I do not understand this.

This leads me to another conclusion. If Wes, sitting in the
middle of
LVK with sucessful planes all aroud, chooses to go a
different route,
what does that tell you about builders working on their own,
isolated
and out of touch? I now understand why this is not so simple
and brace
yourself fellas, we're in for more!

These comments are in no way intended to bring heat on Wes but to
simply make a couple of points as to why Q pilots are still
smacking up
their planes. Hopefully this turns on the lights for someoen
who don't
want to get hurt.

Regards,
Jim Patillo N46JP Q200 800 hours in type. So far, even in severe
crosswinds, this plane has never deviated from the center line.
Bingo! There's a reason.

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