Re: Taxiing before first flight. (long editorial)


Allan Farr <afarr@...>
 

I suggested a locking tailwheel about a year ago, but nobody seemed to think very much of the idea.
Allan F

----- Original Message -----
From: David J. Gall
To: Q-LIST@yahoogroups.com
Sent: Friday, 20 October 2006 19:27
Subject: RE: [Q-LIST] Taxiing before first flight. (long editorial)


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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