Winter project, tow out gear - wing wheel

bildan wrote:

I've watched people try to perfectly align non-castering wing wheels
for years. The slightest mis-alignment will get worse the further the
glider moves until they have to stop and re-align it. With a flapped
glider, this has to put unnecessary force on the flap hinges.

My non-castering wing wheel cuffs the right aileron which is free to
move. But a snug fitting cuff will *not* move if on straight. I
achieve a perfect and snug fit by molding the cuff in place on the wing
with Great Stuff insulation foam spray. Again, one quickly learns how
to put the cuff on straight first try. It's no problem, really.


The tow out bar itself will need some careful re-design. The
accidents I have seen and read about all seem to involve a failure of
some kind with the tow out bar. Mostly this is the glider tailwheel
jumping off the bar leading to a jackknife-type accident. I like a
tow-out bar that lets the glider roll on the tail dolly wheel. Maybe
this means vertical-only compliance so the bar remains straight aft
eliminating the possibility of a jack-knife.

Some have expressed concern over the diameter of the wing wheel. Any
concerns over the diameter of the tail dolly wheel?

I have built a tow-out bar for a LAK-12 and have no concern about things
coming loose such as the tailwheel jumping off the bar. Not exceeding 5
mph is part of that lack of concern.

One thing I'm not in love with, regarding the conventional tow-out bar,
is the torque applied to the fuselage between the tail dolly mounting
location and the tailwheel location. This torque is a function of the
weight of the empty glider at the tail and the lever distance between
the tail dolly cuff and the tail wheel. Your idea of allowing the
fuselage to ride on the tail dolly wheel eliminates this torque. I've
thought about this before but haven't tried anything.

Regards,

-Doug

Lak has produced a design for tail Dolly and tow bar that seems to overcome
all the above concerns. I bought one with my Lak 17.

Sorry I can’t paste a picture, so a lot of words will have to do instead.
The basic tail dolly is like many, having a cuff that goes round the boom,
also engaging with the front of the fin to stop it rotating, and a
castoring wheel.

The clever part is a horizontal tube welded to the underside of the Dolly,
with an internal diameter of about 1 inch. The two piece towbar provided
has another tube welded to its side at the end that goes on the Dolly, at
right angles to the general direction of the towbar, and it slides into
the tube on the Dolly.

Even harder describe is the way it locks in. The outside of the tube on
the bar has a ridge halfway round it at the bar end. The tube on the
Dolly has a groove. The Dolly end of the bar is inserted into the Dolly
from the left-hand side of the fin, with the bar part at about 10 o’clock
rather than horizontally at first. It slides fully in so that the ridge
on the bar is ready to engage with the groove in the Dolly. You then
rotate it, to the three o’clock, or towing, position. With no need for any
other locking devices, it now cannot come out. The other half of the
towbar slides on, an integrated spring-loaded peg holds them together, and
the outer part of the bar of course has a coupling to go on the car
towball. This arrangement keeps the fuselage and towbar in a rigid line
in plan view, allows the bar to flex up and down if the ground is uneven,
avoids any risk of jackknifing, and imposes no undue strain on the
glider.

Chris N.

On Mon, 10 Nov 2008 11:30:05 +0000, Chris Nicholas wrote:

This arrangement keeps the fuselage and towbar in a
rigid line in plan view, allows the bar to flex up and down if the
ground is uneven, avoids any risk of jackknifing, and imposes no undue
strain on the glider.

What a neat arrangement. Fairly obvious too, now you've described it.

Is the towbar straight or does it have a vertical bend to make sure it
always passes under the tail when you're turning right?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Chris,

What you are describing is identical to the Stemme dolly, except the
telescoping tow bar (that allows the tow bar to be short enough to pivot
under the horizontal stab) is inserted from the right side of the ship.
Then, if the tow bar is pivoted forward, there's an offset wheel that goes
over-center to raise the tailboom so the dolly catering wheel can be
inserted or removed from the dolly. When pivoted aft, the tow bar can be
extended for towing or pushing the glider.

bumper
"Chris Nicholas" wrote in message
...
Lak has produced a design for tail Dolly and tow bar that seems to
overcome
all the above concerns. I bought one with my Lak 17.

Sorry I can’t paste a picture, so a lot of words will have to do instead.
The basic tail dolly is like many, having a cuff that goes round the boom,
also engaging with the front of the fin to stop it rotating, and a
castoring wheel.

The clever part is a horizontal tube welded to the underside of the Dolly,
with an internal diameter of about 1 inch. The two piece towbar provided
has another tube welded to its side at the end that goes on the Dolly, at
right angles to the general direction of the towbar, and it slides into
the tube on the Dolly.

Even harder describe is the way it locks in. The outside of the tube on
the bar has a ridge halfway round it at the bar end. The tube on the
Dolly has a groove. The Dolly end of the bar is inserted into the Dolly
from the left-hand side of the fin, with the bar part at about 10 o’clock
rather than horizontally at first. It slides fully in so that the ridge
on the bar is ready to engage with the groove in the Dolly. You then
rotate it, to the three o’clock, or towing, position. With no need for any
other locking devices, it now cannot come out. The other half of the
towbar slides on, an integrated spring-loaded peg holds them together, and
the outer part of the bar of course has a coupling to go on the car
towball. This arrangement keeps the fuselage and towbar in a rigid line
in plan view, allows the bar to flex up and down if the ground is uneven,
avoids any risk of jackknifing, and imposes no undue strain on the
glider.

Chris N.

Bumper, I didn’t realise that the Stemme towbar was that similar, never
having seen one.

Martin, the outer part of the towbar that connects to the car is cranked
in two directions, sideways and upwards, so that it never fouls the
tailfin or rudder. (It posed an interesting storage problem when I made
up a rack to hold it in the trailer!)

I think the original idea was that the sideways crank means that they
coupling end of the tow bar should be on the extended centre line of the
fuselage. I suspect that my tail Dolly was misassembled, and the towbar
fits on the wrong side, so actually the tow ball is well offset from the
centre line of the fuselage when it is all set up ready to tow. It does
not seem to affect the towing dynamics to any extent at sensible speeds.

Incidentally, I found it was possible to add some brackets so that the
towbar can be fitted to the wheel portion of my one-man wing rigging Dolly
when that is separated into its component parts. With some additional bits
of wood, it turns it into a trailer that can carry four full water
containers. I will probably take it over to Gransden Lodge some time, so
you could see it there if we happen to meet up. That said, I have not yet
tried it with a full load of water over uneven ground, so I don’t yet know
quite how well it will work in that configuration.

Chris N.

Wing Wheel thoughts:
1) Combining trailer cuff is a good idea to save space if you can make
it work.
2) Caster wheel is not needed, the $150,000 ships' wing wheels don't
swivel. My previous and current wheels never needed it.
3) Tire/wheel: larger diameter is better for runway/grass transition.
Misalignment comments are correct: see it skip in the mirror, fix it
once or twice, won't happen again. Loose fit allows use on either
wing, saving you a 100' walk.
4) Runway/taxi lights versus wheel location and height deserves
consideration: If on the tip, height should be low so the wheel will
be on the pavement and other wing rides over the lights. If mid-span,
make the ship level so both tips clear the lights by an uncomfortably
small amount.
5) Due to #4, weight is a good idea.
6) Strut? My first one w/20" wheel didn't have or need a strut,
current one w/14" wheel has and needs it. It's a very tiny strut.

I think the weight, diameter and strut are inter-related considering
the need to overcome bumpy transitions. You can avoid one of the three
parameters by perfecting the other two. Large wheel, fixed leg, heavy
weight (my previous one) will bounce over most things. Small wheel,
strut, light (current one) will flex over the same obstacles.

~Bruce