Winter project, tow out gear - wing wheel

Udo Rumpf wrote:


I have used a trailing arm design successfully for over 12 years
on various gliders . There attribute: it makes the wing /
wheel combination very stable even in rough grass. Here is a picture.
http://www.ssa.org/myhome.aspmbr=825...5&show=gallery


Hi Udo,

I am sure your design and fabrication are superior to what I am using.
Although I find that my wheel, once put on straight, stays straight.

Here is a photo:

http://members.talkamerica.net/dougl...lkamerica.net/
Look at the file named Wing Wheel.jpg I have since covered the foam
with felt.

The construction is very simple with no glassing or welding required.
Btw, I could not get your URL to work.

Regards,

-Doug

I've been convinced that I do need to provide some vertical
compliance, if not exactly "shock absorption". I have a collection of
gas struts in my shop that should work fine.

My goal is to absolutely minimize the "stuff factor" since I already
have way too much "stuff" to transport with the glider. That means
using the trailering wing cuffs as part of the wing wheel assembly and
as small a wheel as I can get away with. Storing and transporting
small wheels and struts should add minimal hassle. I wouldn't plan to
use the tow-out gear on rough airfields.

I plan to make only one wing wheel but retain the option of making
two. I've had some heart stopping moments when an unexpected wind
gust nearly tipped a watered-up glider onto the wheel-less wing.
Using two wing wheels prevents this but adds the requirement for
vertical compliance.

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. I will
make castering wing wheels using industrial-strength lazy Susan
bearings. The bearing, strut and wheel will slide into a pocket on
the trailer wing cuff and snap in place.

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.

This March while at the Marfa Wave Camp, I was helping Dick Johnson move his
glider and he said just grab the tip and we will move it. Someone said
something about that and he said, "you can't put enough load on it to hurt
it"; if Dick says it is ok, it is ok in my book.
Fred
"brianDG303" wrote in message
...

Martin Gregorie wrote:


"Thats why you NEVER push a glider by its wingtips."

From DG:

Pulling an aircraft
von friedelweber am Do Okt 25, 2007 1:36 pm
Is it okay to pull an aircraft on the wingtips?

The question tends to pop up in the gliding society. There is always a
concern that wings could suffer damage if the ground crew pulls on the
wingtips. Therefore we’d like to give you a short insight – without
going into too much detail – on the force that can be applied in a
longitudinal direction. Water tanks and other specific items are left
out in this abbreviated description. The length of the lever enable to
calculate the max. force a wing has to withstand without suffering any
damages.

The building describes a force of 400N as specification for every
glider. This will result in a force of 40kp (longitudinal). A normal
adult will commonly not be able to develop a force of 40kp in a
longitudinal direction without any problems. In other words – it is
save to pull a plane on its wingtips.

On Nov 7, 8:38*am, ContestID67 wrote:
...Does this apply to all gliders? *Hmmmm.

No, I don't think that it does. It's probably safe to pull on the
wingtips of most modern 15m and Standard class ships, but other than
that I would be at least a bit cautious, especially with longer wings
or wood construction.

At issue is the certification requirement for drag and thrust loads
applied at the tip. I don't have Basic Glider Criteria handy, but I
believe that the FAR part 23 and JAR22 requirements are both around
100 lbs applied longitudinally at the tip both forward and aft.

However, and this is important, the fine print of the certification
rules says that this requirement can be waived or reduced if a lower
maximum force can be rationally defended. Many such exceptions are
applied in the type certification of sailplanes; a good example is the
reduced maximum input force applied in pitch at the control stick.
There are a lot of gliders out there where if you applied the standard
FAR part 23 maximum pitch input force you'd come away with the entire
control sticl mechanism in your hand.

The gliders I'd want to be especially careful around are those where
the lift pins are not cross-pinned or otherwise captured, and where
the lift pins are longitudinally fairly close to the main spar. The
Libelle comes to mind in this regard, especially as regards thrust
(forward-pulling) forces at the wingtip. Because of the Libelle's
short fuselage, the transverse tube for the forward lift pins is
snugged right back against the spar, and allow for a fairly great
force multiplication as the wings are pulled forward. Of course, the
Libelle has that nice stout tongue-and-fork spar joining that is
probably capable of absorbing a great deal of the moment applied by
drag and thrust on its own. But still, I've seen forward thrust at
Libelle tips open a substantial gap where the aft lift pin plugs into
the wings. It didn't seem to break anything but it did and does give
me the willies.

Also, consider that a lot of Experimental gliders are not subject to
any substantial portion of FAR 23 or JAR22, and maybe nobody knows at
all what sort of drag or thrust loads are safe. I remember back when
they were selecting the whirled, er, World Class glider, and somebody
submitted a glider that looked a bit sketchy in terms of drag and
thrust resistance. The judges crunched some rho-vee-squared and
decided that the glider probably had inadequate thrust resistance to
safely winch launch.

Thanks, Bob K.
www.hpaircraft.com/hp-24

On Fri, 07 Nov 2008 07:40:04 -0800, toad wrote:

Check your tail dolly's also. Mine had the wheel attached to the cuff
with screws, not through bolts. One day this spring they separated
while towing behind my truck and the rudder hit the bumper and was
damaged.

I also smashed a rudder when the wheel + towbar detached from the cuff,
letting the glider run forward and hit the back of the car. This was a
tail dolly that came with the glider and looked good from the outside.

It turned out that:
- the swivel platform was made from polyester car body filler
- the swiveling wheel's base plate was 'glued in place' by dropping
it onto the partially set up platform. This was left a bit late,
with the result that the base plate adhered over less then 20% of
its surface.
- a single layer of 200g (6 oz) open weave glass cloth was bonded
over the the baseplate, down the sides of the platform and onto the
cuff after poking holes for the wheel's mounting bolts.
- when all was cured the glass was painted and the wheel bolted on.
- err, that's it.

The tow bar followed normal practise and clipped onto the wheel axle,
so all towing loads were handled by one layer of glass cloth and a dodgy
partial bond between a flat steep plate and a lump of car body filler. No
bolts had ever been used to transmit tow loafds from wheel to cuff.

I wonder how many other tail dollies are as badly made as this one.

Pictures of the original and how I made what I consider an adequate
assembly are he

http://www.gregorie.org/gliding/dolly/index.html


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

On Fri, 07 Nov 2008 09:47:27 -0800, bildan wrote:

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.

Be careful. Be very careful. If I understand you, you're intending to
attach the tow bar to the tail dolly's swiveling wheel so that the wheel
carries the tail and there's nothing to lock the tow bar on the fuselage
axis.

I've seen this sort of rig 'Z-fold' when the towing golf cart merely
slowed down. Admittedly this was on a glider with little weight on its
tail dolly wheel (ASK-21) but it was a very gentle slow down and
fortunately no damage was done.

There's a good chance that that damage would have resulted if the same
had happened on wet grass with normal braking and the towbar not
perfectly inline with the fuselage axis.

IMO having a pivot at each end of the towbar (i.e. swiveling tail wheel
and tow hitch) you have an accident waiting to happen.


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

On Nov 7, 6:31*pm, Martin Gregorie
wrote:
On Fri, 07 Nov 2008 09:47:27 -0800, bildan wrote:
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.

Be careful. Be very careful. If I understand you, you're intending to
attach the tow bar to the tail dolly's swiveling wheel so that the wheel
carries the tail and there's nothing to lock the tow bar on the fuselage
axis.

I've seen this sort of rig 'Z-fold' when the towing golf cart merely
slowed down. Admittedly this was on a glider with little weight on its
tail dolly wheel (ASK-21) but it was a very gentle slow down and
fortunately no damage was done.

There's a good chance that that damage would have resulted if the same
had happened on wet grass with normal braking and the towbar not
perfectly inline with the fuselage axis.

IMO having a pivot at each end of the towbar (i.e. swiveling tail wheel
and tow hitch) you have an accident waiting to happen.

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

Exactly. This is the input I'm looking for.

My thought is to eliminate the pivot at the car so the tow bar cannot
swing left and right relative to the towing vehicle thus making the "Z-
fold", which we call a jackknife, impossible. It will, of course,
make the glider swing a wider turn than the car but that may be
useful. Even though the tow bar cannot pivot about a vertical axis at
the car, it would be able to pivot about a lateral axis to accommodate
uneven ground.

This scheme would locate the tail dolly an exact distance behind the
towing car and on it's extended centerline. The tow bar would be long
enough that the tailplane couldn't touch the back of the car.

On Fri, 07 Nov 2008 18:46:29 -0800, bildan wrote:

My thought is to eliminate the pivot at the car so the tow bar cannot
swing left and right relative to the towing vehicle thus making the "Z-
fold", which we call a jackknife, impossible. It will, of course, make
the glider swing a wider turn than the car but that may be useful. Even
though the tow bar cannot pivot about a vertical axis at the car, it
would be able to pivot about a lateral axis to accommodate uneven
ground.

This scheme would locate the tail dolly an exact distance behind the
towing car and on it's extended centerline. The tow bar would be long
enough that the tailplane couldn't touch the back of the car.

I can visualize what you're describing and can see that it would be
gentle on the glider. However, when you go round a corner the dolly wheel
will be forced sideways. The longer the tow bar the more the wheel will
be driven sideways and the larger the forces involved will be.

The attachment at the front end will need to be fairly meaty to keep the
towbar straight. What sort of vehicle will this be attached to and how
would it be attached?

What type of glider is this for?


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

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.