Automotive shocks in aircraft

Can automotive shocks or McPhereson struts be used in landing gear struts? If
so how would one determine which ones to use?

Dan, U. S. Air Force, retired

On 14 Jul 2003 07:27 PM, B2431 posted the following:
Can automotive shocks or McPhereson struts be used in landing gear
struts? If so how would one determine which ones to use?

Consider giving this guy a shout:

http://www.telusplanet.net/public/jbrmhall/

----------------------------------------------------
Del Rawlins-
Remove _kills_spammers_ to reply via email.
Unofficial Bearhawk FAQ website:
http://www.rawlinsbrothers.org/bhfaq/

"Corky Scott" wrote in
message ...

You can fabricate a set of landing gear
using springs only (it'd be a bit boingy). . .

Corky.........

Is this just a gut feeling or a pronouncement? )

The Emeraude has springs only (main and rebound) and isn't the least bit
boingy.

Rich S.

On Tue, 15 Jul 2003 07:44:26 -0700, "Rich S."
wrote:

"Corky Scott" wrote in
message ...

You can fabricate a set of landing gear
using springs only (it'd be a bit boingy). . .

Corky.........

Is this just a gut feeling or a pronouncement? )

The Emeraude has springs only (main and rebound) and isn't the least bit
boingy.

Rich S.

It was an attempt at a humerous way of saying continual undamped
motion. :-) Undamped springs can lead to uncommanded oscillations.
You see this all the time with cars that have worn out shocks. In the
worst cases, you see them come to a stop or drive over a bump and just
keep on bouncing up and down.

This is most often the result of undamped (shocks with little or no
oil in them anymore) coil spring suspensions. Leaf spring suspensions
actually have some damping built into the several leafs and the action
between them when the suspension gets deflected so they will damp down
motion a lot quicker than a coil spring.

Bungee cords also have a certain amount of damping effect so you don't
often see that type of landing gear with shocks too.

I'm guessing that the undamped wittman springs and leaf springs work
because the landing gear has very little unsprung weight compared to
automobile suspensions, and also because the suspensions are
relatively short and fairly stiff.

I'm betting that if you were to somehow attach shocks to your
Emeraude, you'd notice a smoother ride across grassy strips. Don't
know if it's worth the weight/drag penalty though.

Corky Scott

"Corky Scott" wrote in message
...

I'm betting that if you were to somehow attach shocks to your
Emeraude, you'd notice a smoother ride across grassy strips. Don't
know if it's worth the weight/drag penalty though.

The Em has coil springs that are pretty stiff. Over the years I've repainted
the lower leg until there's only about 1½" of shiny leg left showing. Very
seldom does the spring compress enough to scratch the paint. The leg does
extend about 2" from the at-rest position when unloaded. There's about 4" of
overall travel in the strut. The ride on grass, instead of being bouncy,
resembles my yard tractor.

But I notice the lack of boingyness most on wheel landings. I can bounce a
Cessna 140 ten feet in the air without half trying; but the Emeraude? Uh-uh.
To tell the truth, I was really surprised. I figured like you that the
undampened springs would be bouncy. It may have something to do with the
rebound spring.

A friend, Col. Russ Russell in Florida, has an Emeraude with gas struts. His
gear has about 6-8" of travel and rides like a Caddy on the grass. But he
has had constant problems with leaking seals and has had to rebuild both of
them several times over the years. While they don't weigh much more than the
plans-built gear, he would swap just to avoid the maintenance.

Rich "Preparation 'H'" S.

"Corky Scott" wrote in message
...
On Tue, 15 Jul 2003 07:44:26 -0700, "Rich S."
wrote:

SNIP
This is most often the result of undamped (shocks with little or no
oil in them anymore) coil spring suspensions. Leaf spring suspensions
actually have some damping built into the several leafs and the action
between them when the suspension gets deflected so they will damp down
motion a lot quicker than a coil spring.

Bungee cords also have a certain amount of damping effect so you don't
often see that type of landing gear with shocks too.

I'm guessing that the undamped wittman springs and leaf springs work
because the landing gear has very little unsprung weight compared to
automobile suspensions, and also because the suspensions are
relatively short and fairly stiff.

SNIP
Corky Scott

Don't forget the scrubbing action of the tires on the surface - that
provides a dampening effect. The sideways scrubbing (outward and inward)
tends to dampen the effect if the wheels remain in contact with the surface.
If the rebound causes you to go airborne, you have another chance to make a
landing. :-) IIRC, the wheels tended to slide more easily on grass than on
a hard surface. Morning dew seemed best of all if recollection serves me
right.

Michael Pilla

"Rich S." wrote in message ...
"Corky Scott" wrote in message
...

I'm betting that if you were to somehow attach shocks to your
Emeraude, you'd notice a smoother ride across grassy strips. Don't
know if it's worth the weight/drag penalty though.

The Em has coil springs that are pretty stiff. Over the years I've repainted
the lower leg until there's only about 1½" of shiny leg left showing. Very
seldom does the spring compress enough to scratch the paint. The leg does
extend about 2" from the at-rest position when unloaded. There's about 4" of
overall travel in the strut. The ride on grass, instead of being bouncy,
resembles my yard tractor.

But I notice the lack of boingyness most on wheel landings. I can bounce a
Cessna 140 ten feet in the air without half trying; but the Emeraude? Uh-uh.
To tell the truth, I was really surprised. I figured like you that the
undampened springs would be bouncy. It may have something to do with the
rebound spring.

A friend, Col. Russ Russell in Florida, has an Emeraude with gas struts. His
gear has about 6-8" of travel and rides like a Caddy on the grass. But he
has had constant problems with leaking seals and has had to rebuild both of
them several times over the years. While they don't weigh much more than the
plans-built gear, he would swap just to avoid the maintenance.

Rich "Preparation 'H'" S.


The Aeronca Champ had spring oleos without any shock fluid or
whatever, and wasn't boingy. It had a somewhat leaky cylinder
arrangement, and I suppose the air flowing in and out probably damped
things a bit. It had a lot of travel and was too soft, making
crosswind operations hairy.
My Jodel was designed with spring oleos, but was modified to use
steel leaf gear. The leaf's OK, but added a horrendous amount of
weight. Someday when I have nothing better to do (yeah, right) I'll
make a set of oleos for it.
Dan

Corky Scott wrote:

On Tue, 15 Jul 2003 07:44:26 -0700, "Rich S."
wrote:

"Corky Scott" wrote in
message ...

You can fabricate a set of landing gear
using springs only (it'd be a bit boingy). . .

Corky.........

Is this just a gut feeling or a pronouncement? )

The Emeraude has springs only (main and rebound) and isn't the least bit
boingy.

Rich S.

It was an attempt at a humerous way of saying continual undamped
motion. :-) Undamped springs can lead to uncommanded oscillations.
You see this all the time with cars that have worn out shocks. In the
worst cases, you see them come to a stop or drive over a bump and just
keep on bouncing up and down.

This is most often the result of undamped (shocks with little or no
oil in them anymore) coil spring suspensions. Leaf spring suspensions
actually have some damping built into the several leafs and the action
between them when the suspension gets deflected so they will damp down
motion a lot quicker than a coil spring.

Bungee cords also have a certain amount of damping effect so you don't
often see that type of landing gear with shocks too.

I'm guessing that the undamped wittman springs and leaf springs work
because the landing gear has very little unsprung weight compared to
automobile suspensions, and also because the suspensions are
relatively short and fairly stiff.

I'm betting that if you were to somehow attach shocks to your
Emeraude, you'd notice a smoother ride across grassy strips. Don't
know if it's worth the weight/drag penalty though.

Corky Scott

The tires, which are typically inflated such as to provide "cushioning
and floatation" and which look underinflated when compared to modern
automobile tires, also provide a great deal of damping. They also
provide a modest amount of drag when taxiing on pavement as well as low
footprint pressure footprint pressure for operation from grass. If you
overinflate the tires so that the mains do not have their normal 25% to
30% deflection, they are decidedly "boingy"; although this is in the
range of a couple of inches, rather than of visibly bouncing the aircraft.

Peter

"Peter Dohm" wrote in message
...

The tires, which are typically inflated such as to provide "cushioning
and floatation" and which look underinflated when compared to modern
automobile tires, also provide a great deal of damping. They also
provide a modest amount of drag when taxiing on pavement as well as low
footprint pressure footprint pressure for operation from grass. If you
overinflate the tires so that the mains do not have their normal 25% to
30% deflection, they are decidedly "boingy"; although this is in the
range of a couple of inches, rather than of visibly bouncing the aircraft.

Agreed. And if you're going to Arlington for the fly-in, blow them up to
thirty pounds. You'll be landing on the concrete - but you'll be pushing it
on the grass. A well inflated tire makes that job a lot easier.

Rich S.

"Rich S." wrote
It may have something to do with the rebound spring.

I think you're probably right. From what I can see of the pictures
and drawings, it's probably sized to damp the natural frequency of the
big spring, which is sized to damp the natural freq. of the small one.

It might be possible to screw up the gear with differently-massed
struts, wheels, brakes, tires, etc. so that the resulting natural
freqs synch up, yielding undamped divergent feedback. (Remember the
films of Galloping Gertie, the Tacoma Narrows bridge?) We had those
kinds of problems in my Statics and Dynamics classes back in college.
But they always made my head hurt. ;-)

Rich, in the Emeraude plans, how specifically are the springs called
out? Do they specify the stiffness (or the spring constant, may be
the same thing), as well as the dimensions? (By way of contrast, the
gear-retraction system of the CA-65 uses a garage-door spring , hardly
a tight specification. But there's not likely to be much occillation,
as the spring is just a mechanical assist while cranking up the gear.

BTW, a newbie question. What's a Wittman undercarriage? I know Steve
Wittman's name and that he was a major contributor to the field, and
I've seen lots of references to "Wittman undercarriage" but what
exactly IS this wonder-invention?

Corrie