Backup for dropping the gear

(Paul Tomblin) writes:

Yeah, it looks like it would need to be "Do not leak hydraulic fluid in
excess of max. gear extension speed."

On the Lance (which is the only plane I have any experience with), even if
you lost hydraulic fluid, air pressure would hold the gear up against the
springs until you dropped down to max gear extension speed. I suspect
you'd get a bit more drag from the gear drooping slightly into the
airstream, though.

Now *that* makes sense. I caught that there was a sensor connected to
the pitot tube that interacted with the gear, but it's clever to use
airflow to keep it from dropping on its own too soon. Thank you for
explaining it.

(BTW, some of my confusion with hydraulics is because of my background
with farm machinery where the oil flows through the valve when "off" -
not because the explanations in this thread were misleading.)

--kyler

In a previous article, "Peter Duniho" said:
"Paul Tomblin" wrote in message
...
[...]
Actually, reading that again, it's possible that what is happening isn't
that it comes on at a lower speed if you have power on, but that the
propellor slipstream holds the gear up against the springs if you have
power on.

I don't know how the "propeller slipstream" could hold the gear up. The
only "propeller slipstream" I'm aware of is the helical movement of the air
around the fuselage, and compared to the other forces involved I doubt it's
all that strong.

The air coming off the propellor is going faster relative to you than your
air speed. If it wasn't, your propellor wouldn't be producing power.
It makes perfect sense to me. Being behind the propellor disk, the nose
gear (and possibly the mains) is experiencing more dynamic air pressure
when the engine is producing power than if you were gliding at the same
speed.


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Paul Tomblin http://xcski.com/blogs/pt/
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"Paul Tomblin" wrote in message
...
The POH for the Lance isn't 100% clear on this, but my understanding is
that with the emergency gear extension, the gear will come down if you go
below a certain manifold pressure at one speed, or will come down
regardless of the manifold pressure at a lower speed.

Well, if it's manifold pressure based, then an engine failure would only
cause the gear to come down if the failure was caused by a blocked air
intake, or if the pilot later pulled the throttle to idle. Again, an engine
failure would not, in and of itself, necessarily cause the gear to extend.

[...]
Actually, reading that again, it's possible that what is happening isn't
that it comes on at a lower speed if you have power on, but that the
propellor slipstream holds the gear up against the springs if you have
power on.

I don't know how the "propeller slipstream" could hold the gear up. The
only "propeller slipstream" I'm aware of is the helical movement of the air
around the fuselage, and compared to the other forces involved I doubt it's
all that strong.

I think it's interesting that the POH puts "propeller slipstream" in
parentheses after "engine power", because to me it's not at all clear what
they mean. The two aren't exactly equivalent, so it's odd they would use
them as if they were. The parenthetical comment ought to add information,
but to me it just confuses things.

Pete

"Paul Tomblin" wrote in message
...
The air coming off the propellor is going faster relative to you than your
air speed. If it wasn't, your propellor wouldn't be producing power.
It makes perfect sense to me. Being behind the propellor disk, the nose
gear (and possibly the mains) is experiencing more dynamic air pressure
when the engine is producing power than if you were gliding at the same
speed.

Hmmm...I suppose so. It still seems to me like the gear enjoys such a small
portion of the overall prop thrust, and that the difference in prop thrust
airspeed and airframe airspeed is greatly reduced at or near cruise
airspeeds. But I admit, I can't imagine what else the POH could be talking
about.

Still, it doesn't explain why an engine failure would in and of itself cause
the gear to extend. Presumably the hydraulic pressure is still holding the
gear up; the prop thrust would be a backup for that, not the sole mechanism
for holding the gear up. The extension mechanism still would need to be
triggered by airspeed or MP changes.

Right?

Pete

In a previous article, "Peter Duniho" said:
Still, it doesn't explain why an engine failure would in and of itself cause
the gear to extend. Presumably the hydraulic pressure is still holding the
gear up; the prop thrust would be a backup for that, not the sole mechanism
for holding the gear up. The extension mechanism still would need to be
triggered by airspeed or MP changes.

Right?

If the hydraulic system was still holding pressure, then I think you're
right.


--
Paul Tomblin http://xcski.com/blogs/pt/
What happens if a big asteroid hits the Earth? Judging from realistic
simulations involving a sledge hammer and a common laboratory frog, we
can assume it will be pretty bad. -- Dave Barry

Paul Tomblin wrote:
In a previous article, john smith said:

Paul Tomblin wrote:

On the Lance (which is the only plane I have any experience with), even if
you lost hydraulic fluid, air pressure would hold the gear up against the
springs until you dropped down to max gear extension speed. I suspect
you'd get a bit more drag from the gear drooping slightly into the
airstream, though.

Okay Paul, I have to ask you to explain those two statements.


What don't you understand? In the Lance, the hydraulic system is there to
raise the gear. Besides manually lowering the gear, there is an automatic
gear extension system and a manual emergency gear extension system.

If you activate the manual emergency gear extension system, springs and
gravity bring the gear down - I'm not entirely clear if hydraulic pressure
helps bring them down if you use the normal gear extension. The
description of the emergency gear extension system in the POH says that
the speed the gear will come down is lower if the plane is developing
power because the prop slipstream holds it up.

I'm told (I haven't verified it yet) that if you activate the emergency
gear extension when you're above max gear speed, the gear won't come down,
but will come down slightly into the airstream developing drag.

I do not believe the air flowing under the wings will hold the gear up.
If the hydraulic pressure is released, the gear will come down.
If you are flying fast, the air may grab the gear doors and possibly rip
them off or otherwise torque/twist the gear (whichever comes first).
Look up stories about the P-51B (or C?) and the redesign of the gear
uplock. Granted, you do not have the Mustang's cruise speed, but the
principle is the same if you exceed Vge/Vgo.

The override prevents the gear from automatically deploying.
You still have to push the lever to release the hydraulic pressure to
enable the gear to drop. The POH further says to kick the rudder right
and left to create a sideward airload on the gear to lock it into
position. In some cases, you must stall the aircraft to get the nosegear
to lock by penduluming forward as the nose drops and airload is decreased.

On Thu, 26 Feb 2004 12:34:18 +0000 (UTC),
(Paul Tomblin) wrote:

big snip

Actually, reading that again, it's possible that what is happening isn't
that it comes on at a lower speed if you have power on, but that the
propellor slipstream holds the gear up against the springs if you have
power on.

little snip

A crude "pitot" head (located within the propeller slipstream)
provides the combined airspeed/prop stream pressure input to the
"super high tech auto extension device"-which consists of a crude
diaphram balanced against a pair of ground adjustable springs attached
to a parking brake valve plumbed between the "up" and "down" hydraulic
lines.

The diaphram sees lower airspeed + higher slipstream the same as it
sees higher airspeed + lower slipstream. If the combined "sense"
pressure is higher than the spring pressure, the gear stay up. If the
sense pressure drops below the spring pressure, the mechanism trips,
and the park brake valve opens, allowing hydraulic fluid trapped in
the "up" line to flow into the "down" line. The mains fall out via
gravity, the nose does the same with a spring assist.

As others have mentioned, the trapped "up" pressure is what holds the
gear in the wells, there are no mechanical up-locks. There are
strictly mechanical spring-loaded down-locks that hold the drag/side
braces in an over-center position. With normal or emergency extension,
these mechanical downlocks are all that is holding the gear down,
there is no "down" pressure in the system with the gear
down-and-locked. The switches that turn the green lights on (and turn
the electro-hydraulic pump off) are driven directly off of these
mechanical down-locks.

Turning the auto extension "off" mechanically pins the trip linkage
into the park brake valve closed position, irrespective of "sensed"
diaphram pressure.

The SB to remove the auto extension feature removes the diaphram from
the loop, and the valve stays in the closed position during normal
operation. The lever that useta be used for auto extension, is now
used to mechanically open the valve to allow emergency extension if
desired.

The really, really neat part is adjusting the balance springs standing
on your head in the back of the airplane while the test pilot
repeatedly accelerates and decelerates (airspeed pressure), and varies
power setting (prop stream pressure) until you get the durn thing
adjusted so it works...

Hope some of this helps;

TC

P.S. the "smallest" hydraulic gear airplane that I am aware of with a
totally independent back-up emergency extension system is the Cheyenne
II XL. It has a secondary set of pneumatic actuators that un-up-lock
and assist/extend the gear pneumatically using 4 cute little CO2
bottles (Mr. Laird very likely has a similiar bottle)

In a previous article, said:
On Thu, 26 Feb 2004 12:34:18 +0000 (UTC),
(Paul Tomblin) wrote:

big snip

Actually, reading that again, it's possible that what is happening isn't
that it comes on at a lower speed if you have power on, but that the
propellor slipstream holds the gear up against the springs if you have
power on.

little snip

A crude "pitot" head (located within the propeller slipstream)

Oh duh. Of course. I forgot the pitot was in the slipstream as well.


--
Paul Tomblin http://xcski.com/blogs/pt/
"It is my prayer that other Americans will fully realize that to condone the
whittling away of the rights of any one minority group is to pave the way for
us all to lose the guarantees of the Constitution" - Harold L. Ickes

wrote in message
...
[...]
Hope some of this helps

Sure, thanks. Nice to hear the full description, so we can finally stop
guessing.

"Paul Tomblin" wrote in message ...
In a previous article, "Peter Duniho" said:
And actually, while I haven't flown the Pipers you're referring to, my
understanding is that the gear extension was based on airspeed, not engine
power. Your comment about the behavior of such systems in icing conditions

The POH for the Lance isn't 100% clear on this, but my understanding is
that with the emergency gear extension, the gear will come down if you go
below a certain manifold pressure at one speed, or will come down
regardless of the manifold pressure at a lower speed.

The Arrow auto extend is simply a mechanical belows on the second pitot (just
behind the clear prop window) that dumps hydraulic pressure when you get slow.