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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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 Posted: Fri Jul 03, 2009 4:53 pm Post subject: self weighing our planes |
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The C-130s I flew at the tail end of the Vietnam war all had fancy and expensive weight and balance computers. The system measured the weight on each wheel and displayed the airplane's gross weight and center of gravity.
I don't know of any that worked, however. The common belief was that the computer went out of calibration after the first landing. All loadmasters I knew used a wooden slide rule specially designed for the C-130. So far as I know, the slide rule never went out of calibration.
On the other hand, one of our primary engine gauges was the amount of torque the jet engine was applying to the propeller. I don't know, but it may have been based on strain gauges. The torque meters always seemed to be accurate and stable.
Dennis
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BobsV35B(at)aol.com
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| Posted: Fri Jul 03, 2009 5:28 pm Post subject: self weighing our planes |
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Good Evening Dennis,
Most engine torque meters get their information from sensors mounted within the reduction gear housing on the engine. It measures the amount of pressure applied to the ring gear. Works quite well and is very reliable. First developed for large radials during WWII. Works good on turbo props as well.
Happy Skies,
Old Bob
In a message dated 7/3/2009 7:54:38 P.M. Central Daylight Time, pinetownd(at)volcano.net writes:
| Quote: | The C-130s I flew at the tail end of the Vietnam war all had fancy and expensive weight and balance computers. The system measured the weight on each wheel and displayed the airplane's gross weight and center of gravity.
I don't know of any that worked, however. The common belief was that the computer went out of calibration after the first landing. All loadmasters I knew used a wooden slide rule specially designed for the C-130. So far as I know, the slide rule never went out of calibration.
On the other hand, one of our primary engine gauges was the amount of torque the jet engine was applying to the propeller. I don't know, but it may have been based on strain gauges. The torque meters always seemed to be accurate and stable.
Dennis
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aerobubba(at)earthlink.ne
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| Posted: Sat Jul 04, 2009 5:13 am Post subject: self weighing our planes |
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| Quote: | Fascinating. Do the docs give any clue what the data source is for
this calculation?
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No, it's just a one-liner that says it happens. I was curious as to how
this is accomplished as well, but that detail doesn't seem to be in the
pilot docs. Or, at least my non-exhaustive search didn't turn it up.
glen matejcek
aerobubba(at)earthlink.ne
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nuckolls.bob(at)aeroelect
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| Posted: Sat Jul 04, 2009 7:06 am Post subject: self weighing our planes |
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At 08:24 PM 7/3/2009, you wrote:
| Quote: | Good Evening Dennis,
Most engine torque meters get their information from sensors mounted within the reduction gear housing on the engine. It measures the amount of pressure applied to the ring gear. Works quite well and is very reliable. First developed for large radials during WWII. Works good on turbo props as well.
Happy Skies,
Old Bob
In a message dated 7/3/2009 7:54:38 P.M. Central Daylight Time, pinetownd(at)volcano.net writes:
The C-130s I flew at the tail end of the Vietnam war all had fancy and expensive weight and balance computers. The system measured the weight on each wheel and displayed the airplane's gross weight and center of gravity.
I don't know of any that worked, however. The common belief was that the computer went out of calibration after the first landing. All loadmasters I knew used a wooden slide rule specially designed for the C-130. So far as I know, the slide rule never went out of calibration.
On the other hand, one of our primary engine gauges was the amount of torque the jet engine was applying to the propeller. I don't know, but it may have been based on strain gauges. The torque meters always seemed to be accurate and stable. |
Yes, these are situations where the strain sensors are
housed within protected environments and VERY important,
spend a lot of time with zero stress on them. i.e. the
sensors are much less likely to creep (acquire offsets
to calibration), offsets are easily spotted if they
happen (gage doesn't read zero when then engine is
stopped). Most important, the elements in which strain
is measured can be tailored to the task. I.e. The
full scale capabilities of the strain gage can be
exploited.
I think the earliest torque measuring systems were
pure hydraulic. The reaction force in a gearbox was
impressed on a piston that compressed a fluid
where pressure gages at the other ends of plumbing
could be calibrated in values of torque. I think
they're still hydraulic but with a pressure transducer
mounted on the gear box as opposed to taking a tube
into the cockpit.
This is an example of an ideal application of
a strain gage where the force to be quantified is
unidirectional by design and relatively free of
noise.
The vast majority of pressure transducer designs
use some form of strain sensor on a diaphram.
http://www.aeroelectric.com/Reference_Docs/Patents/Silicon_Pressure_Gage_4287772.pdf
Since 1960 or so, a new silicon chip strain
sensor has been under development that has a much
higher output voltage. I suspect that very few
new transducer products use wire-wound or foil
strain gages any more.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
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echristley(at)nc.rr.com
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| Posted: Sat Jul 04, 2009 8:53 am Post subject: self weighing our planes |
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Robert L. Nuckolls, III wrote:
| Quote: | I suspect that very few
new transducer products use wire-wound or foil
strain gages any more.
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Most now use piezoelectric elements. Squeeze a piece of quartz crystal
and its resonant frequency changes in proportion to the pressure
applied. A relatively simple circuit measures the difference between
the strained piece of quartz and a reference piece.
--
http://www.ernest.isa-geek.org
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retasker(at)optonline.net
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| Posted: Sat Jul 04, 2009 9:34 am Post subject: self weighing our planes |
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Only most of the high precision sensors use the quartz crystal sensing
technology. Very accurate and stable but expensive technology for
pressure sensing.
The vast majority of pressure sensors out there at moderate prices ($50
to $500) use some form of piezoresistive silicon strain gauges - either
mono-crystalline or poly-crystalline designs. And as Bob noted, sensors
based on silicon have significantly higher sensitivity. The typical
bonded strain gauge bridge has a full scale sensitivity of 3mV/V (mV of
output at full scale strain/pressure per volt of excitation) while the
typical mono-crystalline silicon strain gauge bridge has at least 10
mV/V and 20 mV/V is relatively easy. Bridges using poly-crystalline
silicon fall somewhere in the middle of the above two technologies.
The low end pressure sensors still use resistive elements - cheap
technology but not very accurate or linear.
Dick Tasker
Ernest Christley wrote:
| Quote: |
<echristley(at)nc.rr.com>
Robert L. Nuckolls, III wrote:
> I suspect that very few
> new transducer products use wire-wound or foil
> strain gages any more.
>
Most now use piezoelectric elements. Squeeze a piece of quartz
crystal and its resonant frequency changes in proportion to the
pressure applied. A relatively simple circuit measures the difference
between the strained piece of quartz and a reference piece.
|
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Please Note:
No trees were destroyed in the sending of this message. We do concede, however,
that a significant number of electrons may have been temporarily inconvenienced.
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ainut(at)hiwaay.net
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| Posted: Sat Jul 04, 2009 2:27 pm Post subject: self weighing our planes |
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That is one reason I tried to come up with something. What if your runway isn't long enough for you to hold that 9 degrees until liftoff?
I would like to know my weight per wheel (and can then easily calculate CG) *before* I even start moving, in case I have to leave someone behind. 
Or maybe dump a little fuel. Don't know how I'd to that on an OBAM but I'd figure something out rather than take off way too heavy. If I'm just going around the block that means I'll be landing way too heavy, too.
David
BobsV35B(at)aol.com (BobsV35B(at)aol.com) wrote: [quote] Good Morning aerobubba,
Any of us can do the same if we wish to go to the trouble of getting highly accurate calibration of all factors involved. For any specific weight and any specific angle of attack there will be just one speed at which the aircraft will leave the ground.
If we note that speed and adjust for all the other pertinent factors we will have the weight.
I have used that technique to some degree when flying the DC-8 in a charter operation. I found that if my weight really was what the paperwork said it was and I initiated rotation when the book said I should, the DC-8 would leave the surface at a nine degree nose up attitude. When I started to find airplanes in the charter operation that didn't do what I expected, I began to evaluate each takeoff very carefully.
If I rotated to nine degrees and it did not fly, I held the nine degrees until it did lift off. I would then add two thousand pounds to whatever my weight manifest said for each knot above the normal lift off speed that I attained by holding the nine degrees. If I then added that extra weight to my performance calculations, all became normal.
As an example, if the lift off speed was ten knots higher than had been calculated I would be twenty thousand pounds heavier than the paperwork showed. By using that new weight to establish climb and cruise numbers, the airplane would deliver the performance required. Hopefully, modern techniques for gathering the data for weight manifests have improved drastically in the thirty-five plus years since I flew those charters, but it worked well for me!
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman N3977A
<<<snip>>>
[b]
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BobsV35B(at)aol.com
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| Posted: Sat Jul 04, 2009 7:45 pm Post subject: self weighing our planes |
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Good Evening David,
Sorry if you got the impression that I was suggesting using my procedure as a means of checking the weight of your aircraft prior to a takeoff!
My comment was in response to a note which said that the Airbus supplied a weight figure following takeoff. The submitter seemed to wonder how that could be done, I was just mentioning one way such a determination could be made.
As to what to do if I found find nine degrees wasn't going to get me airborne, that really depends on how much runway is left and how bad the crash will be if I abort.
Fortunately for me, I was flying a four engine airplane and our performance numbers were based on having three running and the most critical engine failed at the most inopportune time. That meant that if all four were still running, we had some wiggle room if the aircraft was over the weight shown on the weight manifest.
It was my experience with some very poor data being provided on charter trips that got me to develop my little checking procedure and, as I could verify sloppy work by those who provided the data, I could take steps to get the quality of the data improved.
I do not recommend ever operating intentionally at any loading not approved for the airplane, but there are simple methods that we can use to help keep everyone honest and on their toes.
Happy Skies,
Old Bob
In a message dated 7/4/2009 5:29:04 P.M. Central Daylight Time, ainut(at)hiwaay.net writes:
| Quote: | That is one reason I tried to come up with something. What if your runway isn't long enough for you to hold that 9 degrees until liftoff?
I would like to know my weight per wheel (and can then easily calculate CG) *before* I even start moving, in case I have to leave someone behind.
Or maybe dump a little fuel. Don't know how I'd to that on an OBAM but I'd figure something out rather than take off way too heavy. If I'm just going around the block that means I'll be landing way too heavy, too.
David
BobsV35B(at)aol.com (BobsV35B(at)aol.com) wrote: | Quote: | Good Morning aerobubba,
Any of us can do the same if we wish to go to the trouble of getting highly accurate calibration of all factors involved. For any specific weight and any specific angle of attack there will be just one speed at which the aircraft will leave the ground.
If we note that speed and adjust for all the other pertinent factors we will have the weight.
I have used that technique to some degree when flying the DC-8 in a charter operation. I found that if my weight really was what the paperwork said it was and I initiated rotation when the book said I should, the DC-8 would leave the surface at a nine degree nose up attitude. When I started to find airplanes in the charter operation that didn't do what I expected, I began to evaluate each takeoff very carefully.
If I rotated to nine degrees and it did not fly, I held the nine degrees until it did lift off. I would then add two thousand pounds to whatever my weight manifest said for each knot above the normal lift off speed that I attained by holding the nine degrees. If I then added that extra weight to my performance calculations, all became normal.
As an example, if the lift off speed was ten knots higher than had been calculated I would be twenty thousand pounds heavier than the paperwork showed. By using that new weight to establish climb and cruise numbers, the airplane would deliver the performance required. Hopefully, modern techniques for gathering the data for weight manifests have improved drastically in the thirty-five plus years since I flew those charters, but it worked well for me!
Happy Skies,
Old Bob
Downers Grove, IL
LL22
Stearman N3977A
<<<snip>>>
====================================
List href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List
====================================
ms.matronics.com/">http://forums.matronics.com
====================================
tp://www.matronics.com/contribution">http://www.matronics.com/contribution
====================================
|
|
A Good Credit Score is 700 or Above. See yours in just 2 easy steps!
[quote][b]
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ainut(at)hiwaay.net
Guest
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| Posted: Sun Jul 05, 2009 4:29 pm Post subject: self weighing our planes |
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No problem. I understood and thanks for the info.
And thanks everyone for the help! This is great stuff.
David
BobsV35B(at)aol.com (BobsV35B(at)aol.com) wrote: [quote] Good Evening David,
Sorry if you got the impression that I was suggesting using my procedure as a means of checking the weight of your aircraft prior to a takeoff!
My comment was in response to a note which said that the Airbus supplied a weight figure following takeoff. The submitter seemed to wonder how that could be done, I was just mentioning one way such a determination could be made.
As to what to do if I found find nine degrees wasn't going to get me airborne, that really depends on how much runway is left and how bad the crash will be if I abort.
Fortunately for me, I was flying a four engine airplane and our performance numbers were based on having three running and the most critical engine failed at the most inopportune time. That meant that if all four were still running, we had some wiggle room if the aircraft was over the weight shown on the weight manifest.
It was my experience with some very poor data being provided on charter trips that got me to develop my little checking procedure and, as I could verify sloppy work by those who provided the data, I could take steps to get the quality of the data improved.
I do not recommend ever operating intentionally at any loading not approved for the airplane, but there are simple methods that we can use to help keep everyone honest and on their toes.
Happy Skies,
Old Bob
<<<snip>>>
[b]
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