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Old 11th Dec 2012, 18:14   #1 (permalink)
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Question On-board weighing systems

What are the best weighing systems for real-time use? ie: not in the workshop. The only one I can find is for external loads. Does anyone have experience of built-in weighing systems for the whole airframe?
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Old 11th Dec 2012, 22:42   #2 (permalink)
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AFAIK doesn't exist because it is not technically feasible. If you mean in flight, I can't see how it could be done. If on ground eg load cells on the skids, with rotors turning and collective right down, the rotor system will still be producing quite a bit of residual lift so it wouldn't work unless you were shut down.
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Old 12th Dec 2012, 00:47   #3 (permalink)
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On-board weighing systems

Thanks for that, helicomparator. Doesnt exist? hmmm.... must be why I cant find one!

I was thinking of "on the ground" to eliminate the paper & pencil & head scratching weight additions that I see going on everywhere...
You make a valid point about rotor lift.- will keep searching.
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Old 12th Dec 2012, 09:08   #4 (permalink)
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Well it does exist.
I try to find out who the manufacturer was and come back to you.
But it was on a heli with gears and not skids!
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Old 12th Dec 2012, 13:43   #5 (permalink)
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There is a way (no pun intended) to get the aircraft weight once you get airborne, but it requires a power required to hover chart - which no civil helicopter appears to have.
Using that chart, you can effectively 'weigh' the helicopter in a hover - but that's not much help to figure out weight before you get started.
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Old 12th Dec 2012, 19:48   #6 (permalink)
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How have we managed all these years without such a gadget?
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Old 12th Dec 2012, 20:07   #7 (permalink)
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Shawn, yes you could do that but would need exact data for current wind, hover height and nature of ground. It would presume that rotor systems were functioning optimally but I suppose if they weren't, you would get a "performance equivalent mass" which might be more useful than actual mass.

Last edited by HeliComparator; 12th Dec 2012 at 20:08.
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Old 12th Dec 2012, 22:05   #8 (permalink)
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Why not have a system with load cells in the MRGB mounts, and an accelerometer on the airframe to calculate the in-flight g-load and compensate for for flight movements on the weight of the aircraft? Barring the weight of the system, this could work...or have I not thought this through...
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Old 13th Dec 2012, 00:09   #9 (permalink)
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Possible, but fitting load cells to MGB mounts might compromise their integrity. Also, in the hover the down wash impinging on the airframe would affect the forces on the load cells, and in fwd flight the fuselage might be generating positive or negative lift. And fuselage drag would increase the force with increasing airspeed.

So really, probably not!
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Old 13th Dec 2012, 10:16   #10 (permalink)
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If the Rrpm dies off in the hover; it's too heavy.
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Old 13th Dec 2012, 16:45   #11 (permalink)
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The complexity of making load cells on the transmission mounts account for lateral and longitudinaly CG variations would make any mathematician's head spin...
Let alone account for lateral and lift imbalance of the rotor blades...
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Old 14th Dec 2012, 02:37   #12 (permalink)
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Use the torque gauge.

Lets use a Bell 206 for example. External gross weight is 3350lbs.
That's 100% torque to hover.

If you're using 90% torque, that's 3000 lbs. More or less.
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Old 14th Dec 2012, 14:25   #13 (permalink)
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That's way too over-simplified. Power required to hover is a function of weight, density altitude, hover height above ground, and wind. At a higher density altitude, the power required for the same weight will increase, rendering your calculation invalid.
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Old 27th Dec 2012, 22:18   #14 (permalink)
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Found this on a blog today. Maybe it can shed some light on the issue:

Boeing Assigned Patent for Onboard Aircraft Weight and Balance System

ALEXANDRIA, Va., Dec. 27 -- Boeing, Chicago, has been assigned a patent (8,340,892) developed by Michael A. Long, Freeland, Wash., and Geoffrey E. Gouette, Stanwood, Wash., for an "onboard aircraft weight and balance system."

The abstract of the patent published by the U.S. Patent and Trademark Office states: "An onboard system and method for determining the instantaneous weight and balance of an aircraft simply, reliably, accurately, and requiring a minimum amount of calibration includes a memory for storing previously determined breakout friction data of the aircraft's landing gear shock struts, sensors for sensing the pressures in the struts, the vertical loads exerted by the landing gear on the aircraft, and the attitude of the aircraft relative to the horizontal during loading or unloading thereof, and a computer for computing the vertical load in each of the landing gears from the stored calibration breakout friction data and the shock strut pressures, landing gear vertical loads and aircraft attitude sensed during the loading or unloading. The computer then computes the gross weight of the aircraft and the location of its center of gravity (CG) using the computed vertical loads in the landing gears."
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Old 29th Dec 2012, 00:30   #15 (permalink)
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A system such as described was attempted many years ago, but perhaps the technology wasn't quite ripe enough. It may work for fixed wing, but I doubt it will ever work for skid equipped helicopters.
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Old 29th Dec 2012, 06:29   #16 (permalink)
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Is there really a need for such an accurate weight system, afterall a helicopter at slightly under MAUW flies in a very similar fashion at a little over MAUW, and another helicopter of similar make and model but more abused may not even get off the ground at those weights.

The current system works. These extra gadgets are just introducing more cost and complexity for limited benefit.
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Old 29th Dec 2012, 07:39   #17 (permalink)
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The problem with airplane auto weight systems is they all can be defeated with 15 knots of wind over the wing.

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Old 30th Dec 2012, 22:12   #18 (permalink)
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You are correct. The Lama has a pitch indicator that you input presure altitiude and indicated pitch in OGE hover and then read total weight in pounds.
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