C of G Question

[MightyGem]

To work out the aircraft's C of G, I need to know the arm of a
passenger seat with/without anyone sat in it.

I do the following. I remove the seat from the aircraft, and balance
it on a narrow piece of wood until it balances, ie it does not tip
forward or back. That is the C of G of the empty seat. I do the same
with someone sat in it. That is the C of G of an occupied seat.

I put the seat back in the aircraft, measure those positions from a
known aircraft station point and I have the arms for the seat empty
and occupied.

Is the above correct or a load of b******ks? The reason I ask is that
the figures I get are somewhat different to the company's figures.

[Genghis the Engineer]

There are more than one way to skin this particular cat, but the method you've just described is a perfectly valid one, and so long as you've done it all sufficiently carefully, and have remembered to ensure that your measuring line from datum is horizontal in the flight attitude, I think it's absolutely correct.

G

[Q max]

I think:

Your statements are correct (enough).

But the positions you are deriving like that are probably not what you are trying to find.

Subtly different but aren't you trying to find the:
1 Station of the empty seat
2 Station of a person when seated BUT NOT with a seat attached to him.

Your Station of person plus seat is not giving you the station of the person.

I presume you want to find the station of the person - so that you can then apply the assumption that any person sitting in the seat is at that station.

The 'person plus seat' station will lie between the seat station and the person station.

Question:
Does the 'Standard passenger' (male/european/onshore/shorthaul - for they are all different would you believe?) have a standard CG location.

(Just curious.... but does it actually matter?)

[tgrendl]

Does this aircraft hover pefectly level?

Will adjusting your CG plotting to the likely nose up attitude in a hover come up with the company figures?

Good question

[GLSNightPilot]

ISTM you're getting too picky. In most helicopters, the seats are in the ship when it's weighed, so you know the empty CG, & exactly where the seats are doesn't matter when they're empty. When you put pax in them, all you need is an approximation - are they leaning back, leaning forward, are they fat, or thin, are they full back in the seat or sitting on the edge with a lose belt? All you can do is get an approximate central location & go with that. In any case, the CG won't change appreciably, & there's nothing you can do about them moving anyway.

[sycamore]

MG
A good place to start would be the Flight Manual!!:)

[MightyGem]

Thanks for your replies guys which confirm that I am basically correct.

Sycamore: the FM is somewhat vague. Because the position of our seats is variable it gives the arm as: as reqd. Despite saying that the seats should be in acertain position for landing and take-off.

Qmax: because the seats can be moved around so much, we really need the arm of seat plus pax. Plus we use actual pax weights.

The arm and station points can be considered to be in the same plane. My figure is just over 6 inches further forward than the company's.

[GLSNightPilot]

What type aircraft is this?

Do you mean the seat station is 6" different, or the aircraft CG is 6" different? 6" is a big difference in CG, but 6" difference in the station of the seat should have minimal effect on the actual CG.

[Arraitch]

GLS is right - in an S76, moving a 90kg pilot by 6 inches results in a cg change of only 0.1 inches.

So unless we are considering an R22 where the pilot can detect a cg change induced by the instructor moving his feet back off the pedals, a change of 6 inches in the arm won't make much difference. Are you talking about a BK or something bigger?

_______________________
Helicopters --- a triumph of science and technology over common sense.

[400 Hertz]

In my experience, moving an S76 Pilot by 6" can only be done by towing the aircraft 6" further away from the tea room. Or you could try moving his sticky buns a further 6" away in the cockpit, that should do the trick.

[The Nr Fairy]

400 Hurts :

Pissed myself.

[Q max]

Gem: "Qmax: because the seats can be moved around so much, we really need the arm of seat plus pax. Plus we use actual pax weights."


- so you need:
Seat station
And
Person station ... where Person Station=Seat Station + x"

(x might be minus 3)


(But I still can't see that it actually matters .... unless of course it actually does matter.)

[Thomas coupling]

MG: serves you right! can of worms springs to mind. As discussed we went through this like a dose of salts 3 years ago. You have to accept the median because there are fat piggies and thin piggies:D :D

The important issue here is that the yellow line drawn across the floor boards in your aircraft, is in the correct place. Then the seats need to be positiioned around that to remain within C of G..
capiche;)

[Capn Notarious]

So now I know why some helicopters have hooks So that you can hang the real biggies in a net, directly under the main rotor disc! :D

[Nick Lappos]

Mighty Gem,

The C of G is an approximation for all aircraft, since the critical flight maneuvers that establish the limits are based on flight tests where the cg naturally moves with fuel burn. For the S-76, we try to flight test within 1/2 inch for critical maneuvers, but 1 inch is allowed (out of a total range of 17 inches). We often start the tests 1 inch outside of the limit, and burn to 1 inch inside the limit. The 1 inch is therefore about 6% of total CG, quite within the common sense range of variability of flight maneuvers, considering all the things that influence the behavior of the craft.

I would take as a rule of thumb that 5% or so is the likely accuracy for CG determination, so take your aircraft's CG range and divide by 20 to yield the likely tolerance band for such variables as whether the pilot slouches in his seat, or if the fuel is 6.3 lbs/gal instead of 6.0, etc.

None of this should be construed as encouraging a lax attitude concerning limits, but rather as showing that basic rules of thumb are acceptable for the calculations that we all depend on.

[Kyrilian]

Nick,
Is that with a 'symmetric fuel load'? :D
Couldn't resist!
Just watched a piece on the 76 going to 1.11Vne last week. Cool stuff

[Nick Lappos]

Kyrilian,

I'll bet Walsh put you up to that jibe! See if I recommend you for another job! The "1,11 Vne" film was fun, did you note the skinny test pilot and his mentor with the full head of dark hair?

Nick

PS It WAS a symetrical fuel loading, wasn't it?

[Kyrilian]

When I told your other apprentice that I'd posted this he said this may be a record for the shortest career here! I hope not! ;)

Washuta found it and showed it to us 'younger folk', though everyone enjoyed it. They said they poke fun because that line sounds staged in the way it's spoken, though it seems feasible.

Yeah, it was a neat film. And without putting myself in even more hot water, I'll admit that you guys do look a bit different.
:D

[Nick Lappos]

Kyrilian,

Looking different means looking older, huh? For test pilots, getting older is a sign of success.;)

[Kyrilian]

wiser :)

[MightyGem]

Thanks for all your replies gents. The final a/c C of G position only varies by 3/4inch, depending on which figure I use which, as stated by someone, is not a great deal. However with 4 heavy crew on board, the company figure puts the a/c just in limits, whereas my figure puts it just outside.

Nick says that the limits are an approximation, but what do you say to the nice AAIB/NTSB man when he asks why you took off out of C of G limits.

Apologies if I'm making a mountain out of a molehill, but it's more the fact that there appears to be a large discrepancy in the seat position rather than the final C of G. If the engineers have got that wrong, are there any more errors. And yes, I am trying to get an answer from them.

Finally, TC, there's a similar thread on the Air Support Forum.

[Nick Lappos]

Mighty Gem,

If I understand your issue, it is that you feel that the seat's centroid, as you have estimated it, and the pilot's position in the seat leave the CG at different places than that which is calculated when using the procedure in the weight and balance section of the flight manual.

I suggest that you follow the flight manual, and calculate the position using the stations listed there. If the calculated cg is within limits, you are compliant. That is what a government examiner will do, of course. That is what was flown in flight test, too, I am sure.

Hypothetical slouching pilots, seat removals and cross checking, and the like are working the problem to a very fine scale, and probably not what the manufacturer did when he flew the points and published the data, and probably not what the FAA did when they checked the manufacturer's data. It is possible that your personal method will introduce an error that could cause you to be out of CG, because you did not follow the conventions that everybody before you followed, thus making you the non-compliant exception!