Lateral weight and balance question (R22 / R44)

[jsh]

Hi. Am a new helicopter pilot convert after 20 years of fixed wing flying (but don't hold that against me). Am confused about something I've found in the W&B records of both an R22 and an R44. (These are the most recent W&B's records that are in the RFM / POH.)

 

In the case of the R44, as an example, the weight (basic empty weight) is 1507. The lateral arm is listed as 0.109. However, the lateral moment is listed as -164.2. In other words, the W&B says the arm is positive but the moment is negative.

 

Can this be right? Given that the math is weight times arm equals moment, shouldn't the arm and moment be either both positive or both negative? According to the rules of algebra they should be! Am I missing something? (I've dealt with longitudinal W&B for years, but the lateral one is new to me.)

 

I've asked two different CFIs and they can't explain this. One said that he thinks he heard that there is something peculiar to Robinson's that is the reason here, but he can't remember what it is.

 

Does anybody out there have an answer?

 

Thanks!

 

jsh

[markl]

Have you made sure that you are not trying to compute an arm for the left side of the ship? All lateral moments on the left side are a negative value where as all lateral moments for the right side are positive. Does this help explain anything or am I just stating something that you have already checked into?

[JumpPilot]

In the case of the R44, as an example, the weight (basic empty weight) is 1507. The lateral arm is listed as 0.109. However, the lateral moment is listed as -164.2. In other words, the W&B says the arm is positive but the moment is negative.

...

Continuing to use the numbers in your example, the weight (1507) gets divided by the lateral moment (-164.20) to determine the lateral arm (-0.11); that is a positive divided by a negative must be a negative (simple math with no imaginary number in this case - sorry, math minor here ). I use only the two decimal places in this because you are not going to be able to mark more than one decimal position on the graph anyway.

 

My best guess:

The person filling out the W&B form is human too - looks like a math error on their part that you found because math is math in this case. Either the moment was to be positive or their math was wrong.

 

Another possible is that this is an updated W&B after some modification and the number got "blindly copied" into the form; garbage in = garbage out. Take a look at the date on the W&B to find out...

 

This needs to be corrected - heavy loading on the right side (with little loading on the left) could put you out of the CG allowance if the moment should have been positive (instead of the negative listed).

 

Please let us know what you find out!

[thrilsekr]

It is that way for when you are carrying helium balloons. It is a upward moment rather than a downward moment as when you have a pile of rocks in the other seat.

 

I could be way off base...

[Goldy]

Does anybody out there have an answer?

 

Thanks!

 

jsh

 

I think I am confused that you are confused. The pilot side is a positive number, the passenger side is negative. The negative gets subtracted from the positive and gives you a total moment.

 

Let me give you an example off my excel sheet:

 

 

Weight Arm Moment Lat. Arm Lat. Moment

1468 105.98 156286 0.5 733

248.00 49.50 12276.00 12.20 3025.60

0.00 44.00 0.00 11.50 0.00

160.00 49.50 7920.00 -10.40 -1664.00

0.00 44.00 0.00 -11.50 0.00

0.00 79.50 0.00 12.20 0.00

0.00 79.50 0.00 -12.20 0.00

1876 94.07 176482.00 1.12 2094.60

In short, yes its right.

 

It might help you to see the excel WB version. Send me a PM with your email and I'll be happy to send you a copy for the R44.(or the 22).

 

Fly safe,

 

Goldy

Edited September 25, 2009 by Goldy

[kodoz]

Hijack okay?

 

I developed R-22 [link] and R-44 [link] W&B calculators, and put them out "there" for anybody to use. One user emailed me back saying my numbers for his R-22 spreadsheet didn't match mine. Since I don't like screwing up, I ran ours side by side and figured out where the differences were coming from:

 

The first thing I noticed was that they were using a lateral arm of 104.3 for the aux tank...that's compared to 103.8 from the POH I have. Do component arms really vary between individual R-22s?

 

That one seemed to be splitting hairs maybe, but the other guy's calculator had this for the W-A-M:

 

BEW: 867, Arm: 103.6, Moment: 91,611

 

Notice that the moment is not equal to the BEW*Arm. This isn't the first time I've seen this. I've looked at W&B sheets and seen discrepancies between the stated longitudinal moment and the calculated moment. In this case, the discrepancy was 1.3 inches. With 2 big guys, this would put the CG well out of limits using the W*A method, but within limits using the given moment.

 

So...does it matter? Is an inch of CG a problem (because I've not flown with guys over less than an inch of CG)? When you see a discrepancy between the calculated moment and the one given in the W&B sheet, how do you deal with that? Go with the W&B sheet, go with the calculated moment, or turn a blind eye to it?

[Mikemv]

Hijack okay?

 

I developed R-22 [link] and R-44 [link] W&B calculators, and put them out "there" for anybody to use. One user emailed me back saying my numbers for his R-22 spreadsheet didn't match mine. Since I don't like screwing up, I ran ours side by side and figured out where the differences were coming from:

 

The first thing I noticed was that they were using a lateral arm of 104.3 for the aux tank...that's compared to 103.8 from the POH I have. Do component arms really vary between individual R-22s?

 

That one seemed to be splitting hairs maybe, but the other guy's calculator had this for the W-A-M:

 

BEW: 867, Arm: 103.6, Moment: 91,611

 

Notice that the moment is not equal to the BEW*Arm. This isn't the first time I've seen this. I've looked at W&B sheets and seen discrepancies between the stated longitudinal moment and the calculated moment. In this case, the discrepancy was 1.3 inches. With 2 big guys, this would put the CG well out of limits using the W*A method, but within limits using the given moment.

 

So...does it matter? Is an inch of CG a problem (because I've not flown with guys over less than an inch of CG)? When you see a discrepancy between the calculated moment and the one given in the W&B sheet, how do you deal with that? Go with the W&B sheet, go with the calculated moment, or turn a blind eye to it?

 

Dear Kodoz/Chris, The WAM numbers are certainly incorrect as stated! Many Aviation Maintenance staff that weigh/mod aircraft make errors when inputting numbers in a calculator and do not check their math!!!! You should approach someone that made the entry and ask them to verify it? They will correct it. Be tactful with this.

 

Beyond this, the bottom line on larger aircraft (B206, L, 407, EC120, AS350's, etc.) may show some slight decimal place discrepencies due to the rounding of numbers. This is common but can be verified by starting at the top of the W&B form and working your way down.

 

I also suggest that training on any make/model include an understanding of the empty weight CG envelope/limitations and that the pilot know/realize where the actual CG is in relation to the extremes of the envelope. I know I will get feed back that these comps are the responsibility of the A&P/IA's but it is an area that I have knowledge of and train my students/pilots in. I normally due completions(Avionics, paint, interiors, floats, etc) on the aircraft I operate and ensure that ALL CG numbers are valid.

 

Best to All, Fly Safe, MikeMV

Edited October 6, 2009 by Mikemv

[kodoz]

some slight decimal place discrepencies due to the rounding of numbers.

 

This is in fact the case in the R22 POH--a rounding error causes a small error in their sample calculation.

 

I also suggest that training on any make/model include an understanding of the empty weight CG envelope/limitations....

 

We've had the mechanics remove equipment (eg, the GPS or VSI) for maintenance. It doesn't hurt to know where to find the W&B info for the installed equipment that make up the "empty" W&B so you can calc this type of CoG change.

 

Thanks Mike.

[Brentd2183]

Another thing you have to remember about Robinson WB and im sure others is that they all use a generic WB for the aircraft they produce. There is no weighing or actually finding of the CG anymore. Theory is if they build them all the same then difference should be fairly small. But that is why the calculated vs the original WB number are just slightly off. We have one aircraft, an R22 IFR ship that has just had its 5th over haul completed and this aircraft has had so many changes over the years, including layers of paint that we wanted to actually weigh and balance the aircraft for ourselves to see if 10 or more years of assumed numbers still lined up.

 

We ended up getting the load cells and weighing the aircraft and then suspending it and using a water level as described in the R22 MM and found out that our aircraft was 16 pounds lighter and CG was maybe .2 inches farther forward. Which made sense since there was a little bit of gel coat work done to the nose due to a birdstrike adding weight out there. But the fact that we gained addition 16 pounds of payload was pretty nice. Every once and while you just have to start from scratch again because no matter how accurate you document ever item that comes on and off the aircraft sometimes those little decimals will add up, combine that with the original WB is not exactly perfect and you start to see differences.

[jsh]

I think I am confused that you are confused. The pilot side is a positive number, the passenger side is negative. The negative gets subtracted from the positive and gives you a total moment.

 

Let me give you an example off my excel sheet:

 

 

Weight Arm Moment Lat. Arm Lat. Moment

1468 105.98 156286 0.5 733

248.00 49.50 12276.00 12.20 3025.60

0.00 44.00 0.00 11.50 0.00

160.00 49.50 7920.00 -10.40 -1664.00

0.00 44.00 0.00 -11.50 0.00

0.00 79.50 0.00 12.20 0.00

0.00 79.50 0.00 -12.20 0.00

1876 94.07 176482.00 1.12 2094.60

In short, yes its right.

 

It might help you to see the excel WB version. Send me a PM with your email and I'll be happy to send you a copy for the R44.(or the 22).

 

Fly safe,

 

Goldy

 

Goldy: Thanks for this post, but the issue/confusion I have is not with the spreadsheet and calculations (including pilot side positive, passenger side negative), but with the fact that in the POH, the BASIC EMPTY WEIGHT and associated lateral arms/moments have a positive / negative conflict. The lateral arm is listed as 0.109. However, the lateral moment is listed as -164.2. This is before I run any spreadsheet calculations on my planned loading configuration. So I am confused as to whether the basic empty lateral C.G. is on the left side (as implied by the empty lateral moment of -164.2) or on the right side (as implied by the empty lateral arm of +0.109). In reviewing all these posts, the only one that seems to have an answer that makes sense to me is the one attributing this to human error by whomever completed the latest CG update in the POH, but I'm still wondering if there is some other explanation as I have observed this issue in two separate robbies - an R44 and an R22.

 

Thanks all!

[kodoz]

In the case of the R44, as an example, the weight (basic empty weight) is 1507. The lateral arm is listed as 0.109. However, the lateral moment is listed as -164.2. In other words, the W&B says the arm is positive but the moment is negative.

 

Can this be right?

 

No, it can't. A positive x a positive is always a positive, even for helicopters! So either the arm is incorrectly given as to the right (positive) or there is a misprint in the calculated moment, putting it left instead of right.

 

I've asked two different CFIs and they can't explain this. One said that he thinks he heard that there is something peculiar to Robinson's that is the reason here, but he can't remember what it is.

 

Heh, that's kinda funny. This is what I found:

 

1/ If you are doing W&B before most or every flight, you are not the typical student. To most of them, W&B is something you figure out before check rides, cross-country flights, and your 1st solo flight (this might be fine if you are at a 1-instructor, 1-helicopter school). Ditto with most other performance calculations--those are for check rides.

 

2/ For the most part, the numbers given for BEW, arm, and moment are god-given and unquestionable, even if they are clearly incorrect. There was a little problem at the school I went to: somebody made up Excel spreadsheets for all the helicopters, but being that most guys there weren't terribly computer proficient, the spreadsheets were corrupted by the users. These spreadsheets were printed out and provided in the ready room for pre-flights. The result was that, when W&B calcs were done, they were done with BEWs for "doors-off" on spreadsheets labeled "doors-on" (and vice versa), with moments that were not = BEW * Arm, and other errors that nobody questioned. And this went on for many months before me (and in some cases, nothing changed after). I figure few noticed and those who did assumed it was something they were doing wrong.

 

3/ In the R22, it is difficult to be out of CG laterally. Watch that longitudinal CG though. Before I flew a new helicopter, I'd make up my own spreadsheet and confirm the weights and arms with the POH. I ignore the moments, since those involve somebody punching a number on a calculator (one more step for error).

 

4/ I asked about how far out of CG you have to be to think about it. I've seen the "fat pencil" technique for plotting CG data: if you are 0.2-0.3 inches out of forward CG at MGW, take off some fuel, redo the calculations, plot the new take-off CG with a fatter pencil, and your point will be on or at least touching the line. You can go fly. By programming the plotting into Excel instead of hand plotting the data, there is no fudge factor. Brentd's post about the inherent inaccuracies was interesting and implies that there must be some tolerance, but you should probably assume those don't work in your favor (I heard airlines assume takeoff distances will be 20% or more longer than what their calculations say). My feeling too is that limits are limits, and if you accept being 1% out of limits today, tomorrow it'll be 5%.

 

end rant. didn't really mean for it to happen.