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Pugachev Cobra
30th Sep 2010, 19:12
Hello PPRuners...

I'm getting concerned with a situation with a weight and balance calculation of a Piper Arrow IV.

The CG limits are 85 to 93 inches, and starting from 2400lbs up to 2900lbs, the forward limit gets closer to the aft limit, with MTOW having 89 to 93 in of buffer.

The aircraft in question has a BOW of 1808lbs since its last weight and cg determination, and the CG was at 79.96 inches.

I found it strange that the BOW of the aircraft had the CG so off the envelope, since the chart just went back to 81.8 in. The envelope starts at 85, but the cg is off the charts!

Still, I redid several times with different calculations as to how the aircraft is normally operated (full tank, half tanks, 3 POB, 2 POB, no baggage, etc.)

Everytime the CG is off limits, too forward.

Now, the common conception (what I think is a misconception) around here, is a culture where people disregard doing W&B for small aircraft, that is inaccurate, etc. etc. etc., and that a few inches off the envelope doesn't do harm, you just rotate with a little higher airspeed and such...

My question is, is it 3 or 4 inches forward of the cg limit (if the forward limit is 85 inches, I'm talking here 81, 82 inches) not to worry about? Is it not that a bigger deal?

Or are we seeing here a future accident?

What worries me more is that, with fuel burn the CG moves further forward, so in landings it's gonna mean trouble. People constantly tell me that the roundout to touchdown the aircraft has a heavier nose and it's hard to pull, to trim up, etc. But that does seem normal with some aircraft.

Should I worry about this or not?

All inputs are appreciated.

If a CG more forward than the certificated limit is acceptable and the aircraft has been flying for a long time without any incident, how does one measure the actual true value of the CG limits of light aircraft's AFM/POH?

How do you convince the pilots and operatores of this small aircraft to respect the manual and the W&B procedures, when they always operate out of the envelope and nothing dangerous ever happens?

Thank you in advance!

27/09
30th Sep 2010, 19:47
Have you considered that the empty C of G calculation is wrong?

What was the C of G before the latest calculations were done?

Compare the empty C of G with another Arrow of the same type.

Unless you are making an error in your calculations I would be seriously questioning the current C of G calculation.

Checkboard
30th Sep 2010, 23:10
It is common for two people in the front of an Arrow to be outside the forward limit without ballast. Yes, you should be worried about it.

john_tullamarine
1st Oct 2010, 02:54
TCDS here (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/21ae845820577b998625774d005339f2/$FILE/2A13.pdf) for those interested. Can't immediately see which model you are looking at but the comments are generic.

The CG limits are 85 to 93 inches, and starting from 2400lbs up to 2900lbs, the forward limit gets closer to the aft limit, with MTOW having 89 to 93 in of buffer.

Not too sure what you mean by "buffer" but let's not get bogged down with that one.

The aircraft in question has a BOW of 1808lbs since its last weight and cg determination, and the CG was at 79.96 inches. I found it strange .. but the cg is off the charts!

Empty CG entirely dependent on build and configuration. Generally see forward-ish CG for light 4 place singles otherwise the loaded case can get a tad aft-ish.

We are not all that concerned with the empty CG .. rather the ZFW, TOW CG and what the fuel burn might be doing in between.

I redid several times with different calculations ... Everytime the CG is off limits, too forward.

This suggests one of

(a) the empty weight data is wrong

(b) the configuration is inappropriate and either the aircraft needs to be reconfigured or ballasted to remove the problem

(c) your sums are wrong

a culture where people disregard doing W&B for small aircraft

you MUST do whatever is appropriate to determine that the flight is inside the envelope. This doesn't necessarily mean you have to do a calculation every flight but you must be able to satisfy yourself (and demonstrate to the Regulators, Insurers, etc) that you are inside the envelope throughout the flight

a few inches off the envelope doesn't do harm

might not do any harm other than killing you and your passengers ....

is it 3 or 4 inches forward of the cg limit (if the forward limit is 85 inches, I'm talking here 81, 82 inches) not to worry about?

I do hope that you are not serious with this question.

From the regulatory/legal/insurance viewpoint, you have a problem as soon as you are forward/aft of the relevant limit.

From a more practical concern, the limits represent compliance with the Certification Design Standards. It follows that, as you move outside the limits, the Standards are being compromised in one or more respects. Progressively you eat into the margins until you kill yourself .. pretty simple.

People constantly tell me that the roundout to touchdown the aircraft has a heavier nose and it's hard to pull, to trim up, etc. But that does seem normal with some aircraft.

Increased stall speed - what margins are you carrying ?
Increased structure loads on the tail and nose wheel. Does that worry you at all ?
Increased stick loads associated with increased long static stability. Do the handling implications bother you at all ?

Should I worry about this or not?

.. and then some.

If a CG more forward than the certificated limit is acceptable

that's the basic problem .. it's NOT acceptable.

and the aircraft has been flying for a long time without any incident,

your luck might be about to change ...

how does one measure the actual true value of the CG limits of light aircraft's AFM/POH?

one does that by referring to the information in the weight and balance section of the POH. The aerodynamicists have done the sums and come up with the figures. The TPs have gone out and verified/amended those figures .. and the end result is published in TCDS and POH. There is NOTHING left for the pilot to measure .. just a matter of complying with the rules.

How do you convince the pilots and operatores of this small aircraft to respect the manual and the W&B procedures, when they always operate out of the envelope and nothing dangerous ever happens?

The owner should be worried about his insurance when he ends up with the bill after the accident .. whenever it might happen .. as it eventually will almost certainly do.

PBL
1st Oct 2010, 05:51
I support john_tullamarine's comments wholeheartedly.

The Arrow IV is a high-tail airplane and is known to have reduced elevator authority at slow speeds compared with similar aircraft (say, Arrow III). If it is being operated at a balance which is forward of the forward limits, that means that its nosedown tendency exceeds the required nose-up authority of the elevator at low flying speeds.

The operators may routinely fly it off quite happily, as PugCo hints, until the day comes when a tailwind comes in at 30 ft AGL on takeoff and the nose drops. Then they will shortly be very unhappy, assuming they would still be alive.

Operating any aircraft outside of cleared envelope is something for test pilots in carefully-delineated scenarios, not for routine operations.

PBL

Pugachev Cobra
1st Oct 2010, 22:12
John and everyone, thank you for your attention to this.

Can't immediately see which model you are looking at but the comments are generic.

Aircraft in question: PA-28RT-201T

Actually it's built in another country under license from Piper, but the specs are the same.

Not too sure what you mean by "buffer" but let's not get bogged down with that one.

I meant CG range, sorry, english isn't my native language.

I've got another PA-28RT-201T w&b form to compare its empty weight CG.

I'm doing unit conversions here, so I'm going to be very specific in decimal places.

- Aircraft I'm worried about:

Basic Empty Weight and CG: 1808 lbs (820.24 kg) / 79.96 in (2.031 m)

- Other comparison aircraft:

Basic Empty Weight and CG: 1 876.6 lbs (851.2 kg) / 82.24 in (2.089)

According to both aircraft serial numbers and the AFM, they are the latter ones produced, so I suppose there shouldn't be a large discrepancy.

As you can see, the comparison aircraft has a BEW CG closer to the CG range, but it's not that different from the 1st aircraft.

I was actually putting in question the maintenance personnel who did the latest Empty Weight and CG measurement, but after comparing to this other aircraft, I don't think its a miscalculation anymore, since they were weighed on different places.

Since there is the fact my calculations must be wrong, I'll give you 2 examples of how the aircraft is being operated.

The units will be metric, hope you don't mind:
(You're welcome to redo the same calculations in Imperial units)

Basic Weight - 820.24kg - Arm 2.031 m
Pilot & Copilot - 130kg - Arm 2.04 m
Pax Rear Seats - 0kg - Arm 3.0 m
Fuel 190L*0.72 - 136.8kg - Arm 2.41 m
Baggage - 5kg - Arm 3.63

Total - 1092.04 kg (2407 lbs)- Total Moment - 2278.9454

CG = 2.09 m (82.28 in) -> Out of CG range envelope

Another common configuration used:

Basic Weight - 820.24kg - Arm 2.031 m
Pilot & Copilot - 157kg - Arm 2.04 m
Pax Rear Seats - 60kg - Arm 3.0 m
Fuel 291 Liters*0.72 - 209.52kg - Arm 2.41 m
Baggage - 3kg - Arm 3.63

Total - 1249.76 kg (2755 lbs)- Total Moment - 2682.02064

CG = 2.15 m (84.64 in) -> Out of CG range envelope!
If the weight was below 2240lbs, since it's more heavy, the CG further forward of the limit, and this configuration is with a full tank, used for at least a 2 hour flight, meaning that at landing, the fuel burned will be approximately 191L (50L per hour), meaning the CG will have moved further forward to 2.13 m (83.86 in), endangering the operation even more.

According to john's cited TCDS:
Center of Gravity Range (+89.0) to (+93.0) at 2900 lb.
(+85.0) to (+93.0) at 2240 lb. or less
Straight line variation between points given.
Moment due to retraction of landing gear (+819 in-lb.)

"is it 3 or 4 inches forward of the cg limit (if the forward limit is 85 inches, I'm talking here 81, 82 inches) not to worry about?

I do hope that you are not serious with this question. "

John, since all I hear here from supposedly more experienced pilots is that W&B for these aircraft are all wrong, or when I question them about this, they tell me that there was probably an AFM revision update, so I shouldn't rely on my AFM copy at hand (and I wondered, I never really knew of any aircraft that had such a manufacturer AFM revision to CHANGE the envelope of the aircraft, or in other words, to actually change the CG range. But that's what I keep hearing.)

And then, the original AFM is nowhere to be found, but I honestly don't believe the envelope did change.

One other person told me his instructor once told him that just a few centimeters (or inches) off the limits isn't a concern, since all you have to do is rotate with a little higher airspeed, I dropped my jaw.

But with so many persons here telling me this, I was questioning my judgement.

I'm coming here to this forum as a last resort, since no one here really bothers even to listen to my concerns.

I think it's a dead serious matter, but I can't call the attention to anyone here, specially the ones who fly this aircraft.

john_tullamarine
3rd Oct 2010, 05:02
I meant CG range, sorry, english isn't my native language.

I guessed as much. Don't worry about the native tongue .. there have been occasions (generally late at night) when no-one has been able to understand my talking in tongues ...

Basic Empty Weight and CG: 1808 lbs (820.24 kg) / 79.96 in (2.031 m)
Basic Empty Weight and CG: 1 876.6 lbs (851.2 kg) / 82.24 in (2.089)

I've weighed quite a few of these over the years - unfortunately I don't have my archive records with me so I can only be generic - but those figures look pretty typical in range for this class of aircraft. The CG, especially, will depend on configuration .. ie what is fitted and where it is bolted down.

According to both aircraft serial numbers and the AFM, they are the latter ones produced, so I suppose there shouldn't be a large discrepancy.

If we are looking at the basic model build as it comes out of the factory, the numbers should be similar. However, each owner tends to spec his/her bird up with this and that and the end result can become significantly variable across the fleet.

I was actually putting in question the maintenance personnel who did the latest Empty Weight and CG measurement,

Sure, the weigh or calculation can be in error but probably not the root cause of the apparent discrepancy. Best thing is to go back to the folk who weighed the aircraft and get them to double check the records. For instance, what was the previous weigh data ? and can any variation between weighs largely be accounted for by weight variation records ? In any case a reweigh is pretty easy and cheap to do .. if the doubt lingers redo it.

all I hear here from supposedly more experienced pilots is that W&B for these aircraft are all wrong

I suggest that pilots are not the font of wisdom for such matters.

they tell me that there was probably an AFM revision update

simple enough to check with the local OEM agent what the current POH status is.

I never really knew of any aircraft that had such a manufacturer AFM revision to CHANGE the envelope of the aircraft, or in other words, to actually change the CG range

That would be unusual but not at all impossible. However, the TCDS would be expected to change as well.

the original AFM is nowhere to be found

then what POH is in the aeroplane ?

I dropped my jaw.

so would I .. and I certainly would cross that instructor off my list of acceptable instructors

But with so many persons here telling me this, I was questioning my judgement.

Why ? From the sound of things you are adopting an appropriate level of concern. Main thing now is for you to resolve that concern -

(a) check the status of whatever POH you are using - make sure it is applicable and current

(b) check with whoever did the last reweigh or calcs re any silly errors

(c) if necessary, reweigh the bird

I think it's a dead serious matter

Yes, indeed. Plenty of folks have been killed very dead as a result of weight and balance errors.

Machinbird
3rd Oct 2010, 06:17
Pugachev Cobra
I once owned a C-210 and was programing a small computer to do weight and balance for various fuel, passenger, and bagage loadings. Using the latest weight and balance info for the aircraft, I kept finding that what should have been perfectly reasonable loads were giving me a CG out of the aft limit.
I audited all the weight and balance changes since the original factory weight and balance. On the 5th weight and balance form depicting changes in aircraft equipment (of 8 such changes), I found a transposition of digits in a moment calculation. After correcting that error and all subsequent calculations, the aircraft CG was well within limits.

Light aircraft are rarely re-weighed unless they require major repairs. Current weight and balance numbers are usually the result of a chain of calculations by various mechanics adding and removing equipment. All you need is one mathematically challenged person in this chain to have problems with your weight and balance information.

On the Arrow with the forward CG question, have you observed handling problems such as higher than normal nosewheel liftoff speed compared to POH data? If so, it points to an actual CG problem, if not, more than likely a weight and balance calculation problem.

john_tullamarine
3rd Oct 2010, 07:24
All you need is one mathematically challenged person in this chain to have problems with your weight and balance information.

Add this to the inherent difficulty in doing the sums to a reasonable accuracy in many cases and you end up with an unacceptable delta after a while.

Hence, the sensible owner reweighs every now and again to clean the slate - regardless of what the local rules might mandate.

Mind you, unless the folks weighing know what they are doing, the weigh itself can introduce significant errors, especially if one is using jackpad loads cells.

Indeed, if the folk are out of their routine patch, quite silly errors can come into play. Years ago, a local chap over here had rebuilt a Stampe and wasn't able to contact me for a weigh. Not unreasonably, one might think, he contacted one of my airline colleagues to fill in. Now, the airline chap, used only to weighing heavy iron, thought no more about the differences and proceeded to weigh the Stampe with a set of 50k lb Cox and Stevens cells (as I recall).

Now, keeping in mind that the tail load is something in the order of not many kilos - TOTALLY incompatible with high range cells - my owner mate subsequently frightened himself witless on the test flight. When we reweighed on platforms, the empty CG moved forward about 4 inches as I recall.

Lots of (generally simple) traps in weight and balance work.

Pugachev Cobra
16th Nov 2010, 19:01
john_tullamarine, you being the person who showed me the TCDS online charts, I found one interesting problem with the Arrow IV there.

The TCDS in question lists that the CG envelope has a linear change after 2240 lbs of weight.

I found that strange since my POH specifies 2400 lbs, not 2240. I was confused, but then I found a scanned copy online of a Turbo Arrow IV original AFM, and to my surprise, it was also specifying the change after 2400 lbs.

So, do the online TCDS suffer from typing errors? 2400 to 2240 seems like one...

But my main question here is, for anyone that cares to help:

Why does an aircraft manufacturer decides to use a T-tail?

I can understand that in turboprops and jets in general, the exhaust gases heat would damage the empennage, but I really cannot see the benefit (instead of maybe, I don't know, visual appealing marketing?) of a T-tail in a conventional piston powered aircraft.

If my theory is correct, a T tail horizontal stabilizer (and elevator... or stabilator for that matter) is outside the propwash (specially in single engines).

Being outside the propwash means less energy on the tail surface's boundary layer, and less lift is produced.

Since posters here have told me that a T tail have less pitch authority (can anyone elaborate on that?), and our Turbo Arrow IV in question even has slots in its stabilator design (meaning it probably had control problems maybe?), why did the manufacturer choose to use the T tail?

Thank you once again!

Rickford
16th Nov 2010, 20:51
Reason for T tail

It acts as an end plate for the rudder & thereby improves efficency which means you can have a smaller rudder. Regrefully this is compromised by the need to make the rudder stonger to hold the T tail so really the reason was the need to look different

Look at ac of that era.

Bonanza backwards sloping rudder
Mooney V tail
Cessna boringly conventional
Piper um I wonder what we could do ..... Tomahawk, Seminole, Arrow IV ..

fdr
16th Nov 2010, 22:16
Hi PC, concur with John and PeterL, there is an issue. I own a 28RT-201 in basically standard IFR configuration, and the CG is fairly forward, but not excessively so. With 2 people, you will run up against the forward CG limit as discussed without adding some ballast. Will check my OEW/I and Pm you later... for a cross reference.

(WRT the T tail, it is different but not that much, it requires slight change in rotate practice and some discipline in approach speed maintenance, compared to the low tail. My biggest complaint is that the design was done for questionable aesthetic reasons against good engineering, with just the mere chance of a drag reduction from end plate effect... but the implementation is so klunky, (modifying a low tail rudder) drag change is minimal).

Keith.Williams.
16th Nov 2010, 23:53
In order to provide longitudinal stability the tailplane must experience a change in angle of attack when the aircraft pitches nose up or nose down.
The greater the change in angle of attack, the greater will be the stabilising moments generated (provided of course that the tailpalne does not stall).

A low tailplane tends to be enveloped in the propwash. As the aircraft pitches nose up and nose down, the propwash direction pitches with it. In a nose up pitch for example, the tailplane and the propwash are rotating down at about the same rate. This tends to reduce the amount by which the angle of attack of the tailplane changes. This in turn reduces the stabilising effect of the tailplane.

A T-Tail tends to locate the tailpane above the propwash. So as the aircraft pitches up and down, the tailpane experiences a greater change in angle of attack. So it produces a greater stabilising effect.

So T-Tails tend to increase longitudinal stability.

john_tullamarine
17th Nov 2010, 02:22
So, do the online TCDS suffer from typing errors? 2400 to 2240 seems like one...

I can't answer the question explicitly. However, if you have a current OEM POH and it disagrees with the relevant TCDS entry .. then, quite possibly, that indicates an error. If that be the case, suggest you refer the question to the local OEM agent (or email the OEM directly) with the question and see what transpires. Typically, one wouldn't be too surprised to see

(a) no direct response

(b) a subsequent TCDS revision.

Peter Fanelli
17th Nov 2010, 02:28
Bonanza backwards sloping rudder
Mooney V tail


I think you have that bass ackwards!

barit1
17th Nov 2010, 13:49
The Piper Arrow's not the only type to be out-of-limits with only a pilot or two on board. My family flew a Howard DGA for a few decades, and we placarded the ship to always maintain:

One or more pax in back seat
or
65# baggage
or
10 gallons (US) in the aft belly tank

Meeting any of the above would always satisfy the fwd CG limit.

Rickford
17th Nov 2010, 19:02
Thank you Peter,

The mind was willing but the body was the other side of a bottle of wine......

aerobat77
18th Nov 2010, 09:04
the company i fly for has a flight school attached and we have an arrow IV there.

when the theme is actual i can take a look into the aom of our arrow regarding this .

what i can tell you now is that in a turbo-seneca with two 85kg guys in the front and nothing in the back you are hard on the front limit. not outside, but hard on the edge.

i doubt that the arrow even with a passenger in the back is outside the envelope, i would also point for a miscalculation. but like said - be careful and clarify this.

john_tullamarine
19th Nov 2010, 00:40
There is no requirement or expectation for an aircraft to be in the envelope for any given loading you might start with - the requirement is that you MAKE it end up in the envelope. This might require

(a) reloading to suit

(b) a bit of temporary ballast in the nose or the boot for a problem this flight

(c) a bit of permanent ballast in the nose or boot if the problem is a continual pain in the neck

(d) there is no necessary reason for a given Type/Model to exhibit the same problems - all depends on the starting empty weight and CG and that depends very heavily on the specific aircraft's configuration and the sort of kit it might have fitted.

cwatters
19th Nov 2010, 17:31
I know zip about this aircraft but the calculator here appears to give similar results. eg out of range in the forward direction without rear pax...

N204GD PA28RT-200 Piper Arrow IV Weight And Balance Calculator (http://www.csgnetwork.com/pa28rt-200arrowwbcalc.html)

contractor25
17th Dec 2010, 13:26
So T-Tails tend to increase longitudinal stability.

Must be the reason why the t-tail arrows are a bit prone to fishtailing...

AFTA
17th Dec 2010, 18:57
Did a skill test in another brand multi engine many years ago. 3 candidates same examiner. All planned a trip including W/B. No problems for the other but i was unable to load within limits. The other two plus several earlier candidates with same examiner used sample data from AFM. Plane grounded for investigation. Found out there was two procedures for weighing either on wheels or jacks. Weighted on one calculated on the other.

Checkboard
17th Dec 2010, 23:20
I worked at a school with a 150 Aerobat, with a 172 engine, which had the same problem. As we were spinning it - it was a serious problem.

In thge end I asked the engineer if he sould move the aircraft battery further aft in the tail to correct the problem -that's one solution.

In the end the school installed a 150 engine, though....

john_tullamarine
18th Dec 2010, 11:47
As we were spinning it - it was a serious problem

What was the problem ? On the surface, I think you are suggesting that the bird had a too-forward CG. This should make recovery easier ?