W&B and CoG Jabiru 170D/230D

[turboplanner]

The whole training is supposed to be competency based assessment

How many of the 10,000 RAA members would you say have been trained on W&B (as against a shouted suggestion over the shoulder to buy a book). How many have been tested until the errors stop?

[facthunter]

I'm not in a position to answer that one. Few would see a big enough sample of that 10,000 and there's probably large variations of compliance and knowledge.

CASA require their W&B experts To renew their qualifications each 2 years even if they have done large numbers of W&B certified determinations through that period. IF a pilot has a plane with an accepted load sheet it's meant to be filled in (a copy) and be there if a ramp check is done . IF he/she flies another plane the POH should have enough info to do a satisfactory load assessment. Do all people have a copy of the POH adjusted for their particular plane (I doubt it but my opinion is just that and no more). It could be graphical (which I prefer) or just numbers being in an allowable range. More abstract..

People only have to cope with what they fly, to be safe. Errors in the basic Wt and index can be a cause of unpleasant surprises, even in larger planes Loading errors may happen. or the load move. I think having a reasonable knowledge of the effects generally and being "aware" is a big help. Nev

[turboplanner]

Well we are only talking about RA, so simple aircraft.

Of the 10,000 the ones who join from GA will have been trained.

I have no idea what percentage that is, but let's say it's 3,000; that leaves 7,000 and my guess is you'de be lucky to find a hundred that have been trained.

[Yenn]

Reading here it appears that RAAus do not have a standard of competence for understanding W & B. I thought that there must be instructin and testing on that as part of the curriculum to get a pilots certificate.

I don't recall ever seeing changes to acceptable C of G with changes of gross weight. I certainly cannot think why it would be necessary. The gross weight has to be within limits and the C of G has to be within limits. If either one is out of limits, the plane is not airworthy, there is one trap often missed by pilots and that is C of G moving during flight due to fuel consumption. The friend who I mentioned earlier could have had that happen, as the fuel tanks are to the rear. He tells me that he checked W and B and as he also tells me he worked as an aeronautical engineer with De Haviland, I assumed he had some idea of what could go wrong.

[facthunter]

That's why you must have the CofG within limits at TO wt, landing wt and zero fuel wt so you can't rely on fuel to keep you balanced unless it's in a tin in the hold like any other fixed load. The further fuel is from the balance point (say mid point of permitted CofG range) the more change/effect using it will have. IF it's forward you end up more tail heavy as you use it.. Since the reserve is there to be used occasionally it has to still be safe to fly with fuel exhausted. The vertical edges of a graphical presentation are never just straight. You'd have to begiving some consideration to performance envelope to get massive changes with high weights. That's just after you take off. I don't GET that one. Nev

[turboplanner]

The Nett CG of the aircraft moves during the flight.

Firstly, before the flight it can be in several places based on the PIC flight plan - whether to take full pax at full pax mass, and reduce fuel to stay within MTOW, or reduce baggage to carry more fuel, or load full fuel and reduce the number of pax etc. The aircraft you're going to fly could have quite a different pre-takoff Net CG depending on wht you are going to do.

 

Prior to this you will have calculated your fuel burn for the flight.

While you and your passengers will hopefully weigh the same at the end of the flight, the aircraft's net CG will vary during the flight as fuel is burnt off. An RV with fuel tank behind the engine is going to get lighter at the front and heavier at the rear, and an aircraft with a fuse tank behind the passenger cabin with do the reverse, so with these aircraf you may be legal when you take off, but have to choose where to seat which passengers so you are safe throughout the flight.

If the aircraft has wing tanks they are usually close enough to the fore/aft CG dimension that you have a lot of margin, but if you only burn fuel from one tank, the Net CG is going to move towards the other tank, and so on.

 

Piper and Cessna use a calculating method which shows where this moving CG is going to be, shows where each item such as baggage, front pax, rear pax, fuel etc - any moveable objects- are going to be on a chart represeting the safe envelope, so this has the benefit that you can see immediately if your CG will fall outside the envelope.

 

Jabiru use a different chart layout, but the base calculations - Mass and Moment Arm length vs Maximum will produce the same end result.

 

I'd suggest it takes about five nights of lessons to be successfully calculating W&B for your flights.

 

Here's an Envelope type chart set up for a fictional RA Aircraft

EXX16.pdf

Edited August 18, 2020 by turboplanner

[RFguy]

but but but it's just a set of scales with time variable weights. (for me)

but I appreciate many would not have my background.

 

thanks for all the write-ups here everybody, this is good to run over. some of this is in the Bristell discussion, and also a few other accident discussions

 

it's good that you guys are writing detailed responses, since people come and search for answers in all this. When I write answers in forums, I am always writing for a wide audience.

[RFguy]

What are people using for plane scales, IE those that you can roll a wheel onto ? (for empty CoG verification ) the attitude of the plane would need to be specified also, presumably ?

 

IE level or not level for the measurement. If it's CoG and balanced when held by a string, then surely level, but then something have to be level! IE a bubble is going to have to be on some surface somewhere that is in the correct level plane (plane I mean plane of axis not airplane) .

???

[turboplanner]

What are people using for plane scales, IE those that you can roll a wheel onto ? the attitude of the plane would need to be specified also, presumably ?

 

IE level or not level for the measurement. If it's CoG and balanced when held by a string, then surely level, but then something have to be level! IE a bubble is going to have to be on some surface somewhere that is in the correct level plane (plane I mean plane of axis not airplane) .

???

You can calculate it based on any angle in 3D

[RFguy]

You can calculate it based on any angle in 3D

Hi Turbs, that would imply you would need to know what inclination it was on I guess. Actually I just found in some POH manuals procedures for this- they do specify where to put a bubble level in the fuselage when doing the 3 wheel scale measurements and nose height adjustment .

[turboplanner]

I was looking at designing something that didn't yet exist, but same thing, however, for routine operations in RA or private GA level is fine, just doing the calcs even better.

 

What does catch people out are cantilever loads where mass is levered off but can't stay up in the air and comes down on top of the existing load at the other end of the balance. Even light items can significantly effect balance in that situation.

[RFguy]

for your own plane, in your own hanger you'd have all the locating registrations for your scales, lines of stations and datums etc painted to your hanger floor like a miniature tennis court. Hmm maybe not painted in case you change it !

Not painted..... how about projected on the floor . did someone say overkill ?

 

Talking about cantilever (in flight) loads, Do long planes deflect from front to rear with changes of weight ? Does this occur (appreciably) with any of the LSAs ?

[Old Koreelah]

What are people using for plane scales, IE those that you can roll a wheel onto ?...

I've used digital bathroom scales, which are a pain to read, but it was easier to use borrowed larger cattle digital scales. Whatever you use, you'll need at least one assistant.

 

... level or not level for the measurement. If it's CoG and balanced when held by a string...

Glen most aircraft manuals should specify how it's to be done.

Mine requires having the top longeron level (not hard to find its outline under the paint and plywood)- this roughly equates with cruising attitude.

 

The measurements and calculations have to be spot on; another reason for a reliable assistant.

 

Since I've made so many modifications, my original datum is no longer usable, so I built strong points under the spar and simply lift the aircraft so it balances on a roller. With me sitting in it, my assistant can simply roll it fore and aft until it balances perfectly. She marks this location on the belly, then uses 2.5kg bottles of water to simulate changes in fuel and luggage loads, re-balances the aircraft and marks those locations.

In this way I've been able to ensure that all likely load combinations fit well within the CoG range.

 

I realise few aircraft allow that method, but it shouldn't be hard to build a lightweight balancing platform that would do the job. Maybe even suspended from the roof; after all, they called them Hangars for a reason.

 

I'd tend to trust the result more than I trust my dodgy maths.

[facthunter]

You could do it easily with a Laser as you strike levels anywhere on a building site. Without a stated condition DATUM easily duplicated you don't get the same answer re each wheel. The aggregate mass will be the same splitting it up to each wheel and using actual projected distances vertical to the floor will derive the balance point.

You could also slide a point of suspension along the aircraft axis and where it balanced at the (level) attitude you want, the point it's suspended at will be the Cof G. Remember your models? It will be about 1/3rd back from the wing leading edge. on the mean chord. As an example but use the designers figure. Nev

Edited August 18, 2020 by facthunter

[Old Koreelah]

...What does catch people out are cantilever loads where mass is levered off but can't stay up in the air and comes down on top of the existing load at the other end of the balance. Even light items can significantly effect balance in that situation.

Turbs can you expand on that?

[turboplanner]

Maybe some other time; this has got way out of control into aircraft design where what I was trying to do was get some more people to comply with their obligation to do a few simple calculations before every flight.

[RFguy]

I think I have satisfied my original topic posting intentions and questions, also. Certainly have got the discussion out in the open.

[facthunter]

Indeed you have and very logically presented if I may say so.

Turbs,That's completely fine IF the right references are available. If a plane is built from plans and altered much Even Painted you need to do a new balance determination. A greasy grip of the principles helps but then apply it to the actual machine and evaluate it. Planes are rebalance checked many times in their lifetimes . Some can remove and shift item s that can be adapted using just calculations. and derive a new weight and index legally but I don't think that's widely practiced with U/Ls in Australia. A good load sheet is not difficult to use amend and check and often gives you a trim figure. Nev

[turboplanner]

Indeed you have and very logically presented if I may say so.

Turbs,That's completely fine IF the right references are available. If a plane is built from plans and altered much Even Painted you need to do a new balance determination. A greasy grip of the principles helps but then apply it to the actual machine and evaluate it. Planes are rebalance checked many times in their lifetimes . Some can remove and shift item s that can be adapted using just calculations. and derive a new weight and index legally but I don't think that's widely practiced with U/Ls in Australia. A good load sheet is not difficult to use amend and check and often gives you a trim figure. Nev

We finished up having two discussions in one.

 

(a) Setting up the Chart - OK's method is as good an example as any, but once the parameters of the Envelope Chart are established, then....

 

(b) Using an Envelope Chart before each flight kicks in.

Say you are a hirer of OK's aircraft, that's what you do.

If you are doing the same thing over and over again - say circuits, you might only do it the first time, then keep a copy for identical flights.

It's particularly holidays, fly ins, and new routes/distances that will catch you out.

Edited August 18, 2020 by turboplanner

[facthunter]

Well, I just replied to two people specifically in one post. as clearly as I can. Nev

[Downunder]

An interesting read for those interested in W&B in accidents.

https://www.atsb.gov.au/publications/investigation_reports/2017/aair/ao-2017-081/

[RFguy]

Reading that, putting aside the POH error, sounds like a number of rookie mistakes made by the pilot.

Care of airspeed with heavy aircraft and high drag / low lift flying scenarios (Xctl side slip, steep turn ) And those passengers were heavy. and with a PPL, pilot would have been trained and competency tested of W&B. (or was that only RPL from RA transfer ????) Point (3) is an important one, also. "During a slipping manoeuvre the indicated airspeed may not be accurate" Does this depend where the Pitot is ? Ah yes would orientation dependency would creep in ? is the effect more than a few % due to the small angle? or does the pitot get air washed or shielded?

 

geeeeez. These sort of mess-ups IMO gives GA a bad name. mutter mutter.

Edited August 18, 2020 by RFguy

[Bruce Tuncks]

You have to look at W&B from the manufacturers point of view. Certifying costs squillions and a single muck-up can ruin the effort.

I well remember Harry Schneider putting on special sandbags on the back of a prototype glider. He needed to prove spin recovery with the c c of g some distance REAR of the aft limit to provide the public with safety when flying at the aft limit. His sandbags had a string-pull emptying system so he could jettison the weight in flight if the need arose . I don't think he used this ever.

From my model plane experience, I know the c of g too far aft can give sailplanes the "tuck-unders" where a dive steepens without any elevator input. (Radio controlled models are a great way to explore illegal effects)

 

From my gliding experience, I know that a c of g too far forward can hurt the climb. In my club, there was an otherwise nice glider owned by a fat guy and I had the strong impression that it had a problem with the climb. But when it was flown by this much lighter guy, it climbed very well. Now this was not a weight thing because the gliders all had wing ballast. It was a c of g thing.

Once I asked this real successful pilot ( he became a national champion ) who had just sold his Libelle at what c of g did he fly it at..." an inch behind the aft limit " was his reply.

Personally, l am too chicken with full size planes to experiment and so adhere to the official figures. But that's what they are, they are made with a bit of science and a lot of fearfulness of legal implications.

[facthunter]

That accident is a good example to learn from. They didn't mention the possibility of a tailplane stall which is also possible when it's tail heavy. If you're familiar with sidelipping in a particular plane you LEARN what indicated speed is safe for it and it always increases with extra weight. Nev

[turboplanner]

For OK, shows what cantilever is and does.

WX00145.pdf