W&B and CoG Jabiru 170D/230D

[RFguy]

question oh wise forum from a neebee who has never had to calculate W&B.

 

I was reading some of the PIlot Handbooks for all their planes.

Since the 170D and 230D share a wing, and based on the payload schedule for the 230D, I was guessing the wing moved back for the 230 relative the the front seats. And it has.

 

Now, from reading the CG envelope / loading diagrams for the 170 and 230 .

The 230 has the same fore and aft limit for all weights up to MTOW. The 170D has additional criteria, weight dependent. The fore limit is different for 440kg v MTOW.

 

At MTOW the 170D CoG limit is only 272 - 255 = 17mm wide ! golly. better get that right. The 230D has a fore-aft limit of 277-99 = 178mm.

 

Makes me think W&B and CoG goes awry for many flights going from full to empty fuel loads.

 

I see there is a graphical proceedure to calculate this loading scenario in the manual . More study required. Obviously the CoG must remain inside limits.

 

The 230 looks much less fussy with loading variation , and where you might put luggage when you have an empty passenger seat (likely in the passsenger seat or immediately behind) .

 

Sure 'just adjust the trim' but my understanding is that aint going to help you in a stall if there is reduced airflow over the tail and the plane is tail heavy, so you must avoid the CG envelope limits.

 

glen

[facthunter]

Weight is about performance. Cof G is about maintaining control. To far to the rear and you may tailplane stall. XXX BAD!! Too far forward, land fast or you run out of UP elevator and can't fully flare. Any load that has IT's C of G inside the allowable Cof G range cannot make it go outside of the Cof G. Useable fuel cannot be used for balance purposes. Cof G must be determined at TO load condition. landing weight and zero fuel weight. You can use calculations and graphical methods as long as it's an accepted format.. If your tanks are in the wings they normally don't give a lot of trouble C of G wise Go by your POH advice and theoretically every flight must satisfy the requirement to have verified that you plane conforms. Taxi fuel can be deducted but usually for simplicity Isn't.

General comments. NOT a summary of all aspects of W & B.

Fuel qty determination and rate of use is another topic you might research.. Nev

[RFguy]

OK got it. " Any load that has IT's C of G inside the allowable Cof G range cannot make it go outside of the Cof G. " . mmmm.

 

For the 170D, as weight decreases, permissable CoG forward limit is increased. . In that aircraft, permissible CoG range increases as fuel depletes. not much.... I guess in the high wing I have to think of exactly what is happening. low load on floor (plumb bob) and depleting wing fuel load. shifting loads a no no.

But 17mm CoG permissible range at MTOW ? How many pilots genuinely satisfy that ? Or is that a typical sort of limitation in this sort of small aeroplane ? I have an analytical math brain, so that's something I can meet as necessary. But knowledge of actual load weight, and all the assumptions about load distribution for an object become important with such narrow limits at MTOW..... Like carrying a CRT glass tube tele. they are heavy at the front !

 

It's been a really good exercise understand similar planes' differences. Sure you can load more into a J170D because it weighs less, but you better know everything about your loads. IN PRACTICE I guess if you passenger weighs 5kg less than they told you, 5kg is not going to push the CoG that far away from what it was, especially with a load of full fuel / baggage. Leaning foreward and back in the seat would be more of an effect. I guess that's why weight shift aircraft work.

 

g

[facthunter]

17 mm seems ridiculously low. When the plane is measured it's in a level fore and aft condition (Pitch). Fuel and pax(or other baggage) that is a considerable distance from the balance point has a large effect and that's where you can come unstuck. You also must make sure your load cannot shift inflight. Nose heavy will cause more drag and raise stall speed. Nev

[Old Koreelah]

Useful discussion. My Jodel D-9's CoG range is only 61mm, but it's only a little aeroplane.

Extensive testing of all the likely fuel and luggage loads shows that CoG stays well within that range.

 

I must admit to one deviation: after installing my lithium battery and re-setting the CoG to suit, I did one flight test with my old AGM battery sitting next to it, not taking enough notice of the extra 4.5kg.

It sure was nose heavy and required a significantly high landing speed. Not clever.

[turboplanner]

question oh wise forum from a neebee who has never had to calculate W&B.

 

I was reading some of the PIlot Handbooks for all their planes.

Since the 170D and 230D share a wing, and based on the payload schedule for the 230D, I was guessing the wing moved back for the 230 relative the the front seats. And it has.

 

Now, from reading the CG envelope / loading diagrams for the 170 and 230 .

The 230 has the same fore and aft limit for all weights up to MTOW. The 170D has additional criteria, weight dependent. The fore limit is different for 440kg v MTOW.

 

At MTOW the 170D CoG limit is only 272 - 255 = 17mm wide ! golly. better get that right. The 230D has a fore-aft limit of 277-99 = 178mm.

 

Makes me think W&B and CoG goes awry for many flights going from full to empty fuel loads.

 

I see there is a graphical proceedure to calculate this loading scenario in the manual . More study required. Obviously the CoG must remain inside limits.

 

The 230 looks much less fussy with loading variation , and where you might put luggage when you have an empty passenger seat (likely in the passsenger seat or immediately behind) .

 

Sure 'just adjust the trim' but my understanding is that aint going to help you in a stall if there is reduced airflow over the tail and the plane is tail heavy, so you must avoid the CG envelope limits.

 

glen

 

 

Firstly, in calculating Mass Distribution if you input the correct moment arm length, the correct dimensions, and the correct masses, you will always get the same accurate result, but if you select a wrong starting point you won't.

 

From that, the correct aircraft datums and masses to use are those for the aircraft you are about to fly, not a generic equivalent. Many people have come unstuck due to different equiment or layout in the same make and model aircraft.

 

With Jabiru, you have to be careful about generalising because they are very good at minimising component variations.

In the examples you've given, the J170D has a 4 cylinder motor and the J230D has a six cylinder engine, and the engine CGs might well be different dimensions from the datum.

 

The J230 is also 11% longer and has a wider cabin.

 

So it's better to treat them as separate aircraft (although its always possible for someone to make a mistake in the manual)

 

Both the J230D and J170D have envelopes printed and instructions on how they want you to calculate.

 

If you're fluent in Maths, Moments etc. and trying to do an aircraft for the first time, I'd advise getting a Piper, Cessna or Becchcraft POH chart and doing work ups on that becaise you can see how the CG jumps around inside the envelope.

 

Jabiru do it slightly differently, but as I mentioned in Para 1, if the inputs are precise the outputs are precise.

 

 

RA aircraft in both two and three axis are very sensitive to weights outside their envelopes, and very little is required to lose control.

 

I was going to use the example of one RF forum member who put a 15 kg tool box in the footwell of his Morgan ahead of his passenger seat and lost control of the aircraft on take off, bening very lucky to get it down without damage.

 

OK has just given us another example of a 4.5 kg battery in the wrong place at the wrong time.

[RFguy]

Hi

 

Looking at half a dozen different lift and drag versus AoA curves, there is for most aerofoils I looked at , not much going on at low AoA. low lift and low drag

 

BUT! What is interesting is the (different shapes of different aerofoils) - the shape of the curve at high AoA at the inflexion point.

 

That is to say, how fast at high AoA you go from increasing lift to decreasing lift (and skyrocketing drag!)

 

Some high lift wings are very sharp indeed.

 

yeah I was specially referring to the Jab pilot handbooks . I have 160,170D,230D POH.

And once you change anything far away from the CoG like a battery weight, numbers in the handbook will go out the window (compared to factory shipped) .

 

But that is all airplane loading 101, isn't it ?????

[facthunter]

Indirectly, but breakaway and boundary layer control is aerofoil shape and the ANGLE of the wing to the RELATIVE airflow. AoA also affect the centre of lift location and therefore the lift /wt couple relationship. IF the allowable CofG is getting less at high wts how does it cope with the added dynamic Wt in a turn or dive pull out?. A "couple" is the turning effect of a force and is the distance from the turning point and the line of action of the force applied.

EACH individual plane is weighed and the Cof G determined.. Your load and balance calculator starts with it's OWN particular BASIC wt and index. This is where it starts to get complex. The datum for the index is not the C of G . It's any convenient reference point on the fore and aft axis and is normally a very forward position like the prop drive face or some point forward of say the firewall that may be in front of any point (station) on the actual plane used as a location reference. for parts and load like pax pilot engine battery tailwheel if it's altered . IF you move, change or add something you need too determine the NEW BASIC Wt and index to load the plane within limits from then on. Even a new paint job THIS is NOT loading. It's how you plane IS BEFORE you put anything in it. to go flying. Look up useable fuel. and what basic weight includes. Nev

[Yenn]

Do RAAus have any requirement for its pilots to understand and be able to work out C of G weight and balance.

A friend today who flies a high wing plastic fantastic RAAus plane asked me about C of G. He had taken a heavy friend flying and when he came in to land he couldn't get enough up elevator and landed nose down. No damage he thinks. He asked me if the regulations allow him to get the elevator settings changed. I cannot remember exactly what his plane is, but the Aussie agent is in Melbourne. He commented that he has never been able to get a stall, full up elevator and closed throttle results in a high rate of descent. The plane is LSA, so I told him it probably wouldn't be possible to alter anything, but he ought to check the actual deflection of the elevator. The pilot and passenger sit forward and fuel is further aft, as is luggage. Must find out what the plane is

[RFguy]

It's in all the pilot handbooks I have read recently under "pilot responsibilities"...

[turboplanner]

Do RAAus have any requirement for its pilots to understand and be able to work out C of G weight and balance.

A friend today who flies a high wing plastic fantastic RAAus plane asked me about C of G. He had taken a heavy friend flying and when he came in to land he couldn't get enough up elevator and landed nose down. No damage he thinks. He asked me if the regulations allow him to get the elevator settings changed. I cannot remember exactly what his plane is, but the Aussie agent is in Melbourne. He commented that he has never been able to get a stall, full up elevator and closed throttle results in a high rate of descent. The plane is LSA, so I told him it probably wouldn't be possible to alter anything, but he ought to check the actual deflection of the elevator. The pilot and passenger sit forward and fuel is further aft, as is luggage. Must find out what the plane is

When you were flying in a paddock maybe, but if you want to enter CASA airspace you need to complete your Performance and Operations obligations, which include W&B before each flight. We've lost a lot of pilots through incorrect W&B which could have been easily fixed with a calculator, fuel exhaustions etc.

[RFguy]

Firstly, in calculating Mass Distribution if you input the correct moment arm length, the correct dimensions, and the correct masses, you will always get the same accurate result, but if you select a wrong starting point you won't.

 

From that, the correct aircraft datums and masses to use are those for the aircraft you are about to fly, not a generic equivalent. Many people have come unstuck due to different equiment or layout in the same make and model aircraft.

 

With Jabiru, you have to be careful about generalising because they are very good at minimising component variations.

In the examples you've given, the J170D has a 4 cylinder motor and the J230D has a six cylinder engine, and the engine CG

 

Both the J230D and J170D have envelopes printed and instructions on how they want you to calculate.

 

If you're fluent in Maths, Moments etc. and trying to do an aircraft for the first time, I'd advise getting a Piper, Cessna or Becchcraft POH chart and doing work ups on that becaise you can see how the CG jumps around inside the envelope.

 

Jabiru do it slightly differently, but as I mentioned in Para 1, if the inputs are precise the outputs are precis

Turbs, Cheers. In fact Jabiru have different datum points for different aircraft ! For the J120, the datum is 1403mm fore of the leading edge, and 170, 230 is the leading edge. Yeah maths , moments, beams etc all OK for me. I'll assume the aircraft doesn't bend appreciably front to back (IE cantileivered box beam ) but I am sure it would (but not to any concern for these calcs) . glen.

[Stuart Erskine]

At risk of being shot down in forum flames.. I'd at least be checking with Jabiru that the 600Kg Fwd limit is not a typo in the POH. If I were a fit trim solo pilot who hadn't have a big breakfast, and found myself in an 'out of fuel' scenario, from my calcs I'd want to see Fwd COG limit not more than 195mm (19.7% MAC) in order to be able to land slowly and safely. I don't know the Jab, and am working from info in POH V4 Feb 20 and guessed the fuel arm, The thread is very interesting, and whatever the answer is, it's a good spot, a great question and a good exercise to go through anyway.

[RFguy]

Hi Stuart. yeah. well there are a few alphabet typos in Jab doco, so I guess its not unlikely there are numerical typos, since 17mm of COG range at MTOW seems tight. Similar tight range at MTOW on the Brumby 610. Since I am new to this, I need to do some more exercises for different planes to get a feel for what is common, beleiveable and what is not. - glen

[Blueadventures]

Do RAAus have any requirement for its pilots to understand and be able to work out C of G weight and balance.

A friend today who flies a high wing plastic fantastic RAAus plane asked me about C of G. He had taken a heavy friend flying and when he came in to land he couldn't get enough up elevator and landed nose down. No damage he thinks. He asked me if the regulations allow him to get the elevator settings changed. I cannot remember exactly what his plane is, but the Aussie agent is in Melbourne. He commented that he has never been able to get a stall, full up elevator and closed throttle results in a high rate of descent. The plane is LSA, so I told him it probably wouldn't be possible to alter anything, but he ought to check the actual deflection of the elevator. The pilot and passenger sit forward and fuel is further aft, as is luggage. Must find out what the plane is

Agree to check deflections and weigh his mate (under / over say 120kG?) Then do calcs of mass and moments of the flight. Even do a current w&b.

[RFguy]

Reading and running the numbers on a few of the lighter LSA class, the narrow permissible CG range for some many aircraft seems common. The numbers in the Jab manual are consistent over their smaller types, also. As a result I have little reason to doubt them. (putting J230 out of the discussion- since its MTOW CG range is large (600kg)- but of course a J230 is a J430 without rear passengers, so we dont really see it up to its design MTOW (760kg) in J230 config. The J170 does stay inside its CoG for two heavy passengers and a light load of fuel. Says something for J170 design I guess.. (My newbee guess) .But some aircraft I calculate need rear ballast in this scenario (depending on fuel load and fuel arm) .

 

Reading some internet posts and forums I conclude many LSA pilots are unaware of the criticality of their CG location at loading combination extremes and or MTOW.

[facthunter]

IF you are operating outside the allowable C of G range your aeroplane is unairworthy and you are a test pilot. It's THAT simple. Nev

[Old Koreelah]

...and people shouldn’t think the “Test Pilot” label is somehow glamorous; it can get quite uncomfortable and bluddy scarey. The possible legal and insurance implications are bad enough, but you can do a lot of damage to your loved ones.

[facthunter]

REAL Test Pilot's are specially trained to handle a plane that might not do what it's SUPPOSED TO, and have a parachute and don't carry passengers at the same time as they are testing a plane. Nev

[Yenn]

Reading some internet posts and forums I conclude many LSA pilots are unaware of the criticality of their CG location at loading combination extremes and or MTOW.

That is what I am seeing and that is why I raised the question. Concerning test pilots it is good to be able to realise that you are a test pilot, far better than to find out by not knowing what you are doing.

[RFguy]

Given how critical it is, I am surprised there is not focus on it in RAaus, and demonstrated competency, at least on flown type. Like many of aircraft accidents , it often takes two things. CoG out of spec might be one of them. Many crash reports I read, (accidents that begin close to the ground - they all end close to the ground)- the pilot is guessed to be confused about what the aircraft is doing, based on their (deduced and seemlingly non sensical) control inputs in recovery attempt. I've been walking through a bunch of LSA POH and indeed there is variation , many aircraft specify a wide ish CoG range at low weights and a miniscule (millimeters !) CoG range at MTOW. Others have broad CoG range over all weights (at least that is what they write in the POH - is that up for any question) ?

 

The planes that are good for 760kg in other registrations are all very relaxed and wide CoG range at 600kg spec. The term 'wide' of course is a variable definition , wide is still not all that far.

[RFguy]

Yeah I would have though RAaus would have required demonstrated competency on flown type of CoG calcs. I read a few more LSA POH. CoG limits do vary considerably at MTOW . From ~ 120mm to about the width of your thumb ... yes width of yoru thumb. Some aircraft designs that have been upsized from similar models from 544 to the 600 MTOW to have a good range at 544 but miniscule range at 600.... Given my actually flying experience , I am hardly the person to cast doubt of whether that is a good thing, but my experience in other industries of what is cool and what is not seems to flag that one.

 

Seems that being out of CoG range many pilots 'get away with' by luck..... As it usually takes 2+ items for an accident, a CoG error leaves just one to occur (and promotes it) .

 

All the 760kg capable aircraft (MTOW when operated under different registration) , they all have 'plenty' of CoG range when operated for 600kg MTOW. (like J230).

[facthunter]

CofG issues are about control. IF you lose control you don't need a second factor unless you take being off the ground as it. Many pilots (most) instinctively pull the stick back if the nose drops. Seeing that the stick controls the wing's angle of attack to the relative airflow you can "see" that action will compound the problem. IF you are also in a turn where you should carry a margin of extra speed for the extra lift required you can again "see" what's going to happen. I can't explain the closeness of the allowable range at the higher weights. Critical and small Cof G ranges are usually caused by short tail moment arms with small tail feather areas. Sometimes you get elevator shielding with large flap settings. The centre of lift on some airfoils at higher angles of attack may have an effect as it changes. Nev

[turboplanner]

Yeah I would have though RAaus would have required demonstrated competency on flown type of CoG calcs.

If you're teaching someone to fly and giving him a Certificate the buck stops with you. The last time I did a Chain of Responsibility check RAA had the duty of care to train the pilots on Performance and Operations including W&B and the RAA structure was incomplete in terms of Compliance and Enforcement so the Flying Instructors were the ones who had direct responsibility for that formal training and assessment.

[facthunter]

The whole training is supposed to be competency based assessment In many conversions(aeroplanes, not the religious kind ) the pass for W&B and fuel system management was 100%. it's obvious therefore that SOME people will fail. The services used to pretty much set a %. who will fail. Too bad if your Course was populated by "ACES". There is an expectation of I've Paid the dough where's my ticket? Also you can teach many the right thing, and on a check many obey all the rules and next day revert to form, "Rules are for the Dumb ones not God's gift to Aviation, ME" . Wisdom, responsibility and airmanship "Wherefore art thou".Nev