weight and balance

[GolfWhiskeyHotel]

Now my Jabiru / Rotax project is nearing completion there is a matter I would like to correct. That is the C of G. placement. Jabiru say that the forward limit of a  (at MTOW)  J160 should be 233mm  behind the wing L.E.

After I had the weight and balance done I see that the C of G is at 217mm  behind the L.E.  the question is  How much weight do I add at the tail end to move the C of G 30mm rearward.  I have all the calculation values at hand , I just dont know how to calculate the value. Weight 2.pdf

weight 1.pdf

[440032]

My question first would be how do you have a 24 registered (factory built type accepted) Jabiru - with a Rotax in it?

[facthunter]

Is the battery located forward? it's often used to achieve balance. You'll have to go back to studying W & B and understand the basic principles.  Have you got a basic weight  and index figure?.   Nev

[GolfWhiskeyHotel]

50 minutes ago, 440032 said:

My question first would be how do you have a 24 registered (factory built type accepted) Jabiru - with a Rotax in it?

Go to MARAP. jump thru required hoops, fly aeroplane

[GolfWhiskeyHotel]

23 minutes ago, facthunter said:

Is the battery located forward? it's often used to achieve balance. You'll have to go back to studying W & B and understand the basic principles.  Have you got a basic weight  and index figure?.   Nev

Battery is so light as to make no difference.  I have necessary weight and balance sheets. (Attached here)   I can see how to calculate aircraft index units. But I want to know how to calculate backwards so to speak , or how to achieve lower Aircraft Index Units by a known amount.  For example ; adding 1 kilo to arm at -3750mm (tail) moves C of G rearwards by ? mm. I'm not a maths expert. I can guess, but I want to KNOW.   Cheers

weight 1.pdf Weight 2.pdf

[kiwiaviator]

7 minutes ago, GolfWhiskeyHotel said:

Battery is so light as to make no difference.  I have necessary weight and balance sheets. (Attached here)   I can see how to calculate aircraft index units. But I want to know how to calculate backwards so to speak , or how to achieve lower Aircraft Index Units by a known amount.  For example ; adding 1 kilo to arm at -3750mm (tail) moves C of G rearwards by ? mm. I'm not a maths expert. I can guess, but I want to KNOW.   Cheers

weight 1.pdf 690.66 kB · 0 downloads Weight 2.pdf 278.15 kB · 0 downloads

Battery may be 'light' however moving it from forward of datum to rear of datum can make a difference. You can use Ozrunways W&B to do what if's. You can add new stations easily. If you do add weight near the tail, you need to be mindful of structural limitations as there is 'less aeroplane' back there.

Edited August 24, 2023 by kiwiaviator

[Thruster88]

30mm x 312.4kg = 3750mm x 2.499kg 

 

You may not need the 2.5kg in the tail. Adding pilot, passenger,  baggage and maybe fuel will move the of G rearward. You need to do wieght and balance from your POH using your "new" empty wieght and moment arm.

[Kiwi]

The weights and C of G in your file "weight 2" are at empty weight,  the C of G you state from Jabiru are at MTOW.

Where does the C of G sit when you are in the pilots seat with both zero fuel and at full fuel?

 

Lots of aircraft are forward of C of G when empty. 

[GolfWhiskeyHotel]

2 hours ago, kiwiaviator said:

Battery may be 'light' however moving it from forward of datum to rear of datum can make a difference. You can use Ozrunways W&B to do what if's. You can add new stations easily. If you do add weight near the tail, you need to be mindful of structural limitations as there is 'less aeroplane' back there.

400 grams a foot in front or behind the c of g will make no difference

[facthunter]

Moving a mass within the CofG range cannot move it out of the range. Moments are a force (weight)x Distance from a designated point. The datum chosen is often ahead of the plane itself. If you chose the mid distance of the desired Cof G  as a datum as a practice run.it would be more easy to understand with Plus and minus moments leaving some residual.. Like balancing a see saw. Otherwise you need a table giving the moment range for any given total wt. I've always preferred a graphical presentation as it's more intuitive.  It's better if you UNDERSTAND what is happening all the way rather than just some Table of numbers. That's my take on it and hope it helps.  Nev

[skippydiesel]

One small point - If it transpires you do need more weight adft, than your existing battery (LiPo ?) will supply, what's stopping you using a heavier battery (lead acid?) ?

Edited August 25, 2023 by skippydiesel

[facthunter]

Nothing. The C-172 Battery was a fair way behind the rear seat the last time I burnt my arm with a metal watchband in the mid 60's. Events like that sharpen your memory..   Nev

[GolfWhiskeyHotel]

10 hours ago, skippydiesel said:

One small point - If it transpires you do need more weight adft, than your existing battery (LiPo ?) will supply, what's stopping you using a heavier battery (lead acid?) ?

THis is true, The battery in question is LIFEP04 not lipo. The problem lies in normal battery weight 7 - 10 kgs. and the position available is some way behind the seats. That may be too much of a good thing!

 

 

[GolfWhiskeyHotel]

 

11 hours ago, facthunter said:

Moving a mass within the CofG range cannot move it out of the range. Moments are a force (weight)x Distance from a designated point. The datum chosen is often ahead of the plane itself. If you chose the mid distance of the desired Cof G  as a datum as a practice run.it would be more easy to understand with Plus and minus moments leaving some residual.. Like balancing a see saw. Otherwise you need a table giving the moment range for any given total wt. I've always preferred a graphical presentation as it's more intuitive.  It's better if you UNDERSTAND what is happening all the way rather than just some Table of numbers. That's my take on it and hope it helps.  Nev

I took your advice and found a J160 W&B sheet. It seems all may be well.  What do you think?  And thanks for the solid advice. G

 

 

W & Balance sheet.pdf

[facthunter]

Couldn't see all of it but that's the format I prefer. The original basic Wt and index MUST be right for YOUR plane . Every aircraft has its OWN.  IF anything is moved that's part of it, It will have a new one.  Check CofG with Fuel at T.O. wt  Planned Landing wt and ALL useable fuel gone.  Has to remain in the range permitted.  Nev

[tafisama]

You do not need to worry about adding any more ballast.Your weigher has done it all.I believe the Rotax  engine with radiator weighs more than the Jabiru engine.That would have put your cog forward so to prevent that,thats why that ballast of 2.5kgs has been added.I have checked your cog with a pilot only with weight of 80kgs with no baggage and no fuel,it is 229.36mm which within both  forward and aft limits.With 100l of avgas,pilot and no baggage ,it is 263.72 which is well within both forward and aft limits.

There is nothing to correct unless you want to reduce weight of your ballast to move cog  more forward.

[facthunter]

How your elevators sit at cruise speed is a Good indication. They should be close to streamlined (level). You might need a mirror to check this If it's well forward the flare might be a bit ineffective at lower speeds. A long way rearwards is quite unsafe. You may have the tailplane stall. Balance is VERY important.  A bit rearwards is more efficient. The tail feathers are doing some of the lifting.  Nev

[Old Koreelah]

On 27/8/2023 at 10:29 AM, facthunter said:

How your elevators sit at cruise speed is a Good indication. They should be close to streamlined (level). You might need a mirror to check this…

An easier way is to install a temporary cardboard “quadrant” next to the stick. With a helper, mark elevator positions on it so you can see where it’s at during level flight.

[skippydiesel]

Elevators position in cruise;

 

Even easier - 

On ground (may need an assistant to advise of elevator position)

Take a piece of flat board (plywood, strong cardboard, etc) of sufficient length to hold one end against a "known" position on bottom edge of instrument panel (or other convenient fixed structure) and covers expected movement range of stick.

Place board end on panel position and along side of stick 

Neutralise elevator (elevator level with horizontal stabiliser)

Mark position of stick

Go flying 

In cruise, mark position of stick on board

Land and park

Place board on panel and bring stick to cruise position - check elevator relationship with horizontal stab

You now know your elevator position in cruise.

Simples!

[RFguy]

For easy weighing for LSA, during rotax install, I bought three of these floor industrial scales. (not RAAus approved but accurate)

https://www.ebay.com.au/itm/285144918758

A fairly inexpensive investment for one wanting to engage in modifications that affect the CG location..

 

For a 912ULS in a J160, I'd expect something would need to be moved to the baggage.....  Ideally any mods should result in a CG that is in the same location and little change to the moment of inertia around the CG location . in order to satisfy the pub test.... Beware though that adding ballast to the rear , for example, I would not recommend as it may change spin recovery behaviour (because it increases the moment of inertia).
If the rotax whole kit weighs more (likely for the 2200) , I would expect it likely you'd need to add weight in the baggage- and the likely best way to do that is move the lead-acid battery to the bags. 
I have seen some Oil cans moved to the rear and I would be careful with that because it will require UPSIZING of the suction side hose for the long length. (which was not done in one I saw) ....

 

Edited August 30, 2023 by RFguy

[Blueadventures]

3 minutes ago, RFguy said:

For easy weighing for LSA, during rotax install, I bought three of these floor industrial scales.

https://www.ebay.com.au/itm/285144918758

 

 

fairly inexpensive investment for one wanting to engage in modifications that affect the CG location..

 

 

 

Have you heard any news on when RAAus will have w&b approvals back; heard was to be September.  Planning to email a question next week.  Cheers.

[RFguy]

updated my post

[skippydiesel]

Just want to be "pedantic on the use of the word BALLAST🙃:

 

Ballast: Wiki "material that is used to provide stability to a vehicle or structure"

 

Leaving aside structures (inc. railway lines) & vehicles, other than aircraft - ballast is not part of the aircrafts structure, equipment or systems needed for flight.

Ballast is a temporary weighting of the aircraft, to satisfy a particular circumstance. 

Relocating the battery, to meet W&B  requirements, is not a ballasting action.

Putting a bag of lead shot in the tail is.

 

Personally, I consider the use of ballast to be an action taken after all other correctional possibilities have been exhausted (see preceding argument/statements on the undesirability of ballasting)

[Blueadventures]

51 minutes ago, skippydiesel said:

Just want to be "pedantic on the use of the word BALLAST🙃:

 

Ballast: Wiki "material that is used to provide stability to a vehicle or structure"

 

Leaving aside structures (inc. railway lines) & vehicles, other than aircraft - ballast is not part of the aircrafts structure, equipment or systems needed for flight.

Ballast is a temporary weighting of the aircraft, to satisfy a particular circumstance. 

Relocating the battery, to meet W&B  requirements, is not a ballasting action.

Putting a bag of lead shot in the tail is.

 

Personally, I consider the use of ballast to be an action taken after all other correctional possibilities have been exhausted (see preceding argument/statements on the undesirability of ballasting)

Bristelle are one example of secured 'Mass Ballast' that is bolted to the lower area of the engine mount.

[Old Koreelah]

Instead of adding lead to correct an imbalance, perhaps you could use something that could some day be useful: extra emergency gear, bottles of water,  more tools, a spare tube, etc.