Empty CofG

[astjp2]

Re: Empty CofG

Just because the tc calls it out, you need to measure it to be right. the 193 was for an oak skid, tail wheel moves it about 4" aft

First off your measurements are off from what the type certificate calls out. They call the mains at 2" not 2.75". The tailwheel is 193 per the TC instead of your 197.75. That being said I would use the 2" figure, but re-measure the tailwheel distance. Even using the TC distances I came up with 18.77, which is still aft of the EW CG range. Really though the fact that it doesn't fall within the EW CG range just means you need to figure the critical fore and aft CG limits for loading. As long as you are within limits when the airplane is loaded and remains so during flight you will be OK.

With my numbers if you are loaded with 3 gallons of fuel and 396 pounds of people you are still within flight CG limits by .25".

[astjp2]

Re: Empty CofG

Ummmm, no, center of mass of the tail wheel is not listed anywhere on anything I have ever seen, it has to do with were you put the scale, The arm is calculated from the center of the axle of the tail wheel and each tail wheel manufacturer is slightly different which changes the arm.

Read chapt 3, it is from the center of each wheel axle...

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-1.pdf

BIGK and all,

The reason that the TCDS says 193" for the tailwheel and we figure something like 198" is because they are two different arms.

The TCDS is telling us that the distance to the center of mass of the tail wheel is 193". That makes sense because the center of mass will be slightly forward of the tail wheel's axle. In other words the center of mass is not located at the axle location.

When we measure to the point of contact on the tail scale we measure to the axle location, a few inches aft of the tail wheel center of mass and get ~198".

In the case of the main wheels (round and center of mass corresponds to the axle location) so if we use the arm for wheels from the TCDS it corresponds (when unmodified or undamaged) to the scale load point.

That's my story and I am sticking to it.

BIGK, I have similar W&B issues as you do. I did an aft loaded CG check and made loading schedules using Microsoft Excel. If you want a copy of that Excel spreadsheet file to modify for yours send me you email address.

Thanks Hank for the kind words.

Dave

[Guest]

Re: Empty CofG

Ummmm, no, center of mass of the tail wheel is not listed anywhere on anything I have ever seen, it has to do with were you put the scale, The arm is calculated from the center of the axle of the tail wheel and each tail wheel manufacturer is slightly different which changes the arm.

Read chapt 3, it is from the center of each wheel axle...

https://www.faa.gov/regulations_poli...A-H-8083-1.pdf

Well, ummm, yes.

You are confusing two things, application of a weight to the airframe such a oil, gas, baggage, components (that's the arm in the TCDS) and determining the CG of the aircraft when weighed (axle locations).

Dave

p.s. - think of it this way, when the TCDS gives an arm to a fuel or oil tank, prop, battery... it's telling how far it is to the center of that load, center of mass

[astjp2]

Re: Empty CofG

Ah but the 193" was for an oak skid, not a tail wheel, when you add the tail wheel you add the 4 inches or so of arm by measuring it to the datum from the center of the axle when doing the calculations. Its like when someone adds a big baggage, to do a true aft CG calculation, you calculate it by putting all of the weight at the very aft location of the baggage, this simulates the load shifting aft because its not secure for example, even though the center of the big baggage is like 10" forward of the aft location. Typically the mass would be centered in the baggage area but it may not always be so.... The tailwheel position is measured from the center of the axle because that is where its arm affects the entire aircraft CG calculation. By itself the center of mass of the tailwheel is forward of the axle because of the fork and mount is forward but how it affects the airplane CG is still the axle...

[Guest]

Re: Empty CofG

I am poor teacher but let me give this a try.

Calculating a CG by weighing the the object is different than calculating the CG of an agglomeration of objects.

That is causing confusion.

To calculate the CG (or center of mass or center of area) you pick a datum then take the distance to the center of gravity of each object then multiply that distance (a.k.a. arm) times the weight.

Do that for each object in the agglomeration.

Then sum up all of the products and divide that by the sum of the weights and you have the location of the center of gravity of the agglomeration.

This is the standard method for the calculation and is just physics or for engineers a statics course. As mechanics we use this method when we add and item and do not weigh the airplane ie. by "calculation" only.

The agglomeration of items in our case is an airplane but the calculation is the same whether you think you are adding an airframe to a radio or a radio to an airframe. Its just a bunch of stuff being put together.

So when you add a tailwheel to an airframe you need to know the weight of it and how far away from the datum its center of gravity is.

That is what the TCDS is telling us or should be. I assume TCDS is correct (193") but I am certain what the number should represent. It should represent the distance from the datum to the center of mass of the tailwheel not the the distance to the tailwheel axle. Please read on for info about the distance to the tailwheel axle location and use.

In real life this can be done in three dimensions but for our interests it is only in the fore and aft dimension that we are told to care about when the airplane is level.

When you weigh an object to calculate its center of gravity your position the object on scales, pick a datum and measure the distance to where the weight is being applied to the scale (a.k.a. arm).

Since we set the plane on its wheels the point where the weight is applied is at the axle location so the arm will be the distance to the axle.

Multiply each scale weight by it's distance to the datum (arm), sum up all the products then divide by the sum of the scale weights and you have the CG location.

The measured distance to the tailwheel axle does not correspond to the arm to the tailwheel's CG as listed in the TCDS that was was needed in the agglomeration calculation above but will be farther aft. They are are two different distances in the case of the tailwheel and have two distinct purposes.

One of the distances, the distance in the TCDS is for the purpose of adding or removing an object from the airframe and corresponds to the distance to the objects center of mass (as in the first calculation described above).

When weighing the airplane (or any object) we need to know the distance from datum to the point where the scale weight is being measured(as in the second calculation described above).

The main wheels being round have an axle location and CG that are the same so the TCDS distance should correspond to the measured distance in this case. Why the measured distance does not correspond is perplexing but I suppose repairs, bends, factory inconsistencies or errors might explain it.

Hope this helps.

Dave

[Ragwing nut]

Re: Empty CofG

nobody has mentioned it but you must weight the airplane in level flight or your cg will also read aft of what it should be as well.

[PA1195]

Re: Empty CofG

From current my W&B. A bit different from the TCDS but I assume it was a result of the actual measurement techniques described above in a level attitude:

Leveling means: Upper surface of horizontal stabilizer
Datum to wing leading edge: 0.0"
Datum to centerline of MLG axle: (D): 3.19"
Datum to centerline of tailwheel axle: 199.44"
Centerline of MLG to tail wheel (L): 196.25"

Gary

[Hank Jarrett]

Re: Empty CofG

What do you mean poor teacher!?! That is about the best explanation I have read (MUCH better than the FAAs). Clear, concise and correct. A LOT better than what I was preparing (long boring and confusing).
It may need to be read a few times, but that isn't bad teaching, it is because the process looks so simple, but is full of traps that will give you a wrong answer.

Good job

Hank

I am poor teacher but let me give this a try.

Calculating a CG by weighing the the object is different than calculating the CG of an agglomeration of objects.

That is causing confusion.

To calculate the CG (or center of mass or center of area) you pick a datum then take the distance to the center of gravity of each object then multiply that distance (a.k.a. arm) times the weight.

Do that for each object in the agglomeration.

Then sum up all of the products and divide that by the sum of the weights and you have the location of the center of gravity of the agglomeration.

This is the standard method for the calculation and is just physics or for engineers a statics course. As mechanics we use this method when we add and item and do not weigh the airplane ie. by "calculation" only.

The agglomeration of items in our case is an airplane but the calculation is the same whether you think you are adding an airframe to a radio or a radio to an airframe. Its just a bunch of stuff being put together.

So when you add a tailwheel to an airframe you need to know the weight of it and how far away from the datum its center of gravity is.

That is what the TCDS is telling us or should be. I assume TCDS is correct (193") but I am certain what the number should represent.

In real life this can be done in three dimensions but for our interests it is only in the fore and aft dimension that we are told to care about when the airplane is level.

When you weigh an object to calculate its center of gravity your position the object on scales, pick a datum and measure the distance to where the weight is being applied to the scale (a.k.a. arm).

Since we set the plane on its wheels the point where the weight is applied is at the axle location so the arm will be the distance to the axle.

Multiply each scale weight by it's distance to the datum (arm), sum up all the products then divide by the sum of the scale weights and you have the CG location.

The measured distance to the tailwheel axle does not correspond to the arm to the tailwheel's CG as listed in the TCDS that was was needed in the agglomeration calculation above but will be farther aft. We see this from experience.


The main wheels being round have an axle location and CG that are the same.

Hope this helps.

Dave

[LostnSpace]

Re: Empty CofG

nobody has mentioned it but you must weight the airplane in level flight or your cg will also read aft of what it should be as well.

Years ago I prepared a Curtiss C-46 for a ferry flight, and in the paperwork was a calculation of the W&B with the plane in a 3 point position, there were all kinds of mathmatical formulas and diagrams, I couldn't prove it right or wrong, but it sure looked impressive, the plane flew fine, and would it ever pack a load, what a weight lifter, gary

[3Dreaming]

Re: Empty CofG

Just because the tc calls it out, you need to measure it to be right. the 193 was for an oak skid, tail wheel moves it about 4" aft

I think that is what I said.


Quote Originally Posted by 3Dreaming View Post

First off your measurements are off from what the type certificate calls out. They call the mains at 2" not 2.75". The tailwheel is 193 per the TC instead of your 197.75. That being said I would use the 2" figure,[B] but re-measure the tailwheel distance.[/B] Even using the TC distances I came up with 18.77, which is still aft of the EW CG range. Really though the fact that it doesn't fall within the EW CG range just means you need to figure the critical fore and aft CG limits for loading. As long as you are within limits when the airplane is loaded and remains so during flight you will be OK.

With my numbers if you are loaded with 3 gallons of fuel and 396 pounds of people you are still within flight CG limits by .25".

[Ragwing nut]

Re: Empty CofG

Years ago I prepared a Curtiss C-46 for a ferry flight, and in the paperwork was a calculation of the W&B with the plane in a 3 point position, there were all kinds of mathmatical formulas and diagrams, I couldn't prove it right or wrong, but it sure looked impressive, the plane flew fine, and would it ever pack a load, what a weight lifter, gary

Thats a whole different beast. Im sure if you could close the doors on it it would fly. I always calculated in level flight, same way I rig the airplane. It will affect CG calculations because in 3 point it will move weight aft on the scales.

[Guest]

Re: Empty CofG

Some clever guy could figure out the transformation of the cg calculated in the 3 pt. position to its location in the level position.

Perhaps if/when the airplane if big enough and difficult to level then they actually provide that transformation rather than attempting to level and weigh a C46?