Tail Weight 46 BC12D

[Scott]

Thanks Rob, though I probably wasn't clear, my understanding is the same as yours.

one step further is that if your empty wieght CG falls outside the range, you need to calculate CG for critical flight phases eg takeoff, landing, and aux fuel tank scenarios.

while it may not be mandatory, weight and balance should be calculated for each flight leg, regardless of the empty CG position. Given the increased weight of the average human, particularly on this side of the pond, it's clear that the average BC12D with two people on board can carry just about enough fuel to get airborne!

[Guest]

Well... I have taken new measurements and found the distance from the DATUM to center of tailwheel axel is 198 inches. 1 inch more than the factory dimension of 197 inches. This is probably due to the Alaska Bush tailwheel I installed. I found the reason my previous measurements were off due to a cloth engineering tape that had shrunk over the years! My steel tape made the difference.
And that being said... I agree that the Factory CG data does not apply if changes have been made. The factory CG was with a wood prop.. I have a McCauley that was installed in the past that certainly cause a different CG range...

The factory dimension is not intended to be the distance from the datum to the tail wheel axle. we must measure that as you did.

The factory dimension is intended to be the distance from the datum to the center of of mass of the tailwheel assembly.

One thing that leads to confusion about this is that the main wheels will measure the same for distance to center of axle as they do for center of mass.

This happens because the are round center of mass and axle center are the same point, so we assume the tail wheel assembly is the same but it is not.

When you calculate a center of mass (CG) of a group of objects of known weights rather than weighing (which is what the arm in the TCDS is used for) you always use each component's center of mass to compute the arm

Hope his helps.

Dave R

[Hank Jarrett]

Dave, sorry to disagree but the tail moment is measured from the datum to the POINT OF CONTACT where the weight is being measured. That is usually measured by dropping a plumb (while the aircraft is level) from the center of the axle to the horizontal plain under the aircraft. Likewise the moment for the mains is to the contact point where the weight is being measured under each main wheel (also usually measured from the datum to the center of the axle to the horizontal plane under the plane. The tail moment has NOTHING to do with the center of mass of the tail wheel. You can actually find the CG from ANY point around the tail, it doesn't even have to be the tail wheel. You only have to measure to the point the weight is being measured to get the arm and measure the weight at that point.

[3Dreaming]

The center of mass would be useful when switching from tail wheel to another if you are re calculating the weight and balance without re-weighing the airplane.

[Guest]

Dave, sorry to disagree but the tail moment is measured from the datum to the POINT OF CONTACT where the weight is being measured. That is usually measured by dropping a plumb (while the aircraft is level) from the center of the axle to the horizontal plain under the aircraft. Likewise the moment for the mains is to the contact point where the weight is being measured under each main wheel (also usually measured from the datum to the center of the axle to the horizontal plane under the plane. The tail moment has NOTHING to do with the center of mass of the tail wheel. You can actually find the CG from ANY point around the tail, it doesn't even have to be the tail wheel. You only have to measure to the point the weight is being measured to get the arm and measure the weight at that point.

What I said is correct.

Hank, you are talking about the case where you weigh the entire object (in this case the airplane) and one must measure to the axle location (that is also the point of contact most often). I said that above also.

However when one adds a tailwheel ASSEMBLY to an airplane one uses the arm from the datum to the center of mass of the assembly being added to compute the new CG of the airplane. I stated that also above, perhaps not clearly enough though.

The measured value that you refer to is used in the case where you weight the airplane and compute CG. However when you add a tailwheel assembly to an airplane you used the distance in the TCDS to compute the new CG. I suspect that the arm in the TCDS will always be shorter than what you measure since the tailwheel assembly CG will normally be forward of the axle.

Tom's comment above is relevant, the distance in the TCDS is to the center of mass of the tailwheels listed in TCDS.

Look at a statics book where they calculate center of mass or center of area of an agglomerate of many component objects.

In the current discussion it would be combining a tailwheel assembly with a airplane of known CG, they both have an arm and center of mass. The TCDS data is for use in that kind of calculation.

When weighing the airplane you are doing a different calculation, perhaps more direct where you use scale readings and arm to the center of the wheels (or whatever contact points you choose).

So I think that you are considering only one case of how to determine CG, weighing and not the "calculated" way.

I tried to make more clear my first post by adding blue text.

Dave R

p.s. Maybe this is a useful summary . The arms in the TCDS are not useful during weighing, they are for adding components and calculating (not weighing) the weight & CG. However the main wheels happen to have the same arm for the weighing method and the adding components method because the wheels are round and symmetrical.

[Hank Jarrett]

I see what you were trying to say now. Not doing an actual weighing, but CALCULATING the change of empty CG and weight. YES, you would do the same if you pulled a wood prop and put a metal one on. EW would go up by the difference in weight and CG would move based on the weight difference and arm to the CG of each part. Subtract the weight and moment of the old and add the weight and moment of the new. That lets you calculate the new CG. On large aircraft it was kind of rare to actually WEIGH the plane after a change. We DID do it in the Navy (on the A-6) with the first plane when we were prototyping a change to be sure we got it right. I mis-read it as if you were saying you would use the distance to the CG of the part as the arm in weighing and balancing the plane.

[Guest]

Hank,

I am so glad we are on the same page.

So here's the thing, the only reason that I injected all that was to shed some light on why the TCDS shows and arm of 193" to the tailwheel assembly and when people measure to the axle location they get a bigger number.

Naturally the center of mass of the tail wheel assembly will be forward of the axle location because the fork, swivel and mount are forward of the axle and they have mass.

The confusion that happens IMHO is that when we look at the TCDS and see that the arm for the main wheel is 2" and that corresponds to the measurement we take on the hangar floor we then expect that the 193" arm given for a tailwheel assembly also should correspond to the measurement we take.

But it does not correspond and should not because the arms in the TCDS are to center of mass so you can do a calculated CG.

The main wheel the measurement matches the TCDS because it's a round object and the axle location and center of mass are the same, not so for the tailwheel assembly.

Thanks for your patience, Dave

[PA1195]

Here's another exception. Recently for experience I helped weigh a C-180 on floats. The mechanic had four electronic transmitting scales. We leveled the plane lateral and longitudinal then dropped a plumb from the firewall datum to the floor at two side by side spots. The scales were placed two per side fore and aft under the floats (step to bow rise section) with a small board as the final contact point mid-scale to float keel. The distances (arm) from the cross plumb line to scale centers was measured (about +5" and +60" per side). The four arms times weights were calculated, the moments added , and the C.G. determined by dividing the total moment by total weight.

Gary

[astjp2]

Well as long as you follow https://www.google.com/url?sa=t&rct=...XmgxZ8Bu6kcGpv you will be OK.

[PA1195]

Great link Tim. Now we have a W&B source with pictures and examples. Better than the other documents I have like AC 43.13-1B.

Gary

[astjp2]

Great link Tim. Now we have a W&B source with pictures and examples. Better than the other documents I have like AC 43.13-1B.

Gary

Gary, it is also posted in the references section, I posted it a couple of years ago.

[swift1b]

Hank made comments in June regarding removing a wood prop and replacing with a metal prop. This would move the empty weight CG forward beyond the 17.9 inches ?

[Scott]

It will move forward. By how much will vary depending on which engine/mount combination you have and the current empty weight and CGOV position.... And of course the weight of the props.

[Hank Jarrett]

The other thing that can help a lot os to take the battery box from behind the seat and move it to the middle of the floor just in front of the kick plate under the seat. Not much but every bit helps. If you decide to move teh battery to the engine compartment be SURE to move the voltage regulator there too. The voltage regulator should ALWAYS be close to the battery so they will be the same temperature. One of the fastest ways to destroy a battery is to have the voltage regulator in a hot engine compartment and the battery in the back. The battery will be overcharging all the time. If the Voltage regulator is cold and the battery is hot the battery will never fully charge. Voltage regulators are TEMP SENSITIVE and so are batteries!!!

[astjp2]

The other thing that can help a lot os to take the battery box from behind the seat and move it to the middle of the floor just in front of the kick plate under the seat. Not much but every bit helps. If you decide to move teh battery to the engine compartment be SURE to move the voltage regulator there too. The voltage regulator should ALWAYS be close to the battery so they will be the same temperature. One of the fastest ways to destroy a battery is to have the voltage regulator in a hot engine compartment and the battery in the back. The battery will be overcharging all the time. If the Voltage regulator is cold and the battery is hot the battery will never fully charge. Voltage regulators are TEMP SENSITIVE and so are batteries!!!

tell that to piper Hank!