weight and balance

[astjp2]

Re: weight and balance

Robert, is there a place to post the template? I cannot add it here because it is not an approved format. It sounds like people want to play with it. Tim

[Guest]

Re: weight and balance

It's hard for me to stay out of this thread, but.....I think WAY too much credit is given early manufacturers for weights, teqhniques, methods, etc. I personally dont believe there was a prayer in hell that EVERY early Taylorcraft was weighed. Probably with most other manufacturers as well. I'll bet the airworthiness certs for that entire weeks production(of the same engine) would vary no more than a few pounds and MAYBE one plane was actually weighed to satisfy the CAA.

Forrest, Do any of the test logs record empty weight?

Mike,

I am only giving them credit for knowing where the wheels are.

I too do not believe they were weighed.

In fact the equipment list with the a/c says that as I recall.

[Robert Lees]

Re: weight and balance

Our distances don't quite match up, your arm to the mains is 2" while mine is 2.5", and your tail moment is 193" and mine is 198".

And

I am perplexed that you get different arms than the TCDS...etc

I came up with the same, in a discussion off-line with Tim. Tim took those figures from A-696 (and Heaven only knows where the then factory got those from, but see below). The four Taylorcraft I have with diligently measured have always been greater that 2", and up to 3" (measuring errors acknowledged).

One reason for the perceived discrepancy may be that for the purposes of the TCDS, the different wheel (and ski) accessories quoted in A-696 item 201 (page 11) are for differences in weight between the factory-fitted equipment and the options. For those purposes, 2" might be perfectly adequate. Same for the tailwheel stuff, although I have a suspicion that the original skid value has been "transferred" to the tailwheel-equipped models.

I personally dont believe there was a prayer in hell that EVERY early Taylorcraft was weighed.

I agree. The original W&B schedule for my BC12D (of which I have the original) is a mimeograph of some mass-produced W&B document. Maybe done in batches.



Using an accurate level is so critical here. "Nearly level" on the horizontal datum (the horizontal stablizer) is not enough, because a small error here leads to a VERY small error on the main gear position (like down to 0.1"), which can in turn give rise to a HUGE discrepancy in empty weight cg arm, due to the weights involved. Accurate plumb-line & floor measurements are a must too. A modern digital or laser level works a treat.

As a casual observation, with something like 16 different Taylorcraft flown (nothing as many as Forrest, I know) I would suggest that a pre-war with a declared empty weight of 770lbs or less is fibbing. Similarly, a post-war B model (excluding those which were actually built pre-war but certified post-war, like one of mine) of less than 800lb is also fibbing.

is there a place to post the template?

I'll post Tim's spreadsheet template on the Foundation website next week (when I'm back at work), but I'll leave off the 2" and 193" TCDS measurements, which will need to be accurately measured by the person carrying out the W&B schedule.

Anyway, all grist for the mill! and Tim says I need a trip to Alaska to wind down. Here's wishing you all a good weekend.

Rob

[Guest]

Re: weight and balance

Here's a thought.

If one was measuring for the tailwheel arm how would one know what point to measure to?

I think the proper arm would be measured to the center of mass of the tailwheel assembly and that should include the additional (as compared to the skid) leaf springs.

Does anyone know where that point is? I don't.

That's why I think the TCDS is the correct arm and has compensated for that.

The arm for the mains is "off" by .5" according to what you guys said so far and this seems like it might be due to leveling and measuring according to what Rob mentioned also landing gear repairs, wear and factory tolerences on the parts that are assembled would effect it too. Finding the center of mass of a circle (wheel) seems pretty straight forward!

Dave

[Robert Lees]

Re: weight and balance

Here's a thought. If one was measuring for the tailwheel arm how would one know what point to measure to?

To the lowest point of the tailwheel tyre. Sit the tyre on the scales, that works.

The length of the tailspring is not relevant. If it is shorter, the weight will be slightly higher, and vice versa. It's the relative positions between the respective measured weights and their distance from the datum that gives the cg position.

That's the purpose of these measurements (the as-weighed values with the aircraft properly levelled). Accurate levelling, accurate weight measurements, calibrated scales and accurate longitudinal references to the datum (l/e of wing) are all crucial in determining the empty weight cg.

[Guest]

Re: weight and balance

I think we are having parallel conversations.

It just clicked, you guys are talking about where to apply the weight from a scale reading, huh?

I am talking about adding or subracting a random mass to an a/c with know c.g.

I that case the arm is to the center of mass of the random object as installed with a/c in the reference position.

Now it does make sense to me.

Dave

[Guest]

Re: weight and balance

BTW- mine measures 198" to the scale contact point of the tail wheel.

[Robert Lees]

Re: weight and balance

That's correct, Dave. Of course, one needs to know both the mass and the position relative to the datum with the aircraft levelled. !


{edit: one of mine is also 198", which is a more realistic figure that the 193" in the TCDS....thinks: 5" difference = tailskid vs. tailwheel}

[Guest]

Re: weight and balance

I am going to choose to beleive that the 193" in the TCDS is the position of the center of mass of the tailwheel and spring assembly.

It makes sense to me because it is used in a W&B sheet that way.

Its about 5" fore of the axle and I wouldn't be surprized to find that is where the c.g. is on the tailwheel with leaf springs attached.

[Hank Jarrett]

Re: weight and balance

OK guys, it doesn't matter a whit what the TCDS says for distances (yea, I know, sacrilege! So burn me at the stake). What you are looking for is where the CG is on the wings airfoil. The datum is the leading edge of the wing. The CG the regs want is within a space behind that datum. The main gear arm is the point the weight on the main gear is reacted. The tail gear arm is the point the tail load is reacted. These points are the center of the footprint of each tire. If your tail wheel spring is shorter or longer and you use the TDCS length you WILL GET THE WRONG CG. It doesn't even really matter if the plane is level if you don't mind running two accurate W&Bs. Each W&B simply (and ALWAYS) tells you the vertical line the CG is on. If you ACCURATELY do one with the mains and the prop hub instead of the tail wheel and a second with the plane hanging vertically from the tail wheel (or any other two positions for the whole plane) you will get two lines that cross at the CG. (Nice thing is it also tells you the exact location WITHIN the plane, not just the fore and aft location). The process the FAA gives us is a simplified method to get the CG for pilots use. The REAL CG is a POINT within the fuselage fore and aft, side to side and up and down, not just a distance from the leading edge. That's why I like playing with measurements of CG. I want to know where my CG REALLY is. If you aren't an aero engineer it really doesn't matter and you should just level the plane and do it their way. Just MEASURE your reaction points because if you miss the distances from reality and use the TDCS numbers you WILL get the wrong CG.
Hank

[Guest]

Re: weight and balance

Hank,

No stake required, I think you are correct with regard to weighing.

I didn't realize you were talking about weighing. I had just done my own spread sheet and used it to do equipment list changes a few days ago and assumed that was the only thing you guys were doing too, not any weighing calculations. bad assumption on my part. BTW-I used 198" (as measured) when I weighed mine.

But in support of the TCDS number I contend it has nothing to do with weighing at all, at least that's my story and I am sticking to it. :-)

I beleive that it (+193) is the arm to the c.g. of the tailwheel and leaf spring assembly and is used when calculating the moment added or substracted from the a/c existing total moment when adding or removing a tailwheel & leaf spring when calculating aircraft c.g. movement due to equipment changes.

I suspect that the leaf spring is included because without a tailwheel you have a skid not a spring and therefore from an equipment list point of view you have to add both so that would make sense but the weights listed seem to light to include the spring. I guess I need to weight a tailwheel alone and compare to the TCDS value to get a better idea. I guess my back up theory should be that the skid and the leaf spring weigh the same.

I just went out and measured 5" foreward of the tailwheel axle (ie +193).

It appears to be a point that could reasonably called the x coordinate of the c.g. of the tailwheel & leaf spring assy.

Makes sense to me because the TCDS info is in the eqipment list section so you would expect to use it as I described not for weighing and calculating the a/c c.g. from scratch.

Of course the main wheel center of mass will/should be the same as the reaction point because its round. Other variations cause it to move a bit. I used the TCDS value for this. I measure too so they must have been the same or very close, was 10 years ago.

Hope that makes sense.

Dave

[astjp2]

Re: weight and balance

WOW, all of this from a little spreadsheet! Oh, that is a funny!!!

I would drop a plub bob off the leading edge, then drop bobs adjcent to the center of the contact area of both the mains and tail when level both laterally and longitudinally.

Gawd, its only a TCraft, not a 747 or helicopter! The CG of most helicopters is about the same distance as the diameter of the mast. Now that is an aircraft that CG is critical, try walking from one side of a Huey to the other when in a hover, Army pilots really love that!

Has anyone ever noticed how the airlines seem to get all of us fat guys spread out throughout the cabin? I am beginning to think they are fortune tellers!

[Hank Jarrett]

Re: weight and balance

I used plumb bobs every time I weighed my plane, whether I was doing a W&B from scratch or changing out components. I never put skis on my plane so I haven't had to mess with that but when I did my W&B I calculated how the CG moved with and without the wind generator and then did a new W&B from scratch to see if it was accurate (it wasn't) It wasn't off so far as to be unsafe, but it WAS wrong. The location for the generator and the weight were wrong in the papers for the plane. That followed for almost everything that could go in or out of the plane. Not very far off, but always off. The one thing that was WAAAAY off was the total weight. The logs showed the basic plane was significantly lighter than it really was. I went all the way back in the logs and couldn't see where it had ever actually been weighed since it was built, all the additions and deletions had been calculated, and the errors had REALLY added up. I tore over 20# of unused wire out of the plane (WHICH IS INTERESTING FOR A PLANE WITH NO REAL ELECTRICAL SYSTEM!) The scariest one was a wire from the +battery terminal under the seat to the back side of the panel that bypassed any fuses. When I was working on the electricals (tearing them out) I pulled the main fuse and got quite a surprise when I pulled the wire out that was wrapped around the pilots control tube. It arced against the fuel tank. Not a good way to ruin a pair of pants. When you work on the electricals, TAKE THE BATTERY OUT! When you make a lot of changes, don't rely on a calculated CG, re-weigh it and figure out where the CG REALLY is.
Hank

[Guest]

Re: weight and balance

Good point on the inaccuracies Hank.

I weighed mine (the first weighing it ever had) and it was heavier too.

I went back and accounted for each repair that added metal and mods that I made and prior owners too. Never could come up with as many pounds as the factory weight and actual weight was. I figure it was 47-50 pounds light.

I plan to remove some stuff later in life to lighten it up by a few pounds.

For the moment I raised the GW by 80 lbs with a partial installation of the much admired Gilberti/Harer STC.

Dave

[Forrest Barber]

Re: weight and balance

Forrest has lots of records I will do some soon! We had a accident here Thurs , no one hurt , NTSB & FAA takes my time on Fri and today. The arms should be measured, I will give full explanation later but basically the first were measured with NO tail wheel only the skid , then somebody forgot the tail wheel stuck out a bit more. THEN we used the Scott 3200 , etc... measure your arms the 2" is correct when the ship is LEVEL. more real soon, back to the elections tonight . working out of the house. I have the "B" book over at the airport , my BC12D , 24 gal fuel ,metal prop, pants,
EW is 815 lbs....