Starduster Too W&B question

timwinters

timwinters

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Okay, Mx gurus...

The Starduster I bought has been totally restored, new covering, new paint, new lots of things. So it needs a new weight and balance.

The P.O.H. states that the "firewall web" is the datum point/plane.

What exactly is the web of the firewall?
 
I've never seen it worded that way on a W&B, but "web" normally refers to the thin inner portion of a structure. In this case it would mean the thin sheet metal of the firewall itself, not any supporting structure.

But ask over on the Biplane Forum, lotsa Starduster knowledge there.
 
timwinters said:
The P.O.H. states that the "firewall web" is the datum point/plane.

What exactly is the web of the firewall?
Click to expand...
Gotta axe...

I can see if you want to, say, put a gun safe in the turtle deck, you would need to measure to find the arm. But, if you are going to weigh it, and if you have the arm of the seats, tanks, and wheels; does it matter where the datum is?
 
Capt. Geoffrey Thorpe said:
Gotta axe...

I can see if you want to, say, put a gun safe in the turtle deck, you would need to measure to find the arm. But, if you are going to weigh it, and if you have the arm of the seats, tanks, and wheels; does it matter where the datum is?
Click to expand...
Trying to establish the arms...gotta have the datum to do that. It's an amateur built experimental...I wouldn't trust that the gear is exactly where the plans say it should be (if I even had them) and I know this plane's tail wheel is on a longer "fork" than standard because the original builder thought it would make it more stable (or so I'm told that was the reason).
 
Dana said:
I've never seen it worded that way on a W&B, but "web" normally refers to the thin inner portion of a structure. In this case it would mean the thin sheet metal of the firewall itself, not any supporting structure.

But ask over on the Biplane Forum, lotsa Starduster knowledge there.
Click to expand...
That makes sense...similar to the web of an I-beam... being the construction guy that I am...

Thanks!
 
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Funny thing is...When I Googled "web of the firewall" and/or "define web of the firewall". I got all kinds of ads for internet security companies!

Even when I qualified it with "aviation, define web of the firewall".
 
timwinters said:
Trying to establish the arms...gotta have the datum to do that. It's an amateur built experimental...
Click to expand...

Perhaps there’s something I am missing but if that is what you’re trying to do why not pick your favorite point and use that as the datum?

One airplane I maintain has the datum line at an imaginary point about 14” in front of the nose of the airplane so anything should be possible.
 
I wonder how thick the web is.

I suppose you could measure forward from a known location and attack it from the other direction. Another idea, and it would take a little work on your part, is to level the aircraft up according to the documentation and basically start from scratch. You could put the datum wherever you want as long as you record it. And you would then know if all the components are where they are supposed to be.
 
Capt. Geoffrey Thorpe said:
The c.g. limits for the aircraft are given w.r.t the datum.
Click to expand...

Good point. But it is an experimental so that can be set where you want. Or he could work his way backward to figure out where the datum was, if he knows where the seats or some other component is. I had to do that with the aforementioned airplane since the type certificate nor any factory documentation had information on it.
 
Capt. Geoffrey Thorpe said:
The c.g. limits for the aircraft are given w.r.t the datum.
Click to expand...

:yeahthat:

mondtster said:
Good point. But it is an experimental so that can be set where you want.
Click to expand...

Legally, yes. Safely, no. The designer specifies the safe CG limits. Regardless of what datum you use, and what the actual numbers are, the CG range must still be in the same place... and to locate that place you have to know where the original datum is, even if you then rework it from a different datum.
 
Dana said:
and to locate that place you have to know where the original datum is, even if you then rework it from a different datum.
Click to expand...
Not really, if you are starting from scratch. Pick a datum and start measuring. That is assuming there is documentation that presents the CG as a percentage of Mean Aerodynamic Chord. Or inches aft of the leading edge, or other such measurement.
 
Greg Bockelman said:
That is assuming there is documentation that presents the CG as a percentage of Mean Aerodynamic Chord.
Click to expand...
I admire your optimism. :D The concept of MAC seems to confuse people used to the typical GA airplane W&B, and makes their heads explode when you start talking about sweep, taper, and/or biplanes. :cool:

Nauga,
and a big MAC attack
 
Greg Bockelman said:
Not really, if you are starting from scratch. Pick a datum and start measuring.
Click to expand...

ummm...no. It'd still have to tie into the original C.G. range which, in this case is 18" to 24" behind the firewall web...if you don't know exactly where the firewall web is how then do you establish your own point? Example W&B out of P.O.H. attached.
 

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  • Generic W&B example.pdf
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timwinters said:
if you don't know exactly where the firewall web is how then do you establish your own point?
Click to expand...
If the firewall is the typical thin stainless sheet with framing behind it it's going to be the location of the sheet metal (like @Dana said). Forward or aft face is splitting hairs, unless you can measure everything to the nearest 0.020-0.032 in. Be sure to make your new measurements with the airplane leveled per W&B instructions, or at least measure them parallel to the level reference (the top longeron in your example).

Nauga,
and a career full of Form F's
 
The problem with this bird is likely that it's outfitted with an O-320 and a fixed pitch prop. The ideal combination according to everything I've found is an O-360 and a CS prop...likely 100# difference out on the nose.
 
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timwinters said:
ummm...no. It'd still have to tie into the original C.G. range which, in this case is 18" to 24" behind the firewall web...if you don't know exactly where the firewall web is how then do you establish your own point? Example W&B out of P.O.H. attached.
Click to expand...

The arms of the seats are given. You can figure out where the datum is. As I said before, sometimes you have to work backwards to get the answer.
 
mondtster said:
The arms of the seats are given. You can figure out where the datum is. As I said before, sometimes you have to work backwards to get the answer.
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There's usually more uncertainty in the measurement point on the (seats, baggage, etc...) than is for the datum :)

Nauga,
whose seat is thicker than his firewall
 
nauga said:
There's usually more uncertainty in the measurement point on the (seats, baggage, etc...) than is for the datum :)

Nauga,
whose seat is thicker than his firewall
Click to expand...

True. But with the description of “firewall web” for a datum and a seat distance I’d think one could get reasonably close.
 
According to Wikipedia, Aircraft Spruce owns the plans. I’d try contacting them. (Obviously you’d probably have to get well beyond a phone salesperson.)

edit: looks like they have a batch of technical tips for $35, and you can get a full set of plans for under $300. They also have a Starduster Forum.
 
MauleSkinner has good advice. You should get a set of plans and follow the instructions on how to weigh your airplane according to those documents.
 
timwinters said:
What exactly is the web of the firewall?
Click to expand...
Had a chance to ask a friend who worked a lot in EAB and his take is the web is the actual firewall sheetmetal. Just as implied above. After a few questions I couldn't answer, he sent link below which had same diagram you posted. On page 3c their example uses the firewall as the vertical reference. He also threw out the Starduster Magazine contained more W&B info in later issues that even got into detailed discussions on MAC, etc.
http://starduster.aircraftspruce.com/magazine/pdf/APRIL76.pdf
http://starduster.aircraftspruce.com/
 
mondtster said:
The arms of the seats are given. You can figure out where the datum is. As I said before, sometimes you have to work backwards to get the answer.
Click to expand...
How would I work backwards if I don't know exactly where the C.L. of the gear is relative to the firewall?

The plane is leveled up and placed on scales (under the gear). And then the calculations start from there. Thus it's critical that I know where the gear is relative to the firewall...or some other establish data point for that matter...but the firewall is the only one I know without the plans. Regardless, establishing were the gear is relative to the rest of the airframe is critical to the accuracy of the W&B and that's what I'm trying to do.

what am I missing?

Edit...but it becomes irrelevant since @Bell206 confirmed what the web of the firewall is!!
 
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MauleSkinner said:
What part of the seat do you measure? Front? Back? Something in between?
Click to expand...
That's definitely another unknown.

The SD Too W&B examples that I've found vary from:

pilot = 63" to 70"

passenger = 35" to 42"

The seat backs are relatively vertical in this plane so my assumption is that the CG of the seated person would be about 6" in front of the face of the seat back (maybe 8"). This would likely be the most rearward possibility...especially since legs are extended forward nearly flat on the floor. Beer guts not considered. :cool:
 
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timwinters said:
The SD Too W&B examples that I've found vary
Click to expand...
You can weigh da plane with your butt in each seat as well as empty (Obviously, you can only plant yer butt in one seat at a time) and calculate the arms for your airplane as built.
Which has brought us full circle...
 
Capt. Geoffrey Thorpe said:
You can weigh da plane with your butt in each seat as well as empty (Obviously, you can only plant yer butt in one seat at a time) and calculate the arms for your airplane as built.
Which has brought us full circle...
Click to expand...
Except that still doesn’t tell you where in MAC the CG is, because flight station distances for seats are based on some form of “average” that I’d bet almost nobody meets.
 
@Bell206 thanks again!

In the case of this plane...unless we mis-measured (which I'm going down to double check tomorrow) the tailwheel on this bird actually measured 186.5" aft of datum on this one (see above commentary about longer fork in post #5).

And, now that I compare various data...I think we did actually mismeasure...

The tailwheel dimension is 3" difference.

An old W&B of this plane indicates that the main gear is 4.5" aft of datum. Last Thursday, we measured 7.5"...the same 3" difference as the tailwheel dimension. I've done all the calcs both ways and, obviously, that 3" makes a huge difference.
 
MauleSkinner said:
What part of the seat do you measure? Front? Back? Something in between?
Click to expand...

That is always a question. When I’ve worked things backwards in the past it is usually evident if a little common sense is used.
 
timwinters said:
How would I work backwards if I don't know exactly where the C.L. of the gear is relative to the firewall.
Click to expand...

That wasn’t the question I was responding to. You wanted to know where the datum line was, which is usually easy to figure out if you have the arm of some component as well as a tape measure and maybe a helper.
 
mondtster said:
That is always a question. When I’ve worked things backwards in the past it is usually evident if a little common sense is used.
Click to expand...
Working it backwards requires you to know where one end or the other of the tape measure goes. If you are measuring between the seat and the datum, but don’t know exactly where either of those points is, it won’t be evident no matter how much common sense you use.

I have determined that a W&B was computed incorrectly, for example they rolled the wheels onto the scales, but used the gear attach point for the arm. But I had at least one exact point from which to measure.
 
MauleSkinner said:
Working it backwards requires you to know where one end or the other of the tape measure goes. If you are measuring between the seat and the datum, but don’t know exactly where either of those points is, it won’t be evident no matter how much common sense you use.
Click to expand...

My point is, when the reference is something like “firewall web” you obviously know it isn’t going to be the spinner. Slide the seat all the way back or forward and lay out the tape measure and figure out what makes sense using the distance and descriptions you have. Or use the arm for another known component if the seat bothers you too much.
 
mondtster said:
My point is, when the reference is something like “firewall web” you obviously know it isn’t going to be the spinner. Slide the seat all the way back or forward and lay out the tape measure and figure out what makes sense using the distance and descriptions you have. Or use the arm for another known component if the seat bothers you too much.
Click to expand...
The original question was essentially “where’s the firewall web?” Depending upon what the firewall looks like, it could be 3 or 4 inches from the forward most point of the firewall to the aft most point. You suggested referencing something that no only could move at least that much in some airplanes, but also has a 12-inch range from front to back of the seat itself. The only thing you’re going to be able to determine is that the firewall web is somewhere in that 4-inch range from the front of the firewall to the back.

so like you say, he’d have to pick something else. Since he doesn’t have the drawings for the airplane, he probably has nothing that doesn’t have the same issue with knowing exactly where it is.
 
mondtster said:
Slide the seat all the way back or forward and lay out the tape measure and figure out what makes sense using the distance and descriptions you have.
Click to expand...
Hahahahahahahahahahaha!

...saying to do that on a Starduster made me LoL!
 
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