Building a Nieuport 11...

[Frank Giger]

Apologies for the text; the forum won't let me put a space between paragraphs.

[Frank Giger]

So I'm hoping the collective brain trust here can explain a few things conceptually to me about MAC, centers of balance, and CG.

Robert is still going to do the W&B for me, but in the meantime I need to understand all this stuff - it's my butt in the seat, after all. Plus I hate not understanding this.

So I dug around and found the formulas and even a nifty tool to find the MAC for biplanes, and using my measurements low and behold mine is 25% of the cord of the main wing (which should be between 24 and 28%, so yea team!)

I get what the formulas are doing and I even understand the placement of the center of balance for the wings inherent in the design - one wants the Center of Balance of the wings (and the lift) over the Center of Gravity.

So I took my MAC placement and translated that to a point on my plane from the datum (my firewall). It's just over 19 inches back from my firewall.

I also, naturally, weighed the aircraft and worked up the CG (empty). I added the meat lump and fuel using the standard methods (fine living now has me at the FAA approved 170 pounds!).

Ten gallons of gas and pilot puts the CG at 22" behind the datum, or three inches behind the Center of Balance.

Playing with fuel loads and pilot weight only moves it about about an inch at the most. So it seems to me everything is pretty much in agreement and I'm okay.

The question is why do the MAC calculations at all? Is Robert just validating his design against my build, or is this standard for homebuilts?

[mcrae0104]

I think you may be confused about some terms (or are using them differently than they are typically used). MAC is a length, not a percentage. I do not know what you mean by "center of balance" if you are referring to something other than center of gravity. Your CG varies with different loading cases, and must be restricted to a forward limit and a rear limit to ensure proper stability. These limits are sometimes expressed as a percentage of MAC length (e.g. Fwd limit at 22% MAC, aft limit at 30% MAC). These points can also be expressed as distances from a datum point. The designer must specify the fwd and aft limits. (You can learn more about how to do this in the book "Airplane Performace, Stability and Control".) What Robert is probably doing is confirming that the fwd and aft limits for the design also apply to your aircraft, which may or may not conform to the dimensions of the design. I'm guessing that is why he asked you to supply him with some dimensions.

[Dana]

MAC is Mean Aerodynamic Chord, no way will it be 25% of the main (by which I presume you mean upper) wing chord. MAC is both a length (the average of the upper and lower wing chords accounting for areas) and a location (somewhere between the upper and the lower wing chords).

"Center of balance" is not a term normally used in aerodynamic calculations, and I have no idea what you mean by it. Normally, the center of gravity needs to be a bit forward of the wing(s)'s center of lift for the airplane to be stable.(actually it's the "neutral point" that the c.g. needs to be forward of as the center of lift moves with different AOAs but that's the general idea). But as mentioned in the post above the c.g. range is often given as a range of MAC so I presume that's what your 24-28% is).

[Frank Giger]

I'm sure I'm gibbering-up the terms - this is why most folks who know airplanes start talking slowly and use small words on technical details!

Okay, so here's the "measurement thingie" that is Center of Balance, taken as a screenshot from Excel:



I'm going to have to track down whomever put this together and bow down to them for making something so darned useful.

And here's the bog standard W&B spreadsheet:



Someone want to help put some ridges in my smooth brain on this?

[jdanneel]

This is not taking into account THE GAP beween THE wings,I' send à Pm next week when I am home

[Dana]

OK, without seeing the underlying calculations, I'm guessing what you have is a spreadsheet intended to help R/C modelers find the desired "balance point" holding the plane with their fingers under the top wing, not something you want to use to determine the proper c.g. of a full scale plane. Most modelers just do a quick balance check of their models that way; if it balances on their fingers at 1/4 to 1/3 of the wing chord, good to go. But it's harder to figure where that point should be on a biplane with stagger, even moss so if the upper and lower areas aren't equal, hence the spreadsheet. The second one you posted helps you find where the c.g. IS, not where it SHOULD be.

The vertical gap between the wings is irrelevant.

[Frank Giger]

I used a biplane formula set, mostly to get an idea if I was going to be way off of CG with my weights. Still waiting on Robert for the definitive answer.

To show at least I did the W&B procedures right, fuel was drained and put into flying position:



Leveled at the longeron by the cockpit.

Then it was a bunch of marks using a plumb bob:



Moving on, the last big thing was mounting the pitot tube. I knew I wanted it on the interplane strut, so while the plane was in flying configuration I bent the tube itself to the angle to match it.

There were three things to consider in my choices:
1) It had to be stable.
2) It had to be simple.
3) It had to be something other than butt ugly.

Well, I've got plenty of 1/4" tubing left over and I did a test piece to see if I could make a mounting flange for the ends. It turned out pretty good.

First I annealed the ends and made two cuts:



The challenge was to make the cuts to the sides just long enough to go flat without going short and getting a split or going long and having a cut past the flange. Sometimes it's best not to think about things too much and just do it....and in this case it worked out perfectly on both ends.

I made vertical cut that fit the pitot tube firmly, painted it, and ran the tubing up through it and into the pitot tube.



Pull down snug and mount to the strut with a couple rivets.



It's solid to the strut and won't be going anywhere.

The ends of the mounting flanges can be bent down a tad to match the curve of the strut, but the hour was getting late and the temperature dropping.

I put an oscillating fan in front of the tube and the ASI jumped around nicely to show it's all connected properly.

I'll post my W&B measurements up later for anyone who wants to chew on them while I wait on the official CG range....

[Frank Giger]

The official word came back from Robert Baslee and....

...I'm ten pounds too light at the tail.

Well, that's better than the other way around! But I've got to figure out how to put some weight back there without doing major surgery. I've a few ideas, some of which aren't Wile E. Coyote in complexity.

[jdanneel]

Àdding 35lb to the pilot weight might have the same effect