Function of rigging datum line?

abufletcher · 14 July 2005, 10:59 PM

Many aircraft (such as the Pfalz DIII) had a riggng datum line painted on the fuse. How exactly was this used?

Tripehound · 15 July 2005, 08:00 AM

I believe it's used, along with a stand under the tail, to get the fuselage level as a baseline in order to check wing incidence angles when adjusting the rigging. The Datafile on the Bristol F2B has a couple pages in the back describing how this is done to the F2B. It's quite a job. The description would more or less apply to any aircraft with wire bracing.

What I've wondered about is aircraft like the "wireless" Fokker Dr.1 and D.VII. Was there some sort of adjustment built into the strut attachment points?

Greybeard · 15 July 2005, 08:11 AM

Quote:

Originally Posted by Tripehound

What I've wondered about is aircraft like the "wireless" Fokker Dr.1 and D.VII. Was there some sort of adjustment built into the strut attachment points?

I don't think so.

Thick airfoils used by these two scouts didn't allow for bending/torsion, reason for presence of wires on all other aircrafts with thin airfoils. This latter chosen for their lower air drag, erroneously neglecting all that generated by wires needed to stiffen them!

Dan_San_Abbott · 15 July 2005, 11:44 AM

Abufletcher:
The riggine datum line painted on the sides of the fuselages is for the purpose of establishing level line of flight and equals zero degrees. Ricrfat that have been disassembled, whatever.. need to be rerigged in accordance in accordance with the rigging manual RFC/RAF with German aircraft, the rigging instructions glued to the left side of the fuselage near the weight table.
On the Fok.DR.I the upper wing incidence is adjustable at the cabane struts. The middle and lower wing incidence are also adjustable at the attachment bolts on the upper longerons for the middle wing, and the lower longeron attachment bolts for the bottom. On the Fokker D.VII the rear cabane struts adjustable attachment bolts set the incidence for the center section of the upper wing and the adjustable attachment bolts on the interplane "N" struts for the upper and lower wings. The attachment of the lower wing to the fuselage establishes the preset incidence at the fuselage. In order to set the correct incidence , the aircraft must be level longitudinally and laterally.
Blue skies,
Dan-San

15 July 2005, 05:04 PM

Thick section airfoils actually have lower drag than undercambered airfoils, rigging aside.

Undercambering was chosen early on because drag wasn't the issue, low wing loading/high low speed lift was, to get the early birds in the air with the low power engines of the time. It was well known, even then, that the high lift at low speeds undercambering gave came with a very high drag penalty as speeds rose.

F=MA

abufletcher · 15 July 2005, 07:55 PM

Does anyone have any further info on how wing incidence was actually measured or set? I mean, did the riggers just pull out a tape measure and measure off certain distances from the datum line (if so from WHERE TO WHERE?) or did they use some kind of external device once the aircraft was level?

Tripehound · 15 July 2005, 08:18 PM

Again, this is out of the F2B Datafile. A long straight edge and something called an "Abney level". Do a Google search on it. There's a bunch of information. In the Austro-Hungarian aircraft book by Peter Grosz there's a picture of a two-seater being trued in this manner; a long straight edge and some instrument held by a guy on a ladder and two others holding the stick. Ground crews must have really cussed at every hard landing.

abufletcher · 19 July 2005, 03:17 AM

OK, now for the other half of the questions!

I have a technical drawing of the LVG VI that shows a VERTICAL datum line as well. What would this line have been used for?

I also notice that the datum line on the LVG VI is not in line with the engine, i.e. both the propellor hub and the tailplane are higher. So what significance does the vertical positioning of the HORIZONTAL datum line have?

allanflowers · 19 July 2005, 08:45 AM

It would be important to have a station reference on an airplane to measure along the length dimension. A vertical line would give this better than just measuring from some object on the plane. As to where these reference lines are placed, I would guess it is not totally arbitrary. It would probably relate to some structure or major assembly in the heart of the aircraft. One would establish a "zero" point in all three axis from which to measure. The datum lines would then be marked on the aircraft, but not necessarily at the zero point - but at least at some convenient position referenced to the zero point (an even dimension away, maybe). So the horizontal datum line might be, for instance, 200 mm lower than the actual zero point.
In the auto industry, each company has its own criteria in establishing the zero. Many use the front wheel location but some might create the zero near the ground, in front of the vehicle - to avoid having to use negative numbers.
A reference system used in the design of the aircraft would undoubtably form the basis for the datum lines used in maintaining/triming the thing in service.
...just my conjecture.
Allan