Angle of Incidence

[Northside]

I am replacing rotten lower longerons near the tail post on a 1946 BC-12D, and want to be sure that I keep the geometry correct. When the fuselage is leveled by placing a level on the horizontal stabilizer mounting stubs, is the angle of incidence at the wing root fittings approximately 5 degrees?

[Guest]

Well attached are measurements that I made about 5 years ago, angles were measured with respect to MAC.

The butt rib is about what you asserted.

However I think you are on the wrong path.

What I have done in the past is to get a bunch of angle iron or old bed frame material and tack weld it to the portion of the fuselage where I am going to cut out tubes.

But tack weld in locations such that you have formed another triangular shape thereby producing a truss that will keep the fuselage rigid at the tube you are doing the cut out.

That way everything stays in the correct geometry while you work. This method force you to work on one tube at a time rather than cutting out many at a time.

Of course remove the tack welded angles when you are done. :-)

Dave R.

[Hank Jarrett]

Dave, are you measuring to the bottom of the wing airfoil or adjusting the angle to the Mean Aerodynamic Chord? It is easier to measure from the flat part of the aft wing ribs, just don't know which part of the airfoil your numbers are measured from. NEAT chart! I want to do that on mine just out of curiosity. When I set the washout I made a rib jig so the level was parallel to the MAC, but that is a lot of trouble for doing a fuselage repair.

[Guest]

Dave, are you measuring to the bottom of the wing airfoil or adjusting the angle to the Mean Aerodynamic Chord? It is easier to measure from the flat part of the aft wing ribs, just don't know which part of the airfoil your numbers are measured from. NEAT chart! I want to do that on mine just out of curiosity. When I set the washout I made a rib jig so the level was parallel to the MAC, but that is a lot of trouble for doing a fuselage repair.

Angles were measured with respect to MAC then referenced to the horz. stab. angle, otherwise they are pretty meaningless.

I posted an account of the entire process in 2016 the thread may still be there , do a search, I recall posting pictures also.

p.s. I searched but cannot find the pictures

[PA1195]

In this report Table 1 airplane 5 suggests the wing incidence to be +3.8 degrees: https://ntrs.nasa.gov/archive/nasa/c...9930082385.pdf

Same here in Table 1 +3.8 degrees: https://ntrs.nasa.gov/archive/nasa/c...9930083935.pdf

The 3.8* incidence closely approximates the best L/D for the 23012 airfoil which makes sense in cruise flight: https://ntrs.nasa.gov/archive/nasa/c...9930091603.pdf

Gary

[astjp2]

In this report Table 1 airplane 5 suggests the wing incidence to be +3.8 degrees: https://ntrs.nasa.gov/archive/nasa/c...9930082385.pdf

Same here in Table 1 +3.8 degrees: https://ntrs.nasa.gov/archive/nasa/c...9930083935.pdf

The 3.8* incidence closely approximates the best L/D for the 23012 airfoil which makes sense in cruise flight: https://ntrs.nasa.gov/archive/nasa/c...9930091603.pdf

Gary

But Gary, is that what Gilbert designed the airplane too?

[PA1195]

But Gary, is that what Gilbert designed the airplane too?

Somebody has the design prints that should indicate what CGT wanted done. In the meantime we can measure AOI to the spar attach fittings, the attaching hardware, or the chord line like the folks above do. Note from the last reference above the 23012 airfoil best L/D AOA is inversely proportional to Reynolds Number....the higher the RN the lower the AOA for best lift over drag. Makes sense as in this case lift increases with airspeed (RN) faster than drag. In general: https://wright.nasa.gov/airplane/lifteq.html. Go faster means lower the nose or AOA to maintain level flight.

Gary

Edit: This is the second round of how many? https://vb.taylorcraft.org/forum/tay...s-of-incidence

[astjp2]

The op needs to put that airframe in a jig before he does any cutting, then its a moot point.

[Edwin Otha]

I would level the horizontal and cut away. Make some measurements so you get the tail post in the right place. Not knowing how much you are replacing, you may need to brace the fuselage better. For a one-off repair you can use wood for a fixture. I don't think you will disturb the tail incidence very much with a lower longeron repair.
EO

[PA1195]

That wood fixture idea might be good. Wouldn't take that much to lock in axles with tires off and tail with a few triangles unless it's already bent.

Gary

[Guest]

Perhaps you are overestimating the effort required to simply tack weld a few angle irons in for bracing this wood fixturing seems like way too much work for me.

[Hank Jarrett]

Dave a wood fixture seems simple to someone who sees welding as a "Black Art". People who weld have no idea how magic and complex it seems to those who don't know how to do it.

[Guest]

Dave a wood fixture seems simple to someone who sees welding as a "Black Art". People who weld have no idea how magic and complex it seems to those who don't know how to do it.

Hank,

LOL, I am thinking that a person that sees welding as a black art perhaps should not be repairing tube fuselages!

You hit my funny bone, thanks!

Dave

[Hank Jarrett]

When it comes to the actual welding, I DON'T repair the fuselages. Where my life depends on it I get someone who is really GOOD at it. I may weld a lawn mower part or something non critical (or visible) on my car, but not my fuselage. With practice I "think" I could be a "credible" welder, but after seeing "my welder" do my 45 fuselage I realized I was in the presence if a true artist. I have inspected welds, but to lay down a bead like he did, EVERY TIME was well beyond my skills. I used to think it was black art. I don't any more. It is just art beyond my skills to do right every time. I know the limits of my skills and I wouldn't accept my own welds on my fuselage.

[N96337]

I really don't think he needs to worry about a "jig" for what he's doing. Like Drude says, tack some bracing on to hold it where you want it (but verify that it's where it's supposed to be first) then get after it. These weren't built to that exacting of a standard! I did a J-3 on a couple wood tables that were bolted together, leveled and screwed to the floor. I ran some angle iron for tabs to catch all 4 gear attach points and another angle with a tail post.