Engine angle

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OhAnElBirds

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Terminology wise, Piper called the four long horizontal tubes on the corners of the fuselage "Longerons" on their plans for the J-3, PA-14, PA-15, and PA-18. (Those are just the plans I personally possess.) Military -2 and -3s for the Taylorcraft L-2, Aeronca L-3, and Piper L-4 use the same terminology.
 

OhAnElBirds

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When the fuselage is leveled per the plans, the stab is -4 degrees. In the leveled condition, the wing attach points are level as well.
My eyes have not been calibrated recently. But I would say the Piper PA-15 plan are about the same. The thrust line is marked on the fuselage plan and the wing attach points are parallel to it. The angle on the stabilizer is not marked but the forward tube is lower than the rear being supported on a bracket under the top cross member while the aft attach point is supported on a bracket over a top cross member. (While the longerons are sloping down toward the tail at an even greater angle.)

Some Aeroncas have a forward bracket with multiple holes permitting a change of the stabilizer's incidence on the ground, and of course the J-3 has a jack to raise or lower the front stabilizer attachment point.
 

Aesquire

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Since the basic nature of conventional tailed aircraft is a see-saw ( teeter totter? ) I EXPECT to see the horizontal stabilizer at an angle to the wing. ( the fuselage can have curves or straight lines at any weird angles, it's the Wing that matters )

"Conventional tailed" ( F-16 and other unstable computer controlled planes are a different critter ) aircraft have the center of gravity in front of the center of lift. A steady nose down pull. The tail is at a negative angle to the wings, creating a tail down pull to balance the weight up front. Consider the center of lift The pivot point.

The important part is the pull from the weight doesn't change. The pull from the tail increases with increased airspeed. ( This ignores fuel burn or dropping armored vehicles out the back ) This is essential for Pitch stability.

Nose goes down, speed goes up, tail downforce increases, nose goes up, plane slows down. Nose goes up, speed goes down, tail downforce decreases, nose goes down, plane speeds up.

It's easier to visualize with diagrams with arrows and labels and twenty-seven 8 x 10 colored
Glossy photographs with circles and arrows and a paragraph on the back of
Each one explainin' what each one was.

Once all that is worked out, the thrust line gets another round ( as in singing repeated verses or fighting the same opponent again ) of calculations to compensate for torque & the swirling angles of airflow off a spinning propeller, with side to side offset angle, and again, in pitch.

Or you can compensate at the tail.

Or both.

There are also special cases, like high mounted engines seen on many flying boat/amphibians and pushers with engines on top of the wing. ( for propeller clearance from the tail boom/fuselage & to get out of sea spray ) That needs it's own thread. ;)
 

challenger_II

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As a side note, while the wing attach points appear to be in line with the centerline of the airframe, do bear in mind the wing's angle of incidence is approximately 2-3 degrees positive, as the aerodynamic cord line of the airfoil runs from the center of the leading edge radius to the apex of the angle of the trailing edge. Never assume the wings' angle of incidence is measured from the bottom surface of the wing.

To add to Aesquire's post: the wings center of lift isn't the pivot point. The aircraft's center of gravity is the pivot point. But, other than that, his premise is quite correct.
 

OhAnElBirds

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It seems that this whole discussion has become a case of blind incidence.
"And there wasn't nothing (we) could do about it, and the (OP) wasn't going to look at the twenty seven eight-by-ten color glossy pictures with the circles and arrows and a paragraph on the back of each one explaining what each one was to be used as evidence against"... changing the factory specifications.
 

challenger_II

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Shades of Arlo Guthrie... :)

"And there wasn't nothing (we) could do about it, and the (OP) wasn't going to look at the twenty seven eight-by-ten color glossy pictures with the circles and arrows and a paragraph on the back of each one explaining what each one was to be used as evidence against"... changing the factory specifications.
 

Appowner

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Did a little research. The Wag-A-Bond is indeed intended as a clone of the Piper Vagabond.

That said, the Vagabond engine offset is 4 down and 2 right. For whatever that's worth.

By comparison the Piper Cherokee has 3 down and 3 right.
 

BJC

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Did a little research. The Wag-A-Bond is indeed intended as a clone of the Piper Vagabond.

That said, the Vagabond engine offset is 4 down and 2 right. For whatever that's worth.

By comparison the Piper Cherokee has 3 down and 3 right.
In reference to what? Longerons? Chord line? (Piper people like to reference things to the generally-flat bottom of the airfoil.)

Offsets really should be referenced to the direction of flight at a chosen (typically, cruise) level, unaccelerated, flight attitude.


BJC
 

Dan Thomas

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Engine offset, wing incidence, and tail plane incidence, is determined from the centerline of the aircraft.
No. Not always. It's decided by the designers.

  • Water line (WL) is the measurement of height in inches perpendicular from a horizontal plane usually located at the ground, cabin floor, or some other easily referenced location. [Figure 3]


1637688620497.png
Waterline here is below the ground.

Maintaining the Aircraft

Then there's this:

1637688706294.png

Waterline, in this case, is that line that runs along the bottom of the forward fuselage. Hardly a centerline. Waterlines are just a reference line, and everything is measured off them.


In this one, the waterline is also somewhere well below the ground. That keeps all numbers positive, even when the aircraft is off the ground with the gear extended. Some light aircraft use an imaginary point out ahead of the airplane for the station datum, to keep weight and balance numbers all positive.

1637689142737.png


Fuselage Stations & Water Lines | The Metric Maven
 

challenger_II

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Your reference is for maintaining the aircraft, not the design of the aircraft.
As for my statement, perhaps I should have used the term "Datum Line", rather than "Center Line".
 

OhAnElBirds

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While I have obviously not seen everything, I do own a number of copies of a number of plans from Aeronca, DeHavilland, B. Pietenpol, Piper, Taylorcraft, etc. Most have some sort of a "reference" line but there doesn't seem to be consensus on what it is called or where it is placed. YMMV.
 

Dan Thomas

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I find that amusing, sir.
Stop deflecting.

If you're going to talk aircraft design, construction or maintenance you MUST use the accepted terminology, or you risk confusing yourself and everyone else.
The Datum is the vertical plane from which measurements fore and/or aft are made. No other reference is called the Datum.

The Centerline is usually called Butt Line zero and is the plane down the middle of the fuselage as viewed from above. It divides the aircraft into two symmetrical halves. No other airframe reference line is called the Centerline.

The longitudinal line you're referring to is the Waterline and has been called that for a long, long time. Wing and tail and engine up/down angles are measured off it.

If you go to mechanic's college and start calling stuff by whatever you want to call it, expect a failing grade.

I am a Licensed Canadian Aircraft Maintenance Engineer, roughly equivalent to the US A&P-IA.

1637706701635.png
 

Pops

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Yes, its called a water line. Just guessing, but maybe a hold over from ships.
 
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