Engine angle

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Aesquire

Well-Known Member
N
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.
Thank you. To clarify, the Center of gravity indeed is where the plane rotates around in flight.
In the teeter totter /see saw pitch stability Analogy, the Center of Gravity is in front of the Center of lift, ( on most "normal" airplanes w/out unstable fly by computer ) pulling the nose down. The tail pushes down ( or canard up ) to counter balance the offset. The pivot in that calculation is at the Center of Lift.

The important thing is that the pull of gravity doesn't ( noticeably ) change, while the tail force varies with airspeed.

One approach to making a "stall resistant" aircraft is to limit tail size so it can't hold the nose up at speeds and ( the related but not fixed ) angles of attack high enough to stall. ( in level flight ) This works, but has consequences. It makes for a lousy bush plane, since you can't slow down all the way to stall in unaccelerated flight. ( but can in a dynamic maneuver like a zoom climb ) Don't know any contemporary designs in production that use that "trick", but it was used in some 1920-30s "Safety Plane" competition designs.

Well-Known Member
Yes, its called a water line. Just guessing, but maybe a hold over from ships.
Must be a carryover. But on ships it's related to a physical thing - the designed flotation level.

Where's the water line on an amphibian ?

BJC

Well-Known Member
In the teeter totter /see saw pitch stability Analogy, the Center of Gravity is in front of the Center of lift, ( on most "normal" airplanes w/out unstable fly by computer ) pulling the nose down. The tail pushes down ( or canard up ) to counter balance the offset. The pivot in that calculation is at the Center of Lift.
Also need to include the pitching moment of a non-symmetrical wing.

BJC

Appowner

Well-Known Member
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
And to what reference is wing incidence measured against? Or the stab?

All three need to be referenced to the same line and not each other. And with an air frame center line any vert stab offset can also be referenced to it.

OhAnElBirds

Active Member
Where's the water line on an amphibian ?
That depends on how many pencil tips have been inserted into rivet holes, how quickly the water is seeping in past loose sheets, and how effective the bilge pump is.

Dan Thomas

Well-Known Member
Must be a carryover. But on ships it's related to a physical thing - the designed flotation level.

Where's the water line on an amphibian ?
Well, there's this one. Waterlines are put where the designer wants them.

Waterlines on ships vary with loading and are not used, to my knowledge, as a design reference plane. One needs a fixed plane for that. Wiki says:

The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy,[1] particularly with regard to the hazard of waves that may arise. Varying water temperatures will affect a ship's draft, because warm water is less dense than cold water, providing less buoyancy. In the same way, fresh water is less dense than salinated or seawater with the same lessening effect upon buoyancy.

For vessels with displacement hulls, the hull speed is determined by, among other things, the waterline length.[citation needed] In a sailing boat, the waterline length can change significantly as the boat heels, and can dynamically affect the speed of the boat.

The waterline can also refer to any line on a ship's hull that is parallel to the water's surface when the ship is afloat in a normal position. Hence, all waterlines are one class of "ships lines" used to denote the shape of a hull in naval architecture plans.

In aircraft design, the term "waterline" refers to the vertical location of items on the aircraft. This is (normally) the Z axis of an X × Y × Z coordinate system, the other two axes being the fuselage station (X) and buttock line (Y).

Numerous nautical terms are used in aviation, and we shouldn't be confusing those terms with the way they're used in sailing. "Dive" or "sink," for instance, connote some pretty different meanings in aviation than they do in sailing.

Well-Known Member
I've got a bunch of boat plans dating from the first half of last century and before. They generally have a datum line that is either at the lowest point of the keel, or some arbitrary point below that.

Now I'm going to have to dig some up and see what they call things.

ps As the person steering an aeroplane is in the cockpit, in nautical terms they would be the coxswain, or possibly bosun. A pilot doesn't steer the ship.

Dana

Super Moderator
Staff member
The location, and even angle, of any datum is completely arbitrary in theory... but in practice, some locations are a lot more convenient than others. Thus we place the zero butt line down the center of the fuselage, the zero waterline parallel to the relative wind in cruise configuration, and the datum is perpendicular to those. If any of those lines are outside the aircraft (e.g. datum in front of the nose, waterline below the aircraft), then all the moment numbers are positive, so you don't have to use negative numbers when measuring arms. Note that the angles do not change regardless of where the zero is located.

Typically (but not always) one or more fuselage longerons will be parallel to the waterline, and the designer will specify which one is used to level the aircraft when weighing and/or rigging it.

Pops

Well-Known Member
Log Member
Started drawing plans and designing model airplanes since about 10 years old. I just got a habit into putting the WL at or near the thrust line. Never liked or built any high thrust line engines designs except one RC pusher. Also always used the prop flange or firewall as the datum, if the firewall is slanted, ( as in the JMR Special ) then used the prop flange face. Negative and positive numbers are not that big of a deal.

Dan Thomas

Well-Known Member
Negative and positive numbers are not that big of a deal.
Not for someone that has some arithmetic skills. But I have found numerous calculation errors in W&B documents, and made one myself, by mechanics failing to remember that removing a heavy old starter or generator and replacing it with a lighter one will get you a positive moment if the firewall is the datum, as it is with the Cessna singles. A negative number (removed weight) times a negative distance (forward of the datum) gets a positive moment; the tail gets heavier.

Some airplanes have the datum way out front somewhere, and distances can be measured from any handy spot on the airframe. If the drawings show the firewall at 64", say, then an alternator 26" ahead of the firewall is at +38". No need to set something up in front of the airplane to measure from.

BJC

Well-Known Member
Negative and positive numbers are not that big of a deal.
Agree that they shouldn’t be, but will bet that all those power plants that you worked on used a a datum line for elevation so that all of the structure had positive elevations. Fewer mistakes that way as opposed to setting the ground level as zero (i.e., the firewall) and using negative numbers for below grade (in front of the firewall).

BJC

Pops

Well-Known Member
Log Member
You are right. All positive.

BJC