Vertical C of G - Blackburn Beverly

[JammedStab]

Apparently the V CofG is limited to 4.71 feet above the datum.

Why?

[DaveReidUK]

Perhaps they're sisters.

[noflynomore]

Logically it must be a pitch couple related to elevator/tailplane/pitch trim effectiveness (probably at low speed, perhaps involving flaps), thrustline and dragline. But anyway, a pitch control/authority argument.

[old,not bold]

So let me get this straight......does that mean that when me and 29 or so others disembarked in well under a minute from the rectangular hole in the boom floor at 1200 ft, which was not a pleasant experience, we were simply helping to get the C of G back to within limits?

[john_tullamarine]

Vertical limitations occasionally need to be specified (especially for physically) larger aircraft. Usual problems relate to

(a) lateral stability matters - dutch roll, roll maneuverability, and spiral stability considerations

(b) longitudinal static stability considerations

Although I have no background with gyros, apparently the vertical CG is important with respect to engine thrust pitch couples. Genghis has some background in this area and may comment in due course, with any luck.

[megan]

The Super Guppy also has a vertical CoG limitation. Carried a 22,000Kg wing box for the Boeing blended wing across the USA and bank angle was limited to 20°.

[pattern_is_full]

On a different note, high-wing aircraft gain some roll stability from the CG being well below the wing. Which acts as a pendulum that tends to "right" the aircraft if temporarily disturbed - e.g. by turbulence. Rather like a lead-filled keel in a sailboat.

Which allows for less need for wing dihedral for stability - you will note a high-winged C172 needs/has less dihedral than a Piper Cherokee with the CG above the wing.

The Beverley is like an overgrown Cessna, high wing with no dihedral in the center wing section, but more in the outer wing sections - net, not a lot. Therefore its design counts on a low CG for stability.

The payload space in the tailboom increases the chance of getting a high CG, if that "attic" space is filled with troops like old,notbold. Thus care must be taken.

As to the Super Guppy, again a comparison to the much smaller Piper, which has "just enough" dihedral for normal loading focused near its CG. If the Piper was given an extra-tall cabin to replicate the Super Guppy layout - without a corresponding increase in stabilizing dihedral, it would be (in another nautical analogy) like standing up in a small boat. CG rises, and boat get more unstable in roll (tips over).

[jonkster]

Many years ago I instructed a lecturer in aeronautical engineering and in one briefing made the mistake of mentioning "pendulum effect" and how it enhanced stability.



I won't ever make that mistake again.

[PDR1]

It can be as simple as the way a high CG on a "tall" aeroplane might make it prone to sitting on its backside if loaded from the back to the front, or while loaded via a rear door with a single heavy item. A high CG can allow the cg to move behind the mainwheels when rocked backwards.

[Vessbot]

High-wings are more laterally stable, but not because of any pendulum effect - there is no such thing. It's because of the local up- or down-wash at the wing root due to the curviture of the airflow around the fuselage, while in a slip.

The air splits at the middle of the fuselage, and the upper half goes above it while the lower half goes below, and then curves around to rejoin. This means that for a high wing, the leading wing is in upwash, which increases AOA; while the trailing wing is in downwash, which decreases AOA. This causes a roll toward the trailing wing, which is stable.

Everything opposite of the above, for a low wing.

More detailed explanation, and with pictures: https://www.av8n.com/how/htm/roll.html#sec-other-slip-roll

(These upwash and downwash components are separate from the upwash ahead, and downwash behind the wing due to lift production that are more familiar to most.)

The false reasoning that leads to the pendulum fallacy is that one considers the plane to rotate around the wing root, with the mass concentrated at the CG wanting to hang down. But that's not so; the CG itself is where the plane rotates around, so the vertical positioning of the wing has no bearing on the act of gravity on the CG.

This is similar to the rocket pendulum fallacy that is based on rotation around the engines.

[Chu Chu]

This is probably the opposite side of the same coin, but it makes more sense to me to think of the lift vector always acting through the center of the fuselage (absent aileron input of course). If the vector stayed normal to the ground when the aircraft rolled, there would be a righting (or upsetting) moment unless the CG was at the wing root. Of course, the vector doesn't stay normal to the ground.