VCG and POH

[PW_Plack]

A general question...

I've never actually looked at a Pilot's Operating Handbook for an experimental aircraft, but I've seen them for several fixed-wing aircraft. Under weight and balance, they all list or graph acceptable weight distribution to stay within a CG range, but only on the pitch axis. Does this assume that the design of the aircraft and its seats and baggage areas brackets the side-to-side and vertical loading within acceptable limits?

In an experimental gyroplane, we have a unique circumstance in which a completed aircraft may have what is today considered an unstable VCG situation as its normal course of operation. Others might have storage bins above or below the CG, or fuel arrangements, which could move the CG out of limits with certain conditions of loading.

Has anyone here addressed vertical CG in the POH prepared for an experimental aircraft? I saw mention in another thread of a suggestion to require this info on a placard in LSA. Thoughts?

[Brian Jackson]

I had often wondered this too, because I am a light person, and soon to be light pilot. So let's say a guy who weighs 150 lbs. builds a CLT gyro and eventually sells it to a 220 lb. pilot. The VCG of the machine is now lower in relation to the thrust line with the new pilot. Is the new pilot required to rework the airframe (besides the cheek plates/hang test) to put the machine back in CLT configuration? And would an adequate handbook cover pilot weight differences in its safety documentation?

[PW_Plack]

Actually, Brian, the handbook should describe a range of pilot weight within which the CG would be considered within limits. 150 and 220 could conceivably both be in that range.

If you include VCG in your weight and balance specs, what do you do with, say, a hypothetical Canadian two-seater with a 12-inch thrustline offset? No combination of passengers, baggage and fuel, short of a roof rack full of bricks, brings it within a stable range.

[Doug Riley]

VCG location in a rotorcraft with a separate thrust source (prop) and modest horizontal tail power is uniquely important.

If the separate thrust source is not dead-on through the CG, then it produces pitching moments on the airframe... either nose-up or nose-down, depending on whether the thrustline is above or below the CG/CM. That much is true of any aircraft. Where the rotorcraft is unique is that (1) control is accomplished by pointing the rotor thrust in various directions and (2) the horizontal tail is apt to be rather marginal in power.

The "marginal HS" problem comes about because of the physical limits of the pusher configuration and because a rotorcraft can fly very slowly, making almost any HS marginal. (Of course, crackpot aerodynamics makes the HS situation worse by encouraging the ELIMINATION of the HS!).

If the rotor thrust is used for control and some or all of the stabilizing forces, then any time rotor thrust is lost, there's nothing left for either control or stabilization. Any moments being created by the thrust source are then unopposed, and over she goes. Rotor thrust can be lost in intentional low-G maneuvers, but it can also be lost in vertical turbulence.

In contrast, a fixed-wing loses neither control nor stabilizing forces even when in zero G. The ailerons still work, and so does the elevator. Therefore, if the FW plane has a thrustline that's offset from the CG, it's not so big a deal -- though in extreme cases it deos lead to unstable trim changes as you add power. I suspect that most ultralight planes nose over and speed up in a kind of runaway mode when you power up. (My Firestar sure does; its thrust line is probably farther above the CG than a RAF's. All you have to do, however, is add a modest amount of back stick pressure as you push in the throttle, thanks to a long tail boom and huge HS.)

As a result, I don't know of any vertical CG ranges for FW planes. Maybe flying wings have them... or should...

[John_Read]

Sorry! Wrong thread! I moved this post to "higher winds" :)

john