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Do forward and rear CG limits depend on mass?

LeSving wrote:

It’s mostly fuel related, but could also be structural/other limitations IMO. At the start it’s just a straight box, moment on the X axis, mass on the y axis. When fuel burns, the CG changes. If the aircraft is to be flyable with a given payload regardless of fuel load, then the upper corners will be cut off depending on how the fuel changes CG.

It’s not fuel related at all. The GC limits are determined without any consideration for GC movement due to fuel consumption. It is up to the pilot to determine that the GC will at all times be within the envelope as fuel is consumed. That’s one reason the w&b diagram generated by software such as SkyDemon and Autorouter show GC for both take off fuel and empty tanks with a line between them.

(PS. If you want a “straight box”, you must have GC position on the X axis — not moment.)

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

The GC limits are determined without any consideration for CG movement due to fuel consumption

Yes, that’s what I said. But, the diagram isn’t due to movement of CG with fuel burn.

I am sure @antonio is right about the considerations about the landing flare. But I am also sure that part of the explanation is structural. A “pancake” landing with a given rather forward CoG is harder on the nose wheel when the aircraft is heavy that when it is light. This could directly explain why forward CoG limit is restricted more at heavy masses.

For this to have logic, the envelope should be turned around on tailwheel aircraft. I have seen a lot of WoB envelopes on nosewheel aircraft, but only a few for tailwheelers. And indeed, on the Dimona motorgliders we used to fly, the forward limit is independent of mass, while the aft limit slopes forward at high mass.

I guess that the nosewheel / tailwheel is never as sturdy as the main wheels, so it will be structurally limiting in many situations, including, sometimes, CoG limits.

huv
EKRK, Denmark

LeSving wrote:

But, the diagram isn’t due to movement of CG with fuel burn.

Right, but you claimed exactly that it did!!

When fuel burns, the CG changes. If the aircraft is to be flyable with a given payload regardless of fuel load, then the upper corners will be cut off depending on how the fuel changes CG.
Last Edited by Airborne_Again at 12 Aug 20:29
ESKC (Uppsala/Sundbro), Sweden

A double negative in here somewhere? What I mean is you start with a box with straight edges. Then, if the fuel isn’t exactly on the CG point for all fuel loads, the corner will be clipped. Otherwise you end up outside the CG limits when fuel is burned, IF you start at too much load and the CG too far in one direction. It’s still straight edges, theoretically, but the corners(s) typically becomes unusable in practice because with fuel burn you wonder off the edges. You can fly with full fuel, but not with half fuel for instance.

I could draw it, and it becomes obvious, hard to explain in words.

There are a few C of G diagrams I have encountered which would allow fuel burn to take you from in limits to out of limits. If the fuel burn is “normal”, I would not approve this. If it would only be the result of an odd fuel selection I might let it pass, if there were a warning. Some airplanes have multiple belly tanks, and wrong selection can put you out.

huv wrote:

I guess that the nosewheel / tailwheel is never as sturdy as the main wheels, so it will be structurally limiting in many situations, including, sometimes, CoG limits.

It’s not a matter of strength of the nose or tailwheel, but rather having enough elevator control to be able to prevent that wheel contacting the ground too hard. In all cases, I insist that I have control to land on the mains, and hold the nosewheel(s) off for a few moments, or, be able to wheel land a taildragger.

(2) With takeoff power, there must be enough elevator control—
(i) For a tail-wheel type airplane, to maintain, at 0.8 , an attitude that will allow holding the airplane on the runway until a safe takeoff speed is reached; and
(ii) For a nose-wheel type airplane to raise the nose-wheel clear of the takeoff surface at 0.85 .Quote

… Is a takeoff requirement, but it will cover landing as if you’re touching down at flying speed, power on or off, you should still have that amount of control.

I’ve had two different planes (one due to control misrigging, the other poor design) in which I could not lower the nose at the point of a stall, as the pitch control hit the nose down stop before the nose lowered, and in both cases, I was not at the aft C of G limit (it could have been worse). Not certifiable.

Home runway, in central Ontario, Canada

Pilot_DAR wrote:

There are a few C of G diagrams I have encountered which would allow fuel burn to take you from in limits to out of limits

The Bonanza F33 springs to mind. With 4 pax (the couple in the rear seats had to be slim), I often found that if fuel would have been used down to a certain amount, the CoG would creep backwards out of the envelope.

@Pilot_DAR. Thanks for the general clarification. Can I ask, what is the unit of those speeds (0,8 and 0.85 of something) ? Is it Vs ? It seems something is missing from your quote.

The (2)(i) condition may explain why the Dimona H36 specifically has a variable aft CoG limit while many other tailwheel aircraft have not. For a tailwheel aircraft, the Dimona H36 has the main wheels unusually far forward – i.e. forward of the wing leading edge. That invariably means there is much weight on the tail wheel. For the tail surface to be able to lift the tail at MTOM at the specified airspeed, it is clear that the CoG has to be moved forward, compared to when the aircraft is lighter.

Last Edited by huv at 13 Aug 06:05
huv
EKRK, Denmark

Pilot_DAR wrote:

There are a few C of G diagrams I have encountered which would allow fuel burn to take you from in limits to out of limits.

Well, our P210 does have a curved aft CG “limit” to account for that. The limit defined by the curved-line accounts for the pilotDAR-like CG criteria where no matter how much fuel you burn, you will be OK.

The “regular” straight-line aft limit implies you need to account for fuel burn during your flight.

On some large aircraft , there are operational minimum fuel limitations when lightly loaded near the fwd CG limit effectively forbidding landing with anything less than x amount of fuel to account for that. It is called ‘ballast fuel’ and avoids having to place ballast in the cargo hold.This is managed operationally.

There are all kinds of different reasons, other than the typical ones, for different weight and balance limitations. In the end, the OEM decides what he wants to do in order to comply with the different certification, performance, commercial and other criteria, (and then hopefully the authority agrees), most o which the pilot/owner does not need to be privy to and fall within the “proprietary information” criteria.

Quite a few commercial aircraft have an MTOW limit defined strictly by how much you pay the OEM, with no airplane changes other than the AFM.
Tip-tank mods frequently also have MTOW variability depending on fuel load.

Quite a world, the one on airplane limitations…

Antonio
LESB, Spain

Sorry, yes, when I copied and pasted from the FAA website, the “Vs1” symbol associated with 0.8 and 0.85 did not paste, as it is not text. That said, the units may be either MPH or knots, depending upon the primary scale for the airplane – yes a 1.16 inconsistency! It’s not the only place this happens in the standards.

Home runway, in central Ontario, Canada

Antonio wrote:

forbidding landing with anything less than x amount of fuel to account for that. It is called ‘ballast fuel’

I find it impossible not to imagine a fuel emergency in this context. So, the unthinkable has happened, the weather has ruined all planning and the engines are drinking into the ballast fuel before a safe landing can be executed. Of course, in order not to render the airplane unairworthy, the crew dutifully shuts down the engines to maintain CoG within legal limits

huv
EKRK, Denmark
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