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Do forward and rear CG limits depend on mass?

Valtime wrote:

trim fuel tank inside the horizontal stabiliser on large aircraft.

‘Could be. Outside the scope of GA planes, and I cannot comment airliner’s fuel systems. I’m sure that the airplane flight manual and type training would cover this well.

Home runway, in central Ontario, Canada, Canada

Pilot_DAR wrote:

If fuel is required to be carried as ballast, and landing is forbidden without that fuel in the correct tank(s), it would have to be considered as unusable fuel, which is fine.

I don’t know any design that requires ballast fuel such that it is non useable, but a similar application would be the trim fuel tank inside the horizontal stabiliser on large aircraft. It is used to provide an aft cg loading during cruise but should be emptied before landing. In this case the fuel is useable but was used as a ballast during cruise.

EDMG, Germany

Antonio wrote:

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.

If fuel is required to be carried as ballast, and landing is forbidden without that fuel in the correct tank(s), it would have to be considered as unusable fuel, which is fine. However, unusable fuel cannot be included in any performance considerations, so it would not be being considered for range or flight planning. Odd to use fuel this way, but not prohibited for a design. I would be very hesitant to certify such a configuration though, as it sort of requires that the fuel quantity indicators be required to read accurately below “zero”, so that the pilot knows to not run that tank dry, or when he is about to.

More commonly, and particularly with STC’d extra wing tanks or a gross weight increase, it’s a certification short cut that any weight in excess of x be carried only as fuel [in wing tanks]. This will alleviate wing bending loads, as there will be no greater lift load imposed on the wing to fuselage connection. An example of this is many older Cessna piston twins carrying the main tank fuel in the wing tips, and beyond that, even when you’re running on the wing aux tanks (inboard), excess fuel is being returned to the tips, to keep the weight as far outboard as possible, and reduce the load seen between the wing and fuselage. Such situations have nothing to do with C of G limits, but are structural. Fuel being used as ballast, and being required would also conflict with the requirement that a tank may be run dry, and the engine must restart easily. During certification testing, I would require that each tank be run dry, and again, there would be a conflict.

Often the reasons for a limitation may not be what they appear at first look.

Home runway, in central Ontario, Canada, Canada

Antonio wrote:

Well, you would have to comply with applicable fuel requirements on top of (ie not accounting for any of ) the ballast fuel.

Yes of course!

huv
EKRK, Denmark

Peter wrote:

Isn’t it easier to raise the nose during the flare anyway (as compared with doing it higher up) because at that point you are in ground effect?
Ground effect is an effect of the downsweep off the main wing (in a relative referential, or cushioning in an absolute one), but the stab is pointing the other way (force is downwards, ie upsweep), even more so during the flare, so it won’t benefit from ground effect.

Peter wrote:

Doesn’t the elevator authority come into the aft limit also?
Potentially, but longitudinal stability is the bigger problem.
ESMK, Sweden

huv wrote:

I find it impossible not to imagine a fuel emergency in this contex

Well, you would have to comply with applicable fuel requirements on top of (ie not accounting for any of ) the ballast fuel.
The alternative would be carrying less fuel and some non-fuel ballast instead…it escapes me how that would be operationally safer

Last Edited by Antonio at 13 Aug 12:36
Antonio
LESB, Spain

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

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, 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.

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

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
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