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Do forward and rear CG limits depend on mass?

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

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

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

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

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

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
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