Cobalt wrote:
Hence, the only way to guarantee that everything remains within the certification limits is to limit the overall G factor, not just the lift generated by the wing.
Correct 
Good explanation! (You would have noted the wink next to my question.)
Cobalt wrote:
In theory it actually increases the MTOW by more
Some aircraft do have increased MTOW if and only if the tip tanks are full.
Slightly off topic – if the stress on the wing roots is the limiting factor in aircraft MTOW, adding tip tanks increases MTOW by the weight of the tanks (including fuel), as long as the extra weight is carried in the tip tanks. In theory it actually increases the MTOW by more, but that would lead to some interesting calculations before every flight…
Archie wrote:
Flying at Va for say 2,000 kg and we pull to the edge of the stall (for Va say ~4.4G) the wing will generate 8,800 kgs of lift force or 88,000 N. This is the limit and we cannot pull anymore as the wing will stall itself before the structure fails.After an hour of burning fuel we are at 1,800 kg and pulling the same amount will only generate a lift force of 79,200 N.
Shouldn’t Va go up as weight goes down?
The lift force is not dependend on the weight of the aircraft. It is dependend on speed, angle of attack and the laminar profile. What you mean is that with decreasing weight the wing might have to produce less lift force to counteract against the weight. But that’s irrelevant in a maximum g load pull.
Secondly, the gross weight figures do not only apply to baggage or seats, but also to engine mounts, wing connections and many others. They get more strained the more force is applied, which will be the case if you decrease the weight of the aircraft.
EDIT, Cobalt was quicker, better explanation, similar outcome :)
When you start the flight with full tanks, the weight is equally distributed between the wings and the cabin. The mass of the wings is higher, therefore the gust needs more force to accelerate it.
During flight, the wings become more and more empty, so the aircraft gets lighter, but also the mass gets more concentrated on the cabin. Because nobody throws passengers or baggage out the door.
This means a gust or lift force applied on the wings will accelerate the wings relatively stronger compared to the cabin, which exercises more strain on the connections between the wings and the cabin. That’s one of the main reasons some aircraft models, i.e. the Bonanzas, but also aerobatic and fighter trainer models have a wing spar that goes from left to right wing straight through under the cabin, while some models have wings which are merely screwed to the cabin structure. Latter construction is less resistant to strain posed by overloads.
This comes up frequently. The certification limit is a load factor limit, and applies to the whole aircraft. In your example, let’s say you pull the now lighter aircraft to the 88,000N of lift force – this will now accelerate the aircraft with 4.9G.
The engine mount was designed to 4.4G, and the engine did not get lighter, so it will be stressed more than allowed for by certification.
Also, with wing fuel tanks, the bending moment where the wing spar enters the fuselage will be higher. The mass was removed from the wings, while the fuselage mass remained the same. So the wing/fuselage link accelerates the same mass by more than before, again creating stress above what the certification allows.
Hence, the only way to guarantee that everything remains within the certification limits is to limit the overall G factor, not just the lift generated by the wing.
EuroFlyer wrote:
In reality, we fly at cruising speed, maybe for quite some time, burn fuel and get lighter (which causes all the V’s to go down)
Flying at Va for say 2,000 kg and we pull to the edge of the stall (for Va say ~4.4G) the wing will generate 8,800 kgs of lift force or 88,000 N. This is the limit and we cannot pull anymore as the wing will stall itself before the structure fails.
After an hour of burning fuel we are at 1,800 kg and pulling the same amount will only generate a lift force of 79,200 N.
Shouldn’t Va go up as weight goes down? 
RobertL18C wrote:
There is probably a Mach speed related to Va and hopefully the jet crew can advise.
Only IAS on mine. Vo BTW is the Part23 concept introduced for newly certified aircraft which is meant to be a more practical manouevering speed with more margin. It can be the same as Va or lower ie it cant exceed Vs*sqrt(load limit) whereas Va can’t be lower than that number.
Martin wrote:
think you can make the structure as strong as necessary to handle the prescribed maneuvers at a Va you chose without having to increase the design limit maneuvering load factor.
Ah – so you could build the aircraft in excess of the certification requirement, say, for 6 G (wing + other structures), but only certify it in the normal category (so you don’t need to meet the aerobatic certification criteria, e.g., spin characteristics) .
So now you can have a Va higher than the minimum (Va min is stall at 3.8G).
Not sure why you would do that in practice, but I see your point now.
Airborne_Again wrote:
It can be lower if roll or yaw are limiting rather than pitch.
In theory, yes. The regulation prohibits this. (23.335)
RobertL18C wrote:
why do aerobatic aircraft have lower entry speeds for flick type manoeuvres?
It could be due to asymmetry of loads.
RobertL18C wrote:
Transport aircraft on the other hand have turbulence speed which is above Va.
Even Part 23 specifies Vb (design speed for maximum gust intensity). When it was introduced is a question. Logically, older designs won’t have it.
Cobalt wrote:
I can’t see how it could ever be higher than this minimum in any typical aircraft.
Since it’s written that way, it has to be possible. I think you can make the structure as strong as necessary to handle the prescribed maneuvers at a Va you chose without having to increase the design limit maneuvering load factor. Does that mean that the whole structure is good for whatever load factor corresponds to that Va?
I think one way to limit it would be to make it so the force required is higher than what the regulation allows (the pilot limit or whatever it’s called).
