Effect of negative-g on aircraft

[sughew]

As I understand, positive G-forces have significant effects upon the aircraft (increase loads etc).

What are the effects of negative G-forces upon the aircraft?

 

[berkshire]

sughew (Mechanical)
Just the same in the opposite direction however most designers take out the normal 1-G on the negative side. So you would have a working load of say 4-G pos 3-G neg.
B.E.

 

[BrianR]

Negative g does wonders for engine oil feed and fuel tank scavaging as the feed points are in the bottom of the tanks.

 

[berkshire]

BrianR (Aerospace)
However if you are designing for negative G you use floppy pickups on fuel and oil.
B.E.

 

[rb1957]

i think BrianR was referring to the physical effects, like fuel starvation on early model spitfires when they flew inverted (because on their carburettors)

 

[berkshire]

rb1957 (Aerospace)
This again comes back to whether, or not, the aircraft is designed for inverted flight. On most aircraft that are not designed for this the engine will cut out very quickly and severe damage can be done when the oil pump runs dry at high rpm. For most designers on normal category aircraft the worst negative loads appear when encountering sharp edged gusts which are usually of short duration.
B.E.

 

[BrianR]

Talking to a couple of our engineers who have been designers of both military and civil aircraft and they said that generally they apply the load cases as needed. For example some military designs might be stressed as high as +9/-3g while civil airliners are much less but having only +3/-0.6g. Consideration has to be given to wings where the top skin is under tension in the -ve case but otherwise it is a matter of applying the loads.

The engine cases are always difficult as even if you maintain fuel feed, using an inverted flight accumulator or something, oil flow is still different and few engines can put up with that for long. I suppose you'd have to consider the cg shift with fuel in the top of the tanks too.