engine operation under negative load factor

[delarocha]

Dear Forum readers

While reading the book "Aircraft Structures by Peery and Azar". On the chapter about flight loads there is a sentence which I don't fully understand
"The negative high angle of attack condition, occurs in intentional flight maneuvres in which the air loads on the wing are down or when the airplane strikes sudden downdrafts while in level flight. The load factors for intentional negative flight atitudes are considerably smaller than for positive flight attitudes, because conventional aircraft engines cannot be operated under a negative load factor for very long..."
Although this is stated as an evidence, what's the main reason why an engine cannot be operated under negative load factor for long?
Thanks for help
Regards

[John Farley]

I guess the most common airframe related limit would be the amount of fuel available in the collector tanks. This can be as little as 15 secs at high powers

Most unmodified engines have a pretty short time limit associated with the interuption to the oil supply and pressure that is likely to happen under negative g. That goes for recips and turbines.

[Al Weaver]

I think that Egypt Air 990 was a good example of this. When the G load goes negative the oil pressure falls due to the oil tank being on top of the engine. This sets off warning lights in the glass cockpit, although the engine can be operated for several minutes completely dry and without oil.

[eaglet-WestBourne]

I confirm: it is mainly a problem of oil circuit (dry sump and wet sump...)

[Dave Incognito]

If you are flying something with a carburettor, then negtive load factors can cause problems in the float chamber.

I believe the early Spitfires had problems with this until they modified the float chamber to incorporate some type of membrane. Not sure how it worked though...

[Genghis the Engineer]

(1) Float type carburettors will only continue to function under negative g for a few seconds. Diaphram carbs solve this problem, but are usually only used in aerobatic aircraft due to the relative cost. SLMG also use diaphragm carbs because they don't mind being at a funny angle when the engine is folded away.

(2) Wet sump 4-stroke engines will, unless modified for aerobatic use, ony have an oil pickup in the base. Aerobatic engines have two pickups and a spring-loaded ball-valve that toggles between then (in Lycomings the prefix "AE" on the engine type signifies aerobatic mods). The snag with these oil systems is that the ball unseats and causes wild oil pressure fluctuations at low (0.5) g manoeuvres at low power - such as pitch changes in a long flight idle descent.

(3) Fuel pickups are usually only near the base (just above the sump) of a fuel tank. So if the above don't get you, you may well hit fuel starvation - typically after 30-40 seconds.

G