Expert: Paul Ruderham - 4/22/2008

Question
QUESTION: hey paul!
it's me again, brandon.  i was wondering if you know how the reverse thrusters on the boeing aircraft redirect the flow of exhaust/air, thus making the plane slow down.  i understand how reverse on planes like md80's work, i just don't understand how moving part of the engine cowling back makes the direction of airflow change so dramatically.
thanks again,
brandon

ANSWER: Hi Brandon,

There are  basically two different types, one is the
clamshell type which is mounted on the outside
of the engine and when deployed it covers the
end of the exhaust and deflects the engine
exhaust forward. The second type is the sleeve
type, they again close over the exhaust of the
engine but their mechanism is contained within
the engine, and the exhaust is directed forward
through grills at the side of the engine pod.
The clamshell type tend to be a bit heavier but
more efficient. They are typically hydraulically
activated and can only be deployed when the
aircraft is on the ground, it has happened in
flight due to a system failure and aircraft have
been lost because of it. On propeller aircraft
reverse is achieved by varying the angle of the
propeller blades to a negative pitch.

Happy Landings
PR  


---------- FOLLOW-UP ----------

QUESTION: sorry
what about the airbus reversers like on the a340 (see this video):
http://www.youtube.com/watch?v=IYdVZiXaCX4
thanks again
brandon

ANSWER: Hi Brandon,

These are usually used on high bypass engine aircraft. Known as petal reversers it's a variation on the cascade reverser. Bypass air is redirected out of a vent and forward by the petals.

regards
PR

---------- FOLLOW-UP ----------

QUESTION: what is a cascade reverser?

Answer
Hi again,

A cascade thrust reverser for a fan-jet engine comprising a two portion shroud surrounding the engine forming a duct there between for the rearward directional flow of bypass gases. The forward portion of the shroud is fixed in position. The rear portion of the cowl translatable from a forward reverser stowed position to a rear reverser deployed position. An opening is provided between the two cowl sections for bypass gases to exit the duct when the reverser is deployed. A plurality of pairs of cascade baskets are positioned around the circumference of the cowl intermediate the shroud walls; one of each pair is positioned over the other and nest in a cavity in the forward wall of the rear shroud portion when the reverser is stowed. The innermost cascade baskets pivot into the duct when the reverser is deployed, and direct the bypass gases through the outer cascade baskets for reversing the direction of the flow of bypass gases. The inner cascade baskets and the linked doors between the baskets block the duct preventing the rearward flow of bypass gases when the reverser is deployed. Linear actuators are provided for translating the rear shroud portion and rotating the cascade baskets.

Regards
PR