Spoilers and reversers?

[measto]

I still have not been able to find a reasonable explanation as to why a nose up tendency occurs when the spoilers and the reversers are deployed after landing. All responses are most appreciated!

[QNH1013]

A downward force is created around the landing gear as this maximises tyre and braking action. This force will cause the nose of the aircraft to seem to pitch upwards.

I am NOT certain about this as this is just an educated guess (or uneducated for that matter) I've actually never felt this 'pitch up' before, just the forward stopping sensations of the decelleration.

[john_tullamarine]

I suggest that whatever effects may be observed on a given aircraft model will be characteristic to that aircraft and very dependent on just when the actions occur during the touchdown sequence.

If spoilers are deployed with a reasonable body angle (ie before the nosewheel is on the ground) then, with the usual (effectively) fullspan spoilers operating (by disturbing airflow on the aft portion of the chord), one would expect that the wing lift distribution would be skewed significantly forward which would account for a reduction in the nosedown pitching moment associated with the landing. The pilot would interpret this as a noseup pitching moment.

The magnitude of the pitching moment contribution associated with reverse will depend on the aircraft configuration (ie whether the thrust line is high, mid, or low geometry - height above the ground) and thrust levels achieved with reverse. The effect will be, however, a noseup moment contribution as reverse is a rearwards acceleration the line of which is located above the mainwheels

[ 16 October 2001: Message edited by: john_tullamarine ]

[BmPilot21]

I definitely noticed it the other day when the Captain kept the nose in the air longer than usual (with a high nose attitude). The selection of reverse definitely caused a pitch up.

Intersesting, as obviously in the air, an increase in thrust causes a pitch up and vice-versa. The aircraft is rotating around it's CofG ABOVE the engines. On touchdown the aircraft rotates about the MAINWHEELS, located BELOW the engines, therefore reverse pitches the nose up.

I don't think speedbrakes give a nose up moment on the Boeing (I think it does on the Airbus). The lift dumpers obviously greatly reduce the lift from the wing. The centre of lift is obviously in front of the mainwheels, so this should cause a nose down moment. The centre of lift may move forward, causing an incresed nose up moment, but I doubt this is significant. I understand lift dumpers can actually cause negative lift (a downforce) - causing the load on the mainwheels to be greater than the (at rest) weight of the aircraft.

[najib]

How about outboard speed brakes/ground spoilers having a downward component (while touchdown with a high noseup attitude). Can vary ofcourse depending upon sweep angle and how farther the spoilers are located from the fuselage.
The gentleman about the reverser is absolutely right as well although its effect comes later than the speed brakes due to the time it takes to deploy compared to spoilers.

[john_tullamarine]

Chaps,

I think that we need to revisit the general lift distribution and just what spoilers do to the airflow patterns around the wing. Please reconsider your thoughts of which way forces are pointing, where they may be, and the sense of consequent moments ......