View Full Version : Take-off thrust question
NWSRG
11th Sep 2003, 20:28
Folks,
With an aircraft like the 777, the engines are mounted very low down, I think the ground clearance is only 18 inches or something like that. On rotation, does the proximity of the engines to the tarmac give the aircraft any benefit? Does the aircraft get any additional 'thrust' due to reaction with the ground?
Just curious.
I'll ask that a good friend of mine who is nuclear phisicist...
(is "phisicist" correct in English??:confused: )
He said he doesn't think so.
Very academic question, given the small interaction angle also...
M.Mouse
15th Sep 2003, 04:39
As you asked:
physicist is the correct spelling.
NigelOnDraft
15th Sep 2003, 15:45
<<does the proximity of the engines to the tarmac give the aircraft any benefit>>
Yes - when the wheels fail to lower, they act as a useful substitute <G>
NoD
I think you may have misunderstood the nature of thrust. The only way the ground would give any assistance to an a/c taking off would be if the generated thrust were to 'reflect' back and provide extra upward moving air to assist the a/c. This would only happen at an angle of approaching 90 degs with the ground.
What can happen is a ground effect where downward moving air (created by the motion of the a/c) 'bounces' off the ground and impinges on the a/c fuselage, wings and tailplane to create extra LIFT. This, whilst a factor during take off is more apparent when landing and can cause an a/c to 'float' along the runway. It is the reason that most commercial a/c autopilots land 'firmly'. I seem to reacall a prototype Trident (first full autoland a/c) crashed due to this phenominum and the Rad Alt and pitch logic was altered to compensate.