Expert: Paul Soderman - 4/1/2010

Question
QUESTION: What is the advantage of manufacturing an movable tail plane of an aircraft? {e.g. learjet 24.}

ANSWER: An aircraft must have variable lift so the pilot can pitch the aircraft up or down as required to fly. Variable lift can be achieved with an elevator or movable tail plane or both.  A movable tail plane has the advantage of more lift potential because of its large size and can be trimmed nicely so in cruise there is no need for the pilot to hold elevator position.  The force on the stick can be set to zero.  A trim tab on the elevator is also be used for trim.

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QUESTION: Thank you for reply sir, why specifically we use adjustable tail plane as we can have trimming by means of trim tab ,as the adjusting mechanism would add more weight penalty?

ANSWER: Yes, you are correct. Movable tails add complexity to the design.  I can only assume that this aircraft needed more pitch control than could be provided by the elevator alone; i.e., the movable tail would add to the pitching moment generated by the elevator.  Perhaps the aircraft was designed to be used with a large CG (Center of Gravity) travel range caused by passengers and luggage.  Another possibility is that the movable tail generates less drag than the elevator for the same pitching moment.

See P. Soderman and T. Aiken: Full-Scale Wind Tunnel Tests of A Small Unpowered Jet Aircraft With a T-Tail.  NASA TN D-6573, Nov 1971 for aerodynamic performance of a Learjet as measured in a wind tunnel.
Paul

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QUESTION: Thank you for reply. how can I see the provided wind tunnel test? And the model which I am learning on is learjet 24 it has elevators and movable tailplane which can be adjusted  on ground...for trimming purposes of course .

Answer
You can get the report online as a pdf file from the NASA Technical Reports Server:

http://ntrs.nasa.gov/search.jsp?R=107812&id=1&as=false∨=true&qs=Ntt%3DNASA

or you may find it in many engineering school libraries.
Paul