THE BIG LIE
so that's a bit harsh. I'll be charitable and call it a Big Fib. You have heard
it, or read it, probably many times over. Seems like we often hear, "It
flew beautifully, didn't even need any trim!". Magazine articles on kit
reviews or new designs like to use the big fib. Too often we read: "It
practically flew right off the drawing board." or, "The maiden flight
was a dream, it didn't even need the transmitter trims changed." A minor
variation is the "All it took was two clicks of left rudder and it flew
great." or, "It trimmed perfectly on the transmitter".
Don't you believe it! It is a rare event
that a new airplane fly's well without trim changes. It may be a proven design,
properly constructed, balanced and set up according to the instructions... But,
the designer can't know everything about the way it was built and the way it
will be flown. It can, and SHOULD BE, trimmed, adjusted, and changed to make it
fly more the way YOU want it to.
The recommendations in the instructions
are a good place to start, and make it highly likely that your new craft will
survive its test flight. So after that first flight, now what?
If some of them are not close to centred, adjust that control surface so they
will be closer to centre. Make small adjustments at a time, one to three turns
on the clevis. Adjust all control
surfaces, and the nose wheel. If all the trim lever travel has been used, it
will probably take three turns to get it close to centre.
CONTROL SURFACE THROW-
Reduce control throws on the rudder and ailerons if they are too sensitive. Make
the reductions a little bit, move one hole on control horns or about three turns
on the threaded stud type control horns often used on ailerons. If some controls
were not very effective, or not sensitive enough, increase the throw a little.
If the elevator is too sensitive, there are two possibilities- 1) Reduce
elevator throw, or 2) Move the balance point, CG or Centre of Gravity, forward.
If the plane flew smoothly, but was sensitive to small amounts of elevator
stick, then reduce the elevator throw a little. If the plane was smooth and
landed going fast with most of the up elevator being used, increase the throw a
little. If the plane tended to climb, would not settle into stable flight,
tended to balloon on landing, or needs down elevator trim, then you likely have
a tail heavy plane and need to move the CG forward. See the next paragraph.
CG/CENTER OF GRAVITY/BALANCE POINT-
Tail heavy airplanes are tough to fly even for an experienced pilot. They have
lots of bad habits- a tendency to lift off at too low an airspeed, balloon on
landing, resist being trimmed for level flight, and so on. Move the CG forward
to reduce those things. Add nose weight, or shift the battery or other equipment
forward in the fuselage. If it means adding weight to your plane, do it. This is
one case where the added weight is well worth it.
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This Site is maintained by
David Probert, Committee Member for Alport Model Flying Club.