Elite Aileron Rigging and Pitch Stability
Posted: Sun Feb 19, 2012 12:03 pm
Guys:
I found the information in my manual regarding aileron rigging on the Elite
to be quite confusing. At one point, they sort of indicate that the
flaps/ailerons should be flush with the top of the cabin, then somewhere
else it says "The Elite aerofoil is in the 5 degree reflex position at
neutral aileron". Trouble is, these are not the same thing. Flush with the
top of the cabin would give you more like 10 degrees of reflex. Then,
Murphy have a picture CD that shows the flap-fuselage fairing. Yes, it's
flush with the top of the cabin, but it looks like somebody has used a blunt
instrument to batter the thing down at the outboard end, so that the TE of
the flap is about 1" down from the cabin roofline. I finally went with the
statement in quotes above and riveted the fairing to the cabin to match,
which results, as I recollect, in about 3/4" difference between the top of
the fairing and the cabin roof at the TE.
On pitch stability, of course the airplane has to have a down force on the
stab to be stable, but the magnitude of that down force depends on both stab
incidence and elevator position. To minimize drag at cruise, the elevator
should be in line with the stab, so the only thing left to adjust is stab
incidence. As somebody pointed out, there are a number of reasons why a
floatplane needs more up elevator than the same plane on wheels. The drag
on the floats tends to pitch the nose down, but the biggest one has got to
be that, since the airplane is flying 25% slower, but the required downward
force at the stab is at least the same as (and due to things like float
drag, somewhat greater than) the force required for the wheeled aircraft.
The stab will have to operate at a 50% greater lift coefficient to achieve
this, hence either up elevator or greater negative incidence on the stab is
required. The only reason that the stab isn't physically at a very negative
incidence is that it's operating in the downwash from the wing, hence its
angle of attack is much more negative than angle of incidence.
The dynamic stability is governed by the size of the restoring force that's
induced when the aircraft momentarily departs from the trimmed condition
(e.g. the stick is nudged). This will clearly depend on the stab area, so it
seems clear to me that a larger stab will enhance pitch stability. For
sure, in the other direction, if the stab is too small, pitch stability will
be adversely affected. In the limit, there'll be none! However, the
effects of the elevator can't be ignored, since it is free to align itself
with the airflow. The spades are de-stabilising. If the elevator moves up,
the effect of spade area ahead of the hinge line is to make it move further
up. That's why cutting the spades in half helps reduce the twitchiness of
the Elite in pitch. However, it also reduces the total stab area, which, as
I've shown is de-stabilising. So, I think a better fix would be to reduce
the size of the spades while maintaining (or perhaps increasing) total stab
area. The cleanest fix to my mind would be to keep them 8" wide, but reduce
their length by 1/3, thus reducing the size of the de-stabilising torque
they contribute by 50%. Mass balance would have to be increased, of course.
Now, if all the Elite builders would chip in to buy a stab kit, I could
probably be talked in to building one and fitting it to my Elite to see if
it improves matters!
Al
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I found the information in my manual regarding aileron rigging on the Elite
to be quite confusing. At one point, they sort of indicate that the
flaps/ailerons should be flush with the top of the cabin, then somewhere
else it says "The Elite aerofoil is in the 5 degree reflex position at
neutral aileron". Trouble is, these are not the same thing. Flush with the
top of the cabin would give you more like 10 degrees of reflex. Then,
Murphy have a picture CD that shows the flap-fuselage fairing. Yes, it's
flush with the top of the cabin, but it looks like somebody has used a blunt
instrument to batter the thing down at the outboard end, so that the TE of
the flap is about 1" down from the cabin roofline. I finally went with the
statement in quotes above and riveted the fairing to the cabin to match,
which results, as I recollect, in about 3/4" difference between the top of
the fairing and the cabin roof at the TE.
On pitch stability, of course the airplane has to have a down force on the
stab to be stable, but the magnitude of that down force depends on both stab
incidence and elevator position. To minimize drag at cruise, the elevator
should be in line with the stab, so the only thing left to adjust is stab
incidence. As somebody pointed out, there are a number of reasons why a
floatplane needs more up elevator than the same plane on wheels. The drag
on the floats tends to pitch the nose down, but the biggest one has got to
be that, since the airplane is flying 25% slower, but the required downward
force at the stab is at least the same as (and due to things like float
drag, somewhat greater than) the force required for the wheeled aircraft.
The stab will have to operate at a 50% greater lift coefficient to achieve
this, hence either up elevator or greater negative incidence on the stab is
required. The only reason that the stab isn't physically at a very negative
incidence is that it's operating in the downwash from the wing, hence its
angle of attack is much more negative than angle of incidence.
The dynamic stability is governed by the size of the restoring force that's
induced when the aircraft momentarily departs from the trimmed condition
(e.g. the stick is nudged). This will clearly depend on the stab area, so it
seems clear to me that a larger stab will enhance pitch stability. For
sure, in the other direction, if the stab is too small, pitch stability will
be adversely affected. In the limit, there'll be none! However, the
effects of the elevator can't be ignored, since it is free to align itself
with the airflow. The spades are de-stabilising. If the elevator moves up,
the effect of spade area ahead of the hinge line is to make it move further
up. That's why cutting the spades in half helps reduce the twitchiness of
the Elite in pitch. However, it also reduces the total stab area, which, as
I've shown is de-stabilising. So, I think a better fix would be to reduce
the size of the spades while maintaining (or perhaps increasing) total stab
area. The cleanest fix to my mind would be to keep them 8" wide, but reduce
their length by 1/3, thus reducing the size of the de-stabilising torque
they contribute by 50%. Mass balance would have to be increased, of course.
Now, if all the Elite builders would chip in to buy a stab kit, I could
probably be talked in to building one and fitting it to my Elite to see if
it improves matters!
Al
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