On 05/14/2010 07:21 PM, Raimo Toivio wrote:
> When trimming that means we are in fact in-lining tailplanes with
> trimtabs to a certain desired position!
Well, I hoped to see a lightbulb switching on, but you lit the whole
X-mas tree!
We had a good laugh over here. :-D
> One interesting point also: obviously all the tailplanes which have been
> built perfectly, are perfectly also aerodynamically balanced.
> Like Franss Europas tailplanes.
This is a compliment to the factory as I got the "accelerated kit" with
the tailplanes built by the factory.
Then again, I don't think you need to have perfectly aerodynamically
balanced tailplanes.
Now we agree about the function of the anti servo tabs, we can go back
to my previous statement about the mass balance weight.
The balance of the tailplane and anti servo tabs is not purely
aerodynamic, it is the result of all forces acting upon the system:
1) If the pilot pulls or pushes on the stick, the tailplane will rotate,
the anti servo tabs rotate even further, and this imbalance is countered
against the force the pilot is exercising. This means we have a stable
position here with the anti servo tabs pushing the system some way, and
the pilot pushing it the other way. The trim tabs stay deflected then,
because they provide the counter force to null out the pilots unconsious
input.
2) If the tailplane is not weight balanced correctly, it has a tendency
to rotate by itself, and the anti servo tabs need to balance this
tendency out. Again we get a situation where weight is trying to get the
tailplane rotating this way, and the anti servo tabs trying to undo
this. There is a balance, but this balance is not with the anti servo
tabs lined out behind the tailplane.
So, in both situations, you won't get the anti servo tabs to line up
with the tailplane.
What can go wrong:
1) The pilot. When I was learning to fly, the instructor had a habit of
ripping my hands of the yoke, which immediately revealed whether I had
trimmed the airplane incorrectly. Often I was sure I had trimmed
correctly, but apparently it is easy to unconciously exercise a force
upon the yoke or stick. So as a student I quickly developed the habit to
"let go of the stick" after trimming to make sure it is really trimmed
and get the tension of my muscles out of the equation.
Maybe everyone does this, maybe not. Anyway, keep this in mind: with the
Europa design you can possible gain a few knots by making sure the
tailplane is perfectly trimmed for the intended speed: let go of that stick!
2) The mass balance weight. As said before, I got through the hassle to
disconnect the pitch tube to really balance out the tailplane. Still
then, it is difficult to balance it with the frictions and inertia
involved. When I thought by moving the trailing edge of the tailplane up
and down that the system was in balance, I double checked with an
elastic band... and discovered that humans are not able to compare a
lifting force against a pushing force very well.
So, tape an elastic band to the trailing edge, put a mark somewhere, and
start pulling the trailing edge up or down. Measure the distance between
the mark and the trailing edge with a ruler, exactly at the moment the
tailplane actually starts moving. Do this also the other way. Goal is to
have the trailing edge moving with the same stretch of the elastic band
pulling up and pulling down.
If you are flying around with the anti servo tabs deflected upwards, it
could mean the mass balance weights are a tad too heavy, or the pilot is
unconsiously pulling on the stick. The weight of the pilot's arm is
enough to disturb the delicate balance.
If you are sure both of these are really ok, then there is a last
resort: You could connect a spring or bungee to the pitch push rod, and
"preload" it in the correct direction. This is however where my
understanding of the system ends: I'm not sure if this could induce
flutter. Then again, it is not different than a pilot unconsiously
pushing or pulling on the stick, and the system seems to cope with that
very well. This corrective bungee gets into the equation and can get the
balance so that the anti servo tabs line up behind the tailplane, and
this could gain you a few knots.
I had it all worked out, assuming that ALL Europa's had this deflected
trim tab defect... and then discovered that my Europa doesn't need it at
all. :-)
> Frans - next time I will consider a bit more before I think you are wrong.
> You are wrong probably very seldom, I assume.
Well, I wouldn't guarantee that. In fact, I just keep my mouth shut if I
do not have a strong feeling that I'm right. ;-) Quite a lot of other
people had the same knowledge about the tailplane, but most of them
didn't bother to convince the rest of us. I was prepared to leave it as
well, but your Donald Duck thing was just too provocative. ;-)
> Would be nice to meet you in Texel next autumn.
Nice to hear that you are coming! We might also show up in Popham next
month.
> I give you some extra present: have you earlier seen a Duck on the wing
> behind a running star engine?
Nice picture! :-D
Frans
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