Date: Mon, 25 Jan 1999 00:36:38 +0200
To: "rossann@azstarnet.com" <rossann@azstarnet.com>,
    "Lancair List" <lancair.list@olsusa.com>
From: "colmar" <jones@colmar.demon.nl>
Importance: medium
Priority: normal
Message-Id: <917220998-0-jones@colmar.demon.nl>
Subject: Re:PITCH SENSITIVITY
Mime-Version: 1.0

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I guess every Lancair driver knows the basic characteristics  - in cruise
the ratio of force/response for ailerons is very high but the force/
response in pitch very low. It is indeed, a very unbalanced situation
which can result in some very sick-making or even head-flattening
manoeuvres in the hands of an un-initiated, and makes an altitude-hold
auto-pilot well worth the investment for long-distance travelling.
In the pattern and on finals, however, with some flaps down, the ratios
are more equally balanced, but the now the stability rather than
sensitivity becomes the issue. Stability turns from positive to neutral
or even negative. In the UK and Australia, this last characteristic
has resulted in the 320/360 being allowed only with the larger tail
option.

Here in Holland, we all have the original small tail, and we are all
adequately adjusted to the characteristics, even though we are certainly
not multi-thousand-hour former-jet-jockeys. The key to the pitch
sensitivity in cruise, of course, is just to hold the stick with finger
tips not palm grip, but in landing I suspect the best training is
probably actually a computer simulator. Just fly-the-picture",
letting it down with the thottle, and don't expect the same force feed-
back from the stick that you got from every other spam-can. (Myself -I
cross-trained on a Katana, landing without flaps, and I found that a
good preparation).

But all this sensitivity issue, and particularly stability, has been
measured on two Lancairs here as part of a MSc thesis of an aeronautics
student at Delft Univ, and we will put it in print some time in the near
future when it is translated into English. Both of the test planes had
the reduced elevator arm distance, of course, but that only increases the
STATIC force/response ratio of the elevator, while what is apparently
needed is a DYNAMIC feedback to the stick when g-force increases. And that,
of course, is the function of the bob-weight - when you pull G's, the
bob-weight's "weight" becomes larger than the static force of the trim
spring which was previously holding it in balance and the stick is pushed
against the pilot's action. So I agree with Ross Colebrook's
approach, but
I think the real solution lies in adjustment of the  bob-weight arm
length, so that for the same amount of lead weight, the rotational
torque fed back to the stick is higher. I haven't done it myself, but
would be interested if anyone has - and (hopefully) "someone" at
Lancair must surely know more than they are saying!

Incidentally, in the testing of two Lancairs, we found differences
between them in their apparent stability in the "pattern" type
configuration which seem to be related to the FRICTION in the elevator
control. We have not tracked it down completely, but I suspect that
the only place where friction, rather than spring force, can vary between
planes is from the free movement (or otherwise) of the trim push-rod inside
the two trim springs. Have you noticed that when the trim
lever is
full-forward (eg. cruise) or full-aft (eg. landing), the forward
attachment point of the trim rod is positioned lower than the elevator
rod itself and hence the trim-rod is not parallel to the elevator
push-rod? This could result in rubbing of the rod against the inside of
the springs. If this is the source of the friction, then the solution is
probably to rotate the teflon clamping device on the elevator rod so
that it holds the trim-rod parallel to the elevator rod at the full
forward and full aft positions, NOT at the center position as might
otherwise seem obvious. I think this is important, because
stability when
the trim is set for the landing (flaps down) configuration is the
critical controllability issue, not when set for the  mid-speed or
take-off position. Anyone tried any experimentation on this issue?

Colin Jones (PH-COL)