Bill, Hartmut
There is no written reference or tolerance for having level sills as an
operational aspiration.
The reason there is a "leveling means" for use in W&B calculations is
to assure a common frame of reference such that all results derived
from "methods acceptable to the administrator" utilizing said common
frame of reference should be an "apples and apples" comparison without
regard to when it was done or by whom.
Ercoupes with the 4" nose tire and wheel, the original 7.00 x 4 main
tires and new rubber taxi cushions (donuts) likely had level sills on a
level area of the ERCO factory floor. This would have been consistent
with the KISS (keep it simple, stupid) concept of efficiency and
economy of (necessary) motion.
In the same sense, when Fred Weick worked out the relationship of the
wing mean airfoil centerline to the centerline of the horizontal
stabilizer/elevator and the resulting assembled airframe in
relationship to the ground, it is logical that he sought the most
benevolent possible handling qualities for both landing and takeoff
conditions. He would have done his best to assure that, as
manufactured, the airframes (as described in the paragraph above)
conformed to such standard.
As time passed, the rubber donuts would compress, take a "set
(compressed, or thinner, from the constant weight of the airframe on
them) and become harder and less efficient in absorbing taxi shocks.
In the process, the tail would become lower and lower; but the effect
during landing and takeoff from the resulting difference in tail height
was of little importance except when pilots who had not "read the
instructions" or had little Ercoupe time or proficiency encountered
significant gusty crosswind. The effect could be humbling, to say the
least.
For the 415-E and subsequent variants a 5" single-fork wheel (6"
optional) and tire replaced the original 4" one on the nose strut.
That raised the nose AND lowered the tail. To such extent as
additional nose tire plies increase total circumference, that would be
even more difference. Difference from what? FROM THE WAY FRED WEICK
DESIGNED IT. That's the "standard" to which we should aspire. I
freely admit that the E and G model likely did NOT sit level when they
left the factory. Nonetheless, if these birds are safer in crosswinds
with level sills, I would want mine level. Personal opinion. Personal
choice.
So, are level sills mandatory? Obviously not. But, outside of
ignorance, laziness or false economy, no one has yet made a convincing
argument against level sills evidencing a well maintained airframe
likely to perform as intended. I happen to strongly believe that if
you own an Ercoupe (et al), it should have the same (or better)
characteristics in flight as when it left the factory.
Can a proficient Ercoupe pilot handle the plane if it has "low tail"?
Sure. But there is increased risk in strong, gusty crosswinds; and the
proficient pilot ALWAYS "stacks the cards" of managing risk in his/her
favor when there is no adverse trade-off. In other words, proficient
pilots seek to maintain their aircraft using adequate resources and
common sense. That is not in the FARs, but they do it with pride.
It should be obvious that tail height is a red herring, given the many
variables possible between wheel/tire combinations of nose and main
gear tires and rubber donuts or Belleville springs. That leaves the
level sill.
What is the acceptable tolerance? That is up to the owner and his/her
standard of maintenance. My standards are high.
In my opinion, acceptable tolerance is to drop from absolutely level
(with new rubber donuts) to whatever is measured in, say, four year's
service. If the drop is noticeable in crosswind performance with that
extent of drop, either replace the donuts at two year's service or
install Bellevilles (presuming this is economically feasible).
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2009)
--
On Aug 25, 2009, at 16:46, Hartmut Beil wrote:
Bill.
"but I have always taken this to mean the reference used to level the
aircraft for the purpose of getting accurate weight and balance data
and not a specific mandate to maintain the ship in level attitude"
You are right.
Window sill level gives you correct data for the W&B calculations.
Having the window sill level on the ground is only good for better
ground handling, but not required by the TC specification. After all,
the CAA cared for flying qualities, not so much the ground handling
qualities. Any tail dragger would be disqualified if that would be so.
Hartmut
To: [email protected]
From: [email protected]
Date: Tue, 25 Aug 2009 17:30:04 -0400
Subject: [ercoupe-tech] Oil Change Detail & Leveling Specifications [1
Attachment]
[Attachment(s) from [email protected] included below]
A few months ago, I listed a few maintenance and inspection tasks
which were in my view made easier and better by dropping the tail
feathers. One item was getting more dirty engine oil out. As a
follow up, on a recent oil change, I measured the additional oil
drained with the tail lowered (see attached pic). It's a tad north of
100 ml. Don't know if it would make any difference in the life of the
engine, but it makes me feel better. For the record, my Mason jar is
traceable to NIST Class A standards and has been maintained at
constant temperature and humidity since initial certification. (big
silly grin)
I also have a question: We have all heard numerous times on this list
about the importance of having our hatch sills level. Can someone
kindly point me to a written reference for that and ideally some
tolerance in either degrees or tail height limits? Someone asked
roughly the same question not so long ago but as I recall the
responses were not specific. I know that the TCDS for aircraft gives
the leveling means, but I have always taken this to mean the reference
used to level the aircraft for the purpose of getting accurate weight
and balance data and not a specific mandate to maintain the ship in
level attitude. I sure could be wrong on that. Gratitude in advance
to anyone that can provide the specification. I think we all
understand the concept of why it makes sense aerodynamically and why
we need a level floor and what not, so I respectfully ask that you
resist the urge to respond by repeating any of that.
Cheers,
Bill
