Ed,
ESB 19 was written about the "later" crank style trim on the
instrument panel that spring-loaded the elevator. There was no
"aerodynamic" (separate articulating) trim tab on THAT elevator. In
item 1. ERCO states that Ercoupes up to Serial 212 with the elevator
disconnected would trim "at a speed of approximately 130 MPH, which
is obviously too high for a safe landing". It then explains that the
"new" system (213-1622) activates the elevator DIRECTLY...and will
therefore provide an additional means of control in case of failure
of the elevator linkage." With this trim system still functional in
event of elevator control failure the pilot could still trim for a 70
MPH glide.
The "aerodynamic trim tab" system installed on Ercoupes 1623 up thru
415-CD production was even more independent of the elevator control
system. Later ESM 55A was issued to install the 415-SK286 "fail
safe" springs and a stop for the trim tab. Should the trim control/
cable break, the springs returned the trim tab to the stop, and the
stop was to be installed such that its lower surface was parallel
with the top of the elevator upper skin top surface.
Your observation is "right on", in that with the trim fully forward
(nose down) the tab is against the stop and, per ESM No. 38, 11.b.
(3): "the ship should be trimmed for high speed flying at 115-125
MPH. With the trim crank handle full back, the airplane is trimmed
for a power off glide at 65-75 MPH and a full power climb of 60-70
MPH." So, with the elevator disconnected, this was the rang of
airspeeds remaining available through operation of the "aerodynamic
trim tab" system.
For those whose birds do not demonstrate these characteristics, refer
them to ESM No.35, item 15: "Effective about Ercoupe No. 3882, the
elevator trailing edge has been bent downward 3ยบ on production
aircraft. It was found that this modification makes it easier to
bring the airplane within...desired glide characteristics."
ERCO did similar tweaking on the split elevator installations. Such
"adjustments" are common (and necessary) for "free
flight" (uncontrolled) model airplanes. Another such "trick" adopted
by Fred Weick was our engine mount right thrust and downthrust angles
(to compensate for "P Factor" and pitch changes between power on and
power off).
I hope this eases your mind on this issue.
William R. Bayne
.____|-(0)-|____.
(copyright 2010)
--
On Aug 30, 2010, at 18:35, Ed Burkhead wrote:
On 2010-08-30 4:02 PM, William R. Bayne wrote:
Don's trim system is discussed at length in Ercoupe Service
Memorandum No. 19 of 4/26/46 (date from ESM No. 38). It says:
"...the natural trim speed in a glide with the controls and trim
device disconnected is now approximately 70 mph, a speed at which
the airplane can be maneuvered to a satisfactory landing by
throttle alone".
That statement concerns me.
When the aerodynamic trim tab is in line with the upper surface of
the elevator, the trim speed on my Coupe was well in excess of 110
mph - not 70 mph. So, how could the natural trim be 70 mph with no
aerodynamic trim tab and no control cables?
To trim any Coupe I've seen to 60 or less mph, the aerodynamic trim
tab must be quite a bit down, on the order of 40-60 degrees down.
The downward trim tab raises the elevator. The raised elevator
pushes down the tail and increases the angle of attack of the wings
which gives you the correct trim for low airspeeds.
Something is not tracking in the discussion of what happens with no
control cables and no aerodynamic trim tab, it seems to me.
Ed
