Hi Harry,
Interesting theory...but further investigation would seem necessary
before acceptance.
Comments interspersed below.
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2009)
On Aug 18, 2009, at 16:56, Harry L. Francis wrote:
Thanks, John.
Let me add that at less than proper tension on the cables allows the
elevator to move down depending on airspeed, as the wind blows over
the elevator. So if one rigs the elevator with less than proper
tension, then at flying airspeeds, the elevator does not move up as
specified.
As I understand it, tension is tension. There is no bias relative to
elevator movement either up or down. If we could somehow measure the
force of the slipstream on the elevator at, say around 60 MPH, which
would approximate the speed at which the pilot would likely wish to
flare to minimum speed before touchdown, we could then measure how much
(if any) any deficiency in elevator cable tension as such down force
would affect the up elevator movement possible with the yoke fully
back.
I think this is one of the problems with conversion to D model, and
need for higher airspeed on landing....as with a looser elevator
cable, the elevator is not rising to 9 degrees, even with the control
wheeel fully back.
If a 415-D is properly rigged as to available elevator movement (9º up
plus or minus 1 degree), I don't think the case has yet been made that
the relatively low air pressure at flare is sufficient to change
elevator movement even a half a degree. That is to say that if it is
rigged for at 10º up with the yoke fully back (9º + 1º is still within
factory spec.), I would be surprised if the elevator did not rise to
9-1/2º up with the full effect of the slipstream on the full up
elevator at 60 MPH.
There is NO "need" for higher airspeed when landing a 415-D. One flies
the exact same approach in either. Landing over a 50' obstruction at
1400# gross ERCO used an approach speed of 75 M.P.H. (p. 10), but a
normal landing at the end of a day's flying or cross country flight is
likely nearer the gross weight of 1260# of the 415-C. The postwar
415-C at 1260 lbs. could glide straight indicating 45 M.P.H. (minimum
speed...at a high sink rate) with the yoke full back. Normal approach
speeds recommended were between 60 and 70 M.P.H. Find out what your
ASI reads at the "burble" near minimum speed at a safe altitude of
2-3000' AGL, and it should not vary significantly just off the runway
when landing.
Because the 415-D has less UP elevator available, its "minimum speed"
at touchdown is at a higher airspeed than a 415-C or CD with 13º of up
elevator available. Since the 415-D Flight Manual lists on page 10 a
minimum, or "Stall Speed" of 58 M.P.H. TIAS (with C.G @ 17.6 M.S.C. and
presumably at 1400# gross. Per Chart No. 1, p. 12, I would interpolate
that the Airspeed Indicator would be reading 54 M.P.H at said "minimum
speed. I would, therefore speculate that actual speed at touchdown
with the yoke fully back would likely be only 5 M.P.H. slower in the
415-C or CD at 1260 # as compared to the 415-D at 1400# gross, all
other things being equal.
I would suspect differences in the straightness and cleanness of
airframes, tightness in control and pitot-static systems, accuracy of
Air Speed Indicators and correct alignment of pitots with the lower
wing surface to be responsible for significant personal recollections
to the contrary. If anyone ever again converts a 415-CD to a D model,
perhaps they would do some testing at 1260# and 13º up elevator BEFORE
and 1400# and 9º AFTER on the same airframe for an "apples and apples"
comparison.
In restoration of my coupe, we found that even tho the elevator
measured 9 deg up, the cable tension was way below specification...
OK, I believe what you say...
thus allowing less than 9 deg on landing.....
still looking for factual substantiation or quantitative observation of
this...
Intersting situation, but points out the need for meeting the proper
specs on cable tension, so when flying, the elevator moves as
specified.
I would speculate that wear (lack of tightness, or slack) in yoke
U-joints or attaching bolts/pins, column chain, column bushings,
pushrod from control column to elevator bellcrank, elevator bellcrank
bushing, and rear elevator arm and pushrod assemblies would likely have
more immediate effect on available elevator movement on an Ercoupe with
the original rigging seal unbroken than any normal loss of elevator
cable tension.
Fly Safe _ Have Fun
Harry Francis
N93530
--
From: John Cooper <[email protected]>
To: [email protected]
Sent: Tuesday, August 18, 2009 2:40:59 PM
Subject: RE: [ercoupe-tech] Rigging of elevator (415-C)
> Or why did they seal that one in the forst place?
The seal is there so someone doesn’t indiscriminately adjust the
cable and adversely affect the stall characteristics.
John Cooper
Skyport Services
www.skyportservices .net
