Hi Ed,
The factory and the CAA have specifics in mind as to stall speed that
we all need to understand and keep in mind.
"Stall speed" and "minimum flying speed" appear to be used to describe
the same, identical speed in the Ercoupe Instruction Manual. I would
think we have interest in knowing the slowest possible "touchdown
speed" while landing with available "up" elevator movement for the
415-C, the 415-D and later Models with the split elevator.
While it is possible to set up a powered approach in an Ercoupe at very
slow speeds, such an approach is quite dangerous at the end from 200'
or so AGL to touchdown. Should the engine quit when the plane is too
low to "nose down" and regain sufficient speed for flight and flare, it
will impact the ground at up to 60 mph at an angle approaching
vertical. Significant damage or destruction of the airframe could be
accompanied with injury, possibly severe, or even death. Accordingly,
I have limited my attention to that portion of any landing I prefer to
accomplish in a power-off glide; including flare.
The "low speed warning cushion" spring was part of the ERCO 415-E
design, described in the Flight Manual for the E & G, and is present in
Forney production drawings. There is no mention of this spring in the
Flight Manuals or Service Manuals for either Forney or Alons. One F-1
I flew had it, and it's presence was a surprise to me on the first
landing ;<)
I don't personally know if it was installed on the F-1A or Alons.
Perhaps owners can help us out here.
On your D Model with the original elevator set for 9º maximum "up" I
would speculate (based on the 415-D Airspeed Indicator calibration
chart) that at touchdown indicating "about 50 mph" you would have
actually have been traveling at least 54 mph. If we presume that this
is the truth of the matter, then the smooth exchange of excess forward
speed energy in the flair permits the plane to touch down smoothly 4
mph below stall speed because (1) you are in "ground effect" and (2)
you have executed an incredibly smooth and controlled "whip stall" that
was so skillfully executed as to be imperceptible to those in the
cockpit.
I don't think this explanation could be stretched to explain what
Hartmut is observing, and so I would tend to agree that the speed shown
by a motor vehicle radar trained on his plane would read 50 mph or
above. Obviously, though, I don't KNOW.
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2010)
--
On Mar 17, 2010, at 19:25, Ed Burkhead wrote:
> The "Stall Speed" given on p. 10 of the 415-E & G Manual
> is 60 mph (power off, with "up" elevator limited to 20º).
> This is verified as being at 1400 lbs. gross weight on p. 11.
Bill,
The term “stall speed” seems to get used with some fluidity in
Ercoupe discussions and documents as minimum flying speed (with a
particular elevator limit) is not quite the same as the wing’s real
stall speed. Heck, even the wing’s stall speed is spread out over a
range as it stalls at the root and the stalled zone moves
progressively outward, developing more and more turbulence which
causes burble (and stall warning) over the tail and interferes with
your ability to push the tail further downward.
I think you’ll find that the 60-65 mph “stall speed” on an E or later
model is what you get when the elevator hits the pre-loaded-spring
low-speed warning cushion.
(Query: did all split elevator Coupes have the low speed warning
cushion or, if not, when was it introduced and/or dropped?)
When you pull through, against the resistance of that spring, you
pull the elevator up to 20˚ and the touchdown speed goes down to about
the same as the 415-C and 415-CD.
Hartmut reports seeing airspeed numbers in the 30-38 mph range when
he’s pulled through the cushion to the elevator 20˚ up travel
position. I suspect those extreme numbers reflect airspeed indicator
deviations due to pitot static tube installation location. That
deviation is designed to be zero at 100 mph but increases slowly the
farther you get from 100 mph.
I’d bet a penny, maybe even a dime, that doing a triangular GPS vs.
airspeed calibration at that speed would show true speeds closer to
48-50 or so. I don’t think the wing, airframe and weight are even
theoretically consistent with a stall speed below about 48 mph. As an
example, with a new ASI, in a plane with 9˚ elevator up travel, my ASI
reported an airspeed of about 50 at touchdown. That airspeed
calibrated within .25 mph at 100 mph.
Having said that, the ASI numbers for minimum flying speed, stall,
etc., should be consistent in that plane from day to day and, once
known, should be a reliable indicator of your aerodynamic condition.
I’ve only got about 10 hours in a split elevator E model but my
observations (and I was experimenting with it quite deliberately)
matched this. Any further data or observations are welcome.
Ed
