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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Bob Patterson
Hi Al !

Where is your C of G - empty, and in your cruise config. today ??
Actually calculated from weights on float wheels ..

That much up elevator sounds like it might be a bit forward, which
could account for some of the odd behavior ....

Might be interesting to put say 80 lb. in the baggage compartment,
with 2 up, and see if there's any difference. (Or, as Wayne often
suggests, add some water to the aft compartments on each side....)
(AFTER judicious calculation, and only a bit at a time !!)

Friction could account for some of it .....
Just thinking out loud -
if the horn is already down, and you push it further down, (back stick
pulse), and release, it would likely tend to stay down, being already
deflected, and having air pressure on it (as well as being trimmed there).
If you pull it up, (forward pulse), you would expect it to
return to neutral, which, with a forward c of g, would give a positive,
increasing, descent......

Maybe, to better evaluate, the trim should be set to neutral,
and the test re-tried --- suspect result would be positive descent
in both cases - again, indicating forward c of g as contributing factor.

Sounds like the drag of the floats is pulling the nose down,
and, since drag increases with speed, that would account for the
increasing rate of pitch change with speed..... and the stab/elevator
combo is not producing enough opposing (downward) force ....
...which suggests - a different angle or increased area for the stab,
OR , back to the c of g being too far forward.... (I suspect most likely)

Please keep us posted on test results !!

......bobp

-------------------------------orig.-------------------------
On Sunday 02 October 2005 08:19 pm, Alan Hepburn wrote:
Scott:

Good point. I'll put a mark on the cable and another on the floor, and
see
if that's what's happening. There is some friction due to these fairleads
along the cabin floor. It's nothing like a pushrod system, but far better
than the rudder loop.

I also notice that, at least in today's configuration, we were cruising
with
abut 5 degrees up elevator. Any comments on whether that's normal?
That's
with two up and of course the amphib floats. Cruises about 97 kts. It
may
just be what it takes to support the floats.

Al

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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Alan Hepburn
Further to my last on this, and following a conversation with Sean White,
the divergent behaviour when pitching down may be due to the low centre of
drag with floats installed. I'd be interested to know if Rebels on floats
have a similar characteristic.

Also, it's not surprising that you need some up elevator when cruising on
floats. The airplane is maybe 300 lbs heavier than on wheels, and cruises
about 25% slower, which means 50% less lift for the same angle of attack.
Then there's also that lower centre of drag, which generates a nose down
pitching moment. Taking all this into account, you'll need a significantly
increased AoA to stay level - i.e. some up elevator. Again, I'd be
interested in the experience of others on floats - not just Eletists. Take
a peek out back next time you're cruising along, or put a mark on the
controls at 0 elevator deflection, and see what the level flight position
looks like,

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Wayne G. O'Shea
Hi Al! Never bothered to measure it on mine (REBEL) 'till you posted this.
Whether it be on wheels or floats my elevator tips have always been about
their width below the leading edge in cruise. Measured it today and that's
5* up Elevator. Most seem to fly this way...except maybe BobP's and Howard's
that I intentionally mounted the stabs on at .7 to .8 degrees instead of the
MAM spec'd 1.0 degrees leading edge up because of this condition I noted on
mine early on.

My Rebel has always stayed in what ever control situation I put it in. I
have my elevator cables a tad tight...but I like it that way. Takes out
twitchiness by friction damping and less worry letting go of the stick on
final..etc.

Wayne

----- Original Message -----
From: "Alan Hepburn" <ahepburn@renc.igs.net>
To: <rebel-builders@dcsol.com>
Sent: Monday, October 03, 2005 7:50 AM
Subject: Re: Elite - Pitch Stability

Further to my last on this, and following a conversation with Sean White,
the divergent behaviour when pitching down may be due to the low centre of
drag with floats installed. I'd be interested to know if Rebels on floats
have a similar characteristic.

Also, it's not surprising that you need some up elevator when cruising on
floats. The airplane is maybe 300 lbs heavier than on wheels, and cruises
about 25% slower, which means 50% less lift for the same angle of attack.
Then there's also that lower centre of drag, which generates a nose down
pitching moment. Taking all this into account, you'll need a
significantly
increased AoA to stay level - i.e. some up elevator. Again, I'd be
interested in the experience of others on floats - not just Eletists.
Take
a peek out back next time you're cruising along, or put a mark on the
controls at 0 elevator deflection, and see what the level flight position
looks like,

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Walter Klatt
Mine is fairly pitch neutral as well. No trouble flying hands
off, and holding altitude. But not sure if it would come out of a
dive by itself.

Walter
-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Wayne G. O'Shea
Sent: Monday, October 03, 2005 5:12 PM
To: rebel-builders@dcsol.com
Subject: Re: Elite - Pitch Stability


Hi Al! Never bothered to measure it on mine (REBEL)
'till you posted this.
Whether it be on wheels or floats my elevator tips
have always been about
their width below the leading edge in cruise. Measured
it today and that's
5* up Elevator. Most seem to fly this way...except
maybe BobP's and Howard's
that I intentionally mounted the stabs on at .7 to .8
degrees instead of the
MAM spec'd 1.0 degrees leading edge up because of this
condition I noted on
mine early on.

My Rebel has always stayed in what ever control
situation I put it in. I
have my elevator cables a tad tight...but I like it
that way. Takes out
twitchiness by friction damping and less worry letting
go of the stick on
final..etc.

Wayne

----- Original Message -----
From: "Alan Hepburn" <ahepburn@renc.igs.net>
To: <rebel-builders@dcsol.com>
Sent: Monday, October 03, 2005 7:50 AM
Subject: Re: Elite - Pitch Stability

Further to my last on this, and following a
conversation with Sean White,
the divergent behaviour when pitching down may be
due to the low centre of
drag with floats installed. I'd be interested to
know if Rebels on floats
have a similar characteristic.

Also, it's not surprising that you need some up
elevator when cruising on
floats. The airplane is maybe 300 lbs heavier than
on wheels, and cruises
about 25% slower, which means 50% less lift for the
same angle of attack.
Then there's also that lower centre of drag, which
generates a nose down
pitching moment. Taking all this into account, you'll need a
significantly
increased AoA to stay level - i.e. some up elevator.
Again, I'd be
interested in the experience of others on floats -
not just Eletists.
Take
a peek out back next time you're cruising along, or
put a mark on the
controls at 0 elevator deflection, and see what the
level flight position
looks like,

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Alan Hepburn
Walter:

This may, of course, be what's left of the Elite pitch stability problem.
It actually doesn't depart from the trimmed attitude on its own, or if it
does, it must do so extremely slowly - I should check that - but when you
give it a prod, that's all it takes to start what would has all the
appearance of being an outside loop, if you left it long enough and the
wings didn't come off first!

And Wayne:

Interesting that only a fraction of a degree on the stab incidence is
enough. You could almost fix that with shims under the stab attach angles.
I don't know how the Rebel stab is attached, but on the Elite it's just
bolted to an aluminum angle front and back.

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Wayne G. O'Shea
Rebel mounting is on a vertical face..vs your horizontal face brackets. If
you are always carrying 5* up elevator in cruise....I'd start shimming those
rear attach brackets up and see if you can get rid of the drag of carrying
up elevator. Oh what I loved about my old C182 and it's trimming stab!

Wayne

----- Original Message -----
From: "Alan Hepburn" <ahepburn@renc.igs.net>
To: <rebel-builders@dcsol.com>
Sent: Tuesday, October 04, 2005 8:20 PM
Subject: Re: Elite - Pitch Stability

Walter:

This may, of course, be what's left of the Elite pitch stability problem.
It actually doesn't depart from the trimmed attitude on its own, or if it
does, it must do so extremely slowly - I should check that - but when you
give it a prod, that's all it takes to start what would has all the
appearance of being an outside loop, if you left it long enough and the
wings didn't come off first!

And Wayne:

Interesting that only a fraction of a degree on the stab incidence is
enough. You could almost fix that with shims under the stab attach
angles.
I don't know how the Rebel stab is attached, but on the Elite it's just
bolted to an aluminum angle front and back.

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:43 pm
by Walter Klatt
You know, I think I tested all that a long time ago, but can't
remember exactly what it did. Will have to try it again next time
I'm out, if I remember.

Walter
-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Alan Hepburn
Sent: Tuesday, October 04, 2005 5:20 PM
To: rebel-builders@dcsol.com
Subject: Re: Elite - Pitch Stability


Walter:

This may, of course, be what's left of the Elite pitch
stability problem.
It actually doesn't depart from the trimmed attitude
on its own, or if it
does, it must do so extremely slowly - I should check
that - but when you
give it a prod, that's all it takes to start what
would has all the
appearance of being an outside loop, if you left it
long enough and the
wings didn't come off first!

And Wayne:

Interesting that only a fraction of a degree on the
stab incidence is
enough. You could almost fix that with shims under
the stab attach angles.
I don't know how the Rebel stab is attached, but on
the Elite it's just
bolted to an aluminum angle front and back.

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:56 pm
by Alan Hepburn
When I was out doing the climb test yesterday, I established that the
divergent response when you prod down elevator is due to the stick not
returning to its original position. The stick in fact stays a good inch or
so ahead of the trim position. There seem to be three possible reasons:

1) Too much friction in the elevator loop
2) The shape of the leading edge of the spades
3) The blunt (1/2") trailing edge of the elevator

There's not a whole lot of friction, so I think (1) is out. Jack Wiebe is
going to a distinctly rounded LE on his spades, so the result of that will
be interesting. I can't see (3) being an issue, since many aerobatic
aircraft use blunt TEs to establis a "break out force", but I don't know
enough about aerodynamics to know. So, looks like I wait an see how Jack's
turns out.

Al





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Elite - Pitch Stability

Posted: Sat Feb 18, 2012 1:57 pm
by Bob Patterson
Hi Al !

I suspect you are correct - that flat leading edge on the elevator
and rudder horns causes lots of disturbed airflow !!

FWIW, the original Elites all had that 'dive of death' thing -
hands off cruise, and a small bump (thermal or cat), and you needed
2 hands to get it back up again !! Good thing the horns are now
half size - now just need the fronts rounded on all 3 !

(Probably "A Good Thing"(tm) for the Moose, too !)

.......bobp

-------------------------------orig.-------------------------
On Friday 07 October 2005 07:42 pm, Alan Hepburn wrote:
When I was out doing the climb test yesterday, I established that the
divergent response when you prod down elevator is due to the stick not
returning to its original position. The stick in fact stays a good inch or
so ahead of the trim position. There seem to be three possible reasons:

1) Too much friction in the elevator loop
2) The shape of the leading edge of the spades
3) The blunt (1/2") trailing edge of the elevator

There's not a whole lot of friction, so I think (1) is out. Jack Wiebe is
going to a distinctly rounded LE on his spades, so the result of that will
be interesting. I can't see (3) being an issue, since many aerobatic
aircraft use blunt TEs to establis a "break out force", but I don't know
enough about aerodynamics to know. So, looks like I wait an see how
Jack's
turns out.

Al

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