[NSRCA-discussion] Trimming question.
Amar Shan
shan at telus.net
Mon Sep 11 16:14:08 AKDT 2006
Tom had exactly the right idea when he thought it was an "air density"
thing.
To begin with, the air density *does* change substantially in going from 15C
to 30C. It's the equivalent of moving from sea level to 1700 feet
altitude!
The second thing to realize is that the plane's air speed (indicated air
speed to the full scale pilots out there) changes with changing air density.
In other words, if the plane appears to be moving at the same speed on a 15C
day as a 30C day FROM THE GROUND, the airflow over the wings is actually
much greater on the cooler (15C) day.
Now, when you trim a plane for flight, you trim for its indicated air speed
(ie, the amount of airflow over its wings). If you are flying slower, you
need a greater AOA, of course. When you take a plane trimmed for 30C
weather, and fly it on a 15C day, the airflow over the wings is much
greater - hence, you need much less AOA, and you have to trim with more down
elevator.
Cheers,
Amar
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Ed Miller
Sent: September 11, 2006 4:54 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Trimming question.
I have the same issue with the EMC I'm flying. At summers 90+
temperatures using a Futaba 9ZAP radio I have 2% up elevator with right
rudder and 4% up with left rudder for straight knife edge flight. I noticed
when the temps cooled to the 70's lately I had to put 2 % down elevator trim
in to fly straight and level at horizontal flight cruise rpm.. Because of
that touch of down elevator trim in cool weather, I end up with a slight
push to the belly in knife edge in both directions. Plane has the DEPS
system installed. I think what is happening is the difference in growth
between the fiberglass/wood fuse and carbon fiber pushrods is causing the
slight elevator trim change during the temperature change which affects the
knife edge flight. Or maybe the cooler denser air requires less AOA in the
wing. I don't see how enabling the trim function in the rudder to elevator
mix will fix this as the rudder is the master channel. Unless I
misunderstand, only a rudder trim change would effect the elevator movement
in the mix. I'm open for suggestions...........
TIA.....Ed M.
----- Original Message -----
From: Rex LESHER
To: NSRCA Mailing List
Sent: Monday, September 11, 2006 6:08 PM
Subject: Re: [NSRCA-discussion] Trimming question.
actually, I think I confused myself... I meant to say, all offset will
be based from the current trim setting, not from the original setup
position....
----- Original Message -----
From: Rex LESHER
To: NSRCA Mailing List
Sent: Monday, September 11, 2006 3:02 PM
Subject: Re: [NSRCA-discussion] Trimming question.
Just curious.... what radio are you using? Do you have mixes set up
for roll coupling? If so, do you have the trims set to "active" in the
mixes? If not, this could be the reason for the pitching. Without active
trim in the mixes, all offset will be based from the original servo position
in setup, not from the current flying position after the trim was moved.
Rex
----- Original Message -----
From: Koenig, Tom
To: NSRCA Mailing List
Sent: Monday, September 11, 2006 2:42 PM
Subject: Re: [NSRCA-discussion] Trimming question.
All-Dean,
I think you could be onto something there. I like to fly on the
edge of nose heavy, and I could sympathize with Colin. My Synergy exhibits
almost the same tendencies. It is not a heavy plane at 4.7 kg, but the CG is
forward. Hmmmmmmm-you boys have me thinking again. I notice it biggest when
travelling as every field I go to is at lower altitude. The biggest and
costliest time was during the Trans Tasman comp in New Zealand-it was like
flying through soup!
I have always assumed it was an air density thing. I have
experienced exactly what Colin describes.
Tom
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Dean Pappas
Sent: Tuesday, 12 September 2006 3:36 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Trimming question.
Hi Colin,
By any chance, is your plane relatively heavy, have a particularly
high wing loading, propped to fly very slowly, or set up relatively
nose-heavy?
The reason I ask is that back in the late eighties, I clipped the
wings of one of my early Turnaround designs in order to fly faster.
This was back when we were in the middle of a wild overcorrection
from flying slow and small in this country to flying fast, fast, fast! Then
things started to settle down.
Actually, I clipped it twice, once from 68" span to 64" and then to
60. I should have left the plane at 64" span. When I made the second clip,
the plane developed
almost the same symptom you described. It pulled to the canopy in
the vertical down line when the elevator was trimmed for a hot day.
It also looked like it flew droopy-drawered (tail low) on hot days.
I moved the CG back until the elevators looked like they were in exactly the
same spot, and lived with what little was left.
There was no suitable electronic fix back then, with either a Futaba
PCM 512 or a JR PCM9.
My hypothesis is that the sensitivity to air density happens when
the plane requires lots of "up" trim to fly. What do the elevators look like
when trimmed on a hot day?
later,
Dean Pappas
Sr. Design Engineer
Kodeos Communications
111 Corporate Blvd.
South Plainfield, N.J. 07080
(908) 222-7817 phone
(908) 222-2392 fax
d.pappas at kodeos.com
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of colin
chariandy
Sent: Monday, September 11, 2006 9:53 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Trimming question.
No, I'm using the MK bellcrank.
If the problem was only related to the length of the pushrods,
then when I re-trim the knife edge performance should return to normal.
Thats not the case. The plane pushes to the bottom in the cold and is dead
straight on hot days.
Colin.
Lance Van Nostrand <patterndude at comcast.net> wrote:
My guess is you have the deps system. carbon fiber does not
have as much shrinkage as the rest of your plane.
--Lance
----- Original Message -----
From: colin chariandy
To: NSRCA Mailing List
Sent: Sunday, September 10, 2006 11:09 PM
Subject: [NSRCA-discussion] Trimming question.
I noticed recently that I need to add a bit of down trim as
the air temperature drops....maybe as much as 4 beebs going from 30+ days to
15C. That upsets the knife edge trim on the aircraft.
Is that normal, or do I have something set a bit marginally,
like CG or wing incidence?
Do you guys typically have a "cold weather set-up" ?
Colin.
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