[NSRCA-discussion] Trimming question.

Tue Sep 12 06:07:39 AKDT 2006

Yes.

Depends on the design.

IMHO, the "best" designs and "best" setups are more tolerant (relatively) of changes in weather and airspeed.  Endless combinations and compromises as many things in pattern.

Regards,

Dave

-------------- Original message -------------- 
From: colin chariandy <cchariandy at yahoo.ca> 

I tend to agree. So what's the best way to neutralize that effect? Is there a way to set-up the airplane so that the trim changes as the airspeed/air density changes will be minimized? 

I get the feeling that there are set-ups that are conditions specific and those that are more versatile.

Which are the key variables for lift/pitch and airspeed....cg, wing incidence, tail incidence, engine thrust line, aileron droop etc.? 

I'm going to play with cg and aileron droop this weekend. I assume it will be a bit cold...it is Canada after all.

Colin.

Amar Shan <shan at telus.net> wrote: 
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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