[NSRCA-discussion] Rudder mix instead of right thrust?

[Dean Pappas]

Hi Bob, 
What you describe is pretty much what I have done for ages.
My real tack, these days is to make a plane that requires less right thrust, because when the thrust angle is low enough, the plane will fly properly with the rudder straight.
Unfortunately, some designs need all of three degrees and then some more rudder. Thes airplanes fly funny.
I think Nat has the right idea.
 
Oh, your paragraph describing the necessary throttle management ... BINGO!
later compadre,
Dean
 
 

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 Bob Richards
Sent: Tuesday, February 21, 2006 1:37 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Rudder mix instead of right thrust?


Dean,
 
Thanks for that information. I'll have to chew on that for a while.
 
FWIW, I have always taken a simplistic approach to right thrust and rudder trim.  With the plane properly balanced (laterally) and with the rudder at neutral, I pull the plane to vertical and see which way, if any, the plane deviates. This determines the need for thrust change. After the thrust is adjusted, I fly the plane from horizon-to-horizon with the wings level, and see that the plane tracks a straight line. Trim the rudder accordingly.
 
I repeat the above exercise several times, preferably with no wind, and with the plane in different orientations (left-right, right-left) in case there is wind.
 
My viewpoint is that the right/left thrust is most critical when the prop is heavily loaded, ie: vertical climb. In level flight, the right thrust alignment has little to do! with yaw trim, except for the slipstream effect which seems to always be taken care of by the vertical climb test.
 
The slipstream effect applies whether the plane is upright or inverted. IMHO, slipstream is best taken care of by engine thrust, NOT rudder trim, since it will always change with airspeed.
 
Any other rudder inputs needed are usually (maybe always?) a result of gyroscopic precession, which is a dynamic situation that can't be trimmed out by a static control mix. This is why you might need left rudder input on an inside corner at the top of a square loop. I had a plane that required lots of right rudder on the outside corners, and slight left rudder on inside corners. No trim could take care of that, I just had to learn the right stick inputs (muscle memory).
 
As for the bias on spin entries, inverted vs upright, this could be affected much more by tiny differences in the left/right! wing panels, incidences, etc. Basing a rudder trim setting on this could be risky.
 
The programmable mix that you incorporated, above 2/3 throttle, is interesting. It does require learning proper throttle management, as you suggest. But, again, the airspeed will dictate how much mix you will need. So, don't give throttle above 2/3 until you have pulled the nose vertical and are at the vertical terminal velocity, THEN give full throttle. :-)
 
I'm not in any way disputing your results. I have a tendency sometimes to play the devil's advocate.
 
Bob R.
 


Dean Pappas <d.pappas at kodeos.com> wrote:

Hello Vicente,
The trimming adjustment procedure for right rudder with throttle couple I will describe was developed with the Excelleron that I reviewed for Model Aviation. I synthesized this method, based on the several different methods I heard described. As Jason said, some of them did not fly right. The Excelleron is a very good flier, and with a dependable muffled 120 two-stroke I would use it a low-hassle practice ship. Like most airplanes, it required about 2-3/4" of right thrust and a small amount of right rudder to make the plane trim normally. The predictable unwanted effects in spin entries and loops (especially the outsides) were present but not unusually bad.
 
So what I did was 
1) Set the rudder dead straight and not touch the rudder trim lever again.
Confirm that this is really the middle with repeated upright and inverted stalls/spin entries, to make suer the plane does not have a bias to either side.
2) reduce the right thrust to a "guess" setting that is enough for whatever throttle setting and speed you fly your normal level lines.
You test this with shallow climbs at 2/3 throttle,  and small loops at that same throttle. I ended up with about 1-3/4 degrees. 
3) Set up a throttle into rudder curve mix and keep the percentages at zero from idle up to 2/3 throttle. Then add a slop from there to full throttle,
and adjust those upper coupling ratios to get a straight vertical line.
 
 
Limitations: 
Never fly above 2/3 throttle in level flight.
Never push the throttle up too soon before pulling or pushing to a vertical.
Avoid sudden throttle changes
 
In other words, this is not appropriate if you have not mastered purposeful throttle management.
 
Hope this helps,
Dean P.
 
 
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 

 

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