Aerobatic Trimming Chart

From RCU - Chip Hyde addresses his view of Engine/Motor thrust.

 I run almost no right thrust in my planes and use the thottle to rudd mix at 2% left rudd. to throttle at idle. Thottle is the master rudd the slave. This makes the planes fly so much better when inverted at idle, they do not have that funny yaw through the corners. I have all but stopped using right thrust and do it all with the mix. What happens is the fin moves to the opposite side when inverted but the engine and prop don't know it, so the plane starts to yaw really bad due to spiraling slip stream.

To set this up just go out and fly the plane straight away from yourself at full throttle and then go to idle and watch to see which way it yaws start with 2% and start it from half throttle on down.

Trim it a full throttle with you rudder trim, and when you go to an idle if the plane yaws left you have to much mix, yaws right there's not enough. Have this screen up at take off and a buddy that can adjust it for you. Once you have it going nice and straight at idle and full go out and do a stall turn at the end of the box, on your way down do a half roll and exit inverted, you will be amazed how well it will go through that corner.

Do that first before any other trimming. After that go with CG(45 degree upline 1/2 roll and it should stay on that line with little or no down elev.) then ail differential(1 Degree More up the Down) verticals(wing incidence .5-1 degree positive, then put the stab where ever it needs to go get the elevators to be neutral, you will at this time mix 1-2% down elevator with throttle for your down lines), and knife edge last (start with 5% up elevator with rudd.)

Let me know how it goes and remember trim smarter not harder<G> 

                                             Chip Hyde

 

Before you get to the chart, on March 30, 2004, Buddy Bramer, of Nederland, Texas, wrote one of the most concise summaries I have EVER read concerning the interaction of elevator pitch trim, CG, and incidences.  I felt his version was so concise and clear that I'd post it here for all to read.  He graciously consented....contact him for any questions, but he deserves credit for doing a terrific summary!!

I am no aerodynamics expert, but I have learned some basic's over the years that work for me.


1. The wing must have a positive angle of attack to create lift.

2. The stabilizer and elevators steer the pitch attitude of the airplane resulting in more or less lift.

3. Balance can change the angle of attack which will change the pitch attitude.

4. Aileron's up or down will change the pitch attitude.

5. Up or down thrust will change the pitch attitude.

Therefore:

A. Increasing the positive wing incidence will cause the airplane to climb, which will require down trim to achieve level flight.

B. Likewise adding tail weight will cause the airplane to climb and require down trim to achieve level flight.

C. Added weight does not alter the vertical down line BUT elevator trim added to maintain level flight because of the added weight will effect the down line. Tail weight will require down trim which will steer the airplane more to the belly in a down line, and added nose weight will require up trim which will steer the airplane to the canopy.

D. In the vertical up line elevator trim and the engine thrust line will effect the vertical and/or yaw direction 

All of the above will effect the knife edge flight of the model.

Combinations of adjustments to any or all of the above trim items done at the same time will leave you confused and frustrated.      (This is the MOST important Principle of Trimming!!!! - Bob P.)

Most models will require some mixing to achieve near perfect lines in all flight attitudes in order to minimize / eliminate control inputs by the pilot to maintain a perfect course.

I have yet to see the perfect airplane should such an animal exist, but I keep looking and building since I truly believe that nothing is impossible.
When trimming an airplane make a written log of every change no matter how small so you don't have to start over when changes are not correct.

Buddy  mailto:BUDDYonRC@aol.com 

 

These tests assume that the plane has been built perfectly aligned, wings square to fuse, stab in line with wings, vertical fin is exactly 90 deg. to horizontal stab.  Rudder is perfectly aligned with fin chord-line.  Wings are not warped (checked with an incidence meter) or bowed, ailerons are perfectly aligned with the chord line of the wing airfoil and elevator halves are aligned with the stab chord line. Both ailerons and both elevators have identical travels and "track" (matched movement throughout their entire travel range) when measured.  Engine thrust, wing and stab incidence, and balance (CG) is set per the designer/ manufacturer's recommendations. 

FLIGHT  tests should be done in near calm conditions, as ANY wind can dramatically effect various observations. Double check each of the following tests before making any changes.  (If you have a capable helper, have him perform the SAME tests, before you change anything!)  

There is not a more critical component of aircraft trim setup than PROPER Center-of-Gravity!!!!  It MUST be correct, and is ONLY correct on a given wing/model design for a particular weight.

 These tests are listed in the recommended order for best results.

 

Procedure

Results

Adjustments
Control Neutrals Response to centering each control stick Adjust trims for straight & level flight Adjust clevises to center transmitter trims
Control Throws Apply YOUR preferred stick deflection to each control Check for response of each to suit YOUR desired rate Change control horns, ATV, and Dual Rates as needed
Center of Gravity Roll into nearly-vertical banked turn (at least 60 deg.) A. Nose Drops

B. Tail Drops

 A. Add tail weight

B. Add Nose weight
Up/ Down Thrust, #1 Straight & Level [90 deg to any wind] at NORMAL cruise speed then cut throttle.  

NOTE: EITHER change B or C requires retest of Decalage and Verticals

 A. Model continues level flight with a gradual drop

B. Model abruptly dives

C. Model abruptly climbs

 A. No Change

 

B. Remove UP thrust (or add DOWN)

 

C. Add UP thrust (or remove DOWN)
Up/Down Thrust, #2 Straight & level, then pull up [90 deg to any wind] sharply at NORMAL cruise speed to vertical; neutralize elevator

NOTE:  EITHER change B or C requires retest of Decalage and Verticals

 A. Model continues straight up

B. Model goes to canopy

C. Model goes to belly

 A. No Adjustment

B. Add down thrust

C. Reduce down thrust
Decalage, Angle of Incidence Power off vertical dive [90 deg to any wind] from high altitude (NEUTRALIZE elevator); see NOTE: at bottom.

A. Model continues straight down

B. Model goes to canopy

C. Model goes to belly

A. No change needed

B. INCREASE wing incidence

C. REDUCE wing incidence
Knife Edge Pitch Normal Cruise speed pass; roll to knife edge, left and right, use rudder to hold model level.

NOTE:  EITHER change in B or C requires retest of Decalage and Verticals

 A. Model does not change pitch

B. Model pitches to canopy

C. Model pitches to belly

 A. No adjustment needed

B. Either move CG aft; or increase wing incidence; or mix down elevator with rudder

C. Reverse of "B"
Tip Weight - Test1 Straight & level, normal cruise speed, roll inverted, release aileron stick A. Model does not drop a wing

B. Left wing drops

C. Right wing drops

 A. No adjustment

B. Add weight to right tip

C. Add weight to left tip
Tip Weight - Test 2 Fly model directly into any wind toward you or away from you; Pull tight inside loop, repeat with outside loop  (be certain rudder trim is correct, and that NO aileron stick is inputted!) A. Model comes out with wings level

B.Model comes out with right wing low

C. Model comes out with left wing low

 A. No adjustment

 

B. Add weight to left tip

 

C. Add weight to right tip
Side Thrust Fly model away from you INTO any wind; Pull to straight vertical at  normal cruise speed.  DO NOT use rudder.

NOTE:  Rudder trim offset (if any) will cause IMMEDIATE yaw after radius.  Thrust problems will be obvious later in the line...and are usually more subtle.

 A. Model continues straight up

B. Model yaws left AFTER line established

C. Model yaws right AFTER line established

 A. No Adjustment

 

B. Add right thrust

C. Reduce Right thrust
Aileron Differential Fly model toward you;  pull to vertical; neutralize controls, half roll.

NO ELEVATOR/RUDDER during the ROLL!!!.

 A. No Heading Changes

B. Heading change opposite to roll command

C. Heading change in direction of roll command

 A. Differential settings OK

B. Increase differential

C. Decrease differential
Dihedral Normal cruise pass, roll to knife edge, left and right, use rudder to hold model level A. Model does not roll

B. Model rolls indirection of rudder

C. Model rolls opposite to rudder

 A. Dihedral OK

B. Reduce dihedral

C. Increase dihedral

NOTE - This portion of the trimming chart may be unclear for the following reason: 

In order to maintain UPRIGHT level flight, the wing of a plane with a symmetrical airfoil wing needs to have a positive Angle of Attack (AOA) usually less than 1 degree.  This positive angle provides the lift required to cause the plane to fly level.  This angle is established by the shape of the airfoil, and the weight it must lift.  A plane balanced slightly nose heavy (for pitch stability) will require slight up elevator trim for level flight. A plane trimmed in this manner will have a tendency to pull to the canopy on a straight, controls neutral, down line because the elevator is controlling the AOA of the wing. 

This positive AOA may ALSO be achieved by positive incidence change which requires down elevator trim to achieve level flight. Thus, a power-off down line should fall straight down with the controls neutralized. There are significant interactions between wing incidence changes and CG, therefore, it is most important that the CG of the airplane be established first.

 A zero/zero/zero, wing/elevator/thrust angle (decalage) will need MORE up elevator trim for level flight.   This will exaggerate the canopy pitch on down vertical lines.

Flight trimming an airplane is YOUR "personal preference" after the basic essential items are set.
 

Bob Pastorello and Verne Koester, July, 2000

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