FT 3D - trimming for knife edge?

nickf

Member
Hi,

I have just finished one of these and am setting it up to have ago at 3D. It's built true and flies straight and level with neutral controls at the recommended cog. If I do a vertical dive, it pulls out slightly but tracks straight down and as far as I can tell does the same with a vertical pull up.

When I come to try and do a knife edge (not skilled in this!) it needs alot of down elevator to stop it turning 'in'. I was expecting it again to be quite neutral and surprised at the amount of down needed, given that the elevator is basically trimmed straight.

I'm not sure this is right and if its not what do I need to look out for, given that it seems to be trimmed ok for everything else?

Any comments welcome!!

nick
 

PsyBorg

Wake up! Time to fly!
Sounds like your CG is not actually perfect and you have some down in your base elevator settings to keep it level in straight forward flight. Or your elevator is not truly in line with the main wing.
 

nickf

Member
Sounds like your CG is not actually perfect and you have some down in your base elevator settings to keep it level in straight forward flight. Or your elevator is not truly in line with the main wing.
Hi,
Thanks for the reply. I'll check with a again.
So the way of trimming this properly would be to, with a straight edge trim the ele until its in-line with the rest of the stab and then adjust the cog/battery position until it flies level?
nick
 

PsyBorg

Wake up! Time to fly!
The center line of the wing and elevator from what I understand must be level and in line with each other to be true neutral for 3d. Since the wing is "symmetrical" (human and foam aside) it should be perfectly in line with a 0 angle of incidence (maybe angle of attack) I believe is the term?

Anyway if the elevator is not perfectly level and inline with the main wing it will not track straight on all axis. If you think about it. You say your trim is inline on the elevator (flat and straight), that your plane flys level, which makes it good on the pitch axis in level flight. But when you roll over on its side it tracks in a manor where you have to use the elevator to keep it in line. The input direction downward tells me that your elevator (not the control part) is at a slight downward angle in relation to the wing.

So to me I would think you have compensated for that misalignment by setting CG more nose heavy to keep it from dropping in straight forward flight if the elevator is in fact angled down slightly. I am not totally sure but it could be possible that the motor has an upward thrust component possibly producing the same result however I would think that would effect forward level flight with a rise on throttle inputs.

I hope that maybe one of the more fixed wing experienced guys pop in to confirm some or any of this or if it may be wrong they can guide you better.

Angles.jpg
 
Last edited:

nickf

Member
Thanks PsyBorg.

I checked the trim last night and it was out by a few degress which I guess with a large elevator would make a big difference. Its windy here until the weekend so I'll give it a shot then!

Do you have an FT 3d?

nick
 

foamtest

Toothpick glider kid
I build and "flew" a FT3D a while ago but I don't have much experience with the bird yet. (flew is in quotations because I took the receiver out of another model and didn't know that the elevator had 150% throws... the rest of the story plays out from there)

I am planning on flying it next week around this time however so I'll try the same with mine and see if it does the same thing. Also have you tried flying inverted with it? If what PsyBorg is true then the plane will try to do a split S once you go inverted.

I hope this helped
 

PsyBorg

Wake up! Time to fly!
I have not officially flown fixed wing yet as I have not completed a full pack flight. that said I have not tried as often or as much as I should. I was just thinking of your conditions and what could possibly be causing them. I have build an FT 3d but have run out of props (only started with 4) before I found a flyable CG.
So when I get around to repairs and get new props on order I will film the flights and thus further embarrass myself.

This is what it started as.. I can build em well enough I think.

FT3d 1.jpg
 

mrjdstewart

Legendary member
thats a good looking plane Psy.

i built my 3D from DTF and messed up the wing incidence. it would fly, but @ a stupid high alpha. it is now the prettiest failure i have hanging from my roof. beautiful plane, spent hours making it so, then bit the dust when i burnt the day lights out of my hand glueing in the main wing. the rest is history. will try again someday...

laters,

me :cool:
 

Bricks

Master member
Here is the best trimming wright up I have seen it may explain what is happening. Have to take into consideration you are dealing with foam which does not stay true or build true. I run thru most of this for my Nitro, gasser planes and a few of my 3D foamies with symmetrical wings.

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. Thrust, incidence, and balance (CG) are set according to the designer's recommendations. The wings are not warped as checked with an incidence meter, and the elevator halves are moving together as checked by a "Throw Meter". These flying tests should be done in near calm conditions. Double check each of the following tests before making any changes. The most critical component of aircraft setup is finding the proper Center-of-Gravity. It must be correct for each airplane, regardless of differences due to building variables and weight. Because of this requirement, it is important that this trim chart be followed in the order in which it is written.

Test for


Procedure


Results


Adjustments
Control Neutrals test response to each control Adjust trims for straight & level flight adjust clevises to center xmter trims
Control Throws
Apply full deflection of each control
Check for response; Aileron hi rate 3 rolls in 3 secs. Elevator, square loop corners Rudder, 35 to 40 Deg.
Change control horns, ATV, and Duel Rates as required
Center of Gravity Method 1
Method 2
1. Roll into a vertically banked turn
2. Roll into inverted flight
1. A. Nose Drops 1. B. Tail Drops
2. A. lot of down required to hold level flight
2. B. up elevator needed to hold level flight
A. Add tail weight
B. Add Nose weight
(see Note A at bottom)
Up/ Down Thrust, test 1
Fly model straight & level, 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. Increase down thrust
C. Reduce down thrust
Up/Down Thrust, test 2
Fly model straight & level, then pull up Note Either change B or C requires retest of Decalage and Verticals
A.Model continues straight up
B.Model pulls to canopy
C.Model pulls to belly
A. No Adjustment
B. Increase down thrust
C. Reduce down thrust
Decalage, Angle of Incidence
Power off vertical dive from high altitude (neutralize elevator) (see Note B at bottom)
A. Model continues straight down
B. Model pulls to canopy
A. No change needed
B. Increase wing or stab incidence
Page 1
Trimming Chart
Incidence
(see Note B at bottom)
C. Model pulls to belly C. Reduce wing or stab incidence
Knife Edge Pitch
Fly model on normal pass, roll to knife edge, left and right, use rudder to hold model level
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
Fly straight; level, 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 towards you / away from you, pull tight inside loop, repeat with outside loop
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 and pull up to vertical
A. Model continues straight up
B. Model veers left
C. Model veers right
A. No Adjustment
B. Increase Right thrust
C. Reduce Right thrust
Aileron Differential
Fly model toward you, pull into a vertical climb before it reaches you. Neutralize controls then half 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
Fly model on normal 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
Page 2
Trimming Chart

Note A:These two methods for determining the C.G. of a model will give approximate results only. Start out with the C.G. where the Designer suggested, or somewhere between 25% to 35% of the Mean Aerodynamic Cord. The optimum C.G. for your model will require further testing while performing maneuvers. The results will only be an approximation at best.
Note B:This portion of the trimming chart may be unclear for the following reason; In order to maintain level upright 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. If the plane is balanced slightly to the nose heavy side (required for pitch stability), it will require a slight up elevator trim to hold level flight. A plane with a zero/ zero wing to elevator angle will also need a slight amount of up elevator trim to hold level flight. Therefore, a plane trimmed in this manner will have a tendency to pull to the canopy on a straight, thumbs off, down line because the elevator is controlling the AOA of the wing.
This positive AOA may also be achieved by a positive incidence change, which requires an offsetting down elevator for level flight. Thus, a power-off down line should fall straight down, with neutral controls. There are significant interactions between wing incidence changes and CG, therefore it is most important that the C.G. of the airplane be established first.
In the final analysis, flight trimming an airplane is a personal preference issue after you have taken care of the basic essentials
 

PsyBorg

Wake up! Time to fly!
Nice Post Bricks. I copied that and have it on my desk top so when I get back around to the FT3d I will have a bit more of a clue what I am doing with it. Thank you good sir!
 

AcBates

Junior Member
Does sound like the model is nose heavy... I think as I was flying it, it was rather coupling free until you got near extreme rudder input then it started to have some slight roll coupling (which isn't really worth the bother to remove). In general, if in knife edge the plane is pulling to canopy (requires down elevator to stay straight), move the pack back a bit and repeat... if it's pushing to the gear, you can bump the pack forward a bit. In general I fly my planes a little ahead of "neutral" (pull up 45 degrees and then roll inverted... my planes will gently fall in a long arc, so require a smidge of down elevator to stay level inverted).

The list from Bricks is pretty solid, though lengthy, and some of the changes are pretty drastic (changing dihedral can be a hassle)... if you have a computer radio, you can use programmable mixes to correct for these issues. (for example, if you like the current balance, but the plane is tucking in knife edge, you can add a mix that tweaks elevator whenever you apply rudder). Can provide more details if needed.
 

nickf

Member
Hi, thanks for all the replies.
The 3D seems very sensitive to cog and just moving the battery a cm resulted in it changing from pulling to canopy to pretty much neutral. Great trimming guide, really makes a difference to setting up a plane properly!
nick