BD3SX FAQ for Newcomers

[rbei312]

Isn't putting the offset in sort adding a torque precomp so the tail won't have to work as hard to hold the heading thus helping to eliminate any tail wag?

[rhodesengr]

Originally Posted by rbei312

Isn't putting the offset in sort adding a torque precomp so the tail won't have to work as hard to hold the heading thus helping to eliminate any tail wag?

My answers is no. Other may have different opinions. The tail has to provide sideways thrust to balance the torque of the main rotor. Without this thrust, the helicopter will spin in the opposite direction of the main blades. When the helicopter is hovering, some sideways thrust is required in the same direction as applying right rudder. If the tail blades are neutral when the tail slider is centered, the gryo will move the tail slider to provide enough right rudder to keep the nose from turning. So at hover, the slider will be positioned part ways towards the boom (usually). There will now be less right rudder range left and more left rudder range. Setting the tail rod so there is some right rudder when the slider is centered is an estimate so the slider is more near center during hover. This gives more nearly equal control range in both directions. It has nothing to do with wag.

[ArchmageAU]

Originally Posted by rbei312

Along those same lines as the last post. Does the BD require an offset for the tail neutral position. I know some FBL units do and some don't.

Essentially no. Rudder input changes direction the FBL unit thinks the heli should be pointing. The FBL unit uses the gyro to detect the rotation in the tail, then adjusts the tail servo more than 50 times a second to try ensure the commanded direction is attained/maintained.

Follow the models mechanical setup for mid-servo throw, then set the BD3SX to know the limits of the tail servo. Ideally the limits of the tail servo should be pretty even if the mechanical setup is right.

[rbei312]

On my 550L and generally all of my align helis call out an offset for neutral tail position. On the 550 it's 10mm. On the 450's and 250 it's 4mm. I've also seen people tell others with tail wag issues to do this.

[rhodesengr]

Originally Posted by rbei312

On my 550L and generally all of my align helis call out an offset for neutral tail position. On the 550 it's 10mm. On the 450's and 250 it's 4mm. I've also seen people tell others with tail wag issues to do this.

If you want a broader discussion about this, consider posting in the main forum why helis specify a tail offset. It is sort of OT for this thread. The reason for the offset is for the reason I explained above.

Wag is when the tail control loop becomes underdamped. Lower loop gain or increased loop damping will control the wag. Offset will not effect either loop gain or damping.

[spinner55]

So the bd3sx will do what is required to stabilize the tail. By having the slider in the middle at stable hover you get equal rudder left or right.

[ArchmageAU]

Originally Posted by spinner55

So the bd3sx will do what is required to stabilize the tail. By having the slider in the middle at stable hover you get equal rudder left or right.

Yep.

The input slider on rudder shows how much you are telling the unit to turn. (This is why you tune the TX to the unit). The BD unit uses a feedback control loop many times a second to ensure the tail stays where you told it to be.

Mechanical setup (smoothness/straightness) and tail gain determine slop, locked or wag.

[rhodesengr]

Originally Posted by spinner55

So the bd3sx will do what is required to stabilize the tail. By having the slider in the middle at stable hover you get equal rudder left or right.

yes but my point has been that with the slider in the middle, the blades are not neutral but effectively providing right rudder to balance the main blade torque at hover. Some guys go as far as to put the Tail gain negative (rate mode) for tail tuning. The tail is lock locked like it would be in heading Hold Mode. They will vary the tail control rod to get perfect torque balance in rate mode and then go back to HH. It is really not needed to get it that exact but it does illustrate the point.

[ArchmageAU]

Originally Posted by rhodesengr

yes but my point has been that with the slider in the middle, the blades are not neutral but effectively providing right rudder to balance the main blade torque at hover. Some guys go as far as to put the Tail gain negative (rate mode) for tail tuning. The tail is lock locked like it would be in heading Hold Mode. They will vary the tail control rod to get perfect torque balance in rate mode and then go back to HH. It is really not needed to get it that exact but it does illustrate the point.

How the loop works.
- What is our current heading (X)
- What is our commanded heading (Y)
- What is out current turn command (Z)
- What is the difference from our last heading (P)
- Add the P to the accumulated Integral (I) of the last n observations
- Add the adjusted Intergral to the accumulated derivative (D) square of differences.
- Work out adjustment to current position by a combination of: Z, P, I and D
- Add adjustment to current servo output (wherever it currently is).
- Repeat.

It does not matter if mid-servo is no tail pitch or positive tail pitch, the result is the same. Ideally the travel of the tail pitch from mid-servo should the same in each direction (but it's no catastrophe if it is not). Some natural counter-torque at mid-servo is normal.

Where it does matter is the extremes of tail travel. If the mechanical setup is too far out, then the tail will blow out quicker in one direction than the other.

You also need to ensure there is no servo binding at either extreme (and that each extreme can be reached).

Once the tail gets up to a speed it is affecting heli direction, then the PID feedback loop should keep the tail pointing where it should. The mid-servo reference point on the servo is mainly a guide on the deck.

[DEMONjoe]

My simple answer: Set the tail symmetric, done.
In case, some helis may need a but of pre-comp, but never more than 2 turns out on the rod.

[rbei312]

What is the meaning of symmetric? Zero pitch at neutral?

[ArchmageAU]

Originally Posted by rbei312

What is the meaning of symmetric? Zero pitch at neutral?

Yes

[jrvetter]

From FAQ above:
"21.Can I use my DX6 or DX7?Yes but you would be much better off with a DX8, DX9 or DX18. The DX6 does not have enough channels having only 6. To use the DX6i, you have to fix both the tail gain and head gain in software. This frees up Channel 5 (Gear) to control SL and CR. However, you only get a 2-position switch so you have to choose two of the three modes (Rigid, SL, and CR). So you could have Rigid and CR and SL and CR but not all three. Even though the DX7 has enough channels, I believe Aux2 is fixed to a 2-position switch. Channel 5 has a 3-position switch so if you give up TX control of tail gain, you can have all three modes, Rigid, SL, and CR.Top"

Will the new version of the DX7 have the limitation outline above? I have been all over the new manual and I cannot tell. It implies that there are 4 position switches and the R knob seem like it could be helpful as per the 3SX manual but I am not sure. I will be using just Sat's if possible on a 450 for space reasons but does that limit me? It is tough as a newbie to discern what is not current since so much changes.
Thanks!

[rhodesengr]

The new DX7 should be fine. I wrote the FAQ long before the new DX7 came out. You need 7 channels so you are ok there. If you are intent on running Sats-only, be sure to bind in DSMX with 11 ms frame rate. Looks like the US version of the new DX7 does have 11ms frame rate. The EU version does not. So you are OK there. You also need to be able to run the mixes listed in the DX18 sticky so you can have Rigid and SL on a switch and Rescue on the PB (switch I). You don't really need a 3-position switch but you do have them. I just use Switch A which gives me Rigid, and one level of SL and then Switch I for Rescue.

Since you seem intent on running Sat-only, also be sure you have a "stiff" 6V power setup. Talon ESC's would be ok or something else with a big BEC. Or an external BEC or even an LiFe RX pack. You should steer clear of ESC's with BEC's rated at only 3 or 5 amps and running at 5.6 V.

[rdlohr]

Originally Posted by rhodesengr

The new DX7 should be fine. I wrote the FAQ long before the new DX7 came out. You need 7 channels so you are ok there. If you are intent on running Sats-only, be sure to bind in DSMX with 11 ms frame rate. Looks like the US version of the new DX7 does have 11ms frame rate. The EU version does not. So you are OK there. You also need to be able to run the mixes listed in the DX18 sticky so you can have Rigid and SL on a switch and Rescue on the PB (switch I). You don't really need a 3-position switch but you do have them. I just use Switch A which gives me Rigid, and one level of SL and then Switch I for Rescue.

Since you seem intent on running Sat-only, also be sure you have a "stiff" 6V power setup. Talon ESC's would be ok or something else with a big BEC. Or an external BEC or even an LiFe RX pack. You should steer clear of ESC's with BEC's rated at only 3 or 5 amps and running at 5.6 V.

Although I trust Rhodes to be correct, I chose to get a full rx rather than a separate BEC after having multiple brown outs with Sats only.

Rick

[robini]

is there a video in english walking throught the new setup wizzard? The only one I can find is in German....

[rhodesengr]

did you watch the videos in this sticky?
https://www.helifreak.com/showthread.php?t=469594

It is not the latest version but there is really nothing different other than the appearance of the software and maybe the names of a few things. The process has not changed.

[Tapgorup]

Great thread. I have a 3sx on my 600 and love it. I currently have a 3D-Hv2 and plan to switch that out for a 3sx for my 450. I plan to use 1 Spektrum sat with my DX7 gen 2. So input voltage looks like an issue here. I currently have a YEP 40a 5.5v 5a ESC. I have a question in with HK tech support to see if this is a continuous 5.5v supply. I do not have HV servos. But, reading the threads, I might be cutting this close if there is any drop in voltage. Anyone else have experience with YEP Esc? Will this provide continuous 5.5v? I am also considering a UBEC from HK. It's a turnigy switching 5v or 6v, with 5a, 7.5a burst. Or a 250mah 6.5v Life rx pack. Any thoughts what is my best route here? Thanks for any help. Terry

[rhodesengr]

look at something like the Talon 35. The talons have high current BEC's and can be set a 6V or 8.4V if you have HV servos..

But the LiFe would also be a good way to go if you don't mind charging it and your servo will be ok.

[blade_crasher]

I'm running a 3sx on my stretched 450. I have a about 15 flights on it so far, and stewed about the voltage issues, etc. with SAT's in my setup also.

First off, I am not a believer in high end electronics - at least my skill set. Why spend a bunch of money on servos when the FBL unit is correcting any centering or other issues for me.

I decided on using a HK Blue Series 50 amp 5.5v 4amp stock sbec output. It has worked fine for me. My servos are TowerPro SG90MG's in Analog mode. The tail is an Align 520 (I think). I'm running 5v. The 3sx is SAT only and I'm using two.

Amp testing during an early flight showed I was pulling close to 50 amp, so I lowered my collective to 11 deg. Now that I have a bit more flight experience I"m going to install a YEP 60 to get back to 12-13deg collective.

Hope this info is of some limited value.