Throttle Hold How-To for mSR Stock Transmitter MLP4DSM - HeliFreak
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mSR Blade (eFlite) Micro SR Helicopters Information and Help

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Old 12-20-2010, 01:28 AM   #1 (permalink)
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Default Throttle Hold How-To for mSR Stock Transmitter MLP4DSM

In order to avoid developing the bad habit of throttle chopping every time I crash my heli, I decided to put a throttle hold switch on my stock mSR transmitter. Thanks to mikeonthebike for the idea. The following method is not the only way to do it, nor is it the simplest, but it will work for everyone without the need for a multimeter and can be adjusted post-installation if necessary.

SPDT mini toggle switch (RadioShack #275-635)
1k-Ohm 15-turn cermet potentiometer (RadioShack #271-342)
Wire -- I used wire from a scrap of ethernet cable about 20cm long
Soldering iron and solder
Dremel tool with #115 cutter bit
Hot glue gun
Screw drivers
Hobby knife

Remove the batteries.

Remove battery terminals by sliding them out of their sockets.

Remove the 4 screws from the back of transmitter and open the case. Be careful with the antenna.

Remove the 4 screws from PCB (circuit board) and remove it. It will take some gentle force, and the power switch will fall out the front of the transmitter when the PCB comes out.

Break loose the glue holding the power LED in place and remove it with the PCB

Reassemble the empty case and drill a hole (6mm or 1/4") in upper right hand corner for the switch. NOTE: do not center hole on the seam, but slightly in front of it (see pic), otherwise the switch will not fit correctly.

Open the case and use a dremel tool to mill out space for switch inside case. Do not mill all the way through the plastic.

Since the switch is too tall to fit perfectly, bend contacts over, all in the same direction. Make sure they do not touch each other.

Fit test the switch, with the contacts bent towards the front of the case. Continue milling the case until the switch fits well with the case closed.

Solder a wire (about 20cm long) to each contact on the switch. DOUBLE CHECK to make sure that none of the contacts are touching each other after soldering.

Hot glue the switch into front of case with the bent contacts FACING DOWN INTO THE CASE. If you glue the switch in upside down, the case will not close all the way.

Solder a wire to each of the outside contacts of the potentiometer (pot).

Glue the pot to case with screw facing up. Use pictures for location reference, and make sure it is not in the way of the PCB.

Trim and solder the wire from the top contact of the switch to the middle contact of the pot.

Re glue the power LED in place and install the PCB. Don't forget your trim buttons!

Cut through and remove the copper trace identified in the picture. Do this by carefully cutting through the trace on either side, and then lifting the trace off the board with the tip of the knife. Be careful not to cut the wrong trace, or to dislodge or damage the small resistor next to it.

Note when routing wires: don't just route them straight across the board from the switch or pot; they will get pinched between the board, battery wires, and battery tray. Instead, route them around the bottom. Also be sure that they don't interfere with movement of the gimbals.

Solder the wire from the center of the switch to contact #1.
Solder the wire from the bottom of the switch to contact #2
Solder the wire from the top of the pot to contact #3
Solder the wire from the bottom of the pot to contact #4

Turn screw on pot clockwise 15 times or until it stops. I think it's supposed to stop, but mine didn't, so I'm not sure. Still works though.

Without reassembling case, install battery contacts and batteries. Turn on transmitter, lower the throttle and center throttle trim. Turn off throttle hold (TH) (off is towards the back of the transmitter).

Plug in your heli and anchor it to the table with tape or a weight of sorts. Raise and lower the throttle to ensure everything works correctly. Turn on TH. If everything is wired correctly and the pot has been turned clockwise fully, the blades will not spin regardless of throttle position.

Now turn over your transmitter, and turn the screw on the pot counter-clockwise until the blades start to spin. Then turn it clockwise until the spinning stops and the motor does not buzz or whine, and turn it an additional 1/2 turn clockwise.

Unplug the heli, turn off the transmitter, and assemble the case. Power everything up again and make sure everything works. With the switch off, everything should work normally. With the switch on, the blades should not spin regardless of the throttle position, but you should still have full control of the cyclic.

From now on, any time you crash, focus on hitting that switch, and not on chopping the throttle. This habit will pay off when you make the move to collective-pitch helis. Throttle chopping is a terrible habit to form. With a CP heli:
Chopping throttle also chops collective and you'll have the heli tryin to bury itself really fast.
Any improvement ideas? Put 'em out there.

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StrikeEagleCC is offline        Reply With Quote Quick reply to this message
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Old 12-20-2010, 05:36 AM   #2 (permalink)
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Agree with the objectives and appreciate the effort and your skillset, but it's also worth pointing out to others that if someone plans to have more than one model over time or migrate up to CP it would be simpler and better and even cheaper in the long run to buy BNF models and a Spektum Tx to begin with instead of RTF and needing to modify the Tx.

Also to avoid confusion you aren't adding a "hold" switch you are adding a "cut" switch. But I can understand why you use the term "hold" to describe it.

A Spektrum has both "cut" and "hold" switches. A spring loaded Throttle Cut button on the Spektrum Tx designed for killing nitro engines. But it doesn't work for electric models very well because an electric motor starts again when the button is released if the throttle is up.

For electric FP models setting the Throttle Hold level on the Throttle curves (used for setting idle / power-up throttle on nitro models and CPs) to 0% makes the T.HOLD switch at the right and back to function as a throttle cut switch. Flipping it up cuts "holds" the power to the motors at 0% cutting the engines and keeping them off regardless of where the throttle stick is set.

I've gotten in the habit of keeping my Tx T.HOLD switch up whenever my model isn't in the air as a safety precaution. It eliminates the chance of bumping the Tx throttle stick by accident and spinning up the motor while on the ground or in your hand.

As for the cutting the throttle habit and breaking it, I understand the difference in the controls on a CP, but if you don't cut the throttle on a CP which is heading for a crash into the ground or a pole and it crashes with the rotor turning full speed wouldn't that cause as much or more damage as cutting the engine and letting it drop?

Except for mechanical failures crashes are, in the final analysis, the result of poor piloting skills -- including cutting the throttle in a panic. Adding the throttle cut button to hit in a panic will help prevent damage, but it really won't improve piloting skills, which will in time allow a skilled pilot to maneuver out of situations that cause a beginner to crash. For example, when beginning I would instinctively cut the throttle when heading for a crash because didn't have any other ingrained reaction. I now in the same situation I would instinctively flare with back elevator and add throttle to stop, spin the tail around 180 towards open space and fly out of trouble avoiding the crash.
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Old 12-20-2010, 10:27 AM   #3 (permalink)
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Welcome! Good to see another tinkerer in these parts....

I'm gonna add this to my sticky if you don't mind. Once I gather the other threads that is cuz I don't have admin rights... so I gotta PM an admin with the changes.
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Old 12-20-2010, 06:21 PM   #4 (permalink)
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Vicovaludemero, feel free to sticky, thanks!

Roundwing -- +1 on getting another transmitter if one plans to continue in this hobby. A computerized transmitter makes all the difference when flying and especially learning to fly, nevermind the cost benefits. My DX6i is the best thing I've bought since I started this hobby, I wish I had done it at the beginning. However, for those who can't drop the extra cash right now, or just want something to tinker with, this mod's for you!

Also, good call leaving your throttle hold on when you're heli's not in the air. A good habit.

Couple of things though, this mod is a throttle hold switch. In the instructions, I just explain how to set it at zero, like you would for any electric model as you explained. You could (don't know why you would) set it at any throttle level you choose by adjusting the pot. On a nitro heli, the throttle cut button will actually kill the motor, while the hold switch will just lower the throttle to idle, thereby disengaging it from the blades. Kind of a mute point though with electric birds.

And when I refer to "throttle chopping" being a bad habit, I'm not suggesting you let the motor continue to run when you crash. I realize now that I was a bit ambiguous in my post, so I'll try to explain myself better for those who are new to this game.

"Throttle Chopping" is the habit of slamming the throttle stick down to it's lowest setting every time you crash in an effort to minimize damage to the helicopter. It's a good thing to do on small fixed pitch or coax heli from a damage standpoint, but it's a terrible, dangerous, and expensive habit to have when you make the move to collective pitch helis. Here's why:

On a larger, collective pitch helicopter, the throttle channel (motor power setting) and the collective channel (blade pitch setting) are both controlled by movement of the throttle stick. If you're flying in normal mode, with the throttle stick at the lowest position, the motor will be off, but pitch of the blades will be at least a little bit negative. If you're flying in idle-up mode, then putting the throttle stick at it's lowest position will actually give full power to the motor and full negative pitch to the blades. This is not what you want if you're trying to minimize inevitable damage. If you know you're going to crash, there are two things you want to do: kill power to the blades (motor) in order to minimize the damage, and apply pitch to the blades to try to soften the blow as they spin down. That's where throttle hold comes in.

By flipping the switch on, it kills power to the motor regardless of stick position, but still allows control of the blade pitch so that you have some amount of control as the blades spin down. While none of this has any direct application to a fixed pitch or coax heli, It's best not to develop the habit of simply slamming the throttle stick down in an effort to save your heli. Use the throttle hold switch instead!
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