possible to put reverse on a pixie?

[jeff262]

ok winter has finially hit Mineesota so i'm done flying for the next 4 months so i need to keep busy so i was thinking that i would really like to put reverse on my GWS A-10 since the landing are so long and it always hits the nose on the curb before its done rolling. is there any easy way of doing this with a pixie 20, i really don't want to build an entire esc. i have 3 open channels on my remote so i was thinking of a relay but all the ones i have seen are to heavy, so i thought i would ask the good people of the DIY forum so see if they knew any way to do this
thanks jeff

[jeffs555]

There is no easy way. You either need a DPDT relay that will handle the current, and will probably be heavy, or else you need a full bridge with 4 mosfets. Then you would also need a decoder for the reversing channel, probably a small PIC or other micro. Maybe there is a schematic somewhere on the net, but I haven't seen one.

[Cas123]

Try converting the ailerons to flaparons by driving each aileron with separate micro servos (It will depend if your Transmitter is a computer one and can do this) These should create extra lift and allow slower landings, and increase the drag, so shorter landings.

Ron...

[Warske]

For lighter weight than a relay, you might want to buy a reversing ESC, like what is used on some R/C cars. I raised a similar question here (http://www.rcgroups.com/forums/showthread.php?p=3058277) about a brushless ESC.

Also, if you don't have steerable landing gear on your plane, you might try that. I found that with a nice short turning radius, it was much easier to avoid the curb in a parking lot at the last second. I used a tricycle gear and the plane could turn around one of the wing tips.

Also, there is the possibility of using brakes on the landing gear.

Or a drag chute.

I have read about all of this being done except for the reverse thrust, so if you do it, please let us know how it works out.

Warske

[Hooters Driver]

You can use the electronics from a micro servo as a forward/reverse speed control. To do so you will have to replace the feedback pot with two resistors and replace the H-bridge transistors (avoid the servo boards that have a single IC instead of the 4 H-bridge transistors) with 4 fets that will handle the power you plan to feed through it.

[killer_d76]

You can use the electronics from a micro servo as a forward/reverse speed control. To do so you will have to replace the feedback pot with two resistors and replace the H-bridge transistors (avoid the servo boards that have a single IC instead of the 4 H-bridge transistors) with 4 fets that will handle the power you plan to feed through it.

..what type of H-bridge transistors could you recommend on this mod?
thanks!, and merry christmas everyone! :)

-kenny

[Hooters Driver]

It depends on how many amps your motor will draw. IRF7456 N-channel FETS will handle 16 amps but are SO-8 chips so they are too largs to be mounted in place of the stock chips and will have to be mounted on a separate PC board.

The 2502 chips that guys are using in bit car receivers will work as a direct replacement but I'm not sure how many amps they will handle.

[killer_d76]

It depends on how many amps your motor will draw. IRF7456 N-channel FETS will handle 16 amps but are SO-8 chips so they are too largs to be mounted in place of the stock chips and will have to be mounted on a separate PC board.

The 2502 chips that guys are using in bit car receivers will work as a direct replacement but I'm not sure how many amps they will handle.


thanks for the reply, using a seperate board for the SO-8 chip is fine with me, i found this FET lying around my desk from my previous project,

http://mselektronik.gdynia.pl/i/irf7343.jpg

.. it's Dual N and P channel Mosfet,will this work? or an N-channel FET only will work for this modification?, if so, which pins goes to which? because i'm not familiar with the servo board set-up or those three pins transistors attached on the servo board,thanks in advance! happy holidays!.. HAPPY NEW YEAR TOO!

[Scoobyvroom]

for all the problems and weight you would be better off taking a small servo mount a flat piece of metal on it and have it act like a drum brake. It would be pretty simple and I don't think it would take much to bring it down to speed..

Heya Jeff whats new??

Chris

[Hooters Driver]

.. it's Dual N and P channel Mosfet,will this work? or an N-channel FET only will work for this modification?, if so, which pins goes to which? because i'm not familiar with the servo board set-up or those three pins transistors attached on the servo board,thanks in advance! happy holidays!.. HAPPY NEW YEAR TOO!I didn't see an amp rating for that fet but if it will handle the power it should work even better than just an N-channel fet as you will only need 2 of those instead of the 4 N-channel fets that would be needed.

It will take me a while to draw up a diagram but I'll try to post it tonight. In the meantime it will be helpful if you could take a picture of your servo board and post it.

Also you need to be aware that you'll have to use a separate BEC as servos are only designed to run on 4.8 volts.



for all the problems and weight you would be better off taking a small servo mount a flat piece of metal on it and have it act like a drum brake.This is a project that requires fairly good soldering skills and a good understanding of electronics but once the speed control is built it should be as light or lighter than your current speed control and it won't take any mechanical engineering to accomplish.

[killer_d76]

I didn't see an amp rating for that fet but if it will handle the power it should work even better than just an N-channel fet as you will only need 2 of those instead of the 4 N-channel fets that would be needed.

It will take me a while to draw up a diagram but I'll try to post it tonight. In the meantime it will be helpful if you could take a picture of your servo board and post it.

Also you need to be aware that you'll have to use a separate BEC as servos are only designed to run on 4.8 volts.



This is a project that requires fairly good soldering skills and a good understanding of electronics but once the speed control is built it should be as light or lighter than your current speed control and it won't take any mechanical engineering to accomplish.


...Thanks, Hooter_driver, my camera is not in good conditon as of now, but i'm using a PC-09 servo board.

[Hooters Driver]

Here is a basic diagram using your FET's. The signal wires should be soldered to the gate pin of the h-bridge transistors on your servo board.

[jeffs555]

Kenny,
The mosfets you drive from a servo board will need to handle logic level switching. With a Vdss of 55 volts, I would guess that they would need 12 volts for switching and wouldn't work, but can't tell with the partial specs you posted. If you post a part number it would help.

Jeff

[Dan Baldwin]

Another problem is that the RDS on is VERY high for use as a speed controller. At 5 amps the fets would be dissipating almost 4 watts, which is way too much for that package.

Dan

[KillerWatt]

Why not have some "clamshell" thrust reverser's buckets to dropdown behind the ducted fan tubes.............kw

[Hooters Driver]

I found the specs on the FET's you have (IRF7343). It looks like they will only handle 2.7 amps continuous.

I'm not sure what the gate trigger voltage is on your PC-09 servo board but if it's 1 volt or greater then an IRF7319 would be a better N-channel P-channel FET to use. It will handle 3.9 amps and the RDS is significantly lower (.029 ohms and .058 ohms, .087 ohms total). If you run two of these in parallel for each side (4 total) it should double the current capacity (7.8 amps) and halve the RDS resistance (.0435 ohms total).

Neither of these compare very well to the IRF7456. It will handle 13 amps, the gate threshold voltage is only 0.6 volts, and the total RDS for two of them in series is only .013 ohms.

[Hooters Driver]

Here is a diagram of the h-bridge using IRF7456 FET's.

[jeffs555]

Unfortunately, a bridge with all N-channel like that won't work without high voltage drive to the upper N-channel mosfets. The specs for the IRF7456 say the gate-source threshold voltage could be as high as 2 volts. In order to turn on the upper mosfets, the gate needs to be at least this much higher than the source, and to guarantee the 13amp rating it needs to be 4.5volts above the source. Since the servo signal driving the gates is a 5volt level, the source on the upper transistors would normally never pull higher than 2 volts under load. In order to use N-channel for the upper mosfets in a bridge, you would need a driving signal that goes at least 2.8volts above the positive battery voltage. This is normally done with a charge pump driver, which would add expense and complexity. Simpler to use P-channel for the upper mosfets. Even with P-channel for the upper mosfets, you would still need high voltage drivers if the battery voltage was higher than 5volts. Simple NPN drivers with pullups would work, or even simpler, cross coupling from the N-channel with pullups.

[Hooters Driver]

I guess I misunderstood the specs. I am currently using a bridge circuit with 7456's like the one I drew and it does work but it's only driving an actuator motor so I hadn't even bothered to check the voltage at the motor.