# Homemade Autopilot with a Raspberry/computer

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#### Merlin

##### Well-Known Member
I was thinking about how nice it would be to have autopilot/auto stabilizer for long journeys. Garmin autopilots are too expencive for my taste.
Has anyone done it? If not i'll start to build one myself from scratch.

#### Daleandee

##### Well-Known Member
I'll be following along ...

#### Jay Kempf

##### Curmudgeon in Training (CIT)
Pixhawk pairs with a pi. There are others based on single board computers. Very powerful. Easy to port to your needs. Cheap starting point. There is a ton of open source stuff that can be combined up to make as much automation and screen bling as you want. I am on a project that is well down this road.

#### Merlin

##### Well-Known Member
Pixhawk pairs with a pi. There are others based on single board computers. Very powerful. Easy to port to your needs. Cheap starting point. There is a ton of open source stuff that can be combined up to make as much automation and screen bling as you want. I am on a project that is well down this road.

#### Jay Kempf

##### Curmudgeon in Training (CIT)

Your work comes in that you want to convert the RC signals to actual analog or digital to analog. But you're in luck cause there are off the shelf servo testers that do that... All cheap off the shelf stuff that is as reliable as anything on my 70 year old Bonanza.

So you need to buy a Pixhawk kit off of Amazon for $60 and start playing. #### galapoola ##### Well-Known Member There is a fellow who was interviewed at Osh, He used RC or drone hardware, it's on YouTube WoodPecker #### Hephaestus ##### Well-Known Member Pixhawk (it's a full drone control system) can run natively on a pi. The tweaking for GA use... That could be challenging. But we have lots of baseline ideas for autopilots based off large scale RC servos etc. Just make sure the pilot can override it easily as you test it. #### Pops ##### Well-Known Member HBA Supporter Log Member Homemade autopilot in my SSSC many years ago. Giant scale RC servos ( now much larger RC servos on the market). Wing leveler and pitch with electric trim on each. Auto pilot worked the yellow and black trim tabs on the elevator and aileron. Flew it for a couple years before removing. Worked great. Might install in the JMR in the future. #### Attachments • 92.7 KB Views: 42 • 101.6 KB Views: 42 Last edited: #### rv7charlie ##### Well-Known Member Doable for not a lot of money (just a fairly large amount of work), and small model servos can run servo tabs on the ailerons instead of driving the ailerons directly. But if all you want is a wing leveler, you should just watch ebay and the various experimental a/c type forums for a used NavAid. You can probably pick one up for under$200, if you're not in a hurry, and it will follow an analog CDI, or with the optional 'smart coupler', it will follow a GPS course. I flew one in my RV4 for many years; works fine.

And for your tidbit of the day.... Both a unique homebuilt on my airport called a Messersnipe and a Thorp T-18 I used to own, had wing levelers based on...air. They used a small jet of air, blowing between two thermistors (mounted in a coffee can). When the plane turned, inertia of the air stream meant it cooled one thermistor more than the other, and an analog circuit comparing their resistance would output a varying voltage driving a model airplane servo, which drove a trim tab on an aileron.

BTW, the Messersnipe is a really sweet flying plane; that EAA forum thread might be a fun read for some of us that like older a/c.

Charlie

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Doable for not a lot of money (just a fairly large amount of work), and small model servos can run servo tabs on the ailerons instead of driving the ailerons directly. But if all you want is a wing leveler, you should just watch ebay and the various experimental a/c type forums for a used NavAid. You can probably pick one up for under $200, if you're not in a hurry, and it will follow an analog CDI, or with the optional 'smart coupler', it will follow a GPS course. I flew one in my RV4 for many years; works fine. And for your tidbit of the day.... Both a unique homebuilt on my airport called a Messersnipe and a Thorp T-18 I used to own, had wing levelers based on...air. They used a small jet of air, blowing between two thermistors (mounted in a coffee can). When the plane turned, inertia of the air stream meant it cooled one thermistor more than the other, and an analog circuit comparing their resistance would output a varying voltage driving a model airplane servo, which drove a trim tab on an aileron. BTW, the Messersnipe is a really sweet flying plane; that EAA forum thread might be a fun read for some of us that like older a/c. Charlie My pitch autopilot work with a pint glass jar, 2 thermistors in parallel on a small PC board in an small cavity with 1/8 brass tube on each end mounded to the jar lid. One end of the brass tube to the outside air and the other to the inside of the glass jar. Go up , air come out of a tube on the jar lid and cools the first thermistor more that the second that is downstream. Go down and the reverse. Balance analog circuit that get un-balanced. Drives a LM-556 to make the square pulse train for the servo. If I remember correctly it would hold a pressure altitude to within about 20' when adjusting the gain before going into oscillation. Used a 10K pot for up or down trim. Autopilot also had LED indicators when the servos were moving up or down. Wing leveler used a solid state rate sensor ( output 0-5v) also unbalancing an analog circuit that went to the 556 for the square pulse train for the RC servo. Used 10K pot for roll trim. Also I installed a turn command switch for a standard rate turn to right and left. Also had LED's on roll. Maybe$150/ \$200 in everything.

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BJC

#### pfarber

##### Well-Known Member
You can get very strong servo motors (well over 1000oz) for very cheap. The real issue is the amount of current they require. Most AC servo's are geared, therefore can run on less current.

The rigging to 'over ride' the servo is basically springs, stiff springs, or a clutch mechanism.

With a AHRS and barometer you should be able to cobble something together. But scaling the outputs and writing code for sane limits would be where I put most of the effort. It takes very little to turn on a servo. It takes a lot more to know which way to turn it and with how much force.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
These servos are geared and the current and all the specs are listed. No problem with over riding if you use the servo to operate trim tabs where you can make large enough to control the airplane but easy to override. I make the tabs a little large at first and then adjust the area and travel to suit. No clutches or springs.
What I use on for the elevator trim on the SSSC and the JMR.
Have been using these servos since about 2010.

Stalled current-- 650 mA @ 12 volts
Max force-- 44.9 lbs ( 200n )

#### pfarber

##### Well-Known Member
These servos are geared and the current and all the specs are listed. No problem with over riding if you use the servo to operate trim tabs where you can make large enough to control the airplane but easy to override. I make the tabs a little large at first and then adjust the area and travel to suit. No clutches or springs.
What I use on for the elevator trim on the SSSC and the JMR.
Have been using these servos since about 2010.

Stalled current-- 650 mA @ 12 volts
Max force-- 44.9 lbs ( 200n )

HORRIFIC CHOICE You missed the 20% duty cycle.

OP wants an autopilot, not electric trim. Trim tabs are not designed for continuous movement. They also are not designed for full control. You will have a hard time keeping straight and level when your control inputs are 10 seconds late.

#### berridos

##### Well-Known Member
INAV could also be useful.
It has GPS tracking and preprogrammed flight path and Matek boards have gyroscopic autostabilisator.
Press a button and go sleeping.

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#### Pops

##### Well-Known Member
HBA Supporter
Log Member
HORRIFIC CHOICE You missed the 20% duty cycle.

OP wants an autopilot, not electric trim. Trim tabs are not designed for continuous movement. They also are not designed for full control. You will have a hard time keeping straight and level when your control inputs are 10 seconds late.
Been using these servors without any problems since 2010. If I was to say 0, you would say 1. That duty cycle would be for the max force. Very little power required to operate a trim tab controlled by the autopilot.
I planned to coupling my auto pilot to a GPS and VOR but never did. I just looked at the GPS heading and set the manual trim on the autopilot to hold it on that heading.

#### Hephaestus

##### Well-Known Member
HORRIFIC CHOICE You missed the 20% duty cycle.
Check out the duty cycle on the servo's garmin sell with their systems.

Clearly they've made similar horrible choices, you may want to clue them in.

#### 103

##### Well-Known Member
Been using these servors without any problems since 2010. If I was to say 0, you would say 1. That duty cycle would be for the max force. Very little power required to operate a trim tab controlled by the autopilot.
I planned to coupling my auto pilot to a GPS and VOR but never did. I just looked at the GPS heading and set the manual trim on the autopilot to hold it on that heading.
Almost home made leveraging the same concept

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
See they are using good quality RC servos. OK for smaller aircraft with light trim tab loads. When the load is to large for the servo the servo is fighting to hold position from the force coming back through the gear train and wants to move the motor, the servo will jitter trying to hold position causing a high current drain, heat and a short servo life. At first I was using the RC servos for giant scale RC models and worked OK for a small LSA as the SSSC.