Legal paperwork trail to add limit switches for electric flaps

[sdmideas]

Title says it all;)

From a mechanical and electrical perspective this seems simple enough. I’d like to update my “unlimited” electric flap system in my 1971 M20c to a limit switch based design.

My real question is whether or not anyone else has done this, what approved data was used etc.

Can anyone point me to an older thread that contains the info I’m looking for? I’ve searched but haven’t found what I’m hunting for yet.

My relatively strict A&P IA is hoping I can track down an official procedure or letter from Mooney etc.

[N201MKTurbo]

I would document the changes you want to do. Most likely using the parts from a later model Mooney. And have your IA send them to his principal inspector for their opinion. Send a cover letter stating why you want to make the change and your basis for approval is the design changes Mooney made in later models. And see what they say. 
 

The inspectors here have stated that they hate it when somebody does a mod, then sends in the paperwork for approval. They would much rather you talked about it before you do anything.

[bluehighwayflyer]

My IA says the same as Rich.  He says the “why” should always be “in the interests of safety”.  He said it jokingly but I don’t think he was kidding.  

[AndreiC]

Out of curiosity, why do you want to do this? I also have a limitless flap system in my ‘70 E and would be curious of the pros and cons of the two systems. 

[Hank]

1 hour ago, AndreiC said:

Out of curiosity, why do you want to do this? I also have a limitless flap system in my ‘70 E and would be curious of the pros and cons of the two systems. 

I just raise the flaps in my C until I hear the motor. Almost never go to Full Down, so it's not a problem at that end.

[sdmideas]

20 hours ago, AndreiC said:

Out of curiosity, why do you want to do this? I also have a limitless flap system in my ‘70 E and would be curious of the pros and cons of the two systems. 

When a motor stalls the current through the windings increases dramatically. Enough cycles of this can have a negative effect on the motor. Limit switches are a relatively low cost way to protect the system.

[AndreiC]

8 hours ago, sdmideas said:

When a motor stalls the current through the windings increases dramatically. Enough cycles of this can have a negative effect on the motor. Limit switches are a relatively low cost way to protect the system.

On my 1970 E, the motor does not stall. It continues to turn, just that it sounds as if a winch-like mechanism is engaged which allows the motor to spin without moving the flaps. It is some sort of mechanical limiting device: while the flaps are in travel the motor runs with a smooth sound, and when they reach the limit the system starts making a "clack-clack-clack" sound, as if the motor axle is spinning but it does not engage the flap mechanism. It sounds like this is how it was designed to work, and everyone I talked to said the system is quite reliable.

[Matthew P]

On 5/9/2025 at 11:01 AM, sdmideas said:

Title says it all;)

From a mechanical and electrical perspective this seems simple enough. I’d like to update my “unlimited” electric flap system in my 1971 M20c to a limit switch based design.

My real question is whether or not anyone else has done this, what approved data was used etc.

Can anyone point me to an older thread that contains the info I’m looking for? I’ve searched but haven’t found what I’m hunting for yet.

My relatively strict A&P IA is hoping I can track down an official procedure or letter from Mooney etc.

have you contacted Frank at Mooney, he's pretty responsive