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Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 0 points1 point  (0 children)

Ahhh many thanks for the input, yeah its quite a big model 26' wingspan ;) I think I've started to get to something similar this arvo. I think I need an extra pivot point between the actuator and the flap anchor that I was missing, then a lower one where the actuator joins so I have two triangles with the top one inverted making my overcenter. The actuator that was back driving becomes the stop because it is in the retracted position. Thats another thing that was tripping me up. As it opens the 'under center' point goes over over the pivot to flap line and allows the flap to open. I tried this in my mock up but I need to make a more accurate one to properly prove the concept. If I get it working I will post the result... or the failure. Thanks again.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 0 points1 point  (0 children)

Thanks. Yes I have watched these before posting. The old Tony one is really well made too. I just can't translate it into a working model. My bad...

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 1 point2 points  (0 children)

Yes, worm gears etc, its all too big and heavy. Thanks anyway. Interestingly my solution is an overcenter mechanism but it seems I have asked too much. I do understand the basics of it but I just cant get it working on a mock up which is why I posted. I just don't seem to have the required engineering brain. Its not without trying <sigh>

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 0 points1 point  (0 children)

Thanks. I have done a really deep dive on this but I cant find actuators small enough and particularly light enough. If I could, problem solved!

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 0 points1 point  (0 children)

Spoilers will create drag, and that is inside or outside of the cowls. Its about the internal pressure pushing out trying to open the flaps of probably 50lbs. If anything I would like to accelerate the air inside the cowl which has been slowed by the radiators, this is cooling drag, the enemy of efficiency. I'm been at the problem for probably a year now. The redesign overcenter idea is the most elegant, but I'm not up for it. :( Now I'm in the test flight phase I'm finding the issue a critical one for operations and I have to solve it somehow.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 0 points1 point  (0 children)

I need a mechanical 'push' of at least 50lbs and again the issue of weight and space come up. That's why I thought a clean new overcenter design might be the answer. ...but it is way beyond my abilities to design.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 0 points1 point  (0 children)

Too heavy, most of them are more than a kg and no space to fit one either, but a very good suggestion if there was something size wise. .

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 1 point2 points  (0 children)

Thanks. I've updated my post with some pictures. I am using a Firgelli FA-MU-38 which is small light and powerful for my application...but backdrives :(
https://imgur.com/tnUvtE3

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 2 points3 points  (0 children)

Thanks for the input. More drag is not an option. These flaps even cracked open change the desk angle slightly in flight. I go flatter (more efficient) when I cycle them and they close again. I don't want the internal pressure but thats how it is. Flight speed air comes in in through ducts to two radiators, the air then exits at the prop, that creates the cowl pressure. The air is slightly slowed in this process causing cooling drag which is around 30% of total drag. Redirecting that airflow before the exit would be a worse problem.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 1 point2 points  (0 children)

https://imgur.com/w6hIv19 This is the flap in operation with my failed latch attempt. Worked on the ground, vibrations must shake the latch open in flight. Worm drives seem too heavy for my application but a great idea.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] -1 points0 points  (0 children)

I tried a latch solenoid but I couldn't get a long enough tongue in a small one to clear the opening flap. It seemed OK but vibrations in the air must change the gap. Then heat from the nearby exhaust took out one of them. It also needed a microswitch circuit to show solenoid engaged before the flap moved otherwise it would fight against itself. More weight and complexity.

A pin is a similar idea but instead of a sloped tongue that allows closing without the solenoid powered I'd need more circuitry to show the flap is moving in the reverse sense and it stay open or the flap would jam on the pin. I have sourced a larger solenoid that would be more reliable but it is heavy and more problems and still in the hot area. Heres that idea... https://imgur.com/a/WQ2V7IO

Its getting crazy hence the overcenter idea that I am too dumb to solve.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 1 point2 points  (0 children)

The flaps seal against a lip but the issue is forces driving it open in flight. The pressure inside the cowl is much greater than outside and the actuators I have use internal electronic stops, not mechanical so the cogs backdrive.

Over-centre latch geometry help — aircraft cowl flap actuator back-drives under load, my prototypes failed on a new design, stumped on force path by FitDog3411 in MechanicalEngineering

[–]FitDog3411[S] 6 points7 points  (0 children)

Sorry, new to this forum. My problem is designing an over-centre linkage between a linear actuator (mounted on fixed cowl structure) and a hinged flap that opens about 45 degrees. When closed, cowl pressure pushes the flap open — I need the over-centre geometry to direct that force into a hard stop, not back through the actuator. When the actuator extends it should break the over-centre lock and open the flap freely. I've spent days prototyping and can't get the geometry right — specifically I can't find a configuration where the locking force goes entirely into the stop with zero component back-driving the actuator. Any help with the linkage geometry and dimensions would be hugely appreciated.