Flying Ravens - Trim forces?

[justin.schoeman]

I am still faffing with the trim system on my Raven. For simplicity, I am contemplating using an electric trim. It seems a R150 RC servo has the same thrust and motion specs as a R2000 Ray Allen servo, and I am looking at the pros and cons of using one.

My primary concern would be that a failure in the servo not endanger the flight in any way. I have flown a Tecnam with the trim stuck full forward, and although my arm hurt like hell the next day, the excess stick force did not affect the flying in any way.

I would like to know if the same applies to the Raven. Back of the envelope calcs say the stick force at full trim should be around 20kg @ 150mph, decreasing rapidly as speed reduces.

Is any current Raven owner willing to test how much stick force full forward, and full back trim induce in level flight in the cruise? No numbers needed, just a basic 'Can I easily control the aircraft even with these stick forces?'

Thanks,
Justin

[Morph]

Justin,

Call Johan Froneman at Saldanha. He is an instructor on Ravens, and he has built a few of them for himself and clients. He will be able to help

082 four57 nine509

[justin.schoeman]

Thanks Morph.

Just spoke to Johan, and he confirms that the trim is easy to override. Much less than the calculated 20kg of stick force (which was, admittedly a worst case calculation).

I think I will give RC servos a go, and see what happens. This has a big advantage, in that I will be controlling it from a PIC micro. I will eventually connect the PIC to the airtalk bus, and add an 'altitude hold' mode to the trim . If this works, a similar trim tab will mystically appear on an aileron for 'heading hold' and perhaps more advanced AP functionality...

[Stephan van Tonder]

Just a couple of things I must say. Use the RC servo and I honestly have no problem with doing that as I also would not pay R2000 for a trim servo. There are several different versions of RC servos and there are some serious ones being used on the large scale models. Those have metal gearboxes and I suggest you use one of those.
2Ndly - PLEASE devise some way by either a spring or a bungy or such that if the servo fail the trim tab will be held or pulled safely into some default position. If you have either a link failure or a gearbox failure and the tirm tab is allowed to flutter you might find yourself sitting with a control surface that flutter and that you cannot control. THIS IS USUALLY fatal in a very short time. Flutter will kill you.
If you look at a cheatah you will see a VERY simple and safe way of doing that. Simple Bugee on a tab on the opposite side of where the trim tab control sits.
I assume you are moving a tab not the entire control surface. If you are moving the entire surface then plase ignore this post.

[justin.schoeman]

Thanks Stephan. I was definitely going to use a metal gear servo - basically for the failure mode. When a plastic gear servo brakes a gear, it will generally jam. I have never seen a metal gear servo brake the gearbox - not even in the worst prang - when they fail, the motor burns out, and the control surface (trim tab in this case) can blow back into a neutral position.

As to flutter, it isn't a serious issue in this case, seeing as the deflection is self damping. I will however follow your advice, and add additional damping, just to be safe.

Thanks,
Justin

[German]

I have seen on the Dynero a simple RC servo.

[justin.schoeman]

I have seen on the Dynero a simple RC servo.

Excellent. So it has been done (safely) before. Glad to know that it really does work.

Once I am done, I will open-source the servo calculator, schematics, PCB layouts, and source code - so if anybody else wants to experiment/contribute, they can. Should end up with a complete electronic trim + single axis auto-pilot for around R500-R700 of parts, and a few hours of labour (the servo controller will have two channels for the eventual addition of an aileron trim, and possibly 2 axis AP).