How to set/trim downthrust on a pusher?

[Tommy D]

Ok Guys on a tractor it’s no big deal . However on a pusher I would think if you shim the “top” of the motor to face more downward <add a shim in my case> you would create more downthrust. That said any idea the thickness or amount I should shim this?

I have a Axi powered Electrajet that heads for the clouds at anything above ½ throttle. Some nose up is normal but this is bad! I have a fair amount of REVERSE reflex dialed in now!

So what’s the expert’s opinion??

Tommy

[stirlingnut]

cg is off?motor mount flexing?thrust line?too much power?(just kidding)with reverse reflex,I suspect a cg issue,hope you get it fixed
I just saw your whole plane in another thread(http://www.rcgroups.com/forums/showthread.php?t=354959&page=3,)
and I feel like your thrust center line is too high.you might check some other pics of pushers,like the concrusher thread,these motors are dead center of the thrust line.I set my 'crusher up this way and heavy throttle just makes it go faster,no pitch up or down.good luck

[Model Aero]

Tommy,

I've been struggling with the same issue on a pusher jet of my own design. I did the same thing you did, which is to lower the prop end, and raise the front end of the motor. This made matters worse, so I did the opposite, even though it goes against my better judgement. :rolleyes:

I raised the prop end, and lowered the front end, and while it didn't totally solve the problem, it made it much better. It helped inverted flight as well, as it doesn't require nearly as much down elevator while inverted.

www.ModelAero.com

[Tommy D]

Thanks for the comments.

While messing around with the CG at the field today it felt "better" with the pack shifted rearwards. I have come to the conclusion I think it felt better because I made what little "throw" I had on the elevons more responsive with a rearward CG.

I have moved the CG forward and increased the elevon throw a good ammount.

As for the thrust line. Im very close to where the stock canned motor sits.

Still I have to ask, if I shim the top of the Axi mount outwards will this in fact trim the thrust line downwards?

It looked pretty neat up in the air so I hope to get it right with some trimming!

Tommy

[Tommy D]

Tommy,


I raised the prop end, and lowered the front end, and while it didn't totally solve the problem, it made it much better. It helped inverted flight as well, as it doesn't require nearly as much down elevator while inverted.

www.ModelAero.com


Wow.... well I didn't expect THAT! Thats completly "backwards" in my way of thinking, but... what the heck do I know!

Very neat looking birds you have there!!

Tommy

[Model Aero]

Tommy,

I didn't expect it either! I have experimented a lot with this.

Think of it this way: With a conventional airplane, downthrust means pointing the prop towards the ground, correct? Well, that's in effect what I did with my pusher. I pointed the prop towards the ground. I know it sounds crazy, but it seems to work.

Thanks for the nice comments on the planes. Those birds will soon be available as kits.

Scott

www.ModelAero.com