Noob With COG Questions

[Derrell]

First post on the FliteTest forums so I'll start off saying hello to everyone here. Like a lot of others it seems that I was inspired to try RC flying by watching videos of the FliteTest guys. A big thanks to them from me.

To get the point of this post after building and crashing a nutball and FT flyer, a lot, I decided to try and build a high wing trainer with a pusher prop setup. The photos are what I came up with. It has a wing like the FT Old Fogey but a bit smaller on the chord, only 8 inches with a 43 inch span and a carbon arrow shaft for the tail boom. It's like the smash drone that CStence wrote the articles about on the main FT site. I actually had the idea for it from looking at the Blu Beagle on RC Groups. But it is basically the same thing. Three channel rudder, elevator and throttle using the same Blue Wonder that I was using in the FT Flyer.

I've flown it several times and three of those times didn't end in a crash. I was able to fly it and land it unlike the flyer and nutball which seemed to go up then come back to meet the earth violently I am still having a problem though. It doesn't want to glide. So long as it has some throttle it seems alright aside from really wanting to climb like crazy, I think some down thrust will help that. But back to the not gliding thing. If the throttle is cut it starts to porpoise really quickly. Is this a COG problem or is there something else I need to be looking at? Angle of the wing etc.. I've been playing with COG some, it is set right now just forward of the fold in the wing thats about 30% of the chord.

Aside from the whole woop woop up and down thing with no power it seems to fly good. Nice and slow and floaty but sort of sensitive to the wind of course. Waiting for the wind to calm back down so I can give it another try.

 

[eagle4]

Do you have a slight down thrust angle? If not this might be the case, as when you're under powered flight your motor is pushing some torque onto your plane which I'm sure you've trimmed out so it can fly straight, however without this torque from your motor your plane wants to pitch. I suggest angling down your motor just a bit.

If you've already done this, we'll I'm out of ideas

 

[Craftydan]

30% cord may be a bit tail heavy, and the eratic elevator implys that a bit, but with he motor so high above the centerline the thrust angle could be fighting you too.

Take some measurments and plug them into this:

adamone.rchomepage.com/cg_calc.htm

It's a nice calculator -- fairly well featured and will give you the theoretical "safe range" of CGs by adjusting the stability factor according to their recomendations. It even has versions for canards and flying wings, IIRC. It's not perfect, but it' get you to a stable, trimmed point more quickly and safely with fewer the fly-and-try-not-to-crash flights.


As for thrust angle, everything reverses between high pushers and low tractors. the typical low tractor, like a Flyer or Old Fogey, would need downthrust to move the line of force up and eliminate the wrench the motor is putting on the center of mass (engineering term for force + torque, not the metal tool). A high pusher will need up-thrust to angle the line of force toward the COM.

 

[Derrell]

Thanks for the replies guys I'll try calculator you linked Dan and see what it recommends.

 

[Blackbird]

You should do some throttle mixing so when you want to glide the elevator balances the plane.

This is probably your best option without changing the design.

 

[xuzme720]

The motor should be mounted so the thrust line passes through the center of lift. On this plane, with the motor mounted in a pusher config and mounted below the wing, it needs to be tipped down at the rear slightly. You probably don't need very much.
Or you could go with Blackbirds' suggestion and mix a little elevator in under throttle. That would work just as well, and like he said, that would need no changes to the airframe

 

[Derrell]

Used the calculator that Craftydan suggested and it showed that the CG should be about a 5/8 of inch further forward than what I had it. So I added weight to the nose to correct that. I can shim the motor mount to change the thrust angle when I get a chance to try flying again. Thunder storms are moving through today so it's not looking like I'll get a chance to try anything. Stupid rain. At least it gives me a chance to go find some more youtube videos about programing my turnigy 9x. I have to admit I didn't research that enough before I bought it. It really surprised me when I opened up the box and there wasn't any sort of manual in there. Live and learn I suppose.

Thanks again for the suggestions. I'll try to get some amusing newbie flying video when I try out the different changes.

 

[xuzme720]

When it comes to the 9X, the internet will be better than a manual anyway. Have you done any mods to yours yet?

 

[Derrell]

No haven't tried anything with it yet. I haven't had it all that long. I've seen where people do all sorts of things with them but I don't even know what might be a good thing to try and what wouldn't. Don't even know what I would want to try.

 

[xuzme720]

If you feel your soldering skills are up to it, one mod I would recommend is the flashing the firmware to the open version or ER9X version. It's easier to program and gives you more model memory. There is even a way to do it without soldering by using the Smartieparts board. I used that one because I wanted it to be reversible in case I changed my mind. Not a chance! I love the radio with the changes!

Do a search for 9X modifications or ER9X and you'll get a wealth of info. If you need more specific info, let me know and I can post a few links.

 

[Derrell]

Looked at the. ER9X site on Google code and watched a couple of videos about it. Does seem to be simpler to program. Once I get some flying time built up I think this is definitely something that I want to look at. Right now though I don't think that I know enough to know what I don't know yet. Just being able to do away with beeping makes it sound like an attractive upgrade.

 

[Derrell]

Managed to get out this morning and try some more flying. Unfortunately the super cool cap cam ball cap mod was a massive fail. So I set the camera up on a mini tripod on the pickup cab and even managed to get the plane to fly in front of it a couple times when it wasn't nosing into the ground. The camera quit recording before I ended that last flight the crash would have been off camera but the nose dive into the plum thicket after I lost it in the sun sure would have sounded amazing at least I thought it did at the time.

I moved the CG forward to 1.700" from the leading edge of the wing and it was really nose heavy couldn't get it to do anything but nose straight into the ground. Moved it back some not as far as it was to start with but it let me get into the air. Something else was going on though because it was wanting to do clockwise circles. It wasn't doing that the last time I flew it. Don't know what was going on and its pretty hard to tell now after the whole plum thicket incident.

Going to have to do some major repairs and maybe rethink the whole thing. Right now I'm thinking an Old Fogey may be a good idea.