CG & stability
 

I designed a plane for 3D practice (it's the one in my avatar) and am completely happy with it except for the extremely aft CG location. It's at approx 50% of MAC, which makes landing very touchy. If the CG is moved forward from this location, you can't just apply up elevator during landing to bring the nose up/flare, power is required as well. I want to build another one, but would like to be able to place the CG in a more conventional location and have the elevator be effective during landing. Any suggestions?

RE: CG & stability
 

Make the horizontal 50% bigger. Looks like the tail span is good, but you might increase the chord. This does 2 things. It expands the allowable CG range. So, you can keep the CG where it is on the wing, and have a better behaved plane. The second thing is it increases the pitch damping. That is, even when it's balanced agressively the response will be slower and less twitchy. Think about holding a piece of cardboard in your hand and waving it like a fan. THe larger the fan, the slower you can move it. That's not to say elevator response will decrease, just the twitchiness. The good news is, you could try his mod just by building a bigger elevator and sticking it on there.

Can I ask how you determined that the best CG location is 50% for this plane? I move my CG on similar planes until I get level flight upright and inverted. Usually that doesn't involve going back as far as 50%.

RE: CG & stability
 

I guess this will actually do three things if you add all the area in the elevator:

1) expand CG range
2) increase puitch damping
3) increase elevator authority

Sounds like all 3 will be good for your plane's behaviour.

RE: CG & stability
 

And one more thing!

One other thing I notice is the tail distance from the wing. I know you want to keep the tail arm short for maneuvering, but moving the stab an inch or 2 back wouldn't hurt.

RE: CG & stability
 

Thanks John! Yes it was determined through upright & inverted flight. Still requires 15% down elevator or so to hold striaght & level inverted, but if you over do it a little bit, she'll go in to an inverted loop very quickly. She's a wild child, but man can she do some tricks, just wish it was a little more stable. BTW, the WS is 48" and it has the same tail moment as my TopCap now, roughly the same horz. tai lsurface area, and far more control surface area. I think the avitar pic might be a little on her back. This might help. One thing that I wanted to do was drop the wing down, thinking it would increase airflow over the tail, but was advised not to do this by Frank Noll. Everything is in line - motor, wing & hoz. tail. One more thing - airfoil is a NACA 15, a little thin by fun fly standards. Stalls great, but I was going to bump it up 3% or so on the next one to decrease the stall speed. At this point, I'm stuck with trial and error, so appreciate any help!

RE: CG & stability
 

In your picture it appears as though both ailerons are deflected upwards. Are you using any aileron-flap mixing?

RE: CG & stability
 

No - just the way they ended up at the time of the photo. They are centered for flight and I've not messed with mixing on anything other than rudder to elevator.

RE: CG & stability
 

How about moving the wing back?

RE: CG & stability
 

I agree with Johng. The overall stabilizer/elevator area appears too small for such a short coupled airplane. How does it stall at altitude? when approaching the stall from straight and level flight, can you raise the nose to a landing attitude, or does it stall practically level? The thin section might contribute to the problem. Does it pitch up at the stall, or just drop it's nose. These will all give a clue as to what the problem is.

RE: CG & stability
 

Stall characteristics are great. Will not drop a wing, just mushes nose up IF you have CG back far enough. With the CG forward, it keeps flying, but nose down.

RE: CG & stability
 

Here's a picture of the plane I'm basing my judgments on: It's a Stinger with enlarged tail surfaces. If you look at the horizontal and vertical, the lime grees parts are 4" wide extensions that I added to the rudder and the elevator. That is, the tail per-plans is the blue and white part, and the green is area I added on myself. 2x elevator extensions means I increased elevator span by 8" - which gave me a stab span at least 1/3 of the wing span.
http://www.rcuniverse.com/forum/upfi.../2485_2790.jpg

This plane balaced neutral right around 40% chord. That is, I could flip it inverted and level, and it would stay level. A little more tail heavy, and I'd flip it inverted and it would push toward vertical. However, it was completely stable and well mannered otherwise.
In fact, I once had 4 oz nose weight (on a 20 lb plane) break lose and slide all the way to the tail cone. Then it got a little divergent, but not real squirrely - that's what a big tail does for you.

With the length of the tail boom on this plane, it seemed to have a good balance between tracking and tumbling maneuvers.

RE: CG & stability
 

Alright John, you convinced me! I'll give it a shot! I really thought that the tail was large enough already given the amount of authority it has. It'll do a waterfall on command, at any speed (sorta cool!) and KE spins, well at half throttle you just roll her on her side & give 'er full down, walla, a KE spin. I'll do it though, and see what happens. :) Thanks!