900 series t28 build, gonna need help

[krob]

Model is all done. I'll probably add more detail and fix some things after successful flights. AUW (with rubber) is 22.5 grams. From my rough calculations, this just squeaks in as acceptable weight for the wing. CG is almost 50% back from LE. At first it seemed this was too far, but my initial test glides seem good. I was doing some short tosses in the house getting farther and farther (not wise, I'm just to impatient). Well before I knew it, I was about 15' back from the couch. One toss went a little too good, and she flew into the window behind the couch. Luckily the blinds were down and it absorbed the head on quite nicely. Well hopefully I'll get a dry day with no wind soon, so I can trim her out proper for gliding. I live in the NW right by the water, so this doesn't happen often! There is an indoor fly in later in the week. I'll try to make that and start adding some power.

[krob]

Just wondering what people use for trim tabs? Any thoughts on my rubber too? I have about a 11" loop of 1/8". Prop to hook is only about 9". I know this is short, but it's all I had for good rubber. My LHS only carries a small selection of Guillow rubber. I had the 1/8" rubber from a peck polymer kit. my AUW with rubber is 22.5 grams. If I manage to not destroy the model, I'll get some proper rubber.

[krob]

Well, I of course got too carried away in the house had a bad toss and crunched the wing on the dog toy basket. Fixed picture below.



Test glides were going good. I made up a dummy weight, to get cg right and added a little right rudder to get her to glide straight with out the prop. Then with prop and rubber back in, I had to add 2 grams up front to get the cg back. AUW is now 24.5 grams. I put an air intake on the nose and stuffed it with magnets, epoxied closed and into place, and then painted to get the 2 grams up front. I started adding power. I'm up to about 300 turns, and the flight profile is same with no matter how many turns straight with a slight climb. Then it starts a mellow turn to the left when power starts to die that slowly gets more progressive till it hits the ground, which is usually in a cartwheel at that point. I'd already added a 1/16" shim top and left to give it a little down and right thrust. That helped. Not sure what to do at this point. Lots I can try but not sure which one. One problem is my right wing has about an 1/8" to 3/16" more dihedral. I think this might be causing the increasing left turn after power starts to die. I'm thinking of maybe making some aileron trim tabs to shallow up the left turn, or a right rudder trim tab. The plane already seems to fly with some yaw though, not sure I want to put more in. What to do? Too many choices, variables!

[krob]

It was about as still as it gets around here tonight, so I thought I'd start over from glide. Well with the prop out and re weighted for cg it glided nice and straight. When I added power, I could tell the left turn was actually starting at the beginning. I think the slight breeze was hiding the turn from happening earlier. I'll read up and and see what to add next, but I'm guessing it'll be more right thrust.

[Scott]

That finished T-28 Trojan looks great,Krob.

[wingnut]

Hey Krob
While I'm no expert at flying these models myself, I do have some education in flight stability and control, plus a little experience with the models heh Anyways, the left turn you're noticing under power is completely normal, it occurs because of the reaction torque from the spinning prop. I would trim out the plane to glide a gentle right turn by maybe adding an aileron trim tab to the left wing. no matter what you do the plane will never fly a straight line due to the varying power of the motor (unless you also add a variable trim tab..but that's not easy to do on a free flight model), but if you start with a gentle left run with lots of power and transition to a gentle right turn with no power you should be alright.

[krob]

Do you normally just add one trim tab to the left wing? Or do you put them on both wings? When you put one on, do you go for it and just glue it on, or do you tape it on first and experiment? Thanks for the help.

[scigs30]

I always put a trim tab under the left wing tip. I figure this is how the plane wants to fly so why fight it. Great job on the build.

[krob]

I cleaned up my thrust adjustments and put the trim tab under the left wing and was pretty happy with the results. The air wasn't very still tonight, so it's hard to say what I learned. With about 300 winds it does a right turn that seems a little tight but not unreasonable Then it seems to still want to turn left towards the end of the flight. That seems backwards to me. Everything I've done so far is to counteract the tendencies of the prop to pull the plane to the left so why when the torque is at least does it want to go to the left. Then again there was some wind tonight, so I'll have to wait for a calm evening and start over. Pretty excited though it's flying. Oh ya, I broke my motor too. I'm gonna have to order some online, since my lhs doesn't carry good rubber. I'm thinking of playing with the prop, rubber combo. By the way it glides, it seems like it should be able to climb and fly slower than it is right now. What are other people using for rubber prop combos on the 900 series?

[krob]

Well I got my rubber in the mail, and my plane is getting heavy. Orginally AUW was 22.5 grams. Then I had to add 2 grams to the nose to get the cg right. Trimming was going good, but plane seemed a little underpowered. Then my motor snapped. The dog got ahold of the rubber. I inspected: seemed okay, worked for awhile, then snapped. Now I have two strands of 3/32 at about 18". The extra rubber through the cg off again, so I swapped the 5" Guillow prop for the 6" to get the cg back. That is one heavy prop. It's like 2 more grams than the peck prop. So now my AUW is 29.2 grams. Motor seems strong and smooth now, though. Weather has been non stop storm here with no end in sight. Not going to get a chance to test her out for awhile.
Just think If I could have made my plane about a gram lighter in the tail, I wouldn't have had to gone down this road and my plane would be 7 or 8 grams lighter. Oh, well!

[krob]

Got my new rubber. Second flight I went for about 300 winds, flew great untill power wound down, and it went into it's left hand death spiral again breaking the wing off. I'll be able to fix it. I think I'll try putting my trim tab all the way at the end of the left wing. Hopefully that'll keep her in the right turn.

[krob]

Finally! It flies and doesn't crash for a landing. My limited time off and calm dry weather have not been coinciding lately, so I had to wait a long time to get to fly it. Awhile back, I moved the trim tab all the way out to the end of the left wing, hoping to get rid of the left death spiral. Well first launch, it banked hard right and tore the right wing off. Took me awhile to make that repair. Tonight I started with the trim tab in close and slowly worked it out the left wing till the left turn at the end of the flight disappeared. I'm not sure on how long it was up but it flew one big right circle on about 350 winds. I should easily be able to wind the motor up to 600 winds.

[krob]

http://www.youtube.com/watch?v=IeLy1U_IrzI

Short flight. I think I could get it upto 20 seconds winding the motor all the way up, but I'm happy to just see it flying at all. Time to move onto the next project a rc conversion of a 600 series Cessna 180.