My Ultra Stick 40 pitches down whether I'm flying right side up or inverted. The plane balances at the main spar with the tank about 1/2 full. The plane pitches down even with the tank low on fuel. The engine thrust angle seems ok regarding pitch. I'm new at some of the aerodynamic issues but what else can cause this?
r/c spender.

re-balance the stick with no fuel in it as per manual

as far as pitch/porpoising check the elev servo might start their

check for poss glitch points

I can see people logic on wanting to balance with a half tank. What people fail to relies is this has been accounted for in the design. Follow the directions and balance the model empty. And do this with any model you own. People that are telling you to balance with a half tank is simple spreading their own BS thought process.
When in doubt always follow the manufacturer


If and when your worried about fuel effecting flight performance, I wouldn't worries about this except in compitition pattern or 428 racing, then get a bladder fuel tank and a pump relocate the tank to the CG point and rebalance the model. In this case you may want to move to the rear of the CG recommendations to compisate for the fuel not being there, you would do this becuase your not going by the manufacures build spects which would need to be altered, also before you alter the CG point flie it stock first then make an adjustment

What is the actual method you are using for setting the balance? When you say 'the plane balances at the main spar' this is only helpful if you specify exactly how the balancing is done.

What size engine is actually mounted on this plane?

If you have a .40 engine mounted and if you mean when the plane is right side up you must feed in excess UP elevator to fly level and when the plane is inverted you must feed in excess DOWN elevator it sounds like the plane is nose heavy, meaning there is excess weight ahead of the actual lifting point of the wing. But I am assuming you are flying at normal operating speeds for that model.

If you are flying at a very slow speed then the excess deflection of the elevator is probably required just to angle the wing to make the wing produce enough lift to keep the plane level.

E-Mo
When I meant pitching down I meant towards the ground (level flat ground). I'm not having any problem with it porpoising and it is not glitchy. The elevator servo is smooth operating and has no play as I have noticed that problem on another plane before and replace the servo for that. The plane heads towards the ground when it is right side up or inverted.

redfox, I did balance the plane empty originally (not doubting the manufacturer) but when this pitching problem was noticed, I moved the cg back to where now it happens to balance on the spar EVEN with the tank about 1/2 full (with about 5 oz in it). So now the plane is even a little more tail heavy than when I set it up originally. So shouldn't that help to remedy this problem?
I like the idea of the bladderless tank at the cg but my problem is that the plane pitches towards the surface of the earth even with the slightly tail heavy plane low on fuel.

KJohn, I'm balancing the plane with 2 fingers on the main spar just outside the center sheeting where you are first able to feel the spar. I did this indoors so no wind effecting it. My engine is a Saito 72 (one that the manufacturer recommends) weighing about 17 oz but I didn't think the weight of the engine mattered here since the plane was balanced with the engine on it. Also this problem occurs at mid to full throttle too.

Thanks y'all for weighing in on this problem.

If you have not done so already, place the wing on the plane as if you were ready to fly it. Then place it on a surface like a table, but make sure the table top is level. Set the elevator trim so that the elevator surface is level to the horizontal stab. Then shim the back end of the fuselage up until the horizontal stab is level.

Now check to see if there is any upthrust or downthrust. You might be able to 'eyeball' that by placing a piece of cardboard on the surface of the table that has a 90 degree edge (straight up edge). Align that straight up edge to the propeller when the prop is straight up and down. From a few feet away eye along the cardboard and see if there is any significant difference between the two vertical cardboard edge and the vertical propeller. If there is, try to determine the gap distance. This should be pretty close to no difference, but you might see a little bit of down thrust which is probably acceptable or even designed into the model.

Then measure the distance of the extreme front of the leading edge of the wing to the table and measure the distance from the trailing edge of the wing to the table when the ailerons are aligned to the airfoil correctly. How much of a difference is there? You might find a little higher leading edge measurement compared to trailing edge measurment and this might be acceptable and part of the design.

If all or these alignments look reasonable or are per the design then I guess it is still pointing to a balance problem or the plane is just not being flown fast enough.

If the stab is horizontal, the prop is perfectly vertical and the leading edge of the wing and the trailing edge of the wing is the same distance off the table then you are in the ballpark for being set right in those components. The wing could have a little decalage (the leading edge could be higher than the trailing edge) by design. And there might be a little down thrust by design. So you have to allow for these. But any gross differences should be suspect. Also a little error in each of these might add up to a problem in total.

I am assuming by 'it pitches down' you mean it continually tries to nose down more and more as you fly forward. I assume you do not mean the fuselage simply LOOKS like it is going down hill when the plane flies level.

If you set set the throttle at 3/4 of full then trim the elevator for level flight, then throttle up to full throttle, does the plane climb UP or does it being to lose altitude?

OK, silly suggestion but why not add up trim to achieve level flight? But assuming you did this and trimmed for level hands off flight at some point then what are you doing to produce the pitch down effect?

Also if you trim for level hands off flight at mid to low throttle and then dive the model and release the controls what happens? This is the dive test that you can use to set the best CG for your model. Dead nuetral won't pull out of the dive or tuck the nose. CG forward will make the model pull out of the dive. Cg aft of the neutral point will make the model tuck under. Most folks trim to slightly positive or perhaps right on the neutral point.

Any model with some positive stability will try to pitch earthwards when inverted. If your model is doing that then it tells me the model is positively stable and you just need to add forward stick pressure for some down trim to get the elevator to mimic the upright position when inverted. It is this part combined with the upright nose down pitch that has me asking if you are trimming for level flight at mid throttle as well. Please pardon the presumption of something so obvious but since I don't know what flying background you have it seems like the right thing to ask with what you gave.

I'm guessing that this is both a C-of-G and a thrust-line issue. If the model has down thrust, it will require up-elevator trim while in normal flight. When rolled inverted, the dialed-in up-trim is now down-trim & the model will dive inverted, despite having what is now up-thrust. A forward C-of-G, if combined with down thrust, will exacerbate this behaviour.

Thanks guys for your responses. I haven't been out to test fly since my last posting.
But in regards to wing incidence, up or down thrust angle, or elevator trim position, would any of these cause the plane to lose altitude when right side up AND lose altitude when inverted??? Again, the plane loses altitude when right side up or inverted, even from mid to full throttle. Also, varying the throttle does not change the pitch in this range. Can this really be caused by anything but CG, other than replacing all the balsa in the plane with lead?

Just a little more clarification to narrow down the problem.......

Are you trying to trim the plane such that it will maintain level flight both upright and inverted?

Did you have to add weight to the tail to achieve balance? This airplane is notoriously nose heavy unless the battery pack and/or servos are moved back behind the wing. MEASURE how far back the balance point should be, then place strips of tape on the bottom of the wing close to the fuselage. Then your finger tip method might be precise enough to be accurate. Jim

My worries are over!
It was mostly resolved adding 3/4 oz to the tail. Also, I found as britbrat mentioned, the trust angle was down a hair. So after bringing the CG back 4 1/4 inches from the LE (book says 3.5 to 4 inches) and bringing up the thrust angle about 1 degree and re-trimming, it flew as nice as I would expect from a stick. In fact, it was getting more fun than ever when, on the busy Sunday at the airfield, someone flew right through my plane... rekitted both of them.
Thanks everyone for all your advice and support.
rc spender