poprlite

My Feedback: (25)

Hey Guys

First off I just wanted to say I'm new to twins and this only my second one so I'm getting different inputs at my flying site as to what the problem is so I'll ask you guys for a little help. The plane is a 126" Cessna 310. Ok here's the deal. On landing approach as it slows it becomes really sensitive to elevator input, with only about an inch of throw on the elevator and a pretty good bit of expo dialed in it's still really hard to keep from ballooning up. I am not using any flaps because the first flight with them resulted in about a week worth of repairs. Not good. Normal regular flight feels fine and the stall seems to be quite neutral only dropping a wing and falling away. Oh other thing is when gets slow and pitchy it will balloon up if I gas and go around for another attempt.

I have not done anything as far as play with the cg because it feels ok in the stall and that makes me want to leave it alone until I get some suggestions. I'm sure I've some details out, kinda hard trying to remember on the spot.

Thanks

speedracerntrixie

My Feedback: (29)

Sounds tail heavy. Check to make sure you have the expo set the correct direction.

poprlite

My Feedback: (25)

Yeah tail heavy is also my thought too. I am positive that expo is traveling in the correct direction. When I show people they're amazed at how little they travel.

Carlos Murphy

My Feedback: (72)

Sounds like you may be a bit tail heavy, try a ounce of lead forward of the CG and play with it.

91zulu

My Feedback: (2)

I have the same problem with my P47. Coming in to land and the slightest touch of elevator it balloons up. Add about 3 to 6 ozs at separate oz pieces. Fly and see what happens. Keep adding until she feels good on the landing. Right now you are on the balance point where it is dead level or there is a better term for it , but my mind is blank now. So the slightest down pressure at the tale pitches the nose up. It is similar to the scale with the 2 disc dangling by 3 chains and when you add a penny on one side it drops.

speedracerntrixie

My Feedback: (29)

I don't think that is going to have much of an affect. The OP may want to see if there is any way to get weight off the tail and/or move equipment forward. The 310 has a fairly short nose os just adding weight to get a CG shift may be closer to 1/2 pound. IMO it would be better to remove 4 oz out of the tail.

gerryndennis

It could be an aft cg, but it might be your elevator throw and expo as well.

Twitchy can be caused by excess elevator throw as well as aft cg, or both

It is possible that your elevator throw is too big (even though you say it doesn't move far), this is tamed by the expo around neutral elevator as in normal flight. But when you slow down for landing you are adding up elevator and moving out of the expo area, any elevator inputs are now larger than before.

You say the cg is OK at the moment (or at least fly able) so why not try changing the throw and expo first?

Do you have dual rates? You could try setting up the rate switch to give you maybe 75% of the throw you have now and reduce the expo.

If no dual rates I would mechanically reduce the throw (shift the clevis out one hole on the elevator) and reduce the expo.

The reduced throw should reduce the twitchiness, and the reduced expo should restore the controllability around neutral.

There are checks you can do for cg position, eg how much down elevator is required for inverted flight. If its very little or none then maybe your cg is too far aft.

If the cg is ok then you need to alter the throw to fix elevator sensitivity.

Hope that helps,

Dave H

ronwc

My Feedback: (404)

It is probably your cg, but you may also need some upthrust in you engine. if you look at your elevator now is it perfectly level or slightly up or down, it may be slightly up to compensate for too much downthrust in engine. Most people trim their planes at full throttle, and if your engine is pulling down, you trim elevator up. When you go to idle, you still have the up trim but engine not pulling plane down, which can cause problems when landing. Trim the plane while your up at about half throttle for level flight. Once trimmed, it should stay about the same or slight climb at full throttle, and go slightly nose down descent at idle. Just something to check.

chymas

My Feedback: (1)

As everyone here has mentioned, it sounds like you have an aft CG. Fly a 45 degree upline and roll it to inverted; if it maintains the same path or climbs you are tail heavy, it should fall off gradually, if the nose falls right away you are nose heavy. Part of the ballooning when you throttle up could be associated with thrust angle. The over sensative elevator is almost always associated with a plane that is tail heavy.

Use Peter Goldsmith's trim chart and follow it from 1 through 10, starting somewhere in the middle just messes things up, each step relies on the previos one.

I realize that this is used more with IMAC and presision flying but you will be amazed what it will do for a scale airplane. One thing to add - when trimming a scale plane with flaps, I will do a flap to elevator mix between steps 4 and 5 flying at 1/3 to 1/2 throttle (depending on the airplane) and start by mixing 3 to 8% down elevator to keep the plane from ballooning when the flaps are deployed.

CK1

My Feedback: (60)

I highly agree with this statement especially with regard to the engine thrust line . If your thrust line is downward you have added up trim to the elevator to counter the downward pull of the engine at speed so that when you reduce power for landing you no longer have the down ward thrust of the engine(s)and the plane now pulls upward and additional elevator to flare causes the plane to baloon. This situation is also compounded by an aft CG and elevator control linkage geometrry.
I would verify CG and move it foreward slightly, reduce engine down thrust (add up thrust appx 2 degrees) move the elevator linkage outward on the cotrol horn (1 or 2 holes if possible). and then......Reduce the amount of expo. If you have more than 30% you need to change your linkage setups. When you flare for landing you may be getting more elevator than you want or need because of the amount of expo you are using
Trim the plane for level flight at 1/2 to 3/4 throttle intead of full throttle.

speedracerntrixie

My Feedback: (29)

Coming back to this there are some things that I did not think about. The first is expo. As some of the other guys have mentioned if you are running too much you may hit the " Step " right when you are flaring for landing. As my rule of thumb I never run more then 40%. I would agree that reducing the amount of expo may be what you want to do after verifying the CG. I do not agree with the engine thrust senario. To cause this much of an issue on this type of airplane it would have to be excessive. I'm going to assume it has been built per plan so the thrustline is not going to be very far off if any. As for the climb to 45 degree upline and roll inverted for for a CG test........This is a viable test with an airplane that has the power to do so but seriously, with a 126" civilian twin? May I suggest that before people comment, google the airplane in question. Some answers read as if the posted did not even know the airplane was a twin or took the size of the airplane into consideration. The picture below is a C-310 close to size as what the OP has.

poprlite

My Feedback: (25)

Thanks for all the replies. I hope to get out tomorrow and do some testing and move the cg forward by moving the batteries forward because I do not want to add unnecessary weight. Maybe this will fix the problem and it's what I'm thinking and hoping it is. As far as flight trimming it was done at half throttle because this is where it cruised along with the best look and feel.

Chymas; An inverted 45 line is how I begin the trimming for my aerobatic planes to set a base line then adjust a little here or a little there but on a twin and in the very beginning stages of test flights I don't think I'm ready to flip 'er over and hold inverted. I also wondered if it would be as accurate seeing how the wing has dihedral and that itself is a built in self correcting feature. Kinda.

As for the travel and expo It is at a minimum. Low rate is less than and inch of travel with some expo to soften things up around center.

speedracerntrixie; I see what you are saying about pulling the stick as the plane slows and then, bam, you have all that throw dumped in at once around half stick. This is not the case because I can hardly even move the stick off of neutral cause it starts looking like a porpoise. I'm talking super twitchy. In flight it is calm, feels normal can complain one bit, but....... slow down and it becomes unfriendly. I will never use the manuals high rate, ever, using it would be equivalent to a monster 3D machine.

Let you know what happens tomorrow.

Thanks

CanDo

I'm betting the aft CG guys are right. Adding some nose weight for the next flight is a safe thing to do, the worst that will happen is you will need a little more speed for landing. On the next flight, take the plane up high to test the slow speed and stall before you land, just to be safe. Good luck!

poprlite

My Feedback: (25)

Update on todays test.

I moved the battery from just behind the cg all the forward next to the nose wheel gear and its much better. I don't think engine thrust is an issue with this plane. I checked it buying flying along at a fast pace above half throttle and without touching the elevator pull the throttle back to idle and it does not behave in any odd way, no pitching up or diving so I feel that the thrust is in good shape. I also made the approach steeper and faster so I would not be around the stall speed. I still need to add the spinners then I think it will be right on the money.

Thanks for all the input.

chymas

My Feedback: (1)

I am quite familiar with what a 310 is, I am also a private pilot and have PIC time in a 310. If the plane is anything like a typical RC plane it is grossly overpowered I am sure it would have the power to fly a 45 deg upline if this is the case.

About expo - 35% expo is just about linear do to the radial track that the attachment point of the linkage to the servo arm makes the "step" fills in the "fall" of the last 20 deg of servo rotation.

speedracerntrixie

My Feedback: (29)

Being that the OP did not state what he was powering the airplane with that would be a bit of assumption on your part. You are obviously familiar with IMAC airplanes. As am I ( 2006 advanced season runner up in the SW ) An IMAC airplane of the same size and weight would be sporting a 170cc. I doubt this airplane has half that power. Moot point now as the OP seems to have everything in hand.

gerryndennis

Glad you are getting somewhere.

If you are uncomfortable or the model is incapable if inverted flight another good test for cg is a 45 degree down line. Trim for upright level flight at half to three quarter power. Push over into 45 degree down and release. If the model continues down or steepens move balance forward. If the model pitches up slowly cg is about right, if it pitches up rapidly you could go back a bit.

As I and others say, once your cg is where you want it adjust throw and expo to give you the feel you want.

Dihedral wont affect pitch stability.

Dave H