Author Topic: Not tail heavy but flys like it is (Read 4910 times)

Bob Reeves · « **on:** May 24, 2008, 06:07:36 PM »

Any ideas on what would make a stunt ship fly and glide like it's tail heavy but the numbers say it isn't. The CG is 21% at MAC and the tail to wing ratio is 22%. According to everything I've been taught this should not be a tail heavy ship but it sure flys and glides like it is

Bootlegger · « **Reply #1 on:** May 24, 2008, 06:11:37 PM »

Bob, Does it "hunt" in level flight? Might add 1/4-1/2 oz nose weight and see what it does...

FLOYD CARTER · « **Reply #2 on:** May 24, 2008, 06:27:03 PM »

Bob. If it's unsteady in level flight, and slows up too much on the glide, then it IS tail-heavy, regardless of what the plans say. Nose weight is the easiest "fix".

Floyd in OR

Bob Reeves · « **Reply #3 on:** May 24, 2008, 06:56:12 PM »

Stab has about 1/2 deg positive incidence, just to be sure it wasn't neg.. engine has about the same down thrust. It really balloons bad coming into the wind on a glide.. Can fly it level with the engine running but have to concentrate. Seems to turn OK and not bobble out of a square corner.. Weird.

Bob Reeves · « **Reply #4 on:** May 24, 2008, 07:55:33 PM »

This one...

http://stunthanger.com/smf/index.php?topic=8989.0

Dennis Adamisin · « **Reply #5 on:** May 24, 2008, 08:08:39 PM »

I would try dialing in about 1/8" down elevator (i.e., withn elevator is neutral the flaps will then be down just a hair...)

Robert McHam · « **Reply #6 on:** May 26, 2008, 11:35:51 AM »

Quote from: Ty Marcucci on May 24, 2008, 06:22:25 PM

Before you put it back in the air, please check all three indences, engine, wing and stabilizer. If any one of them is off, it will fly like it is either tail or nose heavy. Especially check the stab incidence.

If it flys like it is, then check the engine, but if it glides the same, then it almost always is the stab. This is of course assuming it is in fact balanced at the right spot. Usually balancing it at the high point of the airfoil at the root is a good starting point, even knowing the MAC.

All very important indeed. If these check out properly then I would assume tail heavy regardless what the plans say.
Most plans indicate where the designer found to be the best for him and his prototype/s. For many this point will also be correct but for some it won't "feel" right or in your case does not fly as though it were well behaved. Therefore use the indicated CG as a starting point and in your case I would start adding small amounts of nose weight till the plane feels better. Then add some more and see if it makes it change. If it gets worse with some other undesirable behavior then you know how much to take back out.
It really helps flying both sides of the CG on your own model to find your own "sweet spot"!

Record your findings so that if someone else has the same troubles you will have the ability to back your opinions if someone is a nay sayer.

Many sport designs even show a CG range to get you close. One still needs to fine tune. As for many of the better designers for stunt, when they indicate a CG you better be really close to it for a start if not dead on it because this mark usually is more accurately supported by past experience. A mark that can be trusted more so than the sport designs for the masses.

Robert

Jerry Tarnofsky · « **Reply #7 on:** May 26, 2008, 01:53:18 PM »

I was just out in my yard test flying a new plane, which was very twitchy and 'floated' on engine shutoff. It too had been bench tuned to according to Ted Francher’s numbers. It even was ‘flipping’ the wing as it passed on the upwind side. This was my first plane that did this after the bench setup.

Turned out 1/2 oz. of nose weight helped, and 1 oz. got it flying much better. Made appropriate lead out guide adjustment, 1” behind the new C.G. Still turned well with a good wingover, etc. Had to quite ‘cause the wind really started to blow.

My conclusion, no ones ‘numbers’ are exact for ever plane. Just throw in some nose weight and see how it fly’s and turns.

Good luck,
Jerry

Dennis Adamisin · « **Reply #8 on:** May 26, 2008, 02:30:18 PM »

Quote from: Bob Reeves on May 24, 2008, 06:56:12 PM

Stab has about 1/2 deg positive incidence, just to be sure it wasn't neg.. engine has about the same down thrust. It really balloons bad coming into the wind on a glide.. Can fly it level with the engine running but have to concentrate. Seems to turn OK and not bobble out of a square corner.. Weird.

Hmmmm, should have read this closer before

: Bob if you are trimmed for straight & level power on - with downthrust - when you go power off you lose the downthrust, and the airplane will balloon...

Bob Reeves · « **Reply #9 on:** May 26, 2008, 04:43:48 PM »

Thanks guys,

Didn't get a chance to fly it this weekend, will think about the responses and hopfully get a chance to try a few things next weekend.

RC Storick · « **Reply #10 on:** May 26, 2008, 04:57:27 PM »

Quote from: Dennis Adamisin on May 26, 2008, 02:30:18 PM

Hmmmm, should have read this closer before : Bob if you are trimmed for straight & level power on - with downthrust - when you go power off you lose the downthrust, and the airplane will balloon...

Thats why I said with flaps level add some down elevator.

EDIT: Different post but it will still work!

Jim Kraft · « **Reply #11 on:** May 26, 2008, 05:52:00 PM »

Hey Bob; Maybe it needs a little Motorola "Mo-Trac" installed somewhere.

Mark Scarborough · « **Reply #12 on:** May 26, 2008, 06:38:16 PM »

Bob, If I recall this was an RC plane you converted to control line? if that is the case you really just are going to have to adjust to fly right in stead of by what the "plans" say, or numbers say. If you already have incidence in the horizantal stab,, (leading edge up) then adding more elevator down trim is unlikely to be a solution. It really does sound like a weight and balance problem. If your stab was 0 incidence I may agree with adding the down trim. The symptoms you describe are classic tail heavy REGARDLESS of where it balances it is flying tail heavy. It really should just go straight to glide without changing pitch angle when the engine powers off. As noted, Paul Walkers (to Brett to Scott) trim chart is very helpfull in this direction.
I have had two Orientals back to back, and they fly at different CG locations. incidence is straight up on both. Just do what it takes to make it fly right and remember the plans are a guide. Especially if its RC plans
My 2cents worth.

Doug Moon · « **Reply #13 on:** May 26, 2008, 06:42:10 PM »

Keep adding down elevator until you can sleep while it is in level flight. Or you can keep adding until it starts to fly bad again. But I think that will do the trick. Most RC planes have tons of down thrust but once the right amount of elevator trim is achieved dead stick glide is just an extension of power edflight.

Bob Reeves · « **Reply #14 on:** July 18, 2008, 08:30:56 AM »

First chance I have had to fly the silly thing was the other day... I have been adding in down elevator and it does seem to help a little.. It's now drooped farther down than what looks like enough. The big problem is still in level flight.. Joe Gilbert and I both flew it and came to the same conclusion.. It's a bear to fly level but it turns like it's a little nose heavy.. No way it is tail heavy, also even though this started as an RC kit it has no resemblance, numbers wise to the kit.. The numbers are based on a proven design including wing, stab, cg and moments. The controls are all Tom Morris ball link and are not stiff by anyones standards..

It however does fly like it has stiff controls.. If you take your mind off it it will grove but if you try to maintain level flight it starts wandering and you can't help but over correct.. If it didn't have ball links I would add some slop to the elevator and that would probably fix it.

Any other ideas? Maybe slowing down the elevator in relation to flaps?

BTW: Never try adding nose weight around the engine with modeling clay

Dennis Adamisin · « **Reply #15 on:** July 18, 2008, 08:58:49 AM »

To get rid of the heavy control feel, if the flaps are full span - clip the last 4"-5" and set as stationary. The effect will be about the same as a ounce of tailweight...

john e. holliday · « **Reply #16 on:** July 18, 2008, 06:46:34 PM »

Every bodies missed it in my opinion. My Falcon flew like it was tail heavy. Had CG at 25% of mean chord. Tried narrower spacing on handle was no help, in fact made it worse. Tried adding lead by filling back plate of the engine and lead under it also. Even had Higley heavy hub on it. Finally cut into the fuse and removed bell crank bolt so I could drill new hole closer to pivot point and moved the push rod. After that I had to remove the hub and most of the lead. Also got to go back to my E-Z Just style handle. Plane flew like it was on rails, but, outsides suffered a little for some reason. DOC Holliday

Steve Helmick · « **Reply #17 on:** July 18, 2008, 11:08:53 PM »

Bob...is the elevator thinner than the stabalizer? Pretty standard for R/C...may not be the hot setup for CL stunters. Are the flap and elevator hingelines sealed? Any chance there's a flutter? If none of the above, move the LO's back 1/4" and try that.

Steve

Bob Reeves · « **Reply #18 on:** July 19, 2008, 04:58:21 AM »

Humm.. Wouldn't a thinner elevator make it less sensitave around neutral, maybe that is what it needs? Also no the hinge lines are not sealed, I understand the reason for it but sealing the hinge lines might (probably would) increase the efficiency of the control surfaces, I don't think it would solve the problem and if anything make it worse.

BTW: Other than the problem around neutral it doesn't have any trim issues, turns flat and tight if I ask it to, any feeling of nose heaviness was put in to try and solve the hunting problem. Easily undone like going back to a Brodak spinner instead of the Great Planes.. That would knock an ounce off the nose real quick.

Think I'm going to block it up on the bench again and go over everything for the 3rd time just to be sure I didn't miss something.. Have a new digital incidence meter I'm dying to try out.

Thanks for the tips gang.. If I ever find the cause I will for sure let everyone know what fixed it.

Steve Helmick · « **Reply #19 on:** July 19, 2008, 06:53:47 PM »

Bob...the thinner elevators (than stab) make the elevators less responsive around neutral, but when you want a slight correction, you don't get anything. So you have to give a bit more control, and all of a sudden, you got more than you wanted. The TE shape makes a difference, too. What works for RC doesn't matter, if it doesn't work for CL. There's lots that doesn't.

Steve

Bob Reeves · « **Reply #20 on:** July 19, 2008, 08:07:55 PM »

OK, that makes sense but isn't the problem as the stab and elevator are identical to another ship that is fine and as I said this ship has nothing in common with the RC kit except it's looks. Looking like one of those things that is almost impossible to troubleshoot over the internet but thought I would give it a shot.

Doug Moon · « **Reply #21 on:** July 19, 2008, 09:07:42 PM »

Bob,

Adding down elevator is not helping. So onto the next thing.

I am confident you have all the bench trimming correct as you know what you are doing. The next thing to do is add a TON of nose weight. It will take alot more than you think to even move it 3% at MAC. Dont use hubs or spinner weights as they are rotating masses and create serious Gyro effect and cant load up the handle considerably. If it flies like it is tail heavy and glides like it is tail heavy then it is tail heavy. You know the saying If it walks like a duck, looks like a duck, and sounds like a duck, it must be a duck.

Another thing to watch for as you work it over is the inside and outside turn rates. Even though it is twitchy and giving you trouble you should be able to get the in and out turn rate close with pushrod adjustment.

Keep us posted...

Dennis Adamisin · « **Reply #22 on:** July 20, 2008, 07:39:17 AM »

Quote from: Bob Reeves on July 19, 2008, 04:58:21 AM

...sealing the hinge lines might (probably would) increase the efficiency of the control surfaces, I don't think it would solve the problem and if anything make it worse.

Bob:
OK, BEFORE you cut the flaps (!) I think sealing the hingelines is a fundamental step in ERASING any impact they have on flight aerodynamics. It CAN improve the grooviness, it CAN improve the handle heaviness, but more importantly it gives you a proper benchmark point for trimming: the effect of no gaps is easier to predict than the effect of having gaps... (huh?)

I recommend sealing the gaps before you do anything else.

Ted Fancher · « **Reply #23 on:** July 20, 2008, 08:52:20 AM »

Quote from: Dennis Adamisin on July 20, 2008, 07:39:17 AM

Bob:
OK, BEFORE you cut the flaps (!) I think sealing the hingelines is a fundamental step in ERASING any impact they have on flight aerodynamics. It CAN improve the grooviness, it CAN improve the handle heaviness, but more importantly it gives you a proper benchmark point for trimming: the effect of no gaps is easier to predict than the effect of having gaps... (huh?)

I recommend sealing the gaps before you do anything else.

Hi Bob,

Having been reawakened about the obvious reasons to maximize the efficiency of our lifting surfaces by none other than Mr. Adamisin over lunch about a bazillion years ago, I heartily endorse the above. It seems like a pain in the neck the first time you do it but it quickly becomes a fairly minor task that should ALWAYS be done prior to first flights. Ever since returning from that lunch and sealing the hinge lines on a pretty good stunter (which then became a very good stunter) I've never flown a ship for the first time that didn't have the hinge lines sealed. I've never regretted doing so.

The single most important thing to remember about doing so is that, unless you screw up the installation and stiffen the controls, there is "NOTHING" bad that can come from doing so. If you DON'T seal them, something unpredictable IS going to happen whether it is immediately noticeable or not. If the hinge lines aren't sealed and you have trim problems that seem to defy solutions the proper solution to those problems can almost certainly become more easily correctable by eliminating the potential source of a myriad of problems that can develop as a result of changing lift co-efficients resulting from the gaps.

Only thing I'd add to Denny's comments is that when you seal the controls you bring everything else back to neutral (as much as possible giving the built in biases) and start anew.

Ted Fancher

Just curious and not intending any criticism but, how certain are you of the accuracy of the engine down thrust and the stab incidence?

Charlie Pate · « **Reply #24 on:** July 20, 2008, 03:01:59 PM »

Bob, is the front leadout up or down elevator ?

I defiantly agree with TF and DA about sealed hinge lines (my experiences). ( Boy! that should make their
credibility ratings rise!)

Bob Whitely · « **Reply #25 on:** July 22, 2008, 02:21:23 PM »

Might add one more thing that taping the hinges does. Once taped you are getting 100% efficiency from the control surfaces which leads to the effect of making the elevator loads heavier as it takes more effort to overcome the flap deflection pressure. If all you do is to tape up the hinges and go fly you will feel the difference. As the rest say one must never, never fly a model without taping the hinges as it just wastes fuel and time better spent on all the rest of the trimming process. RJ

Jeff Reeves · « **Reply #26 on:** July 23, 2008, 01:35:24 AM »

Do what Bob Whitely and Ted Fancher say, even if you don't understand the reason at first.

Howard Rush · « **Reply #27 on:** July 23, 2008, 02:39:55 AM »

Much as I dislike not being original, I'd recommend taping the hinges. It's an easy way to eliminate a bunch of bizarre effects, none of which you want. Any anomaly left after the hinges are taped will probably be pretty straightforward to fix.

Bob Reeves · « **Reply #28 on:** July 23, 2008, 11:10:59 AM »

Ok OK the hinge lines will be taped before it's next flight...

minnesotamodeler · « **Reply #29 on:** July 23, 2008, 03:41:41 PM »

I think you oughtta tape the hinge lines...don't know why no-one's mentioned it before.

minnesotamodeler · « **Reply #30 on:** July 23, 2008, 08:05:03 PM »

Eight through Howard Rush, in other words not counting our last two...Spose he got the message?

Chris McMillin · « **Reply #31 on:** July 23, 2008, 10:16:43 PM »

Stunt News, August 94 there is an article by some guy talking aboout his big stunter being a dog until he taped the hinge lines. Says Ted Fancher and Bob Whitely told him to do it.
Chris...

minnesotamodeler · « **Reply #32 on:** July 24, 2008, 03:52:38 PM »

Nine...

john e. holliday · « **Reply #33 on:** July 25, 2008, 07:31:56 AM »

Now you know why I like cloth hinges. DOC Holliday

Bob Reeves · « **Reply #34 on:** July 28, 2008, 06:20:46 AM »

Well as suspected, taping the hinge lines didn't improve anything. I have done it on a couple of my ships, mainly to satisfy the sealed hinge advocates but never had it make any difference in the way they fly. My gaps are almost non existant and consistant enough sealing them doesn't seem to be the magic bullet others have found it to be.

This ship has been a jinx from the start, so many things have gone wrong I'm about ready to give up and just chalk it up to experience. This weekend the canopy melted from sitting out in the sun and just not sure it's worth trying to fix.

I did add two ounces of lead to the nose and it improved the level flight characteristics but now it really flys like it's nose heavy, which it is. On top of all this it has the exact same tank that is in my Score, both Saito 56's, same fuel, prop everything... This engine runs 4 minutes on 4 ounces, the one in my Score runs 6 1/2 minutes on 4 ounces. Easy to fix with a 6 ounce tank but just another issue to deal with.

Dennis Adamisin · « **Reply #35 on:** July 28, 2008, 09:19:28 AM »

Some birds seem more sensitive to the hingleline treatment than others - it does not seem to be related to JUST the gaps. Don't have any idea why and don't really care anymore - Just know that I can ELIMINATE all issues by sealing.

Bob, maybe (as you noted) the bird is destined just to be a "life lesson" but then again, maybe you can use it to try some different ideas. A few posts ago I recommended clippint the last 4"-5" off the flaps (making the ends stationary) as a means of reducing control load and improving the corner. I have had it work and seen it work for others

Maybe you can try it here...?

Bob Reeves · « **Reply #36 on:** July 28, 2008, 11:55:58 AM »

Thanks Dennis, had high hopes for this ship but am at the point of pulling the engine and hanging it up till I decide to put more time in it or forget it ever existed. Kinda want to stick the engine in my Score just to see if the short run problem follows the engine.

It's pretty frustrating when your best flying PA ship is an ARF you threw together just as a 4 stroke test bed. The good news is my new Ringmaster 576/Saito 40a is comming through the trim stages great and my Skylark/ST 51 is finally settling down into a diecent flying classic.

Your flap idea is a good one and if I do decide to take it farther that's one of the things I'll do. I'll have to cut the canopy off and refinish the fuselage anyway so a little more slicing won't be a big deal..