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Uncontrollable 100

Old 10-04-2010, 04:52 PM
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vh2q
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Default Uncontrollable 100

I have this rather large 100" balsa glider of unknown design with flat-bottom wings, 2 channel. I finally got it into the air with a Cox Tee Dee 09 with a 7x4 prop ... the 049 I tried at first didn't have enough power. Problem is the glider banks right and won't respond to full left rudder. If I let off the full left, it banks even steeper and wants to dive. It just keeps turning clockwise all the while drifting downwind at treetop level over the neighbor's place or toward the nearest tall tree, with me hanging on for dear life all the while giving it full opposite rudder. Talk about white knuckles! If I give it some down elevator (I am normally trying to gain some altitude while under power so I have it trimmed with a little up elevator) ie increase the airspeed I can get it to obey the rudder and come out of that death loop. Funny thing, once the engine dies, it will glide perfectly level and it's quite floaty with no sign of banking even at low speed. Now for some history ... it used to do the same thing but turn to the left (anti clockwise) despite full right rudder. The left wingtip seemed a little warped (washed in), so I straightened it out. I also put a little right english on the motor mount. Then it started with the clockwise turning.

So I think at low speed I am losing lift on one or other wingtip (making it bank right or left depending on which wing is stalling). There is almost no dihedral on these wings. Plus at these low speeds it seems the rudder has very little effect (it's quite large see att pic with not a very long lever arm). Once I let it get up some airspeed, it starts to obey ... somewhat but it's still a struggle to keep in control.

I have stopped flying this while I try to figure this out. Anyone got any tips? I am going to take the right english off the motor mount for a start and put a bigger prop on it (to get the airspeed up a bit while under powered climb). Every time I fly this I am rolling the dice ... I put it in a tall tree once already, which ripped the wings off. When I was trying to get it to go with an 049, I took the nose off a couple of times as it would just roll left into the ground. It won't survive another major crash and I just hate to turn those long beautiful wings into matchsticks.
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Old 10-04-2010, 05:12 PM
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fudi50
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Default RE: Uncontrollable 100

Seems to me that probably the incidence on one wing could be off or perhaps the motor is not centered and or down thrust needed.
Ah! adding English try some other country may work. Ok! so I am trying to be funny.
Real snooker players [pool is a derivitive of snooker] dont use the term English its FYI top = above center of cue ball, bottom= below center of cue ball, check side = L/H side of cue ball. Running side - R/H of cue ball.
I am full of useless information as you can see.
The Brit
Old 10-04-2010, 06:16 PM
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da Rock
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Default RE: Uncontrollable 100

If it shows it's bad habits only when the motor is running, there is a good probability the motor is the cause. Turns one way or the other under power suggest the motor is pointed that way.

If your motor mount is one with 4 screws at 4 corners take some washers with you next time out and shim to see how changing the thrust line works for your problem.
Old 10-04-2010, 06:17 PM
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da Rock
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Default RE: Uncontrollable 100

Oh yeah, it's also a good idea to look at the plane from the back to check for warps. Warps in the wing, tail, and twists in the fuselage.

And a good idea to find a more open area for test flights.
Old 10-04-2010, 06:36 PM
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Default RE: Uncontrollable 100

Well, this is going to sound stupid. You didn't mention any damage to the rudder yet the photo clearly shows the upper rudder hinge pulled out of the vertical stab. The bottom hinge doesn't look so health either. If you have fixed that then disregard. If you have been flying it like that then that's likely a source of your problems. The rudder could become misaligned under the wind load of the running engine, and somwhat less bothered under an unpowered glide. Like I said if you have fixed that then disregard my post.
Old 10-04-2010, 07:15 PM
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Default RE: Uncontrollable 100

Yes I have fixed the rudder tabs, that's not the problem. The rudder works. I think the problem lies in the wings (warpage) as well as the motor alignment. I used the washer trick to point the motor right after the urge to spin anti clockwise became evident. Motors are supposed to be offset to the right as far as i am aware, to counteract induced twist, but perhaps with such a large wingspan twist is immaterial. I will try going back to the motor aligned straight on and see how that works.

Yes it would be nice to have 100 acres of hay field but I only have about 12. Problem is trees on 3 sides and 11KV on the other side!
Old 10-04-2010, 08:07 PM
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Default RE: Uncontrollable 100

It seems a case of insufficient dihedral versus too much rudder.

There is a helicoidal airstream from the propeller, which hits the right wing from above and the left wing from below.

Because the roll stability is marginal, it may be disturbed by that airstream.

I would suggest trying to increase the dihedral or adding ailerons (regular or independent spoilers), in order to increase the roll stability or control.

It will good to verify wing wraps and symmetry and lateral balance, as well as taping the hinges' lines.
Old 10-04-2010, 08:08 PM
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Tall Paul
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Default RE: Uncontrollable 100

There appears to be a lack of sufficient dihedral for a rudder airplane.
It needs more, if there's no ailerons.
Old 10-04-2010, 09:16 PM
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vh2q
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Default RE: Uncontrollable 100

I appreciate the insight and it does make sense, but I am not sure what this means:

I would suggest trying to increase the dihedral or adding ailerons (regular or independent spoilers), in order to increase the roll stability or control.

It will good to verify wing wraps and symmetry and lateral balance, as well as taping the hinges' lines
Old 10-05-2010, 12:59 AM
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Default RE: Uncontrollable 100

NICE BIRD,
Try 2 degrees down and 2 degrees right thrust. ( washers under left upper motor mount screw).

I second the lack of dihedral. So ailerons might help. Really not that hard to do but some research here for pics of aileron instalations won't hurt.
Old 10-05-2010, 07:45 AM
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Default RE: Uncontrollable 100


ORIGINAL: vh2q

I appreciate the insight and it does make sense, but I am not sure what this means:

I would suggest trying to increase the dihedral or adding ailerons (regular or independent spoilers), in order to increase the roll stability or control.

It will good to verify wing wraps and symmetry and lateral balance, as well as taping the hinges' lines
Sorry, it is my poor English..........

Additional ailerons can be regular (requires modification of the wing's structure), or added to the top or bottom of the trailing edge (I believe that the proper name is split aileron). In the case of split ailerons on the bottom surface, they create undesireable adverse yaw.

Some full scale airplanes use spoilers functioning as ailerons, which rise up at the top surface of the wing independently one from another. If the spoiler of the right wing i srised up, the lift is spoiled and the right wing falls, initiating a right roll for banking and turning.

Regarding the hinges: it is good practice to cover the gaps with flexible tape or plastic cover. This avoids air leakeage from the high pressure side (the one towards the control surface is deflected) into the low pressure side. This increases the effect and finese of the controls. In your case, the hinge lines of the elevator and rudder are the ones recommended to be covered.

Regarding the dihedral: it may be possible bending the wing tubes upwards a little to gain some degrees of additional dihedral.

Regarding wings wraps: it was a typo, I meant to write warps or twisting along the span. Sorry.

Regards!
Old 10-05-2010, 07:30 PM
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vh2q
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Default RE: Uncontrollable 100

How much dihedral is necessary on a glider with flat wings like this? 10 degrees each side?
Old 10-05-2010, 10:21 PM
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Default RE: Uncontrollable 100

There is no question at all that you have not got anywhere near enough dihedral. All your woes are related to trying to walk a tightrope of stability due to a lack of dihedral. There just isn't enough stability or yaw to roll coupling authourity to be stable or to turn well or even to respond to the rudder inputs. After all with a rudder control it is not the rudder that turns the model. The rudder YAWS the model and then the yaw angle in conjuction with the dihedral effect is what rolls the model and allows it to turn. Not enough dihedral means little or no control. And that's what you have.

On a 50 inch panel you should have a good 7 inches at the tip of dihedral. My calculator tells me that 7 inches is 8.5 degrees for a single angle panel. Let's call it 9 degrees and go for a touch more angle. So the question becomes how best to increase the dihedral? If you have a blade style joiner at the center you could make a new blade with more dihedral angle. If your wing uses a round rod joiner that is flat and extends through the fuselage and into the wings then you won't be able to get 8 degrees out of the center joint and it would get VERY ugly if you try. And before you ask under NO CIRCUMSTANCE can you just bend the rod to get your angle. Because unless that rod is rigidly fixed with some extreme measures into the fuselage it WILL twist at the worst possible time and the wings will do really odd things. And no, just epoxying the rod into the fuselage will not do the trick. It may hold for a while but with the very first hard landing the leverage of the wings will break the joint loose and you'll have a total mess on your hands. So without a blade style joiner you're stuck. That means your best bet is to cut into the panels about 2/3 of the way out and make the wing into a polyhedral wing. You want to raise the outer tips enough so that your total dihedral rise is still 7.5 to 8.0 inches. You can get away with slightly less because tip dihedral is far more effective than simple V dihedral. So likely that means the outer panels would lift up somewhere around 6.5 to 7 inches from the inner panels to get things to add up.

Do all this and you'll find that not only is the model far more tolerant of slight thrust offset issues and slight differences in washout warps but it is more responsive to the rudder inputs.

While you're at it your hinge lines are as leaky as a seive with that big gap. Get some covering material and iron it on over the top side of the elevator and one side of the rudder's hinge line. To do this so it doesn't bind disconnect the elevator and rudder from the pushrods and angle the elevator down away from the seal stripe much further than you would ever use for down elevator and then iron on the sealing stripe over the hinge line. Same with the rudder but angle it over to one side about 60 to 70 degrees. Because of how the strip gets ironed on with bits facing inward that are sealed down this anti leak seal will fold into the hinge line neatly.

Do these things and you'll have a winner of a plane.
Old 10-05-2010, 11:45 PM
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vh2q
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Default RE: Uncontrollable 100

No wonder the person who built this plane put it away after flying it once. When I got it, it had been in the attic for a long time. It's hard enough for me to risk those gorgeous wings, but for the poor gent who spent hours and hours building them, it must have been even harder. You would think that whoever designed this plane would know about dihedral.

There are 3 steel rods glued thru the fuse, bent into a V shape (but obviously not enough V). You can see them in the first pic if you look closely. These fit (tightly) into holes in each wing root, each hole is grommeted with a metal tube. I don't know how long those tubes are, as they are buried in the wing, and the first portion of the wing is balsa sheeted. Anyway, I can see how each of these rods could twist if subjected to a force other than lift on the wings. It's hard for me to see how all three could twist together though, as they are pretty firmly anchored in a parallel layout in the wing root. The wing would have to rotate and partially detach into a swept position. Maybe I could silver solder them to a rod running thru the apex of the V and anchor that rod to a bulk head though, or something similar. That would prevent torqueing. Or just set the apex of the V into a plywood stringer with a notch for each metal rod. This stringer would run between bulkheads.

I was wondering, per the suggestion of others, how hard it would be to cut a 3/4" strip off each trailing edge (solid balsa as best I can tell) for ailerons , and then bury a servo in each wing. I can make a balsa box mount for the servos without a lot of trouble, laying them in flat. I have a bunch of HS 85-MG+ servos here, that are designed to lay flat. The servo would sit inside this box, perhaps foamed into place with "Stuff", or screwed into a gusset of plywood in the top surface of the wing. However that would also require snaking the servo extension wires thru the wing, which means recovering the wings, and I just did that after the tree incident.

I will take a look at the wing construction to see how hard it would be to cut off and make dihedral tips ... these could be doweled so I can take them off for storage. These wings are loooong. Should the cut be along the chord, or should the tips be swept slightly?

And I will tape the gaps with monokote as you suggest. I think the elevator is a bit skimpy for this plane anyway. I won't be able to tape the top of the rudder though ...



PS Mr. Matthews, for a motorcycing moderator, you know a heck of a lot about gliders!
Old 10-06-2010, 10:40 AM
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Default RE: Uncontrollable 100


ORIGINAL: vh2q

How much dihedral is necessary on a glider with flat wings like this? 10 degrees each side?
One of my books recommends 9 degrees per half wing for a 100" span without ailerons, which is about 7.8" at each tip.

If you can add ailerons, that dihedral angle can be reduced in ~25%, I believe.

Matthews advices are priceless.

Those gorgeous wings don't belong to an attic.
Old 10-06-2010, 12:34 PM
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Default RE: Uncontrollable 100

If you wish to go with ailerons instead of polyhedral that is another option. But ailerons raises the workload on the pilot when flying out at distances where you can barely judge the attitude of the model. If you're OK with that then slicing off 3/4 of an inch for ailerons is an option. That's not much control surface though. Response will be soft due to the aileron being so narrow for a model of this size. But if you're OK with that then give 'er a try.

You could make a rib boring tool from a piece of steel rod and brass tubing. Get a piece of 1/4 inch steel that is nice and straight. Onto that solder a 3 or 4 inch length of 1/4 ID tubing with about a 1/2 inch joint overlap. Sharpen the end of the tubing to a knife edge and file a few saw tooth like cuts. YOu'll also need to make a oval hole in the side of the tube back near the solder joint to allow getting in and poking out the rib cutouts. To use the tool lay the wing on a table and line the "drill" up with the table using some shims so it is aimed to cut holes in the middle of the ribs. To "drill" the holes just twist the steel rod in your fingers while applying firm but light pressure. About every three ribs you'll need to pull it back and push out the waste. Some patience and holding your tongue in your teeth just right will soon have the wing drilled for the wiring. Once the drill is inside the wing by a good 5 or 6 ribs you can pinch the covering together and feel the tubing. With this you can aid in correcting any wandering by recentering the cutting edge for each rib. If the covering has dimples in it after this you can just shrink them out.



Frankly I'd say that adding polyhedral breaks isn't really any more trouble than adding ailerons. With the polyhedral joints you would only need to strip the covering back for two rib bays to provide room for the joint and spar joiners. That sounds like the same or even less work to me. And if you have a table saw available you could even set it up to cut the angles nice and neatly with a bit of setup and some calculation for the angles.
Old 10-21-2010, 09:23 PM
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vh2q
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Default RE: Uncontrollable 100

Well I added some dihedral, she now has between 8 and 9 degrees either side, and after getting the balance right (nearly killed myself in the process when this big bird turned and came straight at me, Cox Tee Dee 09 going full throttle and no way to stop it, had to dive out of the way), she flies. Level, straight, and obedient. Reminds me of the spruce goose though, not exactly nimble. But boy does she glide forever. Interesting to land without flaps. I put in an extra throttle servo to allow me to control the throttle, got tired of breaking props. Now I need to get a new Tee Dee piston, as I managed to bend the con rod on mine by flooding it, knew better but patience is not my forte.

Next sunny day, she's going up with a full tank and full throttle!

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