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Old Aug 06, 2014, 08:36 PM
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Help me trim my Sig Smith Miniplane

Hello, I recently test flew my newly built Smith Miniplane. To my surprise it was badly out of trim in the pitch axis, I had to apply all of the down trim just to maintain level flight. I have the CG set per the plans and zero degrees of thrust on the motor (converted to electric with an Eflight 46). Any ideas to help this problem? Someone in another thread suggested adding negitive incidence on the top wing or negitive incidence to the stab. If this is true I can manage the top wing adjustments but the stab is built into the plane and no adjustments can be made with out totaly reworking the rear of the fuse, something I am not easily willing to do since it is covered in Solortex and painted. Any suggestions? I am a seasoned kit builder with 40-50 builds but I'm no expert and have not experienced this problem before.
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Old Aug 06, 2014, 08:46 PM
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United States, TX, Leander
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A few of my bipes needed quite a bit of down trim to fly level. Negative 1.5deg on the top is good to have, helps lower the stall speed. You try some down thrust, about 2 or 3 deg to see if it helps some. Some positive 1.5 to 2 deg in the stab would definitely help, but might not be practical without a lot of hacking. You could just live with it. It's not like it's a 3D plane.
Edwin
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Old Aug 06, 2014, 11:07 PM
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Reducing the positive wing incidence may help. Normally the top wing has more positive incidence than does the bottom wing. The result is that the top wing will stall out first, dropping the nose slightly, and then fly on the lower bottom wing.
Perhaps try reducing the incidence in both wings (lift the trailing edge up) by one or two degrees.
Having to use "down" elevator trim would also suggest that the stab would like to be a little more positive, not negative.
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Old Aug 07, 2014, 05:31 AM
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Traverse City, Michigan
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Are we talking about two different things,,here?

Edwin is saying negative top wing incidence. In my mind, that would raise the tail, just like positive incedence in the stab.

Am I missing something ? I'm getting ready to set up a bipe myself, and want to get it right.
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Old Aug 07, 2014, 05:55 AM
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United States, VA, Fredericksburg
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My Smith Miniplane flies the same way as yours, I ran out of downtrim on the maiden. The recommendations I saw while building it were to put 2 degrees positive incidence in the horizontal stab , or 2 degrees negative in the top wing. That should keep it from "ballooning". Like you, my stab was epoxied in place. What I wound up doing was manually shortening the linkage to the elevator, give it about 1/8 inch or more down angle at neutral on the trim. She flies really well this way. She still has a tendency to lift when I go to full throttle, but not bad and I usually don't fly at full throttle, she does well at half to 3/4.

At some point in the future I plan to add the negative incidence to the top wing, but for now I'm having too much fun flying her as she is!

Joe
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Old Aug 07, 2014, 06:32 AM
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On my three bipes, two flew best with -1.5deg on the top wing, 0deg on the bottom. The plans on both had the engines at 0-0, and the stab at 0. The other plane was a sig sun dancer, the big one at 70". Everything is at 0. It flys alright and is a bit twitchy but way more maneuverable than I need. The other two bipes were Ziroli stearman and busa phaeton 90. I flew both those planes originally with different incidence setups for the maiden.

The P90 wings were at 0-0, the stab at 0, engine at 0-0. It flew ok but with 1/4" down elevator trim for straight and level flight. The biggest problem was landing, it would float in ground effect for a long way. You had to force the plane down to the ground before it stalled. Changing the top wing to -1.5 fixed the ground effect problem. It gave me a nice gentle predictable decent rate and slowed down the landing speed to almost a walk. The P90 was a 14lb plane.

The Ziroli stearman is built with +1.5 on the top wing, 0 on the bottom, 0 stab, and 0-0 engine. Total weight is 20lbs, this is the small stearman at 77" ws. This was a half built swapmeet purchase years back. It's maiden was similar with 1/4" down elevator trim, but I had no problem landing. At that weight it landed heavy, but not bad. A very nice flying plane. Built per plans gives you +1.5 on the top wing, that's how I flew the maiden, seemed fine to me. I changed the top wing to -1.5 as an experiment. I noticed no difference in flying. It still 1/4" down elevator for hands off and has a nice predictable decent rate to the ground. It just changed which wing stalled first. I left it with -1.5 since I'm comfortable with the way it flys like that and don't want to back out the changes.

On my next bipe (another P90) I will give the stab about +1.5 deg. I think this will fix the down elevator trim issue. The stearman and (I think) the P90 have semi symmetrical wings which may explain why the sun dancer was sort of different. The sun dancer has symmetrical wings and flys with a G-62. The other two fly with sport type power.

I like bipes and like to experiment with setups. They may not be scale setups, but the flying is a bit better. My favorite bipe is the busa P90. Modified with ailerons on both wings, it does great slow speed aerobatics. You go full throttle you can do some yahoo kind of flying too, a pretty wide flight envelope. I powered my first one with a saito 1.20, a perfect match. It met its end with a dumb thumb doing a low inverted pass. Made a 50 yd debris field, spectacular.
Edwin
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Old Aug 07, 2014, 04:43 PM
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Thanks for the responses guys! I said in my origional post someone suggested negitive incidence on the stab, I ment positive. I am going to try a little negitive incidence on the top wing and see what happens, I must admit I am a little dissapointed in the way it flys, it's one of my better looking builds and don't want to hack it up
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Old Aug 07, 2014, 06:12 PM
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Some quarter inch spacers on the back struts and cabanes should do it.
Edwin
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Old Aug 07, 2014, 11:46 PM
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that is by far one of the better Mini's I have seen! I built one a couple life times ago, that I got too the paint stage, covered in natural Solartex!

lost in a divorce!

that is why I am still a fan of the mini, and would love too build another!

if it where me, I would put a washer under the back of the wing! and buy some stuff too make a new set of cabanes, with the proper incidence!
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Old Aug 08, 2014, 04:44 AM
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I'm going to do some math (frigonometry) and see what kind of spacers i will need an like whiskeykid says make a new set of N struts if I have to. If I had only known while building it I have a bunch of pictures throughout the building process, I may do sort of a built type thread soon.
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Old Aug 08, 2014, 06:38 AM
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Thanks for the detailed replies. They confirm my plan for the incidence set up.

Green River. My brother used to live there. At the time, he didn't fly RC, but he used to stop by, and watch you guys fly.

He carried that interest when he moved to Colorado, and now has 5-6 flying models.
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Old Aug 08, 2014, 12:04 PM
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Only change one thing at a time so that you know if your on the right track...
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Old Aug 08, 2014, 04:54 PM
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I find it hard to comprehend that after all of these years SIG would still be distributing a kit that has grossly misaligned wing and tail incidences. Meaning no disrespect, but I think that your problem has more to do with your flying technique than with the airplane parameters. The relative angle between the wing and the stab determines the angle of attack at which the wing moves through the air.

For any given wing angle of attack there is only one power setting at which the airplane will fly at a constant altitude. Exceed that power setting and the airplane will ascend, decrease that power setting and the airplane will descend. The next time you fly the airplane, instead of adding down elevator trim to make the airplane fly at a constant altitude, reduce the throttle setting. Once the throttle is set to a power level where the airplane holds a constant altitude you are at the power setting and airspeed at which the airplane was designed to fly.

If you prefer to fly the airplane at a different power setting and airspeed than designed, either faster or slower, you need to change the relative angle between wing and the stabilizer so that the wing will move through the air at a different angle of attack. The simplest way to do that is to input up or down elevator trim. The hard way is to change the physical incidence of the wing and/or stabilizer. Even simpler is to fly the airplane within its design parameters.

That being said, there is one other item that can change the flying characteristics of the airplane. That is CG location. Moving the CG rearward will make the airplane less longitudinally stable and at the same time make the elevator more sensitive. With the decrease in longitudinal stability the airplane will still climb when the power level exceeds the level required for constant altitude flight, but it will not pitch up as much with increased power. Also with the elevator being more sensitive it will not require as much down elevator trim to make the airplane fly at a constant altitude with a higher power setting. The only caution here is not to move the CG aft of the aircraft's aerodynamic center. Doing so would make the airplane longitudinally unstable and very difficult to fly.

Larry
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Old Aug 09, 2014, 02:31 AM
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Quote:
Originally Posted by TomCrump View Post
Thanks for the detailed replies. They confirm my plan for the incidence set up.

Green River. My brother used to live there. At the time, he didn't fly RC, but he used to stop by, and watch you guys fly.

He carried that interest when he moved to Colorado, and now has 5-6 flying models.
Perhaps we have met, I'm not good with names and faces though HA!
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Old Aug 09, 2014, 06:25 AM
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Quote:
Originally Posted by green river rc View Post
Perhaps we have met, I'm not good with names and faces though HA!
He's a tall, thin guy. White hair hanging down to his rear end. He was probably riding a motorcycle, or driving a Mazda Miata when he visited your field.
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