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Problem establishing sweet spot trim in mod Easy Star

Old 06-30-2010, 07:12 PM
  #1  
jluther
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Default Problem establishing sweet spot trim in mod Easy Star

I use Easy Stars in my aerial photography business. I've modified them to carry my cameras and perform well. With all the modifications I still use the specified CG and a 1 1/2 oz. tail weight to achieve very satisfactory power and glide trim. But, after a few hard landings and constant use the planes will go to diving off launch, tight circles that will go to moderate spins etc. I go back through my standard set up and look for the changes but to no avail. I think the CG is moving around and I don't have a solution for establishing and trimming to a new CG. Any ideas would be very welcome.
Thanks.
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Old 06-30-2010, 08:51 PM
  #2  
JetPlaneFlyer
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It's easy to see if the CG is in the right place.. Just balance the model on your fingers and see if it balances level with your fingers on the correct position as indicated in the plan/instructions. Also check lateral balance (i.e. one wing heavier than the other)

If it used to fly ok and you have not added anything or taken away anything then it's hard to imagine the CG has shifted. It's more likely that something has got bent or the wings and/or tail surfaces have become twisted... You should be able to spot any bends/twists by careful visual inspection. Check the tail boom is not bent and that the prop/motor has not got bent out of alignment. Make sure the wings and tail surfaces are twist free. Also check that the control surfaces centre correctly and move freely and equally in both directions.

Steve
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Old 06-30-2010, 09:02 PM
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jluther
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It feels and flies like an angle of attack problem, but when I put straight edges on the stab and wing they line up as specified. I'll give all the leading edges a check for warp and twist.
Thanks you.
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Old 06-30-2010, 10:50 PM
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Or have you added 'stuff' to the model and made it overweight?.. That would certainly explain the diving when launched. If you try to fly slowly with an overweight model would explain the spins too ... what is the weight?

Steve
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Old 06-30-2010, 11:30 PM
  #5  
jluther
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The plane specs out at 28 oz. and is a very light floaty trainer with a wing-loading of 10 oz. per sq. ft. My modifications including the camera take it to 32 oz. with a wing-loading of 12.5 oz. per sq. ft. equipped with a brushless high thrust/low rpm motor. The design is solid and proven in 4 years of aerial work. Low alt. low speed flying is critical and I operate the plane accordingly. You can see the results at www.calljerry.biz

Never the less I go through these periods of not being able to bring the plane back into perfect trim and balance. I measure my control throws by degreed compass and balance the plane on a post jig with pre-sets in the wings. But I feel like I'm missing some advanced systematic procedure that pilots that fly more advanced aircraft and do aerobatics might use.

I fly a major a construction site in and around cranes, rapidly moving dump trucks, while over water and cris-crossing the highway. I have to time my landings with passing water trucks and in gusty winds to land between surveyor stakes on gravel roads. I require a lot of my working aircraft and my flying challenges are extreme if not precision.
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Old 07-01-2010, 06:56 AM
  #6  
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The wing loading looks ok.. All i can say is if the plane balances ok and all flying surfaces and engine thrust line are 'straight' and 'as new' and control surfaces move and centre ok.. then there is no reason it should not fly just like it did when it was new.
For the plane to fly differently some physical aspect of the plane must have changed.

Cant the turn you mentioned be trimmed out with some rudder trim?

Here's the procedure procedure (assuming CG is already checked and ok)....

Take the plane up to safe altitude. Cut the throttle and take your hands off the controls what does it do?.. If it dives steeply, stalls or turns then use the trims until you have a nice steady slowly descending, straight glide... the model is now 'in trim'.

Now add full power.. hands off.. what does it do now? It should climb gently still in a straight line. If not then you have a thrust line issue. If this is the case let me know what the model does and maybe we can figure it out.

Steve
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Old 07-01-2010, 08:19 AM
  #7  
Larry3215
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I agree with Steve 100% on everything he has said and suggested.

Something physical must be changing.

Id also check for anything that may be developing any flex, slop or looseness after those hard landings. Motor mount, servo mounts, wing mounting, linkages, hinges etc.

Maybe hold the model and go to full power and look for motor flexing or moving. Tug on the wing, tail etc to see if anything gives or moves. Be sure the servos still center perfectly etc.
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Old 07-01-2010, 02:39 PM
  #8  
jluther
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Great suggestions. I'll test flight this morning, looks like still air, and report the results.

This weekend I have three flight missions over water and one off and on a small area and over a 500 foot cliff face to shoot a new house on a mountain top.

I am appreciating this exchange.
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Old 07-02-2010, 02:44 AM
  #9  
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Gentlemen,
So todays test. I closely examined my balance points and found that they were 1 t0 1.5 mm forward from specification. Interestingly it's that critical, the balance is a few degrees nose heavy which is good for this glider with a payload.

I have up elevator trim of several degrees established in prior tests.

I have left rudder mix when the throttle is 50% which goes to 18%.

The plane leaves my hands into a 1.5 wind and goes almost straight for 100 yd. climbing steadily and gradually taking a right course.

At 200' power is cut, the plane glides smooth and straight with long slow left turn which is desirable. When I apply full power from a steady glide the plane takes a much sharper right turn than on launch, the climb is steep. The motor is a pusher and prop wash has more effect on the up elevator trim than during glide.

I think I should make the throttle to rudder mix start earlier in the acceleration curve so 20% throttle will provide enough left rudder to keep the plane at an even altitude in a big circle hands off the controls.

I feel confident in the current performance and reliability to fly my missions. Thanks for your advice and feedback.
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Old 07-02-2010, 06:53 AM
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Sounds like you are nose heavy. The model should balance dead level when supported on the position indicated on the plan. Even then the plan positions are often very conservative and moving the CG back a little more will often improve flight characteristics.

Here's the test for CG:

Take the model up to safe height and make sure she is trimmed for a good straight glide as described earlier (the glide should be straight, not turning by the way). Now with the throttle closed put the model into a 30 degree dive and take your hands off the controls.
  • The ideal behaviour is that the model should gently recover from the dive without any human intervention.
  • If the model recovers quickly then ends up climbing into a stall, or series of oscillations then the CG is too far forward.
  • If the model shows no sign of recovery or actually steepens its dive then the CG is too far back.
Here is a diagram: http://www.southernsoaringclub.org.za/a-CGdivetest.html

The fact that you need up elevator trim and that the model climbs steeply under power point to a CG that's too far forward (too much stability). This could also explain why the model might sometimes dive after launch.

As for the left rudder mix.. I don’t much like this sort of mixing, I'd much rather resolve by offsetting the thrustline but on a plane like the EasyStar where the prop is 'mid-ship' then thrust offset may be less effective than a 'normal' layout plane, mixing may in fact be the only solution. If it were me though and if the motor mount allows adjustment then I would try some thrust offset. Viewed from above you would want to rotate the motor slightly anti-clockwise (left thrust). The prop wash on the vertical tail will then cause the plane to come to the left under power.

Steve
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Old 07-02-2010, 04:37 PM
  #11  
jluther
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Steve,

That's got my concern focused now. If the plane is balanced why do I have so much up elevator trim? I'll do that test when the rain lets up here. I would rather have a thrust off-set than a digital mix setting. I tried it on my back-up plane but maybe not enough.

Let's agree on the thrust angle direction. Looking down and from the tail toward the nose, I had shimmed the right side of the motor aiming the axes line to favor or push air over the left side of the fin and stab (looking to the nose). I only shimmed with one 1/8" washer. I remember when I flew 1/2 A free-flight as a teen, that we put like 20 down and right thrust to off set the high pylon wing lift, so maybe I'm too conservative. My T-28 has a huge amount of down and right thrust.

Again, I really appreciate the time you're taking with me.

Jerry

PS How do I add to your "thank yous" I saw on your profile?
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Old 07-02-2010, 05:08 PM
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Here's what i mean by the thrust offset, this is viewed from above:

(ps.. 'thank you' button is bottom right corner.. thanks)
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Old 07-02-2010, 05:37 PM
  #13  
jluther
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Great diagram. If the indicated thrust angle shown is the one suggested than I had it wrong on my first attempt with my back-up plane.
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Old 07-02-2010, 06:19 PM
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Thrust offset is a tricky business on a plane where the motor is near the CG. The effect of the offset is less easy to predict than on a conventional plane where the prop is on the nose.

If it's reasonably easy to do give it a try, i think it should help at least reduce the need for rudder mixing in not eliminate it altogether.

Steve
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Old 07-04-2010, 03:02 AM
  #15  
jluther
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Steve,

I got a chance to test fly this afternoon. I made no changes to any settings and did the dive CG test. It went into a steep stall and continued to oscillate. So I kept reducing the up-elevator trim until I got a gradual pull out of the dive into a level glide.

I brought the plane down and and the elevator was even with the surface of the stab. I threw the plane and it dove into the high grass. I gave it one or two clicks of up elevator which barely raised the elevator above the plane of the stab and threw the plane again. I left my hand into a 2 mph wind and flew straight as an arrow at a 15-20 climb.

I did the dive test and its recovery was less gradual but continued to a steady level glide with altitude gain from the head wind. I threw the plane two more times and did the dive test with the same results.

I have been reading the plane incorrectly and really grateful for your advice and written instructions. I feel like I have advanced my abilities a lot in the past two days.

Thanks,
Jerry
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Old 07-04-2010, 09:16 AM
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The dive test is for establishing the correct CG position. If it pulled out quickly then the CG needs to move back... Adjusting the elevator without moving the CG just means the plane is trimmed to fly faster which is not what you want.

Once the CG is moved back re-trim the elevator (take out some of the existing up trim) for a nice slow gide and re-do the dive test.

Keep going through this loop until the model recovers from the dive gently, as shown in the diagram I linked to previously.

Steve
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Old 07-05-2010, 04:03 AM
  #17  
jluther
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I moved the CG back 1.5 mm for a smooth recovery to glide. Than slowly lowered the up-elevator trim to a normal flat position while maintaining the even recover to a glide.

jerry
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Old 07-05-2010, 08:14 AM
  #18  
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Jerry,
You will almost certainly need to move it a lot more than 1.5mm (1/16"). Move the CG in steps of around 3mm (1/8") but be prepared to repeat this adjustment a number of times. It's quite possible that the CG will have to move 12mm or more to achieve the 'sweet spot'.. Just repeat the adjustments until the dive test looks right. You do have to be carefull when the CG approaches the 'sweet spot' dont move it back any further otherwise the model could become very tricky to fly.

I suspect what you have now is simply added down elevator and trimmed a faster glide.. Is the power off glide still good or does it glide fast in a slight dive?.. If so then re-trim the elevator for a nice slow 'floaty' glide and repeat the dive test.

Steve
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Old 08-12-2010, 04:53 PM
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jluther
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Steve,
It's been a few weeks since our last exchange. My plane is flying perfectly now with all your help. I've flown over 20 missions several over water and one in 7 mph gusting winds with complete reliability and control. Thanks again.

I now have a new boat-plane with a design decision of whether to replace my boat hull with a duck bill bottom or raise the motors on the wing. It's a modified Multiplex Twin Star that is a proven conversion to boat-plane. My question will regard how to compensate for built-in down thrust when raising and slightly altering the thrust angle of the motors. Do you care to engage in this design issue and is this the thread to do this.

Jerry
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Old 08-13-2010, 07:37 AM
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jerry,

Great to hear that your model is performing well.

As for the seaplane... My instincts would be to add floats to lift the whole model out of the water. Failing that some sort of extension to the bottom of the fuselage to form a boat hull that would lift the model high enough out of the water to give adequate prop clearance without changing the position of the wing mounted motors?

Do you have any photos or sketches of what you have in mind?

Steve
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Old 08-13-2010, 03:38 PM
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jluther
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Steve,

Yes I'll send I'll upload a slide show and send you the link.

Jerry
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