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Old Apr 15, 2013, 07:18 AM
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Control Inputs - less is more?

Have been doing some “calm air” flying at sunset. Plan was to launch high, pitch over & free flight to get the maximum time, not using control inputs until time to come home. This never worked out for long, as typically after a short free glide, slight turbulence would lift one or other tip. Left uncorrected, the model would start a gradual circle, slowly speeding and tightening up. Thought it might be a warp, but noticed this would happen in either direction, depending on the initial disturbance.
Is this the normal result of the outer wing flying faster than the inner in circling flight or a more complex dihedral, CG issue? If normal, what would be the most efficient correction – a quick blip of aileron to unbank, or a few clicks of trim to ease out of the circle?
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Old Apr 15, 2013, 08:37 AM
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More information required - which DLG are you flying, how much dihedral is in the wing, and how do you like your CG - neutral or forward? Also, you'll have to try this on different days, and compare. The location of thermals, and their intensity and spacing can all effect your DLG's straight-line behavior.

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Old Apr 15, 2013, 09:52 AM
G_T
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I'd say move the CG forward a couple mm and try again. Free flight requires a bit more stability if you are not going to be making the control corrections yourself. A CG change should help for the pitch direction. The rest comes down to the overall design of the aircraft. Plus, any slop in the controls or chatter in the servos could cause the plane to wander around a bit.

Gerald
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Old Apr 15, 2013, 12:16 PM
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Murne,

The biggest issue you may be having with trying to do the tests you suggest is patience. Over all the time of flying sailplanes, I have always tried to tell people that radical blipping of controls to right a plane path is generally not recommended. In FreeFlight, the CG is only one component of the flying. In DLG we address the testing of the CG via the dive test and its related variations. You can find many threads on the subject on rcgroups.

In freeflight, if your plane is slowly starting to turn, then you would suspect a warp or twist somewhere on the plane. On a DLG, your servos may not re-center the same every time a significant input is added. If your do the blip method, then if you servo does not return to the same spot, then the first setting that initiated your blip response is not the same setting it returned to. In this case using clicks of trim is better, as you are correcting the initial setting slowly. This is where the patience thing comes into importance. Add click or two, then wait for the plane to slowly respond. Once a plane is disturbed, then often it will radically change speed, direction, or whatever. This is something you should explore. In the short time, the effects of different wing speeds, left or right, are minor compared to the warp or twist. Keep in mind that the turning issue may not be in the wing. It can be in rudder deflection, the front fin offset, consideration of whether you are using a non-symmetrical airfoiled rudder, , or perhaps even a bent boom. It can be that the wing is offset with respect to the fuselage, so while the wing is not warped, the rudder can be affecting the turning via dihedral turning reasoning.

In freeflight, the plane has to be trimmed to react to the air, move the plane into a thermal, and retain the proper thermaling circle. There is no outside input available. One should not expect a DLG to necessarily behave like a freeflight.

One thing that I have found useful for myself is to soften the input around the TX stick movement. One can do this by adding exponential to the stick response. Be aware that this is a pilot preference and threads of this nature stir up quite a discussion here on RCgroups. Each pilot is different, so just know that there are many things you might try in the TX as well as the plane.

Good luck.
Chris
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Old Apr 16, 2013, 01:42 AM
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Thanks for the inputs. I am flying a B3 with CG at 85mm, and a Long Shot with CG at 90mm.Dihedral 6.5 degrees on B3, 6 degrees on LS.Good servos & linkages (DS 09 & JR 385 - 285 in USA?) The effect is more noticeable with the LS, but also occurs to a lesser extent with the B3.

Perhaps I did not describe the effect well. The models are set up to fly straight - have trimmed rudder & ailerons to get this in each phase. I was aiming to be able to launch and fly with hardly any control inputs once stabilised after pitch over, and expecting a long straight path,perhaps meandering in response to occasional turbulence encounters. What actually happens, is the planes do not seem to be able to recover from any roll disturbance. Once settled into a straight slow glide at the top, the first minor disturbance in roll, initiates a slight turn in which ever direction the plane has been banked by the disturbance - all expected behaviour.(This can be either direction - if it was from a warp, I would expect a consistent turn to one side).If the air is perfectly still, it does fly straight. Once the turn has commenced, if I do not correct it, the turn slowly tightens, so that the model is circling tight enough to need elevator and aileron correction before the end of the flight.
Maybe this is normal behaviour for a non polyhedral model, and I should simply be prepared to blip the control to counter the disturbance, or have I got the CG/dihedral setup wrong?
Thanks for the inputs
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Old Apr 16, 2013, 07:34 AM
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murne:

The behavior you are describing sounds a great deal like what is refered to as "spiral instability". It was one of the conditions we tested for when I was learning the flight test business at Northrop, on F-5E aircraft. Among others, the classic Lear Jet exhibits the same behavior: left to its own devices and what we used to call "controls fixed", a properly trimmed Lear Jet will slowly begin to turn, then spiral down. Under autopilot (which is most certainly not controls fixed) it will fly straight. Under normal pilot control, it feels just fine.

As a comparison, the F-5E aircraft will be spirally instable under some configurations, but not under others. It depends on CG and the de-stabilizing affect of different stores, flight conditions and flap combinations.

For our gliders, I have done (and will be doing soon) the same sort of analysis - comparitive flights in relatively calm air (the air is never calm, but one does one's best) using a pair of Taboos. I will be doing so again here this summer, using a Blaster 3, like yours. Indeed, my Blaster 3 sometimes shows the same behavior: departing straight flight after an initial disturbance. But, this behavior is indeed normal. And no, you don't need to put up with it.

For my Blaster 3, I finally realized the behavior meant that I was flying too slowly, too near the stall. When I'm trying for maximum duration (minimum sink, really) I've learned to trim a click or two nose down, or 5 grams maybe in the nose, to bring the speed up a bit. Then, the airplane usually does fine. The basic configuration of the airplane will indeed allow it to do what you want, which is the fly at a trimmed, steady speed.

My advice: fly a bit faster when trying for those control-free flights.

Yours, Greg
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Old Apr 16, 2013, 07:55 AM
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I was making free flight models for some time. For free flight models airfoil is not the same at the root and at the tip of the wing and they have bigger dihedral. This leads to loss in lift but helps to compensate turbulence. For DLG I think this is normal, their wings must have maximum lift and pilot should compensate air turbulence. All my DLGs needs small inputs to keep them straight even in calm conditions.
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Old Apr 16, 2013, 11:30 AM
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Quote:
Originally Posted by murne View Post
Thanks for the inputs. I am flying a B3 with CG at 85mm, and a Long Shot with CG at 90mm.Dihedral 6.5 degrees on B3, 6 degrees on LS.Good servos & linkages (DS 09 & JR 385 - 285 in USA?) The effect is more noticeable with the LS, but also occurs to a lesser extent with the B3.

Thanks for the inputs
Get some lead and test your DLG with the CG's 10mm forward. 90mm on a Longshot is (IMHO) way back. Take some time and go to the extremes like CG on the blaster at 70 and see what happens. DLG's increase the pilot load as the CG move backwards.
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Old Apr 16, 2013, 07:41 PM
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I seem to remember some comments from Mark Drela that the fin size on a dlg is sized for the launch phase, not for spiral stability. Anyone remember this?

Could be that the typical dlg is spirally unstable, because the fin is too large?

Gary
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Old Apr 16, 2013, 08:14 PM
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Quote:
Originally Posted by GaryO View Post
I seem to remember some comments from Mark Drela that the fin size on a dlg is sized for the launch phase, not for spiral stability. Anyone remember this?

Could be that the typical dlg is spirally unstable, because the fin is too large?

Gary
The fin is large, but so is wing dihedral, and a typical DLG is spirally stable at "normal" speeds. See page 13 of the attached document for more.

RCSD-2004-08

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Old Apr 16, 2013, 08:24 PM
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I have tried many time to get any of my DLG's in a flat spin. Can't do it.
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Old Apr 17, 2013, 04:33 AM
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Hi Greg,

That all makes sense - thanks. I was flying as slow as I could - will try a few clicks more speed - and take the CG foward on the LS.

Thanks for all the inputs
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Old Apr 17, 2013, 05:18 AM
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The only way I can get 2 minute flights in "still" air is to do what you are trying for. I have a CG that is well forward and go into my thermal mode right out of launch. Turns and corrections are done using only rudder. I mentally note to myself DO NOT TOUCH A/E controls, let the plane fly. Seems to work well, hope you can get it sorted.

Remember that on the polars, there is a peak in performance just above stall speed. Too slow and stall then you have big problems, but going a bit faster is only slightly less performance. Better to shy a bit faster IMO and know stalling will not occur, than push the low-speed boundary and risk loosing lots of altitude fast.
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Old Apr 17, 2013, 08:13 AM
Thermal, where art thou?
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Originally Posted by closedloop View Post
Remember that on the polars, there is a peak in performance just above stall speed. Too slow and stall then you have big problems, but going a bit faster is only slightly less performance. Better to shy a bit faster IMO and know stalling will not occur, than push the low-speed boundary and risk loosing lots of altitude fast.
Matt,

I've been hunting for a polar for (predicted performance of) a DLG, similar to those for full-size gliders. Not even Mark Drela posted one back in the Supergee days, and I've never heard of anyone instrumenting a DLG and producing an empirical polar.

What's your strategy for establishing minimum flying speed - trim the elevator until the plane almost stalls, and then back off a few clicks?

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Old Apr 17, 2013, 10:57 AM
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Quote:
Originally Posted by closedloop View Post
The only way I can get 2 minute flights in "still" air is to do what you are trying for. I have a CG that is well forward and go into my thermal mode right out of launch. Turns and corrections are done using only rudder. I mentally note to myself DO NOT TOUCH A/E controls, let the plane fly. Seems to work well, hope you can get it sorted.

Remember that on the polars, there is a peak in performance just above stall speed. Too slow and stall then you have big problems, but going a bit faster is only slightly less performance. Better to shy a bit faster IMO and know stalling will not occur, than push the low-speed boundary and risk loosing lots of altitude fast.
Does your thermal mode have 6mm of camber? If so it is not the best to switch to it right at the top of the launch. Use cruise mode (2 to 4mm) until you find some lift. If this is a z2 wing the camber is used according to flight speed. If your in a thermal you want to fly slower and stay in it. If there is calm air the Z2 works better in cruise mode and moving much faster.

If this is a AG wing the same technique applies but at slower speeds.
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