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157" Discus Glider

Old 06-07-2013, 02:11 AM
  #501  
da Rock
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Default RE: 157

Why the range of CGs?

CG isn't a simple deal. When there are lots of details, there will be lots of differences in the way people go about figuring "the" location out.

For example, you chose to use 15% static margin. Got any idea if the other CG locators even knew what static margin is? or cared about it?

Some modelers don't even know they can or often should change the elevator throws to match the CG location otherwise the model would be sensitive or sluggish. Some do. They all get a shot at telling where it should be. In fact it doesn't have to be anywhere specific but can easily be somewhere in a range of locations.
Old 06-07-2013, 07:06 AM
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AMA-69405
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Default RE: 157


ORIGINAL: da Rock

Why the range of CGs? [snip] In fact it doesn't have to be anywhere specific but can easily be somewhere in a range of locations.
Agree on most points, but I found it very difficult to explain the vast differences in ranges, i.e. 55 to 80mm actual vs 114 to 122mm calculated. Just didn't make sense. After sleeping on it, this morning I did a ground-up rethink of the situation, and for whatever reason,a fact that had escaped me earlier jumped up and hit me between the eyes.Take a look at the photos attached. One is a blown up version of the inset photos on the box label, the other shows the wing thatactually came out of that box. Look at theforward sweep of the trailing edgeof the wings in the label photo, compared to the aft swept leading edge of the actual wing. This significantly altersthe aircraft neutral point, due tosignificantlydifferent aerodynamic centers of the two wings. And of course, the recommended CG for first flights should always be forward of this neutral point.

I would therefore conclude that at some point, the wing design was changed, but they didn't update the manual showing a different CG. Or - maybe all of the manufacturer's CG recommendations were hugely over-conservative. They also apparently kinda forgot to update the box labels.

FWIW, I see both wing types represented in the forum messages I perused, so there were definitely three wing types produced (forward swept with spoilers,aft swept with spoilers, and onewith flaps), in addition to the "early" and "late" fuselage and tail feathers... early with flying stab, late withconventional stab/elevator setup. Someof this I suppose,might have been due to different manufacturers getting involved... I dunno.

In any case, I would bet there are some 4m Semi-Scale Sailplanes flying around out there with CG's that are in the neighborhood of 30 to 40 mm too far forward...that's a bunch. This might also explain some of the "1.5 lbs of lead added to the nose...." comments I saw when researching this problem. In some cases at least,part of that added lead was almost certainly unnecessary.

I haven't flown mine yet, but unless someone convinces meotherwise, I'll probably be doing the first flight with the CG at least 100mm back from the root leading edge.

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Old 06-07-2013, 11:23 AM
  #503  
Danosoar
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Default RE: 157

Howdy Gang!

Good thinking Gary, I think you got something there! Good luck with the test flights!

I've lurking/following this thread from near the beginning.

I have had one of the big beautiful beasties for bout 3 or 4 years (ebay $98 + $20 shipping) I started setting it up, but life got complicated and it had to be put into storage. I expect to be getting it out soon and finish it up. I'll post some photos as I work on it.

I am doing some stuff that most R/C glider guys will think are crazy, but it is my plane and I will do crazy if I want to!

For instance, The pilot will be famous WWII Flying Ace Snoopy (with a modern fighter pilot helmet) The pilot has requested some special modifications to the aircraft so I will be installing a full F-15C cockpit with an ejection seat (a glide first?) The cockpit that I am using is a BBI 1/6 scale cockpit like this one http://jbwid.com/art33c.htm I got mine for $150 on ebay missing a couple parts and a little baet up (hey a lot of guys spend $150 on cockpit details, but they don'y get details like these!) These BBI cockpits are really tough to find and typically sell for $250 to $500. For paint I am doing U.S. Sail Mail... Registration Number 5N00PY (That's right, Snoopy works for the Post Office delivering Sail Mail) The pilot has also requested some armaments (for defensive purposes only of course, the mail must go through!) so I will probably add some machine guns sticking out the leading edge of the wing - P-51 style, and a couple of bombs under the wing (what? a Fighter/Glider??? another first??)

I'll be looking forward to hearing what you guys think of my crazy scheme,

Danosoar
Old 06-07-2013, 01:25 PM
  #504  
da Rock
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Default RE: 157

69405,
Good thing going for all us glider guiders is they are gliders. The first flight can be a gentle glide from our hand as we jog along. No matter where we decide to place the CG by theory, we have a method of testing the trim that is dead safe.
Hold the fuselage just under where we've placed the CG. Jog straight up wind. Feel what the glider tells you. If it's trying to lift off, you'll want to try some down trim. If it's pressing down into your hand, try some up trim. Don't glide it until you're sure of it's initial elevator trim setting. Do these "test runs" where you have room to glide the plane from your hand straight away and not have to turn, where the plane can go straight across the field to a gentle landing. Be optimistic about how much field you need.

The distance of the trimmed hand glide and the amount of elevator trim you wound up with will tell you if you've got a good CG.
Old 06-07-2013, 01:33 PM
  #505  
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Default RE: 157


ORIGINAL: AMA-69405

Agree on most points, but I found it very difficult to explain the vast differences in ranges,

There are so many variables possible with model airplanes, and so many are unknown, why would you expect to be able to explain half of them?

It sounds like you think they all honestly apply to your model. A single production run of an ARF will wind up with a range of CGs that work for the range of modelers who installed a range of equipment in a range of ways in a range of models that had a range of AOIs, weight distributions, etc etc etc.
Old 06-07-2013, 02:45 PM
  #506  
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Default RE: 157

Jogging and feeling. Really? With an 8 pound airplane that probably stalls at twice jogging speed? In any case, when you turn69, try that and let me know how it works out for ya.

Seriously...call me a perfectionist, but with a plane that has hundreds of builds behind it, there is no reason not to have the CG within 8 or 10 mm of "right" before I even walk out the door with it.Those ranges I talked about (114-122 mm calculated vs55-80 mm in real life)were so disparate, there was obviously somethingseriously wrong. I was simply trying to understand which ball park was the right one in which tostart. I now understand. And I'llguarantee you, there are many of these4m birds out there that flew very disappointingly, because the builders followed the instructions (and the forum chat) and ended upwith the CGover an inch and a halftoo farforward.
Old 06-08-2013, 05:03 PM
  #507  
da Rock
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Default RE: 157

ORIGINAL: AMA-69405

Jogging and feeling. Really? With an 8 pound airplane that probably stalls at twice jogging speed? In any case, when you turn 69, try that and let me know how it works out for ya. [img][/img]
Just did it the other day for a buddy and it worked out fine for me.

I've already passed the threshold you mention, btw. Lots of 'old people' have discovered that too much reliance on theory stops them from practicing.

In theory, practice and theory are the same. In practice, they often aren't.

BTW, my AMA is 11568. Gives an idea just how old I am...



.............back to the suggestion... stall speed doesn't enter in to it.

You should read through what I suggested again. The technique doesn't include stalling the plane. Get the trim in the ballpark by feel. Then launch into a straight ahead glide and judge the glide slope to get a feel for the appropriateness of the CG.

Also, any modeler who winds up with a lousy flying glider (of any wingspan) because he accepts the poor performance because he accepts a recommended CG location and goes no further because the source of the recommendation is unimpeachable to him, probably would date a French Model he met on the internet.
Old 06-09-2013, 07:26 AM
  #508  
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Default RE: 157

As planned, maiden flight was made with CG at 100mm aft of root LE. During a13+ minute flight on 4 brief (20-30 second)motor runs we got in 3 decent dive tests; these tests indicate the plane isstill a bit nose heavy. We'll probably leave the CGas-is for the moment, as we hadsome otherissues (loose monocote,Tx programming, etc) that kept us fromreally trimmingthe plane out. Felt pretty good though,overall I'm a happy camper.

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