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Old Jun 26, 2012, 10:09 AM
Mr. Wiz is offline
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So I went out this morning in the clam air and re-trimmed the Fr3aK after the boom repair and pod servo re-installation. I now have the CG at 71mm and an AUW of 10.8 oz. Not too shabby considering I'm new to making these types of repairs. I found I had to trim the elevator for a lot more down that I would have expected. Same with the launch preset and the Elevator to Flap mixing. I got it all dialed in and flying smoothly but as I was leaving it occurred to me. The Fr3aK has a boom that is slightly curved. I however just "clam shelled" a 3" section of that with straight boom pieces. That straightening likely cause my entire stab to now be slightly negative in relationship to the wing. That would cause all the trim adjustments I just mentioned. I don't think I have enough time between now and leaving for the Bruce to fix this. In general it seems to fly nicely but I'm sure it's now carrying more drag than is ideal. I guess I'll have to live with it for now. At my level of flying it probably doesn't matter too much anyway. I'm continually amazed at how meticulous you have to be with these planes to keep them flying in their sweet spot.
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Old Jun 26, 2012, 10:26 AM
BavarianCharles is offline
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Originally Posted by Mr. Wiz View Post
So I went out this morning in the clam air and re-trimmed the Fr3aK after the boom repair and pod servo re-installation. I now have the CG at 71mm and an AUW of 10.8 oz. Not too shabby considering I'm new to making these types of repairs. I found I had to trim the elevator for a lot more down that I would have expected. Same with the launch preset and the Elevator to Flap mixing. I got it all dialed in and flying smoothly but as I was leaving it occurred to me. The Fr3aK has a boom that is slightly curved. I however just "clam shelled" a 3" section of that with straight boom pieces. That straightening likely cause my entire stab to now be slightly negative in relationship to the wing. That would cause all the trim adjustments I just mentioned. I don't think I have enough time between now and leaving for the Bruce to fix this. In general it seems to fly nicely but I'm sure it's now carrying more drag than is ideal. I guess I'll have to live with it for now. At my level of flying it probably doesn't matter too much anyway. I'm continually amazed at how meticulous you have to be with these planes to keep them flying in their sweet spot.
Shim the stab until the fixed and moving surfaces are in alignment. I prefer to do this for speed mode, to get the lowest possible drag from the horizontal, and add up-elevator for cruise and loafing around.

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Old Jun 26, 2012, 10:39 AM
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Mike,

Do as Charles says. Yes, the boom has curvature to it Very few people have actually noticed this. There are three ways to get the tail and wing in the correct alignment... tilt the wing saddle, tilt the stab mount, or curve the whole boom. I chose to do the last. Assuming the rounded tip horizontal... in speed mode if you lay a straight edge across the top of the stab, the aft edge of the movable elevator should just barely touch it. In cruise, if you lay the straight edge across the BOTTOM of the stab, you should see the aft edge nearly touch it. Of course there's some wiggle room... depending on CG and some other factors.

Trim drag is very very nearly negligable with our DLGs. Don't sweat that little bit of trim. It could cause some slight pitchiness in the ballistic portion of the launch but at any "normal" flying speeds you shouldn't notice it at all.
Last edited by tom43004; Jun 26, 2012 at 11:12 AM. Reason: corrected for the tail airfoil
Old Jun 26, 2012, 10:57 AM
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Which airfoil is used on the horizontal stab? If it is one of the Freaks, then in speed mode it should be set up with more down than the ruler on the bottom indicates, by a couple of mm.

The trim drag won't be different. What will be different is the drag in tight thermal turns.

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Old Jun 26, 2012, 11:09 AM
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Good point Gerald. The round tip one (that Mike has) is the Freak Horizontal.
Old Jun 26, 2012, 11:09 AM
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As I said above, at this point I've just got to live with it. I don't have any extra time right now for fiddling around. It seems OK to me. I can't hold the launch preset and watch the rotation before releasing now as I did before.(I liked it that way, BTW) I have to instantly release it. Other than that, I didn't notice any real degradation in performance. But heck, now the launch preset is like the B3 which is actually good. I don't have to remember which plane I'm launching anymore, if you know what I mean. When I get some time, I'll trim the stab properly on the Fr3aK and re-do the launch preset on both planes such that I can control the climb angle.

The Fr3aK and the B3 are good to go and I'm as happy as I could be about that. A month ago the B3 was in for serious repair and 2 weeks ago the Fr3aK was. I guess that's probably par for the course with these planes but I was starting to stress out about it a little. These are the only 2 DLGs I own.
Old Jun 26, 2012, 02:01 PM
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The launch preset should be independent of adjusting the mounting angle of the stab. You can shim the angle with a piece of tape. Then adjust the servo center using the transmitter programming to trim it back out in any of the flight modes. Now all the modes, including launch preset, should be fine. YMMV of course since different transmitters program different ways. But in essence, if one changes the angle of the front of a horizontal stabilizer then the angle of the rear section will change in the reverse direction. If changed correctly for any single mode, the correction is essentially exactly the same for the other modes.

There is a stab Cm change in this process so it is not EXACTLY the same but it should be close enough for any real world application.

Just FYI.

Gerald
Old Jun 26, 2012, 02:16 PM
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Thanks Gerald. I'll be approaching this adjustment in the coming weeks but right now. I'm set... or at least close enough.
Old Jun 26, 2012, 03:54 PM
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I think it should take about 5 minutes to take a few pieces of tape and use them as shims under the stab mount, unless yours is a non-removable stab. Very easy to do at the field. It may take several attemps to get it close but that will only take 15 to 20 minutes.
Old Jun 26, 2012, 03:57 PM
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It's not so much the time putting on the shim as it is the time re-trimming the plane. We'll see...
Old Jun 26, 2012, 04:23 PM
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Dont you almost retrim the plane every time you go to fly. A click here a click there
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Old Jun 26, 2012, 04:25 PM
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Originally Posted by Mr. Wiz View Post
It's not so much the time putting on the shim as it is the time re-trimming the plane. We'll see...
Trimming a plane is like tuning an instrument. Do it at least once per flight session, and more frequently to adapt to changing conditions.

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Old Jun 26, 2012, 04:28 PM
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Dont you almost retrim the plane every time you go to fly. A click here a click there
Gavin
Yeah, but I don't retrim the launch preset and the elevator to flap mixing. Those two usually take me a few times each to get where I'm happy with them. In fact, I generally do those two in calm air, which we had this morning. Again, we'll see. Maybe I can fiddle with it down there on Friday. Otherwise, I don't think I'm going to make it out to a flying field before then.
Old Jun 26, 2012, 04:32 PM
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Trimming a plane is like tuning an instrument. Do it at least once per flight session, and more frequently to adapt to changing conditions.

Little things here and there..... But by in large, no. Unless I'm having trouble I don't mess with it. I just fly it. At the Bruce I'll be taking it all in so as I said before. We'll see.
Old Jun 26, 2012, 04:40 PM
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You won't need to adjust the elevator->flap mix or the flap->elevator mix. The launch preset should still be "such and such precentage of up elevator added" to the trimmed speed setting. So if you do launch preset via mix then you have no changes to make. If you fly something like an 8FG then you'll do your retrimming with the servo subtrim not with the trim tabs. Then all trims are adjusted at the same time.

Retrimming for changing conditions is an extremely frequent occurrance. Calm air, one flies slower. Turbulent air, one flies faster. Windy turbulent conditions, one typically flies with a touch less camber. Settings are not constants, not if you want to get the most performance out of your plane. Learning and being able to judge when airspeed or camber is incorrect during flight - priceless.

Gerald


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