CG, decalage, and trim - Page 18 - RC Groups
 Jan 28, 2013, 04:36 PM IT'S NOSE HEAVY!!!! My nose heavy planes went from dogs that had to fly fast to stay up, stalled harshly, and constantly changed pitch with varying speeds, and with better CG turned into real floaters that could be slowed way down, barely stalled at all, and took to lift SO much better. My tailheavy plane was very hard to fly and would stall with only tiny up elevator, and would kamakazi into the ground in a dive if I didn't pull up quickly.
 Jan 28, 2013, 04:41 PM Chuck 'Em and Chase 'Em If a Cg is set anywhere but the neutral point, will the have only ONE set speed when hands off? What I mean is that if I give some elevator input on a plane that is slightly positive, won't it eventually return to the speed set during setup? If you want to go faster/slower, you need to HOLD some quantity of elevator input from the sticks on non neutral Cg set plane. True? If the Cg is at the neutral point, once the sticks are released, the plane will continue on that pitch until something disturbs it. True? Performance: 1. is it the magnitude of the pitch change the changes with Cg or what is it that idictates a plane indicates lift well? Is it the rate of indication, a more neutral plane will react faster to lift while a more positive will be slower? 2.Is performance then the combination of fast response to input AND fast, large response to variations in air (turbulence, lift, sink) ? 3. Is there any effect on L/D or min sink when performance increases or can it decrease because of pilot workload? Frank
 Jan 28, 2013, 04:49 PM Chuck 'Em and Chase 'Em I have often been told that I fly too fast and that I could be plowing through indications of lift. If performance is partly defined by the magnitude of the pitch change in response to turbulence/lift and flying faster tends to reduce the magnitude of a plane's response to turbulence, could you have 2 planes having the same magnitude response where one plane flies faster but at a more neutral setting and the other be more positive but fly slower? We ballast to fly faster so as to penetrate into the wind. If a plane is also positive in stability, is it possible that we will not see many indicaitons of lift? I have NEVER heard anyone say to become closer to neutral when ballasting I guess it is because many are already close or at neutral and this effect is minimal?
 Jan 28, 2013, 06:30 PM IT'S NOSE HEAVY!!!! As for indicating thermals, think of a high-wire balancing act. A long pole with large effective "force" on the ends provides resistance to changes in "pitch". A short broomstick will "pitch" wildly and the guy will fall off the wire. A nose heavy plane with opposing elevator force will mush right through a thermal and you might not even see any waggle at all. If balanced, it'll take less force to "upset" it and it'll waggle much more pronounced. As far as overall speed through a thermal being a factor, I'd say sure, given it is fast enough although a couple mph may not matter. However, a nose heavy plane can't really reach best min. sink speed and won't be nearly as "floaty".
Jan 28, 2013, 11:04 PM
Registered User
Quote:
 Originally Posted by Fly2High Tom and Mike and several others have commented that as the Cg is moved closer to or at the neutral point, the performance improves? 1. Can you define what is performance? 2. How can I determine I have reached the maximum performance of a plane? 3. If Cg AND elevator trim setting determines if I pull out of a dive, What should be done to determine if maximum 'performance' is reached? How do I know I am flying a plane too slowly and should not move the Cg further back? Isn't it possible that I chose the wrong speed to start with? Are we trying for min. sink or best L/d speed when we are trimming? Frank
All your questions were answered early on in this thread - by Gordy I suggest you re-read his posts starting with post 25 then read the attachments that have his balancing system in them - around post 33 or 34 I think.

If you can get past his tendency to be somewhat abrasive, he has laid out a simple system and explained all the whys.

In the posts immediately after Gordy's we got side tracked by numerous arguments from those who disagree with Gordy or maybe he just ticked them off

Then several of the "experts" and contest winners came in to the thread and basically confirmed everything Gordy said. For some reason or other they didnt piss off as many folks as Gordy managed to

In any case, the bottom line is - Gordy was right all along!

One point I'll make - Gordy says he does NOT balance for "performance" or "feel". The goal is communication. He wants a model that tells him what he needs to know about lift. NOT a model that tells him when it's speed changes.

If you follow Gordy's balancing system, you will end up with a model balanced very close to the neutral point. You can always move the CG forward from that point if you're not comfortable flying a model that responsive yet.

Instead of doing that, Id recommend you reduce throws/add expo etc and keep practicing.

Once you get to where you are comfortable flying with the CG further back you will find that its actually EASIER to fly the model - it wont be fighting you any more AND mysteriously, you will find more lift more often
 Jan 28, 2013, 11:07 PM Registered User One more thing - I have changed Gordy's system in one major way. I do my test flights at about 100 ft up instead of at ground level. Thats largely because my field is rough, full of horse doo-doo, mud holes and black berry bushes. I cant be landing just any old place I also want to be high enough to safely recover if I have a nasty stall reaction initially.
 Jan 28, 2013, 11:10 PM Registered User A confession - I had to re-read Gordy's stuff several times before I "got it". I had a lot of "yeah, but's...." for quite a while. Its not easy to un-learn something you KNOW IS TRUE
 Jan 29, 2013, 12:45 AM IT'S NOSE HEAVY!!!! And that perfectly sums it all up, thank you! And perfectly worded too! No need to drag it all out even further, Google will land folks here. Last edited by cityevader; Jan 29, 2013 at 12:51 AM.
 Apr 17, 2013, 12:03 PM IT'S NOSE HEAVY!!!! One more thing I've found out regarding why dive testing is only PART of the testing, and how the same cg can give a climb OR a dive result. The trim on my transmitter has enough of a range of throw that one click is enough to make it climb OR dive...Now that my cg is right about perfect, the effects of trim are far more pronounced. The dive test is, as stated before, good for seeing if the TRIM (not CG) is grossly off (but since cg affects trim, one can then infer if cg is off), and NOT a way to fine tune cg as some believe. For what it's worth, I discovered my Futaba 9c can change the sensitivity of the trim, so now I keep it way down so the trim hardly moves. (But for maiden flights I turn it way up to get a lot of trim throw as rapidly as possible to avoid a hundred clicks to get it to fly level) Ok...now this thread will stay permanently open.
 Apr 17, 2013, 12:18 PM Red Merle ALES VI Ahhh, welcome back my good friend! Curtis Suter; formerly CloudyIFR
 May 15, 2013, 11:48 PM Registered User This has been one hell of an informative thread. I appreciate everyone's input to this subject as I have been fighting an unbalanced/untrimmed/wonky plane after just getting back into the hobby after a 15 year hiatus. My experience is as follows.... I have a newly purchased powered Decathlon (I know, its not a glider, but I will be taking what I learned and apply it to my old G.Lady , which needs help as well). The new plane is great, except it would pull up sharply when the air speed increased (both with throttle and without, like the infamous CG dive test). I moved the CG back, re-trimmed it, performed the dive test, and it still pulled out of the dive. I did this about 3 or 4 times and the results were the same, the only change being the plane was more sensitive in the pitch axis. I noticed that after all these CG moves and retrimmings, there was a lot of down elevator needed for level flight at cruising speed. That's when the light bulb went off! Since this model does not have a full flying stabilizer, and the existing stab was securely glued in place, I decided to change the wings angle of incidence to adjust the decalage. I raised the trailing edge of the wing by 2 or 3 degrees (1/8") and took her out again. Take off was a handful as it needed massive amounts of up elevator to keep from being a lawn dart, but after trimming it out it was damn near perfect! Changes in airspeed (during level flight) resulted in nearly zero pitch change. The dive test resulted in the plane ever so slightly pitching up as it approached earth, but I did have to pull out of it after a few hundred feet of altitude loss otherwise it would have been lawn dart time. All I can say is changing the CG alone did not fix my pitch change with airspeed change. What fixed it for me was changing the decalage; in this case the angle of incidence of the wing relative to the stabilizer. I can't wait to attack my old glider to improve its flying characteristics, there is no doubt it will need it. Thanks again to everyone!
May 17, 2013, 10:50 AM
Red Merle ALES VI
Quote:
 Originally Posted by daveoxide My experience is as follows.... I have a newly purchased powered Decathlon (I know, its not a glider, but I will be taking what I learned and apply it to my old G.Lady , which needs help as well). The new plane is great, except it would pull up sharply when the air speed increased (both with throttle and without, like the infamous CG dive test). I moved the CG back, re-trimmed it, performed the dive test, and it still pulled out of the dive. I did this about 3 or 4 times and the results were the same, the only change being the plane was more sensitive in the pitch axis. I noticed that after all these CG moves and retrimmings, there was a lot of down elevator needed for level flight at cruising speed. That's when the light bulb went off! Since this model does not have a full flying stabilizer, and the existing stab was securely glued in place, I decided to change the wings angle of incidence to adjust the decalage. I raised the trailing edge of the wing by 2 or 3 degrees (1/8") and took her out again. Take off was a handful as it needed massive amounts of up elevator to keep from being a lawn dart, but after trimming it out it was damn near perfect! Changes in airspeed (during level flight) resulted in nearly zero pitch change. The dive test resulted in the plane ever so slightly pitching up as it approached earth, but I did have to pull out of it after a few hundred feet of altitude loss otherwise it would have been lawn dart time. All I can say is changing the CG alone did not fix my pitch change with airspeed change. What fixed it for me was changing the decalage; in this case the angle of incidence of the wing relative to the stabilizer. I can't wait to attack my old glider to improve its flying characteristics, there is no doubt it will need it. Thanks again to everyone!
So what was the aerodynamic issue? Perhaps fuselage drag? Poor horizontal stabilizer airfoil when the elevator was slightly deflected for straight and level flight? A combination of both? Other?

Curtis
 May 17, 2013, 12:27 PM Registered User It sounds like a lot of down elevator trim wasn't enough to get him trimmed how he wanted, so he raised the trailing edge of the main wing without removing the down elevator trim, providing for an exciting test flight. Glad to hear you got it worked out Dave.
 May 17, 2013, 04:43 PM Registered User One other thing to consider is control surface blow-back. I would advise powering up the plane and trying to deflect the elevator by hand to see how much resistance there is. If the servo or linkage is not up to the task it will produce the same lack of pitch stability with speed. Once the decalage is changed and the elevator no longer needs to hold a deflected position, the problem disappears. Analog servos sometimes require considerable deflection before they get to peak holding power.
 May 17, 2013, 05:32 PM IT'S NOSE HEAVY!!!! On one plane, I had to set up the elevator to pull on the "pushrod" for up elevator due to so much flex. Granted, not having enough down is not good, but i felt better having full up at least.