Setting the proper CG

[flyin'fish]

I've been trying to get back into flying RC and have gone thru my share of "trainer" aircraft. One of the hurdles I've encountered is finding the proper CG setting. My latest plane is a HK Tuff Trainer II...4-channel tail dragger, 880mm wingspan, EPP foam, high wing, and a good amount of dihedral. I'm running it on a 3S 2200 pack. The manual states that the CG is 70-80 mm back from the LE, so I went for the midpoint and set it dead neutral @ 75mm. I maidened it yesterday, did a ROG takeoff and almost got 1 full circuit around the field. In the air, the plane is unpredictable at best. My question is - should I use the 70mm mark for CG (this would make it LESS tail-heavy, right?) and secondly, when setting the CG, should it be slightly nose-heavy or neutral? Thanks in advance!

[TiredIronGRB]

Slightly nose heavy is my preference.

[T-CAT]

How did the plane react in various flight settings, such as any pitching during application of increased throttle, ESC.?

Also, how did you balance the plane, as in the orientation of it (upright, upside down), did you make sure everything was in the plane exactly how it would fly before measuring, and what did you use to verify it (CG machine, fingers, etc.)?

A general rule of thumb that tends to work well for me with most airframes (others have their preference and methods as well) is to measure the CG of a high wing plane upright, and a mid/low wing plane upside down. In addition, if you have retracts (I know you don't in this case) you can look at their movement to see which way to measure the CG with the gear up/retracted or down/extended. For an airplane that has the retractable landing gear extending/deploying forward the CG can be checked with the gear up/retracted. For an airplane that has the retractable landing gear extending/deploying rearwards the CG is checked with the gear down/extended (since this scenario has the weight of the retracts moving aft/rearwards, a tail heavy situation can make for a less predictable behavior than a nose heavy situation in landing configuration) . However, on some airframes a person may not notice much difference in the weight shifting regardless of the retracts being up or down, although that depends on the plane, it's size, it's weight, location of the gear, etc.

[flyin'fish]

Thanks for the replies, guys! I balanced the plane upright (it's a high-winger). I used a marking pen to scribe "X"'s on the underside of the wing on either side of the fuselage, and used the fingertip method, shifting the battery pack to attain dead neutral at 75mm, and then marking the inside of the fuselage for the battery location. When the manual gives a range of CG measurements, I guess it's better to use the value that's more-forward?

The best way to describe how the plane reacted in the air is "wishy-washy"? Seemed uncontrollable most of the time. I think I'll need to shift weight fwd to get away from what I suspect is a tail-heavy condition. I did get it on the ground without damage to the airframe, just a busted prop. Waiting for spares to arrive now.

I have two 1100mm Durafly warbirds (T-28 Trojan and A-1 Skyraider) that are my incentives to improve as a pilot. I'll use your advice, T-Cat, on the retracts and CG once I'm confident enough to fly them - they're too pretty to smash up! LOL!

So, what I'll do is re-balance this trainer at the 70mm mark and make sure its a little nose-down at that point. Thanks again for our expertise and insight

[xviper]

I have found that when the manual states a given CG range, I aim for the average. However, a "range" is what it is. It's a suggestion of what you should try to suit your flying circumstances. Having said that, the plane "should" fly adequately within that range and only in a few rare cases, have I found the manual wrong and only in a very few cases, has a few mm either way, made that much of a difference. I've seen some pilots obsess over 1mm but I've never found even 5mm or 10mm to make a plane unflyable.
I also believe CG is only one factor that determines how a plane flies. Experience and skill or lack thereof has a tremendous affect on how the plane flies. A perfectly calibrated plane that flies for one person might not last 5 seconds in the air for another. Then there's TX settings. Are you using the correct "rates and expo"? Are you using any at all? I flew with a fellow who, at first couldn't fly any of his planes in a consistent manner. Every flight ended in a crash and some flights lasted seconds. I asked him if he used low rates and expo. He said "yes". This kept going on for weeks until finally I asked him if he could show me his TX settings. He considered 90% to be low rates and -50% to be high expo. It turned out that once dialed down to 50% or less on the rates, he did much better. And of course, he should have had +50% expo. So, talking CG placement may be just too simple.
Then there's the matter of how the plane is put together. Are the wings on straight? Is the dihedral the same on both sides? Is the tail straight? Do the control surfaces center themselves correctly each time or do they "stick"? Does one aileron work the same as the other? The list can go on and on.

[flyin'fish]

xviper2 - I totally get what you're saying, but this plane was simply uncontrollable the way it was balanced. Wings, tailfeathers, control surfaces, etc. are all square and aligned properly. Trim buttons for elevator, rudder, and ailerons are centered with the control surfaces fully neutral, so have some adjustment options once I get it in the air. My Tx is a 6-ch but, alas, does not feature dual rates or expo. I do, however, have the servo arms and control horns set up in a "mild" configuration with respect to the holes used. I'll be the first to admit that I'm green on the sticks, but this one particular plane has me stumped. I've flown others, including the early-on Banana Hobby version of the FW Pandora with some success. I fly, I crash, I fix, and I fly again :-) Yet this one still has some wrinkles I need to iron out. Thanks for the input!

[T-CAT]

It looks like others when I searched online may have found this plane to have the published CG incorrect. If it were me, I'd start by trying out a different CG slowly one flight after another and check for other aspects on the plane that could be affecting it, but that would be me. Fly at your own risk though :D, as that's experimenting a bit, and you have to be comfortable knowing that things could go south. I would do more research to see if any other folks found the published CG too far aft.

[xviper]

I fly, I crash, I fix, and I fly again :-)

There's a saying at some of the fields I fly at. If you're not crashing, you're not having enough fun. Believe me when I say that I've had a LOT of fun and continue to do so.

[xviper]

From some online searches about the plane you're describing, I've found some folks saying the published CG was incorrect.

I can believe it. Sometimes, I think the people who make these planes, haven't actually flown them and just put a guess in the manual. One example from my past is the HobbyKing Durafly Vampire. The published CG was so far off, that a great many of the first owners crashed them on the first circuit, myself included. After weeks and months of forum chatter about where it should be, did HobbyKing actually come on board to say how wrong it was. My second Vampire with the new CG flew very well.

[T-CAT]

Yeah, that always mind boggles me too. It's as if no test is done at all in those situations. All those aspects should be figured out well before a manual gets printed, especially with how important it is.

[flyin'fish]

Thanks guys - I'll check out any reviews, and continue to have fun! Appreciate it!

[WintrSol]

Perhaps the control surfaces were just set at too high a rate, and you were over-controlling; you said you hadn't flown in a while, so maybe you should reduce the throws until you get the feeling back.
If you can get it to trim, level flight at middle throttle setting, then cut the throttle, it should begin to dive, with the dive getting shallower if the speed picks up, but not return to level; this would indicate a nose-heavy CG. If you cut the throttle and it just starts to fall, it is closer to tail heavy than you may want.

[Arcangel]

I can believe it. Sometimes, I think the people who make these planes, haven't actually flown them and just put a guess in the manual. One example from my past is the HobbyKing Durafly Vampire. The published CG was so far off, that a great many of the first owners crashed them on the first circuit, myself included. After weeks and months of forum chatter about where it should be, did HobbyKing actually come on board to say how wrong it was. My second Vampire with the new CG flew very well.

Hey xviper2,


I've just about got my Durafly Vampire ready for test flight and I wanted to compare notes since the manual doesn't give you much to work with.

Where did you set your CG on your second Vampire and is there a specific place that you used to measure the CG from other than the wing root? {Manual suggested CG being 311mm measured from the tip of the nose of the model}

What did you use for your servo throw settings??

I'm going with aileron Hi rates at 1/2" deflection and the dual rate set at 80% and the low rate 60%. The elevator Hi rate is going to be 3/4" to 1" deflection if I can get the travel out of it and the same percentages as I used with the aileron dual rate and low rate.

I made a few structure mods such as added carbon fiber tubes in the booms and horizontal, changed out the stock fan with the Freewing V2 70mm 12 blade fan, and upped the ESC to 65amps.

I'm hoping that this will turn the little guy into a real fun flyer as my dad had one and he really enjoyed flying it.

[xviper]

The Vampire goes back 3+ years. I don't remember specific numbers. Just that it dove like crazy and the battery had to go way back. I gave it away a long time ago when the Venom came out.