tail heavy or nose heavy
05-23-2009, 02:43 PM
#4
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RE: tail heavy or nose heavy
If it shows proper CG..then it might just need a washer or two of down thrust. I am a beginner, so take this comment with some salt. Rich
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HighPlains (11-22-2021)
05-23-2009, 04:20 PM
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RE: tail heavy or nose heavy
Hi tree
You will learn a lot more if you try to figure it out yourself. If it was nose heavy the nose would tend to drop and you woulf have to add up elevator to fly level. In your case you had to add down elevator to fly level. Look at your plane and put in some down elevator. The elevator moves down in order to raise the tail. Logic then tells you that the tail must be heavy. There are trim charts that you can search for (Google) that will suggest when you should put in down trim on the engine, or shift the CG back a little. Hope this helps some.
You will learn a lot more if you try to figure it out yourself. If it was nose heavy the nose would tend to drop and you woulf have to add up elevator to fly level. In your case you had to add down elevator to fly level. Look at your plane and put in some down elevator. The elevator moves down in order to raise the tail. Logic then tells you that the tail must be heavy. There are trim charts that you can search for (Google) that will suggest when you should put in down trim on the engine, or shift the CG back a little. Hope this helps some.
05-23-2009, 04:33 PM
#6
Senior Member
RE: tail heavy or nose heavy
Trim needed to fly level upright reallydoesn't tell you much about your CG. The trim needed could simply be a result ofto the wing'sincidence. It could also be the result of the horizontal stab's incidence. You could have your aileronsriggedincorrectly. The engine's thrust line might be wrong. All those things are part of the puzzle.
Where your elevatorwinds up for one conditionisn't really a good judge of any one thing. Aerodynamics isn't sound byte simple. It can tell you to look for a number of things, however. But that's another story, and a long one too.
But you've gotan inkling of a way to judge your CG. Flying level is a start.
Trimthe plane to fly level. All those things mentioned before AND the CG are then all corralled into one. They're all making their contribution and the pitch trim is dealing with all of them. Now.......... roll the sucker over inverted, keeping the same speed, and see how much down elevator you need to fly level. There should be a bit needed, but not a lot.
If UPis needed on the inverted leg, then your CG is too far aft.
If just a little is needed, the CG is ok fore/aft.
If a lot is needed, the CG is too far forward.
Want to know how all that works? If you plane needed some up trim to flylevel uprightAND needed down input to flylevel inverted, the tail was having to work hard both ways to carry a load.And the same is true if your plane needed down trim to fly levelupright AND needed up input to fly level inverted. Where did theload come from? The CG. When it's farther back thannecessary, the tail has towork harder than normalto compensate and to carry that load both inverted and upright. When it's farther forward, same again onlyin the other directions.
Is this upright/inverted test perfect? Not really. Aerodynamics isn't sound byte simple. Butthat test isa good start.After all, those other things still can be messing withthe airplane.
05-25-2009, 01:22 PM
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RE: tail heavy or nose heavy
Just to add to Da Rock's excellent answer. If this is a model with a cambered airfoil (like if it has a "flat bottom" airfoil) then even if the CG is correct you will generally need quite a bit of forward stick to hold level when inverted.
Before I get into my own suggestion I'll start by agreeing strongly that adding elevator trim means less than nothing in regards to CG at this stage in flying your new model. Now it may well be a CG issue but assuming you balanced the model according to instructions it is just as likely one of the other reasons or could be that you got too aggresive with the sanding block at some point and did not put in the correct wing incidence or stabilizer incidence angle and now you have to compensate with some elevator trim. Or it could be, and is far more likely to be, a combination of all of the above. Now on to how I prefer to test my CG location.
Instead of going inverted I prefer to do a dive test. For this trim for level flight at about 1/4 throttle where the model is able to fly level and comfortably over the stall speed but not at a fast cruise. Then push the stick forward to force a 45 degree dive. Let go of the sticks and watch the flight. If it strongly lifts the nose up to a whip stall then your CG is too far forward. If it lifts the nose smoothly and recovers to level within about 50 to 100 feet of altitude loss that is pretty good and you can fine tune it to your preference from there over the next few dozen flights. If it is very slow to lift the nose or worse, tried to tuck to a steeper dive then your CG is too far back unless this is a hard core aerobatic model or a 3D model. If this last option happens remember to pull back before your model turns into a lawn dart
The beauty of this method instead of checking how it flies inverted is that A) it doesn't matter what kind of airfoil the model is using and B) if you're not comfy flying upside down at this stage of your flying you don't have to learn aerobatics to test your model.From the wording of your question you will obviously think my dive test instructions are whacky. But they are not. When your CG is too far ahead you end up adding a lot of up trim to compensate for the load in the nose. The CG location is not speed sensitive but the elevator trim IS speed sensitive. So when you add enough trim for slower flying it is too much at higher speeds and the nose wants to lift strongly, TOO strongly. Alternately if the CG is too far back then the stabilizer is asked to generate very little or no lift to keep the model level. Or worse if it's WAY too far back you may be asking the stabilizer to lift too much. Again, because stabilizer lift is speed sensitive if it is lifting more than the wing to hold the model level then in a dive it will lift TOO much and try to force the nose down. Somewhere between the model lifting the nose gently during the dive and just barely not lifting it at all is the happy zone. Where in this zone you want your model will depend on the use of the model. For a trainer you want it to have a nice positive but not very strong lift to the nose in a dive. TOO fast a pitch up will make the model harder to control and it'll hobby horse itself around the sky under the control of the confused trainee that thinks the model is possessed. But not enough and the trainee will end up with it diving to high speed all the time. You want a nice positive but gentle lift to the nose to give positive pitch stability that acts like an autopilot for them. On the other hand a hot aerobatic model should go just where it is pointed until the pilot tells it otherwise. That's your other extreme. A good sporty sport model should generally have a bit of lift during the dive but it should be just a little to the point that if you start the dive at 300 feet up and then let go you will see it lift the nose back to level but likely you'll need to help it with a bit of back stick before it scuffs the ground. That's close enough to neutral stability that it's fun and you can fly inverted with only a whisp of forward pressure but it'll fly hands off in gentle air when upright.
Another indicator that your CG may be TOO far forward is during the landing when you slow down to flare just before touchdown that you run out of up elevator early. On the other hand another indicator that it is far too back is if the elevator is extremely touchy. Like CRITICALLY touchy with the model trying to over rotate in pitch during a fairly minor input.
That make sense to you? The moral of this is that the CG position for a model is tied directly to the elevator trim. Also there is nothing at all wrong with your elevators being at an angle to the fixed portion of the stabilizer. It may offend your artistic sensibilities but the model will not care at all if it has some angle, be it up or down, as long as the horizontal tail controls the model and it flies well and reacts to the dive test as I've described. If it bugs you enough then the solution is to examine the angle of the line that goes from the stabilizer leading edge to the elevator trailing edge. Then cut into your new model and make the stabilizer line up with this line and remove the elevator trim. It should fly very, very close to how it was before.
But again, this would be the LAST thing you would do once you've flown the model a bunch and checked out a lot of other options and only through a bunch of flying you find that you prefer a specific CG location.
