jradley

My Feedback: (3)

New extra 300s ARF, 60 size with 61 tower engine, On take off it is very tipsy but once flying speed is reached feels good, CG was slightly nose heavy without fuel, reajusted but not flown again yet, would nose heavy have caused this tipsyness?

da Rock

Nose heavy doesn't usually cause "tipsyness". It usually causes the elevator in particular to be sluggish and have less authority.

In truth, the CG location usually doesn't make the airplane more or less stable, just gives the elevator more or less effect. Since most of us don't change the elevator rigging or change the amount of throw our TX asks the RX to move the elevator servo, we often think the airplane is getting more or less stable. It's actually just reacting differently to what amounts to a set TX stick movement. Change the rigging or TX and the airplane can be made to feel however you wish it to feel, fast responding or less fast or whatever. It won't be more or less stable.

The CG versus the NP (neutral point) adjusts how much force the elevator has to carry when trimmed for any certain speed. If the airplane is very nose heavy, the elevator is forced to spend a lot of it's potential balancing that load. It then has to work extra hard to pitch the airplane against that artificial increase. And we think the extra deflection and longer TX stick movement means the airplane is more stable. It's not. It's just less responsive to elevator stick movement. If you move the CG back, the elevator is freed of some of the load and has less of it's available power wasted and we need less stick movement. If we were to then trim the rigging or the TX stick movement, we could restore the slower feel, but we'd actually have the more comfortable feel with the same stability AND more range of pitch available.

What do you mean that it was tipsy on take off. Tipsy???? Did it try to nose over? Turn hard left?

jradley

My Feedback: (3)

By tipsy It took a lot of aileron movement back and forth to keep the plane level, maybe it just did'nt have enough airspeed, It did look kind of slow when it lifted off, It has a 12/6 master airscrew , maybe it needs a 12/7 to get it up to speed, what do you think?

Mike Connor

My Feedback: (5)

It sounds like you may have been close to minimum controllable airspeed. I would take the aircraft to altitude and get the feel of it as it slows down to stall speed. Forcing the aircraft off the runway before it is ready to fly would cause a mushy feel.

Shoe

In truth, the CG location always makes the airplane more or less stable

da Rock

In truth, the CG location usually doesn't make the airplane more or less stable, just gives the elevator more or less effect.

The design formulas used when working out the planform for a new design often give an insight into what features are important and why. For example, when sorting out the area for the horizontal tail, there are a number of measurements and areas that are used. When designing a tail, there are a lot more things to consider than just the area and moment of the tail and the chord and area of the wing, but that's all that are used for one of the most basic and useful of the formulas. The formula that gives the primary feel for how big and how far back the horizontal tail needs to be for pitch stability uses a number of measurements. And nowhere does it use CG location. So in truth, the CG location isn't actually part of the "why".

The CG location does however play an extremely major part in how effective the elevator will be. And it's misplacement can in fact overpower the ability of the horizontal tail to stabilize the pitch.

But with any of our models, the movement of the CG doesn't usually change the stability of our models nearly as much as it changes how controllable our model is with normal movement of the TX stick. Why? Because usually our model's tails provide adequate stability to deal with the CG we usually wind up with after putting the bits and pieces where they were suggested to go. And because most modelers nowadays almost never re-rig elevator connections when changing CG locations. And a lot of people interpret an evil-quick elevator response (that's quick simply as a result of being VERY effective) and super sensitive elevator trim as stability.

B.L.E.

da Rock, are you trying to say that you can move the CG as far back as you want as long as you reduce elevator throw?

I believe that CG location is a major factor in stability, not to be confused with elevator response. If the center of mass is ahead of the center of lift, then the inertia of a direction change from a pitch up or down fights the manuver, negative feedback in other words, e.g. stability. A nose heavy plane needs down force on the elevator to balance the weight of the plane ahead of the center of lift. This causes a plane to automatically pitch down with a drop in airspeed resulting in a shallow dive which causes the airspeed to recover. If a sudden increase in airspeed occurs, like in a sudden wind gust, the plane automatically pitches up into a climb until the plane once again reaches its "trim speed". The tendency of trainers to "balloon" when hit by a sudden head wind is due to excessive stability, not from the lift of a flat bottom airfoil as is widely believed.
Move the CG back and along with more control authority comes less trim stability. Move it back far enough and there is no such thing as trim. It's like backing a trailer that constantly wants to jack-knife one way or another unless the driver makes corrections before it reaches the point of no return.

I have done some experimenting with CG location. Too much stability is nearly as bad as too little. A like just enough stability to be able to tell how close I am to a stall by how much back stick I have to hold, but no more.

da Rock

I've tried to say what I'm trying to say about 3 different ways. And no, as nearly as I can tell from rereading my stuff, I've simply said what I said. That USUALLY what all of us call "stability" is simply how effective our elevators feel to us. And that most of us aren't usually adjusting our elevators to match changes we make to our CG locations.

And here's what usually happens.
An inexperienced flyer brings a tail heavy airplane to the field and for whatever reason, the CG isn't corrected. And the elevator throw is very often a bit muore than sensible but passes inspection also. Even if it's set to the recommended throws, it's going to have lots more authority than the designer expects from his suggested range of throws.

When that airplane takes to the air, the elevator is usually at neutral straight back. Wherever it's neutral, it's most probably wrong and most probably will need a fair amount of trim to fly the airplane level. And remember, the TX stick is going to move the elevator LOTS MORE than it's going to need to be moved now that it's sensitivity has been greatly increased by the aft CG.

So the airplane is taking off with BADLY out of trim elevator, and that elevator is going to be a hair trigger elevator.

Unstable? Jeez, who knows how stable the sucker is, because it's so responsive to the elevator the poor sucker flying it has almost zero chance of controlling the sucker before it either stalls like a brick or bucks itself to death.

It wasn't lack of stability that kills those airplanes. It's impossibly sensitive elevators combined with their being way out of trim. And everyone strokes their chin and wisely pronounce that it was just not stable enough to be flown safely.

da Rock

I just recently flew a brand new airplane that had it's CG almost slap on the NP. It was a shocking surprise on takeoff to say the least. It was also a major embarassment. We had forgotten to check the CG.

After the "climb out" and "level off" it was still alive, and I started to breathe again. I flew the airplane for 3 or 4 minutes sorting the trims. It was VERY obvious to me how hair trigger the elevator was and that trim could be gotten close, but wasn't going to do better than that. Everything else trimmed ok. I ran it up to full throttle and also slowed it down enough to see that it was going to slow nicely for landing, although it was going to be risky to slow it much. We were flying in wind that most of the regulars sit out. But it was almost down the runway. The flight had caught everyone's attention and there were bets being made on the life of that airplane. Darned if the landing wasn't so good that there was applause. Truth is, with the help of the wind, the landing didn't worry me a bit. The sucker was as stable as a rock as long as I didn't move the elevator more than a couple of degrees. On the approach, the wind blustered enough to roll the airplane a couple of times, but it didn't pitch worth mention. Heck, it hadn't lost any stability, it'd just gotten a very powerful elevator that unfortunately was also having to carry an artificially aft weight that the wing was too far ahead of to carry entirely.

Stability of the horizontal tail is a function of the size and leverarm of the tail and is affected by the size and chord of the wing. It doesn't go away when the CG is misplaced.

da Rock

Model designs usually evolve such that the final versions of the model have a couple of characteristics.

The elevator will wind up trimmed "straight" when the CG is located close to the suggested location. And the suggested elevator throws will be sufficient that they don't make the airplane evil to fly. They usually are sufficient on high rates to adequately stall the wing depending on the purpose of the airplane, and to not stall the wing on low rates but still adequately fly the airplane. And the horizontal tail will be adequate in size/moment to produce a stable airplane.