CG vs. Trim

[Capt_Beavis]

my Sport Cub S is going down at a fairly steep angle at 50% throttle on Expert mode. It seems balanced to me and I have move the battery around to no effect. The elevator looks like it is at the right level. How do I best trouble shoot this?

 

[nerdnic]

Does it react different at +/- throttle? Usually CG plays out like this:

-Tail heavy may fly 'okay' while maintaining speed but as you drop in speed the erratic behavior gets very obvious. This is assuming your plane is only slightly tail heavy to the point where it still is flyable. A very tail heavy plane will not fly at all.
-Nose heavy will balloon at high speed and want to dive fast if you drop throttle.

Based on what you're describing it sounds more like you need a good trimming more than anything.

 

[quorneng]

Any Cub type plane is never going to be a high performance glider so its going to depend on what is a 'fairly steep angle'.
I suspect what is happening is that in beginner mode the gyro is automatically adding a bit of up elevator but in expert mode you have to do it so a bit of up trim should improve the low power/glide performance.
The only way to find the best set up of trim & CofG is to "try and see" but only change one thing at a time!

 

[AkimboGlueGuns]

Most full scale pilots trim their airplane to the speed they are flying at. That is something that should be adopted into RC flight, but often isn't. To me it sounds like a decent trim is all you need, but you can experiment a bit trying to get it to fly good.

 

[Florida Air]

Interesting- this sounds exactly like the problem I just posted on the Sport Cub S Owner's Thread. I have two SCS planes (one to crash and the other one to crash). Today #2 was flying just fine in Beginners mode, but as soon as I switched to Experienced mode (while in flight) it heads straight down, nose first. I next switched to #1 plane and experienced the same issue but this time I was up high enough where I was able to switch back to Beginner mode and it promptly leveled out and flew on.
I guess I'll check the COG and then add a little elevator and see how it responds. I was absolutely caught off guard at the difference between Beginner and Expert. It went from a steady level flight to nosediving into the earth.

 

[Capt_Beavis]

My elevator and rudder both seems to get thrown off over time. Mine was definitely a trim issue.

 

[quorneng]

How a bout a simple test?
With the plane switched on but sitting on the ground does the elevator move (it only needs to be a slight down 'twitch') when you switch from beginner to expert more?

 

[Florida Air]

Simple Test Completed

How a bout a simple test?
With the plane switched on but sitting on the ground does the elevator move (it only needs to be a slight down 'twitch') when you switch from beginner to expert more?

Well I don't want to steal from Beavis' thread but as I indicated above I seem to have the exact same issue (at least the symptoms are the same). Hopefully you can help both of us simultaneously. (Make sure your seat-belt is securely fastened and return your tray to the upright position. )

I did the simple test on both my Sport Cub S birds and got the same result from each one. (I'm assuming you meant for the tester to activate the SAFE system with a quick blip of the throttle because otherwise the control surfaces do nothing when switching between modes.) When you move to Intermediate or Expert mode the elevator moves slightly upward. Also the ailerons do a bit of a twitchy movement - however they move in opposing directions, which I assume, tells me that the surface the plane is resting on isn't quite level.

I'm also a bit confused about the COG. I read an excellent post by Wanderlust in the thread entitled "CG Marks" (the entire thread is good) but I'm still confused. In this thread Wanderlust said:

"Actually, you need to get the centre of aerodynamic pressure (which isn't quite the same as the centre of lift, but it's close) behind the centre of gravity.

Think about a dart: heavy streamlined bit at the front, light draggy bit at the back. Whenever an object is moving through a fluid (think of gasses as extra-thin fluids), it will tend to rotate to place the CoG in front of the CoAP. If the CoG is behind the CoAP, the plane will try to fly backwards.

So why not put the CoG at the nose and CoAP at the tail? Because the control force required to disrupt stability and manoeuvre is proportional to the distance between the two. If your CoAP is too far back, you don't get a plane; you get a lawn dart.

For maximum controllable manoeuvrability, you want CoAP right on top of CoM. But manoeuvrability and stability are in tension; maximum agility equals minimum stability. So, usually, the CoAP is pulled back a bit from the CoG. The more trainer-focussed the build, the greater the CoG/CoAP offset.

The FT guys place the CoG mark where it is when they build the plane, with the CoAP unmarked, but presumably somewhere behind it. But when you build at home, you may use heavier foam or servos or just more glue, which can shift the CoG from where it was intended to be. This is what the CoG marks are for; once you finish building your plane, you need to move weight around until the CoG is where it was designed to be. Often all this requires is to shift your battery a smidgeon.

I wasn't able to find where the manufacturer has published where the COG is supposed to be, but with the battery installed both of mine the COG is about 1.25" behind the leading edge. However without knowing where it is supposed to be, this really doesn't tell me anything.

As far as trimming, I've tried to stick with mechanical at this point, especially since the issue is huge...there's no way a little electronic trimming will correct this issue. (I mean it's night and day when you switch out of Beginner mode - it immediately wants to nose dive.) I've tweaked the control rods to get the surfaces as neutral as possible. When doing this I raise the tail of the plane about 1.5" to simulate actual level flying.

I'm beginning to think it's user error. As a newbie I tend to fly a bit slower than most probably do and I wonder if that is causing the dive. However when I've switched to Expert I am around 50% throttle, so I'd think it would be OK..?

Any recommendations?

Thanks!

 

[quorneng]

Florida Air
Whilst the content of that post is quite correct the thing to remember is that the CoAP is not a fixed point - it moves a bit depending on the air speed (actually the angle of attack) and its actual position depends on how accurately the flying surfaces are set up.

I am reminded of the fact that for some time many Mondeos tended to pull to one side. This is despite the fact it is made of metal and built on accurate jigs. It was eventually fixed but only after it almost became a joke in the motoring press.
It begs the question just how accurately can a foam plane be made?

It follows that it is reasonable to expect a bit of trial and error testing will be required before a CofG position is found that best suits how you want to fly but it won't be far the recommended spot.

As far as the effects of changing between 'beginner' and 'expert' mode are concerned it comes down to the characteristics of the electronics rather than the plane itself.
What is needed is for the plane to bee set up so it flies satisfactorily in expert mode - by trim and if necessary CofG adjustment. This would then confirm that the basic aerodynamics are satisfactory and that it does not rely on the abilities of the gyro to maintain level flight.

Just remember before the advent of 'safe' technology this particular problem simply did not exist!

 

[TeckRC]

If you look close to your control horns on it, they might be a little loose, after a bunch of flights with mine i noticed the rudder and elevator control horn both started getting loose and made it so i wasnt able to adjust my trims right, i had to put a little glue on them and get them secure again, and then havent had any problems since