coindex

I built an Extra 300S 60size which has a OS91 fs engine ivnerted. On my maiden flight I noticed my controls were so touchy that the plane was almost impossible to fly. I also noticed it seemed like it was tail heavy resulting in a constant lift. I managed to crash land it with minor damage to the wing to fuselage connection. I balanced my model on the cg machine at 4 1/8" as per instruction. I did modify the model and put two servos in the tail section but on my fixup i moved them to their original position in the middle of the a/c.
Any suggestions for my next flight??

BMatthews

Use the online CG calculator that is linked in the sticky at the top of the listings for this aerodynamics forum to double check the balance point data.

da Rock

Suggestions?

Most certainly. You know the pitch sensitivity is very touchy. So setup your transmitter with dual rates and use low rate if needed after your changes. The mfg's suggested CG and throws are a starting point. You've discovered they didn't work very good for you for some reason.

There is an unbeatable way to verify CG before the first flight. It takes about 10 minutes and you need a yardstick to do it. Simple and obviously would have been worthwhile in your case. It wouldn't hurt to give it a shot before your next flight. http://www.geistware.com/rcmodeling/cg_super_calc.htm Run the calculator with 5%, then 20% static margin to get the CG range for your model. If your balance is toward the rear of the range you get, then figure to take off on low rates.

You don't saw what model nor what elevator throws. I've got a 60 size Extra 300 and it's CG range is 4.1" - 5.86". Good chance our 300s are very, very close to the same layouts. Seldom does any mfg slap too tiny a tail on a semi-scale model that's wildly popular throughout the hobby. So I'd also suggest you have an experienced modeler look at the balance, throws and whether or not the pushrods are sticking and whatever else he sees.

What is the mfg?

TimBle

had a similar problem with a yak 55 sp in size 10e. two things, were out, the static margin was too low and the elevator had way too much authority. These small aerobats are sensitive creatures. Reduce the throws, add lots of expo, maybe 50% on the ailerons and elevator.

but do check the actual cg as well using the advised tool

coindex

I took my measurements plugged the chart and this is what I came out with.
At 20% my ideal CG aft of root LE is 3.94" and at 5% my CG aft... is at 5.67"
20% desired static margain gives me a 27.2% ideal center of gravity %MAC which looks like this would work the best for my model
5% desired static margain gives me a 42.2% ideal center of gravity %MAC

I did do some modifications to the horizontal stab and the vertical stab by extending them .5" bigger then original which is where I think im getting my differences in my balances now.

Therefore, my ideal CG Aft of root LE is between 3.94" - 5.67"

I'll set the CG at 4" and see how that works.

Does this sound like a good idea?

My model is the Great Planes Extra 300S 60 size.
Elevator throws are 2" deflection with rudder at same. (set for aerobatic flight) low rates set at 50%
exponential is all set at 20% + at the present time

BMatthews

Well this IS supposed to be a hot and sensitive aerobatic ship. Selecting a stability margin at 20% is putting the CG at what a basic trainer would fly with or maybe even further ahead than that. Most aerobatic models are going to be in at around the 5 to 0% stability margin and from there the pilots just deal with it by using lots of exponential and a very light touch on the sticks.

Just so we know are these numbers you got in terms of the MAC position compatible with the kit instructions or is the kit spec out in left field?

coindex

The kit states the CofG at 4 1/8" from leading edge so that would put the original build at approximately around the 15% MAC in terms of the cG calculator. So it seems that the kit instructions/specs are right with them being around the higher end of the MAC scale(15%MAC)

Now, if I do set it up at 5%MAC this would make my tail heavy thus having to rebalance the whole inside closer to the engine plus I think I would have to add some deadweight in the engine mount area.
I have it all balanced now at 4" off LE and I had to move my battery to the furthest it could go to the firewall under my tank, and moved two servos (ones that will run my elevators) 3" past the center of the electronics bay thus making them closer to the engine firewall also. It balances well now with my CG machine. With 1/8" difference from manufactures specs it seems that there was alot of adjustments just to rebalance.

da Rock

You think your plane is tail heavy already, right. Yet your balancing was at 4 1/8" and that would have a 15% SM.

If your balancing is correct, then it's not actually tail heavy and something else is causing the overly sensitive pitch response. How many clicks of elevator trim did it take to fly level? Looking at the elevator on the ground how far from neutral is it's throw when the elevator stick is at neutral. From what you described for flight characteristics, it should be trimmed significantly and it should be deflected quite a bit. How much, and which direction?

As somebody else mentioned, fly the sucker with reduced throws and see if it flies like you wish. What was your previous model?

coindex

I can't remember how many clicks it took since I was to involved in keeping it flyable and giving it down elevator so i could land the plane instead of adjusting the trim. I tried some adjustments and i think i put around 8 clicks or around there. It still wouldn't fly level though, i never got it to stabalize. When I look at the elevators on the ground when my sticks are neutral, everything is inline with my horizontal stab. I put a ruler on my stab and my elevators are perfectly inline flush.
Here are some pictures how my stab looks

BMatthews

If that's the case you may want to use something like a Robart incidence guage to check that the wing to tail are sitting at close to or at zero degrees to each other. And check your propeller plane to the wing as well. For a model of this style the setup should be 0-0-0 and you add what little decalage is needed for level flight with a couple of clicks of up trim.

coindex

Well.....I rebalanced my plane at 4 1/8" c of G and on maiden flight only took a couple clicks up elevator to level it off.

Plane is VERY VERY touchy though. I have my exponential on all settings at 25% (+) and was using a 14x10 prop and noticed my plane needed alot of runway to take off and on approach the speed was always to fast. I tried slowing the plane down but due to the controls being so touchy I was having a real hard time trying to "harrier" the plane on the approach.
Flight was successful though my landing gear took a beating so I reinforced the area and bulkhead with fiberglass.

da Rock

No matter why, if your model is too touchy try reducing the throw for low rate elevator and see if that'll work for you. It works for thousands of us.

A lot of flyers base their throws on how and when the plane unexpectedly snaps out of a loop or hard pitch change. They might reduce the throw for low rate, or simply move the pushrod out the horn, depending on what they see and want. The idea is to keep enough high rate throw to be able to snap, and reduce the low rate so it won't.

TimBle

THese birds are just overly sensitive to the elevator. I have mine at 60% exponential and its now managable. Watch out for the stall, it bites you hard if you don;t have altitude. Fortunately I did

da Rock

Which birds?

My Ultimate isn't. Ultimates are VERY quick devils, but don't have to be overly sensitive. I did adjust the throws of course.

I hope no beginners get the idea that big expo settings are the way to deal with what they think is overly sensitive design. That amount works for 3D of course. Not all brands of radios do the same with expo. Some actually reduce the throws when you've dialed in the big numbers. It's worthwhile making a quick, simple test if you plan to use lots of expo. Turn it off or dial it to zero and look at your throws. Then turn it way up and look. Some brands give you a graph on the screen when setting expo. Look at the end points as you click in the expo. You may discover the expo actually reduced your throws. In any case, doing the test will show you absolutely what you are going to be flying.

Foxman

Determining the CG is very easy if the model is already flyable.
Fly along and roll to inverted. Keep it level with the use of elevator. Get a friend to check the stick position if you can't tell.
If you have to push a small amount of forward stick in to hold level flight then the cg is correct.
Having to hold lots in means it's nose heavy.
Having to hold none or very little means it's a bit marginal and should be moved forwards a bit. Small amounts at any one time.
When you have this sorted you can then adjust the throws so that you can use full "up elevator" without it snap rolling. This is important.
If you want to do snap rolls then fly on low rate for normal flight and high rate for your snap rolls
About 20% expo will give a linear effect to the control feeling.
The above advice applies to every aircraft I have ever flown but I haven't flown every aircraft.

Tall Paul

When you say "+ expo"... on Futaba and Hitec, that makes the controls super sensitive around neutral.
On JR and Spektrum, "+ expo" makes the controls less sensitive around neutral.

Foxman

I can never remember which makes of radio are + and which are - to get expo the right way.
A servos natural action gives an exponential movement of the surface the wrong way, i:e it moves it more around the centre and less at the end of it's travel.
Adjusting in about 20% expo the correct way makes it linear.
Jim

TimBle

Futaba logic is servo command based. All of it.

JR logic is user based.


So for Expo's Futaa says reduce the initial travel therefore you use "-" expo to reduce initial travel.

JR says more expo means less initial travel therefore apply positive numbers....


To me the Futaba logic just seems more natural. You want the surface to move less so you use negative...

It just appears one product is deisgned by engineers and the other by marketers..