CG Blues - Decathlon
 

Hi One and All,

I have just assembled an EPO Decthlon 765-1 model. Luckily for a novice like me it has 2 CG markers under its wings to use middle or index fingers to measure CG. From my discussions with more seasoned folks and with research on the internet, the plane should tip slightly forward for optimal config. I got this right by adding tape and coins to the cowling around the motor. It now hangs slightly forward no more than 5 degrees.
When i wanted to take it off from the ground [ cricket pitch with very short grass] it would pitch over onto the prop. It would do this even when applying very little throttle. I then took it out on a road and applied half throttle and the plane stayed on it wheels but when i cut the throttle , the plane would flip over onto the prop.
Is my CG correct?
Surely it should be able to take off in very short grass?

Thanks guys.

Small foam planes don't typically like grass. If the grass is higher than the tires, there is a lot of drag down low on the wheels while the prop is pulling the plane forward, causing it to nose over. However, there could be several issues making the problem worse. First, does the plane roll freely (i.e., no binding or stiffness in the wheels) if you give it a push on a flat surface like your garage floor? Also, when taking off, do you hold full up elevator until the plane if rolling fairly fast and the tail is ready to lift?

It could also be a poor design that places the main landing gear too far rearward. If the first 2 items are not the problem, you can try bending the main landing gear slightly more forward but it may not be much help in thick grass.

Use a CG calculator to make sure you have the CG in the right place. You normally measure CG where the plane balances flat, not nose down. If the CG is good, next check for how much weight is on the tailwheel. If there isn't any with the tail down, the plane will never handle well on the ground. A perfect gear setup will have the plane ever so slightly tail heavy when in a level position on the mains.

Make sure the wheels are free spinning, don't use tape to add weight to the cowl. If the tape comes loose you need a new airplane.
i recommend you go to the nearest RC field and learn from the guys there.

Your cg is not where it was intended and and actually is considerably forward thus a chief contributor to your problems among some of the other lesser problems listed by the fellows.

The old sage advice to always balance your airplane with just 'a smidge' nose down for 'mother and country' will result every time with the CG forward of your real target CG point. The problem is there is no way to accurately communicate how much is 'just a smidge' or even a practical way to measure degrees. Your five degrees down is way to much.

When actual weight and balance measure is done to full scale airplanes great care is always taken to level the aircraft and with our model aircraft its my belief that it too should always be done with the airplane level and never with any nose down.

John

Question... how does one determine a ever so slight tail heavy condition when level on the mains?

Also he should check that the front of the wheels are just past the leading edge when looking straight down.

What I meant by slightly tail heavy when level on the mains is that if you hold the tail up so the plane is level on the ground and let the tail go, the tail should fall gently to the ground. The plane should almost balance on the mains at a normal flying attitude but not quite. Since the weight shifts back when the plane is nose up, it will still have plenty of tailwheel weight to steer properly. When balanced right, the plane will nose over if you tip it nose down and let it go. If the tail falls hard, which indicates the main gear is too far forward, you'll have a plane that doesn't want to lift its tail on the takeoff which leads to ground loops and premature takeoffs which can cause snaps. It will also have a greater tendency to bounce on landings. If the plane tips forward from level (main gear too far back) it will have a bad tendency to nose over and may be erratic in ground handling. Taildraggers aren't any big challenge to taxi, takeoff and land when they are set up right. But they aren't as forgiving as trike gear planes of a bad setup, so it's worthwhile to check everything and get it right.

Makes sense, but I haven't setup tail draggers this way previously. With one one just about complete and a Citabria next, i'll give it a try.. thanks !

Hi Folks,

Thanks for all the great advice! I removed the ballast from the cowling and then changed the wheels on the undercarriage to 3 inches. The CG of the plane is about neutral. And presto, with the elevator on full and then released when there is enough momentum, the decathlon has taken to the skies :) Now just have to figure out how to land her gracefully.

Presto almost like magic Huh:)

Enjoy he ride and we look forward to your further adventures along the way.


John:cool:

Jester.. as I was reading this again, I had a question on what you wrote:

"Since the weight shifts back when the plane is nose up, it will still have plenty of tailwheel weight to steer properly. "

Why would weight shift back when the nose is up for when balanced properly you state, "letting go of the tail it should fall gently to the ground", I assume based on it's current balance setup.

Are you saying at more extreme angles of attack, when the endpoint distances of the nose and tail are closer to the balance point, it takes more 'effort" (elevator) to bring it back into balance (level flight)? Weight shouldn't shift as the balance point stays constant. I hope you don't think I'm nitpicking, I just want to understand as my tail draggers are always light in the tush and very squirrely. I understand mostly due to torque and prop wash over the rudder but any little bit I can understand the better.

Think of the plane as a teeter-totter. When the plane is flying, the fulcrum of the teeter-totter is the CG, there is equal weight on both sides of the fulcrum, and the plane flies level. When the plane is on the ground, the fulcrum of the teeter-totter is the main wheels, there is now more weight behind the fulcrum than in front and, consequently, the tail drops to the ground. The further the main wheels are ahead of the plane's CG, the greater the weight behind the fulcrum and thus the heavier the tail is.

There is also the fact that the plane's vertical CG is above the main gear wheels. So when you lower the tail, the plane rotates around the axis of the wheels. That effectively shifts the plane's horizontal CG backwards relative to the main gear wheels. The taller your main gear and the shorter your tail wheel are, the more pronounced the effect is. If you don't believe me, stack a few books and set your main wheels on them. See how much weight you feel in the tail with the nose up like that versus how heavy it feels when you hold the tail in flight position as it would be just prior to takeoff.

To amplify some of the previous posts. Typically the CG should be over the wheels when the nose is down 15 degrees from level. This can vary slightly depending on operation on grass or pavement. This does not meant change the balance of the airplane because that depends on flight dynamics, but rather sometimes the location of the landing gear is wrong.

Gear too far forward make the airplane more prone to ground loops, and bouncing on landing. Too far back, difficult to taxi and turn in high cross winds. Also a bit more prone to nose over.

A bit finer point, the so called CG point on plans indicates a balance point for the airframe with the fuselage level. The actual CG is usually above or below that mark, which is why the 15 degree down balance tests for possible problems. One might think gear location is always well worked out, but if the designer always flys from grass, that masks problems that pavement exposes.