I have an Ecomrc 1/4 tiger moth with some cg issues. The manual is grossly off when it comes to the cg. I did some rough calculations, and set the cg more appropriately. The tiger moth however has a BAD nose over tenancy. I have read that the cg can be placed farther back, but my question is just how far back can the cg of a tiger moth be? Is there a formula, or an adaptation of the standard biplane cg formula that works for swept wing biplanes?.

Well, a Tiger Moth has a fairly constant chord and relatively little sweep. So, the mean chord should be fairly close to the distance between the leading edge of the top wing at the cabane to the trailing edge of the bottom wing at the tip. So, your CG should be about 28-33% of that, measured from the leading edge of the top wing at the root. I'd say anything in that range should be relatively safe, but ultimately testing it in flight will give you the best answer. Move it back a tiny bit at a time until the plane starts doing things you don't like, or the nose over tendency stops.

What does a tail heavy tiger moth do besides go nose up and stall?

I didn't realize that it is safe to go back to 33% of the MAC.

At 33%, you 'should' be fine on most planes. The telltale sign that you've found the rearward limit of where the plane is happy is to see how it acts when you reduce power. It shouldn't change pitch the instant you reduce to idle, but should slowly pitch down as it looses speed. If it doesn't pitch down at all as it slows down, or worse yet pitches up, you need to move the CG forward a little bit.

Thanks for the info.
I am not yet used to dialing in the cg of biplanes yet.

It all comes with practice! Here are a few more pointers if you're new with messing with the CG:

- Conventional tail airplanes don't fly the way many think. The tail actually pushes down. The CG of the entire airplane is forward of the wing's center of lift, also known as the center of pressure. So, with out the tail the plane would pitch straight down. The tail is designed to push down to counter this. It works well because normally as the plane slows down, the tail generates less downforce so the plane pitches down and stops loosing speed. This arrangement helps ensure the plane is constantly trying to find an equilibrium, or as an Adam Sandler movie would call it, a "happy place ."

- There is a difference between an aft CG and tail heavy. Aft CG means it is still ok, but getting close to the limit. Tail heavy means you have in fact gone too far.

- Be prepared for it to be less stable, but still controlable if the CG is a little aft. That's why moving the CG a little at a time is better

- Usually an aft CG means the plane is still technically stable, meaning it pitches down when it slows down, but is much less so than normal. You are neutral when the plane doesn't pitch one bit as it slows. In reality, truly neutral is almost impossible but we can get very close. Tail heavy means the plane is actually unstable, so it pitches up as it slows down. This creates a big problem since as it slows down it pitches up which makes it slow down which makes it pitch up.... You get the idea.

- A little taiheavy is usually still controlable though, but will require near constant small inputs to keep it where it's going. If you can smoothly control one that's a little out of trim, without trimming, you can control one that's a little taiheavy.

- Speed is your friend when a little tail heavy, but don't go flying around at full power. Rather, if you notice you're having to add a lot of down elevator to keep it from pitching up, add a little bit of speed.


This isn't an all encompassing write up on CG and static/dynamic stability, but rather gives you a few things to keep in mind as you get your airplane sorted out.

When balancing a large scale aircraft using the fingertip method, when properly balanced when I lift the plane straight up should the aircraft assume flight attitude, or should it stay at the attitude it is in when on the ground.

Flight attitude. It doesn't have to be within a fraction of a degree or anything, but it should be pretty obviously level.

You may have figure this out already but I have to ask: When you say it "has a nose over tendency" do you mean:

It wants to nose over into a dive in flight?

or

It wants to nose over when landing?

A lot of classic era taildraggers have nasty ground handling characteristics due to the the layout- tall landing gear placed very close to the CG. It has nothing to do with the CG being too far aft, even balanced perfect they do this because the weight is up high and right over the wheels on touchdown. Cubs are like this somewhat, and WW1 aircraft like the Sopwith Camel and Dr.I Triplane are the worst.


Get your CG close by the math and then flight trim it perfect. Because its a semi or flat bottom airfoil and asymmetric layout its not going to react to the classic pattern trim tests the way an Extra will, so expect a decent amount of down elevator needed inverted, even when its tail heavy. The better test will be flying the 45deg down line - you are looking for it to gently fly back to level hands off.


BTW - A Moth has both swept wings AND a fair amount of stagger. Your best bet is to look at both wings and use the biplane formulas.