tele

After determining the CG at 28% of chord and the plane is tail heavy, how tail heavy is too tail heavy?
Should the horizontal stabilizer be on the level mark (zero bubble)? No plans or instructions came from this garage sale plane.

JohnW

My Feedback: (6)

The plane is too tail heavy when it flys better backwards than forward.

28% isn't an extreme rearward CG in my opinion, but it does depend upon the plane some. What plane is it?

Red B.

tele,

Is 28% the recommended c.g. position? If so, you should definitely add ballast in the nose to get the c.g. right, especially if you are a beginner. Do this no matter how much lead it takes! The last thing you want as a beginner is a tail heavy aircraft.

Putting the stabilizer on the level marks should be fine.

If your aircraft is glow powered, I would reccomend balancing it with an empty tank.

Tip: If you need lead, go down to the local garage and purchase a strip of self adhesive lead weights used for balancing car tires, inexpensive and very handy.

/Red B.

Geistware

My Feedback: (16)

Red, what I do is go to a local tire repair place and get the used weights. They are free!

Askalon

My Feedback: (1)

I just add more CA to the firewall! (Just kidding, tire weights rule)

tele

This is an airplane picked up at the Puyallup Expo. No plans, no instructions, already built. The local club members recommended a CG of 28-30% chord last year and I had a few flights at 28% with it. It flew well with little trim needed for level flight. Changed battery to remove some lead ballast from the nose and it balances at 28% without the lead by moving the battery forward and aft. Has engine and empty fuel tank when doing this. I've heard that a tail heavy plane will fly but a nose heavy plane will only fly once. My question is: what needs to be level in order to find the CG? It has a flat bottom airfoil so do I measure from the floor to the leading edge and from the floor to the trailing edge and get them to be the same measurement? Don't know what kind of craft it is. Kind of looks like an Alpha Trainer but it isn't. It runs a .61

bassmanh

My Feedback: (5)

tele,

you have that backwards = a nose heavy plane will fly and a tail heavy plane will fly once...........

as i was told about finding the CG is 25% of the wing is a good starting place or at the spar in the wing for most trainer type planes.



bassman

DBCherry

My Feedback: (4)

bassman's right, you had that backwards! You do not want a tail heavy plane, and 28 to 30% is leaning towards tail heavy on virtually every trainer I've ever seen.

Put the location of the CG on the underside of the wing where it meets the fuse, pick the plane up and balance it on two fingeer tips (one under each wing). You want the plane to sit level, or very slightly nose down. If it hangs tail down, then shift some weight forward.
Dennis-

DBCherry

My Feedback: (4)

Thought I'd add a bit more info.

With a low wing plane, you balance it the same way, except you do it with the plane upside down.

I'm posting a graphic that will let you determine the Mean Aerodynamic Chord (MAC) of almost any wing, and with the MAC you can ge the CG (which is usually 25% of MAC).

Good luck,
Dennis-

Razor-RCU

My Feedback: (50)

In everyday terms, the nose should tilt down slightly when you are on the main spar if the wing has a straight leading edge- (tank empty)

photoniq

at last - an answer i can almost understand. Can you tell me what the main spar is again? How do I find it? Which bit is it?

Sorry to ask it three different ways. I just want to be clear on what I am asking.

C_Watkins

My Feedback: (10)

It's typically at the thickest point on the wing, or very near there.

FHHuber

The main spar is the heavy support "stick" in the wing that runs from wingtip to wingtip.

25% MAC CG starting point is a known forward safe starting point... almost always you will find that to be noseheavy when flight testing.

See the thread about doing aerobatics with a Hobbicl SuperStar for more about "playing with" the CG, and re-trimming an airplane for higher performance. (when you no longer really need a trainer...)