ameyam

Dear All,

I have recently purchased a Pheonix Tiger III 40 size on OS55AX. I found that this plane is slightly unstable and is buffeted around by the wind. Doubting the CG location, I checked the CG and found the following (max wt is 2600~2800 gms)

Wind cord 270mm
Dry tank (with and without plane inverted) CG: 90mm from leading edge
Full tank CG: 70MM from leading edge

The mfg manual recommends CG at 80mm from leading edge. With this base, plane is tail heavy when dry and nose heavy when fuelled. I am told that the CG range should be between a third to a fourth on the cord. So CG lies within the suggested range.

Can you tell me if the problem lies with CG location or somewhere else? Admittedly I fly very close to the sea but I found that other planes did not seem to have this problem

Ameyam

w8ye

My Feedback: (16)

If the plane has a very light wing loading, it will be buffeted by the wind more than a plane with a heavy wing loading

ameyam

This is a light wing loading alright, but so is a high-wing trainer and on the same day, I didnt have any problems with that one

w8ye

My Feedback: (16)

The may be more control movement on the plane that jumps around making it more difficult to control?

ameyam

Actually I am flying at 50% throw. The plane however buffets on its own

w8ye

My Feedback: (16)

Low wing planes are not as stable as high wing planes yet are much more maneuverable

Delta3

I had one of these with a TT 46 Pro in the nose, flew just fine without any extra weight in the nose, with an OS55AX up front it should be fine. I always fit my motors as far forwards as possible though and that may make a little difference. These planes are a bit twitchy with the controls so dial your throws back a little and set up dual rates just in case you need them. I sold mine ages ago and now fly the Tiger 60 which is a much better and more stable aircraft

JohnBuckner

My Feedback: (1)

Yes your airplane handles poorly you are flying it with the CG 10mm to far aft. Forget fuel tank always referance with an empty tank unless your are flying a pusher with a rear tank. This airplane will fly beautifully balanced at quarter chord (25%) 30% is too far aft. Balance the airplane at 80mm at least or even further forward at 67mm which is quarter chord of your listed chord 270.

Make it happen and do what ever you have to do to balance it there. Also when you balance it at your selected point it should sit level not nose low for mother and country, if you do that then you have not balanced the airplane at your selected point but instead at some unknown point further forward.

John

ameyam

John,

just to check, cord of 270mm is listed including aileron. Is that correct?

Ameyam

JohnBuckner

My Feedback: (1)

Yes ailerons are included in measurement for cord.

ameyam

Dear John,

I did check for addition of weight to the nose at the time of checking dry CG. However, I could not measure the weight numerically as yet. I will do the same this evening and revert. Hope to hear from you then


In the meantime, the original Carl Goldberg Tiger 40 has a 61" wing span. The Phoenix has 58" span. Will that have an impact on the handling?

Ameyam

ameyam

Hi,

I found that a 90gm weight placed on the engine balances the model with dry tank at the manufacturer's recommended 80mm from leading edge. Since (obviously) I cant place the weight outside the fuselage, I am thinking something in the range of 100gms ((3.53oz), by trial and error) in the fuse above tank. What say?

Ameyam

JohnBuckner

My Feedback: (1)

That should work, you could also use a weighted spinner nut. Where is the battery located? perhaps it could be located forward under the fuel tank. Here are two extremes that I have gone to in avoiding the use of weights. In one case the battery is under a hatch almost under the horizontal stabilizer and on the other the battery is all the way forward along side the engine.

John

JohnBuckner

My Feedback: (1)

Here is an example of a weighted brass spinner nut that weighs 3 ounces:

http://www3.towerhobbies.com/cgi-bin...?&I=LXE191&P=7

ameyam

Dear John,

I am concerned about adding a rotating 100gm weight to the engine. Besides reducing throttle response, wont it cause gyroscopic effects?

Frankly I am thinking of adding the same weight in "m-seal" epoxy putty. I dont know you are familiar with the same: it is a epoxy mix used to seal gaps in household items such as leaking water pipes, cracks in floor and wall tiling etc

Ameyam

w8ye

My Feedback: (16)

Actually the rotating weight gives you a better idle. The other effects you read about in physics do not seem to bother you in this case

ameyam

John,

after much trial and error I balanced with 90+50mg putty and 3Nos AA batteries. Total weight could be about 250gms, (8.8oz). What say?

Ameyam

JohnBuckner

My Feedback: (1)

The bottom line is you gotta do what you gotta do to balance at a minimum of 80mm actually I prefer a little further forward of that. The 90mm you were flying at is 33% MAC and that is a dangerous point with this airfoil.

Actually the effects as mentioned by w8ye in using a weighted spinner are negligible, while hardly noticible the effects are two fold: The weighted spinner will allow a slightly slower reliable idle and will cause a slightly slower throttle response. In both cases negligible. As far as gyrosocpic effects on flight you would never be able to sense a difference.


John