One of our club members has restored a Phoenix 8. He doesn't have any documentation for the plane therefore he estimated the C of G location. Although the plane flies, it stuggles for takeoff. Does anybody know where the correct CG is located? It would certainly solve a big mystery.

tia,
-alan

might ask some guys in the classic pattern forum

Precision Aero composite : http://www.precisionaerocomposite.com/ Kits the Phoenix 8. Give them a try. I am sure that they will help you out.

Alan,

The CG on a P-8 is 6 and 3/4" from the wing leading edge AT THE FUSELAGE !!! That will get you in the ball park. I liked flying mine about 7" from the wing L/E at the fuse. A Phoenix 8 that struggles to get airborne must be HUGELY nose heavy. Easy way to find out.. get her up and roll her inverted. If she climbs w/o elevator.. there will be NO DOUBT she is nose heavy. One other thing... a P-8 w/an empty fuel tank will sit on her tail. That's how you get those beautiful landings... (even though they are a bit fast).. I have plans... and if you need help.. just ask. PAC still makes the kit and I am sure they would be an excellent source of information if you need it. Good luck with the plane...

Dan

aka deadstik
Carolina Custom Aircraft

Hey great! That's exactly what we are looking for. I'm sure it is nose heavy although we'll try the test to be sure. Thanx again for your help and I'll let you know how we make out.
-alan

Alan, I'm not sure how many companies molded fuselages for P8's but ours has the CG molded into the INSIDE of the fuse above the wing saddle, if you look closely maybe yours does as well. Dan, no disrespect, but if you roll inverted and she climbs wouldn't that make her grossly tail heavy?

I have the plane on my bench now. What would the mark above the saddle look like? Right now it is balancing at 7". Should I balnce upside down or right side up? I never understood the difference on that.

-alan

On a low wing plane, the C/G should be done with the plane upside down--I think it has something to do with the main mass of the plane hanging underneath making it easier to balance. It works, it's easier to balance that way.

I think if the plane is noseheavy, it will want to dive while inverted.

Eness,

I know it sounds really unusual.. but.. it is true. I test flew a Killer Kaos for a friend that, as he said, "Just doesn't fly right..." He had purchased the plane and installed his equipment as normal but had installed a 4 stroke engine. At any rate.. the plane climbed a LOT when inverted.... I had seen this before ..so.. landed the plane.. put the battery pack behind the servos in the fuselage moving the c/g BACK about 2 inches.. and.. the plane flew perfectly. I can't explain why this works.. but it does. A tail heavy pattern ship.. will tend to actually fly tail low in turns and have a very difficult time holding a good vertical line and doing a decent stall turn. I'm sure there are some aerodynamics folks who can explain why... but.. I just try to get them to fly right.

Dan

Alan,

I going back over your posts.. it seems to me that the first thing you need to do is to make absolutely certain the plane is assembled correctly. This means not only is the wing and stab straight to the fuselage.. but is the stab/wing incidence correct? Having a wing/stab out of correct incidence is a trimming nightmare. It is very possible that the plane was not assembled correctly. Putting in a stab in a glass fuselage is not the easiest thing to do and it is possible that it wasn't done correctly. I don't have the actual measurements but I believe the P8 is a zero/zero/zero airplane meaning that wing/stab/engine are all a zero degrees relative to a straight line down the fuselage. This means that the center of the wing LE and TE are the same distance from the ground and the same for the stab. To do this.. you first block up the airplane so that the wing is sitting at a zero degree setting with the center of the LE and TE the same distance from the building board/floor. Then measure the distance from the center of the LE and TE of the stab. They should also be equadistant. It is doubtful that engine thrust would make the plane "struggle".. but you can also put a bubble level on the prop shaft (assuming you have the wing perfectly level) and that will give you the engine thrust angle. It should be level with the wing and stab. Let me know how it goes with your project. I hope it is just a weight issue.

Dan

Carolina Custom Aircraft

Alan it should look like a cross with a circle around it, like the typical CG mark on a set of plans. Dan, that's interesting about the kaos, I've never heard of this before, not saying you're wrong, but all my personal experience has been just opposite. It's usually very apparent with 3D aircraft, while creaping the CG rearward to improve 3D performance, they will typical require less and less down or even climb while inverted.

It looks like I'm on the right track! I checked the alignments with an incidence meter. The wing,stab, & engine are zero/zero. BUT, when standing on the gear, the plane is sitting almost 1 degree negative. That's just a matter of a longer nose gear. Another note about the engine, it is an inverted mounted, side exhaust, OS61FX turning an APC 11/7 @ 12800. I think it should be spinning faster than that. I can't see how to put a tuned pipe on it because he extended the main gear to achieve a higher stance but now an under wing mounted pipe hits the main gear. I can't find a header to mount the pipe above the wing which is where he originally wanted it. So......I'm planning to try one f those Jett tuned mufflers. Any thoughts?? ( I have creative license with this project) BTW.....I just weighed it at 9lbs.

I can't find any mark or anything resembling your description on or in the fuse. It does balance upright at the 7" mark though.
-alan

yeah there must have been different manufacturers of fuses. Ours has the mark and has a date as well, I think it says 1979, but I'd have to ask my brother, it's over at his house. Sounds like you've got it with the negative incidence. It probably takes forever on the takeoff run and then all of a sudden jumps into the air?

Commit,

That negative stance while on the ground will certainly cause the airplane to "stick" and then suddently jump into the sky. As for the OS... getting 12800 with an 11/7 is about right. You are not running enough prop for that engine. You need to start with an 11/8 and go up to probably 11/10 and see which way it flies best. With the bigger pitch, the engine will turn slower but give you more torque. This is where you just need to fly the plane and see what you like. You can get an offset header so that you can run the header/pipe under the wing. The pipe will give you about 1000 RPM. Before I go thru that, I would probably simply bolt on an OS .91 FX. 1/3 more cubic inches and 1oz less weight. Should make the plane REALLY haul and go vertical without having to go with the pipe setup. Just a thought... have fun.

Dan

Carolina Custom Aircraft