My CG

[Tsemler]

Well... after all the carry on about the CG and thrustline awhile back i decided to bite the bullet and rebuild my machine and lift it 340mm???i think it was to see how much it changed the CG, well finally tested it this weekend and the way it was as a straight thru keel it was 24 inches!!! high thrustline, the way it is now its 10 inches high still, and im happy with that, a fortune teller told me that now when i will apply full power from a 50 knot cruise it will mush a bit then start to climb with no pitching moments and the opposite for power reduction, so i"ll be happy with that! The only true CLT fix for it would be to lift it further up or chuck a C box on her and have it the below the crank line.

[asra]

Regards SamL..........

[Tsemler]

Hmm Thanks Sam seem i might have cocked my lines up on the print out as it doesnt look that bad there does it, if i chuck my full tanks ones up could you please have a look at them too? ThanksTrent

[Tsemler]

without checking my paper work i believe that between empty and full the hang tests changed 1 degree is that what other people found as well?

[Brian]

Sam, what hight did you measure there ? [above CLT]Congratulations trent for making a change. Have you flown your gyro since

[asra]

Hey Trent.Sam is right on the money there. I drew some lines on the photos you sent me and also sent them to Mark Regan for "in depth analysis"We came up with a thrust line approximately 3 inches above the COG.Although you will find you machine significantly more stable with your modifications you can further reduce the offset by tilting the thrust line down to intersect the COG a bit better.Its extremely unlikely your gyro was 24 inches out before the rebuild.I would be interested in your opinions after you have done some flight testingWell done on the build..

[Tsemler]

I"ve had a play around today and think with it full it comes out 7 inches high thrustline now. Thanks Brian, I liked my machine how it used to be, it was sensitive due to the high thrustline but didnt feel dangerous to me...Have i flown it? No not till the paper work is in.... but a mate of a mate of mate of mine has something simular and from a slow 50 knot cruise to full power she bogs down for a second (caviating prop i think) and then she speeds up and begins to climb, no pitch movement what so ever. I think one of the most important things is i"ve got the centre of pressure up as well which will help greatly with no pitching moment with and without power.Below is a picture of it full with my lines put in. I See you"ve just posted Murray what do you think of my Full Fuel Lines am i correct?? Also i just check my plans and i shifted the front keel up 340mm so thats nearly 14 inches, when i did that would of i also raised my CG the full 14 inches also or would it of only changed slightly say CG moved 1 inch up for every 2 inches the keel was raised?? All interesting stuff altho it was a bit of a pain it was well worth the effort.

[asra]

Hey Trent, unfortunately the COG change is only a percentage of the amount of weight moved.I will have a look at the pics and measure.I don"t think your COG would have gone down that much (4 Inches) with only a 40 liter tank tho.The fact that a machine "similar" to yours doesn"t pitch around during power changes means you are defiantly on the right track.Although personally I prefer a slight nose up pitch (Low T/L) with an increase in power.As I said in my previous post you can tilt the T/L down a bit (Lift the back of the engine up) to make the Thrustline/COG a bit better.An easy way of doing it.

[danny]

I must say that I am surprised that apparently there are people flying gyros which don"t go up with the application of power and nose down as the power is reduced.

[Brian]

Mark"s measurement would be pretty spot on so thats a great result !!

[asra]

Hey Trent, unfortunately the COG change is only a percentage of the amount of weight moved.I will have a look at the pics and measure.I don"t think your COG would have gone down that much (4 Inches) with only a 40 liter tank tho.The fact that a machine "similar" to yours doesn"t pitch around during power changes means you are defiantly on the right track.Although personally I prefer a slight nose up pitch (Low T/L) with an increase in power.As I said in my previous post you can tilt the T/L down a bit (Lift the back of the engine up) to make the Thrustline/COG a bit better.An easy way of doing it.Just a few options for you1) A new set of heavy Patroney glass blades replacing your current light weight JA"s or JG"s would also help raise your machines CofG. Could even end up CLT 2) Another fix would be to install a larger JG 80lt seat tank. These tanks are deep in the bottom and will raise the pilot acheiving the same effect. Only issue there is moving all your controls to suit. 3) Down offset Rotax C box with adapter plate, which will lower your thrustline to hopefully match your CofG. Issue is having to then raise engine and mod undercart/mounts.Regards SamL.................

[aussie_paul]

As I said in my previous post you can tilt the T/L down a bit (Lift the back of the engine up) to make the Thrustline/COG a bit better.An easy way of doing it.Tilting the engine (thrust line) can be a good way of achieving closer to CLT. The RAF engine mounts made it very easy to experiment with "engine tilting", which I did. Bensen I believe had the thrust line tilted down at the front 3 degrees to achieve closer to CLT.IMHO, the most important part of these "discussions" is not CLT, it is stability. Unfortunately, like me a few years ago, not a lot of people fully understand the meaning of stability in aircraft.Aussie Paul.

[asra]

Tilting the engine (thrust line) can be a good way of achieving closer to CLT. The RAF engine mounts made it very easy to experiment with "engine tilting", which I did. Bensen I believe had the thrust line tilted down at the front 3 degrees to achieve closer to CLT.IMHO, the most important part of these "discussions" is not CLT, it is stability. Unfortunately, like me a few years ago, not a lot of people fully understand the meaning of stability in aircraft.Aussie Paul. Hi PaulYou are correct, it all about stability. But there are 2 types of stability. STATIC, and DYNAMIC. Static is all about the design configurations eg CLT, HTL or LTL. (Acheiving balance regardless of what happend between WEIGHT, THRUST, LIFT , and DRAG) Dynamic is all about a machines ability to right its self after an external disturbance. This is usually acheived via correct sizing and placement of your machines tail feathers. (this stability is always acheived aerodynamically) Cheers SamL...........

[asra]

Hmm Thanks Sam seem i might have cocked my lines up on the print out as it doesnt look that bad there does it, if i chuck my full tanks ones up could you please have a look at them too? ThanksTrentRegards SamL......

[Tsemler]

Thanks Sam,Seems im not too far out, Rough enough"s good enough