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Could some one please explain "in English" how to work out the centre of gravity of my machine.I know I can use the electronic web site calculator, but I would prefer to understand it, and how its calculated.I have seen pictures of suspended machines with lines drawn through thrust line, mast, and main undercarrage, but I am still very confused.Someone please UN-Confuse me !!!!!!!!!!!!!!![?][?][?] Sam.
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Sam have a read of the topic of clt in the Tassy gyro club, that helped me understand the method of doing with the pics with lines dranw through it, best i can offer.Mark
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Hi MarkI have looked into the Tassy Gyro Club on this forum and found C of G discussion, but no indepth explanation.Such an important issue relating to gyro stability and I have not been able to find any detailed explanation any were on the Web!!!!.I feel such information, along with Construction requirements on setting your machine up (section 5.02 para 4. ASRA OPS. MAN.) should be readly available on this ASRA site for all to read and understand. I did find Paul Bruty's RAF Hybrid photo's of it being hung, and lines drawn on the picture, but no explanations of what these lines represent.I guess we have thrust, lift, weight, but what is the line through the rear wheel axles ????? Regards Sam. [V][V][V]Image Insert:
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Hmmmmm.............., I don't want to be picky or sound like I'm stirr'n the pot, BUT, those lines, if they were drawn in the right place, would indicate the thrust line to be bout 3" above the COM, or did you post those pix just as an example Paul.?[ IOW, the line indicating the TL isn't drawn along the TL,[COT is in the center of the prop hub.] and the line indicating the 'hang' set isn't in the same place in both pix.Looks like the lines were moved to give the impression that the machine is actuay CLT.]I mite be wrong, or blind, but thats the way I see it.Sorry Sam, for hijack'n ya thread.A SCG's explanation of the method.If you hang any asymetrical object from any point,gravity will define the place where it rests.A line drawn virticaly through the object, from the point at which it is hanging will show the vertical mass axis.If you then hang the same object from a different point and do the same, it'll give another virtical axis line.[ a gyro is balanced on the mains coz its easer, but solong as the machine is alowd to 'balance', gravity can do its job.]Where the two lines cross is where the center of the total weight is, or COM.If a third line is drawn along the line of thrust, starting from the center of the porp hub,the intersrection of the lines indicate the TL offset.Remember Sam, thats how a SCG sees it, so don't take it as gospel.[now I got a headache]Ignorance is bliss............but only till you realise you were.Ingratitude stinks.......be generous carefully.
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Sam,What you are trying to do is collate the centres of gravity for all the parts into a central location, this can be done as Paul has experimentally by hanging it off the rotor teeter bolt, a plumb line must fall on the centre of gravity of the aircraft in this instance, it gives you the horizontal centre of gravity in respect of its location along the keel, but you don't know how far up the mast the centre of gravity lies. Paul's next trick is to put the rotors on the [this must happen] gyro and put it all up on blocks and tilt it back, or forward, until it is balanced and there is no weight on either the nose or tail wheel. A plumb line to the centre of the mains axle will then tell you where the centre of gravity lies with respect to the wheels, so you have two lines, one coming off the teeter bolt and the other off the wheels, they will be at different angles to each other, and will intersect, this intersect point is the centre of gravity. To do this though you will have to have the main axle centre slightly aft of the horizontal COG, this will force the angle of the wheel COG to be different to the Teeter COG and will give you the intersect point, if the mains are located on the teeter COG you won't be able to pick the vertical position of the COG because there will be no intersect point, the lines will be parallel.To take the photo's put a camera in a fixed location and hang a plumb bob in front of the gyro relative to the camera, make sure they are in line along a beam or similar feature. The main axle must also fall in this line for the purposes of the second plumb line, in fact it would be a good idea to hang two plumb bobs, one either side of the gyro and both along the same beam that the gyro hangs from.Once you have determined where the COG is you then attempt to move the propeller thrust line so that it is in line with the COG to make it into a CLT machine. This is where the computer program comes is really handy, you can fiddle it to the millimetre. If you don't have a computer you may have to keep moving or tilting the motor up and down or the pilot and fuel tank to do it, another thing that can be moved up and down is the battery if you have one, along with the radiator, etc. Any heavy object will help to balance things out quickly, most usually this is the engine or the pilot.Hopefully this helps.Cheers,Nick.
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What I forgot to mention was that the two plumb lines in the picture must overlay each other in the picture - so you can only see one of them, so put them up without the gyro in position, make sure they line up, if they don't - move the camera until they do, a digital camera is good for this.The joy stick must be in the neutral position for the hang photo, both photos must be taken with the pilot on board and at least half fuel capacity in tank.Cheers,Nick.
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Exellent explanations both Birdy & Nick.The penny has now realy dropped and I've got some testing to do !!!!!Thanks Both Regards Sam.[
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Can I add a quick comment, and add a spin to this subject.Looking at the above pic's, if 2 pilots were seated or more fuel added, the C of G would be lowerd somewhat. This means CLT would change depending on both pilot weight and amount of fuel carried.So this shows how important Horizontal Stabs are on Gyrocopters, including CLT machines.Sam. [][][]
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Sam on a single seat it is best to do the test with half fuel load so you have a bit of give each way, not sure about two seats.Also Sam i've got a email that explains that whole thing if you want i could send it through to you.Mark
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Thanks MarkI have sent you my details so you can send it through.Regards Sam.
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Sam,The mathematical method....Your gyro is composed of a series of components which have their mass concentrated at their centre of length, this is so for extruded aluminium rhs and angle sections and tube, so what ever the section weighs, the mass of it is concentrated at the centre of the length of the extrusion. Motors and the like are generally, especially in the case of subaru engines, concentrated on the crank shaft line and at the centre of the crank shaft, the offset items such as alternators, gearboxes and props should be separately accounted for.If you look at the gyro from side on the parts of the gyro are located with respect to the nose a certain horizontal distance, we shall call this the 'x' displacement or the 'x axis' measure, use mm or m or feet but whichever you use you must use for every other measurement for the rest of the exercise.The same parts are located a veritical distance off the ground, we shall call this the 'y' displacement or the 'y axis'.Each part has a co-ordinate with three dimensions, mass, x distance and y distance.When you are trying to figure where the COG is what you do is this:Sum the mass of the parts, say 300 kg, including all the parts of the machine when it is flying, pilot, fuel, motor, etc...Let us say that the Pilot weighs 100kg, motor 80kg, fuel 40kg, etc, and the pilot is at 1.0m back from the nose, the motor 1.8m, fuel at 1.3m..just looking at these 3 things you do this:Add them up - 100 + 80 + 40 = 220 kgdo their moments and sum them up:100 x 1 + 80 x 1.8 + 40 x 1.3 = 296then divide the moment by the mass:296 / 220 = 1.35m, which is the horizontal position of the centre of gravity for those three items, so effectively there is 220 kg acting down at 1.35 m from the nose, there are others so just add their mass to the mass until it totals to the weight of the machine when flying. Add their moments into the moment equation as above until all the moments are added up into the total moments for the machine.m1*x1 + m2*x2 + m3*x3 + m4*x4 + m5*x5 +.......+ mn*xn = mt*xtthen COG is at mt*xt/mtYou do the same thing in the vertical direction using the 'y' displacement instead of the x, that is, where an object has 90 kg at 1 m 'x' and 0.6m 'y' you stick the 0.6m 'y' in instead of the 1.0m 'x' displacement,that is, you substitute the 'y' value for the 'x' value. The masses do not change, so they are unaffected by this. You then sum up all the mass * 'y' moments and again divide them by the total mass of the parts, this gives you the vertical position of the COG.viz;m1*y1 + m2*y2 + m3*y3 + m4*y4 + m5*y5 +.......+ mn*yn = mt*ytthen COG is at mt*yt/mt; where mt*yt is the total moment summed up from the multiplied mass and distances.You do understand that the multipliers must be done first, then you may add the products up, do not just go m+m+m+m+m+m+m+m+m * x+x+x+x+x+x+x = mt * xt, that will yield the wrong result.Confused???Ring me if you need to on 0418 811953.Cheers,Nick.
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Very interesting Nick, but I think I will just hang & balance my gyro.May be something to consider when designing and building a gyro.Sam.
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Hey Nick / BirdyFinaly hung the Dragonfly, and machine came in at 10 degree's,with 30 litres of fuel, then tested for vertical centre of gravity, and hey presto, Centre Line Thrust.Thanks again !!!! Sam [][][]Image Insert:
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G'day Sam, you should chock or wedge the rotor head (between the torque tube and its stops) in the centre position. Also, the thrust line should be parallel to the crankshaft. I noticed it is different on both photos where it strikes the GPS at the front. Both pictures appear to have the thrust line angled down too much towards the front, more noticable on the bottom picture. Perhaps if you moved the oil tank out of the way you could draw a line through the bottom of the cylinder head and then draw the thrust line parallel to that. It looks like it will be very close if not spot on. Whats keeping the gyro balanced in the second photo?Tim McClure
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Sam,Naughty, very naughty! Where is the prop for the forklift mast???? You know they fall on your head when you don't prop them, don't you?I can't see them in the photo but I assume the rotors are on in the second shot? If not you must have them on, it is imperative that they are there for the second balance line, otherwise the mass of the rotors will not be included in the GOG line and your lines will be wrong.Where is/are the plum bob/s?? The second shot seems a little skew as the plumb line does not pass through the centre of the opposite mains axle. Are you just relying on the building being plumb? Don't forget the angle of the camera may affect the angles of objects in the distance.According to the photos you're just below CLT, but it is on the good side of not quite CLT, ie, TL is below COG. You'll get a good nose up on increase in throttle in this circumstance.If this is the first shot at it you've done well to get it to be as close to CLT as indicated, congrats on this.Cheers,Nick.
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