Hi Rob, I will see what I can chase up on it, but I should have mentioned that vibration & balance is only part of the story. The real danger comes from the possible changes to resonant frequencies & harmonics. No doubt you are already aware of the warnings given with most aero engines not to operate them within certain rpm bands? This is because of the presence of unwanted and possibly destructive resonances or harmonics produced within those rpm bands. These can lead to catastrophic engine failure, particularly affecting the crankcase and/or crankshaft. Repositioning the prop to other than the manufacturers recommendation may well result in the generation of dangerous resonances in the engine, perhaps in an rpm band where the engine would normally be operated. If there are no prop timing instructions given in the maintenance notes for the engine /prop then you are probably pretty safe, but where such a requirement is given it will be there for a very good reason!! Regards JGV Senior Technical Officer, Airworthiness GLIDING FEDERATION of AUSTRALIA INC. [email protected] <blocked::mailto:[email protected]>
---------------------------------------------------------------------- The Gliding Federation of Australia Inc invites you to visit the web site www.gfa.org.au <file:///C:/Documents%20and%20Settings/stoa/Application%20Data/Microsoft /Signatures/www.gfa.org.au> newcomers to gliding and soaring are invited to visit www.soaring.org.au <file:///C:/Documents%20and%20Settings/stoa/Application%20Data/Microsoft /Signatures/www.soaring.org.au> ------------------------------------------------------------------------ ------------------------------------------- This email transmission may contain confidential or privileged information that is intended only for the individual or entity named in the email address. If you are not the intended recipient, please be aware that any disclosure, copying, distribution or reliance upon the contents of this email is strictly prohibited ________________________________ From: Rob Thompson [mailto:[email protected]] Sent: Tuesday, 26 October 2010 6:36 PM To: [email protected] Subject: Re: [DOG mailing list] Thoughts on starter motors Thanks John, Interesting indeed. Have you got more reading on that? - I'm always keen to learn a little more of the physics that affects vibration issues. Last re-build I did I got all the engine components balanced by the people who do VW racing motors and I found it surprising that they regarded the original limbach balancing to be less than impressive. I thought the motor after the rebuild was noticably smoother. Vibration control is certainly an art! Rob PO Box 129, Lawson, NSW, 2783. phone 02 47592307 mobile 0429 493828 ............................................. ________________________________ From: STO Airworthiness <[email protected]> To: [email protected] Sent: Tue, 26 October, 2010 1:36:28 PM Subject: RE: [DOG mailing list] Thoughts on starter motors Before repositioning a prop for convenient visibility it should be noted that propeller and engine manufacturers may have specific requirements for the "timing" of the propeller. These requirements are generally based on vibration analyses of the engine/prop combination and can be quite critical on twin & four cylinder engines with two blade propellers. the Limbach/Hoffmann is one such combination and the propeller should only be fitted with the blades vertical and No 1 cylinder on TDC (either stroke: cam position not important). In cases where vibration is high this can often be minimised/eliminated by refitting the prop in the 180 deg position. Kind regards John G Viney Senior Technical Officer, Airworthiness GLIDING FEDERATION of AUSTRALIA INC. [email protected] ---------------------------------------------------------------------- The Gliding Federation of Australia Inc invites you to visit the web site www.gfa.org.au newcomers to gliding and soaring are invited to visit www.soaring.org.au ------------------------------------------------------------------------ ------------------------------------------- This email transmission may contain confidential or privileged information that is intended only for the individual or entity named in the email address. If you are not the intended recipient, please be aware that any disclosure, copying, distribution or reliance upon the contents of this email is strictly prohibited ________________________________ From: Rob Thompson [mailto:[email protected]] Sent: Tuesday, 26 October 2010 7:05 AM To: [email protected] Subject: Re: [DOG mailing list] Thoughts on starter motors I think it is a pretty common practice to use the starter to move the prop out of the pilot's sight line but that is exactly what I'm not recommending and when there is a chance of jamming. Better to install the prop so that it always stops on a compression stroke in an appropriate position. Worth noting here that the prop in the H36 Dimona when feathered horizontally will cause some blanking effect on the elevator at slower speeds. Rob Surely you don't use the starter when airborne? A dive start is much nicer, needn't lose any height and doesn't faze the avionics. Michael From: Ian Mc Phee <mailto:[email protected]> Sent: Monday, October 25, 2010 9:56 PM To: [email protected] Subject: Re: [DOG mailing list] Thoughts on starter motors Not in Dimona but i do know the falke that used to be at the now defunct club at other end had their starter jam in flywheel resulting in an outlanding on the beach one day and another day in a paddock. In both cases it was unstuck once on the ground and flown out. I think the flywheel had chips in teeth and I think they got a new flywheel at some stage and maybe starter was faulty also. Some GA planes have a warning light to indicate starter motor is stuck but in GA you only use a starter on the ground!!!! Ian M On 25 October 2010 07:46, Rob Thompson <[email protected]> wrote: A rainy day and I'm staring out the window contemplating the meaning of life and thinking about starter motors......... A common thing to do after switching off is to tap the starter motor to move the prop out of your line of sight. Be aware that an aging starter motor, particularly one with too much end float, bearing wear or lack of grease can jam in the thrown out position when doing this preventing a restart. It can be dislodged by rocking the prop back and forth but that is not much help in the air. It is rare but has happened. If you are changing the starter the bottom bolt can only be got at with a small ratchet and about a 140mm extension bar. Ours is an 8mm alan bolt. Also worth noting that a rare few starter motors jam over centre if the internal yoke is assembled the wrong way around. The H36 fiat starter is one of those that will happily go back together with the yoke back to front and will work fine on the test bench. In situ however, the starter will crank but it won't stop and if the engine starts it will all become a bit of a mess. If you get your starter overhauled tell the mechanic as many don't seem to know about this. Rob PO Box 129, Lawson, NSW, 2783. phone 02 47592307 mobile 0429 493828 ............................................. ________________________________ From: Nigel Baker <[email protected]> To: DOGS <[email protected]> Sent: Sun, 24 October, 2010 10:50:40 PM Subject: [DOG mailing list] Aileron Stiffness Hi All. As I mentioned before there is stuff in the archive on the subject of the aileron systems. I am sending this again from a February round on this subject. Ian I can save you the trouble on checking if the aileron horns out in the wing are interchangeable. They are not. One system works with the push rods in tension and the other in compression in relation to normal flight load so no they can't be interchanged unless you want to reroute the push rods and turn the Aileron movement around in your head to go stick left for right roll. Cheers. Nige. ----- Original Message ----- From: Nigel Baker <mailto:[email protected]> To: [email protected] Sent: Monday, February 01, 2010 5:53 PM Subject: Re: [DOG mailing list] Dimona 1984 Hi Lasse and others. Well I am confused by the mk1 mk2 stuff. I really don't know where that comes from. There is only the one manual that I know of last issue Nov 1985. About 9 years ago we asked Diamond Austria for info on the springs in the aileron circuit and whether or not they were needed or could the grade of spring be changed. After a short while they responded to the effect they didn't know there were springs used and couldn't imagine why and couldn't see a problem with removal. I am not so sure on the last bit. I have worked on several Dimona's and of note the S/N's 3512, 3535. 3538 and 3539 (ours) All these aircraft are ex the Thai Air Force and part of a group of 14 H36's the Thai's bought in a package deal with Wolf Hoffmann the designer and business owner at the time. There is one main difference between these numbers and a big difference in handling. S/N 3512 and 3535 had what I believe to be the original aileron drive circuit. In these aircraft the push rods worked in compression in normal flight mode and are guided by nylon bushes. It is best identified by looking through the clear inspection panel at the Aileron bell crank pivot point in the lower wing surface. When looking through the inspection panel you will see a bell crank fabricated from steel tube. This system also has centering springs attached to the aileron push rod drive assembly under the fuel tank in the fuselage. S/N 3538 and 3539 had what I believe is the later version. In this instance the push rods work in tension in normal flight mode and are guided by fairlead rollers. This is easily detected again by looking through the clear inspection panel and in this instance you will see a bell crank fabricated from steel sheet instead of tube. The resulting difference in systems is large. Firstly is the difference between compression and tension in the loaded push rods. In the case of the later system with the rollers the friction is less for one main reason. The push rods are pulled straight in normal flight load and the guides are not influencing them much (except for normal wing flex) so the friction is low and they are rollers (well when not seized). On the other hand the earlier version in compression results in the bushes holding the rods straight and this creates friction in the guides. Couple that with the use of bushes instead of rollers and there is your answer. This can be helped with the application of Silicon Spray Lube (works as a dry low friction lube which doesn't collect dust) to the push rods at the points where the bushes work but it is a pain as it requires removal of the push rods. Something that would need to be done yearly to get the best out of it. There is little friction on the ground of course but it is noticeable in cruise (reasonable flight loads) as you can detect the system sticking with small control inputs. Other than flying it inverted there is no way round this situation. Secondly the other difference is in the "differential Ratio" of the ailerons. The Service manual has a broad range of tolerance for aileron deflection which conveniently covers both systems. The older system produces an up value near the top tolerance of deflection for the aileron and down is close to the bottom of tolerance. This delivers a differential ratio of more than 2-1. The newer system produces an up value near the bottom of the tolerance and a down value of near the top of tolerance and this results in a differential of less than 2-1. So what difference does that make. Well as pointed out by some it means that normal flight loads can at certain points of deflection result in dynamic loads driving the ailerons into further deflection rather than less thus a lack of centering force and in fact the reverse. So the springs in the older system are there to supply a centering load and while they do that they are a negative at times. I am still confused by this mk1 and mk2 thing but can confirm that while I have heard of 1 aircraft built after 3539 but very close to it (3541 I think) the change to the Aileron Circuit happened around the 3540 mark somewhere depending on order schedules during the change over. Interestingly one comment was made by Diamond during enquires about the 2 systems when they didn't seem to be able to find records of the earlier system at the time was that Hoffmann Aircraft were not very good at record keeping. Hope this if of help. Cheers. Nige. ----- Original Message ----- From: Ian Mc Phee <mailto:[email protected]> To: [email protected] Sent: Wednesday, January 27, 2010 11:09 PM Subject: Re: [DOG mailing list] Dimona 1984 Interesting about tail wheel mod from Michael - sure gives smoother ride. You mention the heads/valves lasting only 300hrs. I would recommend to all analysis of exhaust at FULL POWER with lamdameter etc. You may find it is running slightly lean on full power (actually plugs look OK) but gas analysis does not lie. More recently i have been using digital CHT and you can really see what is happening. I set them up so full power CHT rises to about 170degC then very slowly falls. If you bring throttle back just a bit in revs CHT will quickly rise to 180degC and beyond. This proves to me you are running rich on full power- also confirmed on EGT. To achieve this it may be necessary to carefully thin out the end 6mm to 8mm of each the needle in carby and thus achieve the low CHT on full power. (do not think of touching jet) Fuel is cheap when compared to repairing heads. Limbach Tech bull 53 makes mention of max on climb of 180degC (forget what max the manual says - that is stupid value) Also Tech bull 44 (11page edition) is well worth a read. Ian mcPhee 2010/1/27 Michael Grimwood <[email protected]> Hi John and Lasse I have owned a Mk1 H36 since 1988 (G-MRG in the UK, now VH-VRG in Australia). It originally came with an un-sprung
