Hi Jarek Ian and all. I have experienced this pitch problem once before on an IS 28 M2 (worst aircraft I have ever flown) and while I knew the answer I didn’t tell the owner as I considered him a loose cannon on maintenance. He wrote the aircraft off during landing which was pretty predictable given what I had noted in his flying and thus I was never asked to fix his problem.
I have been looking for the appropriate data which I have somewhere but can’t put my hands on for the dimension of the pitch change step on the pitch setting nut’ I have found data for the 2 pitch angles between climb and cruise but not the step dimension. The nut I refer to is the round nut which is fitted to the central pitch drive shaft which is positioned beneath the alloy channel with the nylock nut on top to secure it all. That nut has a step in it that the slide plate engages with the nut when de pitched in flight for pitch change. To understand it you need to remove the spinner and action the blades by twisting at the root to move pitch position then move the slide out from Centre and release the blades to lock back on the nut. That is the cruise position. A small twist of the blade allows the slide to return under its spring force towards Centre and on releasing the blades it is locked in climb pitch. Centrifugal force is what courses the slide to move out from Centre hence the need to change to cruise at around 2200 and climb near idle. I hope you have grasped my explanation but toying with it the best way to work out how the function works. The nut sets the climb pitch position so yes 3000 static is good. Bring the RPM back to cruise pitch change RPM and do the change then open the throttle again. If you get a static drop of 200 to 250RPM then the prop is fine. If it is greater and the RPM Is less than 2600-2700 RPM then I suspect the wrong nut was installed at your recent factory service. To check this using the angle data which is listed in the prop manual requires accurate angle measurement so best done by a competent propeller ground engineer. I am not near my aircraft which is over 200ks from me. However if Ian Williams or someone else is in a position they could give us a reference to check to. To do so remove the spinner and set the prop in the cruise position. Now using feeler gauges the distance between the climb position step and the pitch select slide. This propeller is used on many aircraft types and thus settings for fine pitch and cruise vary a lot. The change in amount of pitch is controlled by the length of the step in the nut. The Longeazy is a good example. The RPM change from climb the cruise is significant. Slippery in cruise hence the big pitch change. The ASK14 has a Hirth 4 cylinder 2 stroke engine and a 2 position Hoffman prop. Same assembly but without the slide. I know a couple of this type have had the nut replaced and the slide assembly added so it has 3 positions like ours. I also know someone who has made his own nut with 2 steps so he has 4 pitch positions. I am not suggesting any of this is wise or legal but telling you this so the versatility of the unit is understood. If I am correct about my best guess of the problem from previous experience then Hoffman need to fix their mistake and fit the correct nut. If you are having trouble reading and understanding my descriptions then consider how hard it has been for me to write this. I keep reviewing it to make sure it makes sense and is understandable. I have likely failed. Sorry but the best I can manage. The Limbach and Rotax engines are permitted to run on alcohol blends and is becoming the norm in that class of engine. In the Limbach engine the rubber components being the factory fitted hoses on the engine, the Carb and fuel pump diaphragms and seals etc. are rated for alcohol blends. The engine is certified for alcohol blends. Around the world alcohol blends are becoming the norm for mogas. In Australia there are 2 states where the use of alcohol is mandated in mogas so you really can’t avoid it. Peter Limbach told me the reason the Limbach is certified for Avgas as well as 98 unleaded is because of certification requirements and market acceptance rather than by simple choice. He prefers unleaded. Except for lead, avgas is the purist fuel on the market. Mogas specs change with the season keeping your car running smoothly so it is far from pure. Discussing issues I have found on top overhauls, full overhauls and prop strike bulk strips on engines running on avgas (Including the L2400 DFI 100HP liquid cooled engine prop strike) he agreed with my views and said yes that’s why we prefer you run on unleaded. The lead fouls spark plugs yes, but even worse it fouls valves especially the exhaust and cakes thick on the combustion chamber and piston crown. This build up causes run on at shutdown/hot spots for ignition etc. It also fouls the piston ring grooves and can lock the rings as I have seen happen before. I have also experienced exhaust valve failure caused by it leaking due the build of that horrible yellow cake. Having said all that there are some other considerations to be respected. I almost exclusively use unleaded. Some times I am not able when touring and the mogas point in town is a long way from the airfield so I use the on field FBO reluctantly. I am always wary of the fuel octane with mogas. The fuel companies mess with the formulae a lot to meet the 98RON. This results in volatiles that will at times rapidly evaporate out and reduce the RON significantly. So always be aware of the risk of detonation. It may come from the pump already degraded or often degraded while sitting in the tank if the aircraft sits idle for some time. Avgas can also degrade in the tank over time but the time factor is significantly longer and thus not usually a problem. While our engine is rated for alcohol blends you must also consider the suitability of hoses and boost pump used in the airframe for alcohol blends. That is actually easily achieved in our case. The let down is the fuel tank if it is composite rather than alloy. There is a question mark on the resin used in the tank manufacture. I check the tank each year as per the SB and have not ever detected any degradation. However someone in a different country could well have a different result with their local fuel supply. So due diligence is required to meet the requirements of your local aviation authority, tank inspection, and checks on the other fuel carrying items in the airframe for suitability. All issues must be considered carefully. Meanwhile I don’t use avgas and advise others in OZ to do same but our rules are set by our sporting body the GFA not the government body. We are very different and what we can do here often is not possible in other countries. Hope my ramblings are of help. Cheers. Nige. From: [email protected] <[email protected]> On Behalf Of Ian Williams Sent: Wednesday, November 6, 2019 7:42 PM To: [email protected] Subject: Re: [DOG mailing list] H36 top speed The RPM’s and speeds are what suits us. There are only 2 Dimona’s in New Zealand .. some distance from each other. So it would be good to get data from other Dimona operators. IW Sent from my iPhone On 6/11/2019, at 9:38 PM, Ian Williams <[email protected] <mailto:[email protected]> > wrote: Yes … an L2000 Limbach and an HOV62 prop. What is your static full throttle RPM on the ground ? From: [email protected] <mailto:[email protected]> On Behalf Of Jarek Steliga Sent: Wednesday, 6 November 2019 9:14 p.m. To: Laurie Hoffman via dog <[email protected] <mailto:[email protected]> > Subject: Re: [DOG mailing list] H36 top speed Hello Ian Williams, I am really worried after reading your reply. Are we talking about H36 with Limbach 80PS and Hoffmann HO V62 propeller? If so, what could be the reason for the serious underperformance of my power plant? Regards Jarek On Wed, 6 Nov 2019 at 09:05, Jarek Steliga <[email protected] <mailto:[email protected]> > wrote: Hi Ric, 1600 hrs / 12 atm pretty consistent on all cylinders Regards Jarek On Tue, 5 Nov 2019 at 12:46, Ric Sutton <[email protected] <mailto:[email protected]> > wrote: Hi Jarek how many hours do you have on your engine/ what are the compressions like? From: <mailto:[email protected]> [email protected] [mailto: <mailto:[email protected]> [email protected]] On Behalf Of Jarek Steliga Sent: Monday, 4 November 2019 5:06 AM To: <mailto:[email protected]> [email protected] Subject: [DOG mailing list] H36 top speed Hello, I noticed that at level flight with the prop at cruise angle I can hardly reach the air speed of 150 km/h at RPM 2500 (full throttle). Is this something to worry about? Should not the top speed be around 190 km/h at RPM 3000? With the prop at "start" position the full throttle revs are 3000. Any comments please? Regards Jarek
