Hi All,
Why on a ground run do you limit the CHT to 300 to stop glazing and yet in first flight you go to over 400 ?
I am planning to put the wings on my RV10 and do my first engine start this weekend, fingers crossed it all goes well. I have 2 electric air guns and am planning to use them through the cowl openings after shutdown to help cool the engine cylinders down quickly. I will let you know how I get on with that idea and if it makes a difference.
With regard to the Prop governor. What am I looking for when I cycle the prop? What do I want to check and adjust if necessary?
Thanks
Amer
Amer - cylinder head temperature is one factor. What we haven't been adding to the discussion is ICP - internal cylinder pressure. With high power being generated, you have both higher temperatures and MUCH higher internal cylinder pressures. It's the latter that's pushing the rings out against the walls of the cylinders, wearing in the mating surfaces... this is break in.
What you don't want is high temperature and low pressure. Think of what it's like when you take very fine grit sandpaper and rub it over a course surface very quickly. You'll get that surface up to a shine on the tops of the ridges, but you won't reduce the height of the ridges. That's glazing.
As for the C/S prop, you are checking to see that it will cycle. It will take as much as 2000RPM to get the prop to move. This is not something you want to do on your first engine run. On subsequent low-power runs all you are looking for is that the prop moves. You don't need to cycle it deeply so as to lose a lot of RPM, just enough to know you've got oil flowing up to the prop hub and that the prop hub is working as expected.
Later on, when you're ready to do a full-power run, you'll be checking to make sure the governor actually governs the prop RPM so you don't have an overspeed condition. The prop itself has internal low-pitch stops which mechanically limit the maximum RPM - you should also check to ensure these are set properly. An engine overspeed event can be a very expensive thing indeed.
Agree entirely with Electrogunner - first few engine runs should be done with the cowl off, with a knowledgeable observer nearby, with fire extinguishing equipment available. Keep it short. You're looking for oil pressure coming up to a normal level, that the engine runs, that fuel systems are not leaking. Many folks recommend not going over 1300 or 1400 RPM on this first run, just enough to ensure the engine responds to throttle changes, including coming back to something like an idle. Don't expect it to idle at low RPM - a brand new engine will likely cough and sputter and not want to run below perhaps 800 RPM - lower, smoother idle will come with a warm engine and a fuel delivery system that's been purged of all the "stuff" that accumulates during the build cycle. Do a mag check (at perhaps 1300 or 1400 RPM) to ensure both ignitions are actually firing their plugs, then come back to 1000 RPM to perform a live mag check to ensure you don't have a hot mag. Finally, check to ensure the mixture control will actually cut off the engine at the idle-cutoff position.
Once you've done this, walk away from the airplane, sit down and let the grin take a good set on your face. It will feel VERY good. Then get the airplane back in the hangar and go over everything in the engine bay with a very fine-toothed comb, looking for leaks (oil, fuel, intake, exhaust), loose fasteners, chafing etc. Give it such a good looking over that you are fully confident that your next engine run will be a "clean" run with no problems.