- May 26, 2017
- Merrill, Wisconsin, USA
To my surprise I agreed with everything. I wonder why? This is so not like me! Is that because I’m getting smarter or simply more agreeable?Hmmm. I have looked at a lot of Rotax 912 installs over the last few years. The wife has one. They all have a few things in common:
Oh, and they all run just fine to 2000 hours, the rebuilders rarely find anything out of order then, and no one is having carb ice issues.
- Ducting air over HX for glycol-water and oil has been planned and executed;
- Air flow over the rest of the engine is incidental at best;
- When cowled, the warmed air from the HX flows over the engine while induction air is drawn from that same warmed air;
- Carb heat, where provided, is a 100 W electric heater in each of the two manifolds. Noteworthy is that carb heat is only provided in models used where regulations or consensus standards require it, but not in the rest. Yes, it is there because somebody said to put it in, not because anyone thinks it is really needed.
Next, millions of cars and trucks and agricultural and construction engines are built and sold and cool just fine every year around the world. The vast majority run a HX each for engine coolant, engine oil, and transmission oil in the air inlet to the engine compartment, then this warmed air washes over the engine and finds its way out of the engine compartment. This includes many of the Rotax 912's running inside a cowling. I see no reason to deviate from this scheme in Eugene's airplane. Incidental air cooling of the cylinders will likely be adequate as it is in other Rotax 912 installs as long as enough total air flows through...
The open radiator and oil cooler supplied with this airplane seems to be adequate for allowing incidental air to wash over it. They are doubtless a little oversize when run with a large inlet and a duct that recovers most of free stream pressure. Most likely inlet area can be reduced substantially. I continue to favor the idea of a modest size inlet (substantially smaller than total HX face area) if all of the air at the inlet can only pass through the HX's. Then that air can wash over the engine in an unguided manner, cooling the exhaust pipes and taking modest heat off the cylinders too, just as many other 912's do.
Ideally, that air, upon exiting around the muffler, will be sped up to near free stream velocity. Can we actually achieve that? Maybe. If we have an inlet of X square inches, and the outlet area is increased only enough to account for heat gain, the clearance around the muffler might be too small to both allow for engine motion on the elastic mounts and to prevent the muffler from burning the cowling, so maybe the clearances will be bigger and the outlet less efficient. Nonetheless, the ideal could be investigated and an attempt at approaching it made. Why bother? Any improvements over a way oversize outlet will both reduce cooling drag and improve prop efficiency - which is better performance. Who doesn't like more speed or less fuel burn?
Some have suggested splitting the inlet air over the radiator and the cylinders... Well, I have found no one bothering to baffle the cylinders of Rotax 912's. Has anyone else got a single example of baffled cylinders on a Rotax 912? The second reason not to do this is that we are trying to recover a significant chunk of free stream pressure to push air through the radiator and oil cooler. Any lowering of the pressure in the duct by having a too open path to the cylinders will result in reduced airflow through the HX's and may create overheat circumstances. The analogy is a parallel electric circuit. At minimum, the path to the cylinders will likely have to be restricted and tuned to keep pressure up and enough flow through HX's. Just providing an open duct to the cylinders will rob the HX's of their needed airflow. So, besides not needing to provide a separate cool air path to the cylinders, it could make for overheating of the engine.
As for shapes, several comments. First is that we found out over a century ago that wings should do the lifting, tails should stabilize and control the airplane, and everything else should provide low drag. Little wings were found to be poor lifters and big drag sources, so no little wings please. Maybe some turning vanes to keep flow where we want it... Other than that, our min drag cowling is part of the fuselage, and its drag goal is not to be clean by itself, but is most effective if it messes with the wings the least that it can. We found that out about 80 years ago. That is usually achieved by making the section change as little as possible through the root of the wing and have have nice smooth curves from front to back. Make the outlet area bigger than the inlet based upon how much the air volume is increased by heating, and remember that the opening is the area between cowling and other stuff like the muffler, gearbox, and prop hub. Do the best you can, but recognize that practical limits exist...
Once one achieves the basics, one can see if the airplane handles better, cools adequately, is more pleasant and fun to fly, and then go fly places. One could also could place tufts and temperature and pressure probes around the airplane, do a bunch of experimenting, analyzing data, and tuning inlets, outlets, adjusting prop pitch, VG's on the aft fuselage, etc. Lots of fun, maybe some measurable performance improvement will result or maybe a confirmation that this bird is about as fast as it can be will occur. Either way, get the basics done well, then go fly.
How will it work with this nacelle overlapping your BRS panel?
That seems like a considerable issue. A BRS that gets fouled on the way out can put you in a worse situation than you were in before you pulled the handle. And if you have any uncertainty about whether it will work, I could see it might lead to a reluctance/delay in using it.Peter Garrison suggested at one time that front part will be essentially overhanging over the BRS and should be made so it will fly away if you need. I'm sure in his mind he had clear picture of how to do it, but I didn't get there with my brain at this point.