Pusher cowling ideas

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Eugene

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Seems like doesn't matter how you move things around whole package is still way too fat right in front of the propeller. You need 16 inch extension to make it right.

Screen Shot 2021-07-13 at 22.33.02.pngScreen Shot 2021-07-13 at 22.29.14.png
 

wsimpso1

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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:
  • 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.
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.

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.

Billski
 

Sockmonkey

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What would be really nice is a flywheel on the front end of the crank acting as a little intake fan.
Then ditch the spinner so the faring around the aft end of the crank is your exit vent.
 

Eugene

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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:
  • 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.
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.

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.

Billski
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?

It is very hard to find picture on 912 engine as pusher installation. But those few that I found do look like they really didn’t care about doing anything special for air cooling cylinders.
7011BA4F-B0A8-471E-B52E-8DC569671A3A.jpeg214004F4-7B05-4BCC-9235-17D83D0650F2.jpeg
 

rv6ejguy

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Some observations from my many years flight testing cooling solutions:

1. It's impossible to achieve free stream velocity with the exit cooling air with the inlet sized the same or smaller than the outlet.

2. It's impossible to achieve free stream velocity with the air passing over the engine, mount, muffler etc. These present further drag and momentum losses to the cooling air.

3. The inlet does not have to have the same area as the rad HX. It can be much smaller. Think of the free area between the tubes and fins.

4. Quick and large transitions in duct area/ volume will create flow separation and momentum losses

5. You'll need a dedicated, properly shaped rad duct with no other drag producing junk inside plus a variable area exit door to achieve adequate cooling in the climb AND low drag in cruise. Ditto to even achieve unity or better with the free stream velocity.

6. Cooling air should never be directed into the rad tanks, only the HX matrix. Doing otherwise creates massive turbulence and more momentum loss.
 
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Eugene

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If I let my carbon spinner diameter to get up to 14 - 15 inches or so and about 24 inches in length, then picture below doesn't look that terrible anymore.
However it will steal 2 inches from the blades don’t know how important that is
tempImageIEzbUa.png
 
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don january

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That couple of inch of the prop is the root so shouldn't be that big of a thing other then some thrust gone by but if you look at picture where does the air go next ? I guess the outer perimeter air stream.
 

Vigilant1

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How will it work with this nacelle overlapping your BRS panel?
 

Eugene

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How will it work with this nacelle overlapping your BRS panel?

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.

1B13F7E9-D229-454A-91B0-166473CF184B.jpeg3F52D9CF-E166-4E23-BC5D-2FE85EB2D9AB.jpeg0FF5CD78-BB40-4B2F-ACF6-6C2C550FDD76.jpeg
 

Vigilant1

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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.
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.
Without a factory appraisal and approval of the new nacelle and any fire-through or jetison capability, or a ground test ($$$), would you feel comfortable with this configuration?
 
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Map

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When you buy a new, certified Rotax 912, it comes with a shroud / plenum on top of the cylinders for cooling air. It is hard to see in the pictures, but the duct where the air is supposed to go in (tractor installation) in on the engine right side next to the hub, pointed down. It is supposed to be fed by a separate air inlet in the cowling.
 

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Sockmonkey

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My knee-jerk reaction would be to keep the prop where it is, but have the engine mounted low in the area where the tail boom attaches and drive the prop with a belt. Then you're just extending the pilot pod back a bit to make room instead of making extra bulges in the already problematic area around the trailing edge of the wing.
 

Eugene

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Did some measuring this morning. Looks like 14 inch spinner will not steal anything from prop blades at all. So, should be just about perfect.

tempImageNnTmQ4.png

However if I go this road top of the spinner will be in line with top of engine cowling. Don't know if it's good or bad.

tempImageFLEGAO.pngtempImageXh5ISb.pngtempImageSGeFc1.pngtempImageiQ0B2O.png

Not sure if top of engine cowling should be flat or bowed up. I was thinking if it's going to look like airfoil, it will become another lifting surface. Negative side of it it will get ripped up in flight. But of course I don't know what I'm talking about.

tempImageetXrTD.png
 

WonderousMountain

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You should get about 75% of target lift out of the center section.
Also, a flat surface will still carry some lift, so you need to cross
stiffen it a little. Is 14" spinner necissary?, 12 is already kinda big.
 
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