Engine/Cowling Clearance

[skippydiesel]

I am about to start designing/making the cowling for my Sonex, fitted with Rotax 912 ULS.

 

My son, who has CAD capability, is going to create a 3D picture,that we can digitally modify to suit. When final design has been approved, he can then laser cut the temporary scaffolding/skeleton on which to   "mock up" the new cowling.

 

It is absolutely critical that engine movement/cowl inside clearances are included in the design phase (particularly for Rotax start/stop movement).

 

So, what should I allow for engine movement/clearance to cowling??? 

 

 I will appreciate receiving  your knowledge/experience/opinion/speculation on the matter

[Old Koreelah]

Skip I’m not much use, but it would depend on the geometry of your engine mounts; the closer they are together, the more you’d expect the engine to move on it’s rubber mounts. My 4 cyl Jab moves most on startup and shutdown, but I cannot see any wear marks on the cowl, which is a pretty tight fit (I covered the engine in glad wrap then used klegecell to build up the cowl over that).

 

A good fit helps with controlling airflow but I wish I’d allowed a bit more space for bits I added later.

[turboplanner]

You've picked the hardest job to do.

 

My recommendation is to set datum points in length width and height for the aircraft frame and the engine so the 3D CAD drawing can be made very accurate. This might mean welding up a simple bracket in two or three planes so you can accurately measure from the datum. (beats pulling the engine out 17 times trying to get the fit right.

I would try to get an engine mount test-flex figure from Rotax for a given set of mount locations (OK has made reference to the movement being related to where you place the mounts). You may be able to get a table of flex figures based on mount positions, but there's no point asking other people how much their engine flexes unless they can provide accurate mount dimensions (MANY people have been caught by that one)

Then ALL accessories should be temp bolted on to the engine, and bolted inside the engine bay. (I just recently fitted a larger battery into a very compact car space and had to take the battery out about 17 times, each time shaving this or that bracket or relocating a sub assembly - if you measure correctly that cal all be done on the CAD system.)

Then all the items required to be withdrawn as part of preflight or servicing, for example dip stick, and oil filter - there should be enough room for the item to have a removal path, spanner space and finger space.

 

Then all the things you think you might like to fit down the track should be considered, and manufacturers drawing used to put that into the CAD drawings to save a lot of heartache down the track.

 

To a degree your firewall cross section and prop centreline will determine your cowl clearance, but if everything has deen included in the CAD drawings your can play around with clearances.

[Thruster88]

Not a sonex or 912, my RV6a has only 3mm between the spinner and cowl. It has not touched.  Googled some sonex cowl images and it would seem that there is plenty of room. The advantage of a small dry sump engine.  

[turboplanner]

25 minutes ago, Thruster88 said:

Not a sonex or 912, my RV6a has only 3mm between the spinner and cowl. It has not touched.  Googled some sonex cowl images and it would seem that there is plenty of room. The advantage of a small dry sump engine.  

He could achieve the same clearance as you provided his engine mounts were on the same arc radius, and had the same flex as yours.

[Marty_d]

I'm finding problems at this stage too.  Had the throttles all set up and then when the top cowl was on, found I couldn't move the throttle.  Obviously it's so close to the Bing carbs (or one of them, anyway) that it binds with the cowl.

 

So now I have to put bulges over the carbs to get it to fit.  Luckily I have a spare top cowl to experiment on as far as position and hole cutout etc.

 

[kgwilson]

You find a number of aircraft especially home builds with little lumps and bumps to accommodate bits that poke beyond the rest of the engine. Keeping the internal cowl space to a minimum is the goal from external streamlining as well as internal airflow for efficient cooling. The cowl on my aircraft would not fit if I used the standard Jabiru fibreglass plenums. The old style plenums with the HT leads on the outside have a much lower profile so they fit. Adding a streamlined bump to fit something will usually produce a better overall result than creating a new bigger cowl.

[APenNameAndThatA]

Maybe you could make the cowl the most aerodynamic and aesthetic shape and address internal airflow with baffles. That would provide access room around the engine, and if it grew a little it would not throw your plans out. Disclaimer: I know nothing about this. 

[skippydiesel]

Soooo we have 3mm at the cowl, a lot of wise words and from a non forum corespondent,  5mm. Progress is indeed glacial.

 

The Sonex has a bed mount (bottom 4  Rotax crank case mounting points are in use). To my untrained eye, this will allow most movement (left and right so to speak) at the upper parts of the engine and of course the exhaust system/tail pipe, with minimal droop/sag  at the prop flange.

 

Everything behind  the front twp cylinders (back towards the firewall) will have plenty of clearance simply due to the angle pf the cowling as it comes in towards the front. The exception here is the exhaust tail pipe as it exist the lower cowl - need lots of wiggle room (10 mm each side?)

 

To accommodate the above movement, I would expect a slightly oval prop flange opening, good clearance aground the front two cylinder rocker boxes leading edge (10 mm ?), fuel pump  (10mm ?) and in this case oil cooler (10 mm?) on opposite side to fuel pump

[turboplanner]

You've abandoned CAD?

 

[Old Koreelah]

9 hours ago, skippydiesel said:

…The exception here is the exhaust tail pipe as it exist the lower cowl - need lots of wiggle room (10 mm each side?)

Skip you have a wonderful opportunity to use the exhaust to enhance cooling. Many Cessnas, etc. have the pipes in the middle of the cooling air exit, intoducing some exhaust augmentor effect. I’m sure this works, because their huge engines are effectively cooled with a tiny exit no larger than the one my little 2.2 litre engine uses.

[skippydiesel]

3 hours ago, turboplanner said:

You've abandoned CAD?

 

Definitely not - however my understanding of CAD  is that you impose the parameters you want. So we (including my Son, the CAD person) will start with data from a series of digital photographs. From this we will be able to create 3 dimensional images, that can be manipulated to suit the dimensions we select. One of the most important dimensions will be engine movement clearance. Hence my question . The next  question for the Forum will be inlet air apertures - what size?

[skippydiesel]

8 minutes ago, Old Koreelah said:

Skip you have a wonderful opportunity to use the exhaust to enhance cooling. Many Cessnas, etc. have the pipes in the middle of the cooling air exit, intoducing some exhaust augmentor effect. I’m sure this works, because their huge engines are effectively cooled with a tiny exit no larger than the one my little 2.2 litre engine uses.

OK you must be psychic - the builder of the Sonex (I am but the humble finisher of the project) has designed and installed a venturi hot air extraction system for the standard Rotax muffler.

[turboplanner]

1 hour ago, skippydiesel said:

Definitely not - however my understanding of CAD  is that you impose the parameters you want. So we (including my Son, the CAD person) will start with data from a series of digital photographs. From this we will be able to create 3 dimensional images, that can be manipulated to suit the dimensions we select. One of the most important dimensions will be engine movement clearance. Hence my question . The next  question for the Forum will be inlet air apertures - what size?

Good luck with it.

[pmccarthy]

Mike Patey did two videos on building the cowl for Scrappy in carbon fibre, and how he set clearance. Worth a look if you haven’t already.

[Thruster88]

As PM said, lots of good advice and who doesn't like looking at a lycoming 780.😍 

Skippy is there no factory cowl for the sonex?

 

 

[skippydiesel]

1 hour ago, Thruster88 said:

...........................................................................

Skippy is there no factory cowl for the sonex?

 

Yes there is a factory Sonex cowl and yes I have one.

It would not fit if your life depended on it!

Reason - cowl is fashioned to accommodate an AiroVee engine not a Rotax 912 and not an Airmaster CS prop on the front.

[Thruster88]

4 minutes ago, skippydiesel said:

Yes there is a factory Sonex cowl and yes I have one.

It would not fit if your life depended on it!

Reason - cowl is fashioned to accommodate an AiroVee engine not a Rotax 912 and not an Airmaster CS prop on the front.

Ok, you could go the cut and open or shut as per Mike's video.  As he says practical is often better than CAD.

[tillmanr]

You might consider using your factory cowl as a mould and lay up a thin glass copy to chop around to develop one that serves your purpose.

Don’t forget to use a release on the original.

[skippydiesel]

32 minutes ago, tillmanr said:

You might consider using your factory cowl as a mould and lay up a thin glass copy to chop around to develop one that serves your purpose.

Don’t forget to use a release on the original.

Your suggestion is one of the options we are considering. Other than the top and two sides of the firewall, the Sonex cowl does not com close to fitting.

 

In these two photos you can see the standard  Rotax coolant radiator, mounted below the firewall - all cowl air must exit through the radiator.

 

You can also se how the muffler, and exhaust/venturi tube must be accommodated by the cowl - very different to the Sonex / AiroVee set up.

 

 

 

 

 

 

Edited December 30, 2021 by skippydiesel

[Yenn]

Nobody seems to have answered your question, but you have plenty of alternative info.

The movement depends upon your engine mounts. I would suggest that you loosen the mounts slightly, then move the engine around by hand to get some numbers to start with. You could then set up a jig frame to suit those numbers and check starting and shutting down to see if there is any interference. That way you may get close to the real numbers you are looking for.

[Blueadventures]

I’d place the oil cooler under the gearbox / flange area.  Looks like it is off to port side of gearbox.

[skippydiesel]

2 hours ago, Blueadventures said:

I’d place the oil cooler under the gearbox / flange area.  Looks like it is off to port side of gearbox.

Fair comment BA - this is how the original builder designed things. He made the mounts from scratch & has done an amazing job.

 

I fear the oil cooler will over cool in this position but will give it a go as long as it does not impact negatively on cowl design.

 

The oil cooler looks a bit odd in that photo, but when I put the Airmaster prop/hub on I am fairly sure the cowling, in accommodating the spinner/back plate, will also accommodate the oil cooler.

 

The Rotax fuel pump, on the other side of the gear box, is much more of a problem.

 

I would have put the oil cooler under the crank case and that may be an option for the future.

Edited December 31, 2021 by skippydiesel

[Blueadventures]

2 hours ago, skippydiesel said:

Fair comment BA - this is how the original builder designed things. He made the mounts from scratch & has done an amazing job.

 

I fear the oil cooler will over cool in this position but will give it a go as long as it does not impact negatively on cowl design.

 

The oil cooler looks a bit odd in that photo, but when I put the Airmaster prop/hub on I am fairly sure the cowling, in accommodating the spinner/back plate, will also accommodate the oil cooler.

 

The Rotax fuel pump, on the other side of the gear box, is much more of a problem.

 

I would have put the oil cooler under the crank case and that may be an option for the future.

My choice would be to move; will only need two brackets made and if you do I'd recommend buying the tight 90 degree pipes that floods sell as they will allow being up close under the gearbox neck.  Up to you.  As your needing to design the cowl may as well get things good from the getgo.

Edited December 31, 2021 by Blueadventures

[skippydiesel]

13 hours ago, Blueadventures said:

My choice would be to move; will only need two brackets made and if you do I'd recommend buying the tight 90 degree pipes that floods sell as they will allow being up close under the gearbox neck.  Up to you.  As your needing to design the cowl may as well get things good from the getgo.

Good advice - we will see.