Considerations in Engine Cowl Design

[skippydiesel]

2 hours ago, facthunter said:

Same area as the radiator will be too big. The air is impeded when it goes through the radiator matrix and slows down considerably. Even fly screens slow wind  a lot.. Thicker and finer are the worst offending radiators. At that point the pressure drop must be considerable to get flow. It takes quite a decent sized belt and fan to pull air through a radiator. Have a look at road graders and they are diesel and more efficient.. The prop would impart a swirl to the air in that region and the openings should take account of that. Nev

Not the same area as the radiator (which would be huge)  the same area as the radiators frontal opening. Velocity/volume would be what can be delivered by the big fan up front on the ground (aircraft stationery/taxying) and ten by relativly high speed air when flying (plus fan). No point in trying to force more air through the radiator than it can cope with or thet will exceed relativly efficient transfer of heat from one medium to another (copper fins to air).

[APenNameAndThatA]

Am I right in thinking that if you have a bigger radiator, the pressure and temperature gradient across the radiator will not need to be as big? Does that mean that if you have a bigger radiator, you need less air and can therefore get away with less drag? If so, are LSA radiators optimised for drag or for weight and cost?  

[Bruce Tuncks]

The detail design of the airflow from the opening to the radiator is very important. You need to avoid separations and steps and leakages.

Thats a very good question pen-name. I don't see how a bigger radiator could be less drag, but there may be a way.

I don't think Rotax engines are completely liquid cooled , It is only the heads.

A coolant leak when at a remote location can be very expensive. Often times, you are not allowed to fix it yourself, so you are up for charter flying a lame in n out. 

[Ian]

Radiator cowling design is an extremely difficult subject and most of the work in this space was done in WW2. If you can find someone with a moderately good solutions it's probably the right one unless you have a lot of time to burn.

What most people don't realize is that a really good radiator design can develop thrust (meredith effect), the P52 radiator design is probably is probably one of the best, however this really only works when your airspeed is pretty high. However at the very least good cooling duct design can be advantageous if you have the space available.

The lower the airspeed through the radiator the lower the drag. Aerodynamic drag increases as the cube or the airspeed and drag due to the radiator size increases at the square of airspeed. So you can exchange radiator area for reduced drag.

The duct should be divergent/convergent and there are maximum rates of change in the angles which can be used. Also the duct needs to be slightly larger post radiator as energy has been added to the airstream.

 

 

[RFguy]

Hi Bruce
The rotax bores are aluminium cased (like Gen4 Jab)  and a fair bit of heat gets conducted from the bores into the water cooled heads. The heat will flow to the cooler components. 

APen - yes- and to put some numbers into it, 1kJoule (or 1kW for one second) will heat  the temperature of 1kg of air 1 deg C.

Marvellous eh ?

so 25kW of heat (coming from the heads at WOT) and 1 kg per second of air  (about a cubic meter/sec)  will result in a air temperature rise  of 25kW/1kg = 25 deg C.

Where is gets messy is calculating the water temperature drop in the radiator, which depends on a zillion things in radiator design and engineering. But they are the basics.  IE yes if you can tolerate higher air temperature increase flowing through the heat exchanger (water radiator or head fins) then less air is required.  
And those numbers assume it is an isobaric process, which it isnt exactly.

 

 

 

Edited January 14, 2022 by RFguy

[Bruce Tuncks]

Thanks RF guy. I grappled with the question of glider ventilation air for years before coming to the view that there was no zero-drag solution.

 you take in air at the relative airspeed ( 80 knots say ) and then let it out at zero knots relative to the glider. 

So each kg of ventilation air costs 840 J. This can only come from the potential energy of the glider so it costs a 300 kg glider 0.3 meters of height. Airliners knew about this and so they recirculate the stale main cabin air all day. Only the pilots get fresh air.

Mind you, with the gliders, there are worse ways to get rid of excess ventilation air than a reverse scoop at the fuse fattest point., so the .3m would be the best you could do. 

 

[skippydiesel]

4 hours ago, Ian said:

Radiator cowling design is an extremely difficult subject and most of the work in this space was done in WW2. If you can find someone with a moderately good solutions it's probably the right one unless you have a lot of time to burn.

What most people don't realize is that a really good radiator design can develop thrust (meredith effect), the P52 radiator design is probably is probably one of the best, however this really only works when your airspeed is pretty high. However at the very least good cooling duct design can be advantageous if you have the space available.

The lower the airspeed through the radiator the lower the drag. Aerodynamic drag increases as the cube or the airspeed and drag due to the radiator size increases at the square of airspeed. So you can exchange radiator area for reduced drag.

The duct should be divergent/convergent and there are maximum rates of change in the angles which can be used. Also the duct needs to be slightly larger post radiator as energy has been added to the airstream.

 

 

I am in complete agreement with your theory, however could you be over stating this for a RAA class aircraft?

My last aircraft, an ATEC Zephyr, had no internal (cowling) ducting. General air through two “front “nostrils”. The coolant radiator, located front lower cowling, with dedicated opening. The oil cooler, lower right, fixed to firewall, just to one side of exit air opening (no special opening or ducting). I made and used a “fixed” cowl lap for winter (below 25⁰C) operations. Never had a heating problem in 500 hrs/10 years.

[RFguy]

yeah but it had as you said : " The coolant radiator, located front lower cowling, with dedicated opening. "

 

the opening was a good size, to be sure, but I bet not quite big enough to climb slow at WOT all day in a australian summer.  but you dont need to I guess/

 

My guess is they got clever with the airflow in the cowling for the oil cooler- they knew the airflow, modelled it etc. 

 

Edited January 14, 2022 by RFguy

[skippydiesel]

1 hour ago, RFguy said:

yeah but it had as you said : " The coolant radiator, located front lower cowling, with dedicated opening. "

 

the opening was a good size, to be sure, but I bet not quite big enough to climb slow at WOT all day in a australian summer.  but you dont need to I guess/

 

My guess is they got clever with the airflow in the cowling for the oil cooler- they knew the airflow, modelled it etc. 

 

They may have "modelled it" but the fact remains, no special ducting to ensure cool air flow AND (belatedly realised) much smaller coolant radiator & oil cooler, than the Sonex I now have.

 

I am not smart enough the debate the physics of the matter  (nor do I dispute your maths) BUT I never had a overheating problem (overcooling in winter months, yes) and yet the system, fitted to the Zephyr, is the epitome of  the "KISS"  principal itself.

 

My point is; the Rotax cooling system, that I am used to is, with a little common sense (management) applied, more than capable of dealing with an Australian summer - who wants to do a full power, max, extended time, climb on a 45 degree day ? Not me!

 

There is little point is specifying the cooling system for a situation which is unlikely to occur ("climb slow at WOT all day in a australian summer"). If it did, the pilot will always be able to apply mitigation (heat reduction) strategies such as - lower the nose (when at safe altitude) reduce power (less heat) and increase speed (more cooling air) and/or "step" climb . The best strategy of all - manage your TO to coincide with the cooler times of the day.

 

At this stage (cowling design) my  biggest concern is cooling on the ground (extended taxi/holding) - will my system have sufficient airflow to keep temperatures within safe limits ? AND if TO after extended holding, on a warm day, will the already heat soaked engine have sufficient cooling during initial climb out to safe altitude? No need to answear as I suspect all of this will be determined by flight testing.

[RFguy]

you'll have to build it and fly it....or at least taxi it !

[skippydiesel]

8 minutes ago, RFguy said:

you'll have to build it and fly it....or at least taxi it !

With a little luck, 2-3 months to have it painted and Mk1 cowling fitted & ready to go.

 

My Rotax 912 ULS aircraft design idol ( Robin Austin) suggested that exit air control, was probably more important, in his World Record aircraft VH- SGS, than inlet sizing. (http://www.worldrecordplane.com/)

[Bruce Tuncks]

My experience with the SK jabiru was that the cooling ducts were not good enough to begin with but after hours of tweaking, they were much better. I still, to this day, watch the temps on climbout and do the things skippy says if needed on a hot day. Keeping the CHT's under 160 C is the aim.

I fully agree with the idea of experimentation. That is, proper experimentation backed up by proper measurements. Tufting and photos qualify here, not that I have ever done this but others have and their work was very helpful.  And getting other opinions sure helps too, I spent a lot of time playing with the inlet side of the carby, trying to even out the egt's,  before I was told on this site that the real action needed to be on the downstream side.

[skippydiesel]

On 15/01/2022 at 6:47 AM, Bruce Tuncks said:

My experience with the SK jabiru was that the cooling ducts were not good enough to begin with but after hours of tweaking, they were much better. I still, to this day, watch the temps on climbout and do the things skippy says if needed on a hot day. Keeping the CHT's under 160 C is the aim.

I fully agree with the idea of experimentation. That is, proper experimentation backed up by proper measurements. Tufting and photos qualify here, not that I have ever done this but others have and their work was very helpful.  And getting other opinions sure helps too, I spent a lot of time playing with the inlet side of the carby, trying to even out the egt's,  before I was told on this site that the real action needed to be on the downstream side.

I am not sure how you would "tuft" the inside of a cowl - light & cameras would be needed and you might still need to do area pressure readings. Sounds like a wind tunnel job to me. Way beyond my meager resources

 

Fingers crossed, my intention to go for a combined "nostril" (cool air in) area, equal or slightly larger, than the coolant radiator face area.

My hope, this will deliver a bit more air than required for extended taxi/holding and climb out.

If it works out this may mean too much cool air at cruise & in winter - solution a pilot operated cowl flap.

If I to go this rout, the flap will be (due to location limitations) on the inside of the cowling (rather than traditional exit) and may only cover a portion of the radiator (determined by using temporary tape).

On the other hand , if temps too high, I have the option of enlarging "nostrils" or more likely adding a small (possibly NACA) duct, directing air onto oil cooler and at the same time providing a larger volume of air to the coolant radiator.

 

This will be an exciting (possibly frustrating) project, that will commence when I have a airworthy cowling/aircraft, in about 2 -3 months

[RFguy]

start testing out of high summer 🙂

[Bruce Tuncks]

I liked the stuff I saw using tufts because it showed the airflow to not even be in the expected direction, but in the end, my tool was just a water U tube. the ends of the tubes were in the ducts or the lower cowl, and the U part was in the cockpit.

It was these readings which finally explained why the RHS ran cooler than the LHS, and I got rid of some of the difference by connecting the 2 ducts with a bit of vacuum-cleaner hose. Yes the difference was counter-intuitive.

After figuring that the prop was going upwards past the LHS ( as seen sitting in the cockpit ) I tried "eyebrow" fairings at the duct entrance on the upper cowl to deflect air in but these did exactly nothing. They were just taped on, so their removal was easy.

The standard thing to do on a Jabiru is to put a skirt on the lower cowl exit, and this works but it must increase the drag. I did a very small skirt and it sure helped.

[Old Koreelah]

4 hours ago, Bruce Tuncks said:

…in the end, my tool was just a water U tube. the ends of the tubes were in the ducts or the lower cowl, and the U part was in the cockpit.

It was these readings which finally explained why the RHS ran cooler than the LHS, and I got rid of some of the difference by connecting the 2 ducts with a bit of vacuum-cleaner hose.

Several people have used a tube to join the two side, but this design goes further: 

I’d like to hear how this one performs in real-world flying.

 

Quote

The standard thing to do on a Jabiru is to put a skirt on the lower cowl exit, and this works but it must increase the drag. I did a very small skirt and it sure helped.

As stated previously, the outlets under big Cessna engines are tiny, but seem to work fine. Maybe their higher speed is one factor, but the augmentor effect of having the exhaust pipes in the centre of the outlet cannot be discounted.

[RFguy]

Skippy

while we're online workshopping this 

something I am thinking about right now is whether I  get 2 x motorcycle radiators and put them behind the existing cowling inlets, or use  6"  to 8" dia ducting and pipe the air where I can put the radiators  somewhere simpler.

duct like this : 

https://airtight.com.au/product/hightempflex/

6" duct, $200/meter . 6" ID would be 182cm2. Two of those would be a little on the marginal side. (362- I need 440) 

It might be easier in the cowling to run 2 x 5" ducts per side instea dof a six. each 4" duct is 122cm2, so four in total is 490cm2. not bad.

 

or just some SCAT hose 4" is $40 per foot.

there are some pre formed NACA ducts you can buy , also for racecars etc...

http://www.revolutionracegear.com.au/index.php?PCID=10550

etc

Now, you could use a large scoop area and smaller duct (and it operates at higher pressure/velocity so the heat calcs are different).  like brake scoops with 3" duct. But then the radiator must work ovber a smaller area  or you need to reverse at the radiator end....)

Maybe a couple of preformed NACA ducts on the cowling.   

https://www.chilloutsystems.com/products/4inch-carbon-fiber-naca-duct

there should be 3D STL files ont he internet to get them printed in your favourite material at a 3d print agency

Indeed, search NACA duct on thingiverse, and there is plenty you can print

https://www.thingiverse.com/search?q=NACA+duct&type=things&sort=relevant

 

https://buildingspeed.org/2012/12/21/whats-a-naca-duct/#:~:text=The length and shape of,a lot of extra drag.

https://www.motortrend.com/how-to/naca-duct/

 

but a NACA duct is not goign to convert 400cm2 of duct opening area to 400cm2 of equivilent open facing inlet duct, they dont work that way.

AND the long duct work will have pressure drop and that will drive everything......

 

BTW 100mm duct 1m long , straight , 0.5mm roughness, will be about an inch of water drop at 80 kts. going up to 125mm duct, a quarter of an inch of water for same airflow.  larger openings = higher airflows so higher pressure drops. BUT this tells me 4" duct for 4" opening is OK, just . 5" duct for 4" opening is great. 5" duct for 5" opening is OK. (80 kts). going from 0.5 to 1mm roughness doesnt hurt much.

 

 

 

Edited January 18, 2022 by RFguy

[RFguy]

https://xplrcreate.com/2013/06/15/naca-duct-vs-scoop/

 

4", 4m length  length local

https://racetechseats.com.au/shop/ducting/silicone-duct-hose

 

Edited January 18, 2022 by RFguy

[skippydiesel]

18 minutes ago, RFguy said:

Skippy

while we're online workshopping this 

something I am thinking about right now is whether I  get 2 x motorcycle radiators and put them behind the existing cowling inlets, Europa Aircraft use this system - looks heavy & complicated but must work. Probably lends itself to a smaller frontal area/look or use  6"  to 8" dia ducting and pipe the air where I can put the radiators  somewhere simpler.

duct like this :  Some of the Sonex /Rotax912  installations I have been looking at go with a radiator on one side of engine bay using a composite duct from front and a side exit exhaust/ air vent

https://airtight.com.au/product/hightempflex/

6" duct, $200/meter . 6" ID would be 182cm2. Two of those would be a little on the marginal side. (362- I need 440) 

It might be easier in the cowling to run 2 x 5" ducts per side instea dof a six. each 4" duct is 122cm2, so four in total is 490cm2. not bad.

 

or just some SCAT hose 4" is $40 per foot.

there are some pre formed NACA ducts you can buy , also for racecars etc...

http://www.revolutionracegear.com.au/index.php?PCID=10550

etc

Now, you could use a large scoop area and smaller duct (and it operates at higher pressure/velocity so the heat calcs are different).  like brake scoops with 3" duct. But then the radiator must work ovber a smaller area  or you need to reverse at the radiator end....)

Maybe a couple of preformed NACA ducts on the cowling.   

https://www.chilloutsystems.com/products/4inch-carbon-fiber-naca-duct

there should be 3D STL files ont he internet to get them printed in your favourite material at a 3d print agency

Indeed, search NACA duct on thingiverse, and there is plenty you can print

https://www.thingiverse.com/search?q=NACA+duct&type=things&sort=relevant

 

https://buildingspeed.org/2012/12/21/whats-a-naca-duct/#:~:text=The length and shape of,a lot of extra drag.

https://www.motortrend.com/how-to/naca-duct/

 

but a NACA duct is not goign to convert 400cm2 of duct opening area to 400cm2 of equivilent open facing inlet duct, they dont work that way.

AND the long duct work will have pressure drop and that will drive everything......

Not all, so described, NACA ducts are NACA especially those that come from automotive sources - true NACA have specific dimensional ratios. Be careful - may look "the goods" but not deliver.

I thought you were a died in he wool front of cowling radiator man - what has caused this change of heart?

[skippydiesel]

RF - It might pay you to contact the owner/builder of the Jab/914 based in Cooma - he is very happy with his aircraft - GA registered with 4 seats.

[RFguy]

no change of heart from me Skippy, just thought you might like some more ideas.

radiators are still best to go and face the wind, given that I already have cowling openings ....

[skippydiesel]

1 hour ago, RFguy said:

no change of heart from me Skippy, just thought you might like some more ideas.

radiators are still best to go and face the wind, given that I already have cowling openings ....

I agree  - just like you I have been presented with an existing layout that I would like to try befor making any significant changes.

If mine works GREAT!

If not will be rejigging the whole shebang (probably radiator at front of cowl)

[RFguy]

I've found a mfr that makes some small industrial heat  exchanger high performance radiators, all copper, lots of specs. affordable.

https://www.alphacool.com/alphacool-nexxxos-v.2-radiatoren

https://www.alphacool.com/download/ENG_1015512_NexXxoS_V2_XT45_240.pdf

https://www.alphacool.com/shop/radiators/radiators/240-mm/20477/alphacool-nexxxos-xt45-full-copper-x-flow-240mm-radiator

 

They'll  fit nicely in the airflow in the cowling inlets with a minor enlargement. All shapes and sizes off the shelf, about $100 per unit.  But they weigh a kilo each. alot of copper. 

 

Edited January 19, 2022 by RFguy

[skippydiesel]

7 hours ago, RFguy said:

I've found a mfr that makes some small industrial heat  exchanger high performance radiators, all copper, lots of specs. affordable.

https://www.alphacool.com/alphacool-nexxxos-v.2-radiatoren

https://www.alphacool.com/download/ENG_1015512_NexXxoS_V2_XT45_240.pdf

https://www.alphacool.com/shop/radiators/radiators/240-mm/20477/alphacool-nexxxos-xt45-full-copper-x-flow-240mm-radiator

 

They'll  fit nicely in the airflow in the cowling inlets with a minor enlargement. All shapes and sizes off the shelf, about $100 per unit.  But they weigh a kilo each. alot of copper. 

 

Hmmm! How will they go in a high vibration environment?? 

 

My sons dirt bikes have all had 2 radiators (one each side of frame) designed for pretty harsh environment. One even has something like a computer fan for when he is going too slow for good air flow. Motor bike gear usually transfers quite well to small aircraft.

[onetrack]

The last thing I'd be using as part of an aircraft, is a radiator designed to cool computers. There's no vibration or movement stress levels taken into account in their design.