Considerations in Engine Cowl Design

[RFguy]

yep

FYI Geoff the Reynold's number for the 125mm scat at 41m/s is 343000.   at these sort of surface roughnesses of corrugated hoses etc, friction ~ prop to roughness.

[Geoff_H]

After 40 years of designing constructing and commissioning I have learned that any errors in design and the working end product depends upon the accuracy of your assumptions. The best advice is to do your calcs three times.  Once where you expect assumptions to be then do them  twice with assumptions at their extremes.  The results will be the worst possible outcomes. 

I was the lead engineer in the middle east in the early 90's.  It was an American job and back in Dallas we had PhD people check all of our calculation in a multiday seminar.  They were good calcs.  Got the design running in Syria, it didn't work as expected.  After a lengthy investigation we discovered that our assumptions were wrong! We did modify the plant and it eventually worked to spec. Ever since I question assumptions not the calcs (not as much anyway).

Murphy was an optimist.  My long held saying.

Geoff

[RFguy]

well, I think in Skippys case, the sealing around his radiator will be the difficult thing, the leaks will really hurt,  but the leaks HAVE to be sealed because the air will just go around the radiator if it can . go as large radiator as possible to reduce velocity through the radiator, which will reduce pressure drop, and then there wont be such a huge difference between the radiator path and a leak path(s)

.... and with no leaks, the cowling will be pressurized and will reduce the performance of the oil cooler radiator hung under the prop.

the hot air in the engine bay will preheat the air hitting the radiator reducing its performance, also unwanted. SKip  I'd be ducting the air from the inlets to that radiator.

Edited January 30, 2022 by RFguy

[Geoff_H]

1 hour ago, RFguy said:

yep

FYI Geoff the Reynold's number for the 125mm scat at 41m/s is 343000.   at these sort of surface roughnesses of corrugated hoses etc, friction ~ prop to roughness.

I have never done an airflow through the engine but I think that the Reynolds number varies with fluid density, would these values and assumption be true for both sea level and 10,000ft?  No idea just asking 

[RFguy]

these numbers Geoff are all for sea level, and will hold to 10k feet. the reynolds number falls but its still in the same range of values, nothign much is affected except of coure the mass air flow is down  and thus cooling capability is down but of course engine output is down (no turbo)

 

[skippydiesel]

25 minutes ago, RFguy said:

well, I think in Skippys case, the sealing around his radiator will be the difficult thing, the leaks will really hurt,  but the leaks HAVE to be sealed because the air will just go around the radiator if it can . go as large radiator as possible to reduce velocity through the radiator, which will reduce pressure drop, and then there wont be such a huge difference between the radiator path and a leak path(s)

.... and with no leaks, the cowling will be pressurized and will reduce the performance of the oil cooler radiator hung under the prop.

the hot air in the engine bay will preheat the air hitting the radiator reducing its performance, also unwanted. SKip  I'd be ducting the air from the inlets to that radiator.

I dont actually se the sealing around firewall to be a problem.

 

Exhaust exit another story. I have some high temp material that , with care, can be placed/configured to minimise (best I can do) air loss in this area.

 

My oil cooler will be moved, from directly behind a cowling "nostril" , to below engine, between exhaust pipes. This location will allow for smaller frontal area of cowling and the potential for an additional air inlet direct to oil cooler. I hope the combinator will not a only improve aerodynamics but also facilitate optimum cooling.

 

As for ducting - not in the Mark1 set up but always a possibility down the test track. 

[RFguy]

Skip, just remember, pressurizing the whole cowling will reduce the ability of the oil cooler to work effectively because of the low pressure difference.  I'll be ducting the air from the inlets  to my radiators (as not to pressurize the whole cowling) , and a slight inlet volume for around engine ancillaries cooling.  If you want to minimise frontal area / drag , you have to make the most of any radiator area you  have , which will mean ensuring you get high pressure differentials...

 

[skippydiesel]

5 hours ago, RFguy said:

Skip, just remember, pressurizing the whole cowling will reduce the ability of the oil cooler to work effectively because of the low pressure difference.  I'll be ducting the air from the inlets  to my radiators (as not to pressurize the whole cowling) , and a slight inlet volume for around engine ancillaries cooling.  If you want to minimise frontal area / drag , you have to make the most of any radiator area you  have , which will mean ensuring you get high pressure differentials...

 

Hmmmm!

 

The sealing of the cowling will inevitably result is a pressure rise - how much and what practical effect this may have on cooling, remains to be seen . I do not agree that pressure in itself, is a barrier to good cooling performance, within the cowling, as long as each system has sufficient cool (relative)  air movement/flow from one side to the other (pressure gradient), 

Sealing (as much as poss) of the cowling is a requirement of my coolant cooling system, ie unavoidable.

I agree there must be an air flow from one side of the oil cooler to the other and this will require an air pressure gradient/difference - how much? Further automotive transmission oil coolers have been shown to be effective, even when not mounted in the front of the engine bay, in much slower moving, often pre heated air - I take some hope from this land based example. I do not , in the Mark I version, of my cowling intend to have a dedicated air supply to the oil cooler. I do think it highly likely that such a supply may be required, (provision for this is part of the plans) resulting in the Mark II variant.

I expect the coolant radiator to have a significant pressure variation, from inside cowling to outside air (much depends on the quality/effectiveness of the aforementioned sealing.

It should always be remember that Rotax engine cooling is a combination of oil, coolant and fin surface area. Certainly one (likely a liquid) can have a disproportionate temperature load/rise due to poor cooling performance of its dedicated heat transfer system and this must be guarded against.

 

[RFguy]

Skip. 

Air that exits the radiator/cowling- if the velocity of the exit air is slower than the bypass air, work (drag) needs to be done to speed that air up. Now that's a simplistic take but is an approximation. This will in practical applications require an exit  duct that is smaller than the radiator area.  The exit air velocity needs to be sped back up. So a large radiator with slow moving air (nice low pressure drop), exiting at radiator area, creates drag  because of the low velocity .  

 

What the small inlet, and large cowling plenum (low to high pressure conversion) does for you is improves available radiator pressure at low airspeeds. But generally none of us are interested at flying at low airspeeds at WOT for too long. At high speeds, it is completely unwanted.

 

The trouble with all this is that the greatest mass air flow /cooling is required at sub cruise speeds (80 kts) , and usually NOT required at cruise  (120kts).

 

In our aircraft , where there is plenty of airflow at cruise (1.5 x more airflow than at climb)  and cooling drag is approx velocity cubed, and our cruise power (75%) means less heat made, (although may be at altitude but that is usualyl cooler) there would seem to be a requirement to reduce the cooling drag at cruise. 

We can afford at least at 50% reduction in inlet area cross section in cruise. For my, for example 244cm2 of inlets (2 x 5" round) (Cd = 1 ~!~) drag at 120 kts = 5 hp) to 2.5hp. Hmm not much gain for the effort.

 

The ideal solution would be to change the shape of the inlets. That is - change the frontal drag. That is the elephant in the room. And also, reduce the drag from the opening of the bottom of the cowl. (Yay for adjustable cowling flaps ! )

Changing the shape of the inlets to encourage bypass of the inlet structure is required, but I think on our aircraft a little too much trouble. Have a look how the YF24/SR71 moved the nacelles in and out to change the input air velocity . 

 

Certainly, cowl flaps are the low hanging fruit- and probably why it is done in many aircraft....

 

recovery of the drag power cost with careful design of the exit air would seem pertinant for your quest. The bottom cowled radiator seems non ideal due to the low radiator air velocity. Ideally, both sides of the radiator need to be transition ducted. 

 

[skippydiesel]

IF I get good coolant cooling, the current design/location of the traditor lends itself to an internal cowl flap/shutter

[RFguy]

Skippy, this is what you need, take a look  at the diagrams  :
https://aviation.stackexchange.com/questions/27003/why-does-the-j58-engine-of-the-sr-71-have-a-diffuser-after-the-inlet-spike

 

 

[Bruce Tuncks]

The difference between upper and lower cowl pressures is what matters. This is in the Jabiru manual.

But help, I've tried to understand how a radiator can actually increase the total thrust, but I can't.

The idea that the higher temperature of the exhaust air from the radiator leads to less density and therefore higher velocity is something I could regurgitate for an exam but can't actually believe.

[facthunter]

That's precisely how a jet engine works. It's possible because you are adding energy.. The pressure it the inlet is constant so the gas flows to the lower pressure place as it always does. Nev

[skippydiesel]

In checking some oil cooler parts, I discovered that my oil cooler is the largest, of 3, that Rotax supply - hope this results in great cooling characteristics (always subject to air flow)

[RFguy]

again , cowling / exit air control will be your friend, so you dont have to wait too long to get going on cold mornings.

[RFguy]

Skippy how are you going with your Sonex ? I of course have thought and calculated  (even more) about the problem.
I went back to just slinging the radiator under the front of the prop. The high pressure region permits the smallest  OPENING to be used.

(A minimum opening (for airflow kg/sec)  of 15cmx15cm is required to get the air to do the job in my case) .

 

If you want to make the exit airflow radiator work, and be tolerant of cowling leaks, I'd suggest a shallow, thin but large area  radiator- IE something that requires a minimum of pressure drop to work- otherwise the air will try and find away around a high pressure drop restriction such  a deep core radiator.  

 

As you are going to dam up the air in the cowling, a forward facing oil cooler would have a hard time working well also, because there is a high pressure in the cowling. but there may be enough to make it all work with the low restriction low pressure drop exit water radiator .

 

As you need to minimum frontal everything - I think you would be better off to :

• if there isnt room to mount the radiator under the prop above/below the oil cooler - duct using duct to the radiator, mounted where ever is convenient. you'll need to get creative with transitions from the ducting to the radiator size. And you'll want to ideally duct the airflow out of the radiator , and if you want to go to town, with roughly same size or smaller exits as the inlets- but that's a rather complicated endeavour because the airflow at the nose (inlets) is rather complex and varies with airspeed, attitude, prop etc) . but that's really fine tuning at the edge.
  or
• Sling the radiator under the prop at the front through a> 225cm2 opening, and then open it up behind the opening- IE *  the opening does not have to match the radiator size *  as long as that opening is large enough to get the mass air flow, the opening can expand up to the radiator width behind it. There is a point where more radiator airflow is not useful (airflow behaviour, cooling efficiency), so having the required opening to get the mass air flow, then expanding out, slowing the air down to a larger area is what the doctor orders.....

 

So, install some pressure probes and pipes as part of your installation in order to assess the pressure difference. From my view , its all about understanding the inlet pressure to cowling pressure to assess heat exchanger effectiveness.

 

The drag for your 225cm opening, assuming it was flat plate drag (which it isnt) and nothing exits to offset is approx  50N (3kW 4HP  at 107k). but- its not flat plate, It's probably half that at the front because it is part of a rather complex airflow deflection at the nose.....  and you'll get a fair chunk of that back if you duct it out effectively. 

-glen

 

 

 

[Garfly]

This is a video by Fred Peuzin about the carbon fibre cowl he designed for his Jodel.

He has provided english translations for the titles.

 

And this is an auto-translate of the YouTube description:

 

"New cowl test flight.
1,329 views Feb 25, 2019 A technical video this time, with the first flight test of our new engine cover. It is more aerodynamic and lighter than the previous one.
 It is also the culmination of 4 years of theoretical and practical studies on cooling drag, a step closer to our speed objective of 135 kt true airspeed (250 km/h), without affecting the engine power which is of 100 hp (at sea level and 2750 rpm).
 Incidentally, the aircraft is lightened by 6 kg, the cover should ultimately weigh (after painting) less than 5 kg.
 The cover was made of carbon fibre and epoxy resin on extruded polystyrene male moulds.

 All the details on our blog:
https://speedjojo.blogspot.com/

 

 

[Old Koreelah]

I’m glad I don’t have to do my test flying over such unforgiving territory!

Jodels are amazing aeroplanes, but not immune from engine failure.

[facthunter]

What aeroplane is? That's why a lot of Planes have more than one engine.. The more engines you have, the more chance of ONE (of them) failing. Nev

[RFguy]

did the southern cross have three engines for performance or redundancy (what weighting ?)

Edited June 27, 2022 by RFguy

[facthunter]

Fixed pitch props of the day make engine out performances disappointing. Additionally the Flying boats from Short Bros and Harland used by TEAL had optimist engine out  figures.  The Fokker/Ford trimotor did fly in ground effect (water) with one out but extra drag from the damaged engine.

  Later designs conform to more  realistic engine out performance but twin engined JET planes end up overpowered when on two engines so often perform reduced thrust take offs at suitable airports and weights. Nev

[RFguy]

SKippy, I think,  the (good) thing about using the ram air flow ( putting the heat exchangers (oil, water)( at the front of the plane in the airflow is that it's pretty difficult for it not to run the radiators at maximum performance for their size. 

With your cowl exit air , as discussed, that will need to be very well sealed to work (or need a large, low pressure drop radiator) . 

When you put the cowls on, you'll need some means of tightening up / latching/clamping a seal around the cowl exit radiator  because there will be lots of pressure in there trying to get out any leak.

IE will need some latches or screws are a lip that the cowls slide under etc so the cowl pressure generates the seal etc. and that's warmed air..

 

You could also look at cascaded water/oil radiators .  Assuming intake is 38C and 225cm2 frontal cowl opening,  and 38ms, the air passing the water radiator at full smoke (25kW of heat) will rise about 27deg C. If the intake is ar 38 C, the air will hit the oil cooler behind at 65C, a likely HALVING of temperature difference  (85 oil to 65 air ) compared to (85 oil to 38 air) , which would limit the oil cooler's performance .

 

Oil cooler rotax 912ULS spec is 6kW at full smoke, so the hotter (less dense) pre heated air  (weight = 1.05kg/cuM) passing through would rise about 7deg C.  but again  the temperature difference is LOW (oil = 85C - 65C) compared to (85-38C) so certainly requires a bigger oil cooler than stock. double the surface area perhaps.

 

Now. If you opened up the frontal area , say 2x to 450cm2 (about the full size of a rotax radiator) then the temperature rise would be HALF and the oil cooler would likely be just fine.  There is plenty of pressure to drive two radiators, although pushing through a thick  oil cooler, especially the deep core ones (90mm on my Jabiru!) might take ALOT of pressure away and become sensitive to cowling pressure. Dunno would need to look at the airflow versus pressure drop curves for proposed oil coolers. Thinner one, and large front area would be the fix. The loss of pressure over the radiator set migth not be as bad as we thiink  because there is an upper limit on radiator performance, you cant get keep increasing the airflow with a linear increasing in cooling capability.

 

just ideas....   I think that two coolers each with their own inlet (and not cascaded)  is better because temperature difference rules when  it comes to heat exchangers.  but you could make the cascaded cooler work. 

 

 

Edited June 27, 2022 by RFguy

[APenNameAndThatA]

On 29/1/2022 at 7:29 PM, RFguy said:

Hi SKippy

As I continue to learn about fluid dynamics, some things are now apparent by the numbers...

firstly - 

For liquid cooled aircraft,  using the surface of the leading edges of the aircraft, like using tube forming the wing leading edges,  or the skins being heat exchangers would be fantastic for liquid cooling, and wouldnt require any front area dedicated for cooling... seems a no brainer, but I have not seen anyone doing it.  

 

The leading edge would seem a no brainer, and some aircraft I think the RANS the leading edge is already an alumuminium extrusion. 

The skins, while that only receive boundary airflow (lower velocity etc) , large areas available would make them work !

If I replaced my Jabiru top or bottom cowling with a skin of aluminium with water in behind it, like a flow structure etc (there are many flat, formable aluminium extrusions designed  for intercoolers etc )  , there would be more than enough surface to cool anything.... I dont have time to weld up the tanks on each side of the flat tubes so I'll use draggy radiators.. 

 

Now the bad news :

 

Your small inlet  (suggest min 288cm2) , large  plenum (the cowling) is akin to a hydraulic  amplification.  small  inlet into a large volume. 

This increases pressure of course.

Your exit air mounted radiator will work grandly IF you can get the whole thing sealed, and not otherwise- the high pressure will go hand in hand with sealing challenges.

You'll have some gaps , so the minimum inlet size might need to be increased quite a bit.  In the photos , it look slike a substantial gap will be required around the exhaust pipe. 

 

the next problem you might face is the production of a low pressure region where you want it. This wont be flush with the skin, it will be up a recess/duct. ideally it will work as some thrust.

 

Next thing, parasitic drag is proportional to velocity CUBED . Put a 500cm2 flat plate in the wind at it will consume 10HP at 120 ks !!!

Unfortunately, you are going to have to get air from somewhere to run your radiator, which means having SOME frontal area.

 

 

glen

 

 

I thought drag was proportional to velocity squared. And power required was proportional to velocity cubed. And the difference was because you need more power to go faster for a given amount of drag. P.S. I am amazed by how much you and others on here know. 

[RFguy]

good find, yes I was talking about costs in that context

I was missed inserting the words 'power required to overcome..'

yes power required  to overcome the drag is velocity cubed 

etc.  

What is MORE interesting is propeller dynamics!

read up on that! I have recently..... just a little, it is a complex topic ...

 

and some goodies are : prop thrust proportional to  is RPM^2 . diameter^4

but prop input power is proportional  to RPM^3 . diameter^5

 

so those high RPM props really hurt.

having to accelerate the fluid (air)  to a higher velocity costs energy.

 

 

 

 

 

 

 

Edited June 27, 2022 by RFguy

[RFguy]

one might wonder if drag proortional to  velocity squared , why is the power to overcome it proportional to velocity cubed ?

 

drag is a force. drag is proportional to velocity squared (putting aside Reynolds) 

and work done= force  x  the distance that force is applied over.

 

example. suppose a shopping trolley has a high friction wheel.... 

if you push a shopping trolley 0 m, you have done no work.

if you push a shopping trolley 100 meters you have done work.

 

so workdone  = dragforce x distance 

so, per second......

so  workdone per second = dragforce x distance per second

and

Power is measured in units of  workdone per second (power )

and our distance per second  measured in meters per second (velocity)

so.... substituting in ....

power = dragforce x velocity.