Considerations in Engine Cowl Design

[RFguy]

footnote

 

previous I had that rotax book had 6kW for oil cooler requirement. True in an old manual and for 912S (80hp)

Current : is 10kW for 912ULS and  "experience shown that 160cm2 is required "25sq""

.

and 28kW for water cooler requirement (25kW for 80hp)  "500cm2" ....  and 60 litres per minute which is a really high flow I reckon. 2 m/s in the 25mm radiator hoses. (mix of 18 and 25mm) wow.

 

[facthunter]

And you are only doing the cylinder heads. Nev

[Geoff_H]

Active inlet dampers could possibly be used to regulate the amount of air into cowling for regulated cooling.  Many aircraft use exit dampers, a simpler and better idea to only using as much air as necessary.  But inlet dampers such as that used on some vehicles could be used. Inlet restriction may reduce airframe drag, may, but the complexity and weight penalty may negate their effect. It's just a thought, my design process has always been, "what can we do different" then suggest even stupid ideas then examine possible implementation of the stupid idea.  I then usually end up seeing that things as they were traditionally designed are the best.  I live by the motto that there are no stupid questions just stupid answers.

 

[facthunter]

You don't have to re-invent the wheel either. The planes that have restricted frontal openings appear to be the minority. If the front is blunt it might as well have a large(r) opening. and is inherently more likely to work better at the lower speeds. A more open motor has the ability to radiate heat better and shouldn't be polished.  Cowled exhaust pipes put heat back to close parts and ancillaries.  Nev

[APenNameAndThatA]

On 27/6/2022 at 10:03 PM, RFguy said:

 

 

 

 

 

On 27/6/2022 at 10:03 PM, RFguy said:

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.

 

 

 

I wonder if that means that, roughly speaking, relative to airspeed, your fuel consumption for a given trip between two set points will be squared and your fuel flow will be cubed? 

 

Edited July 6, 2022 by APenNameAndThatA

[Bruce Tuncks]

I have thought about inlet air control for a Jabiru.

Why inlet? well the flaps would not need to seal very well. You could use two flaps, one each side of the upper cowl.

The main downside would be the need to disconnect the drive every time the upper cowl was removed, but the upside of this would be that the connection would be easy to access through the upper cowl air inlet holes.

Has this been done on a Jabiru or similar plane?

[RossK]

Agree with Nev, you don't have to reinvent the wheel.

Pic is the front view of a Sportstar with the 912ULS.

Top openings - air cooling for the cylinders

middle - radiator

lower - oil cooler.

None are that large and we've never had cooling issues.

 

[Bruce Tuncks]

I agree Apenname. But you need to factor in the fact that the fuel used will be for less time as the trip will end sooner.

My son has a Lancair which has retracts and a 360 Lycoming. It is about twice as fast as my Jabiru and the fuel consumption rate is higher. But his trips are faster and the liters per hundred km figure is similar. Bugger huh.

SO...  if you double the airspeed, the drag goes up 4 times ( ie squared) so the engine power must increase to compensate and more fuel will be used PER REV.

Lets say squared too, although this is assuming a lot about the engine.

Now if the revs have also doubled, the fuel used will double again ie cubed..( this is also assuming a lot)

But the trip will take half the time, so total fuel used will be halved.

[facthunter]

The best I've seen used are louvers like a horizontal venetian blind with each piece pivoting 90 degrees from closed to full streamline position when open..The medium sized Russian radials have them  in a radial fan shape full circle.. This could be for snowstorms etc. Nev

[RFguy]

Ultimately, the fuel used will be depending on the aircraft drag, and how much the aircraft burns for a specific rate of power production.  

Bruce, the spirit of your math is right but the numbers are not.

doubling  something from squared is not cubed !

[Bruce Tuncks]

Yep Nev, those radial louvres look good. As you say, the climate there has extreme swings . Mind you, it has been cold here lately but not nearly as cold as a Russian winter.

If they had enough blades, they just might fit into the front of the ram-air ducts and this would get them free from the top cowling.

Another downside though is that the plane is now more complicated to operate.  In the Jabiru, I really like how you don't need to worry about prop pitch, mixture and wheels up lights.

 

[Bruce Tuncks]

Thanks RF guy, you are quite right. I could revisit the math but it's not really the right way to go about the problem. You need prop power curves and engine power vs fuel used etc. But it is still correct that thrust to overcome drag is proportional to V squared  ( at the top end ) and power equals thrust times speed and so power is proportional to V cubed.

This explains why the first "go-fast " item you do may well give you 3 knots more but if you do ten such things, you don't get 30 knots more speed.

Here's a list of "go-fasts" I did on my old Jabiru

1. sharpened the strut trailing edge ( the old struts were very rounded and left a thick wake)

2. sealed up the control-surface gaps and the wing-root gaps

3. Installed an arduino-based cht and egt readout for each cylinder ( Thanks Kevin). Yes this is not really a go-fast but it does allow more engine power to be used without worrying.

4.reshaped the u/c legs for less drag, using tape and foam 

5. changed the prop from a 40 inch pitch to a 44 inch pitch ( thanks Ken and Paul)

6. sealed gaps etc in the tail-end of the plane...  elevator and rudder mainly.

And I can't say for sure how much faster it is...  I think about 3 knots but can't really tell.

[RFguy]

Good one Bruce. yeah I reckon the U/C legs are worth quite a bit in drag with those 90 deg corners / surfaces

3 kts is quite a bit. (say 115 >> 118 CAS)   probably 5 to 8% fuel saving depending on where the Bing is operating....

[Mike Gearon]

11 hours ago, Bruce Tuncks said:

Another downside though is that the plane is now more complicated to operate.  In the Jabiru, I really like how you don't need to worry about prop pitch, mixture and wheels up lights.

 

That’s so true. This is why I’m always going to fly a Quick Silver. Seat of the pants flying. No 6 pack scan. Just make sure the wind in your face continues to be like sticking your head out the car window at 100kph, wings fairly  level and the yaw string fairly straight. Round out and flare…. Simple. When it feels like your arse is going to hit the ground you flare. Simple.

[Ian]

On 06/07/2022 at 7:36 AM, Geoff_H said:

It's just a thought, my design process has always been, "what can we do different" then suggest even stupid ideas then examine possible implementation of the stupid idea.

Good approach to see. There's far too many people who don't ask fundamental questions for fear of looking stupid. Too many times I've left meetings to have people thank me for asking what "XXXX" meant and they've been involved with the project longer.

Or possibly there's something wrong with those who actually ask questions 😉

 

One of the features in the video below which I found interesting was the lack of effect of a spinner on the speed of the plane. The simple high pressure zone in front of a blocking leading edge is actually a somewhat efficient shape. This makes me think that trailing mechanisms tend to be more effective.

 

 

[Mike Gearon]

A couple of things stand out on that video.

 

1. He take a copy of Fluid Dynamic Drag on holiday. I get a kick out of people,who are way more quirky, almost makes me look good.😀 didn’t expect to watch the whole video. Thanks for sharing!

 

2. Flew a 300kg all up fuel and pilot to 170kn plus, set a world record, smashed his aircraft up at that speed and walked away. All remarkable.
 

 

[Garfly]

And then YouTube turns up this terrific sequel:

 

 

[Ian]

I might have to buy a copy to take on my holidays as a bit of light reading.

He died a few years ago and also designed the AR-6 https://en.wikipedia.org/wiki/Arnold_AR-6

According the the video he also built the Amsoil Racer so if anyone wants to go fast efficiently he's worth paying attention to. 

 

[Old Koreelah]

Sad to see Mike Arnold passed away. He sure influenced a lot of builders.

Reading about how fast he went with 50hp must have made an impression: after several phases of modifying my Jodel Babe it started looking a little like his amazing AR-5…if you squint your eyes and ignore the ruff-as-gutz finish.

 

[Mike Gearon]

3 hours ago, Ian said:

I might have to buy a copy to take on my holidays as a bit of light reading.

He died a few years ago and also designed the AR-6 https://en.wikipedia.org/wiki/Arnold_AR-6

According the the video he also built the Amsoil Racer so if anyone wants to go fast efficiently he's worth paying attention to. 

 

We will get along! I met up with an old friend yesterday who problem solves high end jets. He spent 2 hours explaining last weeks problems including a reverse thruster problem nobody could solve for a year. He hooked up all,his sensors and had the guys taxi from 40 to 60 knots. Risk taking there!. If he’d not found the problem there would have been grumbling over that.  Bingo, at high speed taxi he found interference that caused the intermittent problem. A small adjustment snd the interfering part no longer doing so.  All stuff that isn’t in manuals. Anyway, what would probably bore a lot of people I’m fascinated by.

 

Great video. Screen shot of the interference coming up over canopy area. I’ll keep going on video…
 

 

 

[Ian]

9 hours ago, Old Koreelah said:

after several phases of modifying my Jodel Babe it started looking a little like his amazing AR-5

After watching his video on wheel pants I'm noting the flush sides on your wheel pants and the design of the engine cooling