Cooling Vents
Posted: Thu Apr 01, 2021 3:21 amSorting out baffling, mixture and exit cooling slot - with the onset of spring into summer - I'm pretty close to being able to climb full power at Vy on a hot day without cooling issues - but Vx - not so much. (AeroVee 2.1) I've been contemplating adding the Sonex B cooling vents on either side of the cowl. Was measuring up for same - though I was going to make my own. Then in my "spare bits that might come in useful someday box" - I found a couple of smart stainless louvered vents from a yacht project. So I thought I might try them. Couple of quick holes in the cowl and some rivets and I have a working setup. They certainly keep things MUCH cooler. But the louvres sticking into the slipstream adds a bit of a rumble - and I've yet to measure the speed cost - but it is there.
Then while testing some more I heard some whistling tones. I was a little concerned I has a canopy gap issue developing - but after looking around realized it was not that. It seemed to be airspeed and altitude specific (more on that follows). And then I realized - the cooling louvres were acting as "fipples" - like on a penny whistle and resonating musical notes.
After a little head scratching, some discrete and very slight bending of the louvres and more than just a few test flights I discovered a number of things:
- I could get 10 distinct notes from the two louvres (2 x 5 slots each) without the louvres looking markedly different to the eye. Most of the metal bending to tune each louvre takes place inside the louvre.
- The onset of the notes was airspeed specific at 63KIAS (on my EFIS - I'm sure the actual CAS/TAS is slightly different)
- The plane must be absolutely coordinated with smooth air blowing down either side of the cowl. A slight skid or slip negates the effect.
- And actually that is not quite true. I'm thinking the swirl of air from the left turning prop is why it requires I put in an almost imperceptible amount of right foot and skid to get both sides to "sing" equally.
- The notes also required a specific DENSITY altitude of around 3,700ft. Took me a while to figure out it is air DENSITY specific and not MSL altitude specific. There is probably a variable for humidity too - but so far if I get to 3,700ft DA it's close enough to get the louvres to "sing" by climbing and descending slightly till they do.
- It is also possible to "overdrive" the louvres and get them to jump up an octave at 97KIAS. Just like overblowing a penny whistle.
I did a little experimental blocking of each louvre to see how much they affected each other and the answer is "not much". So now I am looking to add small flaps that will open and close each louvre. They will need to be set on some kind of near balanced pins to require the least actuating force and I am thinking of using fast acting RC servos to move the flaps. Then I can wire the servos to a simple set of switches and after climbing out with them all open - and leveling off - possibly play some tunes.
The 10 note range is a bit limiting - but not impossible for some tunes. The possibility of being able to play "Taps" while another plane flies a "missing man" at memorial flypasts being one application that springs to mind.
- Overall Look
Then while testing some more I heard some whistling tones. I was a little concerned I has a canopy gap issue developing - but after looking around realized it was not that. It seemed to be airspeed and altitude specific (more on that follows). And then I realized - the cooling louvres were acting as "fipples" - like on a penny whistle and resonating musical notes.
After a little head scratching, some discrete and very slight bending of the louvres and more than just a few test flights I discovered a number of things:
- I could get 10 distinct notes from the two louvres (2 x 5 slots each) without the louvres looking markedly different to the eye. Most of the metal bending to tune each louvre takes place inside the louvre.
- The onset of the notes was airspeed specific at 63KIAS (on my EFIS - I'm sure the actual CAS/TAS is slightly different)
- The plane must be absolutely coordinated with smooth air blowing down either side of the cowl. A slight skid or slip negates the effect.
- And actually that is not quite true. I'm thinking the swirl of air from the left turning prop is why it requires I put in an almost imperceptible amount of right foot and skid to get both sides to "sing" equally.
- The notes also required a specific DENSITY altitude of around 3,700ft. Took me a while to figure out it is air DENSITY specific and not MSL altitude specific. There is probably a variable for humidity too - but so far if I get to 3,700ft DA it's close enough to get the louvres to "sing" by climbing and descending slightly till they do.
- It is also possible to "overdrive" the louvres and get them to jump up an octave at 97KIAS. Just like overblowing a penny whistle.
- Dimensions that produced the effect
I did a little experimental blocking of each louvre to see how much they affected each other and the answer is "not much". So now I am looking to add small flaps that will open and close each louvre. They will need to be set on some kind of near balanced pins to require the least actuating force and I am thinking of using fast acting RC servos to move the flaps. Then I can wire the servos to a simple set of switches and after climbing out with them all open - and leveling off - possibly play some tunes.
The 10 note range is a bit limiting - but not impossible for some tunes. The possibility of being able to play "Taps" while another plane flies a "missing man" at memorial flypasts being one application that springs to mind.