Cowling engines

Ronnie L

Member
Joined
Jul 12, 2004
Messages
131
Location
Rossendale, Lancashire,England
Aircraft
RAF2000
Total Flight Time
450hrs
I would like to ask a question of Chuck Beaty or one of the other technical folks.

If I cowled the engine on my RAF 2000 similar to a M24 or Zenon.
Would it make a noticeable diference to performance and noise?
It would be wonderful to have more performance but to cut down on the RAFs horrendous noise would be even better especially for the neigbours.
Any of your thoughts would be interesting.

Thanks

Ronnie
RAF 2000 G-BYIN
 
A cowl can make a big change in noise & speed-But it has ti "flow" make tha air do what you want- Did a cowl on a Casset racer-It added 35 kts with just a few tweeks & fences.
 
You mean something like this? On my little single Bee I gained about 5 -7 knots efficiency at 60 mph, meaning my throttle setting to break through the aerodynamic drag threshold of 60 mph was reduced by several hundred RPM and consequently less fuel burn. Before, it seemed like I was really pushing the throttle to break through 60 mph, now it is fairly inconsequential. It handles a hell of a lot better, with much less turbulence at that speed, something that always bothered me previously on the little Bee. On a gyro with a great big side-by-side enclosure like an RAF or Sparrowhawk I imagine you would realize significant improvements in AS, handling, and noise if you do it right.

I can't for the life of me figure out why we don't see more aerodynamic streamlining of gyros, they are the draggiest aircraft out there. The trade-off in weight is insignificant compared to the improvements in speed and handling.

Streamlining the aft section of an aircraft is usually more important than the front. Now, you've got that big bubble that is aerodynamically design for what hits you in the face, why not finish the job and clean up that mess behind you.
 

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Nowings- I was very impressed seeing your cowling at Bensen Days. I was studying it and wondering how you molded it to fit the several parts it seemed to custom fit around. That was real ingenuity on your part seeing that snowmobile cowling and adapting it to wrap your engine. Stan
 
I should use it for a mold and start selling them!
 
Hilberg / No wings

Thanks for replies. It seems like it would be worth the effort if the difference is so noticeable.
I would have to make plugs and take moulds from them as I don't rekon I could find something to adapt.
The RAF would look pretty cool with the engine cowled in too.

Ronnie
 
Some vague opinions

Some vague opinions

In my opinion, converging at more than 7 degrees per side, 14 degrees included loses control of the air and generaly doesn’t work.

I feel because a propeller sucks it is possible that a steeper angle will work but not much steeper.

I would be concerned that the cowl may affect the performance of the rudder so careful first flights are in order.

As I recall an interesting finding in the wind tunnel in Ottawa on the streamlining of bluff bodies, a bus was that making the back 20% converge at 7 degrees reduced the drag by 18%.

Preparing for Bonneville we have taken vehicles on top of a windy hill and used highway flairs for smoke to see what the wind is doing.

We have also used a piece of yarn on a stick to investigate possible areas of improvement.

We have taped short, 3 inch, pieces of yarn to bodies and taken pictures at speed.

Coast down tests are illuminating but would probably not work well on a short wheelbase RAF.

We always found tests to be more instructive and easier to conduct than constructing something based on fantasy and conjecture.

Good luck with you project, Vance
 
Hi Vance

Thank you for your illuminating thoughts.
You have inspired me with thoughts of windy places and smoke generators.
Don't understand why an RAF would be so much more noisey than an M24 though. Unless it is from better airflow into the prop.
What do you think?

Many thanks

Ronnie
 
Hi Vance

Thank you for your illuminating thoughts.
You have inspired me with thoughts of windy places and smoke generators.
Don't understand why an RAF would be so much more noisey than an M24 though. Unless it is from better airflow into the prop.
What do you think?

Many thanks

Ronnie

Probably Propeller tip speed Ronnie.

Aussie Paul. :)
 
Hello Ronnie,

We played with noise a lot.

We found noise production was cumulative.

I feel that tip speed is one of the things that make a lot of noise.

The lower tip speed of the M24 may make quite a difference.

Most of the modified RAFs I have flown seemed loud even at the lower RPMs.

In my opinion the interrupted air flow adds to the cacophony and a cowl may change that, hopefully for the better.

I have found that the study of air flow and sound production is sufficiently complex to hold my attention for some time.

These are not subjects I could master but I did find confusion on a higher level.

I found when testing that a flat surface taped up from cardboard was very close to a nicely curved one as far as drag goes.

I also found it was easy to misconstrue the test results.

Thank you, Vance
 
Vance / Paul

On the subject of prop tip noise.
When I bought my RAF kit I used the 2.37:1 re drive and 4 blade Warp Drive as Paul suggested. There is a considerable reduction is noise using this system, I guess it is due to lower tip speed....but the same machine with a 2.1:1 re drive and a 2 blade Prince P tip sounds less intrusive from the ground but more harsh sounding to the pilot.

Is it possible that some of the noise is generated by shock waves from the prop tips hitting the side of the keel?

A couple of years ago we experimented with a smoke generator, one of the lads put it on you tube. It is titled RAF2000 prop airflow test. There is part 1 and 2. the smoke generator is taped to the keel. At some points the smoke goes through the prop tips the wrong way then through the correct way at about half way down the blade.
This may make sense to you but it confused me.

Ronnie
 
Vance / Paul

On the subject of prop tip noise.
When I bought my RAF kit I used the 2.37:1 re drive and 4 blade Warp Drive as Paul suggested. There is a considerable reduction is noise using this system, I guess it is due to lower tip speed....but the same machine with a 2.1:1 re drive and a 2 blade Prince P tip sounds less intrusive from the ground but more harsh sounding to the pilot.

Is it possible that some of the noise is generated by shock waves from the prop tips hitting the side of the keel?

A couple of years ago we experimented with a smoke generator, one of the lads put it on you tube. It is titled RAF2000 prop airflow test. There is part 1 and 2. the smoke generator is taped to the keel. At some points the smoke goes through the prop tips the wrong way then through the correct way at about half way down the blade.
This may make sense to you but it confused me.

Ronnie

Ronnie, after the 2.37 and the 4 blade WD I swapped the WD to wide chord 2 blade timber props. This configuration was definitely the best. Tried Prince Ptip first and then went home grown here in Australia. The Ptip was better than the 4 blade WD but produced a humming noise around 4100 I think it was. Lonnie and I were going to work on this but I had an incident and wrecked the Ptip!!!!!

I then found a great prop builder here in Australia who really did know his job.

Aussie Paul. :)
 
Alan

It would be interesting to know if the voretex generators had any effect on the noise and I will read the links you have left . Thanks.

Paul

Did the Aussie prop help with noise?

I found that my machine got off the ground better with the P tip but needed a few more revs in the cruise. ( they both gave 5250rpm static)
The P tip made a very harsh sound in the cab a sort of WHA WHA WHA.
Not a very good description but you get the idea.
Have you tried an in flight adjustable. Then you can have quick off the ground and lower revs in the cruise...

Ronnie
 
Alan


Paul

Did the Aussie prop help with noise?

Have you tried an in flight adjustable. Then you can have quick off the ground and lower revs in the cruise...

Ronnie

Yes Ronnie, the Aussie was the quietest I ever used. Unfortunately he does not make them any more as I need one for this RAF that I never seem to have the strength ability now to convert.

No, I have never tried an inflight adjustable. Too expensive for me at that time!!!

I played with vortex generators to get more air over the too small RAF rudder. We did not notice any improvement in noise.

Aussie Paul. :)
 
The loudest aircraft I have ever heard was a Harrier.

The second loudest aircraft was the Golden Butterfly, it's prop noise being so loud it would cause pain to spectators watching a flyby. I can never imagine an aircraft that loud being welcome at any populated airport.

Cowls can help reduce engine mechanical noise but aren't likely to effect exhaust and especially prop noise.

An effective muffler is needed for a quiet engine.

Prop design can make a difference in noise generated.

Some reduction of prop noise, at least at some angles of orientation, might be derived from a shrouded propeller.

http://www.rotaryforum.com/forum/attachment.php?attachmentid=30265&d=1170731682

.
 
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Mostly prop noise can be reduced by two factors, I have found:

1) Lower the rpm to reduce tip speed,
2) Make sure the prop sees as clean air as possible.

Number 1 is easily dealt with by using the largest diameter prop possible and/or increasing the number of blades.

Number 2 is an inherent problem in pushers. There's no ready made answer for this one that I know of.

-- Chris.
 
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