Here are a couple of examples of prop noise.

This is my ASP .91 on the test stand with an APC 11x7 prop.

In the video it is running at about 9200 rpm, with the muffler attached to a "noise trap" (a length of silicone feeds the exhaust into a metal fuel can)
In the sound file you can hear me removing the silicone hose from the muffler about 30 seconds into the recording.

IMO this shows that the best way to keep noise down, is to keep the rpm and the prop tip speed down...

[link=http://users.pandora.be/my_rc_world/IMAG0215.ASF]video sample[/link]
[link=http://users.pandora.be/my_rc_world/IMAG0216.WAV]sound sample[/link]

Winibald,


Isn't an APC 11x7 prop a bit small (typical English understatement... Read; way too small.), for an engine of this displacement?

Is is running at part throttle so RPM is kept down?

You might as well have a .50 sized engine on that prop.

The whole big deal about noise reduction in Europe is to run a larger than normal prop to keep rpm down. This serves three functions. One, the prop noise is less, and two, there are fewer exhaust impulses to contend with, and three, it makes the generated sound of a lower pitch. Whereas a 14-6 is the normal prop for a 91 engine, something like a 15-6 would be in order.

Enjoy,

Jim

I am running a new ASP .91 two stroke as well. It calls for a 12-6 prop. Yes,
it sounds small....that is a .60 size prop. Even this 13-7 was too much prop
for it....it detonates real bad. The reason is the engine has too much compression.
It blows plugs too. I had to drop to 5% nitro, and a cold plug to run it without it
banging away.

It blew a new Fox Miracle plug on the first run, and a new Rossi cold plug on
the second run on 15%. I am running a 12-7 on it now, and it is OK on 5% nitro.

Any load at all on the engine will cause it to detonate because it is over compressed.
That's the reason for the small props. It's really just a bored out .60 engine. It is
a short stroke engine.

FBD.

Hi!
Why don't you just run 80/20 fuel and mount a decent prop on it...12x7 is way to small for most aplications.

Regards!
Jan K
Sweden

Which ASP 91 are we talking about? The round head or the square head (S) version?

Enjoy,

Jim

Dave's words don't make this ASP sound very good.

An engine this size, which is supposed to be an OS clone, should not exhibit any over-compression issues, if the 'original' OS .91 wasn't.

As Jim wrote, anything smaller than a 14x6 (13x8, 15x4, 12x10...) is too small.

Most people flying Tai-Ji and similar planes here, with MVVS (and some other) .91 engines, with tuned pipes, use a 14x8 APC and get in the vicinity of 10,000 RPM. Overheating and detonation are not an issue, even for those using 10% nitro.

Has anyone ever worked with a prop design similar to the one used on the full scale C-130?
These props seem to make less noise and I wonder if it has to do with the fact that the prop actually gets a wider chord near the tip of the prop. Anyway, has anyone experimented with such a prop design?

3dbob

At the other end of the spectrum, my Webra .91 P5 turns an APC 14x12 at 8,400 rpm now with an hour and a half time on it and gets stronger each time I run it. This engine and its seemingly huge muffler are designed for exactly that, a huge prop in the mid 8s, on 5% nitro. It seems like American guys just can't accept that fact. And, Dar, the MVVS .91 comes very close to that.

Bob,


The Hamilton Standard props used on the GE T56 engines are old-tech.

They spin at just over 1,000 RPM, in all conditions and speed and load changes are only done by blade pitch changes.


The new C-130J uses different props. Very different, in fact.

See [link=http://ascpa.public.wpafb.af.mil/Images/News/C130J.jpg]this photo[/link], or [link=http://www.air-and-space.com/20040918%20Pt%20Mugu/DSC_3585%20C-130J-30%2001-1461%20115AS%20left%20front%20in%20flight%20l.jpg]this one[/link].

Boy, they do look different! They almost look like the props used on submarines.

Yes, I am aware that the turbine runs at a constant RPM and that only the pitch changes to increase or decrease thrust.
But, the question remains: Are the old or the new prop shapes quieter than what we are using on models and if so, has anyone experimented with that shape? By that shape, I mean a broader blade at the tip. I can see a lot of questions would come up as for example what RPM would a wide tip prop blade need to operate at and could we achieve that with a model?
This could get into some real Reynolds Numbers equations.

Anyone have any thoughts on this subject.

3dbob

Bob,


Even those old-tech props were very efficient.
The new ones are significantly more so, achieving a 50 mph improvement in cruising speed, with just a modest addition to HP.
At the same time, range is increased by 75%.

Were getting carried away... Back to models, those newer C-130J prop blades resemble APC.

I have a ringed ASP 91 series 3 - I think some older versions are ABC - this engine pulls a 14x6 in the high 9000s on 5% fuel. On a 13x6 the ASP sounds as if it is overrevving. I also have the MVVS 91 which is ABC. On the same prop the ASP pulls more revs. That is on all reasonable prop sizes.

Both these engines work well on 5% nitro or less. Neither respond well to small props: I think a 12x6 on an ASP91 would make a lot more noise than progress. I have had good results from both engines in 3D models using a 16x4 ASP at less than 9000rpm.

The 14x6 is below the recommended size for the MVVS .91,

MVVS 15 GFS/R - ABC - RC #3089 or 3089-H
1.8 kW @11000rpm
552 gr
carb 3222
Mufflers: See .61 engine
props: 14x7 to 15x10

The smallest recommended for my P5 is a 14x12

Not everybody flies "3D"...

It is the newest version of the ASP, the square one... I just got a couple of weeks ago from J'En.

The engine was intended for a prop jet... the heaviest prop it is going to see is probably an 11x9!
The 11x7 is a break in prop, and suits the engine just fine: turning up to 14000 rpm... while still rich. It is a ringed engine after all.

And Dave is right: the engine is over compressed, or perhaps the squish clearance is too large.
I have been running 80/20 castor fuel through it, it does not work on 15% nitro. After break in (if I can ever get it broken in) 2 or 3% of nitro is probably all that's needed to clean up the transition.

My ASP Redhead .91 likes to spin a 13-6 at around 11,500 with 5% nitro. It does good with a 14-6 also, it spins that one at just under and sometimes at 11,000. A .91 two stroke should be pulling a 14-6 well over that of a good .91 four stroke.

I have a .61 HP Gold Head that will spin an 11-7 prop at 13,800 on a good day. IMO that engine will need prop with more pitch to make your plane go fast! Maybe an 11-10 or 11-11.

I have two Super Tiger 90's with ASP 91 Red Head carbs on them. They do 11,000 with a 14-6 APC. They did around 10,400 with the Super Tiger carbs. 10% fuel

Temp 29ΒΊ F

Rudeboy,

Perhaps you are not too familiar with 91 size two strokes and their prop requirements?

Winibald,


In the 70s and up to about 1982, before RPM was pulled down by larger props and longer pipes, the better F3A .60 piped engines, used to spin an 11x7, 11x7.5or 11x7.75 prop at 14,500-15,300 RPM. So having a .90 engine spinning this size prop at this RPM, albeit rich, is nothing special.

If it is an engine intended for very high RPM (CMB, Picco, OPS, Rossi, Jett), it should show close to 16K on this prop size.
Now it becomes a blade throwing risk...