Originally Posted by Professor

I why don't you do this next experiment!

I'd love to.
Dont have my plane back yet and there is no 33x10 to try.

Maybe I am wrong in my thinking but, wouldnt a larger diameter prop have less tip speed because the overall RPM is lower than a smaller diamater prop?
Dont we run the 30x12 to get more RPM's compared to a 32x10?

It's a shame that Mejzlik doesn't make a 31X12

I would bet that the 33X10 will be really loud. Like running a 27X10 Vs. 28x10 on a Da100. The 27 is much quieter due to the tip speeds. I think the 10 pitch isn't enough.

Anyone tired a Vess 32D on a 170 pipe combo?


Nice work Kurt. Can you take a picture of the airmodels 31X12 and post it. I have only seen their 3 blades.

Why is more RPM = to less noise?
Slow the prop down, slow the tips down = less noise right??

What am I missing?

Originally Posted by Wayne

Why is more RPM = to less noise?
Slow the prop down, slow the tips down = less noise right??

What am I missing?

I am curious to, as my 32x10 is definitely quieter than my 30x12.

The radius of the prop disk has a large impact on the prop tip speed per RPM. A 30x12 on my 150 has a different sound, and I consider it to be quiter, than a 32x10. Even though the 30x12 turns faster, the radius is smaller. Same reason why the tips on a full scale prop go super sonic and make lots of noise at closer to 2700 rpm rather than 6000 on our models.

Originally Posted by Wayne

Why is more RPM = to less noise?
Slow the prop down, slow the tips down = less noise right??

What am I missing?

Hi Wayne

Find attached a chart that may help understand the tip speed versus rpm on a prop.

Basically, you have to turn a smaller prop eg, 30x12 at a higher rpm to a 32 inch prop to get the same tip speed.

It is said that a tip speed of 0.7mach is getting loud.

This is why 3 blades are so quiet (perceived) is due to a smaller diameter again.

As the chart suggests:
28 inch prop at 6400rpm
30 inch prop at 6000rpm
32 inch prop at 5700rpm

all equal about the same 'tip speed' at 0.7 mach.

I used the word 'perceived' because a noise meter and our ears hear two different things.


cheers

Anthony

P.S Love your Godfrey 300

Here is a picture of all the props:
From the top_
Airmodels 31x12
Mejzlik 30x12
Mejzlik 32x10
Mejzlik 32x12
Mejzlik 28.5x12 3 blade
As you can see the blade of the Airmodels is narrower at the tip.
Earlier this summer (after running all of the other Mejzlik props) I told DA that I thought a Mejzlik 31x12 would be an ideal prop for the DA170. Maybe they can get Mejzlik to make a few for testing purposes. The only other production Mejzlik prop that I have heard works well on the 170 is the 29 inch N 3 blader.

Originally Posted by MattyMatt

It's a shame that Mejzlik doesn't make a 31X12

I would bet that the 33X10 will be really loud. Like running a 27X10 Vs. 28x10 on a Da100. The 27 is much quieter due to the tip speeds. I think the 10 pitch isn't enough.

Anyone tired a Vess 32D on a 170 pipe combo?


Nice work Kurt. Can you take a picture of the airmodels 31X12 and post it. I have only seen their 3 blades.

Wayne,
Anthony provided the correct info in his chart. Basically the tip speed increases linearly with prop diameter (at a constant RPM) because the distance the prop tip moves per rotation = 3.14 x Prop Diameter. Of course as diameter and pitch increase the rpm drops (and there are no equations that can predict this real accurately - basically we have to do our own experiments).

To make matters worse for testing all of these tuned pipes, the header length should be optimized (tuned) for each particular prop and each type of tuned pipe. The number of experiments starts to become VERY large. If you want to test 3 types of tuned pipes, 6 props, for 4 different header lengths that would be a total of 72 different tests and ground testing alone doesn't really tell you that much since the props have different loads on them in the air. The other option would be ground testing in a wind tunnel.....

Originally Posted by Wayne

Why is more RPM = to less noise?
Slow the prop down, slow the tips down = less noise right??

What am I missing?

Thats why it's nice that you guys do all the testing and we reap the benefits at less expense.
Thanks for doing this guys.

If I would switch from Krumscheid can's to the MTW RE3 tuned pipes, how much would the difference be? (rpm) Any idea?

Thanks.

kurt,

can you give a listing of what the rpms were with each of the props you used please?

i'm guessing that the 32x12 was too much load and thats why you wanted to try a 31x12??

thanks
tony

Hi Guys - just to add what I can, the 29x12N 3-blade Mezjlick was 6100 rpms on my DA170, standard header, Greve pipe combo. The PT Models 32x12 (which looks similar to the Mezjlick 32x12) was 6020 yesterday (cold). All rpms with redline 40/1, 91 octaine.

I would like to know where other DA 170 users have thier low and high speed needles set for the Greve combo.
Thanks,
Jim

Originally Posted by Wayne

Thanks Kurt.

Are the blades on the Air Models pretty thin compared to a Mejzlik?
I am hearing a 33x10 prop is coming from Mejzlik.
I am hoping that will quiet down my 170.

That might actually be louder Wayne. Goes supersonic at a much lower RPM that the 32x10. I found that out the hard way with a 33x10 Fuchs compared to a 32x10 Fuch prop.

Ok I did some simple calculations and I got you a formula for rpms to make speed of sound this is approximate

Speed of sound used was 767 mph

RPM that reaches speed of sound = 257815/prop length in inches


If you want the rpms to be 70% speed of sound multiply the answer by 0.7

So a 32" prop reaches speed of sound at ~8056 RPM and is .7 mach at ~5639 RPMs

33" mach at ~7813 RPM and .7 mach at ~5469 RPM

This is a question for Pe Reivers. I'm thinking about the RE2 tuned pipe for a Kroma 100. You mentioned in another thread that a tuned pipe would do well with a 15 in. header. Would you reccomend a 15 in. header for this engine. If so, where do I measure from & to for the header length? Is it from the piston face to the front of the tuned pipe tube, or from the face of the piston to the baffle inside the tuned pipe?
Thank you in advance for the info.