Flying A Mac @ 3,000 RPM

[Ron Iaconis]

I just got a hand held tach and a tiny tach,,,,and realised that I am flying my Mac at 3,000 RPM !!!!!!!!!!!!!!!!!
How bad can that be????????????
I have been flying my machine and felt that my expansion chambers were not designed correctly,,,,but in fact My home made prop is designed incorrectly,,,,now,,,,I am going to re-shape it,,,,as soon as I understand what my problem is in the question of pitch / diameter, and chord?
I have another blank of white oak that is waiting to be carved but I am going to practice on my red oak one first,,,,,,,,,,
Gee,,,,,having instruments seems to be somewhat of a helper,,,,huh?????

[Passin' Thru]

Hey Ron, What does it turn static, full throttle? Do you have access to a "known" prop you could mount for comparison? It's good to have some sort of base line to work from.

Have you checked your ignition timing? I once had some simular symptoms and found my ignition had "somehow" slipped retarded.

Expansion chambers can be problematical, I often think they are more art than science. The best performing ones are rather "peaky" and work within a rather narrow RPM band. In your instance, You may be loading the engine just enough to prevent it coming up "on the pipe". In such a case the engine will actually produce considerably less power than a more conventional type of exaust. Also, with a true "tuned" pipe even a small bit of damage to the end of the "stinger" can throw it off peak.

Although it may very well be you have just plain over proped the engine, I would run some test to verify what you really need in your new prop.

Keep us posted with photos and specs on the new one. You do nice work!

[Ron Iaconis]

I have thrusted another prop before but my scale broke,,, and I was just " wingen" it! Now I have a 440 pound scale,,,for thrust testing,,,,and I do know that the engine and carb and pipes should be fine,,,,my new prop was never tested regarding thrust but soon I will bench mark it with my new scale. But truely I had a suspicion about my prop,,,,it's either too heavy and leading edge too blunt,,,,or pitch is too much,,,,I will work on it when I get free time,,,and will post results,,,,Thanks for the " food for thought"

[Caribean_gyro]

looks you have too much pitch. With a 50x30 we get 3600. with 50x28 we get 4100.
so your RPM is way low

Chuck

[Ron Iaconis]

Yes, I know I have a prop problem because it's my first 100 percent built from scratch and I am still learning and it's totally enjoyable seeing how/what/why a prop is what it is,,,,so,,,,I will be retooling/carving/sanding the prop before I start on my other started blank,,,,it's very educational and enjoyable to learn about wood props.....
My 50X28 it good,,,,but when I go to the 52X26 it's not working as well,,,,,so , my learning curve will be directed towards the 52 inch props,,,,,,and if all fails I will return to the 50 inchers....

[Timchick]

What's the Max recommended RPMs for a Mac?

[MMorgan]

Tim...it's the rpm right before it has it's "mac-attack."

[Fl90]

4100 rpm is where the 72hp is rated

[Timchick]

Originally Posted by MMorgan

Tim...it's the rpm right before it has it's "mac-attack."

Arrrgh! That's what I forgot during my Mac Attack, to look at the tach. I'll remember to look next time.

Phil, Thanks.

[Rick Whittridge]

Tim, Don`t worry too much about your tach, Watch your EGT ! THis will tell you when your running hot.

[Timchick]

Rick,
My EGT looked OK at the time. Hopefully it was a fluke thing and I won't go through that again.

[Ron Iaconis]

What I " LIKED " about the 3000 RPM is that when flying ( And I have no idea how it flew like that),,,,but the CHT was at 390 to 405 degrees and I thought to my self,,,,,man I have a good prop, here,,,,,,but when only flying at a cruise RPM ( 65-70 percent power) ( comparing 4000 RPM to 3000 RPM is a 25 percent reduction,,,,,,relating to a 70 percent power reduction( an estimate) THAT'S WHY MY CHT WAS SO LOW !!!!!!!!!!!!!!
Flying at 4100 rpm will take a mandatory RE-JETTING of my carb,,,,,from .063 to probably .066 see???????? That's what I am figuring,,,,,,maybe,,,,,,,Because when I had a different prop that produced about 285 pounds of thrust( I didn't know what the RPM's were at that test,,,just the static thrust) my temp was at 450 degrees,,,,,,,,so,,,,as soon as I fix the RPM prob,,,,I most assuredly will need to Re-Jet