Alternative Fuels Are Not The Future

[Ted]

You're missing part of the equation: The radius of the prop. Larger props have to turn at lower RPMS to keep from going transonic, and it's really the tip speed that's causing prop noise, not the RPM. For example, if you took that same PT6-67B/P and spun it at 2700 RPM, it'd sound like you were tearing the sky apart even though it's running at the exact same RPM as many light GA singles. If you took an IO-360 and spun it at 1700 RPM it'd be significantly quieter than the PT6.

It's a combination of the engine itself, tip speed, and RPM. Low RPM props are favored on turbines because you need a gearbox anyway, and the lower RPM is quieter and more efficient. 100% prop speed on a TPE-331 is about 1500 RPM, and on the PT-6s I recall 2200 RPM, at least the ones I flew. A 421 does similar. 2166 is redline prop RPM as I recall, with cruise in 1600-1900 RPM range. Multiply by 1.5 for engine RPM, and obviously a 3k+ RPM prop would be both loud and inefficient.

The 2850 RPM redline on my 520s isn't optimal, but it does wake up the neighbors when I have 4 AM departures.

 

[Henning]

It's a combination of the engine itself, tip speed, and RPM. Low RPM props are favored on turbines because you need a gearbox anyway, and the lower RPM is quieter and more efficient. 100% prop speed on a TPE-331 is about 1500 RPM, and on the PT-6s I recall 2200 RPM, at least the ones I flew. A 421 does similar. 2166 is redline prop RPM as I recall, with cruise in 1600-1900 RPM range. Multiply by 1.5 for engine RPM, and obviously a 3k+ RPM prop would be both loud and inefficient.

The 2850 RPM redline on my 520s isn't optimal, but it does wake up the neighbors when I have 4 AM departures.

Don't you have enough runway to make 2700rpm departures? That would quiet you down considerably.

 

[Ted]

Don't you have enough runway to make 2700rpm departures? That would quiet you down considerably.

I do, and furthermore I think 2700 RPM wouldn't reduce thrust any because of the inefficiency of the props. I've been considering switching to that.

 

[Henning]

I do, and furthermore I think 2700 RPM wouldn't reduce thrust any because of the inefficiency of the props. I've been considering switching to that.

I always reduce power/noise as much as feasible when I'm flying hours I would prefer not to be woken up during. Thats the thing with a twin, as long as you have two running, you have a lot of excess horsepower you don't really need.

I just pull the props back before I advance the throttles for the runnup, then give it 20" and advance the props to runnup RPM and then bring them up to T/O RPM at the end of the runnup. If I have an issue, the amount of time it takes to get full RPM back is insignificant.

I used to fly out of Long Beach at 2400 at night in my Travelair and never tickled the noise monitor mics.

 

[Cpt_Kirk]

You're missing part of the equation: The radius of the prop. Larger props have to turn at lower RPMS to keep from going transonic, and it's really the tip speed that's causing prop noise, not the RPM. For example, if you took that same PT6-67B/P and spun it at 2700 RPM, it'd sound like you were tearing the sky apart even though it's running at the exact same RPM as many light GA singles. If you took an IO-360 and spun it at 1700 RPM it'd be significantly quieter than the PT6.

Yeah, you're right. I did miss that part.

 

[Ted]

I always reduce power/noise as much as feasible when I'm flying hours I would prefer not to be woken up during. Thats the thing with a twin, as long as you have two running, you have a lot of excess horsepower you don't really need.

I just pull the props back before I advance the throttles for the runnup, then give it 20" and advance the props to runnup RPM and then bring them up to T/O RPM at the end of the runnup. If I have an issue, the amount of time it takes to get full RPM back is insignificant.

I used to fly out of Long Beach at 2400 at night in my Travelair and never tickled the noise monitor mics.

Keep in mind I can't hold the brakes to full throttle - the plane is overpowered and the brakes won't hold. So I have to advance power and then reduce RPM, or else have the governors adjusted. On a 2800 ft runway there's a lot that happens in a short period of time, so it's not too wise to mess around with stuff and distract yourself. My safety is more important than the neighbors sleep.

But I do think I'll try it at a few longer runways and see what I can determine.

 

[Henning]

Keep in mind I can't hold the brakes to full throttle - the plane is overpowered and the brakes won't hold. So I have to advance power and then reduce RPM, or else have the governors adjusted. On a 2800 ft runway there's a lot that happens in a short period of time, so it's not too wise to mess around with stuff and distract yourself. My safety is more important than the neighbors sleep.

But I do think I'll try it at a few longer runways and see what I can determine.

You can set your RPM without going to full throttle. 20-21" should be all you need to get into the govenor. You can work your way forward working between the 2 to find you minimum manifold pressure that will get you to the full governor. It was around 20-21" on my 470s to 2400, and I would set the governors at that at the end of my runnup.

 

[Ted]

You can set your RPM without going to full throttle. 20-21" should be all you need to get into the govenor. You can work your way forward working between the 2 to find you minimum manifold pressure that will get you to the full governor. It was around 20-21" on my 470s to 2400, and I would set the governors at that at the end of my runnup.

Nope, not on my plane. 520s and 3-bladed props. 20" is 2200 or so.

 

[mattg]

I want diesels setup to burn Jet-A! I'm thinking I'd like to build an RV-10 around one but so far I haven't found an engine that I think fits.

the turboprops are just too damn expensive and are too thirsty.