How low can propeller RPM really go?

[The Wizzard]

Here is the case. Whirlwind 200RV prop, IO-375-M1B.

2350 RPM, 70% power 164KIAS @ 3000 msl FF 11.3 GPH

2000RPM, 63% power 160KIAS @ 3000 msl FF 9.2 GPH

The operating limits on prop are 2050-2300 no continuous operation but nobody ever talks about 2000 or 1900. An email was sent to Jim the main engineer at Whirlwind and his explanation is there has not been any testing done in that range but he operates his prop routinely around 2200 RPM.

This prop has very low drag and my thoughts are if you can take the biggest bite then why not? Any thoughts on the above? Not good for the engine, to hard on the prop governor, bad for the prop? Help! I'm listening, the fuel saving outweigh the speed lost but what am I missing.

Dave

[RVG8tor]

I wondered about this as well. So far my smooth opertion tends to be 2350-2400 but I have stayed way from the caution zone. Nice to know others operate in there. I would be curious as to the answer to your question.

Cheers

[RV10inOz]

I have no specific knowledge of the WW props, but I would be brave enough to assume it is all about resonance at those speed ranges.

There is nothing wrong with using 1800 RPM on descents, but for normal cruise operations in your case use any RPM that is smoothest around say 2400 RPM- 2500RPM.

I would imagine you have a nicely built engine with good F/A ratios and should be cruising LOP.For high HP operations make sure the LOP ops are at the appropriate amount LOP, that may be 60-80dF for example.

There is a nice band of RPM missing for you unfortunately, but work around it.

This is where a really good education in the combustion process and how MP/RPM/Mixture all inter-relate and what the effects are is useful in working out what is best for your engine/prop on the day and the mission you have that day.

There is no single recipe to offer here.

The fuel savings are there, you just need to operate it the right way.

[The Wizzard]

Dave, you mention the boys in Ada. What are you referring to?

Dave

[jrs14855]

Tornado Alley turbocharging

[roee]

I have a friend who regularly cruises up high, LOP and with the prop (WW 200RV) set to 2000 RPM. Gets good fuel efficiency, and hasn't fallen out of the sky yet. Maybe he'll chime in with more info.

[Walt]

Lycoming recommends the following in their operation manual:

Performance cruise 75% - 2450 RPM
Economy cruise 65% - 2350 RPM

You rarely see a performance chart with anything below 2100-2200 RPM.
I feel like the engine is beginning to "lug" at those low RPM's unless you keep the MP down as well.

[Toobuilder]

Originally Posted by The Wizzard

...This prop has very low drag and my thoughts are if you can take the biggest bite then why not? Any thoughts on the above?..

Why not? Let's take the "biggest bite" to the extreme... Fully feathered, 0 RPM... Or another example.... Is flying your airplane at max AOA (biggest bite) the most efficient way to get around?

I think the real goal is to make the most thrust per unit of fuel, and that has many variables.

[mike newall]

Use the rpm like you would a gearbox........

Oh, I forgot, you drive automatics

Well,

I drive a Toyota Hilux 3.0ltr diesel automatic.

It is like a Tacoma, with an oil burner engine.

So, when you go down hills, use lower gears, when you get stuck in snow, use lower gears.

That's the funny letters and numbers next to D.......

A constant speed prop has a wide variety of selectable rpm's and they are all appropriate to various modes of flight.

Take Off - full power, full fine - 2700 rpm.

Max power climb - 25/25

Cruise climb - 23/23

Fast cruise - 23/23

Slower cruise - 22/22

Descent - ahhhhhhhhh

Bring the prop back with lower power - 2100, 2000, 1900 but with no real manifold pressure.

Into the pattern, bring it back to 2300rpm, but - leave it there unless you are performance limited by either runway length, weight, temperature etc.

Figure this - if you run a fixed pitched prop, it will run 2300rpm a lot of the time.

It is a good figure for an approach, especially with a performance prop like a 3 blade Whirlwind, MT or modern blended airfoil prop.

Finally,

If you are - forgive me - doing a low pass.

Get the prop back to 2000, minimal or moderate power - it sounds soooooo good as you grumble by.

Just a thought

[skylor]

Originally Posted by mike newall

Max power climb - 25/25

Cruise climb - 23/23

Noise abatement procedures aside, why pull it back for max power climb? Max power climb should be WOT, 2700 RPM (maybe 2650 for some props), lean as necessary.

Skylor