Prop Question

[RFazio]

I have an Aymar-Demuth 68" D X 78" P prop on my 6 with an O-360. It's flys great and is very smooth. She revs to 2400 on the ground static. She'll red line at full throttle at 2500 feet, 2700 RPM at 203 MPH or so indicated. I tried climbing to 7700 ft where supposidly full throttle will give you 75% power. I went full throttle, leaned out and she still red lines right around 2700, mabye 2725 RPM. Is that what's supposed to happen? I somehow thought she would be running around 2400 RPM at altitude. How can I cruise at altitude 75% power if it revs that high? Do I need more prop? Or am I going about this the wrong way.

[PCHunt]

I wish mine did that. Sounds like to me you have the perfect prop!

[RFazio]

What kind of numbers are you getting?

[Geico266]

A fixed prop is always a compromise, but I think you nailed it! You might be just a tad "undersized", but I certainly would not spend the money on another prop. What ever you gained would be minimal. Just enjoy the prop & engine combo you have and call it good. Your short feild performance must be awesome!

[N941WR]

Originally Posted by RFazio

...I tried climbing to 7700 ft where supposidly full throttle will give you 75% power. I went full throttle, leaned out and she still red lines right around 2700, mabye 2725 RPM. Is that what's supposed to happen? I somehow thought she would be running around 2400 RPM at altitude. How can I cruise at altitude 75% power if it revs that high? Do I need more prop? Or am I going about this the wrong way.

2700 RPM at that altitude IS 75% power.

With the thin air at that altitude the engine can't produce 100% power. Thus to get 75% you have to spin it up, which is normal. These Lycoming engines are designed to run at that RPM all day long.

Check your engine operating manual.

BTW, Don't compare this to guys running constant speed props. To get the same MP you are seeing they can change the pitch of the prop and load the engine down by slowing down the RPM's.

[Kyle Boatright]

Originally Posted by RFazio

I have an Aymar-Demuth 68" D X 78" P prop on my 6 with an O-360. It's flys great and is very smooth. She revs to 2400 on the ground static. She'll red line at full throttle at 2500 feet, 2700 RPM at 203 MPH or so indicated. I tried climbing to 7700 ft where supposidly full throttle will give you 75% power. I went full throttle, leaned out and she still red lines right around 2700, mabye 2725 RPM. Is that what's supposed to happen? I somehow thought she would be running around 2400 RPM at altitude. How can I cruise at altitude 75% power if it revs that high? Do I need more prop? Or am I going about this the wrong way.

The first question is do you know what your manifold pressure is at those RPMs and altitudes? That might give you a better indication of how much power the engine is producing at various settings.

In the end, I think you'll find that your prop is about right for your airplane. If you increase the pitch, your takeoff and climb performance will suffer.

As a data point, my airplane has an 0-320 and an Aymar/Demuth prop and has a static RPM of 2100-2200 rpm. I can redline the engine at any altitude below about 7,000' under standard conditions. The engine/prop/airframe combo delivers about 175 knots (true) at 1,000'. In cruise, I typically fly at between 2350 and 2450 rpm and 21" or 22" of MP at 8,000'. That translates to 65-70% power and 155-160 knots.

[hevansrv7a]

Originally Posted by N941WR

2700 RPM at that altitude IS 75% power.

With the thin air at that altitude the engine can't produce 100% power. Thus to get 75% you have to spin it up, which is normal. These Lycoming engines are designed to run at that RPM all day long.

...

BTW, Don't compare this to guys running constant speed props. To get the same MP you are seeing they can change the pitch of the prop and load the engine down by slowing down the RPM's.

If it takes full throttle and 2700 rpm to get 75% at 8000 MSL (pretty much true) then if you have a CS prop and you increase pitch to get, say, 2500 rpm at the same altitude, then where did the increase in MAP come from? For a normally aspirated engine, MAP cannot exceed ambient more than a slight effect from ram-air and that's the same for both props anyhow. It seems to me that 2500 rpm at 8000 MSL will not get you as much HP as 2700 rpm at 8000 MSL, assuming that in both cases the throttle is wide open.

[n5lp]

Yes, if you are going for maximum cruise speed, without exceeding engine RPM redline, you have, essentially, the perfect prop. The pilot of an airplane with a constant speed prop would do exactly the same thing for maximum cruise speed at 8,000 feet. They would run 2,700 RPM at maximum MP, full throttle. The MP will be the same at full throttle, regardless of prop type.

By the way, very few people actually run maximum cruise speed routinely. The pilots of airplanes with constant speed and FP props usually dial things back a bit. The difference is that with a FP you have to dial both MP and RPM back while the pilots of constant speed airplanes can leave MP at maximum and just dial back RPM.

[N941WR]

Originally Posted by hevansrv7a

If it takes full throttle and 2700 rpm to get 75% at 8000 MSL (pretty much true) then if you have a CS prop and you increase pitch to get, say, 2500 rpm at the same altitude, then where did the increase in MAP come from? For a normally aspirated engine, MAP cannot exceed ambient more than a slight effect from ram-air and that's the same for both props anyhow. It seems to me that 2500 rpm at 8000 MSL will not get you as much HP as 2700 rpm at 8000 MSL, assuming that in both cases the throttle is wide open.

I should have said the max MP comes at a lower RPM due to the change in pitch. When comparing engines with a CS vs FP prop, the MP will be higher at a lower RPM with the CS prop. With the FP prop, you need to spin it up to get max MP. You are correct, you do not get a higher absolute MP with a FP prop.

[pierre smith]

.....Richard. Exactly what my Catto revs at that altitude and also 202-203 TAS.

Regards,