Power Settings

Most cubs have an O290, O320, or O360 engine all of which (I believe) are rated at 100% at 2700 RPM (no doubt I will be slapped with a wet noodle if I am wrong, and I am ready).

This means that the following (rough) table applies to all of these engines

Power % - RPM
95- 2675
90- 2600
85- 2550
80- 2500
75- 2450
70- 2400
65- 2350
60- 2250
55- 2200
50- 2125

Where do you run your cruise RPM at?

I was told with the borer you can run it higher, but it seems to me it still wears the engine as much as without...

sj

Steve, you are correct. RPM is RPM no matter what prop you have.

I usually run 2500 rpm 8244 prop.

rpm

I run my 0320 with 8241 borer prop at 2450, with an average fuel burn of 8 gal/hr. Just seems to to run the smoothest at that rpm !

Just thought I'd be a Smart A$$ I have a O-200 and set my cruise at 23" of MP. I don't really care what the RPM is.

Matt

Matt,

You told me about this when I was up at Brodhead, and it is intriguing. Is the basis of this that no matter what altitude you are at, you just set the same MP since with (fixed pitch) RPM you have to increase it with altitude?

What MP would set these other engines at? 23" or 24" is kinda standard for CS props, same on fixed?

sj

Someone is going to have to explain to me why any pilot with a fixed pitch prop would want to know what his or her manifold pressure is. I fly both. And I'll restate the obvious that an airplane with a constant speed prop has to have a Manifold pressure for engine power management, especially at low altitudes. Is it curiosity? With a fixed it is what it is any given altitude.

Jeff,

As you know, when you climb in a complex airplane, the manifold pressure drops about 1" for every 1000' you go up. The same thing happens in a fixed pitch airplane. As I understand it (which is a stretch in itself) to acheive the same power % (say 75%) you have to run at a higher RPM at a higher altitude to acheive the same MP. There are little charts that tell you the difference in RPM, I will hunt around for one...

sj

Or just run whatever power you need to do what you need to do that day. Alternatively, if you have a fuel flow instrument (well worth having one of these tiny little jewels, by the way), the fuel flow will tell you something about how much power you are using.

And, by the way, fuel is what generally precipitates these sorts of questions in flight, as in: "do I have enough?"

MTV

I dont think you have to worry about wearing your engine out if you run alot of rpms. The pipeline plane here in town is changing engines this week. 0320A2B, 7300 SMOH, 4000 STOH. Runs 2500+ with a 7454 prop. Probably the only reason their changing it is because weve had 3 days of 40kt wind and its snowing today so they got time to do it. Engine was ran just about everyday the last 5 years and never started cold. Used Phillips XC20/50 oil. Looks to me what wears your engine out is starting them cold and rust from not flying.

94SUPER18 said:

Someone is going to have to explain to me why any pilot with a fixed pitch prop would want to know what his or her manifold pressure is. I fly both. And I'll restate the obvious that an airplane with a constant speed prop has to have a Manifold pressure for engine power management, especially at low altitudes. Is it curiosity? With a fixed it is what it is any given altitude.

Click to expand...

I like to know what my engine is doing and a M/P gauge gives me a lot of information. 1. It's a quick way to detect carb ice (faster than a Tach). 2. Next time your at cruse on a smooth day ease the stick back( RPM will drop ) move the stick forward (RPM increase ) a M/P gauge would stay the same unless you hold a nose up or down position. On a rough day you can chase a Tach. setting all day long. 3. A good rule of thumb for a controlable prop is to square the numbers. On a fixed pitch prop it will tell you if you are over pitched (high M/P preasure low RPM) or the other way around. On the aircraft Matt is talking about at 2000 ft. we get about 100 RPM above the square. This can help answer questions about which is the best prop for my aircraft. 4. A M/P gauge is a fairly inexpensive insturment for a non electric aircraft that will give me a lot of information. If you take the time to check fuel burn at different M/P settings and altitudes it can be a excellent fule management tool in a fixed pitched aircraft. Just my openion but all my fixed pitch aircraft have one.

Si

W1Flier, couldn't have said it better myself!

Tim

Steve

Yes that is correct. As Si stated it is the best way in a non-eletric aircraft to set power settings when calculating fuel burn.

Matt

One thing to remember is that the 100% horsepower rating is not only at 2700 rpm, but also at sea level at 59 degrees.
Roger Borer's STC used to come with paperwork explaining that the longer prop caused lower manifold pressure (longer swing), thus most pilots will bump their cruise rpm up from a stock prop to regain the mp, i.e. horsepower.

One interesting exercise to determine loss of hp with density alitude:

1. determine density altitude
2. subtract 1" of mp for each 1000' of density altitude (for example, 8000' d.a. would be 30"-8=22". This is the MP you would expect at 8000' d.a.)
3. take your total certified hp (i.e.150) and divide by 30 to determine your hp per inch of MP (5 hp per inch man. p.)
4. multiply the 5 by the 22 to equal 110 hp (theoretical) at 8000' density altitude. Just a rule of thumb, does not include humidity or other performance factors.

Also, the experimental racers using 320 engines typically run them way over 2500 rpm, and they don't think it does any appreciable damage, particularly when operating at high d.a.'s when you can't get over 65-75% power anyway. True, though, that you are turning the internal parts faster!

The lycoming 0320 handbook has a very complicated graph to figure all that out. Way to complicated for me. Cubs weren't designed to be flown that way. ( in my opinion )

Dick,

It is also worthy of note, however, reference the racers that run those engines at very high rpm: They all have safety cables between the engine and airframe, in case the engine mounts turn loose, they'll still have the engine for ballast in maintaining cg while landing.

MTV

Ah, yes...very good point!

Can I just move the safety cables I have now up from the gear attach points to the engine mounts? :lol:

sj

steve said:

Can I just move the safety cables I have now up from the gear attach points to the engine mounts? :lol:

sj

Click to expand...

Kind of like wrapping the parachute harness around your neck!...